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US airforce looking at winged-rocket booster 'X-plane'
'VTHL': Cheaper than disposables, proper spaceplanes
Cheaper than anything good, gooder than anything cheap?
Specifically, the current technology objectives are derived from an unmanned, vertical take-off and horizontal landing (VTHL), rocket-powered, reusable booster concept. During normally expected operations of this concept, the reusable booster uses its rocket engines to autonomously perform a post-staging maneuver (i.e., rocket-back) which orients and positions the booster for a gliding landing near the launch site. This concept includes an integral tank and airframe that bears both internal pressure and flight and ground loads. The concept is powered by pump-fed, LOx-HydroCarbon rocket engines.
That said, "responses need not be limited to the subset of the RBS concept space described in the previous paragraph." But the air force does want to see designs using an existing main engine, rather than some kind of expensive new thruster design.
It seems that not only the air force Research Laboratory but also the service's Space and Missile Center and Space Command are in favour of the RBS concept. This presentation (ppt) says that the RBS is seen as the best way to reduce overall space launch costs in the US military, and perhaps for civil space missions too. In terms of complexity and difficulty the RBS idea falls between today's expendable boosters (low development costs but very pricey in use) and plans for completely-reusable launch systems, probably involving stacked rocket-gliders with a shuttle-style orbiter spaceplane on top. The latter would be extremely expensive to develop and not especially cheap to use, as much of the mass fired into orbit would consist of orbiter/lander rather than payload.
By contrast, the RBS combo of reusable winged lower stage plus expendable upper stage(s) is seen as offering a 50 per cent overall saving and useful flexibility. It would be possible, for instance, to throw a bigger upper stack and large payload up to kickoff point by strapping paired RBS rocketplanes to either side of its base.
The space-command people see serious development starting on the RBS plans early next decade. Reportedly, a subscale "X-plane"* demonstrator might fly before 2020. ®
* Particularly arse-kicking US military winged testbed aircraft have traditionally tended to get an X-plane designation. Examples include the famous X-1 (broke the sound barrier) and X-15 (manned flight at Mach 6+). The X-planes rather petered out in the Apollo/Shuttle era, but ambitious military wing-ship plans still occasionally seek to recall the glorious past.