This is the Ram Accelerator

The ram accelerator is a scalable hypervelocity launcher capable, in principle, of accelerating projectiles to velocities greater than 8 km/s (18,000 mph) and was developed in 1983 by researchers of the Department of Aeronautics and Astronautics at the University of Washington. This device operates as an in-bore ramjet in which a subcaliber projectile, shaped like the centerbody of a cylindrical supersonic ramjet, is propelled through a stationary tube filled with a pressurized gaseous propellant mixture of fuel, oxidizer, and diluent. The propellant combustion is self-synchronized with the moving projectile to occur near its base, raising the pressure on the aftbody to generate prodigious amounts of thrust. The chemical energy density and speed of sound of the propellant can be adjusted, via gas pressure and composition, to control the in-tube Mach number and acceleration history. Successful ram accelerator operation has been obtained at gas fill pressures up to 200 bar and projectile velocities up to 2.7 km/s. Scaling has been demonstrated in bore sizes ranging from 13 to 120 mm in research facilities around the world. Potential applications of the ram accelerator include hypervelocity impact studies, hypersonic propulsion research, and direct launch of acceleration-insensitive payloads to low Earth orbit. More detailed information about the ram accelerator's history and various research topics are available on these pages and the links to related research sites.