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Orbital Space Capsule

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When I heard that NASA was about to spend money developing something that they're calling the "Orbital Space Plane", I was worried. The name of the project implied another plane-based design, similar to the space shuttle, I was worried. Today, however, NASA has revealed that they're also considering capsules.

That's a good idea. Capsules have a number of advantages over gliders for space applications. The largest one is that capsules do not have large wings, which take up large amounts of mass for little benefit.


The advantage of the wings on the current space shuttle is that it increases the cross-range of the spacecraft. This was important when the shuttle was being considered for wartime military applications. The original requirement for the shuttle called for it to be launched from SLC-6 at Vandenberg Air Force Base (in California) into a polar orbit, release a satellite, and return to earth before completing a single orbit.

This had some advantages for military use. Specifically, it would allow the launching of spy satellites into an ideal orbit, and wouldn't give the soviets a large target to track, such as a shuttle. (This was back when they expected to fly shuttles once every few weeks, before Challenger was lost.) This mission couldn't be flown from Kennedy Space Center in Florida, as doing so would require the overflight of land. Unfortunately, in the hour and a half it would take, the earth would have rotated thousands of miles, requiring the shuttle to fly back to land.

It would be prohibitive to use the shuttle's rockets to move this far, so the designers chose to use wings to change the path of the shuttle. Large delta-wings were used since the manuevering had to occur at high speed.

The world has changed since the shuttle was designed. The government is no longer in the business of launching spy satellites itself. (IIRC, there's a law requiring the purchase of commercial launch services whenever the special capabilities of NASA craft aren't involved.) There's little need to launch spy satellites on a manned craft.

Without the cross-range requirement, large wings aren't required. This makes a capsule very appealing. A capsule doesn't have to spend mass on the wings and control surfaces needed for hypersonic flight. This allows it to be lighter, which tends to make reentry less severe. To be fair, the forces on the crew of a re-entering capsule will probably be worse than those on a lifting-body or winged design, but they can probably be kept to 2.5g, compared to slightly over 1 for the shuttle. (At least, according to Henry Spencer)

Many of the objections people have with capsules are easily addressed. The first is probably a perception that capsules are inaccurate. This probably comes from the fact that the early US capsules landed in the ocean. In fact, Apollo entries became so accurate that the recovery carrier was moved out of the center of the landing zone, for fear a capsule would hit the carrier.

When doing an ocean landing, there's no fundamental need for accuracy. As a result, Apollo wasn't designed with a steerable parachute. With one, capsules could easily land in a small amount of space. The Gemini capsule was originally intended to land on a runway using a parafoil, but that was canceled along with a second production run.

Probably, the ideal OSP program should develop a capsule that can be launched on an EELV, such as the Delta IV or Atlas V. Once the OSP itself is developed, work can be done on a reusable launcher for it, perhaps starting with the recovery of a first stage and working from there.


I'm not sure if OSP will ever get that far, however. It will be interesting to see what happens with the X-Prize, and follow ons to that. But that's another blog post.

Update: One thing that I forgot to mention is that capsules can be made to be reusable. While historically they haven't been reused (with the exception of Gemini 2), there's nothing intrinsic that would prevent it, and an OSP-capsule would probably be reusable.

Update: Another couple of thoughts about capsules versus spaceplanes:

Capsules can land just about anywhere. While it would be expected that a capsule would land in a small designated recovery area, a failure could knock it off course. Unlike a spaceplane, which must land on a runway, a capsule can land on almost any terrain. While landing in water probably wouldn't be ideal from a reuse perspective, it probably would be a successful landing.

Capsules also minimize the amount of equipment that is not directly used for space purposes. Wings and control surfaces are just dead weight in space. Eliminating them could simplify the spacecraft, reducing weight, improving reliability, and lowering costs.

- Tom | permalink | changelog | Last updated: 2003-07-24 20:59

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