Solid-Fuel Ramjet (SFRJ)

In a solid-fuel ramjet, air from the inlet flows through the "pipe" of fuel (also known as a "fuel grain"), which burns along its length. Air from the ramjet inlet enters coaxially of and flows centrally along a burning tubular fuel grain which releases combustible material into the air for mixing and combustion therewith. In this construction the mixing and combustion occur in a boundary layer of fluid flow along the grain.

Tubular projectiles utilizing solid fuel ramjet thrust augmentation are required to possess low drag while maintaining sufficient kinetic energy and mass to provide adequate terminal momentum and the consequent destructive force upon impact. Such solid fuel ramjet tubular projectiles typically comprise a centralized tubular design with an internal mid-section constituting a combustion chamber annularly lined with solid fuel for effecting thrust augmentation. The rear portion of such projectiles is provided with a thrust generating design comprising a constriction portion and a flared rear section, which operate on the hot expanding gases generated in the mid-section combustion chamber in a well-known manner.

The twofold opportunity of a simpler, more reliable engine having a higher performance was the potent underlying incentive for studying solid fuel ramjet systems. Calculations indicate that sufficient fuel for short (approximately 20 miles) and perhaps intermediate range (100-200 miles) missiles can be stored within the combustion chamber proper without occasioning undue internal drag losses. By this expedient, fuel tanks, pumps, meters, injectors and their associated plumbing, and even the pilot and flame holders, can be eliminated. Means for effecting these component savings in longer range missiles (>1000 miles) were not so immediately apparent.

There are two fundamental ramjet concepts. These comprise subsonic combustion and supersonic combustion, both of which are applicable to vehicles flying at supersonic speeds. In the subsonic combustion ramjet, the fuel is burned in air which has been slowed down to subsonic speeds within the engine. In a supersonic combustion ramjet, the fuel is burned in air which remains at supersonic velocities throughout the engine. Ramjets require a rocket booster, or gun launch, to achieve a flight condition where thrust is greater than drag, which for missiles is approximately Mach 2, at which point the ramjet is capable of accelerating to higher speed. Since the ramjet propulsion system depends only on its forward motion at supersonic speed to compress intake air, the engine flow-path components have no moving parts. Consequently, it has inherent simplicity, reliability, light weight, and high-speed flight capability not possible with other air-breathing engines. These attributes make the ramjet a good choice for propelling medium-caliber cannon ammunition at supersonic speed. The ramjet powerplant for aircraft propulsion, conceived by Rene Lorin of France in 1913, became available in useful form in the 1940s. Its developers focused their effortslargely on liquid fuels, particularly hydrocarbons of the gasoline or kerosene type. Nevertheless, during the period of resurgent interest occasioned by teh Great Patriotic War, there was early recognition that solid fuels might offer certain design and performance advantages.

Although the Germans were chiefly interested in coal and even wood, Sanger and Bredt did suggest the use of metal dispersions to obtain higher flame temperatures and, hence, thrust coefficients. Lippisch and Schwabl were first attracted to solids because of the inherent simplicity of the fuel system in short-duration missiles and artillery or mortar shells. They carried out numerous burner tests with briquetted carbon and natural coal charges, later extending the scope of their work to designs suitable for piloted aircraft. At Great Britain's National Gas Turbine Establishment, Roberson prepared a theoretical performance survey covering a very large number of solid fuels. Actual experimental work appears to have been confined to aluminum.

Packaging the fuel within the combustion chamber to eliminate the fuel system components leads to complications as well as simplification. For example, the problem of obtaining fuel-charge geometries having sufficient surface exposed to the air stream to provide the necessary over-all rate of heat release, and yet remain compatible with the requirement of a low internal drag, plagued most of the early investigators. On the other hand, if the fuel is fed to the burner as a finely divided powder, either in the pure form or suspended in a liquid carrier, not only is the complexity ofthe fuel-supply system enhanced over that originally required for liquid, but much of the density advantage responsible for a high volumetric heat release is also lost. These same con-siderations partly apply to a wire-type feeder.

At the Applied Physics Laboratory of The Johns Hopkins University, Berl developed techniques for improving the output of a lean propylene oxide burner through the addition of aluminum and magnesium powders. At the Jet Propulsion Laboratory of the California Institute of Technology, Bartel and Kannie used carbon tubes in flow combustion tests and Alperin considered the problem from a theoretical viewpoint. Damon and his associates at the Bureau of Mines initially investigated coal for ramjet use, but later began to incorporate the light metals in their formulations. Finally, a solid fuel propulsion test vehicle, after extensive development in free-jet tests, was successfully flown in the Untied States in the velocity range between Mach 1.8 and 2.1 on January 11, 1952.

The use of solid fuel in a ramjet introduces a new problem. As the fuel burns, the effective combustion chamber area increases. This would normally result in a change in thrust. However, if the condition cf constant thrust is imposed, then this change in combstion chamber area would vary the amount of heat required. The change in heat required may be expressed in terms of the burning rate required for a solid propellant ramjet developing constant thrust.

Previous methods of controlling the thrust produced by air-breathing solid fuel rocket motors operated by controlling the flow of ram-air to the engine. These methods have had limited success because they interfere with the stability of air flow into the motor necessary for proper operation of an air-breathing rocket motor. As a result, it is extremely difficult to throttle a solid fuelled air breathing system.

In operation, the inner surface of the tubular fuel member, the combustion surface, burns and said surface will thus act as a combustion-chamber delimiting surface. One condition for initiating and continuing propulsion of the projectile is that a at least a given pressure prevails in the combustion chamber. This condition is achieved in projectiles, by launching the projectiles at a speed which correspond to a Mach number of at least about 2, therewith generating a sufficiently high so-called ram pressure in the combustion air flowing in through the intake. So that the combustion air will not prevent effective combustion of the aforesaid burning surface, a so-called flame holder is provided at the combustion chamber inlet in the most common non-rotary application of ramjets.

Early efforts to burn solid hydrocarbons in the ramjet mode encountered difficulties in reliable flame holding. Ramjet technology, using oxidizer-free solid fuels, developed rapidly after the role of the sudden expansion flame holder as a reliable method of flame stabilization for SFRJ's was recognized.

At first, it might appear that the application of the sudden expansion flame holder to SFRJ's represents no new technology since the technique has been used successfully for many years in liquid ramjet combustors. However, a major difference exists between the two systems. In the liquid ramjet, fuel and air are premixed to the desired equivalence ratio. This assures combustion at proper flow speed by dumping into a sudden expansion chamber. In the SFRJ, air alone is dumped into the sudden expansion region.

In the SFRJ, a recirculation zone is formed in the expansion region which becomes the flame holder. The correct range of fuel-to-air must be internally generated within the recirculation zone to achieve flame stabilization. Fuel is gasified and mixed inside the combustion chamber instead of being premixed with air. Additionally, the fuel-air mixture in the flame holding region is not necessarily at the stoichiometric composition.

The method of fuel addition is in turn dependent upon the combustion process. A turbulent flame transfers heat to the fuel surface (which constitutes the wall of the combustion chamber), causing fuel gasification. The turbulent flame is established between the fuel surface and the boundary layer. Flame holding in a SFRJ occurs in the recirculation zone behind a rearward facing step or sudden expansion region just ahead of the fuel grain. Heat transfers to the fuel surface during the ignition phase and results in a gasified fuel being injected into the recirculation zone. Simultaneously, a small fraction of the air streaming through the injector becomes entrained and a stable flame is formed in the recirculation zone.