Ram Accelerator

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In Jules Verne's classic 1865 story From the Earth to the Moon, protagonist Impey Barbicane sends a group of passengers to the moon by firing their space vehicle out of a giant cannon. Though Verne implies our heroes survive the experience, the concept now seems quaintly naive. No person could actually survive a launch in that fashion.

But Verne's fancy may not be as far off as one might think. Now, over 130 years since Verne published his space-flight fantasy, perhaps we are coming full circle. After visiting the moon several times and routinely launching space shuttle missions into orbit, scientists recognize that further exploration of space will require the development of a large-scale space infrastructure to support the effort. A major obstacle is the high cost of sending materials needed for construction of large space systems up into orbit and beyond. Many of these raw materials can withstand high degrees of pressure and stress during the launching process.

University of Washington researchers have devised a scheme called the ram accelerator , not totally unlike Verne's approach, that has the potential to send acceleration-insensitive cargo into space, and moreover, to launch it in a more cost-effective way than any other method devised to date.

In the ram accelerator, a projectile is accelerated by a combustion wave down a tube filled with propellant gas, achieving very high velocities. Initial aspects of the concept were first envisioned in 1983 by UW professor Abraham Hertzberg.

Hertzberg, together with UW professor Adam Bruckner and research associate Carl Knowlen, all of the Department of Aeronautics and Astronautics, and David Bogdanoff, UW research engineer and later of NASA Ames, were the first to build an accelerator with this particular design. Their efforts have stimulated interest worldwide in hypervelocity launchers of this type. There are now six ram accelerator facilities of various sizes in the U.S., France, and Japan.

Conventional rockets must carry all of their own fuel and oxidizer, and they're designed to protect and shield their contents. As a result, only a small fraction of a rocket's capacity is available for carrying payload. That makes carrying anything on the space shuttle very expensive: on the order of several thousand dollars per pound. Payload costs using the ram accelerator would be much less--perhaps as low as $100 to $300 per pound.Footnote 1

Sited on a mountain slope to attain an incline of about 15°, the accelerator would be a 2.5-mile-long tube through which a 4,400-lb payload-bearing projectile could be launched. The muzzle velocity would reach about 20,000 miles per hour, enough to hurl the payload into orbit 300 miles above the earth's surface.

The idea resembles the way a ramjet aircraft engine works. The ramjet has an external tube and a sculptured centerbody which scoops up air effectively when the ramjet is moving forward. It uses the forward speed of the craft to compress incoming air which is then mixed with fuel and ignited, providing thrust.

In the ram accelerator, the centerbody is now the projectile; it is detached from the tube, which has been lengthened into a long, stationary launching tube. Gaseous fuel and oxidizer are premixed and fill the entire launcher. As the projectile travels the length of the tube, the gas mixture burns behind it, creating thrust. It is as if "the projectile is surfing through the tube on a wave of combustion," says Knowlen.footnote 1

In an experimental set-up at the UW, methane, oxygen, nitrogen, and helium are mixed in various proportions and injected into a 52-ft. long, 1.5-in diameter tube which has been divided into four segments by thin plastic diaphragms. In analogy to shifting gears in a car, the performance of the launcher can be optimized by tailoring the composition of the propellant mixtures in the segments. Recent experiments achieved a launch velocity of nearly 9,500 feet per second, or Mach 8.5 in air. The researchers expect to attain velocities well above 10,000 feet per second with their experimental device. Their work has been funded by the U.S. Air Force, the U.S. Army, NASA, the U.S. Department of Energy, and the Olin Aerospace Corporation.

  1. "Ram Accelerator: Space launch system of the future," Mary Cooksey, The Trend in Engineering, 39 (1) Spring 1988, p. 1.
  2. "Shot Into Orbit," Hill Williams, The Seattle Times, May 23, 1988.
  3. "Ramming Speed," Gregory T. Pope, Discover, March, 1994, p. 50.
  4. "Battle of the Big Shots," Frank Kuznik, Air & Space, August/September 1993, p. 55.
  5. "Ram Accelerator Demonstrates Potential for Hypervelocity Research, Light Launch," Aviation Week & Space Technology, Sept. 30, 1991, p. 50.

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