Reaction Engines Ltd (REL) 's ultra-lightweight heat exchangers are the key enabling components in SABRE engines for Mach 5 cruise and aircraft-like access to space.
Heat exchangers that cool the incoming air are the biggest technical challenge to the realisation of the SABRE engine. At Mach 5 (5 times the speed of sound) the heat exchanger needs to cool air from 1,000°C to minus 150°C, in 1/100th of a second, displacing 400 Mega-Watts of heat energy (equivalent to the power output of a typical gas-powered power station) yet weighs less than 1¼ tonnes.
Using Heat Exchangers in Aerospace Engines
In the 1970s REL's founder Alan Bond started exploring the potential impact of heat exchangers light enough to be usefully integrated into rocket engine cycles. The main idea was that heat exchangers could remove heat where it caused a problem, and the energy extracted could then be used to drive the pumps and compressors that supply propellants to the rocket engine. While the potential to create a significant increase in engine efficiency was clear, the problem was that the weight of heat exchangers using conventional technology was so large that all the potential benefit was lost.
Inspired by the miniaturisation of the silicon chip, and understanding that heat exchangers were far from reaching their physical limit in terms of miniaturisation, developing high power lightweight heat exchangers became the focus of REL when it was founded by Alan Bond in 1989.
The impact of successfully developing lightweight heat exchangers on aerospace propulsion systems is seen as comparable to the impact of the silicon chip on computing: new products, new markets, new capabilities.
Heat Exchangers for the SABRE Engine
In the case of the SABRE engine there are two uses for heat exchangers. The first use is to cool the incoming air (which becomes very hot at high speed) so that it can be compressed to the pressure required for it to enter the rocket combustion chamber whilst concurrently heating helium, which is used to drive the engine machinery. We call the heat exchanger on the Sabre engine a Pre-cooler.
The second use is to cool the hot helium using the cold liquid hydrogen fuel, but this uses more conventional heat exchanger technology.
The team at REL has found ways to produce heat exchangers light enough to be viable for the first time in a flying engine. REL has also pioneered solutions to the other practical problems in this application, such as the control of frost formation (where the water in the atmosphere freezes in the heat exchanger, icing it up).
Although REL has developments in other technology areas of the SABRE engine, its heat exchangers are the key to its success and also represent the biggest advance over existing technology.
