FAQ

1) Why have I never heard of a ram accelerator before?

Technical people that work on combustion or ballistics usually have heard of a ram accelerator; however, there have not been many popular publications that mention ram accelerator technology. In contrast, the railgun technology has been featured in Popular Science, Slashdot, the Fox News channel, and many sci-fi novels.

2) How can you expect to launch anything useful out of a gun?

Complex gun-launched systems are in every day military use. Examples include GPS-guided shells, guided missiles, rockets, and even gun-launched aircraft (UAVs). High-g tests from as early as the 1960's (HARP) have shown that every major satellite subsystem can be made to survive the high accelerations from gun-launch. Data from the 1990's (SHARP) show that commercial off-the-shelf electronics can be inexpensively hardened against high launch accelerations. Additionally, data from both SHARP and railgun research show that conventional ablative heat-shield technology can protect a satellite as it transits through the lower atmosphere at orbital velocities.

3) Why is the ram accelerator cheaper than a rocket to launch satellites?

The ram accelerator represents the ultimate in reusability. Both a ram accelerator and a rocket represent a significant capital investment. The difference is that the rocket is thrown away after it has been used once, while the ram accelerator is good for many thousands of shots. Due to its simplicity and reliance on combustion as its energy transfer method, the ram accelerator also has the lowest capital costs over any other gun-launch technologies (light gas guns, rail guns) by orders of magnitude.

4) What is a ram accelerator, and how does it work?

The ram accelerator is a thick-walled steel tube with rails, filled with a combustive gas mixture like methane and oxygen. A projectile is injected at supersonic speeds into the ram accelerator via a starter gun, and it leaves the ram accelerator traveling greater than 6km/s. The front of projectile acts as a ramjet center body (supersonic diffuser), the rear acts like a nozzle, and the shock waves generated cause the combustion of the gas mixture it is flying through. This combustion and subsequent expansion of gasses through the nozzle area is what causes thrust. In essence the projectile is operating as an inside-out jet engine. Since the "jet engine" fuel is pre-mixed, there can be combustion at extremely high Mach numbers (M 7), thus very high velocities are attainable. You can find more technical information at the ram accelerator's home page. Also, you can find detailed descriptions of launch noise, ram accelerator operation, high-g component selection, dealing with atmospheric transit at Mach 18, and more within the technical risks paper.

5) What is the current state of ram accelerator technology?

There are many ram accelerators of varying size worldwide, with the most advanced facility currently at the University of Washington. Since its invention at the UW in 1983, steady ram accelerator development has strongly indicated that the ram accelerator propulsive cycle will work at the projectile sizes the BFG needs for orbital launch (~500mm bore). Successful ram accelerator operation has been demonstrated with bore configurations ranging from a rectangular 5x10mm launcher at Hiroshima University in Japan, to the 120mm-bore ram accelerator (currently the world's largest) built at the Army Research Laboratory in Aberdeen MD.

Ram accelerators have demonstrated projectile velocities as high as 1.5 times the detonation speed (Vdet) of the propellant, corresponding to an in-tube Mach number of M = 7. These two numbers are the most important metrics for determining ram accelerator performance. While these particular experiments were carried out in methane-oxygen propellants diluted with carbon dioxide (to significantly reduce propellant acoustic speed) at velocities around 2km/s, this translates into 6km/s performance when using a propellant composition with a high detonation speed; e.g., hydrogen and oxygen (Vdet = 4km/s). A lengthier facility to demonstrate this key milestone will require a concentrated injection of capital.