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Conventional rockets create thrust by combining liquid fuel with an oxidizer, usually liquid oxygen. Both the fuel and the oxidizer take up a lot of space, resulting in rockets that must be very large to achieve the thrust necessary to launch a satellite into orbit. For example, 8 grams of oxygen is necessary to ignite 1 gram of hydrogen, a typical rocket fuel. For a rocket to hold both a fuel and oxidizer requires containers for both, further increasing the total weight of the rocket and requiring an even greater amount of fuel to lift a given payload into orbit. Conventional rockets need a complex network of pipes and holes to ensure that the fuel and oxidizer mix evenly and quickly throughout the launch process.
Moving beyond the techniques employed by conventional rockets, the scramjet (Supersonic Combustion Ramjet) uses atmospheric oxygen as an oxidizer, bypassing the need for an onboard oxidizer completely. A large scoop at the front of the craft takes in air, while onboard systems isolate oxygen from the air, compress it, and introduce it to a stream of fuel when then uses the oxygen to combust and produce thrust. For a scramjet to intake sufficient oxygen for self-sustaining flight, it must already be moving at supersonic speeds. For this reason, a scramjet must be coupled to a conventional rocket at the beginning of its flight.
The first successful scramjet trial occurred on 16 August 2002 when the University of Queensland's HyShot team launched their scramjet rocket from a launch pad in Woomera, Australia. Mounted on a Terrior Orion rocket, the scramjet achieved speeds of Mach 7.7 and flew for a total of 6 seconds, enough to demonstrate that the scramjet principle works. NASA has expressed great interest in scramjet technology, launching the Hyper-X program, a collaborative effort between Langley Research Center in Hampton, Va. and Dryden Flight Research Center in Edwards, Calif., for the purpose of making scramjet technology a practical reality.
One day, scramjets could take passengers from Tokyo to New York City in under 2 hours, almost 10 times faster than conventional airliners. Because a scramjet carries no oxidizer tanks, it can be much lighter, faster, and ultimately cheaper than conventional rocket technology. Its only exhaust is water, released from combining hydrogen, the fuel, with oxygen, the oxidizer, and it needn't discard massive empty tanks as conventional rockets do. The scramjet might be the most appropriate tool for taking payloads and passengers up into orbit in a future era of commercialized spaceflight.