J Grannec

L Lozane

J Portier

M Fouchard

P Sorbe

P Hagenmuller

CHEMISTRY OF FLUORIDES OF THE TRANSITION 3d-ELEMENTS IN HIGHER OXIDATION STATES

The structural and electronic contigurations of fluorides of the 3d-elements in higher oxidation states (ionic radii, nature of the chemical bonds and factors influencing it, etc.) are discussed. A comparison 1s made with the fluorides of the 4d-and Sd-elements. The laws determining the chemical, structural, and elec tronic properties of the transition of the 3d-elements in higher oxidation states were derived on the basis of the experimental data.

. The attention of chemists has also been attracted to the investigation of the possibilities of using fluorides of the elements in increased oxidation States: these compounds are comparatively "mild' 1 fluorinating agents, which are finding use in the selective synthesis of organic cornpounds.

.

1

Figure l•prese�ts the oxidation states for fluorides of the 3d-elements. The transition elernent always ha�'àn octahedral configuration, even in increased oxidation states [START_REF] Babel | Structure and Bonding[END_REF], How ever, we should: 1 -ri!tlke a reservation when we are speaking of oxidation states corresponding to the presence of four or nine electrons for high-spin configurations (Cr 2 + , Mn 3 + , Cu 2 + ) or seven electrons for low-spin configurations (Ni 3 + ), for which Jahn-Teller distortions are ob served.

In the case of structures corresponding to the highest oxidation states, usually the Pauling rules are fulfilled: the mutually isolated MF 6 octahedra ensure maximum weakening of the forces of electrostatic repulsion. The �hases formed correspond most often to the compositions M 6 +r6, A+i.i 5 +p 6 , A 2 fi.1: Q ry 6 , A 2 + A'fi.i 3 � 6 , etc, (in Tables 1 and2, a list of the known fluoride compounds of Ni, Cu(III) and (IV), and the alkali metals is cited as an exam ple [START_REF] Grannec | [END_REF][START_REF] Sorbe | These[END_REF][START_REF] Sorbe | [END_REF][8][9][10][11][12][13][14][15][16][17]). Figure 2 depicts idealized structures, corresponding to the formulas A2MF 6 and AsMF6 , on the basis of which the lattices of the abovementioned compounds can be obtained. These structures can be described by face-centered cubic packing of the (MF 6 ) n -octahedra. In compounds corresponding to the formula A2MF 6 , the ions A occupy "tetrahed ral" positions, in which they are actually surrounded by 12 fluoride ions. -In compounds with composition A2MF6, one third of the A ions also occupy octahedral positions; the structure of the A2A'MF6 phases, in which, depend ing on the size of the ions A and A', a change in symmetry may be observed, is analogous, Table 3 indicates the electronic configurations that can be used for 3d -ions contained in fluori d es. Although the ligand field created by fluorine can be consid ered weak as a re sult of the str.Ông ionic character of the bond s (as a result of which, chiefly high-spin con figurations are• •observed) , nonetheless, in elements at the end of the 3d-group, low-spin con figurations begin to appear [Cl(IV), Ni(III) or (IV), Cu(III)]. This effect can be explained by an inten�ification of the crystal field, due to a decreas e in the size of the cation as a result oL:tl\'e increase in the effective charge of the nucleus.

The case of trivalent nickel merits special consideration. There is no doubt that this ion possesses a low-spin configuration at low temperatures: an investigation of the magnetic_ susceptibility indicates that at the temperature 4.2 ° K, Ni(III) possesses an effective morrent equal (depending on the compound ) to from 1.73 to 2.11 µB; the value of this moment 1s close to the value (1.73 �B) calculated for the ion d 7 in the low-spin configuration on the assump tion of the existence only of the spin contribution [START_REF] Sorbe | These[END_REF]. On the other hand, x-ray diffrac tion study of Cs2KNiF6, which possesses a cubic elpazolite structure at normal temperature, indicates the appearance of tetragonal symmetry at temperatures below 140 °K (18], The ap pearance of tetragonal distortion is d ue to the Jahn-Teller effect as a result of nonequiva population of the e g orbitals, It should be noted that the structure of the homolog com pound Cs2KMnF6 also corresponds to tetragonal symmetry; the four d-electrons of the Mn(III) ions are in a high-spin state (Fig. 3), At normal temperature, when this distortion in a nickel-containing compound no longer exists, the effective moment is of the order of 4.3 µB; thus, we might have assumed the existence of a transition from the low-spin to the high-spin state (with increasing tempera ture). In the case of a high-spin configuration, the value of the magnetic moment d rops in 'l' (Structure of the K 2 PtCl 6 type.

the range 3.76-5.20 µB; symmetrical population of the e g -orbitals leads to the appearance of regular octahedral (NiF6) 3 -, and, as a result, to cubic symmetry of the lattice, The cor rectness of this interpretation, however, is placed in doubt by the results obtained by the ESR rnethod in an ±nrestigation of various compounds of trivalent nickel, conducted in [19].

According to [19], a,low-spin configuration might also be preserved at high temperature, since cubic symmetry is due to the dynamic Jahn-Teller effect • . ;

Let us turn.;: ,t'o' the electronic configurations of the 3d-elements (Table 3). Since Ni (III) at room temper-âture occupies a low-spin configuration, it rnight have been expected than an analogous situation would also exist for the Cu(III) ion, to which an even higher charge of the nucleus corresponds. Actually, the situation proves to be different: in all th� corn pounds Cu(III) possesses a high-spin configuration. This effect can be explained as the re sult of the presence of eight d-electrons. In the case of Cu(III), the existence of a low spin configuration would lead to a substantial distortion of the (CuF6 ) 3 -octahedra, which is not compatible either with the weak crystal field created by the fluoride ions or with the geornetry of their arrangement around the Cu atoms. In view of this, it should be men tioned that the isoelectronic ion N1 2 + possesses a high-spin configuration in all compounds with fluorine.

Ionie Radii. Whereas the radii of transition metal ions in the oxidation state +2 are well known, matters are otherwise for the case of the oxidation states + 3 and +4. An analy sis of the parameters of a large number of phases of the type of A2-f-M 4 +r 6 , AlMF6 , or A2 + A 1 + M 3 + r'6 permits us to fill this gap (Fig. 4). A deterrnination of the ionic radius was per formed by cornparing the parameters of these phases with the corresponding parameters for identical structures, for which the value of the ionic radius was already cited in the ta bles of Shannon and Prewitt [20], We endeavored to maximally narrow the limita of values [20] (1, 2) and this work [START_REF] Dance | [END_REF][START_REF] Babel | Structure and Bonding[END_REF].

found, so that the ionic radius could be considered as a linear function of the cubic param eter of this phase. Calculations performed for various phases gave results in sufficiently good agreement for the possibility of determining the values of the ionic radii with an ac curacy of ±0.01 A.

On the whole, it is possible to ascertain the presence of a compression effect with in crease ionic•��mber within the limits of the same series, with the exception, however, of the se 3 i uence

6
from Cr 5+ (d 3 + ) to Mn 3+ (d 4 , high-spin state) and from Ni 3+ (d 7 , low-spin state) to Cu (d , hJgh-spin state ) . The increase in the radius observed in this case, although negligible, :tnight serve as an argument supporting a low-spin configuration (at least partial) for Ni(III) -1 ':ln the. structure of the fluorides considered.

Another comment must be made, For the series of tetravalent cations it could have been assumed that the Cu 4+ ion should be characterized by a larger radius than the Ni 4 + ion t as a result of the appearance of a seventh d-electron on the orbital e g • However, actually this is not so; therefore, we conclude that for increased oxidation states the in�rease in the effective charge of the nucleus more than balances the effects of the crystal field. The in fluence of weakening of the shielding effect of the electrons present when their number de creases consista precisely of this.

Stability of Fluorides of Elements in Increased Oxidation States. It seems advisable to formulate the general laws of stability for fluorides of elements in higher oxidation states. (Here and henceforth the term 11 s tability" will be used in the thermodynamic sense.) Strictly speaking, the ideal method would consist of comparing the standard enthalpies of formation of various fluorides considered. Unfortunately, there is only a small number of thermodynamic data pertaining to binary compounds of the elements in lower oxidation states. Considering this� we had to limit ourselves to a comparison of the empirical data. Influence of the Crystal Structure. Above it was noted that in higher oxidation states a transition element is accommoctated w1thin rnutually isolated octahedra, formed by fluorine atoms. This pertains, for example, to the case of a structure of the K2PtCl6 type, realized in ternary fluorides for manganese, cobalt, nickel, and copper ions in the maximum oxidation state. This effect is explained by a maximum weakening of the forces of electrostatic re pulsion.

An influence of the surroundings, due to the presence of other cat. ions, is also mani fested. For example, it becomes understandable that certain phases, such as Na2NiF6 and Na2CoF6, have not been absolutely reliably synthesized as yet, Actually, if there were no unfavorable factor due to the presence of a alkali metal ion with smaller radius, weakening the covalency of the Ni -F and Co -F bonds, such phases should most likely have crystallized in a structure of the Na2 SiF 6 type. in which the (MF 6 ) 2 -octahedra would be bonded to the (NaF6) 5 octahedra through common ribs, which would lead to strong electrostatic repulsion [ 21] Influence of Covalency of the Metal -. Fluorine Bond. Frorn the standpoint of the simple electrostatic approach, the difficulty of obtaining high oxidation states is intensified in the sequence from titanium to copper as a result of an increase in the effective charge of the nucleus, which 1s manifested in an increase in the ionization potentials.

However, the ionic mode! is not sufficient. And actually, it does not permit an ex planation of the changes observed in the sequence from binary to ternary fluorides. This change is associated with an increase in the covalency of the bonds of the 3d-element fluo rine.

Figure 1 illustrates the stabilizing influence of the diamagnetic cation present in the case when it is monovalent or divalent. In connection with this, we must make some comments: a) Firs t of aii•: in ternary fluorides, the oxidation state +4 is alway s realized, The only exception is the 1 case of iron (the cause of this at first glance, anomalous effect will be discussed below) ; . . ' ., ,/ '•.t

b) The m� � ifaum oxidation state realized in ternary fluorides is always at least equal to the oxidation state obtained in binary fluorides and even exceeds it in the case o� co balt, nickel, and copper. However, there is one exception-the case of chromium. Actually, it seems rather strange that CrF 6 has net been synthesized [22), whereas in ternary êornpounds the highest oxidation state is equal to +5. There are no theoretical arguments permitting an explanation for this fact. It can be assumed that in this case it is a matter of experimen tal difficulties: work with the compound CrF 6 is rather incovenient as a result of the in stability and high volatility of this compound, which hinders the production of a ternary compound, in which CrF6 would be bonded to some other fluoride. However, it is appropriate to raise the question of .what stable structure of the ternary material might accommodate iso lated (CrF6) octahedra.

In the case of cobalt, nickel, and copper the influence of the cation bonded to the transition metal ion is very distinctly manifested and is of a dual nature: a) The corresponding ternary compounds possess exceptionally stable structures, to which high lattice energies correspond. The structural aspect of this question was dis cussed above; b) The formation of a bond between fluorine and the other element competes to a sub stantial degree with the formation of a bond between fluorine and the transition element. The stronger the ionic character of this bond, the higher the electronic charge of fluorine and the more c ovalent the bond between the 3d-element and fluorine be c omes. Naturally, an intensification of the covalent charac ter will promote an increase in the stability of the compound. In c onnection with this, suffice it to recall that the production of ternary fluo rides proves clearly simpler in the case of cesium and for rubidium and all the more for po tassium.

Influenc e of Electronic Configuration. It is known that certain electronic configura tions can be stabilized to a substantial degree either on ac c ount of the energy of inter atomic exchange or by the crystal field. As an example, for the case of octahedral coordi nation we might cite the high-spin c onfigurations d 3 , d 5 and the low-spin configuration d 6

,

The existence of suc h specific stabilization permits, for example, an explanation of the fact that higher oxidation states than +4 (d 3 ) are not known for manganese. The production of Mn(IV) does not present any difficulty even in the case of alkali metal cations that have a small radius, such as lithium or sodium.

An analogous situation exists for trivalent iron (c onfiguration d 5

).

It can be assuroed that the production of this element in the oxidation state +S would be simpler than the pro duction of Fe(IV) (configuration d 4 ), if it were not necessary in this case to consider the existence of a very high ionization potential.

For the same reason, distinct stability is possessed by compounds by Ni(IV) (low-spin configuration d 6 ). From the experimental standpoint this finds reflection in the fact that such c ompounds as Rb 2 Ni 4 + F 6 and Cs 2 Ni 4 fy6 can be produced very easily, whereas the produc tion of Rb3Ni 3 +p6 and Cs3Ni 3 +f6 proved entirely impossible.

Corn arison with Fluorides of the 4d-and 5d-Elements. Table 4 presents the maximum oxidation states observed for fluorides of the 3d-, d-, and 5d-elements, according to the data known until quite recently. This table confirma the well-known fact that high oxida tion states ar•e••more stable for 4d-and 5d-elements than for 3d-metals. This is explained by the fact that 1 the increase in the radial extent of the 4d-and 5d-orbitals leads to a greater overlap. ping of the orbitals of the anions and cations and, c onsequently, to an in tensification of the covalency. Lowering the ionization potentials also permits an explana tion of th � ... ,.p' ârtial transf er of a large number of electrons to fluorine a toms. An anomalous effect is observed for elements of the side subgroup of the firét group (copper, silver, gold), whereas for the other columns of Table 4 the oxidation s.tate remains constant or inc reases with increasing atomic number. Silver has a lower oxidation state than copper and gold, For gold the existence of a valenc e state of +5 was detected by Leary and Bartlett (23). In the case of silver, the highest oxidation state known until recently was the state +3. A study published in 1977 [24] indic ates the possibility of the existenc e of silver in an oxidation state of +5. The silver compound Cs2 AgF 6 possesses a very low mag netic susceptibility, independent of the temperature, which might be due to the simultaneous presenc e of Ag(III) (low-spin configuration d 8 ) and Ag(V) (low-spin configuration d 6 ) within the AgF6 octahedra.

Comparison with Oxides. Table 5 indicates the maximum oxidation states obtained for fluorides and oxides of the 3d-elements. It can be stated that, as a rule, they are the same. However, in the case of manganese and iron the oxide possesses the highest oxidation state; the opposite result is observed for copper, Somewhat naive considerations, based on the greater electronegativity and oxidative capacity of fluorine, would permit us to expect that fluorine should stabilize increased oxidation states to a greater degree than oxygen. Actually this is not so, and the opposite conclusion, confirmed, in pa rticular, in the case of rnanganese and iron, seems even more plausible, Such behavior rnay be due to the higher energy of Coulombic interaction (the forma! charge oxygen is equal to -2, whereas for fluorine it is only -1), This may also be as sociated with the greater covalency of the rnetal-oxygen bond. Covalency leads to a trans fer of electrons from anion to cation, the effective charge of which decreases, and, conse quently, so does the ionization potential necessary for its formation. Another cause of the stability of metal oxides with an increased oxidation state (in particular, in the case of manganese) might be due to the fact that the cation may be in tetrahedral coordination, whereas only octahedral coordination is observed for fluorides, It can also be noted that most of the lower oxides of elements at the end of the 3d series possess metallic-type electric conductivity. Such delocalization of the electrons, absent in the case of fluorides, is another stabilizing factor.

It is interesting to note that transition frorn a low-spin to a high-spin state, mani fested for fluorides beginning with Ni(III), is already observed for Co(III) in the case of oxides. This difference in behavior 1s naturally due to the ligand field, which 1s stronger in the case of oxides, Our consideration of the nature of the chernical bond in fluorides provides evidence of the existence of a definite gap in the knowledge of this question, due to the fact that the thermodynamic properties of these fluorides, and especially the values of the enthalpies formation, are insufficiently well known. In the presence of such data, it would become possible to rnake a more quantitative generalization of various structural and electronic data, obtained in this field of chernistry. On the other hand, from the standpoint of possi ble applications a knowledge of thermodynarnic parameters would permit a sensible selection of these materials for use as fluorinating agents. A systematic investigation of the reac tivity of these substances with respect to various organic cornpounds might be of great in terest. In connection with this, it is appropriate to raise the questions: might the tran sition element present play the role of a catalyst? How does the electronic configuration influence the reactivity?

It is also sensible to think about whether it will be possible in the near future to obtain higher oxidation states in comparison with those known at present. Especially at tractive is the case of iron: whereas the oxidation states +4, +5, and +6 have been ob served for oxides, in the case of fluorides it has not yet been possible to exceed an oxida tion state of +3. To be able to obtain positive results, it will be necessary to use new methods of fluorinât:.": i.on, based on the use of powerful fluorinating agents, as well as sub stantially higher pressures. Nonetheless, it must be considered that in most cases it 1s necessary to develop new methods, permitting avoidance of the difficulties due to the de creasing stabili;y'of binary fluorides and their increased volatility, ,,: -�r Also of great interest is the case of oxyfluorides. Only a small nurnber of oxyfluoride compounds of iron, cobalt, nickel, and copper, corresponding to oxidation states exce• eding +3, are known. It might be hoped that such compounds will be successfully produced by si multaneously utilizing the effects of the electronegativity of fluorine and the oxygen-deter mined stabilizing influence of the delocalization of electrons.

National

  Center for Scientific Research, University of Bordeaux, France, Translated from Koordinatsionnaya Khimiya, Vol. 5, No. 8, pp. 1123-1131, August, 1979. Original article submitted June 8, 1978.

Fig, 1 .

 1 Fig, 1. Oxidation states of the 3d-elements contained in currently known binary (1) and ternary fluo rides (2).

Fig. 2 .

 2 Fig. 2. Idealized structures of compounds with composition A2MF6 (a)and A 3MF6 (b) (A is an alkali metal),

Fig. 4 Fig, 3 .

 43 Fig, 3Fig. 4 Fig, 3. Electronic structures of the ions Mn 3+ (a) and Ni 3+ (b) on the assumption of the existence of an octahedral field.

Fig. 4 .

 4 Fig.[START_REF] Babel | Structure and Bonding[END_REF]. Ionie radii of Mn 4 + (1, 3) and Mn 3+ (2, 4) in fluorides according to the data of[20] (1, 2) and this work[START_REF] Dance | [END_REF][START_REF] Babel | Structure and Bonding[END_REF].

TABLE 4 .

 4 Maximum Oxidation States in Fluorine-Containing Compounds of the Transition Elements 1 Oxidation l 4d-Elerrents Oxidation I l 5d-Elements 1

TABLE 1 .

 1 Compounds of Ni(III) and Cu(III)

		Lia	LbNiFs (5] orthorh g JPbic a-L13 6 type	�--�-
		N.<12	Na2LiNiF6 [6] Na2LiCuF6 [6]	Na3r• iiFa [8] Na3CuF5 [U] Monoclinic of crylolite type
		K1	K2LîC11Fs [6] Cubic of elpazo-lite type	K2NaNiF's [ JO] K2NaC11Fs [7] Cubic of elpazo-lite type	K,NiFs [U] K3C11Fo [Il] Tetragonal of a-K3 Fe Fa type
	-	Rb2 Cs2.	Rb2LiCuFe [6) Hexa � nal of Cs2 nCrFs type I l Li 1 1	Cs2NaNiFo [ 10] Cs2N'aCuFs [7) C!-2NnCrFatype Na 1 Rb�NaNiFa [10] Hb2NaCuF6 [7] Cubic of elpazo-lite type	Cs:iKNiFs [10] Cs2KCuFa [ 12] Cubic of elpazo-lite type K Rh2KNiF& [ °10] Rb2KC11Fe [7] Cubic of elpazo-lite type	Rl>3CuFa [6] Tetragonal of distorted (NH 4 ) 3

AlF6 type Cs2RbCuFa [12]

	Cubic of elpazo-
	lite type
	Rb

TABLE 2 .

 2 Cornpounds of Ni(IV) and Cu(IV)

	Li2 Na:\	Na2NiF& (13)	Rh2	Rh2NiFs * [-15] Rb2CuFa * [ 14]
	Kz	K2NiF&* ( 14]	Cs2	Cs2N il<'s [ 15] Cs2CuF6 * [Hl, 17]

TABLE 3 .

 3 -Electronic Configurations of 3d-Elements in Various Oxidation States*

	3 d-Element	+2	1	Oxidation state + 3 1 1 +5 1 +G H		3dM Element	+21	Oxidation sra te IHI� +3	+e
	Ti Cr Mn	1 2 4(tt) 3 3 5(tt) 4( t t)	0 1 2 3	0 1	0	�	Fe Co Xi Cu	6(tt ) 5(tt ) 7(tt) 6(tt) 8( t t, ) 'i'(trt) 6(H. ) 5(H}1 O(tt) 8( t ) 7(H),
	*ttHigh-spin configuration; Hlow-spin configuration.
									n)	0,	oJ
		a + + +tt + e' 8			0,60 0,7 0	• •2 • li • 0 •	•	•	•	•
		b + -e' ' § ;+ it ;+ tffg			O,ôO Ti.	V	Cr Mn	Il re Co Nt Cu
				Fig, 3

TABLE 5 .

 5 Higher Oxidation States in Fluorides and Oxides

	Oxidation state	Oxidation state
	3d-Elements


domain: spi phys info math

text:
Julie Gauvrit-Ledogar

Nicolas Bérend

Jean Hermetz

FLIGHT EXPERIMENTS FOR THE ALTAIR SYSTEM WITH THE DEMONSTRATOR EOLE AIR-LAUNCH SYSTEM WITH THE DEMONSTRATOR EOLE HERE 1 FLIGHT EXPERIMENTS FOR THE ALTAIR SEMI-REUSABLE AIR-LAUNCH SYSTEM WITH THE DEMONSTRATOR EOLE

Keywords: air-launch system, flight tests, demonstrator

ONERA and the French space agency CNES, in close collaboration, have been studying innovative air-launchto-orbit systems for many years. In that frame, they have proposed a two-vehicle concept where an unmanned dedicated carrier releases a launcher at predetermined separation conditions in terms of Mach number, altitude and flight path angle.

Fully related to these studies, ONERA under CNES support, conducted the development of a flying automatic subscale demonstrator, called EOLE, which aims to be an experimental platform for testing various launch conditions and separation technologies. EOLE's main objectives are to investigate the definition and the implementation of the launch separation conditions and to identify the relative behaviours of both vehicles (carrier and launcher) during the separation phase.

From 2018 to 2019, the demonstrator EOLE was used as a flying test bed within the European Horizon 2020 ALTAIR project. The ALTAIR project is focused on a cost-effective launch system for small satellites up to 150 kg in Sun Synchronous Orbit. The ALTAIR system can be described as a semi-reusable air-launch system made of a reusable automated carrier designed specifically for the launch mission and an expendable launcher using hybrid propulsion for the two main stages and monopropellant H2O2 liquid propulsion for the upper stage. This 4-year project, performed by a consortium of 8 partners from 6 countries and coordinated by ONERA, aimed at demonstrating the economic and technical viability of this concept in order to pave the way for a future available, reliable and competitive European launch service for the access to space of small satellites.

In relation to the overall ALTAIR project work plan, flight experiments are included as complementary means to the design studies. They are used to test representative avionics equipments in the loop in order to meet realistic flight conditions. The flight tests are focused on two specific topics: testing the on-board software designed for the ALTAIR launch vehicle, installed aboard a subscale mock-up of the ALTAIR launcher, and testing the separation phase designed for the ALTAIR carrier, including the release system activation and the launcher release. The flight tests involve 4 of the 8 ALTAIR partners: ONERA, CNES, GTD Sistemas de Información S.A. and Piaggio Aerospace.

In the first part, the paper describes the European Horizon 2020 ALTAIR project. Then, it describes the demonstrator EOLE and its development program. Finally, it details the flight tests campaigns performed with the demonstrator EOLE for the ALTAIR project.

ALTAIR European Air-Launch System and Subscale Flight Tests Approach

European Horizon 2020 ALTAIR Project

As an answer to the increasing market of small satellites (< 200 kg) and the need for Europe to dispose of a launch system fully adapted to this kind of payload, the ALTAIR project was started in December 2015 by a consortium of 8 partners from 6 European countries: ONERA -The French Aerospace Lab (France) acting as coordinator, CNES (France), GTD Sistemas de Información S.A. (Spain), Bertin Technologies (France), Swiss federal Institute of Technology Zurich (Switzerland), Piaggio Aerospace (Italy), NAMMO Raufoss SA (Norway), and SpaceTec Partners SPRL (Belgium). ALTAIR is part of the European Horizon 2020 research program and is funded by the European Commission (3.5 M€) and Switzerland (0.5 M€). The 4-year ALTAIR project aims at demonstrating the economic and technical viability of a future launch system for the access to space (Low-Earth Orbit) of small satellites in the range 50-150 kg. The challenge is both technical and economic, as the system should be not only performant, available and reliable, but also economically competitive. In order to address this challenge, the ALTAIR Consortium proposes an innovative launch system based on the concept of air-launch relying on an automated aircraft carrier designed specifically for the mission, as described in section 1.2.

ALTAIR European Air-Launch System

The ALTAIR system is based on the concept of air-launch. The vehicle part of the system is made from two vehicles, as illustrated in Figure 1 [START_REF] Bérend | ALTAIR semi-reusable air-launch system -current design and status of flight experiments[END_REF][2]:

• A reusable unmanned aircraft carrier.

• An expendable 3-stage rocket. The carrier brings the rocket at a given altitude. The starting of the rocket at a high altitude instead of the ground level has two benefits on the rocket performance:

• Reduction of drag losses.

• Reduction of nozzle losses.

Due to this gain of performance, the air-launched rocket is smaller and lighter than its equivalent ground-launched rocket with the same performance.

Target Mission and Release Conditions

The following reference target mission has been defined according to a market analysis (which has been updated throughout the ALTAIR project). The cost target is 5 M$ per launch, considering a launch rate of 30 launches per year.

Table 1 -ALTAIR target mission.

A multidisciplinary study and sensitivity analysis has led to the following launcher release conditions:

Table 2 -ALTAIR launcher release conditions.

ALTAIR Carrier

Figure 2 illustrates the ALTAIR carrier concept of operations.

ALTAIR Launcher

The ALTAIR launch vehicle has a 3-stage architecture:

• Two main stages: NAMMO hybrid propulsion (same chambers: 7 for stage 1, 1 for stage 2).

• Orbital module (H2O2 monopropellant). (credit: Bertin Technologies)

In addition to the two hybrid propulsion main stages, the ALTAIR launch vehicle includes an orbital module (or "upper stage") using monopropellant H2O2.

ALTAIR Ground Segment

The ground segment definition has included the following tasks:

• Functional analysis of the ground segment.

• Detailed specification of the systems.

• Preliminary architecture design.

• Preliminary concept of operation definition.

• Preliminary cost estimation based on parametric and statistical approaches.

Several possible launch sites have been studied and in the end, two "best candidates" have been identified:

• Guyana Space Center (CSG).

• Andøya Space Center (ASC).

Support of the ALTAIR System Design Studies with Subscale Flight Tests

Beside the system design studies, the ALTAIR project includes an experimental workpackage dedicated to perform flight tests [1][2]. The main objective of those tests is to validate some of the specific elements of the ALTAIR system in conditions representatives of the free flight. For that purpose, the experimental workpackage is directly fed by the design workpackages to develop and test, at a given subscale, sub-systems representatives of the full-scale ALTAIR system. With regard to the particularity of the ALTAIR air-launch system, the flight tests are focused on two of its features:

• The separation phase during which the launcher is automatically separated from the carrier throughout a dynamic manoeuver. The flight tests aim to reproduce the ALTAIR separation manoeuver dynamic, including the activation of a release system representative of the ALTAIR release system. • The launcher on-board system and its new functions specifically defined within the ALTAIR project and mainly resulting of the airborne part of the launcher flight. Those functions concern the navigation and the safety management. For the flight tests purpose, experimental sub-systems typical of the ALTAIR air-launch system have been specifically developed, as described in section 3. Those elements have been then mounted or implemented on the demonstrator EOLE, which has been used as a flying test bed. Section 2 describes the demonstrator EOLE and its development program.

EOLE Flying Subscale Demonstrator

EOLE Demonstrator Genesis

In the frame of activities done in close collaboration between CNES and ONERA during the last years of 2000 decade (namely DEDALUS [START_REF] Bérend | DEDALUS: UAV-based solutions for future air launch systems[END_REF] and L3AR projects), the idea of using a Flying Scaled Demonstrator (FSD) has emerged as a potential technical mean to address the critical phase of the air-launch mission, which consists of the launcher release from the carrier [START_REF] Ledogar | an innovative flying scale demonstrator for air-launch-to-orbit automatic systems[END_REF]. A multidisciplinary working group, set up to evaluate the feasibility of such an idea, concluded positively and delivered a set of top level requirements toward a flying demonstrator suited for this kind of experiments. In the same period, CNES launched the PERSEUS project (https://www.perseusproject.com/) toward students, with the aim to give them the opportunity of designing, building and testing parts of space launch concepts at reduced scale. As air-launch concepts of operation were in the scope of this project, PERSEUS offered the opportunity to support EOLE development and the decision to proceed was taken. For that reason, many students from universities such as Université d'Evry Val d'Essonne (UEVE), engineering school such as ISAE-Supaero, ENSAM ParisTech, IPSA and scientific associations such as GAREF and Planète Sciences, were involved in the development program, through internships or students team's projects. Close to 70 students have participated to help EOLE experimental system to become a reality. The main capability of EOLE FSD is to release in safe and controlled conditions launcher mock-ups in order to capture in-flight relevant data of the behaviour of the two vehicles during various release manoeuvers, using also various experimental release systems. The objective is to help understanding, modelling and simulating this critical part of the air-launch mission. A secondary capability of EOLE consists of carrying prototypes of launcher's on-board equipments in order to test them in realistic flight conditions, for example under load factor. In relation with the full-scale concept of operations studied at the time of EOLE initial definition, some investigations related to scale's similarities were performed to set up a first design of the FSD. According to the objective to catch the vehicles' relative behaviours during the release manoeuver, Froude scaling was of first importance, and served as a basis to define an initial set of characteristics for the FSD. On this basis, a multidisciplinary design process was performed to go from this initial design to a more realistic aircraft configuration, namely EOLE, taking into account operational constraints and French UAV regulations in force at this time. Figure 7 illustrates this process. At the end, EOLE resulted from a compromise between similarity objectives and operational constraints, which leads to manage a lower wing loading than required. As a consequence, no direct transposition was possible, but a methodology based on simulations was set up and consolidated through EOLE development to help the design of a full-scale air-launch concept. Once designed, EOLE was co-patented by CNES, ONERA and Aviation Design. The latter is a French SME specialized in flying demonstrator and UAV manufacturing, acting as CNES subcontractor to develop with ONERA the EOLE experimental system. Hence, the EOLE development project was managed by ONERA, involving Aviation Design as the main partner. It was cadenced in three main steps, with interim review milestones in compliance with CNES program management process. First step, dedicated to preliminary design was done from 2009 to 2010. Then the detailed design, manufacturing and integration were done in two years (from 2010 to 2012). The third and final step consisted of qualification and tuning tests, from ground to flight, and lasted 5 years (from 2012 to 2017).

EOLE Characteristics and Performances

EOLE is fully designed in relation with the aim to carry and release a launcher which represents a large amount of the total mass of the flying system. To minimize disturbances and avoid risks of collision between EOLE and the launcher during the separation, a twin fuselage was selected to carry the launcher in the symmetry plan, easing their center of gravity colocation. A twin engines configuration was also chosen for safety purpose, and located above a gull wing suited to increase ground clearance. This arrangement accommodates a wide free space under a central pod, on which the release system is locked through a standard interface. This central pod includes in its upper part the safety parachutes and a part of the onboard avionics. A V-tail ensures also a free space behind the launcher and limits the tail blowing with the jet engine hot flow. As launcher mock-up weight was expected in the 10 kg to 40 kg range, EOLE was designed for a payload of 50 kg, including a provision of 10 kg for the release system. Considering the full-scale flight conditions and the main release manoeuver which consisted of a constant-g pull-up, the design release performances for EOLE was set at Mach 0.2, 4000 m altitude under a 3g pull-up manoeuver. From those considerations, the design process has frozen the EOLE main features, which are:

• A nominal take-off weight of 150 kg, with structural provision to increase it safely at 200 kg.

• A 6.7 m span wing using an in-house ONERA airfoil, with an area of 2.58 m² and an aspect ratio of 17.

• Two AMT Titan turbojet with a unitary sea level static thrust of 40 daN.

• A demonstrated ability to handle at least 4 g pull-up manoeuver @ at a flying weight of 150 kg and 3 g @ 200 kg. • An operational ceiling greater than 4 000 m, a max speed close to 110 m/s or Mach 0.32, an endurance up to 45 min and a maximum range close to 250 km. • A fully automatic flight except for take-off and landing, which are manually piloted.

• An ability to perform automated Beyond Visual Light Of Sight (BVLOS) flight in segregated areas. The later feature comes from the release conditions, which are clearly out of the visual range of a pilot on-ground. It leads to design an avionics architecture robust to the single failure for any of the systems, and robust to a double failure for equipment involved in the flight safety management. The safety chain is based on two independent operating ways, with their own data uplink, one dedicated to the flight tests center safety officer and another one for the flight tests team. The safety procedure consists of ejecting a safety parachute to stop the flight, stopping the engines through redundant fuel valves, and deflecting ailerons to a spiral turn. Moreover, the activation of the safety procedure can occur automatically on onboard decision from an internal permanent survey protocol, which scans and detects onboard systems failures.

EOLE Development Program

Beyond the design phase, Figure 9 depicts the set of ONERA's multidisciplinary activities performed in order to develop EOLE [START_REF] Ledogar | an innovative flying scale demonstrator for air-launch-to-orbit automatic systems[END_REF].

• Wind tunnel testing (L2 wind tunnel, ONERA center of Lille) in complement with CFD computations to provide a wide aerodynamic database for handling qualities and performance analysis, then flight control laws definition. • FEM computations and static bending tests (done up to the wing primary structure break, with the collaboration with ISAE-SUPAERO) to validate the wing structure (torsional resistance of the fuselage-tail joining section was also computed and validated through experiments on a test-bench).

• Ground Vibration Testing (GVT) in the ONERA center of Meudon facility to feed a flutter analysis.

• Performance assessment and handling qualities analysis then control laws definition, using in-house methods & tools [START_REF] Waitman | Autopilot design for the EOLE demonstrator[END_REF] In close collaboration with ONERA, Aviation Design has performed the detailed design of the airframe, the avionics and the ground station, starting from its know-how built on several other UAV or flying demonstrator projects with the industry. The airframe is fully made in composite materials (either using glass fiber or carbon fiber materials, partly including honeycomb or NOMEX®) and manufactured through molding using molds done by numerical control machining. The specific separation manoeuver guidance law was also experimented with lower dynamic parameters according to the rather limited flight space, in order to validate the corresponding trajectory, which consists of a pull-up manoeuver followed, after the release, by a 180° turn at low speed in order to take an opposite direction. However, no release was done at Saint-Yan airfield as this kind of experiment is prohibited over such an open ground area, despite its low populated density. At the end of this set of flight tests campaigns performed at Saint Yan airfield, a formal review concluded that EOLE was ready to start BVLOS experiments. This review has also stated on how to proceed for future BVLOS experiments. A gradual approach was discussed and validated with the aim to enlarge progressively the flight envelope (higher altitude and higher speed), and to test progressively the ability of the systems -especially the datalinks -to operate at larger range, typically up to 25 km from the ground control station, the EOLE flying demonstrator.

Flight Tests Using the Demonstrator EOLE for the ALTAIR Project

In the frame of the ALTAIR project, the demonstrator EOLE is used as a flying test bench in order to place experimental sub-systems in conditions representative of the free flight environment. For that purpose, experimental sub-systems, typical of the ALTAIR air-launch system, have been specifically developed to be mounted or implemented on the demonstrator EOLE [START_REF] Bérend | ALTAIR semi-reusable air-launch system -current design and status of flight experiments[END_REF][2].

Specific Experimental System Development for the ALTAIR Project

As indicated in section 1.3, the flight tests are focused on two specific characteristics of the ALTAIR air-launch system: on the one hand, the launcher on-board system specifically designed for the ALTAIR launch vehicle, and on the other hand, the separation phase including the particular separation manoeuver performed by the ALTAIR carrier and its release system activation. Those experiments have led to develop experimental sub-systems that are described in the sections 3.1.1 to 3.1.5.

Mock-Up of the ALTAIR Launcher On-Board System

The experiments about the ALTAIR launch vehicle on-board system concern two critical functions: the navigation function and the on-board flight safety management.

For the navigation function, the tests aim to validate the launcher on-board system alignment procedure required just before the launcher separation. For the on-board flight safety management, the tests aim to validate the safety procedures defined for both launcher release and launcher ignition consents, involving the algorithms implemented for the systems checkout and the states estimation (ground impact prediction, launcher state assessment at engine ignition, etc.). Those functions and the associated algorithms are derived from the workpackage dedicated to the ALTAIR launch vehicle design and they are implemented in an experimental set of equipment representative of the ALTAIR launch vehicle on-board avionics. This experimental system is composed of an on-board computer and a VECTOR-NAV 200 dual antenna GNSS-aided inertial navigation component. Those elements are installed into a mock-up of the ALTAIR launcher which is then attached to the demonstrator EOLE. Thanks to EOLE, this launcher mock-up is placed in airborne flight conditions and, at the very end of the flight tests campaigns, is released during a separation manoeuver. In order to record the information collected by the launcher mock-up during its ballistic flight after the release, a twin onboard computer is mounted on EOLE. This twin computer receives the launcher mock-up on-board system data through a WiFi datalink. All the experimental devices concerning the launcher on-board system and their arrangement in the launcher mock-up are represented in Figure 14.

Subscale Model of the ALTAIR Launcher

The launcher mock-up has been developed in order to accommodate the on-board system described in section 3.1.1. This mock-up represents a 1:13 scale model of the ALTAIR launch vehicle. Its body is based on carbon fiber tubes and its internal structure is made of aluminum. The nosecone, the central skirt and the aft part with dummy nozzles are made with additive printing. Figure 13 shows the launcher mock-up overall geometry and Figure 14 illustrates its internal structure and the integration of the experimental on-board system. Finally, the launcher mock-up is fitted with two attachment points. Their design and manufacturing have been the subject of deep investigation in order to ensure the whole experimental system safety in case of disturbances (i.e. minimize risks of collision during the launcher mock-up release from the demonstrator EOLE) and to be compatible with the flight tests operations (i.e. allow quick and easy assembling/disassembling operations). Those attachment points are highlighted in Figure 15.

Mock-Up of the ALTAIR Release System

In addition to the ALTAIR launcher mock-up and its equipment, a mock-up of release system has been developed. This experimental release system is representative of the one defined for the ALTAIR carrier in the dedicated workpackage. It uses the same physical principle based on Hold-Down Release Mechanisms (HDRM). The release system mock-up is composed of two HDRM which allow locking and then releasing the two attachment points that are present on the launcher mockup side. As for the full-scale ALTAIR release system, the mock-up HDRM are positioned at equal distances around the launcher mock-up center of gravity. Figure 16 illustrates the overall geometry of the release system mock-up and Figure 17 shows a photo of the release system mock-up assembled with the launcher mock-up. This last figure brings to light the four lateral stabilizers present to maintain the launcher mock-up body, in the same way as the full-scale ALTAIR release system. Finally, the release system mock-up is attached to EOLE through its interface plate which is screwed under EOLE's central pod. For the flight tests campaigns, a fairing has been specifically manufactured to protect the release system. Equipment related to the ALTAIR release experiment and carried on by EOLE, including the electronics driving HDRM devices, are attached to the interface plate, as illustrated in Figure 18.

Figure 18 -Rear HDRM and associated computer (left), twin computer for launcher data recording (right). (credit: ONERA)

GNC Laws for Reproducing the ALTAIR Separation Manoeuver

Beside the experimental hardware manufactured for the flight tests and described from sections 3.1.1 to 3.1.3, Guidance, Navigation and Control (GNC) laws have also been developed. The main topic of the GNC development focuses on the separation manoeuver with the objective of reproducing the ALTAIR separation manoeuver dynamic with the demonstrator EOLE. This manoeuver, defined in the workpackage dedicated to the ALTAIR carrier design and illustrated in Figure 19, is composed of three steps: an acceleration segment at constant altitude, a pull-down segment and a pull-up segment under normal load factor. The separation flight conditions defined for the ALTAIR system and the related conditions defined for the flight tests with the demonstrator EOLE are indicated in Table 5. Table 5 -Separation flight conditions for the ALTAIR system vs. flight tests.

In addition to the laws about the separation manoeuver, specific GNC laws have also been developed in order to perform the alignment manoeuvers required for the tests dedicated to the launcher on-board system alignment procedure mentioned in section 3.1.1.

Integration of the Experimental System with the Demonstrator EOLE

The mock-up of launcher on-board system is integrated into the launcher mock-up, as described in section 3.1.2. Then, the release system mock-up is mounted under the central pod of the demonstrator EOLE and protected in a fairing. Finally, the launcher mock-up is screwed to the release system mock-up through the two HDRM.

Visual Line of Sight Flight Tests Campaign

The first campaign focuses on a low altitude and low speed flight envelope, restricted to Visual Line Of Sight (VLOS) conditions. This campaign occurred in the French civil airport of Saint-Yan. During this campaign, three categories of tests have been conducted along the several flights. First, the VLOS flights were used to test four different launcher on-board system alignment manoeuvers described in The campaign concluded on the validation of the launcher on-board system alignment algorithms and the selection of the Thach Weave manoeuver to be used for the ALTAIR system. Figure 22 illustrates the data collected during an alignment with a Thach Weave manoeuver. Then, the VLOS flights were used to test the launcher on-board system safety management procedures related to the launcher ground impact prediction. For those tests, the safety algorithms took as inputs either the VECTOR-NAV 200 navigation data or the on-board navigation data previously aligned. Those tests have been performed at several speeds (from 45 to 65 m/s) and altitudes (from 200 to 500 m above ground level) and, as for the alignment manoeuvers, they have considered both nominal and degraded conditions. The campaign contributed to validate the onboard flight safety management through the following elements:

• Evaluation of the real-time performance, accuracy and precision of the impact point and impact area algorithms. • Detection of sensible ground areas to be protected from launcher impact towards release order acceptance.

• Detection of target areas for secure/controlled launcher release towards release order acceptance.

• Detection of dynamic events during ballistic flight towards ignition acceptance.

Figure 23 illustrates the launcher on-board system answer with regard to different categories of ground areas. (credit: GTD Sistemas de Información S.A.)

Finally, the VLOS flights were used to test the implementation of the separation manoeuver and its proper functioning. During those flights, the release system actuators were inhibited in order to prevent any launcher separation, but all the separation manoeuver steps were tested, including the release system actuation order generation. Figure 24 illustrates the speed and altitude evolution of the demonstrator EOLE during one of the separation manoeuver tested in VLOS conditions. The campaign concluded positively, opening the way to go to the next and final step: the effective release of the launcher mock-up in BVLOS conditions.

Figure 24 -Separation manoeuver in VLOS conditions. (credit: ONERA)

Beyond Visual Line of Sight Flight Tests Campaign

The second and last campaign is dedicated to higher altitude and higher speed flight envelope, in Beyond Visual Line Of Sight (BVLOS) conditions. This campaign occurred in the Guyana Space Center (CSG) of Kourou [2][8].

The main objective of this campaign is to perform the release of the launcher mock-up during a separation manoeuver and then collect data during its ballistic flight. In addition, the high altitude and high speed flight envelope available at the CSG allowed reproducing the selected Thach Weave manoeuver and the launcher on-board system safety management tests in more aggressive flight conditions than during the previous VLOS flight tests campaign. (credit: ONERA)

Figure 27 shows the data collected by the demonstrator EOLE during the separation manoeuver and Figure 28 shows both EOLE and launcher mock-up evolution around the release time.

Conclusion and Perspectives

The flight tests performed with the demonstrator EOLE and the analysis of the experimental data collected during the several campaigns provide a valuable source of information toward the design of the ALTAIR semi-reusable air-launch system. First, the flight tests performed about the launcher on-board system have contributed to validate the on-board algorithms and the software strategies. Regarding navigation, the tests have validated the alignment software performance in term of position, velocity and attitude, with an initialization time lower than 30s. Then, they have contributed to select the Thach Weave manoeuver for the alignment before the launcher separation. In addition, the flight tests have participated to the validation of a flight simulator specifically developed for the launcher on-board system. About the on-board flight safety management, the flight tests have confirmed the viability of the real-time on-board autonomous safety approach considering a low power on-board computer. Finally, they have allowed assessing the real-time computation and performances of the safety algorithms about ground safety. Then, the flight tests have confirmed that the design selected for the release system guarantees a safe and secure separation of the launcher from the carrier. The data collected during the tests have been compared to numerical simulations, showing that the global relative behavior of both vehicles was well represented by intermediate level aerodynamics simulations. In addition, it has been highlighted that local phenomenon such as suction effects need high level aerodynamics simulations to be well estimated. Finally, the GNC laws developed and tested with EOLE have proved their high performance to achieve the ALTAIR separation manoeuver.

In addition to all the validations provided by the flight tests on hardware and software, leading to progress on the scale of TRL, the flight tests have also been fruitful about defining the operations of such specific system as the ALTAIR semi-reusable air-launch system composed of a reusable automated carrier and an expendable launcher.

Figure 1 -

 1 Figure 1 -ALTAIR mission profile. (credit: Bertin Technologies)

Figure 2 -Figure 3 -

 23 Figure 2 -ALTAIR carrier operations (COP) vs. autonomous operations.(credit: Piaggio Aerospace)

Figure 4 -

 4 Figure 4 -ALTAIR launch vehicle. (credit: Bertin Technologies)

Figure 5 -

 5 Figure 5 -ALTAIR launch vehicle main flight parameters (left) and trajectory (right).(credit: Bertin Technologies)

Figure 6 -

 6 Figure 6 -ALTAIR orbital module (left) and its mission profile (right). (credit Bertin Technologies)

Figure 7 -

 7 Figure 7 -FSD overall definition process. (credit: ONERA & Laurent Michelet)

Figure 8 -

 8 Figure 8 -View of EOLE overall airframe architecture. (credit: ONERA)

  [START_REF] Jouhaud | Guidance of UAV EOLE for launcher separation maneuver[END_REF][START_REF] Jouhaud | Flight Path Management System of EOLE UAV[END_REF].• Flight control laws were implemented into the onboard flight computer and also used to feed a flight simulator for on-ground pilot training and flight experiments preparation.

Figure 9 -

 9 Figure 9 -Main ONERA activities for the EOLE development program. (credit: ONERA & Laurent Michelet)

Figure 10 -

 10 Figure 10 -Detailed design and manufacturing details of the airframe. (credit: Aviation Design).

Figure 11 -

 11 Figure 11 -EOLE during flight experiments at Saint-Yan airfield, with/without the launcher mock-up.(credit: Laurent Michelet)

Figure 12 -

 12 Figure 12 -EOLE experimental system setting up and qualification at Saint-Yan airfield.(credit: ONERA)

Figure 13 -

 13 Figure 13 -Launcher mock-up (scale 1:13 of the ALTAIR launch vehicle).(credit: Université de Rennes 1)

Figure 15 -

 15 Figure 15 -Focus on the launcher mock-up attachment points. (credit: ONERA)

Figure 16 -

 16 Figure 16 -Release system mock-up. (credit: Piaggio Aerospace)

Figure 19 -

 19 Figure 19 -ALTAIR separation manoeuver. (credit: Piaggio Aerospace)

  Figure 20 represents the experimental system assembled to the demonstrator EOLE.

Figure 20 -

 20 Figure 20 -Experimental system assembled with the demonstrator EOLE. (credit: ONERA)

3. 2

 2 Flight Tests Campaigns for the ALTAIR Project The experimentations have been divided in two flight tests campaigns [1][2].

Figure 21 :

 21 Straight Flight, Coordinated Turn, Wing Rock and Thach Weave. During the flights, each manoeuver have been repeated at different flight conditions, as indicated in Figure 21. Moreover, the tests have concerned nominal but also degraded conditions. Those degraded conditions have been faked by simulating the failure of some of the launcher onboard system components (malfunctioning GNSS, malfunctioning INS, degraded navigation data).

Figure 21 -

 21 Figure 21 -Alignment manoeuvers. (credit: GTD Sistemas de Información S.A.)

Figure 23 -

 23 Figure 23 -Safety output vs pre-loaded prohibited ground areas.(credit: GTD Sistemas de Información S.A.)

  Figure 25 illustrates the flight path of the last flight of the CSG campaign where several tests are visible.

Figure 25 -

 25 Figure 25 -Flight path of the last flight at CSG. (credit: ONERA)

Figure 26 -

 26 Figure 26 -Zoom on the Thach Weave manoeuver (left) and the separation manoeuver (right).(credit: ONERA)

  Figure27shows the data collected by the demonstrator EOLE during the separation manoeuver and Figure28shows both EOLE and launcher mock-up evolution around the release time. Figure29illustrates three pictures recorded during the launcher mock-up separation from the demonstrator EOLE. The campaign concluded successfully on the good functioning of the separation manoeuver and a proper separation of the launcher mock-up.

Figure 27 -

 27 Figure 27 -Separation manoeuver data. (credit: ONERA)

Table 3 -

 3 ALTAIR carrier mass breakdown.

Table 4

 4

-ALTAIR launch vehicle general mission parameters.

Funding Sources

The ALTAIR project has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No 685963. The authors would like to thanks particularly P. Choy, F. Jouhaud and C. Toussaint from ONERA, J. Oswald from CNES / DLA, E. Rantet from Aviation Design, M. Kieffer from LM Design and S. Pernon from Université de Rennes 1 for their strong and essential contribution throughout the development of EOLE and the achievement of the flight tests campaigns. Furthermore, the authors also would like to thank the ALTAIR partners involved in the flight tests: CNES, GTD Sistemas de Información S.A. and Piaggio Aero Industries S.p.A.

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Contact Author Email Address

julie.gauvrit-ledogar@onera.fr nicolas.berend@onera.fr jean.hermetz@onera.fr

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Nicolas Maille

Critical decision making with a highly automated UAVa case study

In transportation and aerospace, more automation and autonomy are continuously added to systems. The ability of human operators to effectively monitor and interact with these systems, poses significant challenges. This research focuses on critical decisions that largely rely on the system capabilities but need to be validated and made under the responsibility of the operator. In the context of unmanned combat air vehicles (UCAV), the experiment focuses on how the communication strategy of semi-autonomous systems modifies the operators' understanding of the situation and the final decision. The study has been conducted in an immersive simulator with a 30 minutes ecological military scenario where the operator had to manage a full mission, including an unplanned missile firing decision. The experiment included the use of physiological measures related to electrodermal and cardiac activities. The paper reports the results of the decision-making performances and the analyses of the physiological parameters. It appears that the communication strategy has an impact on the situation awareness of the operator, the decision taken, and the evolution of the physiological parameters.

Introduction

The development of highly automated vehicles, from autonomous cars for civilian applications to Unmanned Combat Air Vehicle (UCAV) for military operations is profoundly changing the way people interact with these systems. Although the word "autonomy" implies that systems will be able to perform actions on their own, in realworld applications, these autonomous systems must still cooperate with humans who may be responsible for effectively monitoring the behaviour of systems, directing them when needed, or acting as teammates and collaborating on decision-making. The ability of human operators to oversee and manage these systems appropriately when needed is a major challenge. [START_REF] Endsley | From Here to Autonomy: Lessons Learned From Human-Automation Research[END_REF] wrote "an automation conundrum exists in which as more autonomy is added to a system, and its reliability and robustness increase, the lower the situation awareness of human operators and the less likely that they will be able to take over manual control when needed". Questions about autonomous driving and how humans adapt to taking control of these vehicles are currently under investigation [START_REF] Morgan | Handover Issues in Autonomous Driving: A Literature Review (Project Report)[END_REF]Eriksson & Stanton, 2017;Morgan Maille et al., 2018). For example, recent studies investigate the effects of takeover signal lead time or modality on automated vehicle takeover performance [START_REF] Huang | Takeover Requests for Automated Driving: The Effects of Signal Direction, Lead Time, and Modality on Takeover Performance[END_REF].

In the case of drones, the main issue is no longer taking manual control of the vehicle, but rather being responsible for monitoring the mission, assessing the overall situation in relation to the mission objectives, and collaborating in decision-making [START_REF] Barnes | Soldier-Robot Teams in Future Battlefields: An Overview[END_REF]. From an operational perspective, humans may soon act as managers 'on' or 'over' the decision-making loop, rather than in it [START_REF] Mayer | The New Killer Drones: Understanding the Strategic Implications of next-Generation Unmanned Combat Aerial Vehicles[END_REF]. As a result, a new context is emerging, characterized by humans managing a machine-driven decision loop. In the military domain, a strong requirement is the ability to operate in more contested air environments, which implies reduced data transmission, automated on-board analysis of raw intelligence data, and greater autonomy when navigating or tracking targets [START_REF] Mayer | The New Killer Drones: Understanding the Strategic Implications of next-Generation Unmanned Combat Aerial Vehicles[END_REF]. Increasing the level of autonomy allows for more irregular monitoring by the operator and raises the issue of "neglect time" (i.e., the length of time the system can operate autonomously before reporting back to the human) and "interaction time" (i.e., the period of time during which the system and the human communicate and define the next actions) [START_REF] Olsen | Metrics for Evaluating Human-Robot Interactions[END_REF]. Stress is high during the neglect period because of the uncertainty of what the system is doing and whether it will actually return a communication. Stress is also high in the interaction period because communication must be fast, efficient, and accurate [START_REF] Hancock | Stress and Performance[END_REF]. These communication constraints amplify the difficulties for the operator who does not have access to the continuous evolution of the situation but who may have to enter the decision loop at any moment and cooperate in highly critical and urgent decisions. While the operator has only a limited amount of time to weigh, verify and gather all critical information, such an interaction process can create a high workload and stress. It is worth pausing to reflect on the operator's ability to contribute effectively and take responsibility for the final decision.

This study focuses on the effects of the timing of communications on the decision making process. Human computer interaction studies addressing human-drone interaction generally indicate that greater transparency about the drone's behaviour helps the operator to monitor the mission [START_REF] Mirri | Human-Drone Interaction: State of the Art, Open Issues and Challenges[END_REF]. Nevertheless, when communications must be sparse for operational reasons, the effect of communication choices on the operator's supervision and decision is little studied. The main contribution of this work is to evaluate in an ecological simulation how the communication strategy implemented by a UCAV impacts the final decision made by the operator and can change the stress, workload and situation awareness of this frontline operator. The use case involves a UCAV operating in a hostile environment in which a trade-off must be made between stealth (for survivability) and communications with the ground station. This work is based on two main hypotheses: H1: An early communication strategy reduces the stress and workload of the decisionmaking process.

H2: An early communication strategy helps the operator to make the right decisions. This experiment is part of a larger research project, but the results presented in this paper focus on one decision to be made during the mission. Further hypotheses on the evaluation of global awareness and workload at the end of the mission have been defined but are not presented here.

Material and method

Participants

The study was approved by University of Aix-Marseille Ethics Committee (ref. Prop. 2018-24-05-001) and involved twenty students and junior research scientists (5 females, 15 males), aged between 20 and 38 years old (M=27.4; SD=4.9). There were all civilian employees of an aerospace laboratory. All subjects volunteered to take part in the study and gave their full informed consent before taking part in the experiment.

Task

The military operational context used to create an ecological task was based on an armed reconnaissance mission, which is one of the typical missions envisaged for future UCAVs. The main objective of the mission is to obtain detailed information on enemy activity in a given area, with a contested air space (Fig. 1). Even if the mission is not dedicated to attacking predetermined targets, the aircraft must be armed and capable of identifying threats and conducting air strikes on targets of opportunity. The mission usually involves medium-range infiltration into a contested environment, reconnaissance of the area and exfiltration from enemy territory. In this study, the simulated UCAV was equipped with terrain following capacities, air-ground missiles and highly automated features that gave it the ability to autonomously identify threats and adapt the mission if necessary. To increase survivability, communications in foe territory were severely restricted, but the UCAV took pictures of targets and threats and sent them back to the operator when a

Friend airbase

Foe airbases

Target area Air and missile defense systems communication point was reached. The use of the weapon to strike targets of opportunity had to comply with the rules of engagement and was under the operator's responsibility. A set of rules, in line with those used in military battlefields, was defined for this experiment. Briefly, the use of weapons was limited to enemy military targets with hostile intent and permission to fire was given to the crew only if no collateral damage was expected. Otherwise, permission to engage the target was given to a higher hierarchical level and the operator had to request permission to fire.

The participant had to monitor a complete 30 minutes mission. The initial flight plan to achieve the mission objectives was already inserted in the flight management system. This plan contained three targets to be observed by the UCAV. Pictures of these targets were to be taken and sent back to the ground station when communication between the UCAV and the ground station was allowed. Specifically, the flight plan contained sections where the UCAV communicated continuously with the ground station, updating its position, sending all available data (images) and other sections where only intermittent communication was allowed at predefined communication waypoints. Nevertheless, the initial flight plan could be modified by the UCAV if threats were encountered during the mission. Depending on the operational situation, these threats could be avoided (the UCAV moved away from the threat) or engaged (the flight plan was modified to create a missile firing opportunity on this new target). In the second case, the operator was responsible for the final firing decision, which had to be in accordance with the rules of engagement. In addition, throughout the firing window, a continuous communication channel was maintained between the UCAV and the ground station, so that all available data to support decision making was displayed to the operator. On the ground station's touch screen interface, the operator had the option to initiate or deny the attack. A radio communication system allowed direct communication with headquarters (in this case the experimenter) if approval was required by the rules of engagement before executing the action.

During the mission, two threats were identified by the UCAV and led to changes in the flight plan. Only the first, which was a missile firing opportunity, required a decision by the operator and is considered in this article. The operator had to collaborate with the UCAV and decide whether or not the target should be engaged. The process involved three distinct parts:

• Perception: extracting useful elements from the environment to understand the actual situation at that time and place on the battlefield and evaluate possible collateral damages. • Deduction: Select the relevant rules of engagement and decide what to do.

Depending on the situation, the operator could deduce that engagement was not allowed, or that engagement was only possible with the approval of higher headquarters, or that she/he could take responsibility for the missile firing. • Action: abort the attack, contact headquarters, or validate the attack.

All participants had the same scenario and the right decision was to contact headquarters.

Experimental conditions

This study employed a 2 between-subject design (N=20). Two communication strategies (C1 and C2) were used. C1: Early communication strategy. In this condition, the UCAV inserted new communication waypoint whenever new threats were encountered. The operator was therefore immediately informed of the change in the flight plan and knew that a firing opportunity was possible. However, the operator did not have the necessary information at that time to make the decision. This data was only provided at the beginning of the firing window. BioPac (MP150) was used to collect physiological data. Electrodermal activity (EDA) and cardiac activity (ECG) were recorded. Both raw signals were acquired at a sampling rate of 1250 Hz. The Biopac ECG100C amplifier used a band-pass filter of 35Hz and 0.5Hz. The Biopac EDA100C amplifier used with a low pass filter set at 10Hz. As both hand were used for the experiment, we used the recommended foot sites (right foot) for EDA recording [START_REF] Boucsein | Publication Recommendations for Electrodermal Measurements[END_REF].

Procedure

Once the ECG and EDA physiological acquisition systems were set up, the participants were briefed on the operational context of the mission, including the rules of engagement, and installed in the simulator. A presentation of the user interface was given and then the participants performed four training scenarios to familiarize themselves with the UCAV monitoring and the firing decision-making process. If they wished to continue with the other phases of the trial, participants signed a consent form. They then performed the 30 minutes scenario before fulfilling NASA-TLX [START_REF] Hart | Development of NASA-TLX (Task Load Index): Results of Empirical and Theoretical Research[END_REF]) (workload) and QUASA (McGuinness 2004) (situation awareness) questionnaires. Participants received a full verbal debrief. The experiment lasted approximately 2 hours.

Data analysis

A factorial independent measure design was employed. The independent variable was the experimental condition with two levels: early and late communication strategy. So 10 subjects (M=27.5; SD=5.5) managed the UCAV with an early communication strategy (C1) while the 10 others (M=27.3; SD=4.5) managed the UCAV with a late communication strategy (C2). Both electrodermal and cardiac activities were analysed thanks to the AcqKnowledge 4.1 © software. Each participant's raw ECG data was processed using the built-in "Detect and Classify Heartbeats" function to estimate the R-wave peaks (minimum BPM 30; maximum BPM 240; R wave threshold 50% Max R peak level). A visual inspection was used to remove unreliable R peaks and related R-R intervals before calculating the mean R-R interval for the one-minute baseline and for the firing window. The metric used is then the difference, for each participant, between the mean R-R interval for the firing window and the mean R-R interval for the baseline. This normalization allows for comparison between subjects. Each participant's raw EDA signal was visually inspected to remove parts with noisy data (foot movement) and SRC were identified thanks to the AcqKnowledge "Locate SRCs" function. For each participant, the first 500 seconds of the experiment (before the threat was detected) are used as a baseline (extract mean and SD) for a participant z-score transformation. Then, the normalized EDA allows for comparison between subjects. Two measures are used, as shown in figure 3.

• Measure 1: The difference between the EDA value prior to threat identification (mean value for the 60 seconds prior to detection of the new threat and communication of the flight plan change to the operator) and the higher EDA value obtained during the firing decision window. • Measure 2: The largest amplitude of the skin conductance response (SCR) during the firing decision window. It measures the phasic change in electrical conductivity of the skin related to the drone request for the firing decision.

Results

All statistical tests reported are two-tailed with alpha levels of .05. Effect sizes were determined using Cohen's d with ≥.2, ≥.5 and ≥.8 indicating small, medium and large effect sizes. In figures, the central rectangle spans the first quartile to the third quartile, the black segment inside the rectangle shows the median, the diamond gives the mean, error bars represent the "inner fence" and unfilled circles outliers.

Testing Hypothesis 1

The first hypothesis is that the early communication strategy should reduce the stress and workload associated with the decision-making process, compared to the late communication strategy. Changes in stress level and workload are assessed by physiological parameters: heart rate and skin conductance. In both experimental condition, the identification by the UCAV of a new threat and the request to the operator to validate or not an attack on this new target should increase the level of stress and workload during the firing window. In both conditions an increase in heart rate and skin conductance is expected. This first hypothesis will be confirmed if these modifications are shaped by the experimental conditions.

As already stated, the analysis of the heart rate is made on the bases of R-R intervals expressed in seconds. In both conditions, a reduction of the R-R interval is observed (Fig. 4) and corresponds to an increase of the heart rate in beat per minute. Nevertheless, the reduction of the R-R interval compared to baseline is significant only for condition C2 (Paired t-test; C1: t = 1.325, df = 9, p-value = 0.218, Cohen's d= 0.256; C2: t = 3.368, df = 9, p-value = 0.008, Cohen's d= 0.574). So, a significant heart rate increases (in beat per minute, compare to baseline) is observed only for condition 2. The comparison of the reduction of R-R interval between both conditions is not significant (Two Sample t-test; t = 0.940, df = 18, p-value = 0.360, Cohen's d =0.420).

Figure 4. Differences between the mean R-R interval during the fire decision windows and the mean R-R interval during the minute that precedes the detection of the threat, by condition.

To conclude, there is no clear increase in heart rate during the decision making process in the first condition (C1) whereas this increase is significant with the late communication strategy (C2). Although this result is not confirmed by the direct comparison between both conditions, it is therefore likely that with a larger number of subjects, the difference between the two conditions should become significant.

For the electrodermal activity, the statistical analysis of measure 1 indicates that in both conditions an increase in skin conductance is observed (Paired t-test; C1: t = -5.920, df = 9, p-value = 0.000, Cohen's d = 2.135; C2: t = -4.300, df = 9, p-value = 0.002, Cohen's d = 1.372). Nevertheless, no significant difference appears between the two experimental conditions (Two sample t-test; t = -0.566, df = 18, p-value = 0.578, Cohen's d = 0.253). Thus, the detection of the new threat, and the opportunity to attack it, implies an activation of the sympathetic branch of the autonomic system from the operator. Moreover, this physiological response seems to be equivalent, whatever the communication strategy of the UCAV.

When measurement 2 is analysed, the skin response related to the drone request during the firing window is slightly different for the two experimental conditions, although the predefined statistical level is not reached (Two sample t-test; t = -1.554, df = 18, p-value = 0.138, Cohen's d = 0.695). It is likely that with some additional participants these differences become significant as the Fligner-Policello test (U* = 2.465, p-value = 0.015) considers that the difference is significant. The overall increase in skin In conclusion, the physiological parameters indicate that the early communication strategy slightly reduces stress levels and workload, with a smaller increase in heart rate and an increase in skin conductance that spread out over time.

Testing hypothesis 2

The second hypothesis is that the early communication strategy should improve the operators' performance in making a firing decision. Two indicators are considered here, the validity of the response and the reaction time. In this experiment, a complete detection and identification of all elements in the scene, as well as an accurate application of the rules of engagement, should lead the operator to request a firing clearance from headquarters. Of the three possible actions (fire, request permission, abort), the first is a clear violation of the rules of engagement, the second is the expected one and the third is sub-optimal. Table 1 shows the results of the experiment. A χ 2 test with this contingency table indicates that differences between the two conditions are just over the predefined threshold (X-squared = 5.905, df = 2, p-value = 0.052). It is very likely that with some additional participants these differences become significant. From an operational point of view, a majority of operators violated the rule of engagement in experimental condition 2, which is simply not acceptable. Let us now look at the response time, calculated as the time between the request for a shot by the UCAV and the moment when the operator starts an action (fire, request permission, or abort). The t test (t = -1.409, df = 18, p-value = 0.176, Cohen's d = 0.630) indicates that the difference is not significant. From an operational point of view, the average response time is 4.5 seconds shorter in condition 1 (Fig. 6), which can be a real advantage in case of hostile enemy reaction.

Discussion

This study focuses on issues related to supervision and critical cooperative decision making involving an operator and a highly automated system. With the increasing level of automation and autonomy and the diffusion of more and more sophisticated objects in everyday life, it is of interest to better understand and assess their impact on human decisions. A plausible military scenario was chosen to outline a situation where the operator has to take, under strong time pressure, the responsibility of a critical decision that has been largely prepared by the partially autonomous system. Implemented in an immersive UCAV simulator, an ecological experiment was conducted to better understand the impact of the UCAV's communication strategy on the decision-making process and the overall monitoring of the automated system. This article only reports the results related to critical decision making.

Firstly, the physiological parameters recorded indicate that the decision-making process is always accompanied by an increase in electrodermal response and heart rate, reflecting an increase in stress level and workload. However, when the autonomous system warns the operator that a major decision has to be made soon, the increase in stress level (as captured by the electrodermal response) is spread over the Reaction time (s) available time. Heart rate analysis indicates that the overall increase in stress and workload appears to be slightly lower in this condition.

Secondly, the communication strategy has a real impact on the final decision and far fewer wrong decisions are made when the operator knows a few minutes before that he will have to assess a situation and make a decision. With the early communication strategy, the operator has a better perception of the elements of the battlefield and makes more accurate use of the rules of engagement. In addition, the reaction time is shorter.

It is also found that the process modifies the operator's level of alertness and vigilance. Thus, although the time windows in which the operator can acquire the required information and construct his decision are exactly the same, the operator is more effective when he/she has been prepared to act. An interesting observation is also that, in the late communication strategy, the wrong decision was always to fire (rather than to abort the attack), as if the strong time pressure pushed the operator to follow the system's decision. Such a result needs to be studied with other experiments, but it is consistent with the notion of complacency towards automation that has already been studied in the aeronautical field. Finally, analyses of QUASA, NASA-TLX and debriefing data (not presented in this paper) indicate that there is no real difference in the overall mission. Users do not report that the late communication strategy is more uncomfortable and do not realise that their decisions were not in line with the rules of engagement.

Further studies are now needed to reinforce these results but also to determine how to reduce complacency towards automation. One perspective is to work on how the autonomous system can better 'explain' to the end user what is relevant to the decision.

Figure 1 .

 1 Figure 1. Armed reconnaissance mission. The red line indicates the border of the contested air space.

C2:

  Figure 2. UCAV flight simulator (left) and its dedicated touchscreen user interface (right). An immersive UCAV simulator (Fig. 2) was used to run the scenario with a dedicated touchscreen interface allowing (1) the monitoring of the UCAV trajectory and the visualization of the new the flight plan when modified by the UCAV (right part of Fig. 2); (2) the visualization of the target's images sent by the UCAV (central part of Fig 2), and (3) the visualization of a continuous stream of full-motion video during the firing decision process (also on the central part of Fig 2, instead of the image management interface).

Figure 3 .

 3 Figure 3. Measures used to characterize the galvanic skin response induced by the decisionmaking process.

  be considered steeper in the second condition and more concentrated during the decision-making process.

Figure 5 .

 5 Figure 5. Modification, by measure and condition, of the normalized skin conductance related to the threat detection and fire decision-making process.

Figure 5 .

 5 Figure 5. Reaction time by condition Thus, these results broadly support the early communication strategy (C1) where better decisions are made in a shorter time. Nevertheless, other operators should be taken into consideration to confirm the robustness of these results.

Table 1 .

 1 Summary of operators' decisions, by condition

		Condition 1	Condition 2
	Fire	1	6
	Request permission	8	4
	Abort	1	0