What's cute about RASCAL is it builds up its kinetic energy in the lower atmosphere and then does a zoom climb into the upper atmosphere trading off velocity for altitude (NASP would have done the same thing). Then in the upper atmosphere where there is almost no dynamic pressure, the payload is jettisoned from the booster's bomb bay. Doing this removes the unnecessary mass of a payload fairing and a skin around the upper stage's propellant tanks. The down side of the design is the buildup of kinetic energy in the lower atmosphere inflicts a significant heat soak that will not be efficiently convected away while the booster is in the upper atmosphere (pure radiation is very inefficient). The heat soak will cook the booster's innards unless the vehicle has either a thermal protection system (TPS) like the Space Shuttle (silica tiles and blankets) or a hot metal TPS as was proposed for the X-33. Silica tiles have the advantage of being cheap and nasty along with the disadvantage of being nonrobust (except for TUFI tiles which are heavy). A hot metal TPS (titanium shingles) is just hands-down better in terms of reusability and safety but much more expensive to design and manufacture.
I forsee significant expense in getting the air breathing supersonic propulsion setup for this specific airframe and developing the TPS system. Lockheed-Martin is better prepared to deal with these issues than Burt Rutan. Unfortunately LockMart is overly skilled at sucking from the government tit and tends to be over priced (Maybe OSC is a better bet?). Managing this project at the (D)ARPA level would be tricky.
Gary
Space/Science » in reply to DARPA is funding RASCAL development
Thermal protection is a problem
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