Aeronautics and astronautics – Spacecraft – With fuel system details
Reexamination Certificate
2000-07-19
2001-04-10
Jordan, Charles T. (Department: 3644)
Aeronautics and astronautics
Spacecraft
With fuel system details
C244S158700
Reexamination Certificate
active
06213432
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method of putting a space vehicle, such as a satellite, into a target orbit such as the orbit for normal operation of the space vehicle, starting from an elliptical initial orbit that is significantly different and in particular more eccentric than the target orbit.
PRIOR ART
Most artificial satellites are now fitted with a thruster system enabling them to move in space, in particular to correct imperfections of trajectory, due in particular to imperfections with which they were put into orbit, to the gravitational attraction of the moon and the sun, to potential effects due to the earth being non-spherical, to aerodynamic, magnetic, and electrical effects associated with the earth, and to the action of solar radiation. The thrust system of a satellite also enables the satellite to be put on station, to make changes to its orbit, to ensure that it is appropriately oriented in space, or indeed to ensure that the attitude control system remains functional by enabling the inertia wheels fitted to such satellites to be desaturated.
Such thrust systems generally give a satellite the ability to move in any direction, with one direction being generally preferred for performing movements of large amplitude.
In terms of mass budget, the satellite's thruster system generally constitutes a major component, or even the largest component.
Most of the thruster industry became interested very early on in techniques for reducing the mass of a thruster system. Given that specific impulse is a characteristic value of a thruster, specifying the impulse provided per unit mass ejected or consumed, high specific impulse thrusters have been designed, developed, and evaluated. By way of example, mention can be made of “resistojet” type thrusters, closed electron drift plasma thrusters, FEEP field emission thrusters, ion bombardment thrusters, and heliothermal thrusters.
Theocratically speaking, increasing specific impulse is based on transforming zero-mass power, i.e. power whose production consumes practically no matter, into mechanical power applied to particles of matter. In practice, such zero-mass power is electrical power or thermal power obtained from solar radiation, or indeed as obtained from a radioisotopic generator.
The thrust obtained from high specific impulse thrusters of this kind therefore depends on the level of electrical or thermal power that can be supplied to them. On a satellite, such power is limited by the size of its solar panels, by the size of its solar thermal energy concentrators, or by the size of its radioisotopic generator, or indeed by the size of its energy storage means. As a result, the thrust delivered by any high specific impulse thruster is small or very small compared with the thrust from a conventional chemical engine, e.g. 400 N (a typical value for a satellite apogee engine).
The greater the electrical or thermal power transformed into mechanical power applied to a given mass of particles, the greater the resulting specific impulse. Thus, the greater the specific impulse of thrusters, the lower the thrust provided for given consumption of electrical or thermal power. This property is substantially valid for all types of high specific impulse thruster.
This property has the following effects on propulsion systems: for a given total delivered impulse (i.e. the cumulative value or the time integral of the force delivered to the vehicle over the entire duration of firing), there is both a remarkable reduction in the mass of matter consumed by higher specific impulse thrusters and a corresponding increase in the time over which such thrusters operate.
Thrust of the high specific impulse type is suitable for the maneuvers performed when a satellite is on its nominal operating orbit, since the forces that need to be delivered are then low or very low, thus making it possible to achieve real advantages over chemical thrust systems (of lower specific impulse).
The opposite applies when a satellite is initially placed on an orbit that is very different from its nominal orbit and the satellite needs to operate its own thrust system to move from its initial orbit to its nominal orbit.
Under such circumstances, the total duration of the transfer maneuver tends to be lengthy whereas, on the contrary, it would be better to be able to minimize said duration. The longer the total duration of the transfer maneuver, the greater the financial burden, and costs associated with putting the vehicle into orbit also increase (including ground station costs and the cost of tracking teams on the ground). Also, a long duration transfer maneuver increases the risk of the space vehicle being damaged as it passes through the Van Allen belts (which vary in position, but which may be situated, for example, in the vicinity of the following altitudes: 1800 km, 2000 km, 10,000 km, and 21,000 km).
It is desirable to minimize the number of revolutions the vehicle performs on orbits that pass through the Van Allen belts, in particular to minimize the additional hardening of components or solar cells that would otherwise be needed to protect them against the electromagnetic waves or radiation emitted by the protons or electrons present in the belts.
Various examples of satellite maneuvers have already been proposed that make use of thrusters having high specific impulse and low thrust.
Thus, the article by A. G. Schwer, U. W. Schöttle, E. Messerschmid of the University of Stuttgart (Germany), published at the 46th International Astronautic Congress in 1995 and entitled “Operational impacts and environmental effects on low-thrust transfer missions of telecommunication satellites”, shows that a transfer maneuver between a conventional initial orbit constituted by the geostationary transfer orbit (GTO) of the Ariane 4 rocket and a final orbit constituted by a geostationary orbit (GEO). The transfer maneuver comprises a large number of thrust arcs about the apogee of the GTO orbit created by “Arcjet” type thrusters having high specific impulse, such that the orbit of the satellite deforms progressively until it reaches the final geostationary orbit GEO.
Document EP-B-0 047 211 (inventor A. Mortelette) also describes a method of changing orbit by thrust arcs.
For the two methods described in the two above-mentioned documents, apogee altitude is constrained to remain constant or to vary very slowly. The duration required by the maneuvers described is quite large, such that to reduce said duration it is necessary to increase thrust. Also, the number of times the engine needs to be started is also large and this can give rise to major operational constraints. When the satellite is on an orbit with a variable sidereal period which is changing constantly as a function of the extent to which its position is progressing, and which can be different or even very different from the sidereal period of earth rotation, the satellite is not always visible from a given ground station when it is necessary to start its thrusters. This means that any procedure for putting the satellite into orbit that requires thrusters to be started on numerous occasions cannot be performed safely using only one ground station. On the contrary, it must be possible to make use of a plurality of ground stations located at different places throughout the time required for putting the satellite into orbit. The cost of ground operations and rental of ground stations is not negligible.
In order to reduce the total duration of the maneuver in the schemes put forward in the above-mentioned document for putting a satellite into orbit, it would be better to have high thrust thrusters. However, under such circumstances, for given power delivered to the thrusters, the specific impulse would have to be smaller and consequently the mass consumed during the maneuver would be greater. The various solutions that have already been recommended for putting a satellite into orbit in this manner are therefore of relatively low performance.
Proposals have already been made by Irving, to pass fr
Dinh Tien
Jordan Charles T.
Societe Nationale d'Etude et de Construction de Moteurs d&a
Weingarten, Schurgin Gagnebin & Hayes LLP
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