Document created: 14 October 2003
Air University Review,
November-December 1973
Space Propulsion
Let’s do it Better Electrically
Chemical propulsion systems will continue to be the mainstay of space propulsion for the foreseeable future. Many gains still remain to be realized in this area. Such items as improved packaging, reusability, reduced costs, and increased durability are major goals of the current Air Force rocket propulsion technology program. This program is structured so as to make potential improvements a reality within the next five to ten years. Beyond these improvements, it is necessary to look to other than chemical propellants to increase propulsion performance substantially. This is where electric propulsion systems offer promise. These systems, through vastly improved propellant mass utilization, have the potential to serve us better electrically.
At the present time, electric propulsion devices for Air Force space applications are being developed for utilization at low thrust levels. This situation is dictated by the fact that the present shortage of available electrical power aboard spacecraft would prohibit large thrust levels from being attained by electric propulsion systems. Even so, there are a number of space propulsion functions for which electric propulsion systems are becoming prime contenders. Such functions as satellite attitude control and orbital maintenance appear ideal for certain electric propulsion devices. The attractiveness of these devices stems from their ability to utilize propellant mass so efficiently at the necessary thrust levels. For example, a certain satellite propulsion function could be performed using far less propellant through employment of electric propulsion. The savings in propellant consumption could be realized through launch weight savings, increased payload, longer satellite missions, or a combination. These alternatives could potentially reap considerable benefit in terms of launch costs, material utilization, and mission coverage.
But where should electric propulsion ultimately lead us? The answer to this question is that a patient development effort could enable us to perform spatial propulsion maneuvers that are not attempted today. The ability to perform these new maneuvers will be highly dependent upon harnessing more efficiently the vast amounts of energy that are available in the universe. At the present time our energy pioneers such as Hannes Alfvénl are pointing to new sources of untapped energy, solar winds, for instance. These sources could enable us to reduce space repositioning and travel times significantly through employment of specific electric propulsion devices. As we reach these new acceleration regimes, other ways of improving acceleration mechanisms will probably be identified. The main point to remember is that we must retain our pioneer spirit. Indications are that we have a long path ahead of us in the area of propulsion refinement.
This article briefly describes the electrical acceleration processes from the basic electrostatic and electromagnetic aspects through the fascinating theory of magnetic field annihilation. The role of electric propulsion is then assessed in light of postulated military missions. Finally, future propulsion regimes for electrical thrusting systems are hypothesized.
In any discussion of rocket propulsion, two parameters are of extreme importance: total impulse and specific impulse. Total impulse establishes the magnitude of the thrust and the duration of the thrusting time that are necessary to complete a certain mission. For example, in an orbit-changing mission, to transfer from one orbit to another, changes in velocity must be imparted to a satellite. These changes can be accomplished by using rocket engines to provide thrust for a certain time period. Thus, the engines provide the correct total impulse for entering the new orbit.
A very important question now arises: How much fuel will be required to provide the specified amount of total impulse? To achieve thrust, particles of propellant are expelled at a certain velocity. A desired thrust level can be achieved by either expelling more particles at a lower velocity or fewer particles at higher velocities. Naturally, it is more desirable to accelerate particles to as high a velocity as practical, since fewer particles in the acceleration process will result in less propellant weight. In fact, some desirable military space missions, such as sizable repositioning maneuvers, would require optimum propellant utilization in space in order to maintain allowable spacecraft launch weights. Furthermore, launch weights have now been transcribed into dollars per pound.2 For these reasons, it behooves us to use each propellant particle in space in as efficient a manner as possible.
We are now ready to define specific impulse, since it is a measure of how efficiently each propellant particle is used. Specific impulse is generally defined as the velocity imparted to the propellant divided by gravitational acceleration. In meter-kilogram-second (MKS) units, the velocity is greater than the specific impulse by a factor of ten. Thus, by knowing the specific impulse capability of a system, we know how efficiently the system is using its propellant. Specific impulse has the units of seconds in the MKS system.
The practical implication of specific impulse can immediately be seen by considering the amount of propellant needed to provide a certain total impulse. The propellant weight is determined by dividing the total impulse by the specific impulse. Thus, if a system were operating at a specific impulse of 300 seconds, 200 pounds of propellant would be required to provide a total impulse of 60,000 pound-seconds. If the specific impulse were somehow doubled, then the corresponding propellant weight would be halved.
Facets of Chemical and
Electric Propulsion
Chemical propulsion systems derive their energy from internal sources. As
the propellant ignites, energy in the form of heat is given off in great
quantities. By expanding the propellant exhaust through a nozzle, heat energy
is transformed into kinetic energy of the exhaust products. There is an upper
limit to the amount of internal energy available in chemicals. It appears that
chemical propulsion systems cannot exceed a specific impulse of 600 seconds.3
In contrast to chemical systems, electric propulsion devices receive their energy from external sources. This energy is used to furnish a “push” to the propellant particles. Electric systems are therefore not limited to low values of specific impulse. In fact, electric propulsion devices have routinely operated at specific impulses as high as 5000 seconds. Provided that the external energy is available, the attainment of high specific impulses by electric propulsion devices is not a problem.
In space environments, electrical energy may be furnished by several means. A typical example of today’s spatial energy source is the solar array. The Air Force Aero Propulsion Laboratory is currently examining such arrays to generate 10 to 20 kilowatts of power. At this power level, enough energy can be furnished to an electric thrusting device to furnish fractions of pounds of thrust at specific impulses in the range of 1500 to 3000 seconds. Granted, not all satellites will have this much power aboard, and not all the available power will be continually at the disposal of the electric thruster. However, suggested power allocations4 indicate that sufficient power is available for electric thrusters to negate forces which cause orbital perturbations and to perform repositioning functions in which time constraints are minimal. The vast propellant weight savings that could conceivably be derived through employment of electric propulsion could then be used advantageously in many ways.
Looking ahead to the 1980s, we anticipate the availability of nuclear power supplies for space applications. Additionally, solar array technology is expected to increase rapidly over the coming years. Power levels of up to 75 kilowatts have already been predicted for solar arrays.5 Probably equally important, new sources of untapped energy such as solar winds6 are now under close scrutiny. All these facets add up to the fact that electric thrusters could produce substantial thrust levels as well as high specific impulse. The realization of such thrust levels would allow electric propulsion devices to perform repositioning functions in which time constraints were significant.
Thus chemical propulsion systems, while capable of attaining high thrust levels by expelling large amounts of propellant at relatively low velocities, necessitate very large vehicles to contain the required amount of propellant. On the other hand, electric propulsion devices use their propellant much more efficiently but require external energy sources. The further evolution of electric propulsion devices, then, will be heavily dependent on the evolution of energy sources.
Electric Propulsion Devices
High-specific-impulse electric thrusting devices are of two types, electrostatic and electromagnetic. Electrostatic devices operate by accelerating charged particles through a difference in electric potential. Figure 1 depicts the acceleration process occurring in electrostatic thrusters. The propellant is first ionized by such means as application of electric forces or inducement of chemical reactions. Once ionized, the propellant carries a positive charge. Particles of charged propellant are accelerated by means of a plate that is at negative potential. The propellant proceeds outward through symmetrical openings in the plate. To prevent the propellant from circling around and reattaching to the spacecraft, a neutralizing device is utilized. This device produces negative particles, which join the propellant stream. The result is that the mass leaving the spacecraft is neutral and will not have further attractive interactions with the spacecraft.
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In contrast to the electrostatic device, an electromagnetic device works in the following manner. An electrical current is established in a conductive propellant, which is in the presence of a magnetic field. The magnetic field interacts with the current to generate a force on the propellant. This action is identical to the process occurring in an electric motor. In a motor, a conductor is placed on a rotor. The rotor, being in a magnetic field, spins when current is driven through the conductor.
There are presently reasons to research both electrostatic and electromagnetic systems. Electrostatic systems are higher in efficiency, which means they need a lower amount of input power to perform a certain mission. On the other hand, electromagnetic thrusters are marked by simplicity of operation and high energy density. The feature of high energy density allows electromagnetic thrusters to provide high thrust levels with frontal areas of much smaller size than a corresponding electrostatic thruster. Additional attractiveness of electromagnetic systems is that, by virtue of their pulsing feature, they apply equally well to spinning spacecraft and three-axis stabilized spacecraft. Thus, if a spacecraft is spinning for stability purposes, the electromagnetic thruster can be pulsed at a time that will produce thrust in the desired direction. In contrast, most electrostatic systems operate continuously over fixed periods.
Two electrostatic concepts are presently under development by government agencies: NASA is developing ion engines, and USAF is developing colloid engines. The main difference between the ion and the colloid concepts lies in the charge-to-mass ratio of the propellant particles. Each of these devices has its own operating regime. The ion systems operate very efficiently at approximately 3000 seconds specific impulse and up, while the colloid is very efficient at specific impulses of approximately 1500 seconds and below. When power is at a premium, as it will be aboard certain Air Force spacecraft, the colloid engine is indeed attractive because the tight limitations on input power prohibit operating at the extremely high specific impulses.
The Air Force and NASA, in a closely coordinated effort, are also presently working on the development of electromagnetic devices. These devices, known as pulsed plasma thrusters, look very attractive for many space applications, a big plus being the simplicity of their operating concept. Essentially, all that is necessary is to discharge a capacitor bank through a selected load. The propellant is then electromagnetically accelerated. Unlike the electrostatic systems, pulsed plasma systems are relatively insensitive to parameters such as temperature. Thus they are free of complex control system loops.
The Air Force is presently developing two specific pulsed plasma devices: the solid-propellant pulsed plasma thruster and the pulsed inductive thruster. The rationale is that the solid-propellant (Teflon) device appears to be optimum at lower thrust levels while the pulsed inductive thruster appears better at higher thrust levels. As indicated earlier, the electrostatic systems are eliminated at higher thrust levels due to their low energy density. However, the Teflon thruster is being considered at thrust ranges that are in direct competition with the electrostatic devices. The issue at hand is the simplicity and pulsing feature of the solid-propellant thruster versus the lower power requirements of the electrostatic systems. At present, the involved trade-offs make both systems worthwhile to pursue.
The solid-propellant pulsed plasma thruster operates by discharging a capacitor bank across a pair of electrodes. (Figure 2) The energy developed across the electrodes ablates and accelerates the solid propellant. This thrusting concept has already operated reliably in space aboard the Lincoln Laboratory spacecraft LES-6. The function of the solid-propellant thruster aboard LES-6 was to produce micropounds of thrust to counteract forces on the spacecraft caused by the earth’s oblateness. Presently, Air Force efforts are directed at evolving this LES-6 thruster into the millipound regime. At this level, the solid-propellant thruster can perform other vital orbit maintenance functions.
 Figure
2. Pulsed plasma thruster |
The operation of the pulsed inductive thruster is analogous to that of an induction motor. (Figure 3) The propellant, Xenon, is one with conductive properties. After the propellant is sprayed over the face of the inductive load coil, the capacitor is discharged. This discharging capacitor creates a magnetic field. As this field diffuses into the ionized propellant, a current is induced in the propellant. The existence of both current and magnetic field energy generates forces by which the propellant is accelerated. This concept is more complex than the solid-propellant thruster. However, the flexibility of this device makes it suitable for operation at elevated thrust levels. This flexibility is achieved by varying the density of the gaseous propellant and varying the capacitor discharge rate through the accelerating load.
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Figure
3. Pulsed inductive thrusterA new and radical electromagnetic concept presently under preliminary investigation is known as “magnetic field annihilation.” This fascinating theory is based on the phenomenon occurring in solar flares and identifies a more efficient method of using electromagnetic energy as an acceleration mechanism. Solar flares appearing on the sun’s surface are caused by bodies of hot, ionized gases coming together. The result is that hot gases (i.e., solar flares) are expelled outward from the sun’s surface. Petschek first employed his theory of magnetic field annihilation in an effort to explain the vast amount of energy that had been observed in solar flares.7 Based on Petschek’s theory, Charles Lee Dailey has advanced an idea for a magnetic field annihilation thruster.8 The name of the theory derives from the hypothesis that magnetic fields annihilate one another in solar flare processes. The energy that had been stored is then totally transformed into the kinetic energy of the solar flare mass particles. By total utilization of the generated magnetic field in an electromagnetic thruster, it is believed that higher thruster efficiencies can be attained. This will provide pay-offs in terms of the amount of propellant and input electrical energy that are required to perform a certain mission.
Probably the most advanced electric thrusting concept presently under Air Force study is the utilization of the ambient atmosphere as thruster propellant. Gordon L. Cann, under AF contract, is determining if there are sufficient particles available at an altitude of 100-200 miles to be electromagnetically accelerated so as to overcome atmospheric drag.9 If there are sufficient particles, the implications are awesome. This would essentially mean that a low-orbit satellite could be maintained indefinitely by electromagnetically accelerating the particles in the atmosphere to overcome drag. Thus, it would not be necessary to carry along propellant to perform this “secondary” propulsion function.
In summary, both electrostatic and electromagnetic systems are capable of attaining high specific impulse. The electrostatic systems are more efficient at low thrust levels and require less input energy for a specific mission. The electromagnetic systems have the advantages of simplicity of operation, applicability to spinning satellites, and the ability to extrapolate to high thrust levels reasonably. Both types of systems are in development. Questions yet to be answered regarding electric propulsion systems include lifetime and spacecraft interference. Tests to date with the ion engine aboard the NASA spacecraft SERT-II and the solid-propellant pulsed plasma thruster aboard the spacecraft LES-6 indicate that electric thrusters do have the potential to perform orbital maintenance functions successfully and to accomplish repositioning functions in which time constraints are minimal. As more power becomes available for space applications, electric propulsion devices should proceed to even more ambitious functions.
Military Space Applications
A major goal of military research and development is to increase satellite lifetimes significantly. Accomplishment of this feat can realize substantial savings in terms of dollars and manpower utilization. Extension in lifetime will require propulsion refinements. The development effort now proceeding in the electric propulsion area offers promise that the necessary propulsion improvements can be achieved.
Some typical USAF missions in space include communication relay, navigation aid, and data relay. Satellites performing these functions presently operate for not longer than five years.10 Hawk and others have presented some conceivable propulsion functions, along with representative total impulses, which would allow some very ambitious missions of the above types to be performed over a seven-year life.11 They considered the post-1975 time period as the earliest opportunity for incorporating electric propulsion systems into operational USAF satellites. Their findings are listed in Table I.
|
Function |
Orbit |
Total
Impulse |
|
Attitude control |
Sync Eq |
1,000 |
|
E-W stationkeeping |
Sync Eq |
3,000 |
|
Initial acquisition |
Sync Eq |
5,000 |
|
Reposition (D Ú = 200 fps) |
Sync Eq |
12,000 |
|
N-S stationkeeping |
Sync Eq |
60,000 |
|
Control apsidal drift |
i = 30°, e = 0.25* |
100,000 |
☆“i” is orbit inclination; “e” is orbit eccentricity.
The propulsion functions listed in Table I are necessary for acquisition of the correct orbit, overcoming forces that would remove the satellite from the proper orbit, orienting the satellite for means of communication, etc., and performing a small repositioning function. As can be seen, the performance of these functions requires large total impulses. Successful development of electric propulsion will enable extended life without degradation of the satellite mission.
With the advent of the space shuttle, another ideal mission for electric propulsion becomes identified. Through use of the shuttle, USAF satellites could be lifted to an altitude of approximately 100 miles. However, certain satellites for such purposes as communication can operate best from synchronous orbits—those occurring at approximately 20,000-mile altitudes and characterized by remaining fixed with respect to a point on earth. That is, the satellite in synchronous orbit and the earth rotate at the same rate, thus enabling two-way communication between the satellite and fixed antennas on earth.
The transitioning of a 1000-pound satellite from a 100-mile orbit to a 20,000-mile orbit would require approximately 365,000 pound-seconds of total impulse. Heavier satellites would require a correspondingly greater impulse. In this large total impulse regime, the high specific impulse furnished by electric propulsion devices is certainly attractive. As mentioned previously, the limitations on available satellite power mean that electric propulsion devices are presently thrust-limited. This thrust limitation necessitates larger transition times. However, for many orbit-raising requirements, time consideration should be minimal. For these cases, a large payoff in propellant weight savings can be realized through the employment of electric propulsion.
Another advantage that electric propulsion presently can offer is in the area of very fine attitude control. The fact that the capability to point antennas very accurately permits significant reductions in transmitter power lends great impetus to the search for new methods of achieving the necessary degree of control. A likely candidate for this application is the pulsed plasma thruster. This type of thruster can provide very small average thrust levels for very short periods of time, with the result that a satellite can be rotated in terms of arc seconds. Possible competitors to the pulsed thruster are mechanical devices such as gyro-controlled platforms or momentum wheels. Whether or not these mechanical devices will have the necessary lifetime for long-duration satellite missions is under debate.
The employment of electric propulsion devices for high-total-impulse space missions will allow significant savings in terms of utilized propellant mass. Thus, a substantial payoff can be realized through longer satellite missions, launch weight savings, increased payload, or a combination of these. As man learns to harness more effectively the vast amounts of available energy, electric propulsion systems could provide the capability of allowing extensive space repositioning to be accomplished in a reasonable amount of time and for a reasonable expenditure of propellant.
Hypothetical Future Electric
Propulsion Regimes
In his article entitled “The Relevance of Space,” Arthur Kantrowitz strikingly points out the payoff to be derived through the utilization of more efficient propulsion systems.12 He indicates that, if the energy consumed in lifting payloads into space were used as efficiently as consumer electricity on the ground, the cost of the energy necessary to put a 6600-pound spacecraft into low orbit would be less than $150. The high cost of space launches results partially from the relatively low energy available from chemical fuel, which causes the launching vehicle to be much greater in size and weight than the actual payload put into orbit. Through employment of very-high-thrust electric propulsion devices, the launching vehicle could conceivably be reduced to about the same size as the payload.
Once in space, the ability to operate at high thrust levels and correspondingly high specific impulses would enable the realization of sustained, large-scale maneuvering. At present, chemical rockets can rapidly maneuver on a relatively low total impulse basis. After that, their fuel will have been exhausted. The evolution of electric propulsion into the high thrust regime would enable rapid and sustained maneuvering.
The major impediment to the realization of high-thrust electric propulsion devices is the present lack of adequate energy sources for space applications. Solar arrays are presently thought capable of providing between 10 and 20 kilowatts of power. This level would allow fractions of a pound of thrust at a few thousand seconds specific impulse to be produced by an electric propulsion device. As mentioned earlier, this thrust level would allow accomplishment of certain repositioning maneuvers in which time constraints were minimal. At 50 to 100 pounds of thrust, electric propulsion devices would begin entering the rapid repositioning regime. To attain this level, approximately 10 megawatts of input power would be needed. This level represents a multiple of the presently available power of approximately one thousand. Thus we are talking in terms of space power breakthroughs.
Should we be optimistic that these power breakthroughs can and will be achieved? I think so. The reason for this optimism is that man is presently being pressed to develop new energy sources. As Peter Glaser points out, the limitations on fossil and non-fossil fuels will force us to develop options for the future.13 Hopefully, the technology developed to provide the necessary energy options will also allow us to make the desired advancements in space propulsion.
Adequate magnitudes of energy could be furnished to a spacecraft by two means. Energy sources, such as nuclear-electric devices, could be put aboard the craft. Alternatively, energy from a power plant could be beamed to the spacecraft. There is interest in each of these concepts at present.14
A development effort based on any of these concepts should concentrate on weight and size. At today’s level of technology, C. L. Dailey estimates that an on-board energy system capable of supplying 10 megawatts of power would weigh about 80,000 pounds.15 He also estimates that the receiving equipment necessary to implement the beaming concept would weigh about 40,000 pounds but with a greatly increased area requirement. The receiving equipment, which could be “rolled up” during launch, would need a deployed area encompassing about 210 meters on a side. Spacecraft relying on these energy systems could certainly be launched with a Saturn V into a low orbit. However, for purposes of faster response and greater maneuvering capability, it would be highly desirable to have systems reduced by a factor of 10 in weight and size.
Rough estimates of the possible availability of these energy sources have been made by several prominent individuals. Layton estimates that an on-board nuclear power source could be ready for spacecraft utilization in about eight years at a cost of $2 to $3 billion.16 Glaser and Krafft A. Ehricke, although not specifically thinking in terms of spacecraft applications, advocate the development of spatial power systems that could be used for a wide variety of functions.17 Once deployed in space, these systems could furnish beamed energy to a spacecraft when rapid repositioning maneuvers were desired. Through utilization of the space shuttle, Glaser and Ehricke predict that their systems could be ready or near-ready by the 1990s.
Exactly what would be gained by operating in the previously mentioned thrust range of 50 to 100 pounds or alternatively using the available energy to achieve specific impulses as high as 30,000 seconds? The answer to this question is that many new repositioning regimes would become available to military spacecraft. For instance, it might be desirable to station a backup spacecraft in a certain orbit; then, if a certain primary satellite serving a critical function such as data relay became inoperative, the backup spacecraft would be repositioned by using the maximum thrust level. Alternatively, if it were only necessary to reposition slowly, then the higher specific impulse could be utilized at a greatly reduced rate of propellant consumption. It should be remembered that input energy to an electric propulsion device can be traded off between thrust and specific impulse.
The idea of having a backup spacecraft in orbit would also be highly desirable for another reason. In case an unforeseen mission were to arise quickly, the backup spacecraft could be temporarily repositioned to undertake the new mission. It is the flexibility of this concept that could yield substantial payoffs.
The repositioning functions of interest would be orbit plane shifts, orbit raising or lowering, and position shifts in a certain orbit. Dailey has assessed the feasibility of performing large-scale orbital maneuvers in times of the order of a few days to a few months by means of a compact electromagnetic thrusting system.18 The energy for this system is derived from a beamed source. Dailey’s results are summarized in Table II.
|
Function |
Fuel Consumed |
Thrust |
Specific Impulse |
Time |
|
180° position change |
330 |
26.6 |
10,000 |
1.5 |
|
|
|
|
|
|
|
2400-mile orbit |
25,000 |
55.5 |
2,000 |
9 |
|
|
|
|
|
|
|
90° orbit plane |
25,000 |
45.7 |
4,375 |
26 |
Table II. Approximate repositioning times for an electromagnetic thruster operating in 50—100-pound thrust and /or high specific impulses regime. Ten-megawatt input power is assumed; spacecraft weight is 125,000 pounds. The difference between the power weight of 40,000 pounds and the spacecraft weight is comprised of the electromagnetic thruster configuration, propellant, structure, controls, and payload. These figures are based on estimated technology and would require the development of the power equipment and high-thrust electromagnetic device. (Data summarized from C. L. Dailey.)
The figures listed in Table II indicate that thrust levels in the 50- to100-pound range do indeed begin to allow us to enter the regime of rapid repositioning, even for very large spacecraft. However, much is to be gained by moving up to thousands of pounds of thrust. Chemical rockets are already at this level, but with specific impulses in the 200- to 400-second range. Thus, chemical rockets are highly effective for relatively low total impulse maneuvers. The sustained maneuvering capability will require systems that can more efficiently utilize their propellant.
This discussion has concentrated on the payoffs that could be realized through the employment of high-thrust electromagnetic devices for repositioning. But how about the actual launching of vehicles by electromagnetic systems? As mentioned at the first of this section, the use of high specific impulse devices to perform the launching function could reduce the size of the launching vehicle to roughly the size of the payload. For example, to place a satellite into low orbit, an orbital velocity of approximately 8000 meters per second is required. If the launch vehicle propellant were given a specific impulse of approximately 2400 seconds, then the system at launch would weigh only about 1.4 times as much as the weight placed into orbit. If a chemical system operating at 400 seconds specific impulse were used, then the system at launch would weigh approximately 7½ times the weight placed into orbit.
Again, however, the bugaboo of the electric propulsion systems is power. Acceleration levels of approximately 8 g’s would be desirable to place, say, a 1000-pound satellite into low orbit. If an electromagnetic thruster were tasked for the launch phase, then an input power of approximately 1.67 X 108 watts would be required. This power supply would weigh approximately 1.3 million pounds. Thus, means must be devised either to greatly reduce the weight of power supplies or leave the vast majority of them on the ground.
These examples show that learning to harness the vast amount of energy in the universe effectively will be paramount to the continued refinement of electric propulsion. Whereas the previously discussed concepts to harness energy could be considered “classical,” Alfvén and Kantrowitz are presently advocating some totally different ideas. In the area of launching vehicles by electromagnetic means, Alfvén is advocating transfer of readily available power from a ground network to a spacecraft in much the same manner that lightning is transferred between a cloud and the ground. He is calling this concept “energy transfer à la Zeus.”19 The possible advantage of this concept would be that very little airborne weight would be chargeable to either the power or propellant required by the electromagnetic thrusters. As an alternative to this method, Kantrowitz feels that the use of lasers will appear as an important possibility in the next decade. He contends that nothing we now know would rule out transferring the needed amount of energy to a spacecraft via advanced laser technology.
In the area of space repositioning, Alfvén advocates “sailing in the solar
winds.” These winds are caused by disturbances on the surface of the sun and
contain a great amount of electric power. The earth’s magnetosphere extracts
about 1012
Concepts such as these, although still in the elementary thought stage, could someday allow us to enter into the operating regimes for which we are searching. They definitely deserve serious consideration.
Electric propulsion devices now appear ideal for low-thrust, large-total-impulse space missions and missions in which spacecraft pointing must be very accurate. As more power becomes available, it is conceivable that high-thrust electric propulsion devices could become a reality. The advantages of elevating electric thrusters into the high-thrust regime would be that sustained, rapid maneuvering in space would be possible and the costs of spacecraft launches could be reduced. To move into the high-thrust regime, many times the presently available power will be required. The task will undoubtedly be very difficult. Yet all the significant technological breakthroughs of the past have required their fair share of effort. If the same amount of effort is allocated to achieving our high-thrust electric propulsion goals, I feel the necessary breakthroughs will come. I believe the propulsion department of the patent office is going to be busy for many years to come.
Air Force Rocket Propulsion
Laboratory
Notes
1. Hannes Alfvén, “Spacecraft Propulsion: New Methods,” Science, 14 April 1972.
2. Arthur Kantrowitz, “The Relevance of Space,” Bulletin of Atomic Science, April 1971.
3. T. O. Dobbins, “Thermodynamics of Rocket Propulsion and Theoretical Evaluation of Some Prototype Propellant Combustions,” Wright Air Development Center Tech Report 59-757, December 1959.
4. Daniel S. Goldin, personal communication.
5. Hughes Aircraft Company, FHUSA report presented at Space and Missile Systems Organization (SAMSO), December 1971.
6. Alfvén, op. cit.
7. Harry Petschek, “Magnetic Field Annihilation,” Proceedings of AAS-NASA Symposium on Physics of Solar Flares, NASA SP-50, 1965.
8. Charles Lee Dailey, “Magnetic Field Annihilation of Plasmas for Advanced Engine Design,” AFOSR TR-71-0328, 31 March 1971.
9. Peter Glaser, as interviewed in Astronautics and Aeronautics, February 1973, p. 10.
10. Ed Barth, personal communication.
11. Clark W. Hawk et al., “System Study of Electric Propulsion for Military Space Vehicles,” AIAA paper nr 72-493, April 1972.
12. Bulletin of Atomic Science, April 1971.
13. Peter Glaser, “A New View of Solar Energy,” presented at 1971 Intersociety Energy Conversion Engineering Conference, Boston, Massachusetts, August 1971.
14. J. Williams and K. Kirby, “Exploratory Investigation of an Electric Power Plant Utilizing a Gaseous Core Reactor with MHD Conversion,” presented at the Nuclear Power for Tomorrow Conference sponsored by the American Nuclear Society, 24 August 1972; W. Brown, “Transportation of Energy by Microwave Beam,” procedures of the Intersociety Energy Conversion Engineering Conference, August 1971.
15. Dailey, personal communication.
16. J. P. Layton, personal communication.
17. Glaser, Astronautics and Aeronautics; K. Ehricke.
18. Dailey, personal communication.
19. Alfvén, op. cit.
20. Ibid.
Major Richard S. Baty (Ph.D., University of California, Los Angeles) is currently a student, Air Command and Staff College, following an assignment in electric propulsion at the AF Rocket Propulsion Laboratory. Previously Major Baty served in the Minuteman program at Malmstrom AFB, Montana, and in Space and Missile Systems Organization (SAMSO), El Segundo, California. He has advanced degrees in aerospace engineering and control system engineering.
Disclaimer
The conclusions and opinions expressed in this
document are those of the author cultivated in the freedom of expression,
academic environment of Air University. They do not reflect the official
position of the U.S. Government, Department of Defense, the United States Air
Force or the Air University.
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