High-density heat exchanger developed for aerospace

High-density heat exchanger developed for aerospace
Tom Shelley reports on the development of what is believed to be the world's highest performance heat exchanger.

With a thermal density of 1GW/m3, the heat exchangers for the engines of the 'Skylon' hypersonic space plane will cool the engine airflow from intake recovered conditions of up to 1000°C to -140°C in just 0.01s, in order that it can be compressed again when the engines run in jet mode, without raising temperatures so high that they melt everything.

Despite the extreme performance, the first full sized unit, built for testing, is made from quite ordinary, commercially available materials, even though it presses the limits of manufacturing techniques.

According to Mark Hempsell, the Future programmes director of Reaction Engines, which is the company behind the project, the heat exchanger is made up of banks of 20µm thick, 1mm diameter Inconel tubes, brazed together in a 'Swiss Roll' configuration to perform as a cross flow, counter current design. The cooling fluid is helium, which when heated is used to drive the turbo compressor to deliver air to the rocket engines which will power the machine in jet mode.

The project is descended from the HOTOL – HOrizontal Take Off and Landing design, which conceived the original idea of a hydrogen powered aircraft whose engines would function as jets on take off from a conventional runway, and rockets when the craft reached to top of the atmosphere. The new concept is based on the old one, except that new inventions have been made in order to get round the original patents, which were sold to Rolls-Royce, and the new machine is to be made entirely of commercially available materials instead of exotics.

Reaction Engines has got much further than the original HOTOL project, largely because it has benefitted from two tranches of €1 million funding from the European Space Agency, backed by the UK government, which has enabled a further £6 million public/private financed development programme. The primary purpose of the development is to reduce the cost of launching satellites, which is currently around $10,000/kg even after around 50% subsidies. The design is intended to reach Mach 5.5 in air breathing mode and then go into rocket mode at an altitude of 25km. For launching geostationary satellites, there is to be a second rocket stage launched from the payload bay, which will return to earth and, once recovered, will be able to be reused.

Because the machine is to be powered by hydrogen, which is much less dense than conventional jet fuel, most of the interior of the craft will be hydrogen tanks, which are to be made of aluminium lithium alloy. The exterior of the machine will be covered in glass ceramic panels, with 1mm expansion joints between them. This means that they cannot contribute to the overall structural strength of the machine, so the airframe is to be based on silicon carbide reinforced titanium, metal matrix composite, girder trusses. This is inspired by 20th century, rigid airship design, which had hydrogen tanks suspended within an aluminium girder structure, with a fabric covering. However, unlike the old airships, there is a to be a very clever, titanium foil-based insulation system that will protect the trusses and the panel clips from the hot panels.

At time of writing, the engine technology development programme is to be completed by the end of 2011. Two test vehicles are to be constructed in 2016, with production prototypes to be tested in 2018, and commercial operations to commence in 2020.

Tom Shelley

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