The US Air Force will be testing the X-51 on May 25 and it is hoped that this will lead to a new Space Plane and a Prompt Global Strike missile in 10 to 15 years. The X-51 dropped from a B-52 at 50,000 ft will be accelerated to Mach 4 and 70,000 ft by a solid fuel rocket. The scramjet using conventional jet fuel will then power the X-51 to over Mach 6 (over 4000 m/hr) and 100,000 ft plus.
The X-51 engines are envisioned as the power plants of a Global Strike missile that could hit any target on Earth within 30 minutes of launch from California with a kinetic energy warhead doing the equivalent damage of a low yield nuclear warhead.
The hypersonic engines are also to be used on an aircraft that would either carry an X-37B like vehicle to the edge of space or a hybrid craft that would use rocket motors to achieve orbit. The hypersonic plane would be similar to the proposed but never built X-30 National Aerospace Plane from the Reagan administration.
The Obama administration is planning on having the missile ready by 2015 and the space plane by 2025. It is wished that the Strike missile would replace our ICBM’s as deterrence and enable further reductions in US nuclear warheads. The space plane is planed to augment private sector space launch capabilities.