Propulsion: Many Roads Go Up

Feb. 1, 1961

Part I—The Air-Breathers

The technological revolution of the past decade and a half has left its imprint on almost every facet of air and space vehicle development.

Nowhere is this more dramatically illustrated than in the vital field of propulsion—key to range, payload, and speed.

Technical progress since World War II has opened up innumerable types of possible engine systems com­pared to the few that were feasible in the early 1940s.

At that time reciprocating engines were the prin­cipal type in development. Turbojets, ramjets, and rocket engines were just emerging from the labora­tories of a few persistent pioneers.

Today the Air Force is investigating or develop­ing turbojets, ramjets, liquid-fuel rockets, solid-fuel rockets, nuclear rockets, nuclear ramjets, nuclear tur­bojets, ion rockets, and plasma rockets. Specific engines within these general classes include: turborockets, turboramjets, external-burning ramjets, turbofans, lift fans for VTOL use, hybrid rockets using liquid and solid propellants, spherical-shaped solid rockets, rockets with plug nozzles, clustered rockets, seg­mented rockets, direct-cycle nuclear turbojets and ramjets, and indirect-cycle nuclear turbojets and ram­jets—among others.

The uses to which these engines can be put are as varied as the engines themselves. Many types of recon­naissance and bombardment vehicles, for use both in space and in the atmosphere, are being studied by the Air Force. Some of these vehicles have recognizable wings, others derive their lift from their body geom­etry, others are symmetrical and will produce little or no lift. Regardless of their mission the performance of each of these vehicles will depend in high degree on the effectiveness of its propulsion system.

Many improvements in existing engine schemes can be expected, some of them of a radical nature. It is also possible that, in the decade ahead, some funda­mentally new means of propulsion will appear.

Development costs are extremely high. The power plant normally is the principal expenditure item for any weapon system, and its development can run to several hundred million dollars. If a major basic- or applied-research program is involved, the total cost of obtaining a satisfactory engine can go to a billion dollars or more.

Management skill is a key factor, perhaps the most important requirement for orderly and efficient pro­pulsion programs of the future. The lead times for new engines are so long, the needs for men, facilities, and dollars are so large, and the technical choices so diverse that there is small room for major error or omission in the space age. The importance of basic decisions and the long lead times required to repair errors of judgment have been illustrated dramatically by the construction of “small” engines for US ICBMs. These engines perform the ICBM task efficiently but do not meet the corollary requirement for large space boosters. As a result, the US space effort faces a severe payload handicap at least until the mid-1960s.

The extreme payload sensitivity of its booster sys­tem is the primary reason for the stretchout in design and development of Project Mercury. The US is limited to a total capsule weight of about one ton for its first man-in-space program; the Soviet Union apparently has a five-ton leeway for the same mis­sion. Undoubtedly, keeping weight down will become a problem with Dyna-Soar. All new types of vehicles seem to gain weight beyond their original estimates.

The magnitude of the management task for all vehicle systems is certain to bring changes in devel­opment organization and procedures. This has been the theme of suggestions in every critique of the US research and development effort, from Von Kármán’s Toward New Horizons of 1944 to the Stever report of 1959.

The three main suggestions are: (1) longer term planning and financing to cover several years or at least the complete life of a development project; (2) increased specialization and technical competence for officers in R&D posts which require major changes in current personnel policies; (3) greater authority and freedom of action for contractors and military per­sonnel on the working level of research and develop­ment programs. Under this arrangement highly placed commanders and civil authorities will retain close con­trol only over general policy and the selection of major programs.

Regardless of organizational improvements the main task of top management will remain extremely diffi­cult. It will be impossible to develop and operate all of the technically promising propulsion systems and vehicles. Many feasible systems must be bypassed. The only certainty is that this sorting-out process and the search for the best means of using available resources is certain to grow more difficult with time.

The multitude of possible propulsion systems may be broken down into five general classes to allow a closer look at some of the technical decisions which will have to be made in the near future and during the 1960s.

These decisions will have a considerable effect on the military posture of this country, its ability to ex­plore and control space in the 1970s, and on the na­tional budgets.

The five classes of engines are: air-breathers, rockets, electric systems, propellant-collection systems, and nuclear and advanced systems.

The ultimate potential for air-breathing engines does not yet appear to be in sight in any speed range and for any type of aircraft. Improved designs for light­weight VTOL lifting engines and Mach 3 turbofans are now under development by the military and US engine manufacturers. Air-breathing engines which will operate up to escape speeds of 25,000 mph at altitudes of 250,000 feet or better are now considered possible by competent engine designers, although there is little experimental proof to back up their theoretical predictions. Several types of air-breathing engines are considered possible at these extreme speeds and altitudes, including turborockets, super­sonic combustion ramjets, and external-burning ram­jets (see accompanying cuts).

However, the course of air-breathing engine devel­opment -above Mach 6 or so already has been greatly influenced by management decisions. The National Aeronautics and Space Administration essentially eliminated air-breathing engine research from its pro­gram shortly after it was formed around the nucleus of the personnel and facilities of the National Ad­visory Committee for Aeronautics. The efforts of the group at Lewis Research Center, Cleveland, Ohio, one of the world’s largest and most experienced air-breath­ing engine research teams, was redirected at that time.

This was a controversial decision, both within NASA and -among power-plant experts throughout the entire western world. The reasoning behind the decision was that no further basic or applied research was needed by manufacturers to build a new generation of im­proved turbojets for speeds below Mach 4. This essen­tially has proved to be true because several manufac­turers are working in this area, although much of their effort has been of a research rather than a develop­mental nature, and NASA data could have been helpful.

In the hypersonic engine area, it was decided that experimental research should be dropped and all effort concentrated on a wide variety of rocket engines, from more or less conventional chemical engines to nuclear and electric rockets. It was believed that the primary requirement in the 1960s would be for rockets.

Now it appears that the military will not follow this policy lead and that perhaps NASA will reverse itself and return to experimental research with hyper­sonic air-breathing engines. The Air Force is planning to begin applied research leading to a large, one-stage, winged Aerospace Plane that can fly into orbit using air-breathing engines rather than rockets. NASA apparently is going to reenter this propulsion field even if most of the research work has to be done under con­tract ,with Lewis Research Center furnishing the man­agement personnel.

During the two-year hiatus in NASA’s experimental activities small-scale experimental research in hyper­sonic engine technology has been pursued at the Ap­plied Physics Laboratory of Johns Hopkins Univer­sity, at the National Bureau of Standards, by several manufacturers—primarily under modest military con­tracts—and by a few small research firms.

But there is still a great deal of basic propulsion design data which must be gathered before the de­tailed planning of hypersonic aircraft can proceed on a firm basis. The management decision to gather this data through large-scale basic- and applied-research programs is going to be a big and costly step. There are few facilities in the US at which fairly large-scale engines can be tested at supersonic speeds and none for hypersonic speeds. The supersonic propulsion wind tunnel at the Arnold Engineering Development Cen­ter, Tullahoma, Tenn., is the largest, and it will ac­commodate engines of several thousand pounds’ thrust at speeds of Mach 5. This tunnel will be a valuable development tool, but a hypersonic-research program undoubtedly will require as well the extensive use of large flight-test vehicles.

Estimates of the cost of gathering the necessary design data for hypersonic engines are matters for de­bate. Some figures run to several hundred million dol­lars for the supporting research alone, with the total cost of developing a single type of operational hyper­sonic air-breathing engine going to a billion dollars or more. These sums are exclusive of the vehicle develop­ment costs and the costs of aerodynamic and structural research.

Other estimates, which come primarily from outside of industry, indicate that the necessary research and development programs can be accomplished less ex­pensively—on the premise that the goals be carefully defined and the program allowed to proceed at its natural pace. Adequate funding would have to be made available from the start rather than sporadically through a review system by outside agencies. Under these conditions the estimates are for a total orbital airplane cost of about $1 billion with half of the ex­penditure on the power plant.

Regardless of which estimates are correct, it is cer­tain that top-level management in the Administration, the military, and industry will have a great influence on the total cost and ultimate success of any hyper­sonic orbital airplane.

Technically, almost every problem with the hyper­sonic engine arises from high temperatures. A number of proposals are being investigated to reduce the op­erating temperatures of these engines. One of the most important is supersonic combustion (referring to the speed of the airstream in the engine, not to the speed of the vehicle). On convention ramjets of the type now operating on the Bomarc and other supersonic missiles the air entering the engine is slowed to sub­sonic speeds in the combustion chamber. Burning fuel in a subsonic airstream is much more efficient than burning in a supersonic stream.

However, it is possible to reduce the temperature in the engine inlet and combustion chamber by about one-half (from more than 4,000 degrees F to about 2,000 degrees F in a Mach 8 ramjet) if the airstream is not slowed down to subsonic velocities. Slowing the engine airstream down increases its pressure and tem­perature and also raises the rate at which its heat is transferred to the engine wall, so that supersonic com­bustion is a very attractive procedure.

The relative inefficiency of supersonic combustion is overcome at hypersonic speeds by a big improve­ment in inlet duct efficiency because the engine air need not be slowed and compressed to such a great ex­tent. Temperatures in the supersonic combustion ram­jet reach those in a comparable subsonic combustion ramjet at only one point, the nozzle throat. This gen­eral lowering of engine air temperature also reduces the tendency of the air to disassociate and soak up energy which might be used to produce thrust. The disassociation loss occurs when the temperature of the air molecules gets high enough to break the molecular bond and split them into individual atoms.

At high supersonic speeds the total effect of super­sonic combustion in a properly designed ramjet ap­parently will be a more efficient and cooler-running engine. At low hypersonic speeds, from about Mach 5 to 10, the supersonic combustion ramjet is not feasible, but the supersonic combustion engine appears to be a workable device. At moderate and high hyper­sonic speeds (above Mach 10 or 12) the temperatures the supersonic combustion engine get too high for known materials and cooling methods, and a complete change in configuration apparently will be required.

One type of configuration receiving study is the external-burning ramjet which amounts to turning the engine inside out so that its hot surfaces may be cooled by radiation. An external-burning ramjet of typical geometry is shown on page 50. The combustion air is compressed by the bow shock and slowed down to supersonic speed. The fuel is added at the point of maximum thickness, and shape is adjusted so that all of it is burned in the dotted area behind the rearward-facing surface. The static pressure is raised in this region by the fuel combustion, and lift as well as thrust is produced on the slanting surface. For this reason it is more difficult than usual to separate the engine performance from the airframe performance. Through its significant lift production the external-burning ramjet materially improves the lift/drag ratio and the aerodynamic efficiency of a hypersonic air­craft.

Operationally, it has been proposed by Breitieser and Morris of NASA to run the external-burning rain-jet “fuel-rich” at the higher flight speeds. There are three main advantages to this procedure: (1) tempera­tures of the burned gases are lowered because of the excess fuel and there is less disassociation; (2) a vary­ing fuel rate can be used to maintain a match between inlet and exit flow conditions at most speeds so that variable engine geometry will not be required (it is possible that a small movable cowl might be required to maintain efficient operation—see page 50); (3) the excess fuel flow can be used to cool the engine and possibly the airframe.

There is one main disadvantage of running the ex­ternal burning engine “fuel-rich.” Its specific impulse is lowered although it remains considerably higher than rocket engine specific impulses. Some researchers say that the specific impulse will remain high enough to ensure a satisfactory acceleration margin to speeds of at least Mach 20 or about orbital speed. Others ap­parently believe that this type of propulsion could be used to rapidly accelerate an aerospace vehicle to escape velocities as it flies through the upper levels of the atmosphere at altitudes up to fifty miles.

Not all propulsion scientists agree that hypersonic engines will operate as described in the previous para­graphs—or even that these engines are feasible. There is no unanimous agreement that supersonic combus­tion is possible under all necessary conditions even though it apparently has been achieved in the labora­tory over narrow ranges of speeds and pressures.

The first goal of any major research and development program for hypersonic air-breathing engines will be to gather a great deal of basic evidence that there, are no substantive holes in any proposed engine system. Once this proof is available it will be possible to sensibly gather engineering design data.

Several other types of hypersonic air-breathing en­gines are possible and have received some attention. One is the standing wave or detonation ramjet in which the fuel energy is released by passing the fuel-air mixture through a stationary shock wave created by a special shaping of the engine duct. Another is a turboramjet engine (see page 50) in which the exhaust products of a rocket engine may be used as fuels in an air combustion process. This double reaction process can be adjusted to achieve an engine per­formance somewhere close to either the high specific, impulse of the turbojet or the high thrust and low weight of the rocket, or to meet a compromise set of performance requirements somewhere in between.

New fuels are an important requirement for hyper­sonic engines. Petroleum hydrocarbon fuels will not be useful because they are thermally unstable at the temperatures experienced at about Mach 6 and above, and they vaporize and leave deposits on fuel-injection systems, etc.

Hydrocarbon fuels also provide much less range than the high-energy fuels such as the boron-hydrides and hydrogen. The high-energy fuels have higher heats of combustion than hydrocarbons, and they burn effi­ciently at a lower fuel/air ratio to achieve a given thrust. Therefore, for a given fuel weight they provide more range.

Many fuels have been investigated by the Air Force and other agencies to meet a wide variety of per­formance requirements. These include such especially tailored fuels as those with small bits of metal sus­pended in them and others designed to form especially stable combustion products.

Large facilities were constructed to produce boron-hydride fuels, but ‘their use was canceled because of high costs. Today it seems certain that liquid hydro­gen will be the basic hypersonic engine fuel. It is in plentiful supply, and it has the highest energy per pound of any fuel although it is not as attractive as some of the other high-energy fuels from all com­bustion standpoints.

One of the implications of using hydrogen fuel is that unusually large tank space must be provided be­cause of the low density of liquid hydrogen. One pro­posed hypersonic aircraft configuration which would have the necessary tank volume is shown on page 51. This aircraft was suggested last year by Antonio Fern of the Polytechnic Institute of Brooklyn. It is designed to fly into orbit using almost its entire undersurface as an engine after taking off from a normal-sized air­field with turbojet engines. The turbojets would be stopped and their inlets and exhaust nozzles closed at about Mach 3.

Any management decision to pursue a costly de­velopment program for hypersonic air-breathing en­gines will have to be made against a background of continuing improvement in rocket-engine performance. Liquid- and solid-propellant rockets are entering prob­ably their most promising era, and their performance as boosters to “fire” or rapidly accelerate payloads into space or through the atmosphere is improving each year with no sign of abatement.

The nature of these chemical rocket improvements along with a discussion of electric rockets, nuclear engines, and air-liquification engines or propellant-collection systems for use in the upper atmosphere will be presented next month.—End