Other candidates were a liquid oxygen/kerosene-powered air-turbo rocket or a turbine engine integrated with a mass injection pre-compressor cooling (MIPCC) system. In this engine a fluid, usually water, is mixed with the incoming airstream to decrease the total temperature of the flow.
MIPCC systems have been studied since the late 1950s and proposed for previous space-access programs including the Defense Advanced Research Project Agency's Rascal (rapid-access small-cargo affordable launch) initiative in the early 2000s. “The engine has no clue it is flying at Mach 4.5; it thinks it's at Mach 2,” says Bowcutt. The air-launched, two-stage Rascal was aimed at placing a 75-kg (165-lb.) payload into low Earth orbit for $750,000, but was canceled in 2005.
The first stage connects to the rest of the stack via a nose-mounted folding interstage arrangement, which Boeing says is fully reusable apart from frangible pyrotechnic separation nuts. The interstage carries the structural loads while attached to the second stage; after separation, the component folds flush with the nose for return to the launch site. “We have two other [interstage] concepts which we're not sharing,” says Bowcutt.
The second stage is a 37-ft.-long, hypersonic waverider design with “an inward-turning inlet that feeds a circular combustor scramjet,” says Bowcutt. The second stage takes over at Mach 4.5 and accelerates to Mach 10 for the final staging of the rocket-powered third stage. “The wing, fuselage and flowpath are integrated to provide high aero-propulsive efficiency,” says Thomas Smith, Boeing associate technical fellow and co-concept designer. The baseline design has a “sugar-scoop” three-dimensional inlet, but Boeing says the second stage could also employ a two-dimensional planar inlet and combustor such as that used on the X-51A.
The third stage, “nested” in the trough in the upper surface, will be deployed with a trapeze mechanism to ensure safe separation. The vehicle, measuring roughly 16 ft. long, will be powered by a solid- or liquid-fueled rocket with thrust-vector control and a specific impulse of 300 sec. The payload will be carried in a 21 X 38-in. volume within the nose, and is designed with sufficient space for up to four Poly Picosatellite Orbital Deployers (P-POD) or a single payload.
The proposed mission profile would see the SLV carried by the WK2 to a release altitude of 30,000-40,000 ft. Following air-launch, the first stage would accelerate at a constant altitude until the flight dynamic pressure (Q) reaches 2,000 psf, at which point it would climb and maintain this pressure until the first staging point of Mach 4.5 at 61,000 ft. The first stage would then execute a 2g turn and return to land on its own retractable gear.
The second stage will accelerate to Mach 10 at 95,000 ft. using its scramjet before releasing the rocket, or will pull up to a higher altitude and lower Q before releasing the third stage. Boeing's study also includes a possible recoverable third stage with a biconic aeroshell and body flaps for control. The vehicle also would use its own propulsion system and deployable legs for a vertical landing similar to demonstrated concepts such as the DC-X and Masten Space Systems' Xombie and Xaero.
Boeing is agnostic over the whether the concept is developed for commercial or military users, and believes the SLV could be applicable to all comers, says Bowcutt. “At $300,000 per launch, we believe a lot of people will come to market.”