The projectile consists of a two stage rocket and cargo. It is propelled by hot, compressed, light gas. Long-term exposure to hot hydrogen embrittles steel tanks, so helium is preferred. The gun replaces the first stage rocket. Maximum velocity is limited to the speed of sound in hot helium, which is about 2 km/s. The maximum velocity can be increased by additional heating of the propellant prior to launch, either with electric arc pulse, or contact with hot particles. Another variant called the steam gun propels the projectile with a burning mixture of hydrogen and oxygen. The minimum mass is 104 tons.
A. E. Seigel and Z. I. Slawsky, "A hypervelocity gun using a shock-compressed steam-heated propellant," NavOrd Rept. 4345, 1956. (the steam gun)
Dani Eder, "A Low Cost Earth Based Launch System and Its Effects on Space Industrialization," published in Space Manufacturing 4, Proceedings of the Fifth Princeton/AIAA Conference, May 19-21, 1981, pp. 221-229.
J. R. Powell, F. L. Horn, R. Benenati, and A. Lowrey, "Technology of Thermal Hypervelocity Launchers," IEEE Transactions on Magnetics, Vol. 22. No. 6, 1986, pp. 1675-1680.
Derek A. Tidman and Dennis W. Massey, "Electrothermal Light Gas Gun," IEEE Transactions on Magnetics, Vol. 29, No. 1, January 1993, pp. 621-624.
L. R. Bertolini, John W. Hunter, J. R. Powell, and Derek A. Tidman, "SHARP, A First Step Towards a Full Sized Jules Verne Launcher," Space Manufacturing 9, Proceedings of the 11th SSI-Princeton Conference, May 12-15, 1993, pp. 79-86.