If, as Air Force planners logically contend, the atmosphere and space are a single operating continuum called aerospace, the inexorable pressure of operational requirements on technology must eventually marry the airplane to the space vehicle.
The object of the wedding is to conceive a winged offspring which can fly into orbit, rather than being shot there with large rocket boosters, and which can take off from and land on conventional airfields. The first successful flight of such a vehicle, into orbit and return, will truly mark the Kitty Hawk milestone of man’s conquest of space.
The “Spaceplane” concept has an awesome set of general requirements. It is envisioned as a self-contained, one-stage vehicle which uses air-breathing engines to maneuver in the atmosphere and to accelerate itself to satellite speed of about 18,000 mph. It must either carry enough fuel into orbit to maneuver extensively in space or be able to collect this fuel as it orbits in the upper atmosphere. Finally, the Space-plane must be able to withstand the heat of reentry, maneuver at very high speeds in the atmosphere as it returns to the earth’s surface, and land under power at relatively low speeds at any desired airfield.
Militarily, the attractiveness of Spaceplane is unquestionable. However, the first glance from the technical viewpoint indicates defiance of many of the physical laws which govern the design of aircraft, air-breathing engines, booster rockets, and reentry vehicles. It certainly pushes current technology to its limits, and in many areas the concept cannot be proved or disproved until more research is completed.
As with all vehicles which strain existing knowledge, Spaceplane has both strong proponents and strong critics. The argument is primarily over where Space-plane fits into the time schedule.
Few people contend that Spaceplane could never be built, but many question whether it could fly in the next ten years, even if it were given the highest national priority in a crash development program.
These critics point to the host of separate vehicles now under development, which considered in the aggregate could accomplish all of the missions Space-plane can and perhaps better. An example is the fact that large boosters such as Nova can put more weight into orbit in much less time. The reconnaissance and early-warning satellites, such as Samos and Midas, can accomplish these missions as well and perhaps as cheaply as Spaceplane. It is also probable that the Spaceplane, which will need a very large volume to carry its load of hydrogen fuel, will never be able to maneuver as well during reentry as a heavier-for-its-size Dyna-Soar-type hypersonic glider.
In effect, it is the multipurpose aircraft requirement carried over into the space field. It is very difficult to say whether it is better to have a group of high-performance specialized vehicles or a multipurpose vehicle which can do many of the necessary jobs but none of them at top performance. But the increasing cost and complexity of individual weapon systems make the multipurpose approach an attractive one.
The Spaceplane proponents do not suggest that the current space programs be canceled and that all effort be put on the all-purpose vehicle. They do believe emphatically that a single-stage vehicle capable of aircraft-type takeoffs and landings, which can carry men and a sizable payload in between the atmosphere and space almost at will, will be the foundation of the space-vehicle program during the 1970s.
The best indication available today that the Space-plane is feasible and can be flown before 1970 is that a sizable group of aeronautical experts sprinkled through industry, government research agencies, and the Air Force not only believe that it can be done but are enthusiastic about it.
Several manufacturers have already submitted preliminary type Spaceplane proposals to the USAF. These have been evaluated and considered in the light of proposals from government laboratories and from within the Air Force. The USAF budget for fiscal 1962 contains money for more detailed Space-plane studies and for state-of-the-art experiments.
Not even the enthusiasts, however, claim that Space-plane will be an easy technical development. Much of the current US research and development effort, such as Dyna-Soar, the X-15, and state-of-the-art work in high-temperature structures, high-speed stability and control, etc., will feed valuable information into the Spaceplane project. But one field of experimental research vital to the project has been virtually abandoned in recent years, and there can be no sensible hope for a true one-stage Spaceplane unless large-scale research in this area is revived.
The missing technical link in the Spaceplane concept is the air-breathing engines that can operate at hypersonic speeds. Air-breathing propulsion systems theoretically can eliminate the need for high-thrust rocket boosters by drawing their oxidizer supply from the atmosphere. Since the weight of oxygen needed is much larger than the fuel weight, hypersonic air-breathers offer the hope of very light, orbital propulsion systems.
The key to hypersonic air-breathing engines is the ability to burn the fuel externally. In effect, the engines must be turned inside out so that their hot parts will be exposed and can be cooled by radiation. The air entering a conventional enclosed engine at hypersonic speeds would be literally too hot for the engine component to handle. And there would be no way to further raise temperature and therefore produce thrust by adding “fuel to the flame.”
External burning has been studied theoretically for many years, but the only extensive experimental research effort was conducted at the Lewis Laboratory of the National Advisory Committee for Aeronautics in the middle 1950s. This research proved conclusively that external burning would work at relatively low Mach numbers. It also cleared up enough theoretical unknowns to convince many thermodynamicists and engine designers that it would work through the high Mach number range right on up to orbital speeds and at very high altitudes.
The external-burning effort was not continued, however, and it was abandoned along with all other air-breathing engine research when the NACA became the National Aeronautics and Space Administration. The decision to drop all other air-breathing work was perhaps the most controversial one yet made by NASA. It raised strong protests from within industry, the military, European aeronautical circles, and within NASA itself. It forced the professional reorientation of the research scientists at Lewis Laboratory who had achieved worldwide eminence for their efforts with air-breathing engines. The decision not only weakened any Spaceplane or air-breathing booster development but limited hypersonic aircraft configurations to the essentially one-shot, rocket-powered, boost-glide type.
During the past couple of years theoretical work with external burning has continued, primarily in industry. Further experimentation is needed immediately, however, to obtain detailed design data and to bring the Spaceplane onto more solid ground technically.
The development problems of a Spaceplane extend far beyond external burning, and they occur in three of its basic modes of operation, which are flight into orbit, maneuvering in space, and reentry into the atmosphere.
When the Spaceplane takes off in the conventional manner and accelerates to a speed of around 18, mph while climbing to an altitude of 200 miles or more, its flight will resemble much that of a large rocket as of an airplane. An analysis of this flight into orbit must be made from the standpoint of both types of vehicles.
Fundamentally, the rocket vehicle is much easier to analyze than the hypersonic airplane. There are two basic factors which influence the ability of the ICBM-type rocket or a large space booster to accelerate to orbital speed, and they are just as important to the Spaceplane as they are to the rocket. These factors are the vehicle’s mass ratio and the specific impulse of its propellants.
The mass ratio is the total takeoff weight of the vehicle divided by its weight after all fuel has been consumed and the engines stop. Mass ratio is an indication of the lightness and efficiency of the vehicle’s structure, and it is a dimensionless number.
The specific impulse is a measure of the energy released by each pound of propellant. Its definition is the pounds of thrust produced by each pound of propellant burned each second, so that the specific impulse is given in seconds for each propellant combination of fuel and oxidizer.
In terms of practical numbers the propellants currently used in operational rocket boosters have specific impulse ratings of around 250 seconds or a little more This means that a single-stage rocket would have t have a mass ratio of around fifteen to achieve orbit speed if it carried very little payload. Adding a large payload—so that something useful can be done with the rocket after it is in orbit—means that the mass ratio would have to be increased significantly.
Unfortunately, the best mass ratio that can be achieved with any large single-stage vehicle today, using the construction materials which are available, is only about seven or eight. So it is not possible to use single-stage boosters to put even an empty shell into orbit.
The effective mass ratio of large rocket booster systems is increased by using the stage or step principle by which it is possible to discard dead weight in flight. For example, the effective mass ratio of a three-stage rocket is approximately the product of the mass ratios of the separate stages. This powerful design tool makes it possible to take three sturdy, structurally conservative rocket stages with mass ratios of three and connect them with equally sturdy and reliable interstage structures and come up with a complete vehicle that has a mass ratio potential of nearly twenty-seven. Therefore, such a vehicle could carry a sizable payload into orbit using current propellants. Five stages are about the practical limit.
As long as the multistage principle is the only method used to increase performance, the size of the complete booster vehicle goes up rapidly when the payload is increased. The Saturn and Nova vehicles now under development are good examples. While it will be available in several configurations, the Saturn’s capability generally is to put approximately 35,000 pounds up into a low orbit with a total vehicle takeoff weight of around 1,350,000 pounds. Preliminary designs on the Nova show that it will be able to put up about 400,000 pounds in a low orbit for a maximum vehicle weight somewhere around 10,000,000 pounds.
The other route to better rocket vehicle performance is to use improved propellants with increased specific impulse. The liquid hydrogen-liquid oxygen high-performance propellant combination now coming into wide use will give an improvement of twenty-five percent or better over liquid oxygen-kerosene, which is the most common operational combination today. Specific impulse of the hydrogen-oxygen system is over 300 seconds in most engines, but this still isn’t high enough to get a one-stage vehicle into orbit with any kind of a payload.
Big improvements in specific impulse are in the development mill and undoubtedly will become operational around 1965 or shortly thereafter. The nuclear rocket being pursued in the NASA-Atomic Energy Commission Project Rover will have a specific impulse of around 700 and possibly much better. Thus the Rover rocket will be able to put a one-stage vehicle into orbit if it is possible, from a civil safety point of view, to operate nuclear rockets in the atmosphere. If not, the nuclear rocket will be sent into orbit by chemical boosters where it will be started and used to send large payloads farther out into space.
A new chemical rocket (described on page 108), which combines the liquid- and solid-fuel ideas into one engine, has shown the potential of achieving a specific impulse of 500 seconds or so. This hybrid rocket could place a large payload into orbit using a single-stage vehicle, and it is possible that it could be ready for service long before the Rover rocket.
However, strictly from the specific impulse point of view there are no large-thrust engines on the horizon that have the potential of the system planned for the Spaceplane. The Spaceplane propulsion system will burn hydrogen fuel with air, and its fuel specific impulse will be about 6,000 seconds. All air-breathing propulsion systems have much larger fuel specific impulse figures than rocket engines through burning the oxygen in the atmosphere rather than carrying an oxidizer along in the vehicle. Fuel specific impulse of good hydrocarbon-fueled turbojets is about 2,000 seconds, and the hydrogen is better because its energy per pound is much higher.
The very high specific impulse of air-breathing engines does not automatically mean that they have the potential of propelling a single-stage vehicle into orbit. There is the major problem of keeping the air-breathers operating at all of the necessary speeds and altitudes. However, even if this were no problem, hydrocarbon engines probably would not be able to send a one-stage airplane into orbit because the requirements for mass ratio and aerodynamic efficiency would get too high.
Several factors combine to make the hydrogen engines proposed for the Spaceplane marginal for their task of sending the one-stage airplane into orbit. These are the factors which have always plagued aircraft designers when they were trying to reach higher speeds or provide more range. The factors are the, airplane’s lift/drag ratio and the excess power available under all flight conditions.
The lift/drag ratio depends upon the total aerodynamic efficiency of the airplane, its wings, fuselage, tail surfaces, etc. If the lift/drag ratio is high, then the power required is low. The dramatic effect of improving lift/drag ratio was evidenced with the B-70 supersonic bomber. When the design was first studied it was predicted that a lift/drag ratio of four would be available at the Mach 3 cruise speed. This meant that the engine thrust must be one-fourth of the weight of the airplane. It was impossible to carry enough fuel to achieve long range with this sort of aerodynamic efficiency. Through an extensive research effort the lift/drag ratio was raised to eight so that the power required for the B-70 was cut in half, as was the fuel consumption.
The other vital factor to a constantly accelerating airplane is the excess thrust available at all flight speeds and altitudes. If the thrust available is just equal to the total drag in pounds, the aircraft can maintain level flight, but it cannot accelerate or climb. If only a ten percent margin of power is available, then the aircraft will accelerate so slowly that it probably will consume its fuel long before it reaches orbital speed. Modern supersonic aircraft need at least a thirty percent power margin over most of their speed range to accelerate efficiently to their top speeds. It is probable that the Spaceplane will need a substantially higher margin of excess power to reach orbital speed.
In many ways the rocket is the ideal engine for acceleration and climb. Its performance gets better as the altitude increases, and it consumes fuel rapidly so that the vehicle weight goes down quickly. The excess power margin therefore goes up rapidly during a rocket flight. Also the rocket leaves the atmosphere so rapidly that its aerodynamic efficiency can be disregarded in a general discussion.
In contrast, the airplane’s climb and acceleration performance is extremely critical because the thrust of air-breathing engines decreases at the higher altitudes and the lift/drag ratio decreases at the higher speeds. Therefore, the power available decreases as the power required increases. In this situation the thrust margin for acceleration and climb can quickly get too small for efficient flight or can disappear altogether so that a definite limit is placed on maximum speed and altitude performance.
The Spaceplane proponents believe that they will be able to maintain a satisfactory power margin over the entire range of the Spaceplane flight speeds. At subsonic speeds, for takeoff and acceleration through the very high drag region near Mach 1, the Spaceplane undoubtedly will have some sort of turbofan engine. Somewhere near Mach 2 the external burning will be initiated, with the turbofans probably shut down and the ducts closed off somewhere around Mach 3. In theory, it now appears possible for the external-burning engines to maintain an adequate margin of excess power on up to orbital speeds.
It is certainly conceivable that the excess power available will go to zero sometime during the Spaceplane flight either because the engine thrust drops off in a certain speed range or the lift/drag ratio gets very low. If this is impossible to correct, then consideration probably will be given to carrying rocket engines along to provide the power necessary to pass through the critical speed range.
Proper operation of the external-burning engines is keyed to one main question. The hydrogen fuel must be burned in a supersonic flow when the Spaceplane is flying at high hypersonic speeds, and it has not been positively established that supersonic combustion is possible. In the external-burning experiments conducted to date the free stream Mach number has been around 2 to 6 so that the flow on the after portions of the wing has been slowed down through a couple of strong shock waves and is still subsonic. If supersonic combustion proves possible, then the efficiency of conventional enclosed ramjets can also be increased significantly in the Mach number region of about 4 to 8.
External-burning systems must also be integrated into an aircraft configuration with more care than conventional engines. It apparently will be possible with external burning to improve the pressure distribution around a hypersonic airplane and improve its lift/drag ratio considerably.
Once the Spaceplane has achieved an orbit, there are many missions possible for it to perform. These missions include: rendezvous with other space vehicles to either join or inspect them; launch of both offensive and defensive weapons; provide long-term observation and reconnaissance and maintain an advantageous position from which it may launch a glide attack into the atmosphere. All of these missions have one requirement in common, and that is a need for maneuverability. The most effective space vehicles will undoubtedly be those which have the greatest maneuvering capability. Two general categories of engine are now being developed or are available to maneuver in space. First, there is the chemical rocket which will provide high thrust and rapid maneuverability but needs a large supply of propellant. Second, there are the electric engines, the ion and plasma rockets, which provide low thrust and slow maneuvers. These engines do not need a large supply of propellant, but they require a large fixed weight in the electrical-generating machinery, which supplies them power.
It is probable that the Spaceplane and other military vehicles operating in space near the earth will need to maneuver rapidly and will use chemical engines to do this. Most of the Spaceplane ideas being studied today incorporate a novel idea which will provide the Spaceplane with a good maneuvering capability even though it doesn’t carry a large propellant load into orbit. This idea is to carry some light machinery which can liquify the atmospheric oxygen available at an orbital altitude of sixty to seventy miles. The machinery would be run by liquid hydrogen.
Studies of this system show that a Spaceplane with a takeoff weight of about 500,000 pounds can climb into orbit with the necessary machinery and hydrogen fuel load to store about 500,000 pounds of liquid oxygen taken from the atmosphere. There will still be enough hydrogen aboard to burn with the 500,000 pounds of liquid oxygen in a rocket and provide a large maneuvering capability near the earth. Eight times more weight of oxygen than of hydrogen is required in the chemical rockets so that less than 50,000 pounds of hydrogen fuel needed by the rockets plus a much smaller weight of hydrogen fuel for the oxygen collector must be carried into orbit to provide the maneuvering potential. The drag of the Space-plane while it is collecting the oxygen will be overcome by external burning or possibly by a small rocket.
From the military point of view there is still one major drawback to the oxygen-collection system, which has been studied extensively by Antonio Fern of the Polytechnic Institute of Brooklyn and Sterge T. Demetriades of the Northrop Corporation. With the presently proposed systems the oxygen cannot be collected quickly, and it will take in the neighborhood of 100 days to store 500,000 pounds of liquid oxygen with a system that can be carried in the Spaceplane.
Fundamentally there are two basic space maneuvers. One is to change orbital altitude while staying in the same orbital plane, and the other is to change planes while holding altitude. There are an infinite number of powered maneuvers which are combinations of these two.
Changing orbital planes requires considerably more energy than changing altitude so that the plane of the original orbit of a Spaceplane will have a strong influence as to whether it can accomplish any given task. To illustrate the comparative energy requirements, it takes a velocity change of around 14,000 feet per second to change the orbital plane forty degrees at an altitude of 1,000 nautical miles, and it requires a velocity change of about 1,400 feet per second to change from a circular orbit 500 miles high to one 1,000 miles from the earth.
The Spaceplane will make a glider-type reentry, probably similar to what is now planned for the Dyna-Soar. Initially, the angle of attack will be very high, close to ninety degrees, and this will be held during the very high heating period. Consequently, the bottom of the Spaceplane will be of heavier, more heat-resistant construction than the top surfaces. Somewhere below Mach 15 the angle of attack will be reduced, and the Spaceplane will fly more like an airplane.
There is one major design difference between the Spaceplane and the Dyna-Soar. The Spaceplane will be a very large vehicle, probably well over 150 feet long, and it will have a very large tank space for the liquid hydrogen it must carry. Liquid hydrogen weighs only about four pounds per cubic foot as compared to kerosene and liquid oxygen, both of which weigh around sixty pounds per cubic foot. Since a Spaceplane weighing 500,000 pounds at takeoff would have to carry in the neighborhood of 200,000 pounds of liquid-hydrogen fuel, its tank space would be something like 50,000 cubic feet just for the hydrogen, which results in a very large vehicle.
During the reentry, however, this almost empty tank volume aids the Spaceplane considerably. The Space-plane will essentially be a large empty shell on the way to the ground, and its wing loading will be very low. The low wing loading results in a low heating rate, and it is presently believed that the heating rate is so low that the Spaceplane structure can be cooled completely by radiation. If complete radiation cooling is possible, then the skin can be very thin; very little insulation and no cooling system will be required under it. In other words, the Spaceplane can be built with much the same structural concepts used on current Mach 2 aircraft because essentially all of the heat generated by air friction will be radiated away by the skin. Therefore, the main structural problem is to get skin materials which have slightly better radiation efficiency than those available today. It is believed that this will be possible in the next four or five years.
The Dyna-Soar heating problem is more severe because it is a small dense vehicle with a relatively heavy wing loading. The higher wing loading raises the heating rate and requires the heavy use of insulation and cooling equipment for certain types of reentry along with radiation cooling.
The Spaceplane’s very light wing loading will make its landing a relatively simple matter regardless of the configuration that is finally chosen for it. Its landing speed and sink rate will be well below those experienced with the X-15 and the Dyna-Soar.—End
This article marks the editorial debut with AIR FORCE/SPACE DIGEST of J. S. “Sam” Butz, who has joined our staff as Technical Editor. Mr. Butz is thirty-three, a native of Gainesville, Fla., where his family has been in the newspaper business for years. Immediately following World War II, Mr. Butz served a stint with the Army’s airborne troops, with duty as an instructor at the Parachute School, Fort Benning, Ga. In 1952 he obtained his Bachelor of Science degree in Aeronautical Engineering at the University of Florida. He worked on aerodynamic problems for the McDonnell Aircraft Corp. in St. Louis for more than two years, then for engineering-consultant firms whose clients included Canadair and the Martin Company. In 1957 he joined the New York staff of Aviation Week as an engineering writer, coming to Washington the following year. He is married, has four children, and lives in Fairfax, Va. He will contribute regular articles on technical subjects and write a monthly column, “Tech Talk” (see page 108).