Propulsion apparatus and method using boil-off gas from a cryogenic liquid
ORIGIN OF THE INVENTION 
     The invention described herein was made by an employee of the U.S. Government and may be manufactured and used by or for the Government for governmental purposes without the payment of any royalties thereon or therefor. 
    
    
     BACKGROUND OF THE INVENTIOn 
     For various space vehicle maneuvers and experiments in space, particularly in low earth orbits, it is necessary to control the attitude and compensate for the drag on the vehicle while it is in orbit. In particular, these requirements are critical for experiments where telescopes and other viewing instruments are used and it is necessary to keep these instruments oriented in a precise direction. 
     This invention relates to a propulsion system which utilizes cryogenic helium, already on board the space vehicle for cooling an experiment package, for the additional purpose of meeting in-orbit propulsion requirements of the vehicle. 
     Accordingly, an important object of the present invention is to provide a propulsion system for a space vehicle utilizing boil-off gas of the on-board supply of liquid helium as a propellant without adversely affecting the temperature of the experiment package which is being cooled by the helium. 
     Still another important object of the present invention is to provide a simplified propulsion system which has few components and which utilizes components already existing on a space vehicle. 
     SUMMARY OF THE INVENTION 
     In the present invention, boil-off gas from a cryogenic liquid container or a space vehicle is employed to provide the propulsive forces necessary for controlling the attitude of the vehicle and compensating for drag in low earth orbit. The apparatus and method are useful for a space vehicle of the type having an experiment package and a helium dewar for cooling and maintaining the experiment package at a desired temperature. The dewar is vented to enable release of boil-off gas due to the heating of the dewar by solar radiation leaked to the dewar. The vehicle includes a dewar support frame having a support ring manifold from which a plurality of solar-panel-supporting spars extend radially. The above objectives are accomplished according to the present invention by providing thruster modules carried adjacent ends of the solar panel spars for producing a thrust force in a desired direction. An auxiliary heater is operatively connected with the dewar for increasing the amount of helium boil-off gas produced in the dewar to provide an amount of boil-off gas which is adequate as a propellant for controlling the attitude and drag when the boil-off gas due to heat leak is inadequate. A temperature sensor and controller are provided for sensing the temperature of the dewar and actuating the auxiliary heater to maintain the supply of helium boil-off gas adequate as a propellant. A supply line for delivering the boil-off gas is connected to the thrust modules. Distribution control valves are provided for selectively delivering boil-off gas to a desired thrust module and thereby controlling the attitude and drag on the vehicle. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The construction designed to carry out the invention will be hereinafter described, together with other features thereof. 
     The invention will be more readily understood from a reading of the following specification and by reference to the accompanying drawings forming a part thereof, wherein an example of the invention is shown and wherein: 
     FIG. 1 is a perspective, partially cut-away view illustrating a space vehicle propulsion system utilizing helium boil-off gas from an experiment package helium dewar according to the invention; 
     FIG. 2 is a schematic illustration of apparatus for utilizing boil-off gas from a helium dewar cooling container to control spacecraft attitude and drag according to the invention; 
     FIG. 3 is an elevation, partially cut away, illustrating a helium distribution system according to the invention; and 
     FIG. 4 is a thrust module for a helium boil-off gas propulsion system for controlling attitude and drag on a low earth orbit space vehicle according to the invention. 
    
    
     DESCRIPTION OF A PREFERRED EMBODIMENT 
     Referring now to the drawings, a vehicle 10 is illustrated in FIG. 1 for a space experiment in a low earth orbit. Liquid helium 11 is contained in a dewar 12 which comprises a major portion of the spacecraft. The dewar is supported by an aluminum framework 14 which includes frame ring members 14a and longitudinal frame members 14b on the outside of the dewar and attached thereto. Hollow annular support rings 16 are interconnected to form a manifold and are spaced from and carried about the dewar support structure for supporting solar array panels 18. The solar array panels are carried on hollow spars 20 which in turn are spaced around and carried by the support ring manifold. An experiment package 22 in the helium dewar is kept cool by the helium which is pressurized and at a temperature of about 1.6 degrees K. The package may include, for example, a telescope 22a which receives light through a shield 22b and instrument windows 22c. 
     The dewar is encircled by a plurality of heat shields 38 interleaved with layers of insulation 39, which extend beyond one end of the dewar and taper inwardly over the instrument windows. The extended end portions of the heat shields are cooled by means of hollow, interconnected rings 35 (FIG. 3) through which helium is passed on its way to the thrusters. As illustrated, a supply line 40 leads from the heat cooling rings to the support ring manifold 16 which is constructed from tubular aluminum material and is hollow. The support ring manifold is used as a conduit for delivering the helium to the interior of the solar array spars 20 which then deliver the helium to the four-way manifold 26 (FIG. 4) of the thruster modules. In this manner, a supply of helium is delivered to the thrusters 24 utilizing, in part, the existing hardware. 
     Porous plug 41 in the dewar container acts as a phase separator, keeping in the liquid helium and allowing the vapor to escape. Solar heating of the dewar heat shields, lines, frame members, and spars raises the temperature of the helium gas significantly before it reaches the thrusters. 
     According to the present invention, a plurality of thrust modules, designated generally as A, (FIG. 4) are provided with one each of the thrust modules A carried on the ends of each solar array spar 20. Each thruster module includes four thrusters 24 which are arranged mutually perpendicular to each other. Each thruster 24 includes a valve motor 25 which remotely controls a motorized valve 25a (FIG. 2) inside the thruster which is variable so as to emit a variable amount of helium boil-off gas through the thruster. Any suitable throttling valve may be utilized. This allows precise control of the thrust produced by the boil-off gas emitted from the thruster. A four-way manifold 26 is included in each thruster which direct helium into the thrusters, when one of the valves is open and outwardly through the nozzle outlet 24a. 
     A temperature sensor 30 and temperature controller 33 are illustrated in FIG. 2 to sense and control the temperature of the helium 11 in the dewar. The helium sensor senses the temperature of the helium in the dewar and if the temperature of the helium increases beyond 1.7 degrees K., the helium is allowed to vent through two of the thrusters 24 which are arranged at one hundred-eighty degrees to each other so that the resultant thrust is zero. If the temperature of the helium in the dewar decreases below 1.5 K., an auxiliary heater 32 may be turned on to increase the temperature of the helium so that a sufficient supply of the helium boil-off gas is present for propulsion. Heater 32 need only be of about one watt since heat leak into the dewar from solar radiation is normally the main source of helium boil-off gas. It has been found for one experiment that the temperature of the helium may be kept between 1.5 and 1.7 degrees K. without adversely affecting experiment package conditions while providing adequate propellant for attitude and drag control. 
     Operation of the thrusters in the desired manner is regulated by means of a controller 34 (FIG. 2). The controller includes a conventional computer or microprocessor into which are programmed desired parameters derived from dynamic analysis of the system. The controller receives signals from temperature controller 33, along with signals from gyroscopes 36, indicative of attitude of the vehicle, and signals from proof mass 37, indicative of drag, which is related to displacement of the proof mass from the center of mass of the vehicle. Programmed values are then compared with values of the received signals, and a resulting signal is applied to turn on one or more of values 25 for a period sufficient to produce the necessary thrust. Various conventional software and hardward systems may be utilized to control the thrusters, as is well within the purview of one skilled in the art. Further detailed regarding theoretical aspects of thrust control in the above described system may be had by reference to a report, NASA TM-82517, January, 1983, by the present inventor, entitled &#34;Attitude Control and Drag Compensation Propulsion System for the Gravity Probe-B Spacecraft,&#34; available from National Technical Information Service, Springfield, Va. 22161, which report is hereby incorporated by reference in its entirety. 
     While a preferred embodiment of the invention has been described using specific terms, such description is for illustrative purposes only, and it is to be understood that changes and variations may be made without departing from the spirit or scope of the following claims.