The present invention relates to the field of
engines for space vehicles, in particular for satellites.
5 In the context of certain applications, it may be
desirable for a space vehicle to present two modes of
operation, either at high thrust, or else at high
specific impulse. Two examples may be mentioned:
putting a satellite onto station and maintaining
10 its orbit: putting a satellite of a station requires high
thrust in order to transfer it into a geostationary orbit
as quickly as possible; and the satellite requires
propulsion with high specific impulse in order to
maintain it in position during fifteen years;
15 exploration: it could be advantageous to have a
thruster enabling a celestial object to take off, and
then once in space, enabling it to make use of propulsion
at high specific impulse.
In known manner, satellite propulsion is generally
20 obtained by means of two technologies: chemical
propulsion and electric propulsion. These two modes of
propulsion have respective specific domains of operation
in a plot of specific impulse (I,,) against thrust: in
outline, chemical thrust enables high thrust to be
25 achieved, but specific impulse remains limited to
450 seconds (s). Conversely, electric propulsion enables
high specific impulses to be obtained (2000 s), but the
thrust remains relatively low.
For satellite propulsion, Hall effect thrusters are
30 thus used in attitude and orbit control systems (AOCS)
for space vehicles and in particular in the AOCSes of
geostationary satellites. Hall effect thrusters make it
possible to obtain very high specific impulse (I,,), of
the order of 1500 s, thus making it possible to control
35 accurately the attitude and/or the position of the
Translation of the title as established ex officio.
vehicle while using mass and complexity that are
considerably less than would be necessary in conventional
systems using inertial devices, e.g. such as reaction
wheels, in combination with chemical thrusters for
5 desaturating them.
Nevertheless, a Hall effect thruster offering high
specific impulse normally achieves only very low thrust.
Consequently, AOCSes incorporating Hall effect thrusters
are conventionally associated with chemical thrusters for
10 certain fast maneuvers, such as orbit transfer or putting
into position. Nevertheless, this presents the drawback
of increasing the overall cost and complexity of the
space vehicle, to the detriment of its reliability.
In conclusion, neither of the available technologies
15 (chemical propulsion, electric propulsion) makes it
possible to provide propulsion in both of these intended
operating domains, i.e. firstly with high thrust and
relatively low specific impulse, and secondly with high
specific impulse and relatively low thrust.
20 Thus, an object of the invention is to propose a
space vehicle engine capable of providing thrust in both
of these operating domains, and of doing so without
making the space vehicle excessively heavy or complex.
In order to achieve this object, the invention
25 proposes a space vehicle engine including both a chemical
thruster comprising a nozzle for ejecting combustion gas
and also a Hall effect thruster, the engine being
arranged in such a manner that said nozzle acts as an
ejection channel for particles ejected by the Hall effect
30 thruster when it is in operation.
Thus, both technologies, i.e. chemical propulsion
and electric propulsion, are incorporated within a single
engine. By putting certain means in common, in
particular the nozzle, it is possible to make the engine
35 constituted in this way relatively compact.
Consequently, the engine as constituted in this way
remains relatively simple and inexpensive, given its
operating capabilities, which are extended because of the
simultaneous presence of both thrusters.
In an embodiment, the Hall effect thruster has a
magnetic circuit; and in a section on a meridian half-
5 plane, the magnetic circuit is horseshoe-shaped with an
airgap open to the downstream end of the nozzle; in such
a manner that the magnetic circuit is suitable for
generating a magnetic field in the airgap of the magnetic
circuit.
10 The magnetic field generated in the airgap is
preferably substantially radial.
The terms "upstream" and "downstream" are defined in
the present context relative to the normal flow direction
of propulsion gas in the direction defined by the central
15 axis of the nozzle.
The magnetic field is not necessarily generated
throughout the airgap but it is generated in at least a
portion thereof, generally situated at its downstream
end. A meridian half-plane is a half-plane defined by an
20 axis, specifically the axis of the nozzle.
In this embodiment, it is possible to obtain a
magnetic field in the nozzle in particular because,
instead of being hollow and empty like most conventional
chemical thruster nozzles, this nozzle contains a portion
25 of the magnetic circuit. This inner portion of the
magnetic circuit is generally arranged on the axis of the
nozzle and is conventionally of an axisyrnmetric shape or
even in the shape of a body of revolution about the axis.
The meridian half-planes in which the section of the
30 magnetic circuit is horseshoe-shaped are preferably
angularly distributed regularly around the axis of the
nozzle. Ideally, the magnetic circuit presents one such
section in every meridian half-plane, i.e. over 360"
around the axis of the nozzle.
35 Preferably, in a meridian half-plane view, the
combustion chamber of the chemical thruster is arranged
inside the magnetic circuit.
In an embodiment, the nozzle has an axial section of
annular shape, and passes through the airgap of the
magnetic circuit. The airgap is thus also of annular
axial section. The term "axial section" is used herein
5 to mean a section in a plane perpendicular to the axis of
the nozzle.
In an embodiment, the magnetic circuit has at least
one outer magnetic core situated around the nozzle and an
inner magnetic core situated radially inside the nozzle,
10 and in a section on a meridian half-plane, sections of
said inner core and of said at least one outer core form
branches of said horseshoe-shape.
In an embodiment, the Hall effect thruster further
includes an electric circuit suitable for generating an
15 electric field in the nozzle, and the electric circuit
includes an anode and a cathode arranged respectively
upstream and downstream from the airgap of the magnetic
circuit.
The anode and the cathode may be arranged in various
20 ways.
In an embodiment, the anode comprises a portion of
the nozzle. For example, it may constitute a portion of
the wall of the nozzle.
In another embodiment, the anode is arranged inside
25 the nozzle.
In an embodiment, and in particular in the above
embodiment, the anode is electrically insulated from the
nozzle.
The anode may be arranged in the vicinity of
30 injectors for injecting fluids (propellant injectors)
into the combustion chamber, for the chemical thruster,
and/or in the vicinity of particle injectors for the Hall
effect thruster: i.e. as a general rule completely at the
upstream end of the fluid flow path in the engine.
35 In an embodiment, axially at the level of the
airgap, inner and outer walls of the nozzle are made of
electrically insulating material.
These inner and outer walls of the nozzle may in
particular be made of ceramic material, which is
particularly appropriate because of its electrical,
magnetic, and erosion-resistance characteristics. By way
5 of example, the insulating walls may be formed by two
electrically insulating rings that define said airgap
respectively on its inside and on its outside.
In an embodiment, the nozzle presents a combustion
chamber at an upstream end that is connected to a
10 diverging portion at a downstream end.
The Hall effect thruster also includes at least one
particle injector. In an embodiment, the particle
injector is suitable for injecting particles into said
combustion chamber.
15 The particles may be an inert gas, e.g. xenon.
The present invention also provides a space vehicle
incorporating at least one engine as described above.
The invention can be well understood and its
advantages appear better on reading the following
20 detailed description of two embodiments given as nonlimiting
examples. The description refers to the
accompanying drawings, in which:
Figure 1 is a fragmentary diagrammatic view in
axial section of a space vehicle including a first
25 embodiment of an engine of the invention; and
Figure 2 is a fragmentary diagrammatic perspective
view of the engine shown in Figure 1.
Figures 1 and 2 show an engine 10 of the invention.
It forms part of a space vehicle 100, in the present case
30 a satellite.
The engine is a hybrid engine that is capable of
operating both as a chemical thruster and as a Hall
effect thruster. In order to enable it to operate as a
chemical thruster or as a Hall effect thruster, the
35 engine 10 is connected to propellant tanks (not shown;
there may be a single propellant or two propellants), and
it is also connected to a tank of propulsion gas.
The engine 10 is generally in the form of a body of
revolution about an axis X.
It is arranged inside a casing 20 that is
substantially cylindrical about the axis X. A first
5 axial end 22 of the casing, its upstream end, is closed
by a substantially flat end wall 24 perpendicular to the
axis X, while the other end 26 (its downstream end) is
closed in part by a substantially flat end wall 25 that
is likewise perpendicular to the axis X. The end wall 25
10 has a wide annular passage 28 passing therethrough to
eject gas.
The end wall 25 is generally in the form of a disk
perpendicular to the axis X. Because of the presence of
the annular passage 28, the end wall 25 is constituted by
15 a disk 56 and by an annular ring 58 situated radially
around the annular passage 28. The ring 58 is formed
integrally with the casing 20.
The engine 10 includes a chemical thruster 11.
The chemical thruster 11 has a nozzle 30 arranged
20 inside the casing 20.
The nozzle 30 is of generally annular shape about
the axis X. More generally, the nozzle 30 may also be
axisymmetric. Nevertheless, it is possible as an
alternative to envisage shapes that are non-axisymmetric,
25 e.g. of cross-section that is oval or racecourse-shaped.
Whether or not the nozzle 30 is in the shape of a
body of revolution or axisymmetric, the nozzle 30 is
generally annular in shape and thus not only has a
radially outer wall 34, but also a radially inner wall
30 32.
These walls are concentric about the axis X.
The nozzle 30 is closed at the upstream end (on the
left in Figure 1) and open at the downstream end.
From upstream to downstream, the nozzle 30 presents
35 initially a combustion chamber 36, then a throat 38,
followed by a diverging portion 40. These elements are
arranged so as to enable the engine 10 to operate as a
chemical thruster 11.
The chemical thruster 11 also has injectors 42 for
injecting propellants. These are arranged in such a
5 manner as to enable propellants to be injected at the
upstream end of the combustion chamber 36. For this
purpose, they are connected to propellant sources (not
shown) by a feed circuit 44.
The engine 10 also has a Hall effect thruster 50.
10 This thruster 50 firstly comprises a magnetic circuit 52.
The magnetic circuit 52 comprises: the casing 20
itself, which is made of ferromagnetic material and thus
forms an outer magnetic core; end walls 24 and 25 made of
ferromagnetic material; and a central magnetic core 54 in
15 the form of a shaft extending along the axis X. The disk
56 constituting a portion of the end wall 25 forms the
downstream end of the shaft 54.
The above-specified elements of the magnetic circuit
52 are arranged together so as to enable a magnetic field
20 to pass without losses via the magnetic circuit.
In order to protect the downstream portion of the
nozzle from wear and in order to contain the electron
cloud formed in the airgap of the magnetic circuit, the
axially downstream portions of the vralls 32 and 34 are
25 formed by rings made of ceramic material, given
respective references 82 and 84. These rings are
positioned at the level of the airgap of the magnetic
circuit 52.
The magnetic circuit 52 also has an inner annular
30 coil 70 and an outer annular coil 72 that serve to
generate the magnetic field needed to enable the Hall
effect thruster to operate. These two coils are formed
concentrically around the axis X. The coil 70 is formed
around the. shaft 54 (radially) inside the wall 32 (i.e.
35 between the shaft 54 and the wall 32). The coil 72 is
formed on the inside face of the cylindrical casing 20,
and more precisely between the inside face and the outer
wall 34 of the nozzle 30.
Axially, the coils 70 and 72 are placed a little way
downstream from the throat 38 of the nozzle 30. In more
5 generally manner, these coils may be located axially at
any level along the axis X from the combustion chamber at
the upstream end to a position immediately upstream from
the ceramic rings 82 and 84 at the downstream end
The coils 70 and 72 are powered by an electric
10 energy source (not shown).
In the magnetic circuit 52, the central magnetic
core 54 and the outer magnetic core (the casing 20) are
arranged in such a manner as to have opposite polarities.
The circuit 52 is arranged so as to generate a
15 substantially radial magnetic field in the annular
passage 28, thus constituting the airgap of the circuit
52.
In other embodiments, the magnetic circuit may be of
a structure that is different from that of the circuit
20 52. The important point is that the magnetic circuit is
suitable for generating a radial magnetic field in the
ejection passage (specifically the passage 28) of the
Hall effect thruster.
The intensity of the magnetic field decreases
25 progressively from the ejection passage 28 to the throat
38 of the nozzle. In the embodiment shown, the magnetic
field (which is at its maximum axially level with the
passage 28) is attenuated by internal and external
magnetic screens 77 so as to reduce the intensity of the
30 magnetic field in the vicinity of the anode 62.
These screens are formed respectively on the inside
surface of the casing 20 and on the outside surface of
the shaft 54, and they support the coils 70 and 72
mechanically.
35 The coils 70 and 72 are coils of substantially
cylindrical shape, in which each of the turns is
substantially in the form of a circle about the axis X
In another embodiment, the coil 72 could be replaced by a
plurality of identical coils 72, each about a respective
axis parallel to the axis X, the coils 72 being arranged
in axisymmetric manner around the outer wall 34 of the
5 nozzle 30.
The downstream portion of the nozzle 30 passes
through or extends into the airgap 28 of the circuit 52.
In a section on a meridian half-plane (Figure I),
the magnetic circuit is thus horseshoe-shaped, with an
10 airgap 28 that is open towards the downstream end 26 of
the nozzle 30. Going from the end wall 24, the
horseshoe-shape is constituted respectively by the
section of the casing 20 on the outside and by the
section of the central core 54 on the inside, which form
15 the two branches of the horseshoe.
The thruster 50 also has an electric circuit 60.
This circuit comprises an anode 62 situated axially about
halfway along the diverging portion 40, a cathode 64
situated downstream from the end 26 of the nozzle 30, and
20 an electric voltage source 66 connecting the anode 62 to
the cathode 64.
In more general manner, the anode 62 may be located
axially at any level along the axis X going from the
combustion chamber at the upstream end to a position
25 immediately upstream from the ceramic rings 82 and 84 at
the downstream end.
The anode 62 is constituted mainly by the inner wall
34 of the nozzle 30: it is thus incorporated in the
nozzle 30, while being electrically insulated therefrom.
30 The cathode 64 is fastened on the disk 56 on the
outside, i.e. downstream from the shaft 54. In Figure 2,
the cathode 64 is drawn in dashed lines.
The cathode 64 is connected to the electric voltage
source 66 by a cable passing inside the inner wall 32 of
35 the nozzle 30.
Advantageously, this cable passes inside the shaft
54.
Finally, at the upstream end of the nozzle 30, the
thruster includes propulsion gas injectors 75. These are
arranged in such a manner as to enable propulsion gas to
be injected into the upstream end of the combustion
5 chamber 36. For this purpose, they are connected to a
source of propulsion gas (not shown) by an injection
circuit 76. The propulsion gas may be xenon, which
presents the advantages of high molecular weight and
comparatively low ionization potential. Nevertheless, as
10 in other Hall effect thrusters, a wide variety of
propulsion gases could be used.
The engine 10 presents two main modes of operation,
namely electric propulsion and chemical propulsion.
For chemical propulsion, the propellants are
15 injected into the combustion chamber 36 via the injectors
42. They are burnt in the chamber; the combustion gas is
accelerated by the throat 38 and the diverging portion 40
and ejected at high speed via the downstream opening 28
of the nozzle 30.
20 For Hall effect propulsion, the engine 10 operates
as follows.
An electric voltage, typically of the order of
150 volts (V) to 800 V when xenon is used as the
propulsion gas, is established between the cathode 64
25 downstream from the downstream end of the nozzle 30 and
the anode 62. The cathode 64 then begins to emit
electrons, most of which are trapped in a magnetic
enclosure formed by the magnetic field created by the
magnetic circuit 52, which is adapted to the performance
30 desired and to the propulsion gas used, and which is
typically of the order of 100 gauss (G) to 300 G when
using xenon as the propulsion gas. The electrons trapped
in this magnetic enclosure thus form a virtual cathode
grid.
35 Highly energetic electrons (typically 10 electron
volts (eV) to 40 eV) escape from the magnetic enclosure
towards the anode 62, so long as the propulsion gas
continues to be injected into the nozzle 30 via the
injectors 75. The impacts between these electrons and
the atoms of the propulsion gas ionize the propulsion
gas, which is then accelerated towards the downstream end
5 26 of the nozzle 30 by the electric field E generated by
the coils 70 and 72. Since the mass of the propulsion
gas ions is several orders of magnitude greater than the
mass of electrons, the magnetic field does not confine
the ions in the same way as it confines the electrons.
10 The thruster 50 thus generates a plasma jet that is
ejected at extremely high speed through the downstream
end of the nozzle 30, thereby producing thrust that is
substantially in alignment with the central axis X.
The operation of the thruster 50 is analogous to the
15 operation of the thruster described in Document
US 2003/0046921 Al.
Optionally, the engine 10 could also include an
additional nozzle segment downstream from the rings 82
and 84 for the purpose of enabling additional expansion
20 of the combustion gas when the chemical thruster is in
operation.
The annular shaped of the nozzle 30 thus enables it
to be used not only as a channel for combustion of
propellants and ejection of combustion gas, during
25 chemical propulsion, but also as an ion acceleration
channel during electric operation. In particular, the
arrangement of the magnetic core 54 in the form of a
shaft on the axis of the nozzle does not impede in any
way the operation of the chemical thruster 11.
30 Furthermore, the position of the cathode downstream from
the end wall 25 and protected by the end of the shaft 54
(the cathode 64 is in direct contact with the center of
the disk 56) makes it possible to ensure that the cathode
does not come into contact with the stream of combustion
35 gas, to which it cannot be exposed for a long time.
Although the present invention is described with
reference to a specific embodiment, it is clear that
various modifications and changes could be made to this
embodiment without going beyond the general ambit of the
invention as defined by the claims. In addition,
individual characteristics of the embodiment mentioned
5 may be combined in additional embodiments. Consequently,
the description and the drawings should be considered in
a sense that is illustrative rather than restrictive.

CLAIMS
1. A space vehicle engine (10) comprising a chemical
thruster (11) having a nozzle (30) for ejecting
combustion gas, the engine being characterized in that it
5 includes a Hall effect thruster (50) arranged in such a
manner that said nozzle acts as the ejection channel for
particles ejected by the Hall effect thruster when it is
in operation.
10 2. An engine according to claim 1, wherein:
the Hall effect thruster (50) has a magnetic circuit
(52); and
in a section on a meridian half-plane, the magnetic
circuit is horseshoe-shaped with an airgap (28) open to
15 the downstream end (26) of the nozzle;
in such a manner that the magnetic circuit is
suitable for generating a magnetic field in the airgap of
the magnetic circuit.
20 3. An engine according to claim 2, characterized in that
the nozzle (30) has an axial section of annular shape,
and passes through the airgap of the magnetic circuit
(52).
25 4. An engine according to claim 3, characterized in that
the magnetic circuit has at least one outer magnetic core
(20) situated around the nozzle (30) and an inner
magnetic core (54) situated radially inside the nozzle,
and in a section on a meridian half-plane, sections of
30 said inner core and of said at least one outer core form
branches of said horseshoe-shape.
5. An engine according to any one of claims 2 to 4,
wherein the Hall effect thruster further includes an
35 electric circuit (60) suitable for generating an electric
field in the nozzle, and the electric circuit includes an
anode (62) and a cathode (64) arranged respectively
upstream and downstream from said airgap (28).
6. An engine according to claim 5, wherein the anode (62)
5 comprises a portion of the nozzle (30).
7. An engine according to claim 5, wherein the anode (62)
is arranged in the nozzle and is electrically insulated
therefrom.
10
8. An engine according to any one of claims 2 to 7,
characterized in that, axially at the level of the airgap
(28), the inner and outer walls (32, 34) of the nozzle
are made of electrically insulating material, in
15 particular of ceramic.
9. An engine according to any one of claims 1 to 8,
characterized in that the nozzle presents a combustion
chamber (36) at an upstream end that is connected to a
20 diverging portion (40) at a downstream end.
10. An engine according to claim 9, in which the Hall
effect thruster further includes at least one particle
injector (72) suitable for injecting particles into said
25 combustion chamber (36) .
11. A space vehicle including at least one engine (1)
according to any one of claims 1 to 10.