Ramjet

From Free net encyclopedia

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

A ramjet, sometimes referred to as a stovepipe jet, is a type of jet engine. The idea was patented as early as 1908 by René Lorin, but it only became reality with the works of René Leduc in France (whose work was greatly slowed down by the need to evade occupation authorities during World War II) and William Avery in the United States. Leduc's Model 010 was the first-ever ramjet-powered aircraft to fly, in 1949.

1 Design
2 Flight speed
3 Applications
4 Related engines
5 Aircraft using ramjets
6 See also
7 External links

[edit]

Design

Image:Ramjet operation.png 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.

Ramjets try to exploit the very high stagnation 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]

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]

Applications

Ramjets are found almost exclusively in missiles, where they are boosted to operating speeds by a rocket engine, or by being attached to another aircraft (typically a fighter).

Ramjet propulsion is used in the British Bloodhound (no longer in service) and Sea Dart surface-to-air missiles.

The Soviet Krug missile first flew in 1967 is launched with the aid of four solid fuel rocket motors inside boosters attached to the outside of the massive missile. Once it's burned and the missile is aloft, it fires a liquid-fuelled ramjet sustainer engine. It reaches speeds of up to Mach 4 and has an effective range of 50-55 km (31-34 miles) depending upon the version. It carries a 135 kg (300 lb) warhead. Possible engagement altitudes range from 100 m-27 km (330-88,500 feet).

The Bomarc missile in the U.S. used two body pylons underneath the wings each housing a Marquardt ramjet engine capable of producing thrust of 10,000 pounds force (44 kN) in the A version and 14,000 pounds force (62 kN) in the B version. The Bomarc served as part of the North American Defense System between 1959 and 1972.

A number of missile projects currently under development use ramjet engines to achieve better fuel efficiency (and thus longer range) at supersonic speeds than a rocket-driven approach. These include the British MBDA Meteor air-to-air missile and the Russian-Indian BrahMos supersonic cruise missile.

[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 ramjets at high-speeds (Mach 3.2).

[edit]