Aeronautics and astronautics – Spacecraft – Attitude control
Reexamination Certificate
2002-04-03
2003-10-28
Dinh, Tien (Department: 3644)
Aeronautics and astronautics
Spacecraft
Attitude control
C244S164000
Reexamination Certificate
active
06637701
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a system and method for maneuvering a spacecraft. In particular, the present invention relates to a system and method that carries out spacecraft maneuvering with fuel savings as compared to known systems and methods. More particularly, the present invention relates to new thruster configurations.
BACKGROUND OF THE INVENTION
For a spacecraft to perform its mission generally requires control of its in-orbit position relative to the Earth. For example, the orbit of a geosynchronous communications spacecraft is controlled so that the spacecraft remains stationed above the equator at a fixed earth longitude within small tolerances in the north/south and east/west direction. This position control is necessary to allow access to the satellite using fixed-direction ground antennas and to prevent both signal and physical interference with nearby spacecraft.
The orbit control is accomplished by performing periodic orbital correction maneuvers, where thrusters are fired to change the spacecraft's velocity. During maneuvers, the attitude of the spacecraft is controlled so that it remains correctly pointed relative to the Earth and the momentum stored by actuators such as reaction or momentum wheels is adjusted. The fuel efficiency of the system is determined not only by the thruster technology used, but also by how the attitude and momentum control and the velocity change tasks are accomplished. Systems are desired that are as efficient as possible, thereby reducing the propellant required and allowing the payload mass of the spacecraft to be increased.
Various types of thrusters may be used to produce an orbital velocity change. For example, ion thrusters, such as Hall Current Thrusters or gridded ion engines, which accelerate Xenon ions through an electric field to high velocities utilizing electric power supplied by spacecraft solar arrays to generate thrust may be employed. One drawback of ion thrusters is that when the ions impinge on spacecraft surfaces they cause damage by removing surface material. In addition, the ion thruster exhaust components can interfere with communications signals transmitted to and from the spacecraft.
Another type of thruster that may be utilized includes Arcjet thrusters that heat hydrazine decomposition products using an electric arc. Generally, in designing a spacecraft, thruster technology is selected that provides the highest propellant efficiency and is compatible with the spacecraft design.
Whichever types of thrusters are utilized, they typically are mounted directly on a spacecraft or on a boom or other structure attached to a spacecraft to provide components of thrust in at least the east/west and north/south directions.
To minimize undesirable interactions between material emanating from thrusters, whether ions or other exhaust components, and elements of a geosynchronous communications spacecraft, the ion thrusters are typically located towards the panel that faces away from the Earth, commonly referred to as the base panel.
FIGS. 1
a
and
1
b
illustrate such an arrangement. Along these lines,
FIGS. 1
a
and
1
b
illustrate a satellite I that includes Hall Current Thrusters
3
and
5
arranged on gimbaled platforms.
FIGS. 1
a
and
1
b
depicts the thrusters oriented in a position for stationkeeping and firing to produce 45° plume cones
7
and
9
.
The thruster platforms shown in
FIGS. 1
a
and
1
b
are arranged on the north and south sides of the spacecraft. This configuration is also referred to as an aft-mounted ion thruster arrangement. An advantage of this thruster arrangement is that the ions produced during thrusting may be expelled out the aft end of the spacecraft, away from sensitive surfaces and out of the path of RF signals.
Mounting ion thrusters on gimbaled platforms that permit control of the thruster vector orientations as shown in
FIGS. 1
a
and
1
b
can increase fuel efficiency. The gimbaling eliminates the need to fire low-efficiency chemical thrusters such as hydrazine thrusters or bi-propellant thrusters for attitude control. Most satellites today utilize a variant of the aft-mounted arrangement shown in
FIGS. 1
a
and
1
b.
FIGS. 2
a
and
2
b
illustrate in greater detail an aft-mounted thruster configuration arrangement referenced to the body coordinates of a geosynchronous spacecraft
11
. Along these lines,
FIG. 2
illustrates a spacecraft
11
having a north side
13
, a south side
15
, an east side
17
, a west side
19
, an earth facing side
21
, and an anti-earth facing side
23
. Solar arrays
24
and
25
extend from the north side and the south side of the spacecraft. Four thrusters
27
,
29
,
31
, and
33
are arranged in two pairs located at the corners of the spacecraft where the anti-earth side meets the north side and the south side.
Although the thruster configuration illustrated in
FIGS. 1
,
2
a
, and
2
b
is prevalent in satellites today, drawbacks are associated with the arrangement, as described below in greater detail. For an earth-pointing spacecraft, the yaw axis (x) is aligned with the Zenith vector, the vector directed from the center of the Earth towards the spacecraft; the roll axis (y) is aligned with the spacecraft velocity vector, which is directed in the east direction; and the pitch axis (z) is aligned with the orbit normal, which is directed in the north direction. The angle &agr; is the angle of the thrust vector projection in the yaw/roll plane, which is measured from the yaw axis. On the other hand, the angle &bgr; is the angle of the thrust vector from the pitch (z) axis. Typically, &agr; is about 5 degrees to about 20 degrees and &bgr; is approximately 45 degrees as constrained by the location of the center of mass of the spacecraft, because firing a single thruster must produce near-zero torque.
The most fuel efficient and operationally simple thruster arrangement for stationkeeping would allow thrust to be applied purely in the north/south direction (along the pitch axis) and in the east/west direction (along the roll axis). Thrust would not be applied in the radial (x) direction, since thrust in this direction is of limited utility for orbit control. The north/south thrusting provides inclination vector control, and the east/west thrusting provides longitude and eccentricity control.
The aft mounted configuration does not provide the desired thrust direction de-coupling, since a large thrust component is always generated in the unwanted radial direction (along the yaw axis) regardless of which thrusters are fired. For example, firing thrusters
27
and
29
for north/south stationkeeping (inclination control) produces a thrust vector equal to 2F [cos(&agr;)sin(&bgr;), 0, cos(&bgr;)], where F is the nominal thruster force. Clearly, for &bgr;=45 degrees and &agr;=10 degrees the x and z thrust components are nearly equal. Hence, the radial coupling for a north/south maneuver using an aft mounted configuration is approximately 100%. Although this large radial coupling does not significantly affect the fuel required to perform inclination vector control, it does introduce operational complexity since split maneuvers must be executed 12 hours apart to cancel the effects of the radial thrust.
A more serious drawback of the aft-mounted configuration is that it increases the propellant required for east/west stationkeeping. For example, when thrusters
27
and
31
are fired for east/west stationkeeping the resulting thrust is 2F [cos(&agr;)sin(&bgr;), sin(&agr;)sin(&bgr;), 0]. For &bgr;=45 degrees and &agr;=10 degrees, the thrust vector is 2F [0.69, 0.12, 0.0]. Because longitude control can only be accomplished using the roll (y) thrust component, the fuel efficiency is reduced by a factor of 8 (=1/0.12) compared to systems that thrust directly along the roll axis (in the east/west direction). In addition, although the large radial (x) component can be used for eccentricity control, the fuel-efficiency is half that possible using thrusters t
Glogowski Michael Joseph
Goodzeit Neil Evan
Dinh Tien
Lockheed Martin Corporation
Townsend and Townsend / and Crew LLP
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