The Rocket Motor of the Future Breathes Air Like a Jet Engine
There’s a small airfield about a two-hour drive north of Los Angeles that sits on the edge of a vast expanse of desert and attracts aerospace mavericks like moths to a flame. The Mojave Air & Space Port is home to companies like Scaled Composites, the first to send a private astronaut to space, and Masten Space Systems, which is in the business of building lunar landers. It’s the proving ground for America’s most audacious space projects, and when Aaron Davis and Scott Stegman arrived at the hallowed tarmac last July, they knew they were in the right place.
The two men arrived at the airfield before dawn to set up the test stand for a prototype of their air-breathing rocket engine, a new kind of propulsion system that is a cross between a rocket motor and a jet engine. They call their unholy creation Fenris, and Davis believes that it’s the only way to make getting to space cheap enough for the rest of us. While a conventional rocket engine must carry giant tanks of fuel and oxidizer on its journey to space, an air-breathing rocket motor pulls most of its oxidizer directly from the atmosphere. This means that an air-breathing rocket can lift more stuff with less propellant and drastically lower the cost of space access—at least in theory.
The idea to combine the efficiency of a jet engine with the power of a rocket motor isn’t new, but historically these systems have only been combined in stages. Virgin Galactic and Virgin Orbit, for example, use jet aircraft to carry conventional rockets several miles into the atmosphere before releasing them for the final leg of the journey to space. In other cases, the order is reversed. The fastest aircraft ever flown, NASA’s X-43, used a rocket engine to provide an initial boost before an air-breathing hypersonic jet engine—known as a scramjet—took over and accelerated the vehicle to 7,300 mph, nearly 10 times the speed of sound.
But if these staged systems could be rolled up into one engine, the huge efficiency gains would dramatically lower the cost of getting to space. “The holy grail is a single-stage-to-orbit vehicle where you just take off from a runway, fly into space, and come back and reuse the system,” says Christopher Goyne, director of the University of Virginia’s Aerospace Research Laboratory and an expert in hypersonic flight.
The big challenge with a single-stage-to-orbit, or SSTO, rocket is that achieving the speeds necessary for orbit—around 17,000 mph—requires a lot of propellant. But adding more propellant makes a rocket heavier, which makes it harder to reach orbital velocity. This vicious circle is known as the “tyranny of the rocket equation,” and is why it takes a two-stage rocket the size of an office building to launch a satellite the size of a car. Staging a rocket helps because it can shed dead weight once the first stage’s propellant is used up, but it’s still pretty inefficient to have to burn all that propellant in the first place. This is where an SSTO rocket with air-breathing engines would provide a huge efficiency boost.
“The idea is to use air-breathing engines earlier in the launch to take advantage of efficiency gains from engines that don’t have to carry their own oxidizer,” says Goyne. “Once you get high enough in the atmosphere, you start to run out of air for the air-breathing system and you can use the rocket for that final boost to orbit.”
When Davis founded Mountain Aerospace Research Solutions in 2018, no one had ever made a working air-breathing rocket engine before. NASA and aerospace giants like Rolls-Royce had tried, and all the projects fizzled out due to soaring costs and major technological challenges. But Davis, a former Aviation Ordnance technician in the Marines, had an idea for an air-breathing engine of his own and couldn’t shake the idea. “I hired Scott Stegman to prove to me it wouldn’t work,” Davis says. But Stegman, who previously worked as a mechanical engineer at Northrop Grumman, crunched the numbers and didn’t find any showstoppers. As far as physics was concerned, Davis’ engine seemed like it should work.