Layman's Introduction of the Ram Accelerator

Since 1983 research has been carried out at the University of Washington on a hypervelocity launcher concept called the ram accelerator. This device owes little to conventional hypervelocity launchers (power guns, rail guns, light gas guns, etc.). In conventional powder guns, the propellant (e.g., nitrocellulose or gun powder) is burned behind the projectile in a breech, generating high pressure gas which expands as it pushes the projectile down the barrel. As the projectile goes faster and faster, though, the propelling gas must expend more energy to accelerate itself just to keep up with the projectile. Once the projectile reaches a certain speed, the propellant gas pressure exerts only enough force to overcome friction, and thereafter the projectile begins to slow down if the barrel is too long. As a rule of thumb, this velocity limit typically corresponds to ~twice the sound speed of the propellant prior to its expansion (without friction the ideal limit is ~5 times the sound speed). For high performance laboratory powder guns, this upper velocity limit is around 2-2.5 km/sec. (Note: the ratio of propellant gas mass to projectile mass has little effect on peak velocity after a ratio of ~8 is achieved, thus increasing pressure beyond a certain point does not increase peak velocity.)

To reach velocities greater than those possible with powder guns, low molecular mass propellants heated to high temperatures can be used to raise the propellant sound speed, and thus the gun’s upper velocity limit. Two-stage light gas guns accelerate a piston (with either gunpowder or compressed gas) in a “pump tube” to compress and heat hydrogen propellant. The hydrogen is compressed to peak temperature and pressure in an “acceleration reservoir” just prior to its release behind the projectile. The hydrogen is readily heated to ~3000 K in this manner which raises its sound speed to over 4000 m/s, thus muzzle velocities in excess of 7 km/s are possible with these launchers. Because of their hypervelocity capability these launchers have been proposed for space launch applications; however, scaling issues and overall system complexity have precluded, to date, them from being applied in this manner.

In conventional guns, whether they use gunpowder or light gas propellant, the highest pressure in the launcher is always at the breech, where it does the least amount of good. The lowest pressure of the system is always at the base of the projectile, where high pressure is desired, and continuously decreases as the projectile is accelerated to higher velocities. The pressure profile of a conventional gun is shown the adjacent schematic.

In 1983, Prof. Abe Hertzberg and two colleagues at the University of Washington, Prof. Adam Bruckner and Dr. David Bogdanoff, came up with a novel approach to the problem of launching large payloads to hypervelocity: fly a jet engine through the tube! In this situation the launch tube is filled with propellant and the subcaliber projectile is shot directly into it. With a properly shaped projectile, a ramjet-like propulsive cycle can be initiated in which the supersonic projectile ram-compresses and ignites the propellant as it travels through the tube. This process raises the base pressure on the projectile, generating thrust, without any significant acceleration of the propellant. This results in a pressure pulse being accelerated down the tube, self-synchronized with the projectile, as shown in above figure. Thus the highest pressure in the system is always right behind the projectile, where it does the most good, and not in the breech (as in a conventional gun).

Since the projectile must fly through its own propellant, a gaseous propellant is used. You can think of the projectile as surfing on the pressure pulse of a combustion wave that is accelerating down the length of the launch tube.

The flow field around the projectile is similar to a conventional ramjet, with the outer cowling replaced by a stationary tube, and a projectile which resembles the centerbody of a ramjet. The tube is filled to a high pressure (up to 200 atm) with a premixed propellant, usually oxygen, methane, and various diluent gases. The projectile is injected into the tube at supersonic velocities by a conventional single-stage light gas gun (i.e., a compressor pumps helium into the breech). The resulting shock structure initiates and sustains a combustion process which accelerates the projectile down the tube. Different propellant mixtures (separated by thin diaphragms) can be used down the length of the ram accelerator, tailoring the device for maximum performance.

There are a number of different modes of propulsion which utilize both subsonic and supersonic (shock-induced) combustion. Supersonic combustion allow projectiles to accelerate at speeds greater than the Chapman-Jouguet detonation speed of the mixture though which it is traveling. Theoretical modeling of this so called “superdetonative” propulsive mode indicates that velocities greater than twice the CJ detonation speed are possible. Since gaseous propellants can have CJ speeds up to ~4 km/s, this indicates that muzzle velocities of 8 km/s are feasible with the ram accelerator.

The emphasis of most experimental work to date has been on the thermally choked propulsive mode, which is very similar to a conventional supersonic ramjet operating with subsonic combustion. This mode of ram accelerator operation has accelerated 70 gm projectiles at the UW facility up to velocities of 2.7 km/sec in a 16-m-long test section, and demonstrated peak accelerations of ~75,000 g's with 110 gm projectiles. While the present facility is only a 38-mm-bore, the ram accelerator has great scaling potential for applications such as direct space launch and ground-based testing of hypersonic propulsive cycles at full-scale Reynolds number.

Scaling is one of the ram accelerator's key selling points. Unlike light gas guns and EM railguns, the ram accelerator stores its energy source (combustible gas) in the launch tube itself. Hence, as the size of the projectile is scaled up, the amount of energy available increases automatically. Proof-of-principle on the small side has been demonstrated with both a 25-mm-bore and a rectangular 15 x 20-mm-bore ram accelerator facilities in Japan. The scaling up potential has been demonstrated by a 120-mm-bore ram accelerator at the U.S. Army Research Laboratory (ARL) in Aberdeen MD and a 90-mm-bore ram accelerator at the Institut Franco-Allemand de Rescherches de St. Louis (ISL) in France.

These above mentioned research institutes and others from around the world that have constructed ram accelerator facilities are shown on the map below.