A ramjet, sometimes referred to as a stovepipe jet, or an athodyd, is a form of jet engine that contains no major moving parts. Unlike most other airbreathing jet engines, ramjets have no rotary compressor at the inlet, instead, the forward motion of the engine itself 'rams' the air through the engine. Ramjets therefore require forward motion through the air to produce thrust.
Ramjets require considerable forward speed to operate well, and as a class work most efficiently at speeds around Mach 3, and this type of jet can operate up to speeds of at least Mach 5.
Ramjets 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 tip jets on helicopter rotors.
Ramjets are frequently confused with pulsejets, which use an intermittent combustion, but ramjets employ a continuous combustion process, and are a quite distinct type of jet engine.
In 1939, Merkulov did further ramjet tests using a two-stage rocket, the R-3. In August of that year, he developed the first ramjet engine for use as an auxiliary motor of an aircraft, the DM-1. The worlds's first ramjet powered airplane flight took place in December 1939, using two DM-2 engines on a modified Polikarpov I-15. Merkulov designed a ramjet fighter "Samolet D" in 1941, which was never completed. Two of his DM-4 engines were installed on the YaK-7PVRD fighter, during World War II. In 1940, the Kostikov-302 experimental plane was designed, powered by liquid fuel rocket for take-off and ramjet engines for flight. That project was cancelled in 1944.
In 1947, Mstislav Keldysh proposed a long-range antipodal bomber, similar to the Saenger-Bredt bomber, but powered by ramjet instead of rocket. In 1954, NPO Lavochkin and the Keldysh Institute began development of a trisonic ramjet-powered cruise missile, Burya. This project competed with the R-7 ICBM being developed by Sergei Korolev, and was cancelled in 1957.
A ramjet is designed around its inlet. An object moving at high speed through air generates a high pressure region in front and a low pressure region to the rear. A ramjet uses this high pressure in front of the engine to force air through the tube, where it is heated by combusting some of it with fuel. It is then passed through a nozzle to accelerate it to supersonic speeds. This acceleration gives the ramjet forward thrust.
A ramjet is sometimes referred to as a 'flying stovepipe', a very simple device comprising 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.
By way of contrast, a turbojet uses a gas turbine driven fan to compress the air further. This gives greater efficiency and far more power at low speeds, where the ram effect is weak, but is also more complex, heavier and more expensive, and the temperature limits of the turbine section limits the top speed and thrust at high speed.
Ramjets try to exploit the very high dynamic pressure within the air approaching the intake lip. An efficient intake will recover much of the freestream stagnation pressure, which is used to support the combustion and expansion process in the nozzle.
Most ramjets operate at supersonic flight speeds and use one or more conical (or oblique) shock waves, terminated by a strong normal shock, to slowdown the airflow to a subsonic velocity at the exit of the intake. Further diffusion is then required to get the air velocity down to level suitable for the combustor.
Subsonic ramjets don't need such a sophisticated inlet since the airflow is already subsonic and a simple hole is usually used. This would also work at slightly supersonic speeds, as the air will choke at the inlet, but this is inefficient.
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.
The propelling nozzle is a critical part of a ramjet design, since it accelerates exhaust flow to produce thrust.
For a ramjet operating at a subsonic flight Mach number, exhaust flow is accelerated through a converging nozzle. For a supersonic flight Mach number, acceleration is typically achieved via a convergent-divergent nozzle.
Ramjets have been run from as low as 45 m/s (100 mph) upwards. Below about Mach 0.5 they give little thrust and are highly inefficient due to their low pressure ratios.
Above this speed, 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.
Due to the stoichiometric combustion temperature, efficiency is usually good at high speeds (Mach 2-3), whereas at low speeds the relatively poor compression ratio means that ramjets are outperformed by turbojets or even rockets.
In a liquid fuel ramjet (LFRJ) hydrocarbon fuel (typically) 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 an 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. This offers a lower cost approach than a regulated LFRJ requiring a turbopump and associated hardware to supply the fuel.
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, although this must be traded against the reduction in performance compared with that provided by a dedicated booster nozzle.
These are a slight variation on the ramjet where the supersonic exhaust from a rocket combustion process is used to compress and react with the incoming air in the main combustion chamber. This has the advantage of giving thrust even at zero speed.
In a solid fuel integrated rocket ramjet (SFIRR) the solid fuel 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 an 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 performance of conventional ramjets falls off above Mach 6 due to dissociation and pressure loss caused by shock as the incoming air is slowed to subsonic velocities for combustion. In addition, the combustion chamber's inlet temperature increases to very high values, approaching the dissociation limit at some limiting Mach number. In the scramjet, or supersonic combustion ramjet, the ram air is not slowed to subsonic speeds for combustion and as a result, shocks are not encountered and pressure loss is avoided.
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.