Supersonic Combustion Ramjet. The scram engine, unlike conventional jet engines, has no moving parts and uses hydrogen as it's fuel source to power itself. Present rockets use liquid fuel and liquid oxygen which is much heavier limiting the speed it can achieve.
The scram engine uses the oxygen in the atmosphere to mix with the hydrogen, creating thrust that can propel the vehicle much faster than conventional planes of today. So far, NASA has revealed that the scram engine is capable of propelling a vehicle at speeds exceeding MACH 15. Other accounts report the engine can easily approach 25,000 MPH, which is the speed needed to escape the Earths atmosphere.