very high total specific heat capacity, 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, 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 amount 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 of precooling using heat exchangers Robert P. Carmichael in 1955 devised several engine cycles that used liquid hydrogen to precool the inlet air to the engine before using it as fuel. 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 ATREX was superseded by the Preecooled Turbojet (PCTJ) and Hypersonic Turbojet studies.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