SCRAMJETS


Scramjet Missiles
Tech Level: 12
Scramjet Fighters
Scramjet Spaceplanes
Tech Level: 13
Scramjet Airliners
Tech Level: 14

Scramjets are currently being developed under NASA’s Hyper-X and X-43A programs, at the Defense Advanced Research Projects Agency (DARPA,) the Univeristy of Queensland's (Australia) Hyshot program, and by a number of private interests.

THE BASICS

The term scramjets comes from their technical name, Supersonic Combustion Ramjets, or s.c.ramjets. In order to understand how they work, we need to look first at how both standard jets and ramjets work.

Most modern jets are turbojets, so named because they use large, high-speed turbofans to draw in and compress air in the engine ignition chamber, where it is mixed with fuel and ignited. The exhaust is blown out the back of the engine, providing thrust. The denser and faster the airflow into the engine, the more power the engine can generate. However, between Mach 3 and 4, turbojet turbines and fans suffer damage from overheating.

Enter the ramjet, which feeds the air into the engine and compresses it using the extreme forward velocity of the vehicle itself. Ramjets can’t operate below certain speeds (about Mach 2 or 3), so they are usually integrated in with standard turbojets, as in the French Griffon II, an experimental aircraft built in 1959 which set the jet speed record for its time.

Unfortunately, ramjets can’t operate much beyond Mach 6, when the combustion chamber becomes so hot that the combustion products decompose. And this brings us to scramjets.

In a scramjet engine, the compression of the airflow is reduced so that it is not nearly slowed so much. Because the airflow remains supersonic, its temperature does not increase as dramatically as it does in normal ramjets. Fuel is injected into the supersonic airflow, where it mixes and must burn within a millisecond.

The extreme upper limits of a scramjet engine is undetermined, but is thought to be in the range of Mach 20 to 25, fast enough to acquire orbital velocity. Whether the rest of the craft could be built to withstand the structual stress of such in-atmosphere velocities remains to be seen, however. In practical use, it more likely scramjet vehicles will operate in the range of Mach 5 to 10, with higher velocities reserved for unmanned scramjet missiles or orbital insertion vehicles.

Also, scramjets have a much higher operational ceiling than their lower-tech jet cousins. Advanced turbojets have an extreme operational ceiling of about 40 km, while ramjets have a ceiling of about 55 km. Scramjets can operate up to 75 km high without fear of stalling.

Scramjets could use any standard combustible fuel, but the fuel of choice for early models being developed is liquid hydrogen, whose extreme low temperature can be used to help cool both the engine and the craft as a whole. In fact, without this cooling effect, scramjets will most likely be limited to speeds of Mach 8 or below.

Most diagrams for scramjets vehicles I’ve seen places the engine airtake on the bottom of an aerodynamically optimized, vaguely wedge-shaped lifting body. This scheme takes advantage of the vehicle’s bowshock to direct most of the airflow on the bottom of the vehicle into the intake.


SCRAMJET MISSILES


Tech Level: 12

Hyshot scramjet test flight, July 30, 2002

In 2001, DARPA twice successfully test-fired a scramjet-powered projectile, fired from a gun at Mach 7.1. The scramjet, once activated, covered over 260 feet in 30 milliseconds, proving the viability of the technology. Scramjets are far more efficient than rockets at hypersonic speeds within the atmosphere, and could represent a significant leap forward both in missile speed and range.

Like the DARPA test, scramjet missiles could be fired from specially designed guns or launchers that would allow them to obtain the minimum speeds needed for operation. Modern day artillery technology, even advanced stuff like the two-stage, 130-ft long gas gun used in the DARPA test, would probably not be a good idea for everyday application; the projectile underwent 10,000 g’s of acceleration when it was fired. Producing scramjet projectiles in quantity that could withstand such stress would probably not be too cost effective.

ElectroMagnetic Launchers (EMLs) may be a better idea, as they could accelerate the projectile somewhat more gently. However, EMLs are still in the experimental stage much like scramjets themselves, and may not mature soon enough to help out early scramjets.

A more practical method would be a two-stage system, with a rocket-powered first stage to get the missile quickly beyond the Mach 2-3 threshold and a scramjet-powered second stage to deliver the payload to its target.

This was the method used in both NASA X-43's and Hyshot's test flights of a scramjet engine. NASA's test had the X-43 test vehicle attached to a Pegasus rocket booster ferried up to altitude on the wing of a B-52. The rocket detached and shot up to the edge of space. The X-43 then detached from the booster for a descent that would push it to the hypersonic speeds needed to activate the scramjet engine. Unfortunately the test resulted in an uncontrolled descent and crash without a confirmed firing of its scramjet engine. The exact cause is still being investigated. Hyshot's missile was much more successful. The missile shot up to an altitude of 35km, sheddings its first stage and unleashing its scramjet payload for descent. Upon descent, the second stage achieved a velocity of Mach 7.6 and scramjet engine ignition was confirmed.

The first stage of a scramjet missile could be made reusable, basically a drone vehicle that could glide back to the ground unpowered once all its fuel was expended accelerating the second stage to the desired height and speed.

As one can imagine, shooting down a scramjet-powered projectile or cruise missile moving at what may be Mach 10 or more would be near-impossible with today’s ABM tech or techniques. Ironically, their very speed would make them very effective as Anti-Missiles to use against already-existing projectiles, and it may be in this role that scramjet missiles will first be used as the current push for an SDI anti-ballistic missile shield continues.



SCRAMJET FIGHTERS

Tech Level: 13

Speed is everything in air combat; just ask any fighter pilot. Oh, there are other factors--maneuverability, pilot skill, weapons, etc--but if your fighter is not at least in the same speed class as your opponent’s, you’re in very serious trouble.

Scramjet engines would be the greatest leap forward in air combat since jets went supersonic. Besides the tremendous increase in speed, they could also be the first trans-world fighting vehicles, able to take advantage of sub-orbital parabolic arcs (much like ICBMs) to theoretically reach any point on the globe in under an hour or so.

Like with ramjets, scramjets can be integrated into the same engine housing as a standard turbojet, allowing the vehicle to get up to minimal scramjet operating speed of Mach 2 or 3 and then switch over. Thus scramjet fighters would probably to take advantage of almost any already-existing runway or support facility that already handles normal turbojets, including modern aircraft carriers.

Sustained flight at extreme Mach speeds may require a radically different vehicle configuration than that required for subsonic or low Mach speeds, so much so that the hypersonic configuration might have trouble generating enough lift at low velocity for take off and landing. And, of course, the subsonic/low Mach configuration may literally be ripped apart if forced to withstand hypersonic speeds.

One obvious solution would be to have a reconfigurable aircraft, much like swept-wing fighters like the F-16, but involving a number of different surfaces and systems. When the aircraft reaches the transitional speed, it would automatically undergo the structural change, then reverse it when it slows down and crosses over that barrier again.

Integrating a human pilot safely into a scramjet fighter may be as difficult as engineering scramjets themselves. It is very easy with scramjet accelerations for a pilot to undergo an unhealthy amount of G-forces, especially with the tight manuevering sometimes required of fighter aircraft. At 9 Gs even the most experienced and hearty fighter pilot will black out; a scramjet fighter is easily capable of generating twice that in a tight turn.

Computer controls and (hopefully) the pilot’s know-how will probably keep the craft from exceeding tolerable G forces (about 3 Gs or lower) for more than a few seconds. Also, improvements in pilot G-suits (specially-designed flight suits with dynamic air or liquid compression networks that help cushion the pilots in high-G maneuvers), cockpit harnesses, and even G-compensating drugs being researched may help offset some adverse high-G effects.

Needless to say, a scramjet fighter would completely outclass any military or civilian aircraft flying today, and could probably even outrace most modern missiles in a straight run.


SCRAMJET SPACEPLANES
Tech Level: 13

If scramjets ever reach their theoretical extreme speed limit of Mach 25, it would be possible for them to scream up to 75 km altitude, building up all the velocity they could, and then just "coast" up into orbit.

However, it is far more likely that we’ll see scramjet/rocket hybrid spaceplanes. Just as turbojets and scramjets can be integrated into one engine housing, its also possible that a rocket can be integrated into the scramjet combustor to create a combined cycle engine. The rocket would be used for takeoff, subsonic, and low hypersonic flight. Once past the Mach 2-3 threshold, the scramjet takes over, further accelerating the vehicle to Mach 12 or so, when the rocket re-ignites to supplement the scramjet engine. Past Mach 18 the rocket engine takes over completely, propelling the vehicle into orbit and allowing it to maneuver in space.

There has also been talk of a rocket/ramjet/scramjet combined cycle engine, so the vehicle can take full advantage of optimized engine efficiency at each speed stage (rocket up to Mach 2-3, ramjet from Mach 3 to Mach 6, scramjet from Mach 6 to Mach 12, scramjet/rocket to Mach 18, and rocket only beyond Mach 18.) Also, the vehicle may have a turbojet/ramjet/scramjet combined cycle engine for take-off through Mach 12, then uses seperate rocket engines, either integrated onto the vehicle or as modular add-on "boosters," to allow the acceleration to space and to de-orbit.

Such modular rocket boosters could convert standard in-atmosphere scramjet vehicles, such as fighters and airliners, into true orbital vehicles, should such a need for that ever arise (fighters could be so modified to take out enemy satellites or ICBMs in mid-flight; scramjet airliners may be so modified to dock of any "space hotels" that may be in LEO.)


SCRAMJET AIRLINERS

Tech Level: 14

Scramjets will almost certainly be used for military and spaceflight purposes long before they’re integrated into civilian air fleets, but their speed advantage over long distances will probably eventually make them very attractive for trans-world commuters. Flights from Los Angeles to Tokyo would be more along the lines of two hours as opposed to the standard ten hours available nowadays, maybe less than half that if passengers are willing to withstand the higher g’s (up to 3) associated with parabolic sub-orbital arcs.

Like scramjet fighters, scramjet airliners would probably use standard turbojets integrated into a combined cycle engine for take-off, landing, and low-supersonic speeds, switching to the scramjet engine for hypersonic flight. Also like scramjet fighters, scramjet airliners may eventual integrate reconfigurable airframes in order to optimize airlift efficiency at each speed level.

Civilian scramjet vehicles probably won’t be without their controversy, however. The Concorde, today’s only commercial transonic airliner, is only in limited service today in part because of the disruptive sonic booms they create over nearby communities when approaching and taking off from commercial airports. Scramjets will no doubt meet with similar resistence from citizen groups concerned with excessive noise pollution.


RELATED INFORMATION


Details on NASA'a X-43 Project:

http://www.fas.org/spp/guide/usa/launch/x-43.htm

Details on NASA's SCRAMJET propulsion Research:

http://hapb-www.larc.nasa.gov/

Homepage for University of Queensland's Hyshot Program:

http://www.mech.uq.edu.au/hyper/hyshot/

Australia's Space Research Institute's Scramjet Program:

http://www.asri.org.au/research/SCRAMJet/

Diagrams for ramjet and scramjet engines:

http://www.aviation-history.com/engines/ramjet.htm

An excellent article on hypersonic lifting bodies:

http://www.aerospaceweb.org/design/waverider/



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