SCRAMJETS

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.