Ramjet

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For the hypothetical method of interstellar travel, see Bussard ramjet. For the Transformers character see Ramjet (Transformers)

Schematic diagram showing simple ramjet operation, with Mach numbers of flow shown.

A ramjet, sometimes referred to as a stovepipe jet, is a type of jet engine. It contains no (major) moving parts and can be particularly useful in applications requiring a small and simple engine for high speed use; such as missiles. They have also been used successfully, though not efficiently, as tipjets on helicopter rotors.

1 History
2 Design
3 Ramjet Types
4 Flight speed
5 Related engines
6 References
7 Aircraft using ramjets
8 See also
9 External links

[edit] History

The idea of the ramjet (not to be confused with the Pulse jet engine of V-1 flying bomb fame, or with the Scramjet) was patented as early as 1908 by René Lorin. In the Soviet Union, the GIRD-08 ramjet engine was built by Yuri Pobedonostsev and test fired in 1933. In France the works of René Leduc was notable, as was that of William Avery in the United States. Leduc's Model 010 was the first-ever ramjet-powered aircraft to fly, in 1949.

[edit] Design

In its simplest form a turbojet consists of an air intake, compressor, combustor, turbine and nozzle. In a ramjet, owing to the high flight speed, the ram compression is sufficient to dispense with the need for a compressor and a turbine to drive it. So a ramjet is virtually a 'flying stovepipe', a very simple device comprising of an air intake, a combustor, and a nozzle. Normally the only moving parts are those within the turbopump, which pumps the fuel to the combustor, in a liquid fuel ramjet. Solid fuel ramjets are even simpler.

Ramjets try to exploit the very high total pressure within the streamtube approaching the air intake lip. A reasonably efficient intake will recover much of the freestream stagnation pressure, to support the combustion and expansion processes. Most ramjets operate at supersonic flight speeds and use one or more conical (or oblique) shock waves, terminated by a strong normal shock, to decelerate the airflow to a subsonic velocity at intake exit. Further diffusion is then required to get the air velocity down to level suitable for the combustor.

Since there is no downstream turbine, a ramjet combustor can safely operate at stoichiometric fuel:air ratios, which implies a combustor exit stagnation temperature of the order of 2400 K for kerosene. Normally the combustor must be capable of operating over a wide range of throttle settings, for a range of flight speeds/altitudes. Usually a sheltered pilot region enables combustion to continue when the vehicle intake undergoes high yaw/pitch, during turns. Other flame stabilization techniques make use of flame holders, which vary in design from combustor cans to simple flat plates, to shelter the flame and improve fuel mixing. Overfuelling the combustor can cause the normal shock within a supersonic intake system to be pushed forward beyond the intake lip, resulting in a substantial drop in engine airflow and net thrust.

Because nozzle pressure ratios are relatively high, ramjet engines are normally fitted with a convergent/divergent propelling nozzle. Given sufficient initial flight velocity, a ramjet will be self-sustaining. Indeed, unless the vehicle drag is extremely high, the engine/airframe combination will tend to accelerate to higher and higher flight speeds, substantially increasing the air intake temperature. As this could have a detrimental effect on the integrity of the engine and/or airframe, the fuel control system must reduce engine fuel flow to stabilize the flight Mach number and, thereby, air intake temperature to sensible levels.

As a ramjet contains no (major) moving parts, it is lighter than a turbojet and can be particularly useful in applications requiring a small and simple engine for high speed use; such as missiles. They have also been used successfully, though not efficiently, as tipjets on helicopter rotors.

[edit] Ramjet Types

Ramjets can be classified according to the type of fuel, liquid or solid, and the booster.^[1]

In a liquid fuel ramjet (LFRJ) hydrocarbon fuel is injected into the combustor ahead of a flameholder which stabilises the flame resulting from the combustion of the fuel with the compressed air from the intake(s). A means of pressurising and supplying the fuel to the ramcombustor is required which can be complicated and expensive. Aerospatiale-Celerg have designed a LFRJ where the fuel is forced into the injectors by an elastomer bladder which inflates progressively along the length of the fuel tank. Initially the bladder forms a close-fitting sheath around the compressed air bottle from which it is inflated, which is mounted lengthwise in the tank.^[2] This offers a lower cost approach than a regulated LFRJ requiring a turbopump and associated hardware to supply the fuel.^[3]

A ramjet generates no static thrust and needs a booster to achieve a forward velocity high enough for efficient operation of the intake system. The first ramjet powered missiles used external boosters, usually solid-propellant rockets, either in tandem, where the booster is mounted immediately aft of the ramjet, e.g. Sea Dart, or wraparound where multiple boosters are attached alongside the outside of the ramjet e.g. SA-4 Ganef. The choice of booster arrangement is usually driven by the size of the launch platform. A tandem booster increases the overall length of the system whereas wraparound boosters increase the overall diameter. Wraparound boosters will usually generate higher drag than a tandem arrangement.

Integrated boosters provide a more efficient packaging option since the booster propellant is cast inside the otherwise empty combustor. This approach has been used on solid, for example SA-6 Gainful, liquid, for example ASMP, and ducted rocket, for example Meteor), designs. Integrated designs are complicated by the different nozzle requirements of the boost and ramjet phases of flight. Due to the higher thrust levels of the booster a different shaped nozzle is required for optimum thrust compared to that required for the lower thrust ramjet sustainer. This is usually achieved via a separate nozzle which is ejected after booster burnout. However, designs such as Meteor feature nozzleless boosters. This offers the advantages of elimination of the hazard to launch aircraft from the ejected boost nozzle debris, simplicity, reliability, and reduced mass and cost^[4], although this must be traded against the reduction in performance compared with that provided by a dedicated booster nozzle.

In a solid fuel integrated rocket ramjet (SFIRR) the solid propellant is cast along the outer wall of the ramcombustor. In this case fuel injection is through ablation of the propellant by the hot compressed air from the intake(s). An aft mixer may be used to improve combustion efficiency. SFIRRs are preferred over LFRJs for some applications because of the simplicity of the fuel supply but only when the throttling requirements are minimal i.e. when variations in altitude or Mach number are limited.

In a ducted rocket a solid fuel gas generator produces a hot fuel-rich gas which is burnt in the ramcombustor with the compressed air supplied by the intake(s). The flow of gas improves the mixing of the fuel and air and increases total pressure recovery. In a Throttleable Ducted Rocket (TDR), also known as a Variable Flow Ducted Rocket (VFDR), a valve allows the gas generator exhaust to be throttled allowing control of the thrust. Unlike a LFRJ solid propellant ramjets cannot flameout. The ducted rocket sits somewhere between the simplicity of the SFRJ and the unlimited throttleability of the LFRJ.

The Franco-German company Bayern-Chemie/PROTAC are world leaders in TDR technology and have been developing this form of ramjet propulsion since the 1970s. The first demonstration firing of a ducted rocket with a high energy boron-loaded gas generator was conducted in 1981 while a lightweight TDR suitable for use on the planned Franco-German Anti Navire Supersonique (ANS) supersonic anti-ship missile was tested in 1986. Research and development for BVRAAM applications began in 1990, exploring the problems of asymmetric intake configurations. A lightweight BVRAAM motor demonstrated the critical boost-to-sustain transition in 1998.

VFDR technology has been under development in the US by an Atlantic Research (ARC)/Alliant Techsystems (ATK) team since the mid-1980s. In 1997 the ARC/ATK team completed ground tests of a flight weight 180mm diameter VFDR as part of the USAF Air Superiority Missile Technology programme which started in 1996.^[5]

[edit] Flight speed

Ramjets generally give little or no thrust below about half the speed of sound, and they are inefficient (less than 600 seconds due to low compression ratios) until the airspeed exceeds 1000 km/h (600 mph). Even above the minimum speed a wide flight envelope (range of flight conditions), such as low to high speeds and low to high altitudes, can force significant design compromises, and they tend to work best optimised for one designed speed and altitude (point designs). However, ramjets generally outperform gas turbine based jet engine designs at supersonic speeds (mach 2-4). Although inefficient at the slower speeds they are more fuel-efficient than rockets over their entire useful working range.

[edit] Related engines

Ramjets always slow the incoming air to a subsonic velocity within the combustor. Scramjets, or "supersonic combustion ramjet" are similar to Ramjets, but the air goes through the entire engine at supersonic speeds, eliminating the strong normal shock wave in the intake. This increases the stagnation pressure recovered from the freestream and improves net thrust. Owing to the hypersonic (rather than supersonic) flight speeds experienced, scramjet air intake temperatures are too high for burning kerosene, so hydrogen is normally used as the fuel. Thermal choking of the exhaust is avoided by having a relatively high supersonic air velocity at combustor entry. Fuel injection is often into a sheltered region below a step in the combustor wall. Although scramjet engines have been studied for many decades it is only recently that small experimental units have been flight tested and then only very briefly.

A variant of the pure ramjet is the 'combined cycle' engine, intended to overcome the limitations of the pure ramjet. One example of this is the SABRE engine. Another example of this is the Air Turbo Ramjet (ATR) which operates as a conventional turbojet at subsonic speeds and a fan assisted ramjet at speeds below Mach 6.

The ATREX engine developed in Japan is an experimental implementation of this concept. It uses liquid hydrogen fuel in a fairly exotic single-fan arrangement. The liquid hydrogen fuel is pumped through a heat exchanger in the air-intake, simultaneously heating the liquid hydrogen, and cooling the incoming air. This cooling of the incoming air is critical to achieving a reasonable efficiency. The hydrogen then continues through a second heat exchanger position after the combustion section, where the hot exhaust is used to further heat the hydrogen, turning it into a very high pressure gas. This gas is then passed through the tips of the fan providing driving power to the fan at sub-sonic speeds. After mixing with the air it's then combusted in the combustion chamber.

During the cold war the United States designed and ground-tested a nuclear-powered ramjet called Project Pluto. This system used no combustion - a nuclear reactor heated the air instead. The project was ultimately canceled because ICBMs seemed to serve the purpose better, and because a low-flying missile would have been highly radioactive.

The SR-71's Pratt & Whitney J58 engines act as turbojet-assisted ramjets at high-speeds (Mach 3.2).

[edit] References

^ "A Century of Ramjet Propulsion Technology Evolution", AIAA Journal of Propulsion and Power, Vol.20, No.1, January - February 2004
^ "Aerospatiale studies low-cost ramjet", Flight International, 13 - 19 December 1995
^ "Hughes homes in on missile pact", Flight International, 11 - 17 September 1996
^ Procinsky, I.M., McHale, C.A., "Nozzleless Boosters for Integral-Rocket-Ramjet Missile Systems, Paper 80-1277, AIAA/SAE/ASME 16th Joint Propulsion Conference, 30th June to 2nd July 1980
^ "Rocket/ramjet power studied for air-launched weapons", Flight International, 10 - 16 February 2004