Chair Force Engineer

Tuesday, January 10, 2006

Back in the USSR

SpaceDev has gone back to the drawing boards, redesigning its "Dream Chaser" space tourism craft for future orbital applications. The move demonstrates a forward-thinking approach and a hopeful look at things to come. It also draws upon a sound design that was examined by the Soviets in the 1970's.

The original Dream Chaser concept used the basic shape of the Orbital Sciences X-34. When I first saw the proposal, I felt that it was the wrong shape for what SpaceDev was trying to accomplish. SpaceDev wanted a rocket that would blast off vertically, fly on a suborbital trajectory, and return its passengers to earth safely. The X-34 was supposed to be dropped horizontally from an aircraft, then ignite its engine and execute a stressful pitch-up maneuver. The X-34 would then fly the same suborbital trajectory and re-entry projected for the Dream Chaser.

The wings on the X-34 were sized and stressed for the powered pitch-up maneuver, which occurred while the vehicle was flying supersonically and still carrying a sizable load of fuel. These heavy wings would let the Dream Chaser survive an abort with a substantial amount of fuel on board, but they were dead weight during ascent and during a nominal re-entry.

Ultimately, it was thermal protection concerns that led to the redesign of Dream Chaser. The new design will encounter a more benign thermal environment when returning from orbit. It's based on the NASA HL-20 lifting body, which in turn was an evolution of the Soviet BOR-4 from over 30 years ago.

BOR-4 was an early attempt by the Soviets at a reusable space plane. Several models of BOR-4 were launched on suborbital jaunts, while the MiG-105 validated that a pilot could land the BOR-4. Ultimately, BOR-4 was dropped in favor of BOR-5, a copy of the US Space Shuttle. Soviet paranoia over potential military uses of the American shuttle led to the multi-billion dollar Energia-Buran program, which greatly contributed to the bankruptcy of the USSR.

After Challenger was lost, NASA looked at BOR-4 (given the NASA name "HL-20," for "Horizontal Landing") as a backup to the Space Shuttle that would provide assured access to the Space Station. While HL-20 was seen as a successful program, it lost funding in 1992 in favor of the crew-rescue X-38. When NASA's doomsday scenario played out in 2003-2005, space station access was provided by the reliable but payload-limited Soyuz.

SpaceDev's concept art shows the Dream Chaser (aka BOR-4/HL-20) straddling three hybrid boosters, in an arrangement similar to the shuttle. A rocket attached to the Dream Chaser's tail will probably provide the needed push for orbital injection, possibly doubling as an escape system.

At this point, I have issues with the escape system. Unless the Dream Chaser was mounted at the top of the launcher, it will be incinerated in the fireball of an exploding booster, and has a good possibility of colliding with the booster if the rocket fails in a more benign way. The ony reason I can see for this arrangement is because the rocket becomes more unstable as the winged vehicle is moved closer to the nose of the booster rocket.

If th boosters are reliable enough, the debate over an escape system will be moot. Time will be the judge of whether SpaceDev's hybrid engines are really necessary for space tourism. While the engines may be inherently safer than liquid engines, their specific impulse is a lot weaker than liquids (and, dependent on the oxidizer chosen, can be worse than solid rockets.) The case can also be made that liquid engines are "safe enough" for manned sapceflight, especially if engine-out survivability is designed into the system.