Document created: 24 March 04
Air University Review,
September-October 1972
William G. Holder
Major William D. Siuru, Jr.
In September 1969 an elite group of men headed by Vice President Agnew presented President Nixon the results of an investigation that may well set the course for exploration and the use of space during the next two decades. The Space Task Group (STG) proposed the direction for future space endeavors, the goals for future space vehicles, and finally the vehicle concepts that would satisfy these goals. According to the group’s report to the President, our space program should attack the space frontier for many reasons: practical benefits to mankind, advancement of science, exploration of the universe, maintenance of national pride and prestige, and, finally, national security. Since our national budget for space is and probably will continue to be severely limited, these space tasks must be completed with the greatest efficiency and economy. The keys to this efficiency and economy for future space operations and explorations are reusability and commonality of components and the availability of effective advanced propulsion system technology.
A reusable Space Transportation System (STS) was recommended by the STG as a means of decreasing the cost of space operations and allowing exploitation of the space environment for the benefit of mankind. The STS as proposed would include a two-stage space shuttle, consisting of a booster and an orbiter, that would operate between the earth and low-altitude orbits for delivering and returning passengers, supplies, equipment, and spacecrafts. A second element of the STS is a high-energy upper stage, the orbit-to-orbit shuttle or space tug, which would transfer payloads from low earth orbits to high-energy orbits. A nuclear-powered upper stage could be considered that would be used for carrying crews and equipment into lunar orbit and into deep space.
An advanced propulsion system—the high-pressure staged combustion rocket engine—is currently being developed to support the space shuttle. The nuclear rocket is being developed as a potential propulsion system for interplanetary applications. A third propulsion system, the composite rocket/air-breathing engine, while not currently under development, has shown promise as a potential replacement for pure rocket engines sometime in the future.
current rocket technology
The 1960s saw a tremendous advancement in large liquid-rocket engines. These ranged in size from the 205,000-pound-thrust H-1 engines used in a cluster of eight to power the first stage of the Saturn lB to the five 1.5-million-pound-thrust F-1 engines used on the Saturn V’s first stage. While both these engines use liquid-oxygen/kerosene type propellants, equal strides have been made with the more energetic liquid-oxygen/liquid-hydrogen propellant combination. These range from the 15,000-pound-thrust RL-10 engine used in the Centaur stage to the 230,000-poundthrust J-2 used so successfully in the upper stages of both the Saturn lB and Saturn V.
Although these space engines have proved to be highly reliable and extremely efficient, engineers have for some time been looking for ways to improve them. It is reasonably certain now, as it was in the 1960s, that the near-future propulsion systems, like those for the shuttle, will be derived from today’s chemical-rocket technology.
It was realized a number of years ago that one of the best methods of “getting more” from chemical-fueled engines was to design them to operate at higher chamber pressures. (High chamber pressure means more thrust per pound of propellant expended.) To obtain this increase in chamber pressure, it is necessary to transfer the propellants from storage tanks to the combustion chamber under a much higher pressure. This, of course, means more complex and sophisticated plumbing and turbine-driven fuel and oxidizer pumps. In light of the space shuttle application, high-pressure turbopump technology was examined to determine what problems might exist and what if any new technology would be required. It was concluded that there do not appear to be any insurmountable problems in pumping cryogenic propellants at pressures even several times higher than those found in today’s high-pressure engines. Since 1961, over $100 million has been spent by both industry and the government on high-pressure engine technology. The Air Force high-pressure technology XLR-129 engine program provided the base for the shuttle engine development. Probably no other engine development has ever started with such a strong technical base as the engine to be developed for the space shuttle.
The engine proposed for the shuttle’s upper, or orbiter, stage will use an advanced concept known as “staged combustion.” Staged combustion is very similar to that of a turbojet equipped with an afterburner; that is to say, there are two different stages of combustion. Whereas in the turbojet the first combustion occurs in the main chamber, the shuttle engine’s first burning is in the gas generator or preburner. The purpose of the gas generator in the shuttle engine is identical to that of any other pump-fed rocket engine—to provide the gases that turn the turbine(s) that turn the fuel and oxydizer pumps. However, there is a difference in this engine’s operation. In the normal engine, the gas generator gases are ported overboard after driving the turbine. The shuttle engine will use them again. Thus there is little energy lost in the cycle, and a significant increase in efficiency can be realized.
Unlike the current expendable launch vehicles, the shuttle will be used many times, thus cutting costs to the bone. These many reuses will cause stringent requirements on the orbiter’s rocket engines, for they too must be reusable to keep the costs down.
The high-pressure engine currently being designed for the shuttle’s orbiter stage will probably be the most advanced rocket engine ever built. Developing well over 400,000 pounds of thrust, it will be slightly less than one-third as powerful as the F-l engine. But its chamber pressure will be about 3000 pounds per square inch, or about three times that of the F-l. And since it must be usable for many flights, it must survive many firings, which will accumulate hours of total operation. After a certain specified time period, the engine will be overhauled and then start a new life on the shuttle.
The shuttle engine will use a conventional bell-shaped nozzle incorporating a “two-position” extension; that is to say, there will be two distinct parts to the nozzle. The upper portion of the nozzle, which will be the most effective at low altitudes, was originally planned for use on the booster stage. However, recent NASA decisions have dropped the use of this engine in the booster stage. This, however, does not mean that it may not be incorporated in the shuttle at some later time.
For the orbiter stage, the lower part of the nozzle will be stowed during the booster burn. Then when the orbiter is brought to life, the nozzle extension will be deployed into position, thus increasing the nozzle’s exit area and providing better engine performance at higher altitudes. With only the basic nozzle, an expansion ratio of 60 is possible, while with the nozzle extension in place the expansion ratio is increased to about 150. The engine will also have the capability to throttle down to one-half the rated thrust.
All this discussion has probably created the impression that the shuttle’s orbiter engine will be one of the most complicated and sophisticated rocket engines ever built. The requirements placed upon this engine will be severalfold greater than those placed on any previous engine. It will have to operate efficiently and reliably, since it may well be the only new launch vehicle propulsion system for the next decade.
composite engine concept
An advanced concept for space propulsion which may offer certain advantages is the composite engine. This engine is in reality a combination of several different types of propulsion systems. In the composite concept, each engine type would be utilized in that part of the trajectory where it could perform most efficiently. Let’s break down a typical space vehicle’s flight to and from orbit into several phases and look at the individual engines that do the best job for each phase.
First of all, the vehicle must be lifted off the ground. A rocket does this job best, since it provides the high thrust required to start the fully loaded vehicle on its way. After the vehicle is moving sufficiently fast, a very efficient ramjet can be used. Since the ramjet uses air to oxidize the fuel, the vehicle need draw only fuel from its tanks. Up to a vehicle velocity of about 3500 to 4000 miles per hour, the burning of the propellants in the combustion chamber can be done at subsonic speeds. In other words, although the vehicle will be flying supersonically, the airflow through the engine itself will be reduced to subsonic speed. However, after the vehicle is moving at speeds above 3500 to 4000 miles per hour, the airflow cannot be reduced to subsonic speeds, and the burning of the fuel must occur at supersonic speeds. The result is a supersonic combustion ramjet, or what is popularly called a SCRAMJET.
The SCRAMJET would operate at speeds of about 7000 to 10,000 miles per hour. By this time the vehicle has reached such an altitude that the atmosphere is extremely thin, without enough air to burn the propellants. Now a rocket engine, with its self-contained oxidizer as well as fuel supply, must be used. The rocket would power the vehicle the rest of the way to orbit.
Once in orbit, any required maneuvering could be done with the rocket engines. To get out of orbit, the vehicle must be slowed down. This slowing down process could be done with a retrofire from the rockets. After re-entry, when the vehicle is near the landing site, the ramjet could be started up again for loitering and to assist in the touchdown. Or perhaps a turbofan, like those found on many of today’s commercial airliners, could be used. The vehicle using these different engines has a great advantage over a straight rocket vehicle. The propellant load is much less since, while fuel must be carried for the entire trip, only the small amount of oxidizer required for the rockets must be carried.
We could build a future space vehicle with all these different types of engines placed separately aboard. Or, preferably, we could have a single engine that would operate as a rocket, a ramjet, a SCRAMJET, and a turbofan. An engine that could incorporate all these characteristics would be called a composite engine. This engine not only would operate in each of the modes mentioned but also might operate simultaneously in more than one of the modes. Let’s see just how the composite engine might operate.
At lift-off, the rocket would be firing, and the turbofan might also be operating to supply additional air to improve the performance of the rocket. After the vehicle is moving at a greater speed, the ramjet would start operating, and the turbofan would be removed from the airstream. The pure rocket would continue to operate briefly to aid the ramjet. As the speed continued to increase, the ramjet would convert into a SCRAMJET. As the vehicle reaches the outer fringes of the atmosphere, the inlets would be closed, and the pure rocket would be used alone for reaching, maneuvering in, and leaving orbit. To return to base after re-entry into the atmosphere, the ramjet and/or the turbofan might be used separately, or they might be used like an afterburning turbofan.
To improve the performance of such a space vehicle even more, we would like to get away from having to carry any oxidizer for the rocket portion of the flight. In other words, the less propellant that must be stored, the more room there is for payload and astronauts. To accomplish this, an air liquefaction system would be required to convert to liquid oxygen the air that would be scooped in as the vehicle traveled through the atmosphere, and the oxygen would either be burned immediately in the rocket portion of the composite engine or be stored for future use when the vehicle is above the earth’s atmosphere. To convert air to liquid oxygen, a means of cooling the air to a very low temperature is required, and also a way to separate the oxygen from the other constituents of the collected air—specifically, nitrogen. The cooling could be done with the on-board liquid hydrogen that is used as the fuel for the composite engine. This liquid hydrogen would be carried in tanks at temperatures below –400 degrees F. The nitrogen separated from the air could be used to improve the performance of the SCRAMJET. If a performance penalty could be accepted, liquid air rather than liquid oxygen could be used with the liquid hydrogen in the rocket. This would eliminate the need for a separation device, which today requires a rather large advance in technology to make such a device light enough for a flying vehicle and economical enough to achieve a payoff for a reusable vehicle.
While the composite engine is not nearly as far along in development as the high-pressure rocket engine previously discussed, enough basic work has been completed to gain a better understanding of its advantages and problems. The individual components of the composite engine (i.e., rockets, ramjets, SCRAMJETS, etc.) are fairly well understood at this time, but additional work is required to integrate and test them as a single unit.
nuclear rocket
Future engines for space will mate the tremendous energy available from nuclear explosions with the ability of a rocket to operate at high thrust levels in the vacuum of space. While several high-thrust nuclear rocket concepts have been investigated, the one that will probably be used first in an actual space vehicle is a solid-core thermal reactor engine.
The heart of a nuclear engine is the reactor core. The heat given off by this reactor heats the propellant, usually liquid hydrogen, adding energy to it. This high-energy propellant is then accelerated to a very high velocity in the nozzle, thus producing the rocket’s thrust. The reactor must heat the hydrogen to temperatures of almost 4000 degrees F. To keep the reactor core and nozzle from melting at such extreme temperatures, they must be cooled. For this purpose a double-walled nozzle and reactor can be used. Cold hydrogen is circulated inside this double wall on its way to the reactor core. This method of cooling not only takes heat from the nozzle and reactor but also improves the overall efficiency of the engine, since this heat adds energy to the hydrogen even before it reaches the reactor.
The amount of heat the reactor adds to the hydrogen is tremendous. In an engine of the size that might be used in a spacecraft bound for Mars, almost three tons of hydrogen is raised from –300°F to 4000°F every minute. The reactor is made from graphite; however, if pure graphite were used in contact with the hydrogen, the hydrogen reacting with the hot graphite would quickly erode the reactor. To prevent this erosion, the reactor passages are covered with a metallic carbide coating. Not only are the high temperatures a source of problems, but so are the long operating times required of a nuclear rocket. On a Mars trip, a nuclear rocket might have to operate continuously for well over an hour. In comparison, on the Saturn V the longest any rocket engine operates is only about eleven minutes.
The liquid hydrogen is contained in the propellant tank at a pressure of about 30 pounds per square inch; but for the engine to work efficiently, hydrogen pressure must be increased to about 1000 psi. A pump driven by a turbine is used to increase the pressure. The turbine is in turn driven by hot hydrogen that has passed through the cooling walls on its way to the reactor. Thus, some of the energy gained in cooling the engine is given up to pumping more propellants through the engine.
A nuclear engine itself is heavier than a normal chemical-rocket engine because of the shielding required to protect the surroundings from radioactivity, the high temperatures involved, and the longer and more rugged operating durations. Also, because hydrogen is so light, relatively large tanks are needed for propellant storage. Fortunately the performance of the nuclear rocket more than makes up for its being heavier than a normal rocket. The specific impulse of a nuclear rocket is about twice that of even the best chemical rocket. A liquid-oxygen/liquid-hydrogen engine, like the engines used on the S-IVB stage, has a specific impulse of 430 seconds, whereas a nuclear rocket has a specific impulse of over 800 seconds. To illustrate the effect of this difference in specific impulse, one might compare a nuclear-powered reusable vehicle with a chemical-powered vehicle in performing a specific mission. For example, on a mission requiring the vehicle to deliver a payload to lunar orbit I and then return empty to an earth-orbiting space station, a nuclear vehicle could carry three times as much payload for the same expenditure of propellants. For other high-energy missions the comparisons are equally dramatic.
A nuclear-powered space vehicle could perform many roles. One possible nuclear engine application in the future might be in a multipurpose interorbital and planetary shuttle. Such a vehicle would travel from a space station in near-earth orbit to establish and supply space stations in other orbits, including synchronous orbits and orbits about the moon. A nuclear stage with its high performance could easily make the round trip to these intraspace destinations with large payloads and return to the near-earth space station for refueling and reuse. A nuclear stage could have sufficient capacity to place entire space stations in lunar orbit, or earth-synchronous orbits, and still have sufficient energy to return to the home station.
Several of these nuclear stages could be strapped together to form the launch system that could take men to Mars as early as the 1980s. While there are many concepts under consideration for making the trip, they all depend on nuclear propulsion.
In any case, the nuclear stages would have to be launched into space by a chemically fueled launch vehicle. The nuclear stages would be launched totally fueled and ready for operation on top of the chemically fueled launch vehicle, or they could be launched empty with additional stages used to bring up the fuel.
The nuclear stage would also be useful for seeding space with unmanned satellites having numerous applications—for example, communication, meteorology, and earth resource survey. Whatever purposes may be decided for its use—and there are many possibilities—the nuclear engine will not be operational for many years.
The future generation of space travel and exploration presents challenging problems for the propulsion engineer. It appears as though space operations in the near future will depend upon the mainstay of the 1960s, the chemical rocket. However, some new additions to space propulsion, namely, the nuclear rocket and the composite engine, may provide new means for accomplishing space missions in the future.
Dayton, Ohio
William G. Holder (B.S.A.E., Purdue University) is a space systems analyst with the Foreign Technology Division, Air Force Systems Command, Wright-Patterson AFB, Ohio. He has worked with the Boeing Company on the Bomarc B and the Saturn V. As a lieutenant in the U.S. Army, he served three years as an air defense guided missile instructor. Mr. Holder is the author of a number of technical articles and a book, Saturn V—The Moon Rocket (1969).
Major William D. Siuru, Jr., (M.S., Air Force Institute of Technology), is currently working on his doctorate in engineering at Arizona State University. He was a project engineer in the XB-70 program at Wright-Patterson AFB, Ohio, and did advanced space and launch systems planning at the Space and Missile Systems Division, Air Force Systems Command. His last assignment was as Chief, Space Launch Systems Branch, Foreign Technology Division, AFSC.
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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