The Rocket Engine of the Future - Maybe

An engine that could allow a spaceplane to take off from a runway like a jet and accelerate all the way into orbit has been the dream of rocket designers (and science fiction writers) since the dawn of the space age. Now it looks like it might be becoming real. 

The idea behind Sabre is to use the engine’s air-breathing mode to whip a spacecraft up to hypersonic speeds in the lower atmosphere and then switch to a full rocket mode at the edge of space. It’s conceptually simple, but the devil is in the details. For example, as the engine works the aircraft up to hypersonic speeds at low altitudes, the air temperature approaches 1,800 degrees Fahrenheit, which is hot enough to melt engine components. To overcome this challenge, Sabre uses a precooler to lower the air temperature by circulating hydrogen fuel through the engine. This lowers the air to ambient temperatures at altitude, which are around -200 degrees Fahrenheit. “Effectively the core engine does not know it is flying hypersonically,” says Shaun Driscoll, the programs director at Reaction Engines. “The precooler takes care of that.”

Once the air is lowered to a manageable temperature, it’s passed to a compressor to raise the gas pressure, much like in a conventional jet engine. Then it’s routed to a rocket combustion chamber where it is mixed with liquid hydrogen fuel and ignited to produce thrust. By the time the vehicle reaches hypersonic speeds, the atmosphere is too thin for an air-breathing engine and the system switches to its onboard oxidizer tank for the final leg of the journey to space.

Bond retired from Reaction Engines in 2017, but work on the Sabre engine continues apace. Over the past four years, the company has raised over $100 million to develop Sabre, and shortly after Bond stepped back from the company, Reaction Engines contracted with Darpa to develop a test facility for the engine’s precooler in Colorado. Late last year, the company demonstrated that its precooler could handle the extreme heat generated under hypersonic conditions, a major milestone on its path to a full engine demonstration. Around the same time, the European Space Agency concluded its design review of the engine and gave the company the green light to start testing its engine core.