Saturday, April 03, 2004

The X-43, Hypersonic flight, and Scramjets

Recently, NASA's X-43 research plane flew successfully. I have noticed some misconceptions regarding the significance of the flight and I want to clear some of them up.

The X-43 was the first airbreathing vehicle to fly at hypersonic speeds. The transition from supersonic to hypersonic flight speed is not nearly as well-defined as from subsonic to supersonic; there is no hypersonic barrier. The hypersonic flight regime is typically regarded as speeds beyond Mach 3 or three times the speed of sound. By this definition the Mach 3.2 achieved by the SR-71 (first flown in the '60s!!!) qualifies, but just barely. The X-43 on the other hand, flew at Mach 7, undeniably hypersonic.

The propulsion system used by the X-43 to achieve this speed was two-staged. First a booster rocket accelerated the vehicle to high-speed and then something called a scramjet was used. This was the first time a scramjet was operated generating enough thrust to accelerate the vehicle (the Aussies have got one to work, but it didn't actually accelerate the vehicle).

The most important element of any transportation vehicle is its propulsion system. For flying vehicles there are numerous engine types that are particularly well-suited to certain flight regimes. I'll talk about the following six types:

1. Internal combustion
2. Turbo-prop
3. Turbofan
4. Turbojet
5. Ramjet
6. Scramjet

The order of these is important. There are listed in order from slowest to fastest.

The internal combustion engine (like a car engine) is the most efficient system to use for very low speed flight but becomes less efficient the faster you fly. So low-speed general aviation aircraft use this engine.

The turbo-prop uses a jet engine to generate power to turn a propeller like the internal combustion engine, but it is more efficient for moderately fast flight. These are generally on commuter type airplane which fly short distances and hence don't need to be very fast.

The turbofan has a jet engine which develops thrust and also provides power to turn a fan at the front of the engine. The fan acts like a propeller in that it accelerates the air and provides thrust. This is the most efficient engine for high-subsonic flight and even for moderate supersonic flight, although an engine designed for supersonic flight will be different than for subsonic in that it will rely less on the fan to provide thrust and more on the turbojet portion.

The turbojet is what is typically called a jet engine. It provides thrust by burning fuel to add energy to the airflow and expels it out the back. The turbofan was developed as a derivative of this engine to be more efficient. For supersonic flight this is generally the most efficient, though it drops off as the Mach number increases.

The ramjet is basically the same as the jet engine, but more simple. It uses the inlet design to compress the incoming air before adding fuel and burning it for thrust. The jet engine uses a compressor, which is a highly complicated fan to increase the pressure so that the fuel can be burned more efficiently. By getting rid of the compressor the ramjet is more simple and more efficient for higher Mach numbers. The ramjet, however, cannot operate subsonically because it needs to be flying fast such that the inlet can compress the air. So a ramjet-powered vehicle needs another system, be it a rocket or turbojet or whatever, to accelerate it fast enough that it can operate. A big disadvantage.

The scramjet is almost identical to the ramjet except that combustion occurs inside the engine with the air moving at supersonic speeds (scramjet means supersonic combustion ramjet). The ramjet uses the inlet to compress the air and slow it to about Mach 0.5 so that it can ignite the fuel, but the scramjet burns its fuel at supersonic speeds. This has been compared to lighting a candle in the middle of a hurricane; it's a difficult engineering problem, but it allows higher speeds. Like the ramjet it needs something else to bring it up to speed.

The last type is the rocket, which most people are familiar with. It burns its fuel and pushes it out the back to generate thrust. Rockets can go almost any speed but they are horribly inefficient.

It is hoped that a scramjet, combined with a rocket or turbojet or something else for low speeds, could be used to replace rockets to get to space and be much more efficient (and hence cheaper). This technology is a long way for being ready for something like that, but the flight of the X-43 is a step in that direction.

From what I hear, NASA's hypersonics research program is being lost in the shuffle of the reorganization following Bush's space plan announcement. NASA is being organized into a space group working on getting to the moon and Mars and an aeronautics group. Hypersonics doesn't clearly belong in either, but it was assigned to the aeronautics group without any funding. And the aero guys don't want to pay for it so it sounds like hypersonics will go back the backburner.

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