The original ram accelerator is a steel tube filled with a gaseous mixture of fuel, oxidizer, and diluent. The most popular gases are methane, oxygen, and nitrogen. To reduce the length of the tube, the mixture is pressurized. A projectile resting on a sabot is fired from a conventional powder gun into the ram accelerator. The projectile compresses the mixture to the point of ignition. Thrust is generated by the mixture expanding behind the projectile. The ram accelerator is plagued by ablation and premature detonation in front of the projectile. Physical contact between the projectile and the tube erodes both of them.
A. Hertzberg, A. P. Bruckner, and David W. Bogdanoff, "The Ram Accelerator: A New Chemical Method of Accelerating Projectiles to Ultrahigh Velocities," AIAA Journal, Vol. 26, No. 2, February 1988, pp. 195-203.
P. Kaloupis and A. P. Bruckner, "The Ram Accelerator: A Chemically Driven Mass Launcher," AIAA Paper 88-2968, AIAA/ASME/SAE/ASEE 24th Joint Propulsion Conference, Boston, MA, July 11-13, 1988.
Ram accelerator at the University of Washington.
The hydrogen core ram accelerator is a steel tube divided into two parts: the part near the axis is filled with hydrogen, while the outer part is filled with a mixture of fuel, oxidizer, and diluent. Ablation and premature detonation are reduced by immersing the projectile in hydrogen. Annular or helical dividers also prevent premature detonation. There are 3 variants of the hydrogen core ram accelerator:
David W. Bogdanoff, "Ram Accelerator Direct Space Launch System - New Concepts," Journal of Propulsion and Power, Vol. 8, March-April 1992, pp. 481-490.
David W. Bogdanoff and Andrew Higgins, "Hydrogen Core Techniques for the Ram Accelerator," AIAA Paper 96-0668, 34th Aerospace Sciences Meeting and Exhibit, Reno, NV, January 15-18, 1996.
This contraption also has the hydrogen core. The projectile is propelled by a high explosive. Plastic foam protects the steel tube from the explosion. The blast-wave accelerator is more expensive than the hydrogen core ram accelerator. Disposable designs forgo the foam to achieve higher hydrogen pressure.
E. T. Moore, D. Mumma, C. S. Godfrey, and D. Bernstein, "Explosive Gas Guns for Hypervelocity Acceleration," Fourth Hypervelocity Techniques Symposium, Arnold Air Force Station, TN, November 1965, pp. 457-484.
NASA Contractor Reports: CR-982, CR-1533, and CR-2143.
C. A. Rodenberger, "Obtaining Hypervelocity Acceleration Using Propellant-Lined Launch Tubes," NASA CR 10193, 1969.
C. A. Rodenberger, M. L. Sawyer, and M. M. Tower, "On the Feasibility of Obtaining Hypervelocity Acceleration Using Propellant Lined Launch Tubes," NASA CR 108699, 1970.
I. T. Bakirov and V. V. Mitrofanov, "High Velocity Two-Layer Detonation in an Explosive Gas System," Soviet Physics Doklady, Vol. 21, 1976, pp. 704-706.
A. E. Voitenko, "Principal Energy Characteristics of a Linear Jet Engine," Journal of Applied Mechanics and Technical Physics, Vol. 31, 1990, pp. 273-275.
V. I. Tarzhanov, "Massive Body Acceleration on the Detonation Wave Front," Combustion, Explosion, and Shock Waves, Vol. 27, 1991, pp. 130-132.
P. V. Kryukov, "BALSAD-Ballistic System for Antiasteroid Defense," Second International Workshop on Ram Accelerators," Seattle, WA, July 17-20, 1995.
G. Carrier, F. Fendell, and F. Wu, "Projectile Acceleration in a Solid-Propellant-Lined Tube," Combustion Science and Technology, Vol. 104, January 1995, pp. 1-17.
K. Takayama and A. Sasoh, (editors) Ram Accelerators, Springer-Verlag, Heidelberg, Germany, May 1998.