The ram accelerator belongs to the transverse gas gun family because the hydrogen flow is transverse to the projectile direction. The minimum mass (for 1-ton cargo) is 104 tons. The maximum velocity is unknown.

ORIGINAL RAM ACCELERATOR

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.

BIBLIOGRAPHY

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.

Ram accelerator profile

Ram accelerator profile

Ram accelerator section

Ram accelerator section

HYDROGEN CORE RAM ACCELERATOR

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:

  1. Balloon ram accelerator divides the tube with a disposable balloon. It takes lots of manual labor to remove the remnants of old balloon and place new the balloon in the tube.

    Balloon ram accelerator section

    Balloon ram accelerator section

  2. Gas vortex ram accelerator does not have a physical barrier dividing the tube. The gases are separated because they flow in the tube like a vortex. If the tube is horizontal, the flow must be fast enough to offset buoyancy. The flow is turbulent so the separation is not perfect.
  3. Laminar ram accelerator is a vertical tube filled with slow flowing gases. The flow is laminar, so the gases do not mix. To prevent premature detonation, the center of the tube is filled with hydrogen, while the outer layer is made of poorly mixed fuel and oxidizer. The passage of the projectile mixes and ignites the outer layer.
  4. Powder vortex ram accelerator also employs the vortex to divide the tube. A fine powder of ammonium nitrate is used as the oxidizer. The centrifugal force keeps most of the powder away from the center of the tube. A thin, hot boundary layer forms on the nose cone of the projectile. Powder in the center of the tube burns in the boundary layer before impinging on the nose cone. Density of the mixture is lower in the center of the tube, so the aerodynamic forces may be strong enough to keep the projectile away from the walls of the tube. Helical fins maintain the vortex flow and generate turbulence which ensures uniform density of the powder in the outer part of the tube. Ammonium nitrate is hygroscopic and does not produce soot while burning in hydrogen.
    Military application of this contraption must be short, so it must utilize dense mixture of hydrogen, powdered fuel, and powdered oxidizer.

    BIBLIOGRAPHY

    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.

BLAST-WAVE ACCELERATOR

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.

Blast-wave accelerator section

Blast-wave accelerator section

BIBLIOGRAPHY

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.