~™ mi mi ii inn in iiimii ii hi (19) J European Patent Office

Office europeen des brevets (11) EP 0 91 9 464 A1

(12) EUROPEAN PATENT APPLICATION

(43) Date of publication: (51) int. CI.6: B64G 1/26, F02K 9/50, 02.06.1999 Bulletin 1999/22 F02K 9/58 (21) Application number: 98120184.1

(22) Date of filing: 29.10.1998

(84) Designated Contracting States: • Bassichis, James S. AT BE CH CY DE DK ES Fl FR GB GR IE IT LI LU Playa del Rey, CA 90293 (US) MC NL PT SE • Hook, Dale L. Designated Extension States: Rancho Palos Verdes, CA 90275 (US) AL LT LV MK RO SI (74) Representative: (30) Priority: 25.11.1997 US 977759 Schmidt, Steffen J., Dipl.-lng. Wuesthoff & Wuesthoff , (71) Applicant: TRW Inc. Patent- und Rechtsanwalte, Redondo Beach, California 90278 (US) Schweigerstrasse 2 81541 Munchen (DE) (72) Inventors: • Sackheim, Robert L. Rancho Palos Verdes, CA 90275 (US)

(54) Spacecraft attitude and velocity control thruster system

(57) A rocket propulsion system for spacecraft which may also be of the SCAT thruster construction, achieves greater economy, reliability and efficiency that uses the same propellent fuel. Hydrazine and Bini- rocket by incorporating monopropellant RCS thrusters trogen tetroxide are preferred as the fuel and oxidizer, (1a-1f) for attitude control and bipropellant SCAT thrust- respectively. The new system offers a simple conversion ers (5a-5c) for velocity control. Both sets of thrusters are of existing monopropellant systems to a high perform- designed to use the same liquid fuel, supplied by a pres- ance bipropellant dual mode system without the surized non-pressure regulated tank, and operate in the extreme complexity and cost attendant to a binitrogen blow down mode. In the propulsion system such station tetroxide - hydrazine bipropellant system. keeping and attitude control thrusters may function in conjunction with a large thrust apogee kick engine,

LATCMNG BMA1BN VALVE

DUAL SEAT THRUSTER VALVE

FR0U TO P VALVES

CO * If MRE-1 TRUSTERS ATTITUDE CONTROL FIG. I CT> O Q_ LU Printed by Xerox (UK) Business Services 2.16.7/3.6 1 EP 0 919 464 A1 2

Description impulse, lsp which is defined as the thrust developed by the engine per unit of propellant weight flow rate. If the FIELD OF THE INVENTION thrust is measured in pounds and the flow rate in pounds per second, the units for the measurement of [0001 ] This invention relates generally to rocket pro- s specific impulse are seconds. The specific impulse is pulsion systems and, more particularly, to rocket propul- somewhat analogous to a miles-per-gallon figure for an sion systems for placing and maintaining spacecraft in automobile, since it measures how much thrust is devel- planetary orbits. Although the invention has broad appli- oped for a unit fuel flow rate. cation to both manned and unmanned spacecraft, it is [0006] Another measure of performance is, of course, particularly concerned with the launch, insertion and to the thrust force generated by the engine. For the rapid maintenance of satellites in geosynchronous orbits. acceleration that is required in a transition to geosyn- chronous orbit, particularly at the apogee "kick" phase BACKGROUND of a mission, an engine with a relatively large thrust is required, perhaps generating up to several thousand [0002] Placing a geosynchronous satellite into orbit 15 pounds of thrust force. The specific impulse, lsp, of such typically involves three principal mission phases. First a high thrust engine is also important, and should be in the satellite is placed in low earth orbit not far above the the 300 to 400 second range. earth's atmosphere, either as part of the payload of a [0007] For station keeping and attitude control, high space shuttle vehicle or on a conventional non-reusable thrust is far less important, since most station-keeping rocket vehicle. In the second phase, the satellite orbit 20 and attitude control maneuvers can be efficiently has its apogee or highest point raised in altitude by one accomplished with low-thrust burns of the rocket or more rocket "burns" at a selected point in the orbit, engines. However, fuel economy is very important for until the apogee is approximately at geosynchronous rocket engines used in these activities. Hence, the altitude. Finally, the satellite is given an apogee "kick," higher the lSp the better. Present monopropellent rock- i.e. a further rocket burn at apogee that circularizes the 25 ets for these functions achieve an lSP on the order of orbit at geosynchronous altitude. about 225 to 235 seconds. [0003] Once in orbit, rocket engines are called upon [0008] Because of the different requirements, earlier for three additional functions: station keeping, station propulsion systems involved using multiple fuels and changes and attitude control, which are sometimes engine systems for the apogee kick and the velocity and referred to collectively as reaction control system 30 attitude control. For example, a solid rocket was used for (RCS)functions. Satellites are usually required to main- the apogee kick engine and hydrazine catalytic engines tain a particular "station" with respect to the earth's sur- were used for the station keeping\changing velocity and face. Moreover, to satisfy requirements of a particular attitude control system thrusters. There is nothing inher- mission, satellites are sometimes required to change ently incorrect with that traditional approach, except that from one station to a different station. Changing the sat- 35 the use of two separate propulsion systems weighs ellite to another station obviously requires expenditure more, thereby severely limit the size of the useful pay- of energy. Maintaining a given station also requires the load that can be placed and maintained in orbit, and it expenditure of energy, even though the orbit is theoreti- costs more. cally self-sustaining and geosynchronous. Various fac- [0009] Some improvement can be obtained using an tors that create drag and reduce or change the satellites 40 integrated bipropellant system, in which both the apo- velocity, such as the non-spherical nature of the earth, gee kick engine and the RCS thrusters each use a the gravitational influences of the moon and sun, and so bipropellant fuel system, such as monomethyl hydra- forth, require that the orbit be corrected from time to zine (MMH) as the fuel and binitrogen tetroxide time if the required station is to be maintained. To make (N204)as the oxidizer. Even with that, there is still room either station keeping corrections or station changes, 45 for further improvement in the payload that can be the station keeping/changing rocket engines provide a placed in orbit for a given mission. Another way to look "burn" sufficient to slightly change the satellite's velocity. at the matter is that there is room for improvement in the Attitude control is simply the use of multiple rocket lifetime that a given spacecraft payload could be main- engines on the spacecraft to maintain a particular angu- tained in orbit. With a more efficient propulsion system, lar attitude or "pointing" of the vehicle. This may be so a greater payload may be maintained in orbit for a given needed, for example, to point an antenna or other sen- time, or the same payload may be maintained in orbit for sor at the earth, the sun, or a star. a longer time. [0004] Unfortunately, the rocket engine performance [0010] To that end, additional propulsion systems characteristics required for the various functions of were proposed to increase payload efficiency. In U.S. orbital transfer, station keeping/changing and attitude 55 5,282,357 granted Feb. 1, 1994 to one of the present control are not identical. inventors and owned by the same assignee, a space- [0005] A figure of merit often used in the comparison craft rocket propulsion system is disclosed which uses of the efficiency of rocket engines is the specific the same fuel for both a bipropellant rocket engine

2 3 EP0EP 0 919 46464 A1 4 capable of producing high thrust to provide the apogee alternative vaporization procedure for the oxidizer obvi- kick, and one or more monopropellent rocket engines ously somewhat enhances the efficiency of the propul- that deliver low thrust, such as the MRE-1 thrusters, for sion process used in the SCAT thruster, an advantage the station keeping and attitude control functions. By to that engine. employing a common fuel for both the bipropellant and 5 [0016] The cooling effect inherent in the SCAT monopropellant rocket engines, the spacecraft is thruster's bipropellant mode of operation raises an addi- required to stow only one fuel, the Hydrazine, as exam- tional factor of importance for some spacecraft mis- ple, and that reduces weight in comparison to prior sys- sions: durability. Heating to high temperatures, tems requiring different fuels and storage vessels, particularly to temperatures close to the engine metal's thereby improving propulsion efficiency. w breakdown or melting temperature is corrosive of met- [0011] Further, in additional patents U.S. 5,417,049 als and, if possible, is best avoided. Rockets for velocity granted May 23, 1995 and U.S. 5,572,865, granted Nov. and attitude control application are used repeatedly 12, 1996, related to the foregoing '357 patent, and over a mission that may last ten years or more and, issued to one or more of the present inventors, among therefore, engine durability is important. other things, a new propulsion system is proposed and 15 [001 7] Cooling the small monopropellant engines typ- a new bi-propellant thruster construction is described ically used for reaction control functions is difficult due that has dual mode capability. That thruster construction to the engine's small thermal radiating surfaces. Any is presently referred to as a Secondary Combustion prolonged use may raise the temperature enough to Augmented Thrusters or, simply, as a "SCAT" thruster. damage the thrust chamber. Thrust chamber durability [001 2] A propulsion system is also there presented 20 can be improved by using more exotic materials, such that employs a bipropellant engine to provide the high as Columbium, which generally withstands the four to thrust apogee kick and the bipropellant SCAT thruster to five thousand degree Fahrenheit temperatures gener- provide station keeping and attitude control functions. In ated in the combustion chamber. That alternative, how- keeping with the description of '537 patent, both thrust- ever, significantly increases construction costs, a ers in that propulsion system use a common fuel, 25 decided disadvantage. Hydrazine. The bipropellant thruster uses the liquid oxi- [0018] Cooling the thrust chamber reduces thermal dizer, N204, as the second propellant. stress on the metal that forms the chamber walls. The [001 3] The SCAT thruster uses the same oxidizer to lower operating temperature lessens the need for use of both cool the thruster chamber, whereby the oxidizer is exotic materials and coatings. For a given metal, the transformed to a gaseous phase, and, in the gaseous 30 metal is more durable at lower temperature and, hence, phase, as the second propellant for the bipropellant the engine will last longer. The inherent cooling that mode of operation. In its construction, the SCAT occurs in the normal operation of the SCAT engine sug- thruster contains two connected reaction chambers. gests a longer operational life in comparison to other Liquid propellant fuel, such as hydrazine, is fed into the engines, all other factors being equal, an additional first chamber where it reacts with a catalyst and enters 35 advantage to the engine. the gaseous phase in an exothermic reaction, thereby [0019] The SCAT thruster is noted as having dual heating the chamber walls, and the reaction propellant mode capability. It is shown to operate in a bipropellant gas is propelled by the reaction into a second chamber. mode and, alternately, in a monopropellent mode. It is Liquid propellent oxidizer is fed through a heat now realized that a simple SCAT bi-modal thruster can exchanger surrounding the thruster and thereby cools 40 operate in either a bipropellant mode and achieve an lSP the unit, and the associated energy absorbed by the oxi- of over 315 seconds, or, alternatively, can operate in a dizer transforms the liquid oxidizer to the gaseous state. monopropellant mode and achieve an lSP of about 225 The gaseous oxidizer is then routed into the second seconds. chamber and mixes with the gaseous propellant fuel [0020] The SCAT thruster is described in the two cited entering from the first chamber and reacts with the pro- 45 patents as useful for station keeping/changing and atti- pellant to create thrust. tude control functions of the propulsion system, and the [0014] Using the two propellants, the SCAT thruster patents thereby propose the SCAT thruster as a substi- produces an lSP of about 315 to 325 seconds and a tute for the monopropellant RCS thrusters in the single thrust AV that is significantly greater than that available fuel rocket propulsion system earlier described in the from monopropellent RCS thruster. For additional so '357 patent, in which those thrusters serve as a com- details of construction of the SCAT thrusters, the reader panion to the high thrust apogee kick rocket engine. For may make reference to the afore cited patents. Such one, the substitution reduces propellant weight and SCAT thrusters are commercially available from TRW thereby increases the payload size carried on a mission. Inc., Redondo Beach, California, assignee of the [0021] Despite the apparent advantage of the dual present invention. ss mode SCAT engine, Monopropellant rockets, such as [001 5] Using the thermal energy to perform the work the wide range of catalytic thrusters available today in of vaporizing the liquid oxidizer instead of the alternative the industry, despite their lower performance, remain of radiating that thermal energy into space and using an the engine of choice for station keeping and attitude

3 5 EP 0 919 464 A1 6 control functions in satellite systems. The designs of low performance blowdown monopropellant hydrazine those monopropellant rockets are space proven; its per- reaction control system in the velocity control function formance is predictable, it has been reliable; and it with a higher performance bipropellant RCS while con- offers none of the uncertainty of a new product for the tinuing with a monopropellant hydrazine reaction control satellite designer. Those advantages apparently out- 5 system for the attitude control function. weighed the superior performance offered by the SCAT [0028] Significant performance increase is obtained thrusters. The requirements for many space flight mis- with almost no change in RCS operating modes and sions offered neither motive or incentive to change to with the same basic operating conditions, ie. Blowdown another rocket engine for station keeping and attitude pressurization, wide mixture ratio excursions, passive control application and the lack of industry acceptance 10 system cooling, and with no unusual or extra fluid com- suggests the proposed substitution as overly ambitious ponents, such as regulators and check valves. and\or illusory advantage. [0029] Further, since velocity control and attitude con- [0022] Notwithstanding such discouragement, an trol need not be operated simultaneously, as an added important aspect of the present invention is a propulsion feature, the bipropellant SCAT engines are operated in system that relies on the SCAT engine as the important is a monopropellant mode to backup the monopropellant component, providing weight savings with no significant rockets. The mode of the bipropellant SCAT engine is compromise in reliability and hardware compatibility, switched, as and if needed, to monopropellant mode to and offering an advantage of two modes of operation, join with the monopropellant engines to effect the atti- but one which does not eliminate RCS monopropellent tude adjustment or, if the monopropellant engines are rockets. The present invention retains the benefit and 20 found inoperative, as a backup to alone perform the atti- advantage of the monopropellent thrusters and adds tude control adjustment. When needed for velocity con- the advantage of SCAT thrusters in satellite propulsion trol, the control system switches the bipropellant systems. engines to their bipropellant mode and achieve the [0023] Accordingly, an object of the present invention higher thrust. is to provide a novel, highly reliable and more efficient 25 [0030] The maximum achievable performance for a propulsion system suitable for geosynchronous and satellite catalytic velocity control thruster, the monopro- other high-energy mission spacecraft programs. pellant thruster, is characterized by a deliverable maxi- [0024] And an additional object of the invention is to mum specific impulse, lsp, of about 225 seconds. Direct provide a simple means to obtain high performance by substitution of a bimodal bipropellant thruster, referred converting a monopropellant system to a dual mode 30 to as SCAT, increases the velocity control maximum monopropellant and bipropellant system that avoids the specific impulse to about 325 seconds. As a conse- extreme complexity and cost increase attendant to bini- quence of that alone, weight savings of velocity control trogen tetroxide - hydrazine type bipropellant systems. propellant, such as Hydrazine, of almost 45% is achieved. SUMMARY OF THE INVENTION 35 [0031] The foregoing and additional objects and advantages of the invention together with the structure [0025] The present invention divides the attitude con- characteristic thereof, which was only briefly summa- trol functions from the velocity control functions. The rized in the foregoing passages, becomes more appar- attitude control functions are served by the monopropel- ent to those skilled in the art upon reading the detailed lant rockets as in the prior propulsion system. Velocity 40 description of the preferred embodiments, which follows control is served exclusively by the bipropellant SCAT in this specification, taken together with the illustration engines operated in bipropellant mode. thereof presented in the accompanying drawings. [0026] Briefly, and in general terms, the propulsion system of the invention comprises a liquid fuel bipropel- BRIEF DESCRIPTION OF THE DRAWINGS lant high-thrust apogee kick engine or other high-thrust 45 engine, a plurality of SCAT thrusters for station keeping [0032] In the drawings: control, that is, velocity control, a liquid fuel tank and an oxidizer tank connected to the high-thrust engine and Figure 1 schematically illustrates an embodiment of SCAT thrusters, and a like plurality of monopropellant the invention; reaction control thrusters for attitude control. The mono- so Figure 2 schematically illustrates an alternative propellant thrusters are connected to the same liquid embodiment; fuel tank and use the same fuel as the high-thrust apo- Figure 3 schematically illustrates a modification to gee kick engine and the SCAT thrusters to provide a the embodiment of Fig. 1 used to construct a still substantial saving in propellant weight and propulsion additional embodiment of the invention; and system inert weight for given mission requirements. 55 Figures 4a, 4b, 4c, and 4d are comparison charts [0027] The present invention provides a simple, mini- illustrating examples of payloads for various propul- mum cost, minimum complexity, high reliability sion systems and the present invention. approach to substitute for and replace existing simple

4 7 EP 0 919 464 A1 8

DETAILED DESCRIPTION OF THE PREFERRED [0038] The output end of latching control valve 39 is EMBODIMENTS connected via a conduit in parallel to the oxidizer dual seat thruster valves 5a1 , 5b1 and 5c1 for SCAT thrust- [0033] Reference is made to the embodiment pre- ers 5a, 5b and 5c, respectively; and the output of latch- sented in the schematic of Fig. 1 illustrating a propulsion s ing control valve 41 is similarly connected via a conduit system incorporating the invention. The propulsion sys- in parallel to the oxidizer dual seat thruster valves 7a1 , tem contains six (or more) monopropellant thrusters, 1 a 7b1 and 7c1 for SCAT thrusters 7a, 7b and 7c, respec- through 1f, six additional monopropellant thrusters 3a tively. through 3f, which, as becomes apparent, are provided [0039] The outlet of control valve 27 connects via fuel as a backup to the first set of monopropellant thrusters, 10 conduit to the fuel dual seat thruster valves 5a2, 5b2 three bipropellant SCAT thrusters 5a, 5b and 5c, an and 5c2, associated respectively with SCAT'S 5a, 5b, additional three bipropellant SCAT thrusters 7a, 7b and and 5c; and, likewise the outlet of control valve 29 con- 7c, also included preferably as a backup to the first set nects via fuel conduit to the fuel dual seat thruster of bipropellant thrusters, and a high thrust apogee kick valves 7a2, 7b2 and 7c2, associated respectively with engine 2. Propellent fuel tanks 9 and 1 1 are provided to 15 SCAT'S 7a, 7b, and 7c. store the liquid fuel, suitably Hydrazine, N2H4, for both [0040] The foregoing constitutes the essentials of the sets of thrusters, and a propellent oxidizer tank 13 is station keeping velocity control thruster system and atti- included to store the liquid oxidizer, suitably bi- tude control thruster system of a satellite. It is appreci- nitrotetroxide, N204. The fuel and oxidizer tanks are ated that the satellite also customarily includes another non-pressurized, since all thrusters described operate 20 bipropellant thruster 2, referred to as the "apogee kick" in the simple "blowdown" mode. engine, referred to in earlier in the background to this [0034] A fuel conduit 15, represented by a line, leads invention, which, although not necessary to an under- from the outlet end of fuel tank 9 to the series connec- standing of the present invention, is illustrated for com- tion of a fuel filter 17, an electrically operated latching pleteness. Tanks 9, 1 1 and 1 3 also respectively supply isolation valve 19, one arm of a "T" connection 21 , and 25 the fuel and oxidizer to the bipropellant apogee kick branches to each of latching isolation valves 23 and 25. engine 2, normally included on she satellite. A conduit Fuel inlet valve 4 and fuel drain and/or test valve 6 are from the fuel tanks is spliced into the conduit to T-con- connected, respectively, to the inlet and outlet ends of nection 21 and extends to latching isolation valve 36 tank 9, and a pressure transducer 8 is connected to the and therefrom to dual seat thruster valve 2a2 for feeding outlet side of the tank. 30 fuel to engine 2. An oxidizer conduit is also spliced into [0035] Similar plumbing and control valves are pro- conduit 31 and supplies oxidizer to latching isolation vided for fuel tank 1 1 , which is plumbed in parallel to valve 34. The output of that valve is coupled to dual seat fuel tank 9 and serves as an alternative or additional thruster valve 2a2 through which the oxidizer is fed to fuel source. The outlet of tank 1 1 is connected by con- engine 2. duit 1 6 through a fuel filter 1 8, latching isolation valve 20 35 [0041 ] Controller 32 is the satellite propulsion control- to an arm of T-connection 21 . A fuel inlet valve 1 0 is con- ler. It is a known programmed microcontroller or compu- nected to the inlet of tank 1 1 , and a drain valve 12 and ter device which provides programmed outputs when it pressure transducer 1 4 are connected to the outlet end receives predetermined inputs from various sensors, of that tank. including the pressure transducers forming part of the [0036] A pressure transducer 22 is connected to mon- 40 propulsion system. The satellite controller performs itor the pressure at T-connection 21 and the outlet side many of the housekeeping functions and propulsion of valves 19 and 20. From T-connection 21 , the fuel con- system command and control operations on board the duit branches in parallel to each of valves 23 and 25, satellite. It checks the pressure transducers. It receives associated, respectively, with monopropellant thrusters the inputs from other sensors and apparatus in the sat- 1 a through 1 f , and monopropellant thrusters 3a through 45 ellite and/or from ground control that call for propulsion 3f. The fuel conduit also extends to valves 27 and 29 system action, such as a change in velocity and/or a associated, respectively, with SCATs 5a-5c and 7a-7c. change in attitude, and commands those actions. It [0037] A conduit 31 connects the outlet of liquid oxi- interprets the received information, makes appropriate dizer tank 13 through a filter 33 and latching control selections and issues commands to external apparatus valve 35 to the input of a T-connection 37. From one arm so based on the interpretations made. of that T-connection the conduit extends through [0042] When the action sought is to change the atti- another latching control valve 39, and from the other tude of the satellite to a new attitude, controller 32 initi- arm of that T-connection to still another latching control ates operation of the valves associated with the valve 41 . A fill valve 24 connects to the tank's inlet, a monopropellant thrusters to operate one or more of the drain/test valve 26, connects to the tank's outlet. A pres- 55 thrusters for the a pre-programmed interval of time sure transducer 28 connects to the tank outlet and determined by the controller's program. And if thereafter another pressure transducer 30 connects at the outlet the controller determines that the final orientation is not side of control valve 35. quite correct, it determines the additional amounts of

5 9 EP 0 919 464 A1 10 thrust necessary to make the corrections and then initi- compares the temperature and pressure indications ates operation of the appropriate thrusters for another monitored in the present firing with that stored criteria. preprogrammed time interval, repeating the procedure Should any or all of SCAT thrusters 5a-5c be inopera- until the new attitude is properly attained. This entire tive, controller 32, having determined an out of specifi- maneuver sequence can also be implemented open 5 cation firing, instead operates valves 29 and 41 and loop through a human ground controller. The ground valve pairs 7a1 and 7a2, 7b1 and 7b2, and/or 7c1 and control commands firing until the correct attitude is 7c2 associated with SCAT thrusters 7a, 7b and 7c, sensed by the spacecraft and reported back to the respectively, and the backup SCAT thrusters supply the ground controller. requisite thrust. [0043] Each SCAT thruster typically produces a thrust 10 [0047] For attitude control, controller 32 selects and that is at least fifty per cent greater than thrust typically actuates valves 19 and/or 20 and valve 23, associated produced by a monopropellant RCS thrusters, and the with monopropellant thrusters 1a-1c. With the fuel SCAT thruster satisfies the large thrust requirements received at its intake the monopropellant thrusters pro- needed for changing the satellites velocity. Information duce thrust. The controller monitors the satellites atti- on satellite velocity is likewise obtained and assessed 15 tude error, and that error decreases to zero, when the and an appropriate command is issued by the controller selected attitude is attained, at which time the controller to those velocity thrusters, which are operated as 32 re-closes the valves. Should any one or more of appropriate, until the satellite achieves the prescribed monopropellent MRE thrusters be inoperative, the atti- velocity. The controller also functions to operate valves tude error monitored by the controller increases, instead 34, 36, 2a1 and 2a2 and fire apogee kick engine 2 as 20 of decreasing. In response to the increasing attitude desired in the same made of operation used in the prior error, controller 32 instead selects and actuates valves system, which, therefor, need not be discussed in detail. 1 9 and/or 20 and valve 25. And monopropellent thrust- [0044] As those skilled in the art understand, the ele- ers 3a-3f supplies the requisite number of thruster pulse ments of the foregoing system are all known compo- mode firings. nents. The described valves are all electrically operated 25 [0048] As an advantage, if an RCS thruster and its and are under control of controller 32. Electrical leads, backup thruster fails in service, one of the SCAT thrust- not illustrated, are connected from each valve to an ers can be operated in a monopropellant mode to pro- input of Controller 32. Each of the pressure transducers duce the requisite thrust for the attitude control are also electrically operated and via appropriate elec- operation. As example, assuming attitude control thrust- trical leads, not illustrated in the figures, provide electri- 30 ers 1a and 1d fails and its backup thrusters 3a and 3d cal signals, representing fluid pressure, to respective also fails. In such a situation, controller 32 operates inputs of controller 32. That information is received by valve 27 and valve 5b2 admitting only propellant into the controller and is inspected by the controller's pro- SCAT 5a. Valves 39 and 35 to the oxidizer remain gram to monitor liquid pressure, determine the amount closed. SCAT 5a operates in monopropellant mode, of liquid remaining in each tank, and whether the liquid 35 generating the request low thrust. The thrust level gen- is present in the propulsion stage being monitored. erated in the monopropellant mode is essentially the From that information obtained from pressure transduc- same as that generated by the RCS thrusters. This ers 8 and 14, the controller determines which fuel tank matches the thrust produced by the other monopropel- to open or whether to open both fuel tanks when the lant thrusters which are still active. controller receives the appropriate command to change 40 [0049] In this bipropellant mode the SCAT thrusters attitude and/or velocity, and then supplies the appropri- provide the same essential efficiency and effectiveness ate electrical signals to the valves associated with the as the former station keeping thruster systems that respective fuel tanks. incorporate only monopropellant thrusters. As a further [0045] For velocity control, controller 32 selects and advantage, if for any reason the oxidizer becomes actuates valve 1 9 and/or 20 and valve 27 to release fuel 45 depleted prematurely, either by consumption or through into the SCAT'S 5a - 5c; and selects and actuates valves valve failure, it is appreciated that the SCAT thrusters 35 and 39 to release oxidizer to those same SCAT'S. will continue to operate in monopropellant mode to pro- The controller also operates thruster valves 5a1 , 5a2, vide appropriate lower levels of thrust for either or both 5b1 , 5b2, 5c1 and 5c2 to release fuel and oxidizer into attitude and velocity control functions. the SCAT engines. With that fuel and oxidizer received so [0050] The foregoing rocket propulsion system oper- at the SCAT engine's intakes, the SCATs operate in ates in the simple blow-down mode. It does not require bipropellant mode to produce thrust, a greater level of the use of a high pressure tank and pressure regulator. thrust than produced by the monopropellant thrusters. The SCAT engine performs irrespective of the combina- When the satellite achieves the desired velocity, the tion of fuel and oxidizer pressures that may result during control circuit 32 recloses those valves. ss operation of the system. However, as the inlet pressures [0046] Controller 32 retains in memory the tempera- vary from the optimum, the engine's lSP decreases. ture and pressure indications characteristic of a satis- [0051] From the foregoing description, modifications factory firing, and, during operation the controller of the foregoing system and in the manner in which the

6 11 EP 0 919 464 A1 12 system may be used become apparent to those skilled controller selects the alternative oxidizer path through in the art. As example, for a bare bones cost reduced the orificed valve. The foregoing evaluation and selec- design achieving greater weight savings, one might tion is individually made and applied to each of the elect to eliminate the separate apogee kick engine from SCAT engines. the satellite and employ the SCATs to provide the apo- 5 [0056] In normal operation of this alternative embodi- gee kick, in addition to their velocity control function. ment, the SCAT'S are modulated by the controller to pro- [0052] A simple technique to improve operating condi- duce thrust in short bursts. Essentially, the thruster is tions, regardless of how the propulsion system has turned on and off for momentary periods. That is, con- been operated previously, that is, irrespective of troller 32 will operate and close a fuel and oxidizer valve whether the propellant pressures are the same or one 10 for a predetermined period, during which the rocket pro- pressure is higher than the other due to propellant duces thrust, and then re-closes those valves, terminat- usage during the course of the mission, is illustrated ing the rocket's thrust. A period of time later, the schematically in the embodiment of Fig. 2 to which ref- procedure is repeated. Before each thruster is fired, the erence is made. For convenience the elements earlier controller conducts an evaluation of the fuel and oxi- presented in Fig. 1 , which are incorporated within this is dizer pressures and determines whether an adjustment embodiment, are, for convenience, denominated with is necessary. It then operates the proper isolation valves the same numeric designation earlier used for that ele- in each leg to provide optimum pressures and, hence, ment and the description of those elements is not optimum performance. repeated. As inspection of Fig. 2 makes apparent for the [0057] Again using SCAT 5a as an example, if the fuel most part the propulsion system is the same as before 20 pressure and oxidizer pressure are both too high, as and operates in essentially the same way. determined by the controller, the controller initiates [0053] A difference is that each of the fuel and oxidizer operation of valves 43 and 47, which are orificed, leav- thruster inlets to the SCAT'S is equipped with a second ing the associated parallel valves 27 and 29 closed, isolation valve and a series connected orifice that is thereby providing a lower pressure fluid stream to the connected in shunt of the existing valve. Specifically 25 SCAT via respective thruster valves 5a1 and 5a2. series connected orifice and latching isolation valve 43 When, during the course of the modulation operation, are spliced in the fuel conduit circuit in shunt of valve 27; the controller determines that the fuel and oxidizer pres- series connected orifice 48 and latching isolation valve sures has sufficiently reduced to a predetermined level, 47 are spliced in the oxidizer conduit circuit in shunt of it then discontinues operation of the orificed valves, 43 valve 39; series connected orifice 46 and latching isola- 30 and 47, and instead operates the valves in parallel with tion valve 45 are spliced in the fuel conduit circuit in each, 27 and 39, respectively. If the controller instead shunt of valve 29; and series connected orifice 50 and determines that only the fuel pressure is too high, then latching isolation valve 49 are spliced into the oxidizer it selects and initiates operation of valve 43 to supply the conduit circuit in shunt of valve 41 . The orifices add a fuel to the thruster at lower pressure, leaving valve 27 line pressure drop to the associated fluid conduit. 35 closed, and selects valve 39 to supply the oxidizer to [0054] In this embodiment, the operating conditions that thruster, leaving the orificed valve 47 unoperated. are improved regardless of how the propulsion system Although the foregoing example is given for SCAT 5a, has been operated previously. That is, irrespective of the same action occurs for each of the remaining SCAT whether the fuel and oxidizer pressures are the same or thrusters 5b and 5c, or, alternatively, 7a through 7c, if one pressure is higher than the other due to propellant 40 should the backup thrusters be in use, and their associ- usage throughout the mission. Each thruster inlet leg is ated supply valves. equipped with parallel isolation valves. One valve in [0058] As one appreciates some spacecraft propul- each leg is orificed to add a line pressure drop. Before sion systems may mount two SCAT thrusters straddling each thruster is fired, the tank pressures are evaluated the spacecraft center of gravity and requires the firing of by the controller 32, and the proper set of isolation 45 both thrusters simultaneously to achieve the change in valves is opened in each leg to provide optimum pres- velocity. To correct for any slight mismatch or imbalance sures and, hence, optimum performance. in the thrusters performance or alignment or center of [0055] Taking SCAT 5a as an example, if the fuel pres- gravity offsets or any combination of the foregoing as sure sensed by transducer 22 is below a prescribed might result in unacceptable spacecraft attitude distur- maximum, controller 32 operates valve 27 to supply the so bance levels, ie. rotation, thrust modulation is required. fuel to the SCAT. Valve 43 remains closed. However, if To enable modulation, a flow by-pass can be incorpo- the pressure is found to be too high, the controller rated in each SCAT valve in either the embodiment of instead selects and operates valve 43, which contains Fig. 1 or the embodiment of Fig. 3. Such addition is illus- the series connected orifice. The orifice drops the pres- trated in the partial schematic of Fig. 3 to which refer- sure in the fuel line to the SCAT and the fuel is supplied ss ence is made. to the SCAT at the lower pressure through that valve, [0059] In the embodiment of Figs. 1 and 2, the dual while valve 27 remains closed. A like evaluation is inde- seat thruster valve, as example, valve 5a1 , is actually pendently made of the oxidizer pressure. If too high, the formed of two electrically operated valves connected in

7 13 EP 0 919 464 A1 14 series fluid circuit and those two valves are essentially design. Up to thirty per cent thrust modulation is electrically slaved together in operation. That is, control- achieved by choking off fuel and oxidizer flows to the ler 32 provides the "on" or "off" control signals to both SCAT. valves simultaneously to simultaneously open or close [0064] It will be appreciated from the foregoing that the valves, respectively permitting fluid flow or not. In 5 the invention represents a significant advance in the the embodiment of Fig. 3, however, the two valves, field of rocket propulsion systems. The invention does although remaining connected in series fluid circuit, are not require the use of a high pressure tank and pressure operated independently of one another. regulator. The system operates in a simple blowdown [0060] A flow bypass is obtained for the fuel with two mode. The SCAT engines run at whatever combination thruster control valves 53 and 55 are connected in fluid 10 of fuel and oxidizer pressures dictated by the control series and a flow limiting bypass orifice 54 is connected system. As the inlet pressures vary from the optimum in bypass of valve 53 and another flow bypass is design point, the lsp gets lower. obtained for the oxidizer with two thruster control valves [0065] Additional modifications become apparent to 57 and 59 connected in fluid series and a bypass orifice those skilled in the art. As example, the oxidizer used in 56, connected in bypass of valve 57. Each of the valves is the preferred propulsion system is nitrogen tetroxide in controlled by controller 32 by appropriate electrical (N204). However in other embodiments liquid oxygen, signals over electrical wiring, not illustrated, to those nitrogen trifluoride (NF3), nitrogen tetrafluoride (N2F4) valves. To assist understanding of the embodiment, the or combinations of these substances may be substi- foregoing illustrates the combination for only one of the tuted. SCAT engines 5a1 , but it is understood that a like com- 20 [0066] Reference is made to Figs. 4a, 4b, 4c and 4d bination is used for each of the remaining SCAT engines for a comparison of four types of propulsion systems, as well. including that of the present invention, for a typical mis- [0061] The combination of valve with an orifice in sion. The total cargo weight placed into a transfer orbit shunt is essentially a variable flow restrictor having a between a low earth orbit and a geosynchronous orbit, photographic like variable diameter central opening that 25 assumed to be the same for all three cases, namely contains a minimum diameter opening when the valve is 1 1 ,600 pounds. The mission includes the use of an apo- unenergized and therefore closed, so as to allow pas- gee kick engine to effect a transfer to geosynchronous sage of some fluid, despite the valve being closed, and orbit, and an assumed life of ten years of station keep- expands to a maximum diameter opening when the ing and attitude control activities. valve is energized, allowing maximum fluid to pass. 30 [0067] In a conventional approach, shown on the left [0062] In this embodiment, controller 32 includes a of the figure and indicated at Fig. 4(a), a solid-fuel program that allows the bipropellant SCAT thruster to rocket is used for the transfer to geosynchronous orbit, produce a lower average thrust even while operating in and multiple hydrazine catalytic thrusters are used for a bipropellant mode so as to permit the SCAT thruster to station keeping and attitude control duties. The total serve as an attitude adjusting rocket even in that mode. 35 propellant load is calculated at 6,763 pounds. The In the altitude adjusting mode, the controller applies sig- remaining component of the total weight of the vehicle is nals to each of valves 55 and 59 in the respective fuel the inert propulsion system weight, calculated at 742 and oxidizer serial inlet circuits, opening those valves, pounds. The payload delivered to the orbit, exclusive of and, simultaneously, provides a signal periodically and remaining fuel and inert propulsion component, is 4,095 in synchronism to the other valves 53 and 57 in those 40 pounds. two chains to periodically open and close valves 53 and [0068] In an integrated bipropellant propulsion sys- 57 simultaneously. With valves 53 and 57 open, full tem, indicated at Fig. 4(b), the same fuel is used in all thrust is developed by the associated SCAT thruster phases of the mission, namely monomethyl hydrazine 5a1 . With valves 53 and 57 closed, a more limited flow (MMH) with nitrogen tetroxide (N204) as the oxidizer. of the respective fuel and oxidizer is provided through 45 The propellant requirements are reduced to 6,345 the respective orifices 54 and 56, essentially choking pounds and the inert propulsion system component is the fuel and oxidizer flow, and the associated SCAT reduced to 545 pounds. Therefore, the payload is engine develops a reduced thrust level during the inter- increased by about 15%, to 4,710 pounds. val in which the valves 53 and 57 are closed. The effect [0069] In the propulsion system proposed in the prior is to provide a pulsing type thrust. so patent U.S. 5,572,865, earlier discussed, as indicated at [0063] The foregoing SCAT based propulsion flow Fig. 4(c), pure hydrazine (N2H4) is used as the fuel for bypass approach provides the same thrust modulation both phases of the mission, but is used in a bipropellant capabilities for attitude disturbance compensation as mode for the apogee kick phase, and in a monopropel- the standard practice of thruster "off-pulse" modulation lant mode with electrical or chemical augmentation in used for all standard hydrazine and bipropellant during 55 the station keeping and attitude control phase. The total simultaneous two thruster delta V firings. The foregoing propellant weight is reduced to 6,218 pounds, and the modification enables full thrust modulation with mini- inert component weight to 464 pounds. The payload is mum hardware complexity or change in existing system increased to 4,918 pounds, an increase of about 20%

8 15 EP 0 919 464 A1 16 over the conventional propulsion system of Fig. 4(a). said oxidizer tank means to said oxidizer inlet [0070] In the propulsion system of the present inven- of said bipropellant SCAT thruster means; tion using the high performance SCAT thrusters, as indi- controller means for selectively operating said cated at Fig. 4(d) pure hydrazine is used as the fuel in second and third valve means, wherein said each phase of the mission. The total propellant weight s SCAT thruster means produces a thrust to now reduces to 5,830 pounds, and the inert component change the velocity of said spacecraft, and for weight is 457 pounds. The payload is thereby increased selectively operating said first valve means, to 5,223 pounds. This payload is now about 28% more wherein said monopropellant thruster means than the original base line and an 11% gain over the produces a thrust to change the attitude of said payload to orbit of Fig. 4(c) enabled by the propulsion w spacecraft. system proposed in U.S. 5,572,865. [0071 ] It is appreciated that the foregoing system is a 2. The invention as defined in claim 1 , wherein said very simple and expeditious substitute for existing controller means further includes: means for receiv- monopropellant systems and allows conversion or such ing a command to change spacecraft velocity and monopropellant systems to a high performance bipro- 15 selection means for selecting said second and third pellant dual mode thruster system without the extreme valve means for operation, responsive to receiving cost and complexity of a Bi-Nitrogen tetroxide and a command to change spacecraft velocity, and for Hydrazine bipropellant system. alternatively selecting said first valve means for [0072] It is believed that the foregoing description of operation, responsive to receiving a command to the preferred embodiments of the invention is sufficient 20 change spacecraft attitude. in detail to enable one skilled in the art to make and use the invention. However, it is expressly understood that 3. The invention as defined in claim 2, wherein said the detail of the elements presented for the foregoing controller means further includes: purpose is not intended to limit the scope of the inven- tion, in as much as equivalents to those elements and 25 means for determining inoperability of said first other modifications thereof, all of which come within the valve means and for selecting said second scope of the invention, will become apparent to those valve means for operation responsive to receiv- skilled in the art upon reading this specification. Thus ing a command to change spacecraft attitude the invention is to be broadly construed within the full when said first valve means is inoperative, scope of the appended claims. 30 wherein said bipropellant SCAT thruster means operates in monopropellant mode to produce Claims thrust to change spacecraft attitude.

1 . An attitude and velocity control propulsion system 4. The invention as defined in claim 1 , further compris- for a spacecraft comprising: 35 ing:

monopropellent thruster means, said monopro- orifice means and fourth valve means, said ori- pellant thruster means, including a fuel inlet, for fice means and fourth valve means being con- producing thrust in response to fuel supplied at nected in series across said second valve said fuel inlet; 40 means for producing a fuel pressure drop and bipropellant SCAT thruster means, said bipro- providing an alternative fuel route from said pellant SCAT thruster means including a fuel fuel tank means to said bipropellant SCAT inlet and an oxidizer inlet for producing a thrust thruster means; responsive to both fuel supplied at said inlet second orifice means and fifth valve means, and oxidizer supplied simultaneously at said 45 said second orifice means and fifth valve oxidizer inlet and for producing a lesser thrust means being connected in series across said responsive to fuel supplied at said fuel inlet; third valve means for producing an oxidizer fuel tank means for storing propellant fuel in liq- pressure drop and providing an alternative oxi- uid form; dizer route from said oxidizer tank means to oxidizer tank means for storing propellant oxi- so said bipropellant SCAT thruster means; dizer in liquid form; first pressure transducer means for monitoring first valve means for coupling said fuel from fuel pressure of said fuel tank means; said fuel tank means to said monopropellent second pressure transducer means for moni- thruster means; toring oxidizer pressure of said oxidizer tank second valve means for coupling fuel from said ss means; fuel tank means to said bipropellant SCAT and wherein said controller means includes: thruster means; third valve means for coupling oxidizer from means for monitoring each of said pres-

9 17 EP 0 919 464 A1 18

sure transducer means to determine fuel first pressure transducer means for monitoring pressure and oxidizer pressure; fuel pressure of said fuel tank means; means for receiving a command to change second pressure transducer means for moni- spacecraft velocity and generating a veloc- toring oxidizer pressure of said oxidizer tank ity change command; and 5 means; and pressure dependent selection means, wherein said controller means further includes: responsive to a command to change spacecraft velocity, for selectively operat- means for Monitoring each of said pres- ing one of said second and fourth valve sure transducer means to determine fuel means in dependence upon whether the 10 pressure and oxidizer pressure; pressure of said fuel tank means is respec- means for receiving a command to change tively at or below a predetermined fuel spacecraft velocity and generating a veloc- pressure level or, alternatively, is greater ity change command; than said predetermined fuel pressure command receiving means for receiving a level, and for selectively concurrently oper- is command to change spacecraft velocity ating one of said third and fifth valve and for receiving a command to change means in dependence upon whether the spacecraft attitude; pressure of said oxidizer tank is respec- pressure dependent selection means, tively at or below a predetermined oxidizer responsive to a command to change pressure level or, alternatively, is greater 20 spacecraft velocity, for selectively operat- than said predetermined oxidizer pressure ing one of said second and fourth valve level, wherein said SCAT thruster means means in dependence upon whether the produces a thrust to chance the velocity of pressure of said fuel tank means is respec- said spacecraft. tively at or below a predetermined fuel 25 pressure level or, alternatively, is greater 5. The invention as defined in claim 1 , wherein said than said predetermined fuel pressure fuel comprises Hydrazine N2H4; and wherein said level, and for selectively concurrently oper- oxidizer comprises Bi-Nitrogen Tetroxide, N204. ating one of said third and fifth valve means in dependence upon whether the 6. The invention as defined in claim 1 , wherein said 30 pressure of said oxidizer tank is respec- SCAT thruster means comprises at least three tively at or below a predetermined oxidizer SCAT thrusters; and wherein said monopropellant pressure level or, alternatively, is greater thruster means comprises at least three monopro- than said predetermined oxidizer pressure pellant thrusters. level, wherein said SCAT thuster means 35 produces a thrust to change the velocity of 7. The invention as defined in claim 1 , wherein said said spacecraft; and fuel comprises Hydrazine (N2H4); and wherein said means for selecting said first valve means oxidizer comprises Bi-Nitrogen Tetroxide, (N204) for operation, responsive to receiving a wherein Said SCAT thruster means comprises at command to change spacecraft attitude. least three SCAT thrusters; and wherein said mono- 40 propellant thruster means comprises at least three 8. The invention as defined in claim 1 , wherein said monopropellant thrusters; and further comprising: bipropellant SCAT thruster means includes:

orifice means and fourth valve means, said ori- SCAT thruster fuel inlet valve means for con- fice means and fourth valve means being con- 45 trolling fuel flow into said bipropellant SCAT nected in series across said second valve thruster means under command of said con- means for producing a fuel pressure drop and troller means; and providing an alternative fuel route from said SCAT thruster oxidizer inlet valve means for fuel tank means to said fuel inlet or said bipro- controlling oxidizer flow into said bipropellant pellant SCAT thruster; so SCAT thruster means under command of said second orifice means and fifth valve means, controller means. said second orifice means and fifth valve means being connected in series across said 9. The invention as defined in claim 8, wherein said third valve means for producing an oxidizer SCAT thruster fuel inlet valve means further com- pressure drop and providing an alternative oxi- 55 prises: dizer route from said oxidizer tank means to said oxidizer inlet of said bipropellant SCAT first and second fuel inlet valves connected in thruster; series; and wherein said SCAT thruster oxi-

10 19 EPEP0 0 919 46434 A1 20

dizer inlet valve means further comprises: in shunt of said first inlet oxidizer valve means first and second oxidizer inlet valves connected to permit a limited flow of fuel to said second in series. oxidizer inlet valve when said first oxidizer inlet valve is closed; 10. The invention as defined in claim 9, further compris- 5 said controller means including means for ing: operating said second fuel inlet valve to close said second fuel inlet valve and for simultane- flow limiting bypass orifice connected in shunt ously periodically operating said first fuel inlet of said first inlet fuel valve means to permit a valve to periodically close and open said first limited flow of fuel to said second fuel inlet to fuel inlet valve; and means for operating said valve when said first fuel inlet valve is closed; second oxidizer inlet valve to close said second and oxidizer inlet valve and for simultaneously peri- flow limiting oxidizer bypass orifice connected odically operating said first oxidizer inlet valve in shunt of said first inlet oxidizer valve means in synchronism with said first fuel inlet valve to to permit a limited flow of fuel to said second is periodically close and open said first oxidizer oxidizer inlet valve when said first oxidizer inlet inlet valve, whereby said bipropellant SCAT valve is closed. thruster produces thrust that alternates between two thrust levels. 11. The invention as defined in claim 10, wherein said controller means include means for operating said 20 13. The invention as defined in claim 7, wherein each second fuel inlet valve to close said second fuel said SCAT thruster includes: inlet valve and for simultaneously periodically oper- ating said first fuel inlet valve to periodically close SCAT thruster fuel inlet valve means for con- and open said first fuel inlet valve; trolling fuel flew into said bipropellant SCAT and means for operating said second oxidizer inlet 25 thruster under command of said controller valve to close said second oxidizer inlet valve and means, said SCAT thruster fuel inlet valve for simultaneously periodically operating said first means comprising first and second fuel inlet oxidizer inlet valve in synchronism with said first valves connected in series; fuel inlet valve to periodically close and open said SCAT thruster oxidizer inlet valve means for first oxidizer inlet valve, whereby said bipropellant 30 controlling oxidizer flew into said bipropellant SCAT thruster produces thrust that alternates SCAT thruster means under command of said between two thrust levels. controller means, said SCAT thruster oxidizer inlet valve means comprising: 12. The invention as defined in claim 4, wherein said first and second oxidizer inlet valves connected bipropellant SCAT thruster means includes: 35 in series; a first flow limiting bypass orifice connected in SCAT thruster fuel inlet valve means for con- shunt of said first inlet fuel valve means to per- trolling fuel flew into said bipropellant SCAT mit a limited flew of fuel to said second fuel inlet thruster means under command of said con- valve when said first fuel inlet valve is closed; troller means; 40 and SCAT thruster oxidizer inlet valve means for a second flow limiting oxidizer bypass orifice controlling oxidizer flow into said bipropellant connected in shunt of said first inlet oxidizer SCAT thruster means under command of said valve means to permit a limited flow of fuel to controller means; said second oxidizer inlet valve when said first said SCAT thruster fuel inlet valve means com- 45 oxidizer inlet valve is closed; prising: said controller means including means for first and second fuel inlet valves connected in operating said second fuel inlet valve to close series; said said second fuel inlet valve and for simultane- SCAT thruster oxidizer inlet valve means com- ously periodically operating said first fuel inlet prising: 50 valve to periodically close and open said first first and second oxidizer inlet valves connected fuel inlet valve; and means for operating said in series; and further comprising: second oxidizer inlet valve to close said second flew limiting bypass orifice connected in shunt oxidizer inlet valve and for simultaneously peri- of said first inlet fuel valve means to permit a odically operating said first oxidizer inlet valve limited flow of fuel to said second fuel inlet 55 in synchronism with said first fuel inlet valve to valve when said first fuel inlet valve is closed; periodically close and open said first oxidizer and inlet valve, whereby said bipropellant SCAT flow limiting oxidizer bypass orifice connected thruster produces trust that alternates between

11 21 EP0EP 0 919 46434 A1 22

two thrust levels. being greater than said second level thrust.

14. In a spacecraft rocket propulsion system, an atti- 17. The spacecraft rocket propulsion system of claim tude and velocity control propulsion system com- 15, further comprising propellent reservoir means prising: 5 consisting of a propellant liquid fuel reservoir and a propellant liquid oxidizer reservoir; oxidizer delivery monopropellent thruster means, said mono means for selectively coupling oxidizer to said propellant thruster means having a fuel inlet for bipropellant thruster means and to said bipropellant producing first level thrust in response to SCAT thruster means; and fuel delivery means for receiving propellant fuel at said fuel inlet; 10 selectively coupling fuel from said liquid fuel reser- bipropellant SCAT thruster means, said bipro- voir to said bipropellant thruster means, said bipro- pellant SCAT thruster means including a fuel pellant SCAT thruster means and said inlet and an oxidizer inlet for producing a sec- monopropellent thruster means. ond level thrust responsive to receiving both fuel and oxidizer at said respective fuel and oxi- is 18. Rocket apparatus for producing a pulsating thrust dizer inlets and for alternatively producing a comprising in combination: first level thrust responsive to only receiving fuel at said fuel inlet, said second level thrust bipropellant rocket engine means for producing being at least fifty per cent greater than said thrust, said rocket engine means having a fuel first level thrust; 20 inlet and an oxidizer inlet; liquid fuel tank means for storing fuel in liquid an engine controller; form; fuel inlet valve means connected to said fuel liquid oxidizer tank means of storing oxidizer in inlet for controlling fuel flow into said bipropel- liquid form; lant rocket engine means under command of first valve means for coupling fuel from said liq- 25 said engine controller; uid fuel tank means to said inlet of said mono- oxidizer inlet valve means connected to said propellent thruster means; oxidizer inlet for controlling oxidizer flow into second valve means for coupling fuel from said said bipropellant SCAT thruster means under liquid fuel tank means to said fuel inlet of said command of said engine controller; bipropellant SCAT thruster means; 30 said fuel inlet valve means comprising: third valve means for coupling oxidizer from first and second fuel inlet valves connected in said liquid oxidizer tank means to said oxidizer series; said inlet of said bipropellant SCAT thruster means; oxidizer inlet valve means comprising: and first and second oxidizer inlet valves connected controller means for operating said first valve 35 in series; flow limiting bypass orifice connected means to produce first level thrust to effect a in shunt of said first inlet fuel valve means to change of spacecraft attitude and for alterna- permit a limited flow of fuel to said second fuel tively operating said second and third valve inlet valve when said first fuel inlet valve is means for producing second level thrust to closed; and effect a change of spacecraft velocity. 40 flow limiting oxidizer bypass orifice connected in shunt of said first inlet oxidizer valve means 15. In a spacecraft rocket propulsion system, an atti- to permit a limited flow of fuel to said second tude and velocity control propulsion system com- oxidizer inlet valve when said first oxidizer inlet prising: valve is closed; 45 said controller including means for operating monopropellent thruster means for producing a said second fuel inlet valve to close said sec- first level thrust to produce a change in space- ond fuel inlet valve and for simultaneously peri- craft attitude; and odically operating said first fuel inlet valve to bipropellant SCAT thruster means for produc- periodically close and open said first fuel inlet ing a second level thrust to produce a change so valve; and means for operating said second in spacecraft velocity, said second level thrust oxidizer inlet valve to close said second oxi- being at least fifty per cent greater than said dizer inlet valve and for simultaneously periodi- first level thrust. cally operating said first oxidizer inlet valve in synchronism with said first fuel inlet valve to 16. The spacecraft rocket propulsion system of claim 55 periodically close and open said first oxidizer 15, further comprising: bipropellant thruster means inlet valve, whereby said bipropellant rocket for producing a third level thrust to produce a engine produces thrust that alternates between change in spacecraft apogee, said third level thrust two thrust levels.

12 23 EP 0 919 464 A1 24

19. The invention as defined in claim 18, wherein said bipropellant rocket engine comprises a SCAT rocket engine.

10

15

20

25

30

35

40

45

50

55

13

EP 0 919 464 A1

FUEL OXIDIZER t

53 57

5a2

5a1

FIG. 3 EP 0 919 464 A1

S0LID/N2H4 INTEGRATED | PAYLOAD PAYLOAD BIPROPELLANT /INCLUDES AKM\ 1% SOLID o_UUUD \DISPERSI0N CORRECTION / /A TOTAL SULiUI TOTAL mr RCS, CARGO IN GTO L-J CARGO IN GTO RCS 1 1 600-1 11600- PAYLOAD PAYLOAD 10000- (40951b) 10000- (47101b) • BASELINE (BL) '615 lb (15%)>BL 8000- 8000- WFJGHT (lbs) : PROPULSION SYSTEM INLET WEGHT (lbs) ! PROPULSION SYSTEM INERT I 6000-'(TWO SEPARATE PROPULSION 6000- SYSTEM) PROPELLANT 4000- PROPELLANT 4000- (67631b) (63451b)

2000- 2000-

FIG. 4A FIG. 4B

ADVANCED DUAL MODE USING HIGH DUAL MODE | PAYLOAD | PERFORMANCE! SCATS 1

uuuu nRCS Rcs TOTAL □ TOTAL n CARGO IN GTO CARGO IN GTO sc^f_1 11600- 11600-1 PAYLOAD PAYLOAD 10000- (49181b) 10000- (52231b) • 8231b (20%)>BL • 11281b (27.6%)>BL 8000- • 2081b (4.5%)>INTEGRATED 8000- • 5131b (11%)

PROPELLANT PROPELLANT 4000- 4000- (62181b) (5830 lb)

2000- 2000-

FIG. 4C FIG. 4D

17 EP 0 919 464 A1

European Patent Application Number J EUROPEAN SEARCH REPORT Office EP 98 12 0184

DOCUMENTS CONSIDERED TO BE RELEVANT Category Citation of document with indication, where appropriate, Relevant CLASSIFICATION OF THE ot relevant passages to claim APPLICATION (lnt.CI.6) D,X US 5 282 357 A (SACKHEIM ROBERT L) 1,5,8, B64G1/26 1 February 1994 L4-17 F02K9/50 * abstract * F02K9/58 * figure 2 * * column 1, line 42 - line 56 * * column 2, line 30 - line 60 * * column 4, line 20 - line 49 * * column 5, line 10 - line 23 *

GB 1 439 368 A (MESSERSCHMITT B0ELK0W 1,8,14, BL0HM) 16 June 1976 15 * page 1, line 35 - line 76 * * page 2, line 41 - line 62 * * page 2, line 125 - page 3, line 16 * * figure 1 *

D , A US 5 572 865 A (SACKHEIM ROBERT L ET AL) 1,5,14, 12 November 1996 15 * abstract * * column 2, line 45 - column 3, line 16 * * column 7, line 66 - column 8, line 14 * TECHNICAL FIELDS SEARCHED (lnt.CI.6) * figures IB , 1C , 2 * B64G US 3 514 953 A (KEPHART JIMMY F) 1,2,5,14 F02K 2 June 1970 * the whole document *

W0 87 07877 A (HUGHES AIRCRAFT CO) 1,2,5, 30 December 1987 14-17 * abstract * * column 3, line 18 - column 4, line 5 * * column 5, line 34 - column 7, line 32 * * column 13, line 19 - column 15, line 8 * * figure 5 *

The present search report has been drawn up for all claims Place of search Date of completion of the search THE HAGUE 8 March 1999 Calvo de No, R CATEGORY OF CITED DOCUMENTS T : theory or principle underlying the invention E : earlier patent document, but published on, or X : particularly relevant if taken alone after the filing date Y : particularly relevant if combined with another D : document cited in the application document of the same category L : document cited for other reasons A : technological background O : non-written disclosure & : member of the same patent family, corresponding P : intermediate document document

18 EP 0 919 464 A1

ANNEX TO THE EUROPEAN SEARCH REPORT ON EUROPEAN PATENT APPLICATION NO. EP 98 12 0184

This annex lists the patent family members relating to the patent documents cited in the above-mentioned buropean searcn report. The members are as contained in the European Patent Office EDP file on The European Patent Office is in no way liable for these particulars which are merely given for the purpose of information. 08-03-1999

Patent document Publication Patent family Publication cited in search report date member(s) date US 5282357 A 01-02-1994 GB 2293627 A,B 03-04-1996 GB 2293416 A,B 27-03-1996 US 5572865 A 12-11-1996 US 5417049 A 23-05-1995

GB 1439368 A 16-06-1976 DE 2241424 A 07-03-1974 FR 2197118 A 22-03-1974

US 5572865 A 12-11-1996 US 5417049 A 23-05-1995 US 5282357 A 01-02-1994 GB 2293416 A,B 27-03-1995 AU 671402 B 22-08-1996 AU 1505595 A 15-06-1995 AU 663095 B 28-09-1995 AU 2098592 A 25-02-1993 CA 2110057 A,C 04-03-1993 DE 69228068 D 11-02-1999 EP 0620895 A 26-10-1994 WO 9303962 A 04-03-1993 GB 2293627 A,B 03-04-1996

US 3514953 A 02-06-1970 NONE

W0 8707877 A 30-12-1987 EP 0272284 A 29-06-1988 JP 1500531 T 23-02-1989

2} For more details about this annex : see Official Journal of the European Patent Office, No. 1 2/82

19