What kind of "jet engine-like propulsion" does HyperSciences use to accelerate projectiles to sub-orbital altitudes?

Question

The Teslarati article For HyperSciences, geothermal energy builds a path to space describes the use of technology which has been applied to drilling to launching projectiles to sub-orbital altitudes from the ground.

This kind of technology proposition gained NASA’s interest, and HyperSciences has since won a Phase I innovation award for from the agency, the testing for which was successfully completed at Spaceport America in New Mexico at the end of January this year. With this achievement under their belt, Russell’s long-time spaceflight dreams are really starting to take shape.

It links to the Vimeo video HyperSciences - Aerospace NASA Launch and after about 00:36 Mark Russell, Founder and CEO HyperSciences:

Our HyperCore engine, which uses jet engine-like propulsion technology, launches HyperDrone capsules of various sizes up to sub-orbital altitudes.

What is the jet engine-like propulsion technology, and how does it achieve circa 1000 g acceleration of the projectiles?

Answer 1

The term "jet engine-like" is a bit misleading. It's more ramjet than turbojet. This is what happens:

It consists of a long tube (barrel) filled with a mixture of combustible gases with a frangible diaphragm at either end to contain the gases. The projectile is fired by another means (e.g., a light-gas gun or railgun) supersonically through the first diaphragm into the tube. Then the projectile burns the gases as fuel, because it is shaped like a ramjet or scramjet core, and accelerates under jet propulsion.

In a normal ramjet, air is compressed between a spike-shaped centerbody and an outer cowling, fuel is added and burned, and high speed exhaust gases are expanded supersonically out of the nozzle to generate thrust. In a ram accelerator, a projectile having a shape similar to the ramjet centerbody is fired, (often from a conventional gun), into the accelerator barrel, causing compression between the projectile and the barrel's walls. The barrel contains a pre-mixed gaseous fuel-air mixture. As the ram accelerator projectile compresses the fuel-air mixture, it is ignited and the combustion is stabilized at the base of the projectile. The resulting pressure differential generates a prodigious amount of thrust that can accelerate projectiles to in-tube Mach numbers greater than 8. Thus, if propellant mixtures with a speed of sound of 1000 m/s (e.g. fuel-rich H2-O2 mixtures) are used, muzzle velocities in excess of 8000 m/s are possible.

This is a cross-section of a ramjet:

In a ramjet, the bullet-shaped object is in a fixed position (or it can be moved slightly to optimize operation at different speeds).

In a ram accelerator like the Hypersciences system, the bullet-shaped object is fired from a gun and travels through a long tube filled with combustible gas. The bullet compresses the air, and if all goes well, the mixture combusts behind the bullet, accelerating the bullet throughout the tube.