General
Features of the Ram Accelerator
Conventional Gun vs. Ram Accelerator
Conventional Ramjet vs. Ram Accelerator
Staging of the Ram Accelerator
Ram Accelerator Propulsive Regimes
Ram Accelerator with Initial Launcher
How Ram Acceleration Works
![[OpSequence.jpg]](pics/OpSequence.jpg)
Initially the projectile is loaded into the bore of a gun (either gunpowder or light gas guns are used), with an obturator to seal the bore. When the gun is fired, the high pressure gas pushes against the obturator and projectile, injecting it supersonically into the ram accelerator section which is filled with the premixed propellants. The propellants are immediately ignited behind the projectile, causing an increase in pressure that accelerates the projectile down the tube and pushes the obturator backwards. As the projectile is accelerated down the tube, combustion remains attached to projectile base, fed by the propellants that are ram-compressed, ignited and expanded as the projectile travels past.
Conventional Gun vs. Ram Accelerator
![[Staging.gif]](pics/RamVsGun.gif)
In a conventional gun, the projectile is pushed by high pressure / high temperature gun gas released from the breech. As the projectile accelerates and this gun gas expands, the pressure pushing on the projectile decreases. The highest pressure in the system is always in the breech, where it does no good, and the lowest system pressure is always at the projectile base, where high pressure is desired. The gun gas has to accelerate itself as well as the projectile, which leads to rapid decrease in ballistic efficiency (less than 10% at half max speed) as projectile velocity exceeds the acoustic speed of the gun gas prior to launch.
In the ram accelerator, gaseous propellant is distributed along the length of the launch tube. The combustion travels with the projectile, which causes the highest pressure in the tube to always be on the rear of the projectile, where it does the most good. This results in the ram accelerator being a very effective hypervelocity launcher with high ballistic efficiency (e.g., 10%-30% possible up to 6 km/s).
Conventional Ramjet vs. Ram Accelerator
![[Staging.gif]](pics/RamVsRamjet.gif)
The thermally choked ram accelerator propulsive mode operates similar to a conventional supersonic ramjet. The projectile resembles the centerbody of the ramjet and the tube wall acts as its outer cowling. Both devices use a normal shock wave system to decelerate the flow to subsonic for combustion. Unlike the airbreathing ramjet, the ram accelerator does not have a nozzle to mechanically choke the flow. Instead, the flow of combustion products is choked by the heat release itself at full tube area (thermal choking). Another key difference is that the ramjet must carry fuel and inject it into the incoming air, whereas the ram accelerator projectile does not carry fuel. Instead, it travels through an atmosphere of premixed fuel and oxidizer; i.e., the projectile flies through its fuel tank.
Staging of the Ram Accelerator
![[Staging.gif]](pics/Staging.gif)
The composition of the propellant can be tailored along the length of the tube to maintain the most efficient operation. The net thrust decreases in all ram accelerator propulsive modes as the projectile in-tube Mach number increases in a given propellant. High average acceleration can be maintained, however, by entering a new propellant with a higher acoustic speed (which immediately reduces in-tube Mach number). Thin diaphragms separate the propellants of sequential stages. By continuing to make transitions to higher sound speed mixtures, the Mach number can be maintained within the optimal range of the desired ram accelerator propulsive mode, where the thrust is highest, while the projectile continues to accelerate to higher velocities. Staging also greatly reduces the aerodynamic heating of the projectile, since the Mach numbers are kept to a minimum.
Staging is analogous to a transmission shifting gears to increase
automobile performance.
Ram Accelerator Propulsive Regimes
![[Staging.gif]](pics/Regimes-2.gif)
There are several propulsive modes in which the ram accelerator can operate, which are characterized by the relative projectile velocity with respect to the Chapman-Jouguet detonation velocity (VCJ). In subdetonative velocity regime (V less than VCJ), the combustion occurs subsonically and is thermally choked in the full tube area behind the projectile, with the blunt base acting as a flame holder. This mode of operation works best in the Mach number range of 3 to 5. The thrust of the thermally choked modes decreases as the projectile reaches the CJ detonation velocity, where theoretically the thrust goes to zero.
At superdetonative velocities (V greater than VCJ), the combustion has moved up onto the body and the energy release takes place entirely supersonically in the annular region between the projectile and tube wall. The combustion could be an oblique detonation, shock induced combustion, boundary layer combustion, or a combination of these. This mode is analogous to a scramjet or oblique detonation wave engine.
While theoretically the thermally choked mode is limited to below the detonation velocity, experimentally it is observed that a projectile can accelerate from below to above the CJ detonation velocity in a single propellant mixture. This transition from thermally choked to superdetonative operation, called "transdetonative", results from mixed mode combustion occurring on and behind the projectile in both subsonic and supersonic flow.
Ram Accelerator with Initial Launcher
![[Staging.gif]](pics/FacilityFins.gif)
Since the ram accelerator projectile must travel supersonically, the system requires some type of pre-accelerator to provide an initial boost. Typically, a gas gun or powder gun is used to accelerate the projectile to 1 km/sec, or Mach 3, where ram acceleration can begin. By proper venting of the ram accelerator combustion products, the recoil can be completely eliminated. Also, since the projectile is subcaliber, some mechanical means is required to stabilize the projectile in the tube. Either the projectile can have fins to keep it centered, or an axisymmetric projectile can ride on rails in the tube.