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Phase III of the Falcon program, planned to run from 2007 to 2010, is going to further develop the HTV and integrate it with the SLV. The second and third generation HTV designs (HTV-2 and HTV-3) are to incorporate further improved aerodynamic configurations, thermal protection systems, guidance, navigation and control systems. They are planned to be launched on one of the Falcon SLV boosters. The vehicles are going to be about 3.5-4.5 m long and weigh about 900 kg. HTV-1 through -3 designs are to have an increasingly "waverider-like" shape for increased L/D (lift/drag), increasing flight duration and re-entry cross range. HTV-2 will be an expendable Mach 20+ design, but HTV-3 will be a fully reusable vehicle with a design speed of about Mach 10 at 40000 m (130000 ft). Although HTV-1 and -2 will be unpowered, it's possible that the reusable HTV-3 will use a scramjet engine. It is not entirely clear in which direction the HTV development will eventually head - DARPA's original HCV idea (hypersonic cruise) or USAF's CAV (expendable semi-ballistic).