This invention relates to a new propulsion system for satellites. Specifically this invention relates to a reduced toxicity satellite fuel that can be used for both the maneuvering and station-keeping propulsion systems of a satellite.
Current satellite propulsion systems typically use nitrogen tetroxide with hydrazine in bipropellant class thrusters for maneuvering propulsion and use hydrazine in monopropellant class thrusters for stationkeeping propulsion. Unfortunately these satellite propellants are highly toxic and therefore, require special handling, transportation, and storage mechanisms, which add substantial cost to the deployment of satellites.
One of the goals of NASA""s Discovery Program for new planetary exploration missions, is to substantially reduce total mission cost while improving performance. The performance and cost of the on-board propulsion system for satellites can be a significant factor in obtaining the highest possible science value per unit cost.
Consequently there exists a need for lower cost reduced toxicity fuels with thrust per unit mass flow and density characteristics that are sufficient to replace prior art toxic fuels. Reduced toxicity fuels have not been used in the past, due to the fact that candidate fuels are not hypergolic. In other words, liquid reduced toxicity fuels will not spontaneously react with an oxidizer to begin the combustion process as in prior art fuels such as hydrazine.
Thus, to produce a bipropellant satellite thruster for use with a reduced toxicity fuel, there further exists a need for the thruster to have an ignition element consisting of decomposing elements for decomposing a reduced toxicity propellant into hot gases. These hot gases, like hypergolic toxic liquid fuels will spontaneously react with an oxidizer and begin the combustion process.
In addition to being used with bipropellant class thrusters, there is a further need for this reduced toxicity fuel to be used with monopropellant class thrusters. As a monopropellant, the reduced toxicity fuel must have a molecular structure that will decompose into low molecular weight gases without the formation of a solid constituent such as graphite. These monopropellant thrusters must also contain decomposing elements for reforming the reduced toxicity fuel into propellant gases. Satellite fuels that can be used as both a monopropellant and a bipropellant are referred to as dual-mode fuels.
It is an object of the present invention to provide a reduced toxicity propellant for use in satellite propulsion.
It is a further object of the present invention to provide a satellite thruster with the ability to catalytically decompose a reduced toxicity propellant into hot gases.
It is a further object of the present invention to provide a satellite thruster with the ability to decompose a reduced toxicity propellant into hot gases with a fuel cell reformer.
It is a further object of the present invention to provide a satellite thruster with a low weight plasmatron capable of decomposing a reduced toxicity propellant into hot gases without overheating and eroding portions of the plasmatron.
It is a further object of the present invention to provide a reduced toxicity dual-mode propellant that can be used in both bipropellant and monopropellant satellite propulsion systems.
Further objects of the present invention will be made apparent in the following Best Modes for Carrying Out Invention and the appended claims.
The foregoing objects are accomplished in one preferred embodiment of the invention by replacing the toxic fuel used in prior art satellite propulsion systems with a reduced toxicity liquid fuel such as methylamine. The thrusters in the present invention include a decomposing element for converting the reduced toxicity fuel into hot gases. These decomposing elements are included in both the monopropellant altitude control system (ACS) thrusters for stationkeeping and the bipropellant axial thrusters for maneuvering the satellite.
In the ACS thrusters, these decomposing elements are operative to decompose the reduced toxicity liquid propellant into propellant gases. In the axial thrusters the decomposing elements are operative to decompose the liquid reduced toxicity propellant into hot gases which auto-ignite with the second propellant in the combustion chamber of the axial thruster and thereby produce thrust when ejected through a nozzle. The difference between the thrusters is primarily their thrust class or the force generated during firing. The monopropellant ACS thrusters are in a smaller thrust class than the bipropellant axial thrusters because they are required to satisfy a minimum impulse-bit (thrust times time) requirement for precision pointing of the satellite.
The prior art uses a toxic propellant such as hydrazine in both the monopropellant ACS thrusters and bipropellant axial thrusters. Hydrazine is a hypergolic fuel, which means it will spontaneously react with an oxidizer such as nitrogen tetroxide in the liquid state thereby triggering the combustion process in prior art axial thrusters. Unfortunately, as discussed above, reduced toxicity propellants suitable for use with satellite propulsion are not hypergolic. Before the reduced toxicity propellants of the present embodiment will react with a second propellant, they must be decomposed into hot gases. These hot gases will auto-ignite with the second propellant and thereby begin the combustion process.
Propellants can be decomposed by a number of different technologies, including the use of catalytic decomposing elements, fuel cell reformers, and plasmatrons. Each of these decomposing elements is suitable for different reduced toxicity propellants. For example, the amine, methylamine, the nitroparaffin, nitromethane, and the ether, ethylene oxide, can be catalytically decomposed. Alcohols such as methanol and ethanol, and saturated hydrocarbons such as methane can be decomposed with fuel cell reformers. Saturated hydrocarbons such as pentane and octane and jet engine fuels such as kerosene and JP-10 can be decomposed with a plasmatron. Other embodiments use unsaturated hydrocarbons such as 1-pentene, ring compounds such as cyclopropane, and strained ring compounds such as quadricyclane.
In the preferred embodiment of the invention the second propellant is an oxidizer such as nitrogen tetroxide, liquid oxygen, hydrogen peroxide, or oxygen difluoride. Although oxygen difluoride is highly toxic and must be handled as a mild cryogen on the ground, it represents a high performance option. Although hydrogen peroxide has a rather high toxicity, it has unique characteristics in that it is an unstable molecule that can be catalytically decomposed into hot oxygen rich gas. Thus hydrogen peroxide is suitable in use as both a monopropellant in the ACS thrusters and as an oxidizer in the axial thrusters.
In the preferred embodiment of the present invention the decomposing element of a thruster is always active decomposing the reduced toxicity fuel into hot gases. However, in alternate embodiments the decomposing elements could be used in an axial thruster to initiate the combustion process. Thereafter both propellants can be added directly to the combustion chamber and the decomposing element can be deactivated.