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Gaseous fission reactor

A gas nuclear reactor (or gas fueled reactor or vapor core reactor) is a proposed kind of nuclear reactor in which the nuclear fuel would be in a gaseous state rather than liquid or solid. In this type of reactor, the only temperature-limiting materials would be the reactor walls. Conventional reactors have stricter limitations because the core would melt if the fuel temperature were to rise too high. It may also be possible to confine gaseous fission fuel magnetically, electrostatically or electrodynamically so that it would not touch (and melt) the reactor walls. A potential benefit of the gaseous reactor core concept is that instead of relying on the traditional Rankine or Brayton conversion cycles, it may be possible to extract electricity magnetohydrodynamically, or with simple direct electrostatic conversion of the charged particles.

Theory of operation
The vapor core reactor (VCR), also called a gas core reactor (GCR), has been studied for some time. It would have a gas or vapor core composed of uranium tetrafluoride (UF4) with some helium (4He) added to increase the electrical conductivity, the vapor core may also have tiny UF4 droplets in it. It has both terrestrial and space based applications. Since the space concept doesn't necessarily have to be economical in the traditional sense, it allows the enrichment to exceed what would be acceptable for a terrestrial system. It also allows for a higher ratio of UF4 to helium, which in the terrestrial version would be kept just high enough to ensure criticality in order to increase the efficiency of direct conversion. The terrestrial version is designed for a vapor core inlet temperature of about 1,500 K and exit temperature of 2,500 K and a UF4 to helium ratio of around 20% to 60%. It is thought that the outlet temperature could be raised to that of the 8,000 K to 15,000 K range where the exhaust would be a fission-generated non-equilibrium electron gas, which would be of much more importance for a rocket design. A terrestrial version of the VCR's flow schematic can be found in reference 2 and in the summary of non-classical nuclear systems in the second external link. The space based concept would be cut off at the end of the MHD channel.

Reasoning for He-4 addition
4He may be used in increase the ability of the design to extract energy and be controlled. A few sentences from Anghaie et al. sheds light on the reasoning:

 * "The power density in the MHD duct is proportional to the product of electrical conductivity, velocity squared and magnetic field squared σv²B². Therefore, the enthalpy extraction is very sensitive to the MHD input-output fluid conditions. The vapor core reactor provides a hotter-than-most fluid with potential for adequate thermal equilibrium conductivity and duct velocities. Considering the product v² × B², it is apparent that a light working fluid should dominate the thermal properties and the UF4 fraction should be small. Additional electrical conductivity enhancement might be needed from thermal ionization of suitable seed materials, and from non-equilibrium ionization by fission fragments and other ionizing radiation produced by the fissioning process."

Spacecraft
The spacecraft variant of the gaseous fission reactor is called the gas core reactor rocket. There are two approaches: the open and closed cycle. In the open cycle, the propellant, most likely hydrogen, is fed to the reactor, heated up by the nuclear reaction in the reactor, and exits out the other end. Unfortunately, the propellant will be contaminated by fuel and fission products, and although the problem can be mitigated by engineering the hydrodynamics within the reactor, it renders the rocket design completely unsuitable for use in atmosphere.

One might attempt to circumvent the problem by confining the fission fuel magnetically, in a manner similar to the fusion fuel in a tokamak. Unfortunately it is not likely that this arrangement will actually work to contain the fuel, since the ratio of ionization to particle momentum is not favourable. Whereas a tokamak would generally work to contain singly ionized deuterium or tritium with a mass of two or three daltons, the uranium vapour would be at most triply ionized with a mass of 235 dalton (unit). Since the force imparted by a magnetic field is proportional to the charge on the particle, and the acceleration is proportional to the force divided by the mass of the particle, the magnets required to contain uranium gas would be impractically large; most such designs have focused on fuel cycles that do not depend upon retaining the fuel in the reactor.

In the closed cycle, the reaction is entirely shielded from the propellant. The reaction is contained in a quartz vessel and the propellant merely flows outside of it, being heated in an indirect fashion. The closed cycle avoids contamination because the propellant can't enter the reactor itself, but the solution carries a significant penalty to the rocket's Isp.

Energy production
For energy production purposes, one might use a container located inside a solenoid. The container is filled with gaseous uranium hexafluoride, where the uranium is enriched, to a level just short of criticality. Afterward, the uranium hexafluoride is compressed by external means, thus initiating a nuclear chain reaction and a great amount of heat, which in turn causes an expansion of the uranium hexafluoride. Since the UF6 is contained within the vessel, it can't escape and thus compresses elsewhere. The result is a plasma wave moving in the container, and the solenoid converts some of its energy into electricity at an efficiency level of about 20%. In addition, the container must be cooled, and one can extract energy from the coolant by passing it through a heat exchanger and turbine system as in an ordinary thermal power plant.

However, there are enormous problems with corrosion during this arrangement, as the uranium hexafluoride is chemically very reactive.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

title: Gaseous fission reactor


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Nuclear salt-water rocket

The nuclear salt-water rocket (NSWR) is a theoretical type of nuclear thermal rocket designed by Robert Zubrin. In place of traditional chemical propellant, such as that in a chemical rocket, the rocket would be fueled by salts of plutonium or 20-percent-enriched uranium. The solution would be contained in a bundle of pipes coated in boron carbide (for its properties of neutron absorption). Through a combination of the coating and space between the pipes, the contents would not reach critical mass until the solution is pumped into a reaction chamber, thus reaching a critical mass, and being expelled through a nozzle to generate thrust.

Proposed design
Chemical rockets use heat energy produced by a chemical reaction to heat the gas products. The hot products exit through a propulsion nozzle at a very high speed, creating thrust. In a nuclear thermal rocket (NTR), thrust is created by heating a fluid by using a nuclear fission reactor. The lower the molecular weight of the exhaust (hydrogen having the lowest possible), the more efficient the motor can be. However, in this engine the propellant can be any of many fluids having suitable properties as it does not participate in generating heat. In a NSWR, the nuclear salt-water would be made to flow through a reaction chamber and out of an exhaust nozzle in such a way and at such speeds that critical mass will begin once the chamber is filled to a certain point; however, the peak neutron flux of the fission reaction would occur outside the vehicle.

Advantages of the design
There are several advantages relative to conventional NTR designs. As the peak neutron flux and fission reaction rates would occur outside the vehicle, these activities could be much more vigorous than they could be if it was necessary to house them in a vessel (which would have temperature limits due to materials constraints). Additionally, a contained reactor can allow only a small percentage of its fuel to undergo fission at any given time; otherwise, it would overheat and melt down (or explode in a runaway fission chain reaction). The fission reaction in an NSWR is dynamic, and because the reaction products are exhausted into space, it does not have a limit on the proportion of fission fuel that reacts. In many ways, NSWRs combine the advantages of fission reactors and fission bombs.

Because they can harness the power of what is essentially a continuous nuclear fission explosion, NSWRs would have both very high thrust and very high exhaust velocity, meaning that the rocket would be able to accelerate quickly as well as be extremely efficient in terms of propellant usage. The combination of high thrust and high specific impulse is a very rare trait in the rocket world. One design would generate 13 meganewtons of thrust at 66 km/s exhaust velocity (or 6,730 seconds ISP compared to ~4.5 km/s (450 s ISP) exhaust velocity for the best chemical rockets as of February 2023).

The design and calculations discussed above are using 20-percent-enriched uranium salts. However, it would be plausible to use another design which would be capable of achieving much higher exhaust velocities (4,725 km/s) and use a 30,000-tonne ice comet along with 7,500 tonnes of highly enriched uranium salts to propel a 300-tonne spacecraft up to 7.62% of the speed of light and potentially arrive at Alpha Centauri after a 60-year journey.

"NSWRs share many of the features of Orion propulsion systems, except that NSWRs would generate continuous rather than pulsed thrust and may be workable on much smaller scales than the smallest feasible Orion designs (which are generally large, due to the requirements of the shock-absorber system and the minimum size of efficient nuclear explosives)."

Limitations
The propellant used in the initial design would contain a rather large amount of the relatively expensive isotope $235$U, which would not be very cost effective. However, if the use of NSWR began to rise, it would be possible to replace this with the cheaper isotopes $233$U or $239$Pu in either fission breeder reactors or (much better) nuclear fusion–fission hybrid reactors. These other fissiles would have the right characteristics to serve nearly as well, at a relatively low cost.

Another major limitation of the nuclear salt-water rocket design by Robert Zubrin included the lack of a material to be used in the reaction chamber that could actually sustain such a reaction within a spacecraft. Zubrin claimed in his design that the apparatus was created so that the liquid flow rate or velocity was what mattered most in the process, not the material. Therefore, he argued that if the proper velocity was chosen for the liquid traveling through the reaction chamber, the site of maximum fission release could then be located at the end of the chamber, thus allowing the system to remain intact and safe to operate. These claims have still not been proven due to no test of such a device having ever been conducted.

For example, Zubrin argues that if diluted nuclear fuel flows into the chamber at speed similar to diffusion speed of thermal neutrons, then the nuclear reaction is confined in the chamber and does not damage the rest of the system (the nuclear analog of a gas burner). A possible problem in that line of thinking is that neutrons do not all diffuse at the same velocity, but have a broad distribution over several orders of magnitude. It is possible that tails of this velocity distribution would be sufficient to generate enough heat in fuel feeding system (by scattering and fission) to destroy the system. This question can be perhaps answered by detailed Monte Carlo simulations of neutron transport.

The vessel's exhaust would contain radioactive isotopes, but in space, these would be rapidly dispersed after travelling only a short distance; the exhaust would also be travelling at high speed (in Zubrin's scenario, faster than Solar escape velocity, allowing it to eventually leave the Solar System). However, this is of little use on the surface of a planet, where a NSWR would eject massive quantities of superheated steam, still containing fissioning nuclear salts. Terrestrial testing might be subject to reasonable objections; as one physicist wrote:
 * "Writing the environmental impact statement for such tests [...] might present an interesting problem ...".

It is also not certain that fission in a NSWR could be controlled:
 * "Whether fast criticality can be controlled in a rocket engine remains an open question".

title: Nuclear salt-water rocket


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Beam-powered propulsion

Beam-powered propulsion, also known as directed energy propulsion, is a class of aircraft or spacecraft propulsion that uses energy beamed to the spacecraft from a remote power plant to provide energy. The beam is typically either a microwave or a laser beam, and it is either pulsed or continuous. A continuous beam lends itself to thermal rockets, photonic thrusters, and light sails. In contrast, a pulsed beam lends itself to ablative thrusters and pulse detonation engines.

The rule of thumb that is usually quoted is that it takes a megawatt of power beamed to a vehicle per kg of payload while it is being accelerated to permit it to reach low Earth orbit.

Other than launching to orbit, applications for moving around the world quickly have also been proposed.

Background
Rockets are momentum machines; they use mass ejected from the rocket to provide momentum to the rocket. Momentum is the product of mass and velocity, so rockets generally attempt to put as much velocity into their working mass as possible, thereby minimizing the needed working mass. To accelerate the working mass, energy is required. In a conventional rocket, the fuel is chemically combined to provide the energy, and the resulting fuel products, the ash or exhaust, are used as the working mass.

There is no particular reason why the same fuel has to be used for both energy and momentum. In the jet engine, for instance, the fuel is used only to produce energy, and the air provides the working mass the jet aircraft flies through. In modern jet engines, the amount of air propelled is much more significant than the amount used for energy. However, this is not a solution for the rockets as they quickly climb to altitudes where the air is too thin to be useful as a source of working mass.

Rockets can carry their working mass and use other energy sources. The problem is finding an energy source with a power-to-weight ratio that competes with chemical fuels. Small nuclear reactors can compete in this regard, and considerable work on nuclear thermal propulsion was carried out in the 1960s, but environmental concerns and rising costs led to the ending of most of these programs.

Further improvement can be made by removing the energy created by the spacecraft. If the nuclear reactor is left on the ground and its energy is transmitted to the spacecraft, its weight is also removed. The issue then is getting the energy into the spacecraft. This is the idea behind beamed power.

With beamed propulsion, one can leave the power source stationary on the ground and directly (or via a heat exchanger) heat propellant on the spacecraft with a maser or a laser beam from a fixed installation. This permits the spacecraft to leave its power source at home, saving significant amounts of mass and greatly improving performance.

Laser propulsion
Since a laser can heat propellant to extremely high temperatures, this potentially greatly improves the efficiency of a rocket, as exhaust velocity is proportional to the square root of the temperature. Normal chemical rockets have an exhaust speed limited by the fixed amount of energy in the propellants, but beamed propulsion systems have no particular theoretical limit (although, in practice, there are temperature limits).

Microwave propulsion
In microwave thermal propulsion, an external microwave beam is used to heat a refractory heat exchanger to >1,500 K, heating a propellant such as hydrogen, methane, or ammonia. This improves the propulsion system's specific impulse and thrust/weight ratio relative to conventional rocket propulsion. For example, hydrogen can provide a specific impulse of 700–900 seconds and a thrust/weight ratio of 50-150.

A variation, developed by brothers James Benford and Gregory Benford, is to use thermal desorption of propellant trapped in the material of a massive microwave sail. This produces a very high acceleration compared to microwave-pushed sails alone.

Electric propulsion
Some proposed spacecraft propulsion mechanisms use electrically powered spacecraft propulsion, in which electrical energy is used by an electrically powered rocket engine, such as an ion thruster or plasma propulsion engine. Usually, these schemes assume either solar panels or an onboard reactor. However, both power sources are heavy.

Beamed propulsion in the form of a laser can send power to a photovoltaic panel for Laser electric propulsion. In this system, if a high intensity is incident on the solar array, careful design of the panels is necessary to avoid a fall-off in conversion efficiency due to heating effects. John Brophy has analyzed the transmission of laser power to a photovoltaic array powering a high-efficiency electric propulsion system as a means of accomplishing high delta-V missions such as an interstellar precursor mission in a NASA Innovative Advanced Concepts project.

A microwave beam could be used to send power to a rectenna for microwave electric propulsion. Microwave broadcast power has been practically demonstrated several times (e.g., in Goldstone, California, in 1974). Rectennas are potentially lightweight and can handle high power at high conversion efficiency. However, rectennas must be huge for a significant amount of power to be captured.

Direct impulse
A beam could also provide impulse by directly "pushing" on the sail.

One example is using a solar sail to reflect a laser beam. This concept, called a laser-pushed lightsail, was initially proposed by G. Marx but first analyzed in detail, and elaborated on, by physicist Robert L. Forward in 1989 as a method of interstellar travel that would avoid extremely high mass ratios by not carrying fuel. Further analysis of the concept was done by Landis, Mallove and Matloff, Andrews Lubin, and others.

Forward proposed pushing a sail with a microwave beam in a later paper. This has the advantage that the sail need not be a continuous surface. Forward tagged his proposal for an ultralight sail "Starwisp". A later analysis by Landis suggested that the Starwisp concept as initially proposed by Forward would not work, but variations on the proposal might be implemented.

The beam has to have a large diameter so that only a small portion of the beam misses the sail due to diffraction, and the laser or microwave antenna has to have good pointing stability so that the craft can tilt its sails fast enough to follow the center of the beam. This gets more important when going from interplanetary travel to interstellar travel and when going from a fly-by mission to a landing mission to a return mission. The laser or the microwave sender would probably be a large phased array of small devices that get their energy directly from solar radiation. The size of the array negates the need for a lens or mirror.

Another beam-pushed concept would be to use a magnetic sail or MMPP sail to divert a beam of charged particles from a particle accelerator or plasma jet. Landis proposed a particle beam pushed sail in 1989, and analyzed in more detail in a 2004 paper. Jordin Kare has proposed a variant to this whereby a "beam" of small laser accelerated light sails would transfer momentum to a magsail vehicle.

Another beam-pushed concept uses pellets or projectiles of ordinary matter. A stream of pellets from a stationary mass-driver is "reflected" by the spacecraft, cf. mass driver. The spacecraft neither needs energy nor reaction mass for propulsion of its own.

Lightcraft
A lightcraft is a vehicle currently under development that uses an external pulsed source of laser or maser energy to provide power for producing thrust.

The laser shines on a parabolic reflector on the vehicle's underside, concentrating the light to produce a region of extremely high temperature. The air in this region is heated and expands violently, producing thrust with each pulse of laser light. A lightcraft must provide this gas from onboard tanks or an ablative solid in space. By leaving the vehicle's power source on the ground and using the ambient atmosphere as reaction mass for much of its ascent, a lightcraft could deliver a substantial percentage of its launch mass to orbit. It could also potentially be very cheap to manufacture.

Testing
Early in the morning of 2 October 2000 at the High Energy Laser Systems Test Facility (HELSTF), Lightcraft Technologies, Inc. (LTI) with the help of Franklin B. Mead of the U.S. Air Force Research Laboratory and Leik Myrabo set a new world's altitude record of 233 feet (71 m) for its 4.8 inch (12.2 cm) diameter, 1.8 oz, laser-boosted rocket in a flight lasting 12.7 seconds. Although much of the 8:35 am flight was spent hovering at 230+ feet, the Lightcraft earned a world record for the longest ever laser-powered free flight and the greatest "air time" (i.e., launch-to-landing/recovery) from a light-propelled object. This is comparable to Robert Goddard's first test flight of his rocket design. Increasing the laser power to 100 kilowatts will enable flights up to a 30-kilometer altitude. They aim to accelerate a one-kilogram microsatellite into low Earth orbit using a custom-built, one-megawatt ground-based laser. Such a system would use just about 20 dollars worth of electricity, placing launch costs per kilogram to many times less than current launch costs (which are measured in thousands of dollars).

Myrabo's "lightcraft" design is a reflective funnel-shaped craft that channels heat from the laser toward the center, using a reflective parabolic surface, causing the laser to explode the air underneath it, generating lift. Reflective surfaces in the craft focus the beam into a ring, where it heats air to a temperature nearly five times hotter than the surface of the Sun, causing the air to expand explosively for thrust.

Laser thermal rocket
A laser thermal rocket is a thermal rocket in which the propellant is heated by energy provided by an external laser beam. In 1992, the late Jordin Kare proposed a simpler, nearer-term concept with a rocket containing liquid hydrogen. The propellant is heated in a heat exchanger that the laser beam shines on before leaving the vehicle via a conventional nozzle. This concept can use continuous beam lasers, and the semiconductor lasers are now cost-effective for this application.

Microwave thermal rocket
In 2002, Kevin L.G. Parkin proposed a similar system using microwaves. In May 2012, the DARPA/NASA Millimeter-wave Thermal Launch System (MTLS) Project began the first steps toward implementing this idea. The MTLS Project was the first to demonstrate a millimeter-wave absorbent refractory heat exchanger, subsequently integrating it into the propulsion system of a small rocket to produce the first millimeter-wave thermal rocket. Simultaneously, it developed the first high-power cooperative target millimeter-wave beam director and used it to attempt the first millimeter-wave thermal rocket launch. Several launches were attempted, but problems with the beam director could not be resolved before funding ran out in March 2014.

Economics
The motivation to develop beam-powered propulsion systems comes from the economic advantages gained due to improved propulsion performance. In the case of beam-powered launch vehicles, better propulsion performance enables some combination of increased payload fraction, increased structural margins, and fewer stages. JASON's 1977 study of laser propulsion, authored by Freeman Dyson, succinctly articulates the promise of beam-powered launch: "'Laser propulsion as an idea that may produce a revolution in space technology. A single laser facility on the ground can in theory launch single-stage vehicles into low or high earth orbit. The payload can be 20% or 30% of the vehicle take-off weight. It is far more economical in the use of mass and energy than chemical propulsion, and it is far more flexible in putting identical vehicles into a variety of orbits.'"This promise was quantified in a 1978 Lockheed Study conducted for NASA:"'The results of the study showed that, with advanced technology, laser rocket system with either a space- or ground-based laser transmitter could reduce the national budget allocated to space transportation by 10 to 345 billion dollars over a 10-year life cycle when compared to advanced chemical propulsion systems (LO2-LH2) of equal capability.'"

Beam director cost
The 1970s-era studies and others since have cited beam director cost as a possible impediment to beam-powered launch systems. A recent cost-benefit analysis estimates that microwave (or laser) thermal rockets would be economical once beam director cost falls below 20 $/Watt. The current cost of suitable lasers is <100 $/Watt and the cost of suitable microwave sources is <$5/Watt. Mass production has lowered the production cost of microwave oven magnetrons to <0.01 $/Watt and some medical lasers to <10 $/Watt, though these are considered unsuitable for beam directors.

Non-spacecraft applications
In 1964 William C. Brown demonstrated a miniature helicopter equipped with a combination antenna and rectifier device called a rectenna. The rectenna converted microwave power into electricity, allowing the helicopter to fly.

In 2002 a Japanese group propelled a tiny aluminium airplane by using a laser to vaporize a water droplet clinging to it, and in 2003 NASA researchers flew an 11-ounce (312 g) model airplane with a propeller powered with solar panels illuminated by a laser. It is possible that such beam-powered propulsion could be useful for long-duration high altitude uncrewed aircraft or balloons, perhaps designed to serve – like satellites do today – as communication relays, science platforms, or surveillance platforms.

A "laser broom" has been proposed to sweep space debris from Earth orbit. This is another proposed use of beam-powered propulsion, used on objects not designed to be propelled by it, for example, small pieces of scrap knocked off ("spalled") satellites. The technique works since the laser power ablates one side of the object, giving an impulse that changes the eccentricity of the object's orbit. The orbit would then intersect the atmosphere and burn up.

title: Beam-powered propulsion


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Nuclear photonic rocket

In a traditional nuclear photonic rocket, an onboard nuclear reactor would generate such high temperatures that the blackbody radiation from the reactor would provide significant thrust. The disadvantage is that it takes much power to generate a small amount of thrust this way, so acceleration is very low. The photon radiators would most likely be constructed using graphite or tungsten. Photonic rockets are technologically feasible, but rather impractical with current technology based on an onboard nuclear power source.

Energy requirements and comparisons
The power per thrust required for a perfectly collimated output beam is 300 MW/N (half this if it can be reflected off the craft); very high energy density power sources would be required to provide reasonable thrust without unreasonable weight. The specific impulse of a photonic rocket is harder to define, since the output has no (rest) mass and is not expended fuel; if we take the momentum per inertia of the photons, the specific impulse is just c, which is impressive. However, considering the mass of the source of the photons, e.g., atoms undergoing nuclear fission, brings the specific impulse down to 300 km/s (c/1000) or less; considering the infrastructure for a reactor (some of which also scales with the amount of fuel) reduces the value further. Finally, any energy loss not through radiation that is redirected precisely to aft but is instead conducted away by engine supports, radiated in some other direction, or lost via neutrinos or so will further degrade the efficiency. If we were to set 80% of the mass of the photon rocket = fissionable fuel, and recognizing that nuclear fission converts about 0.10% of the mass into energy: then if the photon rocket masses 300,000 kg then 240,000 kg of that is atomic fuel. Therefore, the fissioning of all of the fuel will result in the loss of just 240 kg of mass. Then 300,000/299,760 kg = an mi/mf of 1.0008. Using the rocket equation, we find vf = ln 1.0008 × c where c = 299,792,458 m/s. vf then may be 239,930 m/s which is about 240 km/s. The nuclear fission powered photon rocket may accelerate at a maximum of perhaps 1/10,000 m/s² (0.1 mm/s²) which is 10−5g. The velocity change would be at the rate of 3,000 m/s per year of thrusting by the photon rocket.

If a photon rocket begins its journey in low Earth orbit, then one year of thrusting may be required to achieve an Earth escape velocity of 11.2 km/s if the vehicle is already in orbit at a velocity of 9,100 m/s. Upon escaping the Earth's gravitational field the rocket will have a heliocentric velocity of 30 km/s in interplanetary space. Eighty years of steady photonic thrusting would be then required to obtain a final velocity of 240 km/s in this hypothetical case.

It is possible to obtain even higher specific impulse; that of some other photonic propulsion devices (e.g., solar sails) is effectively infinite because no carried fuel is required. Alternatively, such devices as ion thrusters, while having a notably lower specific impulse, give a much better thrust-to-power ratio; for photons, that ratio is $$1/c$$, whereas for slow particles (that is, nonrelativistic; even the output from typical ion thrusters counts) the ratio is $$2/v$$, which is much larger (since $$v\ll c$$). (This is in a sense an unfair comparison, since the photons must be created and other particles are merely accelerated, but nonetheless the impulses per carried mass and per applied energy—the practical quantities—are as given.) The photonic rocket is thus wasteful when power and not mass is at a premium, or when enough mass can be saved through the use of a weaker power source that reaction mass can be included without penalty.

A laser could be used as a photon rocket engine, and would solve the reflection/collimation problem, but lasers are absolutely less efficient at converting energy into light than blackbody radiation is—though one should also note the benefits of lasers vs blackbody source, including unidirectional controllable beam and the mass and durability of the radiation source. The limitations posed by the rocket equation can be overcome, as long as the reaction mass is not carried by the spacecraft. In the Beamed Laser Propulsion (BLP) concept, the photons are beamed from the photon source to the spacecraft as coherent light. Robert L. Forward pioneered interstellar propulsion concepts including photon propulsion and antimatter rocket propulsion. However, BLP is limited because of the extremely low thrust generation efficiency of photon reflection. One of the best ways to overcome the inherent inefficiency in producing thrust of the photon thruster by amplifying the momentum transfer of photons by recycling photons between two high reflectance mirrors.

Power sources
Feasible current, or near-term fission reactor designs can generate up to 2.2 kW per kilogram of reactor mass. Without any payload, such a reactor could drive a photon rocket at nearly 10−5 m/s² (10−6g; see g-force). This could perhaps provide interplanetary spaceflight capability from Earth orbit. Nuclear fusion reactors could also be used, perhaps providing somewhat higher power.

A design proposed in the 1950s by Eugen Sänger used positron-electron annihilation to produce gamma rays. Sänger was unable to solve the problem of how to reflect, and collimate the gamma rays created by positron-electron annihilation; however, by shielding the reactions (or other annihilations) and absorbing their energy, a similar blackbody propulsion system could be created. An antimatter-matter powered photon rocket would (disregarding the shielding) obtain the maximum c specific impulse; for this reason, an antimatter-matter annihilation powered photon rocket could potentially be used for interstellar spaceflight.

Theoretically, other designs such as spacecraft using a Kugelblitz micro black hole could also be used for interstellar travel given the efficiency of black holes in converting matter into energy.

title: Nuclear photonic rocket


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Fusion rocket

A fusion rocket is a theoretical design for a rocket driven by fusion propulsion that could provide efficient and sustained acceleration in space without the need to carry a large fuel supply. The design requires fusion power technology beyond current capabilities, and much larger and more complex rockets.

Fusion nuclear pulse propulsion is one approach to using nuclear fusion energy to provide propulsion.

Fusion's main advantage is its very high specific impulse, while its main disadvantage is the (likely) large mass of the reactor. A fusion rocket may produce less radiation than a fission rocket, reducing the shielding mass needed. The simplest way of building a fusion rocket is to use hydrogen bombs as proposed in Project Orion, but such a spacecraft would be massive and the Partial Nuclear Test Ban Treaty prohibits the use of such bombs. For that reason bomb-based rockets would likely be limited to operating only in space. An alternate approach uses electrical (e.g. ion) propulsion with electric power generated by fusion instead of direct thrust.

Electricity generation vs. direct thrust
Spacecraft propulsion methods such as ion thrusters require electric power to run, but are highly efficient. In some cases their thrust is limited by the amount of power that can be generated (for example, a mass driver). An electric generator running on fusion power could drive such a ship. One disadvantage is that conventional electricity production requires a low-temperature energy sink, which is difficult (i.e. heavy) in a spacecraft. Direct conversion of the kinetic energy of fusion products into electricity mitigates this problem.

One attractive possibility is to direct the fusion exhaust out the back of the rocket to provide thrust without the intermediate production of electricity. This would be easier with some confinement schemes (e.g. magnetic mirrors) than with others (e.g. tokamaks). It is also more attractive for "advanced fuels" (see aneutronic fusion). Helium-3 propulsion would use the fusion of helium-3 atoms as a power source. Helium-3, an isotope of helium with two protons and one neutron, could be fused with deuterium in a reactor. The resulting energy release could expel propellant out the back of the spacecraft. Helium-3 is proposed as a power source for spacecraft mainly because of its lunar abundance. Scientists estimate that 1 million tons of accessible helium-3 are present on the moon. Only 20% of the power produced by the D-T reaction could be used this way; while the other 80% is released as neutrons which, because they cannot be directed by magnetic fields or solid walls, would be difficult to direct towards thrust, and may in turn require shielding. Helium-3 is produced via beta decay of tritium, which can be produced from deuterium, lithium, or boron.

Even if a self-sustaining fusion reaction cannot be produced, it might be possible to use fusion to boost the efficiency of another propulsion system, such as a VASIMR engine.

Magnetic
To sustain a fusion reaction, the plasma must be confined. The most widely studied configuration for terrestrial fusion is the tokamak, a form of magnetic confinement fusion. Currently tokamaks weigh a great deal, so the thrust to weight ratio would seem unacceptable. NASA's Glenn Research Center proposed in 2001 a small aspect ratio spherical torus reactor for its "Discovery II" conceptual vehicle design. "Discovery II" could deliver a crewed 172 metric tons payload to Jupiter in 118 days (or 212 days to Saturn) using 861 metric tons of hydrogen propellant, plus 11 metric tons of Helium-3-Deuterium (D-He3) fusion fuel. The hydrogen is heated by the fusion plasma debris to increase thrust, at a cost of reduced exhaust velocity (348–463 km/s) and hence increased propellant mass.

Inertial
The main alternative to magnetic confinement is inertial confinement fusion (ICF), such as that proposed by Project Daedalus. A small pellet of fusion fuel (with a diameter of a couple of millimeters) would be ignited by an electron beam or a laser. To produce direct thrust, a magnetic field forms the pusher plate. In principle, the Helium-3-Deuterium reaction or an aneutronic fusion reaction could be used to maximize the energy in charged particles and to minimize radiation, but it is highly questionable whether using these reactions is technically feasible. Both the detailed design studies in the 1970s, the Orion drive and Project Daedalus, used inertial confinement. In the 1980s, Lawrence Livermore National Laboratory and NASA studied an ICF-powered "Vehicle for Interplanetary Transport Applications" (VISTA). The conical VISTA spacecraft could deliver a 100-tonne payload to Mars orbit and return to Earth in 130 days, or to Jupiter orbit and back in 403 days. 41 tonnes of deuterium/tritium (D-T) fusion fuel would be required, plus 4,124 tonnes of hydrogen expellant. The exhaust velocity would be 157 km/s.

Magnetized target
Magnetized target fusion (MTF) is a relatively new approach that combines the best features of the more widely studied magnetic confinement fusion (i.e. good energy confinement) and inertial confinement fusion (i.e. efficient compression heating and wall free containment of the fusing plasma) approaches. Like the magnetic approach, the fusion fuel is confined at low density by magnetic fields while it is heated into a plasma, but like the inertial confinement approach, fusion is initiated by rapidly squeezing the target to dramatically increase fuel density, and thus temperature. MTF uses "plasma guns" (i.e. electromagnetic acceleration techniques) instead of powerful lasers, leading to low cost and low weight compact reactors. The NASA/MSFC Human Outer Planets Exploration (HOPE) group has investigated a crewed MTF propulsion spacecraft capable of delivering a 164-tonne payload to Jupiter's moon Callisto using 106-165 metric tons of propellant (hydrogen plus either D-T or D-He3 fusion fuel) in 249–330 days. This design would thus be considerably smaller and more fuel efficient due to its higher exhaust velocity (700 km/s) than the previously mentioned "Discovery II", "VISTA" concepts.

Inertial electrostatic
Another popular confinement concept for fusion rockets is inertial electrostatic confinement (IEC), such as in the Farnsworth-Hirsch Fusor or the Polywell variation under development by Energy-Matter Conversion Corporation (EMC2). The University of Illinois has defined a 500-tonne "Fusion Ship II" concept capable of delivering a 100,000 kg crewed payload to Jupiter's moon Europa in 210 days. Fusion Ship II utilizes ion rocket thrusters (343 km/s exhaust velocity) powered by ten D-He3 IEC fusion reactors. The concept would need 300 tonnes of argon propellant for a 1-year round trip to the Jupiter system. Robert Bussard published a series of technical articles discussing its application to spaceflight throughout the 1990s. His work was popularised by an article in the Analog Science Fiction and Fact publication, where Tom Ligon described how the fusor would make for a highly effective fusion rocket.

Antimatter
A still more speculative concept is antimatter-catalyzed nuclear pulse propulsion, which would use antimatter to catalyze a fission and fusion reaction, allowing much smaller fusion explosions to be created. During the 1990s an abortive design effort was conducted at Penn State University under the name AIMStar. The project would require more antimatter than can currently be produced. In addition, some technical hurdles need to be surpassed before it would be feasible.

Development projects

 * MSNW Magneto-Inertial Fusion Driven Rocket
 * MSNW Magneto-Inertial Fusion Driven Rocket

title: Fusion rocket


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Bussard ramjet

[[File:Bussard ramjet.gif|thumb|Bussard ramjet in motion. 1. Interstellar medium

2. Collect and compress hydrogen

3. Transport hydrogen beside the payload

4. Thermonuclear fusion

5. Engine nozzle

6. Flue gas jet ]]

The Bussard ramjet is a theoretical method of spacecraft propulsion for interstellar travel. A fast moving spacecraft scoops up hydrogen from the interstellar medium using an enormous funnel-shaped magnetic field (ranging from kilometers to many thousands of kilometers in diameter); the hydrogen is compressed until thermonuclear fusion occurs, which provides thrust to counter the drag created by the funnel and energy to power the magnetic field. The Bussard ramjet can thus be seen as a ramjet variant of a fusion rocket.

The Bussard ramjet was proposed in 1960 by the physicist Robert W. Bussard.

The concept was popularized by Poul Anderson in his novel Tau Zero, Larry Niven in his Known Space series of books, Vernor Vinge in his Zones of Thought series, and Carl Sagan, as referenced in the television series and book Cosmos.

Feasibility
Since the time of Bussard's original proposal, it has been discovered that the region surrounding the Solar System has a much lower density of hydrogen than was believed at that time (see Local Interstellar Cloud). In 1969, John Ford Fishback made an important contribution, describing the details of the required magnetic field.

In 1978, T. A. Heppenheimer analyzed Bussard's original suggestion of fusing protons, but found the Bremsstrahlung losses from compressing protons to fusion densities was greater than the power that could be produced by a factor of about 1 billion, thus indicating that the proposed version of the Bussard ramjet was infeasible. However, Daniel P. Whitmire's 1975 analysis indicates that a ramjet may achieve net power via the CNO cycle, which produces fusion at a much higher rate (~1016 times higher) than the proton–proton chain.

Robert Zubrin and Dana Andrews analyzed one hypothetical version of the Bussard ramjet design in 1988. They determined that their version of the ramjet would be unable to accelerate into the solar wind.

A 2021 study found that, while feasible in principle, the practical construction of a useful Bussard ramjet would be beyond even a civilization of Kardashev type II.

Ram Augmented Interstellar Rocket (RAIR)
The problem of using the interstellar medium as the sole fuel source led to study of the Ram Augmented Interstellar Rocket (RAIR). The RAIR carries its nuclear fuel supply and exhausts the reaction products to produce some of its thrust. However it greatly enhances its performance by scooping the interstellar medium and using this as extra reaction mass to augment the rocket. The propulsion system of the RAIR consists of three subsystems: a fusion reactor, a scoop field, and a plasma accelerator. Fuel is launched ahead of the ship with the accelerator. The scoop field funnels the fuel into another accelerator (this could for example be a heat exchange system transferring thermal energy from the reactor directly to the interstellar gas) which is supplied power from a reactor. One of the best ways to understand this concept is to consider that the hydrogen nuclear fuel carried on board acts as a fuel (energy source) whereas the interstellar gas collected by the scoop and then exhausted at great speed from the back acts as a propellant (the reaction mass), the vehicle therefore has a limited fuel supply but an unlimited propellant supply. A normal Bussard ramjet would have an infinite supply of both. However, theory suggests that where a Bussard ramjet would suffer drag from having to pre-accelerate interstellar gas to its own speed before intake, a RAIR system would be able to transfer energy via the "accelerator" mechanism to the interstellar medium despite velocity differences, and so would suffer far less drag.

Laser Powered Interstellar Ramjet
Beamed energy coupled with a vehicle scooping hydrogen from the interstellar medium is another variant. A laser array in the solar system beams to a collector on a vehicle which uses something like a linear accelerator to produce thrust. This solves the fusion reactor problem for the ramjet. There are limitations because of the attenuation of beamed energy with distance.

Magnetic sail
The calculations (by Robert Zubrin and Dana Andrews) inspired the idea of a magnetic parachute or sail. This could be important for interstellar travel because it means that deceleration at the destination can be performed with a magnetic parachute rather than a rocket.

Dyson swarm-based stellar engine (Caplan thruster)
Astrophysicist Matthew E. Caplan of Illinois State University has proposed a type of stellar engine that uses a Dyson swarm of mirrors to concentrate stellar energy onto certain regions of a Sun-like star, producing beams of solar wind to be collected by a multi-ramjet assembly which in turn produces directed jets of plasma to stabilize its orbit and oxygen-14 to push the star. Using rudimentary calculations that assume maximum efficiency, Caplan estimates the Bussard engine would use 1015 grams per second of solar material to produce a maximum acceleration of 10−9 m/s2, yielding a velocity of 200 km/s after 5 million years, and a distance of 10 parsecs over 1 million years. The Bussard engine would theoretically work for 100 million years given the mass loss rate of the Sun, but Caplan deems 10 million years to be sufficient for a stellar collision avoidance.

Pre-seeded trajectory
Several of the obvious technical difficulties with the Bussard ramjet can be overcome by setting out solid pellets of fuel along the spacecraft's trajectory in advance. This could be done using a different "tanker" spacecraft dropping fuel pellets or using laser propulsion. The method has been referred to as the "fusion runway", ramjet runway or "forward resupply runway".

While most proposals use fusion power, as with conventional Bussard ramjets, fission has also been suggested.

The advantages of this system include:

 * Fusion can be achieved by slamming two fuel pellets together at a speed of at least 200 km/s - this is known as impact fusion. Compared to inertial confinement fusion, impact fusion would not require spherically symmetrical pellets or application of force from all directions.
 * The fuel pellets could use isotopes such as deuterium (hydrogen-2) or tritium (hydrogen-3). A conventional Bussard ramjet will mostly collect protium (hydrogen-1), as this is the most abundant isotope of hydrogen in nature.
 * Launching only ionized fusion fuel so that either magnetic or electrostatic scoops can more easily funnel the fuel into the engine. The drawback is this will cause the fuel to disperse due to electrostatic repulsion.
 * Launching the fuel on a trajectory so that the fuel velocity vector will closely match the expected velocity vector of the spacecraft at that point in its trajectory. This will minimize the "drag" forces generated by the collection of fuel.
 * Although the prelaunched fuel for the ramjet negates one advantage of the Bussard design (collection of fuel as it moves through the interstellar medium, saving the cost to launch the fuel mass), it at least retains the advantage of not having to accelerate the mass of the fuel and the mass of the rocket at the same time.
 * The prelaunched fuel would provide some visibility into the interstellar medium – thus alerting the trailing spacecraft of unseen hazards (e.g., brown dwarfs).

The major disadvantages of this system include:

 * The need to precisely intercept the fusion runway pellets with a spacecraft moving at high speed. Laser pulses could be used to guide the pellets.
 * If using impact fusion, the spacecraft must be moving at a minimum speed in order to achieve fusion. One suggestion is for the spacecraft to use a Sun-skimming maneuver to reach 600 km/s.
 * The spacecraft could not deviate from the precalculated trajectory unless it was critical to do so. Any such deviation would separate the spacecraft from its fuel supply and leave it with only a minimal ability to return to its original trajectory.
 * Prelaunched fuel for deceleration at the destination star would not be available unless launched many decades in advance of the spacecraft launch. However, other systems (such as the magnetic sails) could be used for this purpose.

title: Bussard ramjet


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Antimatter rocket

An antimatter rocket is a proposed class of rockets that use antimatter as their power source. There are several designs that attempt to accomplish this goal. The advantage to this class of rocket is that a large fraction of the rest mass of a matter/antimatter mixture may be converted to energy, allowing antimatter rockets to have a far higher energy density and specific impulse than any other proposed class of rocket.

Methods
Antimatter rockets can be divided into three types of application: those that directly use the products of antimatter annihilation for propulsion, those that heat a working fluid or an intermediate material which is then used for propulsion, and those that heat a working fluid or an intermediate material to generate electricity for some form of electric spacecraft propulsion system. The propulsion concepts that employ these mechanisms generally fall into four categories: solid core, gaseous core, plasma core, and beamed core configurations. The alternatives to direct antimatter annihilation propulsion offer the possibility of feasible vehicles with, in some cases, vastly smaller amounts of antimatter but require a lot more matter propellant. Then there are hybrid solutions using antimatter to catalyze fission/fusion reactions for propulsion.

Pure antimatter rocket: direct use of reaction products
Antiproton annihilation reactions produce charged and uncharged pions, in addition to neutrinos and gamma rays. The charged pions can be channelled by a magnetic nozzle, producing thrust. This type of antimatter rocket is a pion rocket or beamed core configuration. It is not perfectly efficient; energy is lost as the rest mass of the charged (22.3%) and uncharged pions (14.38%), lost as the kinetic energy of the uncharged pions (which can't be deflected for thrust); and lost as neutrinos and gamma rays (see antimatter as fuel).

Positron annihilation has also been proposed for rocketry. Annihilation of positrons produces only gamma rays. Early proposals for this type of rocket, such as those developed by Eugen Sänger, assumed the use of some material that could reflect gamma rays, used as a light sail or parabolic shield to derive thrust from the annihilation reaction, but no known form of matter (consisting of atoms or ions) interacts with gamma rays in a manner that would enable specular reflection. The momentum of gamma rays can, however, be partially transferred to matter by Compton scattering.

One method to reach relativistic velocities uses a matter-antimatter GeV gamma ray laser photon rocket made possible by a relativistic proton-antiproton pinch discharge, where the recoil from the laser beam is transmitted by the Mössbauer effect to the spacecraft.

A new annihilation process has allegedly been developed by researchers from Gothenborg University. Several annihilation reactors have been constructed in the past years which attempted to convert hydrogen or deuterium into relativistic particles through laser annihilation. The technology was explored by research groups led by Prof. Leif Holmlid and Sindre Zeiner-Gundersen, and a third relativistic particle reactor is currently being built at the University of Iceland. In theory, emitted particles from hydrogen annihilation processes could reach 0.94c and can be used in space propulsion. However the veracity of Holmlid's research is under dispute and no successful implementations have been peer reviewed or replicated.

Thermal antimatter rocket: heating of a propellant
This type of antimatter rocket is termed a thermal antimatter rocket as the energy or heat from the annihilation is harnessed to create an exhaust from non-exotic material or propellant.

The solid core concept uses antiprotons to heat a solid, high-atomic weight (Z), refractory metal core. Propellant is pumped into the hot core and expanded through a nozzle to generate thrust. The performance of this concept is roughly equivalent to that of the nuclear thermal rocket ($$I_{\text{sp}}$$ ~ 103 sec) due to temperature limitations of the solid. However, the antimatter energy conversion and heating efficiencies are typically high due to the short mean path between collisions with core atoms (efficiency $$\eta_e$$ ~ 85%). Several methods for the liquid-propellant thermal antimatter engine using the gamma rays produced by antiproton or positron annihilation have been proposed. These methods resemble those proposed for nuclear thermal rockets. One proposed method is to use positron annihilation gamma rays to heat a solid engine core. Hydrogen gas is ducted through this core, heated, and expelled from a rocket nozzle. A second proposed engine type uses positron annihilation within a solid lead pellet or within compressed xenon gas to produce a cloud of hot gas, which heats a surrounding layer of gaseous hydrogen. Direct heating of the hydrogen by gamma rays was considered impractical, due to the difficulty of compressing enough of it within an engine of reasonable size to absorb the gamma rays. A third proposed engine type uses annihilation gamma rays to heat an ablative sail, with the ablated material providing thrust. As with nuclear thermal rockets, the specific impulse achievable by these methods is limited by materials considerations, typically being in the range of 1000–2000 seconds.

The gaseous core system substitutes the low-melting point solid with a high temperature gas (i.e. tungsten gas/plasma), thus permitting higher operational temperatures and performance ($$I_{\text{sp}}$$ ~ 2 × 103 sec). However, the longer mean free path for thermalization and absorption results in much lower energy conversion efficiencies ($$\eta_e$$ ~ 35%).

The plasma core allows the gas to ionize and operate at even higher effective temperatures. Heat loss is suppressed by magnetic confinement in the reaction chamber and nozzle. Although performance is extremely high ($$I_{\text{sp}}$$ ~ 104-105 sec), the long mean free path results in very low energy utilization ($$\eta_e$$ ~ 10%)

Antimatter power generation
The idea of using antimatter to power an electric space drive has also been proposed. These proposed designs are typically similar to those suggested for nuclear electric rockets. Antimatter annihilations are used to directly or indirectly heat a working fluid, as in a nuclear thermal rocket, but the fluid is used to generate electricity, which is then used to power some form of electric space propulsion system. The resulting system shares many of the characteristics of other charged particle/electric propulsion proposals, that typically being high specific impulse and low thrust (An associated article further detailing antimatter power generation).

Catalyzed fission/fusion or spiked fusion
This is a hybrid approach in which antiprotons are used to catalyze a fission/fusion reaction or to "spike" the propulsion of a fusion rocket or any similar applications.

The antiproton-driven Inertial confinement fusion (ICF) Rocket concept uses pellets for the D-T reaction. The pellet consists of a hemisphere of fissionable material such as U235 with a hole through which a pulse of antiprotons and positrons is injected. It is surrounded by a hemisphere of fusion fuel, for example deuterium-tritium, or lithium deuteride. Antiproton annihilation occurs at the surface of the hemisphere, which ionizes the fuel. These ions heat the core of the pellet to fusion temperatures.

The antiproton-driven Magnetically Insulated Inertial Confinement Fusion Propulsion (MICF) concept relies on self-generated magnetic field which insulates the plasma from the metallic shell that contains it during the burn. The lifetime of the plasma was estimated to be two orders of magnitude greater than implosion inertial fusion, which corresponds to a longer burn time, and hence, greater gain.

The antimatter-driven P-B11 concept uses antiprotons to ignite the P-B11 reactions in an MICF scheme. Excessive radiation losses are a major obstacle to ignition and require modifying the particle density, and plasma temperature to increase the gain. It was concluded that it is entirely feasible that this system could achieve Isp~105s.

A different approach was envisioned for AIMStar in which small fusion fuel droplets would be injected into a cloud of antiprotons confined in a very small volume within a reaction Penning trap. Annihilation takes place on the surface of the antiproton cloud, peeling back 0.5% of the cloud. The power density released is roughly comparable to a 1 kJ, 1 ns laser depositing its energy over a 200 μm ICF target.

The ICAN-II project employs the antiproton catalyzed microfission (ACMF) concept which uses pellets with a molar ratio of 9:1 of D-T:U235 for Nuclear pulse propulsion.

Difficulties with antimatter rockets
The chief practical difficulties with antimatter rockets are the problems of creating antimatter and storing it. Creating antimatter requires input of vast amounts of energy, at least equivalent to the rest energy of the created particle/antiparticle pairs, and typically (for antiproton production) tens of thousands to millions of times more. Most storage schemes proposed for interstellar craft require the production of frozen pellets of antihydrogen. This requires cooling of antiprotons, binding to positrons, and capture of the resulting antihydrogen atoms - tasks which have,, been performed only for small numbers of individual atoms. Storage of antimatter is typically done by trapping electrically charged frozen antihydrogen pellets in Penning or Paul traps. There is no theoretical barrier to these tasks being performed on the scale required to fuel an antimatter rocket. However, they are expected to be extremely (and perhaps prohibitively) expensive due to current production abilities being only able to produce small numbers of atoms, a scale approximately 1023 times smaller than needed for a 10-gram trip to Mars.

Generally, the energy from antiproton annihilation is deposited over such a large region that it cannot efficiently drive nuclear capsules. Antiproton-induced fission and self-generated magnetic fields may greatly enhance energy localization and efficient use of annihilation energy.

A secondary problem is the extraction of useful energy or momentum from the products of antimatter annihilation, which are primarily in the form of extremely energetic ionizing radiation. The antimatter mechanisms proposed to date have for the most part provided plausible mechanisms for harnessing energy from these annihilation products. The classic rocket equation with its "wet" mass ($$M_0$$)(with propellant mass fraction) to "dry" mass ($$M_1$$)(with payload) fraction ($$\frac {M_0}{M_1}$$), the velocity change ($$\Delta v $$) and specific impulse ($$I_{\text{sp}}$$) no longer holds due to the mass losses occurring in antimatter annihilation.

Another general problem with high powered propulsion is excess heat or waste heat, and as with antimatter-matter annihilation also includes extreme radiation. A proton-antiproton annihilation propulsion system transforms 39% of the propellant mass into an intense high-energy flux of gamma radiation. The gamma rays and the high-energy charged pions will cause heating and radiation damage if they are not shielded against. Unlike neutrons, they will not cause the exposed material to become radioactive by transmutation of the nuclei. The components needing shielding are the crew, the electronics, the cryogenic tankage, and the magnetic coils for magnetically assisted rockets. Two types of shielding are needed: radiation protection and thermal protection (different from Heat shield or thermal insulation).

Finally, relativistic considerations have to be taken into account. As the by products of annihilation move at relativistic velocities the rest mass changes according to relativistic mass–energy. For example, the total mass–energy content of the neutral pion is converted into gammas, not just its rest mass. It is necessary to use a relativistic rocket equation that takes into account the relativistic effects of both the vehicle and propellant exhaust (charged pions) moving near the speed of light. These two modifications to the two rocket equations result in a mass ratio ($$\frac {M_0}{M_1}$$) for a given ($$\Delta v $$) and ($$I_{\text{sp}}$$) that is much higher for a relativistic antimatter rocket than for either a classical or relativistic "conventional" rocket.

Modified relativistic rocket equation
The loss of mass specific to antimatter annihilation requires a modification of the relativistic rocket equation given as

where $$c$$ is the speed of light, and $$I_{\text{sp}}$$ is the specific impulse (i.e. $$I_{\text{sp}}$$=0.69$$c$$).

The derivative form of the equation is

where $$M_{\text{ship}}$$ is the non-relativistic (rest) mass of the rocket ship, and $$a$$ is the fraction of the original (on board) propellant mass (non-relativistic) remaining after annihilation (i.e., $$a$$=0.22 for the charged pions).

$$ is difficult to integrate analytically. If it is assumed that $$v \sim I_{\text{sp}}$$, such that $$(1 - \frac {I_{\text{sp}} v}{c^2}) \sim (1 - \frac {v^2}{c^2})$$ then the resulting equation is

$$ can be integrated and the integral evaluated for $$M_0$$ and $$M_1$$, and initial and final velocities ($$v_i = 0$$ and $$v_f = \Delta v$$). The resulting relativistic rocket equation with loss of propellant is

Other general issues
The cosmic background hard radiation will ionize the rocket's hull over time and poses a health threat. Also, gas plasma interactions may cause space charge. The major interaction of concern is differential charging of various parts of a spacecraft, leading to high electric fields and arcing between spacecraft components. This can be resolved with well placed plasma contactor. However, there is no solution yet for when plasma contactors are turned off to allow maintenance work on the hull. Long term space flight at interstellar velocities causes erosion of the rocket's hull due to collision with particles, gas, dust and micrometeorites. At 0.2$$c$$ for a 6 light year distance, erosion is estimated to be in the order of about 30 kg/m2 or about 1 cm of aluminum shielding.

title: Antimatter rocket


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Alcubierre drive

The Alcubierre drive is a speculative warp drive idea according to which a spacecraft could achieve apparent faster-than-light travel by contracting space in front of it and expanding space behind it, under the assumption that a configurable energy-density field lower than that of vacuum (that is, negative mass) could be created. Proposed by theoretical physicist Miguel Alcubierre in 1994, the Alcubierre drive is based on a solution of Einstein's field equations. Since those solutions are metric tensors, the Alcubierre drive is also referred to as Alcubierre metric.

Objects cannot accelerate to the speed of light within normal spacetime; instead, the Alcubierre drive shifts space around an object so that the object would arrive at its destination more quickly than light would in normal space without breaking any physical laws.

Although the metric proposed by Alcubierre is consistent with the Einstein field equations, construction of such a drive is not necessarily possible. The proposed mechanism of the Alcubierre drive implies a negative energy density and therefore requires exotic matter or manipulation of dark energy. If exotic matter with the correct properties cannot exist, then the drive cannot be constructed. At the close of his original article, however, Alcubierre argued (following an argument developed by physicists analyzing traversable wormholes ) that the Casimir vacuum between parallel plates could fulfill the negative-energy requirement for the Alcubierre drive.

Another possible issue is that, although the Alcubierre metric is consistent with Einstein's equations, general relativity does not incorporate quantum mechanics. Some physicists have presented arguments to suggest that a theory of quantum gravity (which would incorporate both theories) would eliminate those solutions in general relativity that allow for backward time travel (see the chronology protection conjecture) and thus make the Alcubierre drive invalid.

History
In 1994, Miguel Alcubierre proposed a method for changing the geometry of space by creating a wave that would cause the fabric of space ahead of a spacecraft to contract and the space behind it to expand. The ship would then ride this wave inside a region of flat space, known as a warp bubble, and would not move within this bubble but instead be carried along as the region itself moves due to the actions of the drive. The local velocity relative to the deformed spacetime would be subluminal, but the speed at which a spacecraft could move would be superluminal, thereby rendering possible interstellar flight, such as a visit to Proxima Centauri within a few days.

Alcubierre metric
The Alcubierre metric defines the warp-drive spacetime. It is a Lorentzian manifold that, if interpreted in the context of general relativity, allows a warp bubble to appear in previously flat spacetime and move away at effectively faster-than-light speed. The interior of the bubble is an inertial reference frame and inhabitants experience no proper acceleration. This method of transport does not involve objects in motion at faster-than-light speeds with respect to the contents of the warp bubble; that is, a light beam within the warp bubble would still always move more quickly than the ship. Because objects within the bubble are not moving (locally) more quickly than light, the mathematical formulation of the Alcubierre metric is consistent with the conventional claims of the laws of relativity (namely, that an object with mass cannot attain or exceed the speed of light) and conventional relativistic effects such as time dilation would not apply as they would with conventional motion at near-light speeds.

An extension of the Alcubierre metric that eliminates the expansion of the volume elements and instead relies on the change in distances along the direction of travel is that of mathematician José Natário. In his metric, spacetime contracts towards the prow of the ship and expands in the direction perpendicular to the motion, meaning that the bubble actually "slides" through space, roughly speaking by "pushing space aside".

The Alcubierre drive remains a hypothetical concept with seemingly difficult problems, although the amount of energy required is no longer thought to be unobtainably large. Furthermore, Alexey Bobrick and Gianni Martire claim that, in principle, a class of subluminal, spherically symmetric warp drive spacetimes can be constructed based on physical principles presently known to humanity, such as positive energy.

Mathematics
Using the ADM formalism of general relativity, the spacetime is described by a foliation of space-like hypersurfaces of constant coordinate time $t$, with the metric taking the following general form:

 * $$ds^2 = -\left(\alpha^2 - \beta_i \beta^i\right) \,dt^2 + 2 \beta_i \,dx^i \,dt + \gamma_{ij}\,dx^i\,dx^j,$$

where
 * $α$ is the lapse function that gives the interval of proper time between nearby hypersurfaces,
 * $β^{i}$ is the shift vector that relates the spatial coordinate systems on different hypersurfaces,
 * $γ_{ij}$ is a positive-definite metric on each of the hypersurfaces.

The particular form that Alcubierre studied is defined by:

 * $$\begin{align}

\alpha &= 1,\\ \beta^x &= -v_s(t)f\big(r_s(t)\big),\\ \beta^y &= \beta^z = 0,\\ \gamma_{ij} &= \delta_{ij}, \end{align}$$

where

 * $$\begin{align}

v_s(t) &= \frac{dx_s(t)}{dt},\\ r_s(t) &= \sqrt{\big(x - x_s(t)\big)^2 + y^2 + z^2},\\ f(r_s) &= \frac{\tanh\big(\sigma (r_s + R)\big) - \tanh\big(\sigma (r_s - R)\big)}{2 \tanh(\sigma R)}, \end{align}$$

with arbitrary parameters $R > 0$ and $σ > 0$. Alcubierre's specific form of the metric can thus be written:

 * $$ds^2 = \left(v_s(t)^2 f\big(r_s(t)\big)^2 - 1\right)\,dt^2 - 2v_s(t)f\big(r_s(t)\big)\,dx\,dt + dx^2 + dy^2 + dz^2.$$

With this particular form of the metric, it can be shown that the energy density measured by observers whose 4-velocity is normal to the hypersurfaces is given by:

 * $$-\frac{c^4}{8 \pi G} \frac{v_s^2 \left(y^2 + z^2\right)}{4 g^2 r_s ^2} \left(\frac{df}{dr_s}\right)^2,$$

where $g$ is the determinant of the metric tensor.

Thus, because the energy density is negative, one needs exotic matter to travel more quickly than the speed of light. The existence of exotic matter is not theoretically ruled out; however, generating and sustaining enough exotic matter to perform feats such as faster-than-light travel (and to keep open the "throat" of a wormhole) is thought to be impractical. According to writer Robert Low, within the context of general relativity it is impossible to construct a warp drive in the absence of exotic matter.

Connection to dark energy and dark matter
Astrophysicist Jamie Farnes from the University of Oxford has proposed a theory, published in the peer-reviewed scientific journal Astronomy & Astrophysics, that unifies dark energy and dark matter into a single dark fluid, and which is expected to be testable by the Square Kilometre Array around 2030. Farnes found that Albert Einstein had explored the idea of gravitationally repulsive negative masses while developing the equations of general relativity, an idea which leads to a "beautiful" hypothesis where the cosmos has equal amounts of positive and negative qualities. Farnes' theory relies on negative masses that behave identically to the physics of the Alcubierre drive, providing a natural solution for the current "crisis in cosmology" due to a time-variable Hubble parameter.

As Farnes' theory allows a positive mass (i.e. a ship) to reach a speed equal to the speed of light, it has been dubbed "controversial". If the theory is correct, which has been highly debated in the scientific literature, it would explain dark energy, dark matter, allow closed timelike curves (see time travel), and suggest that an Alcubierre drive is physically possible with exotic matter.

Physics
With regard to certain specific effects of special relativity, such as Lorentz contraction and time dilation, the Alcubierre metric has some apparently peculiar aspects. In particular, Alcubierre has shown that a ship using an Alcubierre drive travels on a free-fall geodesic even while the warp bubble is accelerating: its crew would be in free fall while accelerating without experiencing accelerational g-forces. Enormous tidal forces, however, would be present near the edges of the flat-space volume because of the large space curvature there, but a suitable specification of the metric would keep the tidal forces very small within the volume occupied by the ship.

The original warp-drive metric and simple variants of it happen to have the ADM form, which is often used in discussing the initial-value formulation of general relativity. This might explain the widespread misconception that this spacetime is a solution of the field equation of general relativity. Metrics in ADM form are adapted to a certain family of inertial observers, but these observers are not really physically distinguished from other such families. Alcubierre interpreted his "warp bubble" in terms of a contraction of space ahead of the bubble and an expansion behind, but this interpretation could be misleading, since the contraction and expansion actually refer to the relative motion of nearby members of the family of ADM observers.

In general relativity, one often first specifies a plausible distribution of matter and energy, and then finds the geometry of the spacetime associated with it; but it is also possible to run the Einstein field equations in the other direction, first specifying a metric and then finding the energy–momentum tensor associated with it, and this is what Alcubierre did in building his metric. This practice means that the solution can violate various energy conditions and require exotic matter. The need for exotic matter raises questions about whether one can distribute the matter in an initial spacetime that lacks a warp bubble in such a way that the bubble is created at a later time, although some physicists have proposed models of dynamical warp-drive spacetimes in which a warp bubble is formed in a previously flat space. Moreover, according to Serguei Krasnikov, generating a bubble in a previously flat space for a one-way faster-than-light trip requires forcing the exotic matter to move at local faster-than-light speeds, something that would require the existence of tachyons, although Krasnikov also notes that when the spacetime is not flat from the outset, a similar result could be achieved without tachyons by placing in advance some devices along the travel path and programming them to come into operation at preassigned moments and to operate in a preassigned manner. Some suggested methods avoid the problem of tachyonic motion, but would probably generate a naked singularity at the front of the bubble. Allen Everett and Thomas Roman comment on Krasnikov's finding (Krasnikov tube):

"[The finding] does not mean that Alcubierre bubbles, if it were possible to create them, could not be used as a means of superluminal travel. It only means that the actions required to change the metric and create the bubble must be taken beforehand by some observer whose forward light cone contains the entire trajectory of the bubble."

For example, if one wanted to travel to Deneb (2,600 light-years away) and arrive less than 2,600 years in the future according to external clocks, it would be required that someone had already begun work on warping the space from Earth to Deneb at least 2,600 years ago:

"A spaceship appropriately located with respect to the bubble trajectory could then choose to enter the bubble, rather like a passenger catching a passing trolley car, and thus make the superluminal journey ... as Krasnikov points out, causality considerations do not prevent the crew of a spaceship from arranging, by their own actions, to complete a round trip from Earth to a distant star and back in an arbitrarily short time, as measured by clocks on Earth, by altering the metric along the path of their outbound trip."

Mass–energy requirement
The metric of this form has significant difficulties because all known warp-drive spacetime theories violate various energy conditions. Nevertheless, an Alcubierre-type warp drive might be realized by exploiting certain experimentally verified quantum phenomena, such as the Casimir effect, that lead to stress–energy tensors that also violate the energy conditions, such as negative mass–energy, when described in the context of the quantum field theories.

If certain quantum inequalities conjectured by Ford and Roman hold, the energy requirements for some warp drives may be unfeasibly large as well as negative. For example, the energy equivalent of −1064 kg might be required to transport a small spaceship across the Milky Way—an amount orders of magnitude greater than the estimated mass of the observable universe. Counterarguments to these apparent problems have also been offered, although the energy requirements still generally require a Type III civilization on the Kardashev scale.

Chris Van Den Broeck of the Katholieke Universiteit Leuven in Belgium, in 1999, tried to address the potential issues. By contracting the 3+1-dimensional surface area of the bubble being transported by the drive, while at the same time expanding the three-dimensional volume contained inside, Van Den Broeck was able to reduce the total energy needed to transport small atoms to less than three solar masses. Later in 2003, by slightly modifying the Van den Broeck metric, Serguei Krasnikov reduced the necessary total amount of negative mass to a few milligrams. Van Den Broeck detailed this by saying that the total energy can be reduced dramatically by keeping the surface area of the warp bubble itself microscopically small, while at the same time expanding the spatial volume inside the bubble. However, Van Den Broeck concludes that the energy densities required are still unachievable, as are the small size (a few orders of magnitude above the Planck scale) of the spacetime structures needed.

In 2012, physicist Harold White and collaborators announced that modifying the geometry of exotic matter could reduce the mass–energy requirements for a macroscopic space ship from the equivalent of the planet Jupiter to that of the Voyager 1 spacecraft (c. 700 kg) or less, and stated their intent to perform small-scale experiments in constructing warp fields. White proposed to thicken the extremely thin wall of the warp bubble, so the energy is focused in a larger volume, but the overall peak energy density is actually smaller. In a flat 2D representation, the ring of positive and negative energy, initially very thin, becomes a larger, fuzzy torus (donut shape). However, as this less energetic warp bubble also thickens toward the interior region, it leaves less flat space to house the spacecraft, which has to be smaller. Furthermore, if the intensity of the space warp can be oscillated over time, the energy required is reduced even more. According to White, a modified Michelson–Morley interferometer could test the idea: one of the legs of the interferometer would appear to have a slightly different length when the test devices were energised. Alcubierre has expressed skepticism about the experiment, saying: "from my understanding there is no way it can be done, probably not for centuries if at all".

In 2021, physicist Erik Lentz described a way warp drives sourced from known and familiar purely positive energy could exist—warp bubbles based on superluminal self-reinforcing "soliton" waves. The claim is controversial, with other physicists arguing that all physically reasonable warp drives violate the weak energy condition, as well as both the strong and dominant energy conditions.

Placement of matter
Krasnikov proposed that if tachyonic matter cannot be found or used, then a solution might be to arrange for masses along the path of the vessel to be set in motion in such a way that the required field was produced. But in this case, the Alcubierre drive vessel can only travel routes that, like a railroad, have first been equipped with the necessary infrastructure. The pilot inside the bubble is causally disconnected from its walls and cannot carry out any action outside the bubble: the bubble cannot be used for the first trip to a distant star because the pilot cannot place infrastructure ahead of the bubble while "in transit". For example, traveling to Vega (which is 25 light-years from Earth) requires arranging everything so that the bubble moving toward Vega with a superluminal velocity would appear; such arrangements will always take more than 25 years.

Coule has argued that schemes, such as the one proposed by Alcubierre, are infeasible because matter placed en route of the intended path of a craft must be placed at superluminal speed—that constructing an Alcubierre drive requires an Alcubierre drive even if the metric that allows it is physically meaningful. Coule further argues that an analogous objection will apply to any proposed method of constructing an Alcubierre drive.

Survivability inside the bubble
An article by José Natário (2002) argues that crew members could not control, steer or stop the ship in its warp bubble because the ship could not send signals to the front of the bubble.

A 2009 article by Carlos Barceló, Stefano Finazzi, and Stefano Liberati uses quantum theory to argue that the Alcubierre drive at faster-than-light velocities is impossible mostly because extremely high temperatures caused by Hawking radiation would destroy anything inside the bubble at superluminal velocities and destabilize the bubble itself; the article also argues that these problems are absent if the bubble velocity is subluminal, although the drive still requires exotic matter.

Damaging effect on destination
Brendan McMonigal, Geraint F. Lewis, and Philip O'Byrne have argued that were an Alcubierre-driven ship to decelerate from superluminal speed, the particles that its bubble had gathered in transit would be released in energetic outbursts akin to the infinitely-blueshifted radiation hypothesized to occur at the inner event horizon of a Kerr black hole; forward-facing particles would thereby be energetic enough to destroy anything at the destination directly in front of the ship.

Wall thickness
The amount of negative energy required for such a propulsion is not yet known. Pfenning and Allen Everett of Tufts hold that a warp bubble traveling at 10-times the speed of light must have a wall thickness of no more than 10−32 meters—close to the limiting Planck length, 1.6 × 10−35 meters. In Alcubierre's original calculations, a bubble macroscopically large enough to enclose a ship of 200 meters would require a total amount of exotic matter greater than the mass of the observable universe, and straining the exotic matter to an extremely thin band of 10−32 meters is considered impractical. Similar constraints apply to Krasnikov's superluminal subway. Chris Van den Broeck constructed a modification of Alcubierre's model that requires much less exotic matter but places the ship in a curved spacetime "bottle" whose neck is about 10−32 meters.

Causality violation and semiclassical instability
Calculations by physicist Allen Everett show that warp bubbles could be used to create closed timelike curves in general relativity, meaning that the theory predicts that they could be used for backwards time travel. While it is possible that the fundamental laws of physics might allow closed timelike curves, the chronology protection conjecture hypothesizes that in all cases where the classical theory of general relativity allows them, quantum effects would intervene to eliminate the possibility, making these spacetimes impossible to realize. A possible type of effect that would accomplish this is a buildup of vacuum fluctuations on the border of the region of spacetime where time travel would first become possible, causing the energy density to become high enough to destroy the system that would otherwise become a time machine. Some results in semiclassical gravity appear to support the conjecture, including a calculation dealing specifically with quantum effects in warp-drive spacetimes that suggested that warp bubbles would be semiclassically unstable, but ultimately the conjecture can only be decided by a full theory of quantum gravity.

Alcubierre briefly discusses some of these issues in a series of lecture slides posted online, where he writes: "beware: in relativity, any method to travel faster than light can in principle be used to travel back in time (a time machine)". In the next slide, he brings up the chronology protection conjecture and writes: "The conjecture has not been proven (it wouldn't be a conjecture if it had), but there are good arguments in its favor based on quantum field theory. The conjecture does not prohibit faster-than-light travel. It just states that if a method to travel faster than light exists, and one tries to use it to build a time machine, something will go wrong: the energy accumulated will explode, or it will create a black hole."

Relation to Star Trek warp drive
The Star Trek television series and films use the term "warp drive" to describe their method of faster-than-light travel. Neither the Alcubierre theory, nor anything similar, existed when the series was conceived—the term "warp drive" and general concept originated with John W. Campbell's 1931 science fiction novel Islands of Space. Alcubierre stated in an email to William Shatner that his theory was directly inspired by the term used in the show and cites the "'warp drive' of science fiction" in his 1994 article. A USS Alcubierre appears in the Star Trek tabletop RPG Star Trek Adventures. Since the release of Star Trek: The Original Series, more recent Star Trek spin-off series have made closer use of the theory behind the Alcubierre Drive incorporating warp bubbles/fields into the in-universe science.

title: Alcubierre drive