The Rocket Motor of the Future Breathes Air Like a Jet Engine

"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."

See:

https://www.wired.com/story/the-rock...-a-jet-engine/