dna@dsd.UUCP (02/25/84)
Posted: Wed Feb 8, 1984 7:15 AM GMT Msg: NGIE-1712-8217 From: JKING To: DOCS CC: OFFICE Subj: WATER ROCKET REPORT USING WATER AS A PRIMARY METHOD OF PROPULSION FOR SPACERAFT MODIFYING STANDARD STS ORBITS Jan A. King W3GEY V.P. Engineering The Radio Amateur Satellite Corporation The Space Shuttle has modified the method by which space-bound payloads enter orbit for the forseeable future. The STS offers the promise of lower payload cost and the ability to carry large payloads into orbit to mention but a few of its primary objectives. For very low cost payloads such as those pioneered by the radio amateur community (the OSCAR series), the Space Shuttle poses, however, a number of severe engineering obstacles which have become major stumbling blocks to the exploitation of this valuable resource. Not unlike most free flying satellites, the communications satellites launched by radio amateurs and used in the Amateur Satellite Service are intended to meet long life objectives. In addition, to meet mission objectives for the communications service to be provided either a geostationary transfer orbit or a sun synchronous polar orbit must be attained by the spacecraft. Unfortunately, neither of these objectives can be met by the standard provisions of a Space Shuttle mission. STS orbits, typically 296 km in altitude and inclined from 25 to 57 degrees are unstable. A small spacecraft, with a low surface area to mass ratio, will decay from such an orbit in a matter of months. This class of orbits, with a few exceptions, is also unsuitable for communications experiments of interest to the Amateur Satellite Service. It is therefor a necessary requirement for Amateur Satellites and other free flying spacecraft seeking stable orbits to carry a propulsive capability if launched by the Space Shuttle. Having accepted the burden of a propulsive system as an added spacecraft complexity, yet another problem becomes apparent. Classical propulsion systems employed by satellites are characterized as hazardous devices. Due to the manned presence on board Shuttle safety considerations are necessarily more stringent when using this method of launching a spacecraft. The added complexities and paperwork resulting from the inclusion of hazardous devices on board Shuttle launched satellites conflicts with the low cost nature of these programs and may make such payloads totally impractical or viable only if launched by alternative methods. This problem is exascerbated by the fact that most orbit alternatives can be reached from Shuttle orbits only by multiple delta-V maneuvers. This requires multiple solid rocket engines or a restartable engine on board the satellite, further multiplying the safety hazard problem. A solution is sought to the "Shuttle Dilema." The Shuttle Dilemma may be defined as follows: 1/ A Shuttle payload is always two burns away from the desired orbital elements when separated from the cargo bay. 2/ The cost of NASA safety approval for a propulsive device used aboard Shuttle by a low cost user is approximately a factor of three higher than the entire cost of the payload itself. 3/ As a rule of thumb, the mass of the paperwork necessary for NASA approval of a hazardous device for a Shuttle flight is greater than or equal to the mass of the payload. While the above may seem humorous, these statements are all too true and must be dealt with squarely by would-be Shuttle low cost payload designers. A propulsion system that would solve the Shuttle Dilema could be expected to have the following characteristics: 1/ The propellant used should not be a chemical, pressue or explosive hazard as defined by NASA or the USAF (ref. AFETRM-127- 1). 2/ The loading of propellant into the spacecraft should not constitute a hazardous activity. No special safety equipment should be required. 3/ No portion of the propulsion system should contain hazardous devices of any kind. Certain exceptions to this rule might be taken to include category B electro-explosive devices such as pyrotechnically operated valves. 4/ No portion of the propulsion system should be pressurized or become pressurized even remotely while the satellite is on the ground, during powered flight or during astronaut activities in orbit, including those conducted to separate the satellite from the Shuttle. 5/ No portion of the propulsion system should be susceptable to damage due to the environment of the Cargo Bay during powered flight or in orbit prior to or during separation of the satellite. The Radio Amateur Satellite Corporation (AMSAT), having had practical experience with both liquid and solid propulsion systems on board low cost satellites believes that the above requirements will prove to be virtually mandatory for low cost payloads flown by the Space Transportation System. Two methods have been considered for some time by AMSAT that appear to meet the above five conditions and produce satisfactory performance for space applications. Both involve using water as a fuel and both have been considered by other groups from time to time as methods of space propulsion. PROPULSION VIA WATER ELECTROLYSIS: The propulsion of a space vehicle via hydrogen/oxygen fuel produced from the electrolysis of water is far from a novel idea. Hughes Aircraft Company, Space Systems Division documented the results of an internal research and development IR&D project which developed a working model of a water electrolysis rocket during the first half of 1964 (1,2,3,4). Using this technique a single pressure vessle acts as storage for the water and electolyte, as an electrolysis chamber and finally as a pressure bottle for the combined electolyzed gases. The premixed gas may be fed via a single line into the injection chamber of a small rocket engine or thruster. In their final report on this technology Hughes stated, "The Water Electrolysis Rocket has been explored in sufficient depth to verify the feasibility of the concept. Furthermore, it has been determined that this system offers significant advantages over other presently available reaction control systems. Among these advantages are: 1/ Higher specific impulse 2/ Lower system weight 3/ Lower power requirements 4/ Extended life in space 5/ Improved system reliablity 6/ System simplicity " It is interesting to note that at the time of the writing Hughes did not cosider the safety advantages of the system which are of prime interest to AMSAT. The specific impulse of a small electrolysis motor of the type required for a low cost satellite mission is between 330 and 360 sec. This is considerably better than either an equivalent solid or bipropellant liquid motor system (270 and 305 sec. respectively). Preliminary investigations by AMSAT suggest that the power required to electolyze one kilogram of water is approximately 5,000 WH. This may be related to delta-V for a specific satellite case as shown in Table 1. A minimum system schematic is shown in Figure 1. It is not known why Hughes did not continue to develope this technology to the point of commercial introduction. Clearly, however, monopropellant hydrazine systems replace other methods for reaction control starting about the same time as the Hughes research on water electrolysis motors. Since this work was done there have been dramatic improvements in both electrolysis electrode and thruster technologies. AMSAT has also conducted preliminary studies on an advanced method of drying the hydrogen and oxygen gas which should lead to improved thruster performance. This was one problem reported by the Hughes research team. PROPULSION BY STEAM EXPULSION: A second method of exploiting water as a safe propellant is by means of a small steam engine integral to the thruster in a water fed propulsion system. Water is allowed to superheat in a small chamber adjacent to an expansion nozzle. Thrust is produced by the acceleration of water molecules as they exit the nozzle. The specific impulse of this technique is far poorer than the electrolysis method (107 sec.), however, the system complexity is very low indeed and the energy required to liberate a kilogram of water into steam is only 750 WH, considerably less than with electrolysis. The specific impulse for a motor of this type can be shown to be governed by the equation: | 2 C K Tb (1 - <Pexit/Pchamb.>**C-1/C)| Isp = SQRT | ________ | | (C-1)n m | where: C = Heat capacity of propellant (water = 1.3) K = Boltzman Constant = 1.38E-23 J/K Tb = Gas Temperature = (approx.) 400K n = molecular weight of propellant (water = 18) m = mass of a hydorgen atom = 1.66E-27 Kg Pexit = Nozzle exit plane pressure (assumed = 0.01 Bar) Pchamb. = Thruster chamber pressure (assumed = 5.0 Bar) As can be seen, the specific impulse depends inversely on the molecular weight of the fuel used, taken to the 1/2 power. This favors the use of low molecular weight fuels. As can be seen water is nearly optimum for an engine of this type particularly when the other physical properties of this fluid are taken into consideration in a practical system. While, on balance, a system using steam as a propulsion technique is far from optimum with respect to Isp it is simple enough to be included on even GAS CAN mission and can solve a reasonable number of propulsion problems for small spacecraft. Figure 1 reviews the performance of this system for a variety of missions of interest to AMSAT and gives a comparison to the electrolysis method. ORBIT CORRECTION TECHNIQUE USING WATER PROPUSLION METHODS: A salient characteristic of both propulsion techniques reviewed is that they take electrical energy from the solar arrays of the spacecraft and convery it into potential energy (either in the form of stored gas to be burned or in the form of stored electrical energy). Thrust is best produced in a burst mode rather than with a continuous firing. This is a typical operating mode for a reaction control system (RCS) but is somewhat unusual for an orbit transfer maneuver. In effect, time is traded against the burn duration (power production rate of the satellite) so that a reasonable compromise for the total duration of the propulsion phase of the mission is reached. An important consideration for such a mode of operation is that the total delta-V achieved per day during the maneuver must be greater than the deceleration per day due to drag in the Shuttle base orbit. The orbit transfer strategy for circular orbits with a spinning spacecraft is shown in Figure 2. Two small thrusters at either end of the satellite are employed. Firings always occur at the line of apsidies. Alternate thrusters are used so that a first firing occurs at perigee thus raising apogee and a subsequent firing occurs at apogee now raising perigee. Mini- Hohman transfer maneuvers are repeated until the desired circular altitude is reached. Eliptical orbits can be achieved with a single thruster fired at consecutive perigees thus continuously raising apogee. Inclination changes with this technique are also possible and are most efficiently applied when apogee also coincides with the ascending node of the orbit. SUMMARY: The techniques reviewed have been considered in the past by space research projects and by commercial spacecraft manufacturers. While these propulsion technologies have never been reduced to commercial practice, sufficient study has been done to verify their applicability to space missions. If the STS is going to fulfill its mission as a launcher for ALL space interests then some acceptable methods of propulsion must be found for smaller payloads. These methods must take into consideration the low cost nature of such projects and the very stringent safety constraints imposed by NASA on all STS users. In view of the above AMSAT believes that water propulsion technologies should be revisited because they have the potential of solving the "Shuttle Dilema" for a class of users that can bring significant benefit to the space program as a whole. REFERENCES: 1. Newman, Daniel D., Study of the Water Electrolysis Propulsion System- Final Report, Engineering Record No. 151, Hughes Aircraft Co., S.S.D., Propulsion and Power Systems Laboratory, 5 June 1964. 2. Water Electrolysis Rocket, Hughes Aircraft Company Proposal, 63H-7438/9419 (Dec. 1963). 3. Electrochemical Service Unit, Hughes Aircraft Company Proposal, 64H-2115/A3951-001 (April 1964). 4. Water Electrolysis Rocket, Hughes Aircraft Company Proposal, 64H-2510/A4682-001 (May 1964). .