Heat-exchanger designs for a two-stage launch system that collects and liquifies air while in the cruise are to be tested by the European Space Agency. An ESA study has found it should be possible to build hydrogen-cooled heat exchangers with 85% efficiency.

Efficient heat exchangers are key to a launch system with an air-breathing first stage powered by low-bypass turbofans and hydrogen-fuelled ramjet. This would collect and liquify air while cruising at Mach 2.5. Oxygen separated from the liquified air would be used to power the second-stage vehicle (Flight International, 10-16 July).

In the first-stage vehicle, cryogenic hydrogen fuel would flow through the heat exchangers to chill incoming air, at a mass flow of 53kg/s (116lb/s), reducing its temperature from 206°C (403°F) to about 13°C. "The heat exchanger is on the front of the low-bypass engine inlet. The bypass air is cooled and nitrogen and oxygen is separated. Nitrogen goes to the bypass duct and oxygen goes into the engine," says Patrick Hendrick, a researcher at ESA's Netherlands-based European Space Research and Technology Centre.

The research has examined two types of exchanger, plate-and-fin and shell-and-tube. The shell-and-tube designs involve dozens of 3mm (0.11in)-diameter tubes with 150micron-thick walls that can be brazed together or secured with mechanical joints. A plate-and-fin system has tubes within tubes, with fins that are offset at an angle between each tube wall. The team is testing two types of fins.

Four testbed exchangers, for the alternative designs, are being constructed out of aluminium and will undergo vibration and leak testing later this year.

Hendrick's researchers are working with the ESA-managed, European Commission-funded €7 million ($9.5 million) Long Term Advanced Propulsion Concepts and Technologies (LAPCAT) project team to incorporate future heat-exchanger work into the proposed LAPCAT-2 proposals. LAPCAT is developing concepts for supersonic and hypersonic civil transports.

The US Air Force has a similar study, although it assumes air collection during ascent and acceleration through M5 to M7.

Source: Flight International