Aeronautics and astronautics – Spacecraft – Attitude control
Reexamination Certificate
2001-12-21
2003-05-20
Swiatek, Robert P. (Department: 3644)
Aeronautics and astronautics
Spacecraft
Attitude control
Reexamination Certificate
active
06565043
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention generally relates to orbit inclination and altitude control for spacecraft and, more particularly, to a satellite orbit inclination control system with redundancy to enhance reliability.
It is common for satellites in orbit to use two north and two south pointing electric plasma thrusters for control of the inclination of the satellite's orbit, and for station keeping and altitude control. For example,
FIG. 1
shows satellite
100
, which may be a geosynchronous satellite, in two different positions in its flight orbit
102
about the earth
104
. A geosynchronous orbit is of particular importance for many satellites, including communications and navigation satellites.
FIG. 1
shows geosynchronous orbit
106
in the plane
108
of the equator
110
of the earth
104
. Because equator
110
of the earth
104
lies in plane
108
, plane
108
may also be referred to as the equatorial plane, or the equatorial orbit plane of geosynchronous orbit
106
. As seen in
FIG. 1
, flight orbit
102
of satellite
100
lies in a plane which is inclined to equatorial orbit plane
108
, as indicated in
FIG. 1
by angle of inclination &thgr;. In order to maintain satellite
100
in geosynchronous orbit, it is desired to provide inclination control by actuating north and south pointing thrusters at nodes
112
and
114
, where the plane of flight orbit
102
and the equatorial orbit plane
108
of geosynchronous orbit
106
intersect. For example, by actuating a north pointing thruster at node
112
, a correction in the direction indicated by arrow
116
may be provided, and by actuating a south pointing thruster at node
114
, a correction in the direction indicated by arrow
118
may be provided, in order to correct the inclination of flight orbit
102
back to a geosynchronous orbit
106
in equatorial orbit plane
108
. In other words, north and south pointing thrusters are actuated at nodes
112
and
114
to reduce angle of inclination &thgr; to approximately 0.0 degrees.
The preferred mounting configuration for north and south pointing thrusters on satellite
100
is generally anti-nadir side
120
of satellite
100
, i.e., the side of satellite
100
facing away from, or furthest from, the earth
104
. Anti-nadir side
120
may be the preferred configuration for mounting thrusters due to various engineering constraints and for other technical reasons. For example, mounting thrusters on anti-nadir side
120
of satellite
100
may minimize interference of the thrusters with radio antennas and other communications devices requiring an unobstructed path to the earth
104
, in the case where satellite
100
is a communications satellite. Because the force, or thrust, applied by north and south pointing thrusters to satellite
100
must be directed through the center of mass of satellite
100
to prevent torqueing satellite
100
, there is a radial component to the thrust which acts to change the eccentricity of flight orbit
102
. By actuating first one thruster at node
112
and then actuating an oppositely pointing thruster at node
114
on the opposite side of flight orbit
102
, for example, first a north pointing and then a south pointing thruster, the radial component of the thrust may be effectively cancelled so as not to change the eccentricity of flight orbit
102
, but only the angle of inclination &thgr; of flight orbit
102
. Thus, an operational system for inclination and altitude control of satellite
100
requires, at a minimum, a north and a south pointing thruster.
To ensure mission success, thrusters and their control electronics are currently provided with backup units. For example, where a minimum of two thrusters, i.e., a north pointing thruster and a south pointing thruster, is required, four thrusters may be installed to guarantee access to two thrusters in case of a single thruster failure. The approach of providing more thrusters, or in general more components of any type, than the minimum required in order to enhance reliability, guarantee access to the minimal number of components required in case of single component failure, and to ensure mission success is known as redundant design, or more briefly, redundancy.
Electric propulsion, more specifically plasma type electric propulsion, has been introduced for satellite control as a replacement for chemical propulsion primarily because of the improved specific impulse, i.e. the change in momentum produced using a unit mass of propellant, of plasma type electric propulsion over chemical propulsion. The specific impulse of electrical plasma thrusters is approximately an order of magnitude, or 10 times, greater than the specific impulse of chemical thrusters. Chemical propulsion thruster systems use many thrusters (typically 12) with optimal thrust orientations to reduce propellant use. A number of factors, such as component mass, mounting space and exhaust plume size, for example, do not allow for a simple one-for-one exchange of electrical plasma thrusters for chemical thrusters. In the example that follows, a chemical propulsion thruster system using 12 thrusters is compared to an electrical plasma thruster system using a reduced number of electrical plasma thrusters, i.e., 4 electrical plasma thrusters.
FIG. 2
shows an example of an electrical plasma thruster system for satellite
100
using electrical plasma thrusters
122
,
124
,
126
, and
128
as typically currently mounted at the anti-nadir side
120
of satellite
100
. Two-axis gimbals mechanisms
130
and
132
are used to align the thrust vectors of electrical plasma thrusters
122
,
124
,
126
, and
128
to the center of mass of satellite
100
. This arrangement for an electrical plasma thruster system may reduce the overall weight of satellite
100
, including weight of propellant required for some particular desired life span of satellite
100
and component weight of the thruster system, compared to a chemical propulsion thruster system. The cost of the electrical plasma thruster system, however, may be greater.
For example, in a typical satellite weighing approximately 3,000 kilograms (Kg) with a desired lifespan of approximately 15 years, approximately 1,000 Kg propellant may be needed for chemical thrusters compared to only 100 Kg of propellant for electrical plasma thrusters mounted in the same arrangement as the chemical thrusters. The propellant savings is primarily due to the order of magnitude advantage in specific impulse for the electrical plasma thrusters over chemical thrusters. The two-axis gimbals mounting arrangement using only 4 electrical plasma thrusters instead of 12 is less fuel efficient, however, requiring thruster actuations on opposite sides of the flight orbit as described above, so that 150 Kg of propellant may be needed for the 4 electrical plasma thrusters.
Additional mass inefficiency may be incurred when system redundancy is achieved by duplication of all components, rather than duplication of the functional requirements. Each electrical plasma thruster with its electronic controller and gimbals mechanism typically weighs 40 Kg compared to approximately 0.5 Kg for a chemical thruster. Thus, 12 chemical thrusters may be expected to weigh approximately 6 Kg, a negligible amount, compared to 480 Kg for 12 electrical plasma thrusters. By reducing the number of electrical plasma thrusters to four, the weight penalty for the electrical plasma thrusters is reduced to approximately 160 Kg. Thus, the total propellant and component weight for a chemical propulsion system may be expected to be approximately 1,000 Kg compared to a total weight of approximately 310 Kg for an electrical plasma thruster system using only 4 electrical plasma thrusters.
Typical cost for a chemical thruster system with 12 thrusters may be expected not to exceed approximately $1.5 million whereas typical cost for an electrical plasma thruster system with 4 thrusters may be expected not to exceed approximately $4.0 million. A simple one-for-one exchange of electrical plasma thrusters for chemical
DiPinto & Shimokaji P.C.
Swiatek Robert P.
The Boeing Company
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