Innovators at EADS used ideas from the Astrium suborbital spaceplane project, currently in a slow developmental phase, to come up with the ZEHST concept. A propulsion system that uses several different engines is on the drawing board, some operating on biofuel, others on a liquid hydrogen and liquid oxygen. The combination is expected to enable an initial climb speed of .8 Mach, and a continuous climb speed of Mach 2.5. Once at an altitude of more than 17 nautical miles, the airplane is expected to reach speeds in excess of Mach 4.