CROSS REFERENCE TO RELATED APPLICATION

[0001] This patent application claims priority to commonly owned United States Provisional Patent Application Serial No. 61/074,489 entitled "Ionic Liquid Multi- Mode Propulsion System" that was filed on June 20, 2008. The subject matter of United States Provisional Patent Application Serial No. 61/074,489 is incorporated by reference in its entirety herein.

U.S. GOVERNMENT RIGHTS

[0002] N.A.

BACKGROUND OF THE INVENTION

Field of the Invention

[0003] Disclosed herein is a multi mode propulsion system for spacecraft.

Description of the Related Art

[0004] A multi-mode satellite propulsion system has a high thrust chemical rocket and a high specific impulse electric rocket both operating off a single propellant. Presently, the most commonly used and highest performance multi-mode satellite propulsion system includes a catalytic monopropellant thruster and an arcjet, both operating on hydrazine. This state of the art system is limited in several ways. The efficiency and specific impulse of electrothermal rockets is inherently limited to 30% to 40% by thermodynamic processes and materials. Electrostatic and electromagnetic devices can provide much higher specific impulse but cannot operate efficiently on hydrazine due to the low molecular weight of the decomposition gases. The performance of the chemical thruster is also limited by the thermodynamic properties of hydrazine. Hydrazine is also toxic and requires special handling provisions.

[0005] Published United States Patent Application Publication Number US

2003/0226750 Al discloses an electrospray apparatus as a microthruster for a spacecraft. A fine spray of highly charged droplets is expelled from a fine bore tube. The mass- charge ratios of the electrosprayed droplets are sufficiently high for their electrostatic acceleration to provide greater thrust at lower energy than can the ions of heavy elements that are traditional propellants for ion rockets. The disclosure of US 2003/0226750 Al is incorporated by reference in its entirety herein.

BRIEF SUMMARY OF THE INVENTION

[0006] The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects and advantages of the invention will be apparent from the description and drawings, and from the claims.

[0007] Disclosed herein is a multi-mode propulsion system that uses an ionic liquid as the propellant instead of hydrazine and an electrospray thruster instead of an electrothermal thruster. Electrospray thrusters can achieve efficiencies greater than 80% providing significantly more velocity change (ΔV) capability for a given mass of propellant. In addition, ionic liquids are non-toxic making them more environmentally friendly and reducing the need for special handling and procedures during propellant loading. Ionic liquids also offer the potential for higher chemical engine performance than can be achieved with hydrazine.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] FIG. 1 illustrates a chemical thruster for use in the multi-mode propulsion system described herein.

[0009] FIG. 2 illustrates an electrospray thruster module for use in the multi- mode propulsion system described herein.

[00010] FIG. 3 illustrates an array of electrospray thruster modules.

[00011] FIG. 4 illustrates hardware components of the multi-mode propulsion system.

[00012] FIG. 5 illustrates electrical components of the multi-mode propulsion system.

[00013] Like reference numbers and designations in the various drawings indicate like elements. DETAILED DESCRIPTION

[00014] FIG. 1 shows a high performance chemical thruster 10 as a component of a multi-mode propulsion system as described herein. Fuel tank 12 stores an ionic liquid monopropellant 14 under a pressurized gas 16, such as helium. A conduit 18 communicates the ionic liquid monopropellant to both the chemical thruster 10 and to an electrospray thruster (not illustrated in FIG. 1 , but described hereinbelow).

[00015] A valve 20 controls the flow of ionic liquid propellant 14 into the chemical thruster 10. When the valve 20 is open, ionic liquid propellant 14 flows through an injector 22 that uniformly disperses the liquid through an internal volume of the chemical thruster 10. The internal volume is filled with a catalytic material or thermal bed to promote decomposition and surrounded by a heater 24 or other energy source such as microwave increases the temperature of the bed, typically in excess of 70° to promote decomposition when ionic liquid monopropellant contacts the bed 26. The decomposition products generate a large volume of high temperature gas which is compressed through throat 30 and then rapidly expanded in nozzle 32 generating thrust. The chemical thruster 10 generates thrust of greater than 1 Newton (N).

[00016] For the chemical thruster 10, the propellant must be energetic and it is desired to have the following properties:

[00017] Suitable monopropellants are believed to include: l-Ethyl-3-

Methylimidazolium-CF ₃ Sθ3; 1 -Butyl-3-Methylimidazolium~CF ₃ SO ₃ ; 1 -Hexyl-3- Methylimidazolium-(C ₂ F ₅ ) ₃ PF ₃ ; Trihexyl(tetradecyl)phosphonium bis(trifluoromethylsulfonyl)imide; Trihexyl(tetradecyl)phosphonium tris(pentafluoroethyl)trifluorophosphate; N-hexyl-4-dimethylamino-pyridinium bis(trifluoromethylsulfonyl)imide; l-Ethyl-3-Methylimidazolium-N(SO ₂ CF ₃ ) ₂ ; l-Butyl-3- Methylimidazolium-N(SO ₂ CF ₃ ) ₂ ; l-Hexyl-3-Methylimidazolium-N(S0 ₂ CF ₃ ) ₂ ; l-Ethyl-3- methylimidazolium trifluoroacetate; Methyltrioctylammonium trifluoroacetate; N-Butyl-3- methylpyridinium tnethylsulfate; l-Ethyl-3-methylimidazolium ethylsulfate; l-Ethyl-3- methylimidazolium tetracyanoborate; and l-Butyl-3-methylimidazolium tricyanomethane.

[00018] The traditional approach to monopropellant chemical thrusters has been to use a noble metal catalyst. The current preferred catalyst is an iridium metal on an alumina carrier. While catalytic ignition is a baseline approach, alternate ignition approaches such as microwave may be utilized. Alternatively, a bipropellant approach using ionic liquids for both a fuel and an oxidizer could be adopted.

[00019] Another challenge of higher performance monopropellant thrusters is finding lightweight materials that can withstand a temperature and chemical environments without degrading. Preferred materials for the chemical thruster 10 components are nickel alloys, carbon-carbon and silicon carbide composites, and noble metals and noble metal coatings such as molybdenum, rhenium and iridium.

[00020] FIG. 2 illustrates an electrospray thruster 34 that receives ionic liquid monopropellant 14 from the fuel tank and is a second component of the multi-mode propulsion system. The electrospray thruster 34 has a porous metal preferably tungsten base etched or machined to have at least one, and preferably a plurality of emitters 38 in a two dimensional array. The porous matrix and the emitters 38 wick the ionic liquid propellant 14 via capillary action from an upstream header 40 to a downstream end 60. An electrically insulating spacer 44, such as Teflon, separates the emitters 38 from an extractor grid 46. A voltage applied between the extractor grid and the porous metal emitters pulls the ionic liquid from the emitters as a fine mist of electrically charged particles. An electrically non-conductive spacer 50 separates the extractor grid 46 from a downstream decelerator grid 52. Electric power is provided to the holder bars 36 by line 54, to the extractor grid 46 by line 56, and to the decelerator grid 52 by line 58.

[00021] Emitter 38 is formed from any material effective to form particles of ionic liquid at the surface adjacent gap 42 and to impart each of those particles with an electric charge. In one embodiment, the emitter 38 is formed from porous tungsten where downstream surface 60 is electrochemically etched to form a series of volcano shaped protrusions. Initial testing of porous tungsten substrate emitters has demonstrated pure ion emission with no droplet formation and emission currents per emitter exceeding one micro-amp per emitter and demonstrated that emission currents of 2-3 μA per emitter should be achievable. An emitter density of 2 tips / mm ² or higher should be achievable by electrochemical etching such that a one kilowatt thruster would have a size of approximately 41 centimeters x 41 centimeters.

[00022] The polarity of lines 54 and 56 are maintained in opposition. The charged particles emitted from upstream end 60 are imparted with the polarity of line 54 and accelerated towards the reverse polarity extractor grid 46 generating thrust. Charged particles passing through the extractor grid 46 travel towards decelerator grid 52. By varying the voltage potential and polarity of the decelerator grid 52, the ion exhaust velocity may be varied providing instantaneous I _sp adjustability over a broad range (700- 2,000 seconds).

[00023] An electrospray thruster 34 of the type illustrated in FIG. 2 generates between approximately 50 W and 100 W of power. As shown in FIG. 3, a plurality of thrusters 34 may be arranged in a grid to generate a target power level. Propellant flow to the grid may be controlled by a single common valve. FIG. 3 illustrates a concept of a one kilowatt thruster formed by an array of 18 55 W thrusters. This modular design approach enables a broad range of future thruster power levels with the simple addition or removal of modules to adjust power capability.

[00024] A fifty emitter source operating on EMI-BF ₄ propellant (l-ethyl-3- methyl-imidazolium tetrafluoroborate) has demonstrated an efficiency in excess of 80% at a specific impulse of 3,500 seconds. The high efficiency of an electrospray thruster results from the low ion creation energy (6-7 eV) and narrow ion beam energy distribution (full width half maximum of 7 eV) which minimize grid impingement and cosine losses. The propellant mass in Table 1 of the Example that follows shows Mission 3 based on an efficiency of about 85% at 1,900 second I _sp .

[00025] For many missions of interest, a propulsion device operating efficiently in the range of 700 to 1,000 sec is desired because it can provide high thrust to power at relatively high specific impulse. It has been demonstrated that with a single emitter electrospray source that the ion beam velocity can be decelerated to specific impulses as low as 550 seconds while maintaining a confined beam (approximately 10% of ions lost due to impingement). Through design of grids and careful alignment, similar results can be achieved with multi-emitter sources. The propellant mass shown in Table 1 for Mission 4 is based on achieving an efficiency of 75% at 1,000 second I _sp .

[00026] There are benefits to bipolar operation, that is reversing the polarity of lines 54, 56 (FIG. 2) according to a desired periodicity. Ionic liquids are formed by positive and negative ions with no solvent involved. Therefore, it is possible to electrostatically extract ions in either polarity resulting in ion emission of the corresponding charge, something that is very difficult with any other type of ion source. Emission of positive and negative ions in parallel eliminates the accumulation of net charge on the spacecraft hence there is no need for an electron emitting neutralizer.

Alternating polarity also eliminates the electrochemical reactions that have limited emitter lifetime in electrospray thrusters to a few hours. An electrospray source with EMI-BF ₄ has been operated for an excess of 200 hours with no degradation. Grid erosion is expected to be minimal as well due to the lack of charge exchange ions which contribute to the majority of erosion in gridded ion engines and minimal grid impingement. As the potentially life limiting mechanisms have been addressed, the life capability of electrospray thrusters should exceed thousands of hours.

[00027] For the electrospray thruster 10, to achieve the outlined benefits, the ionic liquid propellant should have certain desired properties:

[00028] FIG. 4 illustrates the hardware for the multi-mode propulsion system 62 described herein. The multi-mode propulsion system includes a propellant storage tank, a propellant feed system for delivering propellant from the tank to the thrusters, two primary propulsion chemical thrusters, one electrospray thruster, and eight chemical thrusters for attitude control.

[00029] Valves controlling the flow of ionic liquid monopropellant 14 include latch valves 64 and solenoid valves 68. Filter 74 removes extraneous material from the ionic liquid monopropellant 14. A heater 76 insures that the ionic liquid monopropellant 14 stays in a liquid state by communicating with temperature sensor 78. Pressure transducers 80 monitor the flow of ionic liquid.

[00030] Prior to operation, service valves 66 are used to charge the system with ionic liquid monopropellant 14 and pressurized gas 16. Latch valve 64 located in a tank isolation module 82 are used to prevent propellant leaks of the system during launch and non-operational periods.

[00031] Solenoid valves 68 direct the ionic liquid monopropellant 14 to desired thrusters 10, 34, the chemical thrusters 10 providing a high thrust option for time sensitive maneuvers and the electrospray thruster 34 providing a high efficiency option for maximum maneuvering capability. Because both types of thrusters 10, 34 operate off a single propellant, either type of thruster 10, 34 can be used depending on the needs of the mission.

[00032] FIG. 5 overlays the electrical systems of the multimode propulsion system 62. The bulk of the power 84 is spacecraft power, typically generated from a solar array. A battery 86, such as a lithium ion battery is provided as a backup or for maneuvers during an eclipse. Spacecraft telemetry/command interface 88 sends signals to a control unit module 90 that controls solenoid valves 68 so that the proper valves are opened. Power line 56 provides power to the extractor grid and power line 58 provides power to the decelerator grid to control electrospray thruster 34 output. Activation of solenoid valves 68 controls the chemical thrusters 10.

[00033] The multimode propulsion system described herein above will be better understood by the Example that follows.

EXAMPLE

[00034] Table 1 compares the performance of the herein described multi-mode propulsion system to the baseline current state of the art. The baseline values are actual, while the multi-mode system values are prophetic, based on Applicants' calculations.

[00035] As Table 1 illustrates the herein described propulsion system will provide a 40% reduction in propulsion system mass over the current state of the art technology. The Table also illustrates that for some sample of missions, the herein described propulsion system will have a substantial mass saving. The electrospray thruster enables this type of mass savings and maximum flexibility because of its ability to operate efficiently over a range of specific impulse from 700-2,000 seconds. Its module nature enables easy scaling for higher or lower power operation. The use of an ionic liquid as the propellant also provides significant advantages. Ionic liquids are non-toxic and require no special handling or storage.

TABLE 1

[00036] One or more embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, no attitude control thrusters are included in the system, the electrospray thruster is a single module as opposed to an array of modules or chemical thruster chamber body and/or catalyst bed are monolithic. Accordingly, other embodiments are within the scope of the following claims.