Ramjet technology is neither new, nor difficult, at least on paper. Implementing such a power plant on a missile, however, poses considerable demands on propulsion designers.
The attraction of ramjet propulsion is that it offers both extended range and increased mean velocity during missile flight, with an effective cruise speed of Mach 3.
Unlike solid-rocket-propellant missiles, however, current ramjet designs, to function effectively need to be ignited at speeds approaching Mach 2.
This has required that ramjet propulsion be used in conjunction with a solid-rocket booster. The ramjet ignition sequence itself poses numerous design issues. The burn chamber has to be cleared of elements of the solid booster. In both Russian and French ramjet designs, this is achieved by ejecting these through the exhaust nozzle. Work is now under way looking at technologies, which would circumvent this difficulty.
Assuming that ramjet ignition is achieved successfully, then airflow down the intakes has to be managed to assure that the engine does not then flame out, at least not until it is so late in any engagement that it will have no effect.
During flight, ramjet-powered missiles would bank, rather than skidded, into turns, since the latter would interfere with intake airflow, raising the risk of an engine flame out. Only in the final seconds of the end game, when instantaneous manoeuvrability is paramount, might skid-turns be used. Even if a flame out does occur, the missile should have enough inertia for a successful conclusion to the engagement.
In France, Celerg (created from the merger of Aerospatiale's propulsion unit and propellant specialist SNPE) is working on both ramjet and scramjet (in which airflow through the combustor is supersonic) missile motors.
The successful flight test in February of the so-called "Rustique" solid-fuelled ramjet-powered missile (in a programme led by French research agency Onera) was a major step forward, says Celerg programmes director Jean-Marie Laurent Laurent.
Laurent identifies two advantages of using solid, rather than liquid, propellant for ramjets: "Low cost and the fact that there is only one order of ignition...in other words, both the booster and ramjet engines ignite at the same time." Celerg is collaborating with US company Atlantic Research on new propulsion grains and a second flight of the engine is due later this year.
Celerg's principal ramjet programme is for the supersonic, conventionally armed, ASMP-C cruise missile.
Studies are being carried out on a single, top-mounted (instead of lateral), intake, to reduce observability. Film cooling of the combustion chamber is also being studied as a way of increasing the missile's range.
This uses inlet air injected through small holes in the chamber wall, to reduce wall temperatures, enabling the engine to become temperature-stabilised so that it can continue burning for longer.
Composite materials are being studied, to reduce the cost and weight of ramjet engines. "We're looking at carbonfibre and phenolic resins as the best compromise between cost and heat, "says Laurent.
The pulse-detonation-wave engine has a turbojet cycle, so that the engine can be started from Mach 0 (typical ramjets have to be boosted to Mach 2-3 before they begin working). Fuel consumption is reduced, allowing a potential 25% range increase. Control of the pulse frequency, which varies between 50 and 100Hz, allows thrust to be varied extremely rapidly for highly manoeuvrable air-to-air missiles.
In Germany, Bayern-Chemie, a subsidiary of Daimler-Benz Aerospace (DASA), is developing a ramjet power plant, which will be used on DASA's A3M advanced medium-range air-to-air missile.
One of the most notable elements of the A3M design is its solid-propellant ducted-rocket (SDR) power plant. The missile also has an integrated nozzleless rocket booster, which accelerates it to a sufficient speed for the SDR to take over.
The SDR engine uses a solid-gas-generator propellant, packed into the missile's mid-section forward of the ram combustor and separated from it by a variable-flow valve and gas-injector tube.
The lump of solid propellant contains about 10% of the amount of oxidising agent needed to burn the fuel it contains. The oxidising agent reacts with some of the fuel, burning the propellant away like a cigarette and producing a fuel-rich gas, which passes through into the ram combustor. Here, the gas mixes with the high-pressure, high-temperature, air coming in through the intakes and burns the rest of its combustible contents, to generate thrust.
The propellant, as has been the case in most of Bayern-Chemie's SDR engines, is based on boron - some 40% of its mass is this element.
Boron has been chosen for its high volumetric-heat content, says Bayern-Chemie development manager Hans Besser, but has always presented a challenge because it does not burn easily. This problem has now been solved, by refining the propellant composition and combustor design.
The rate of fuel flow into the combustor, and hence the missile's speed, is controlled by a valve at the gas-generator exit.
Opening the throat wide lowers the pressure in the gas generator, slowing the propellant burn-rate and therefore slowing the fuel flow into the ram combustor. Similarly, narrowing the throat increases the fuel-flow rate.
This control over fuel flow into the combustor allows a constant Mach 3 to be maintained for most of the missile's flight.
Bayern-Chemie has long experience in designing, constructing and testing SDR engines, dating back to the early 1970s. Between 1979 and 1981, the company worked on a programme called the Experimentale Feststau Antrieb (experimental ducted-rocket engine), which led to two successful test flights of a 240mm fixed-flow SDR missile with an ejectable booster on an Italian air force firing range in Sardinia.
Later, the company developed a 350mm-diameter throttleable ducted rocket, as a candidate for the Franco-German supersonic anti-ship missile competition. This work was concluded in 1987 after the development and bench testing of a flight-weight engine.
Since 1989, Bayern-Chemie has also been working on the joint US-German advanced missile-propulsion technology programme, funded by the German defence ministry and the US Air Force, and involving US Atlantic Research as well as the German company.
Within this programme, Bayern-Chemie has been conducting bench-tests of a half-axisymmetric SDR engine very similar to that required for the A3M.
Illustrating the advantages of SDR as a means of propulsion, Besser says that, if you design two missiles, one conventional, one SDR, for a common mission, the difference in required missile size is dramatic. For a 100km mission at sea level, with a 200kg payload and a 16:1 "fineness (length-width) ratio", you end up with a 10m-long conventional missile, compared with a 5.7m-long SDR missile.