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Scramjets, short for supersonic combustion ramjets, may someday launch satellites more cheaply--or even take passengers from London to Sydney in about two hours, proponents say. The test engines need a rocket boost as they will not ignite until reaching a speed of Mach 5, or five times the speed of sound. The engine sucks oxygen from the atmosphere to burn its hyrdrogen fuel. Scramjet engines are expected to be ready for commercial use in about 10 years.