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) burns in the combustor. Precooled jet engines have never flown in production, 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.[1]:155

Pre-cooled engines avoid needing an air condenser because, unlike liquid air cycle engines (LACE), pre-cooled engines cool the air without liquefying it.

Advantages and disadvantages of precoolers

One main advantage of pre-cooling is (as predicted by the ideal gas law) 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 pre-cooler over a very wide range of flight speeds, thus maximizing net thrust even at high speeds. As a result, 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.

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,[1]:108 including the latent heat of vapourisation, higher than water.

However, the low density of liquid hydrogen has negative effects on the rest of the vehicle, and the vehicle physically becomes very large,[1]:108 although the weight on the undercarriage and wing loading may remain low.

Hydrogen causes structural weakening in many materials, known as hydrogen embrittlement.

The weight of the precooler adds to the weight of the engine, thereby reducing its thrust to weight ratio.

Passing the intake air through the precooler adds to the inlet drag, thereby reducing the engine net thrust, and so reducing the thrust to weight ratio.

Depending on the depth of cooling required, despite its high thermal capacity, more hydrogen may be needed to cool the air than can be burnt with the cooled air. In some cases, part of the excess hydrogen can be burnt in a ramjet with uncooled air to reduce this inefficiency.

Unlike a LACE engine, a precooled engine doesn't need to liquefy the oxygen, so the amount of cooling is reduced as there is no need to cover of fusion of the oxygen and a smaller total temperature drop is required. This in turn reduces the amount of hydrogen used as a heat-sink, but unable to be burnt. In addition a condenser isn't required, giving a weight saving.

History

Precoolers were 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.[1]:155

Interest in precooled engines saw an emergence in the UK in 1982, when Alan Bond created a precooled air breathing rocket engine design he called SATAN. The idea was developed as part of the HOTOL SSTO spaceplane project, and became the Rolls-Royce RB545. In 1989, after the HOTOL project was discontinued, some of the RB545 engineers created a company, Reaction Engines Ltd, to develop the idea into the SABRE engine, and the associated Skylon spaceplane.

In 1987, N Tanatsugu published "Analytical Study of Space Plane Powered by Air-Turbo Ramjet with Intake Air Cooler." part of Japan's ISAS (now JAXA) study into an Air-Turbo Ramjet (ATR, later ATREX after the addition of an expander cycle) intended to power the first stage of a TSTO spaceplane. ATREX was superseded by the Preecooled Turbojet (PCTJ) and Hypersonic Turbojet studies. A liquid nitrogen precooled hydrogen burning test engine was flown at Mach 2 at Taiki Aerospace Research Field in September 2010.[2]

See also

References

  1. 1 2 3 4 Sloop, John (1978). Liquid hydrogen as a propulsion fuel, 1945–1959 (NASA SP-4404) (PDF). NASA.
  2. ↑ Kobayashi, H and Taguchi, H and Kojima, Takayuki and Harada, K and Okai, K and Hongoh, M and Arai, T and Sato, T (6 October 2011). Development status of the hypersonic turbojet engine for Mach 5 flight in JAXA (IAC-11.C4.5.1). 62nd International Astronautical Congress 2011, IAC 2011. 8. Cape Town, South Africa. pp. 6655–6659.
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