Ram accelerator
Encyclopedia
A ram accelerator has the same function as a gun
; i.e., it is a device for accelerating projectiles; however, it is entirely different in that jet-engine-like propulsion cycles utilizing ramjet
and/or scramjet
combustion processes are used to accelerate a projectile
to extremely high speeds. It is thought to be possible to achieve non-rocket spacelaunch
with this technology.
It is a long tube (barrel) filled with a mixture of combustible gasses with a frangible
diaphragm at either end to contain the gases. The projectile, which is shaped like a ram jet core, is fired by another means (e.g., a light gas gun) supersonically through the first diaphragm into the end of the tube. It then burns the gases as fuel, accelerating down the tube under jet propulsion. Other physics come into play at higher velocities.
To span a wide range in a typical ram accelerator system, multiple stages with propellants having different sound speeds are used to maintain high performance. membranes or diaphragms that are easily punctured by the ram accelerator projectile are used to isolate the propellant stages. Each section is filled with a different fuel-air mixture chosen so that later sections have higher speeds of sound. As such, the ram can be maintained at optimal speeds of Mach
3–5 (relative to the mixture that it travels through) during its entire acceleration period. Ram accelerators optimized to use supersonic combustion
modes can generate even higher velocities (Mach 6-8) due to the ability to combust fuel that is still moving at supersonic speed.
). With a ram accelerator, the projectile is propelled primarily by the pressure generated by the reaction of the propellant gases burning just behind the projectile. This leads to constant pressure being put both on the gun and the projectile itself. Consequently, far longer barrels are possible than conventional guns, while still delivering a strong constant acceleration to the projectile.
s, such as those designed in Project HARP
, to achieve orbit. Using rocket
s for upper stages, it is believed that a launch cost of less than $500 (US dollars, 2004) per kilogram can be attained. Using a tether or space tug would further reduce launch costs.
The main competitors to a ram accelerator for direct space launch applications are two-stage gas guns (SHARP)
, multiple sidewall injection gas guns (JVL), railgun
s and coilgun
s. Ram accelerators are currently used primarily for research into supersonic combustion. The scram cannon
science fiction
weapon was inspired by ram accelerators.
As of July 2007, the Ballistic Flight Group has been promoting commercial investment into the ram accelerator technology.
Gun
A gun is a muzzle or breech-loaded projectile-firing weapon. There are various definitions depending on the nation and branch of service. A "gun" may be distinguished from other firearms in being a crew-served weapon such as a howitzer or mortar, as opposed to a small arm like a rifle or pistol,...
; i.e., it is a device for accelerating projectiles; however, it is entirely different in that jet-engine-like propulsion cycles utilizing ramjet
Ramjet
A ramjet, sometimes referred to as a stovepipe jet, or an athodyd, is a form of airbreathing jet engine using the engine's forward motion to compress incoming air, without a rotary compressor. Ramjets cannot produce thrust at zero airspeed and thus cannot move an aircraft from a standstill...
and/or scramjet
Scramjet
A scramjet is a variant of a ramjet airbreathing jet engine in which combustion takes place in supersonic airflow...
combustion processes are used to accelerate a projectile
Projectile
A projectile is any object projected into space by the exertion of a force. Although a thrown baseball is technically a projectile too, the term more commonly refers to a weapon....
to extremely high speeds. It is thought to be possible to achieve non-rocket spacelaunch
Non-rocket spacelaunch
Non-rocket space launch is a launch into space where some or all needed speed and altitude is provided by non-rocket means, rather than simply using conventional chemical rockets from the ground. A number of alternatives to rockets have been proposed...
with this technology.
It is a long tube (barrel) filled with a mixture of combustible gasses with a frangible
Frangible
A material is said to be frangible if through deformation it tends to break up into fragments, rather than deforming plastically and retaining its cohesion as a single object...
diaphragm at either end to contain the gases. The projectile, which is shaped like a ram jet core, is fired by another means (e.g., a light gas gun) supersonically through the first diaphragm into the end of the tube. It then burns the gases as fuel, accelerating down the tube under jet propulsion. Other physics come into play at higher velocities.
Description
In a normal ramjet, air is compressed between a spike-shaped centerbody and an outer cowling, fuel is added and burned, and high speed exhaust gases are expanded supersonically out the nozzle to generate thrust. In a ram accelerator, a projectile having a shape similar to the ramjet centerbody is fired, (often from a conventional gun), into the accelerator barrel, causing compression between the projectile and the barrel's walls. The barrel contains a pre-mixed gaseous fuel-air mixture. As the ram accelerator projectile compresses the fuel-air mixture, it is ignited and the combustion is stabilized at the base of the projectile. The resulting pressure differential generates a prodigious amount of thrust that can accelerate projectiles to in-tube Mach numbers greater than 8. Thus if propellant mixtures having a sound speed of 1000 m/s (e.g. fuel-rich H2-O2 mixtures) are used, muzzle velocities in excess of 8000 m/s are possible.To span a wide range in a typical ram accelerator system, multiple stages with propellants having different sound speeds are used to maintain high performance. membranes or diaphragms that are easily punctured by the ram accelerator projectile are used to isolate the propellant stages. Each section is filled with a different fuel-air mixture chosen so that later sections have higher speeds of sound. As such, the ram can be maintained at optimal speeds of Mach
Mach number
Mach number is the speed of an object moving through air, or any other fluid substance, divided by the speed of sound as it is in that substance for its particular physical conditions, including those of temperature and pressure...
3–5 (relative to the mixture that it travels through) during its entire acceleration period. Ram accelerators optimized to use supersonic combustion
Scramjet
A scramjet is a variant of a ramjet airbreathing jet engine in which combustion takes place in supersonic airflow...
modes can generate even higher velocities (Mach 6-8) due to the ability to combust fuel that is still moving at supersonic speed.
Advantages
The chief advantage of a ram accelerator over a conventional gun is its scalability. In a normal gun, maximum pressure is exerted at the time of the initial charge detonation. As the projectile moves further down the barrel, the amount of acceleration upon the projectile decreases as the gas behind it expands, eventually reaching amounts trivial enough that a longer barrel is no longer justified (see internal ballisticsInternal ballistics
Internal ballistics, a subfield of ballistics, is the study of a projectile's behavior from the time its propellant's igniter is initiated until it exits the gun barrel...
). With a ram accelerator, the projectile is propelled primarily by the pressure generated by the reaction of the propellant gases burning just behind the projectile. This leads to constant pressure being put both on the gun and the projectile itself. Consequently, far longer barrels are possible than conventional guns, while still delivering a strong constant acceleration to the projectile.
Uses
Ram accelerators have been proposed as a cheap method to get payloads into space. Impulsive launched projectiles need some means to circularize their trajectory for orbit insertion, thus typically incorporated into the projectiles are rocketRocket
A rocket is a missile, spacecraft, aircraft or other vehicle which obtains thrust from a rocket engine. In all rockets, the exhaust is formed entirely from propellants carried within the rocket before use. Rocket engines work by action and reaction...
s, such as those designed in Project HARP
Project HARP
Project HARP, short for High Altitude Research Project, was a joint project of the United States Department of Defense and Canada's Department of National Defence created with the goal of studying ballistics of re-entry vehicles at low cost; whereas most such projects used expensive rockets, HARP...
, to achieve orbit. Using rocket
Rocket
A rocket is a missile, spacecraft, aircraft or other vehicle which obtains thrust from a rocket engine. In all rockets, the exhaust is formed entirely from propellants carried within the rocket before use. Rocket engines work by action and reaction...
s for upper stages, it is believed that a launch cost of less than $500 (US dollars, 2004) per kilogram can be attained. Using a tether or space tug would further reduce launch costs.
The main competitors to a ram accelerator for direct space launch applications are two-stage gas guns (SHARP)
Super High Altitude Research Project
The Super High Altitude Research Project was a U.S. government project conducting research into the firing of high-velocity projectiles high into the atmosphere using a two stage light gas gun, with the ultimate goal of propelling satellites into Earth orbit...
, multiple sidewall injection gas guns (JVL), railgun
Railgun
A railgun is an entirely electrical gun that accelerates a conductive projectile along a pair of metal rails using the same principles as the homopolar motor. Railguns use two sliding or rolling contacts that permit a large electric current to pass through the projectile. This current interacts...
s and coilgun
Coilgun
A coilgun is a type of projectile accelerator that consists of one or more coils used as electromagnets in the configuration of a synchronous linear motor which accelerate a magnetic projectile to high velocity...
s. Ram accelerators are currently used primarily for research into supersonic combustion. The scram cannon
Scram cannon
The scram cannon is a hypothetical kinetic energy weapon based on ram accelerator technology. The scram cannon uses a projectile that uses supersonic combustion ramjet technology to increase the muzzle velocity to levels that are impossible with normal gunpowder based guns; firearms are limited...
science fiction
Science fiction
Science fiction is a genre of fiction dealing with imaginary but more or less plausible content such as future settings, futuristic science and technology, space travel, aliens, and paranormal abilities...
weapon was inspired by ram accelerators.
As of July 2007, the Ballistic Flight Group has been promoting commercial investment into the ram accelerator technology.
External links
- The Ballistic Flight Group website
- accelerator home page (archive.org link) at the University of WashingtonUniversity of WashingtonUniversity of Washington is a public research university, founded in 1861 in Seattle, Washington, United States. The UW is the largest university in the Northwest and the oldest public university on the West Coast. The university has three campuses, with its largest campus in the University...
. - Detailed description (archive.org link) of ram accelerator operating modes (from the University of WashingtonUniversity of WashingtonUniversity of Washington is a public research university, founded in 1861 in Seattle, Washington, United States. The UW is the largest university in the Northwest and the oldest public university on the West Coast. The university has three campuses, with its largest campus in the University...
page set).