Precooled jet engine

A precooled jet engine is a concept for high speed jet engines that features a cryogenic fuel-cooled heat exchanger immediately after the air intake, to precool the air entering the engine. After gaining heat and vapourising in the heat exchanger system, the fuel (e.g. H₂) is burnt in the combustor. Precooled jet engines have never flown, but are predicted to have much higher thrust and efficiency at speeds up to Mach 5.5. Precooled jet engines were described by Robert P. Carmichael in 1955.

Unlike Liquid Air Cycle Engines (LACE), precooled engines simply cool, but do not liquify the air, thus avoiding the need of an air condenser.

A potential application for a precooled engine is as part of the powerplant for a space launcher vehicle, or for a very long range, very high speed aircraft.

Advantages of precoolers

One main advantage of precooling is that, for a given overall pressure ratio, there is a significant reduction in compressor delivery temperature (T3), which delays the onset of the T3 limit as flight speed increases. Consequently sea-level conditions (corrected flow) can be maintained after the precooler over a very wide range of flight speeds, thus maximizing net thrust even at high speeds.

Another advantage is that the compressor and ducting after the inlet is subject to much lower and more consistent temperatures, and hence may be made of light alloys. This greatly reduces the weight of the engine, which further improves the thrust/weight ratio.^{[citation needed]}

The fuel typically proposed for precooled jet engines is usually hydrogen, since hydrogen is liquid at deeply cryogenic temperatures, and over its useful range has a very high total specific heat capacity, including the latent heat of vapourisation, higher than water.^{[citation needed]}

However, the low density of liquid hydrogen has negative effects on the rest of the vehicle, and the vehicle physically becomes very large, although the weight on the undercarriage and wing loading may remain low.^{[citation needed]}

Another issue is that the amount of hydrogen needed to cool the air is high, (and using enough to liquifying it which would avoid the need for a turbocompressor greatly reduces the Isp), more than can be burnt in the engine. This means that the engine has a lot of excess hydrogen that must be disposed of, and precooled jet engines may need to use ramjet-like burners around the central precooled engine core.^{[citation needed]}

History and status of precoolers

Precoolers have never flown.

The state of the art is that a lab scale precooler has been developed in the UK, and tested successfully under representative conditions. Rejection of condensed water has been achieved, which is an important achievement in that it prevents ice from blocking the system.^{[citation needed]}

Similar systems

Precoolers were first proposed as part of the research in America on Project Suntan- a liquid hydrogen fuelled aircraft. Robert P. Carmichael in 1955 devised several engine cycles that could be used with hydrogen fuel, and this was one.

Interest in precooled engines saw a brief emergence in the UK in 1982, when Alan Bond (formerly of the Blue Streak missile project) created a LACE-like design he called SATAN. The primary difference of these systems is that the air is only cooled, rather than liquified, and thus the gases are not actually separated, apparently giving significantly greater overall performance, due to a reduction in the amount of hydrogen used for cooling.

At the same time, John Scott and Bob Parkinson at British Aerospace had started some preliminary work on reusable launch systems. The two teams met and created HOTOL, which would use the BAe designed airframe with a Rolls Royce version of Bond's engine, known as the RB545. In 1986 the project was given an official go-ahead to the tune of 2 million pounds for research, but the program was later killed in 1989 when the project encountered problems and the government ended funding.

The principal designers continued development on their own, but the RB545 had been classified top secret and could not be used. Instead Bond developed another version that is more advanced, known as SABRE (ostensibly for Synergic Air BReathing Engine) which is meant for their Skylon design. Funding has not been terribly forthcoming (surprising, considering the design's potential to power an SSTO craft) and development continues at a relatively low level; papers and laboratory work are ongoing. A study of an aircraft powered by a SABRE class engine is underway under LAPCAT partially funded by the EU looking towards hypersonic intercontinental travel (Brussels to Sydney in 2–4 hours non stop).

See also

• Air turboramjet
• ATREX
• Liquid air cycle engine
• RB545
• Reaction Engines A2
• Reaction Engines SABRE
• Reaction Engines Skylon
• Lockheed CL-400 Suntan
• Hydrogen-cooled turbogenerator
• Hydrogen vehicle
• intercooler similar concept