Chair Force Engineer

Thursday, September 10, 2009

The Shape of Things to Come

A lot of Constellation critics (particularly those who have singled out the Orion spacecraft) have questioned why we're going back to the Apollo shape for the capsule which will return astronauts to earth when the mission ends. Why not go back to the Soyuz shape? For that matter, can't we examine the shape of the Mercury/Gemini capsules, or come up with something new?

While some might attribute the Orion shape to romancing the past, there's one very good explanation for the reason why Orion looks like Apollo. It's because NASA and the industry have an extensive database of thermal and aerodynamic data on the Apollo shape, based off the capsule's performance during the Apollo program. Most importantly, this includes data from re-entries at lunar flyby and lunar return velocities.

Could the Soyuz shape make for a good lunar spacecraft? The answer is yes, if history serves as a guide. Soyuz-derived capsules flew around the moon during the Zond program, which was supposed to put a Soviet Cosmonaut around the moon before its cancellation. At one point, the Soviets looked into an upscaled, reusable version of Soyuz. Named Zarya, the new capsule would have fit within the 15 tonne estimate for a multi-man capsule equipped for low earth orbit missions. But one possible drawback to the Soyuz "headlamp" shape for future American capsules is whether American agencies and engineering firms could access the full database of thermal and aerodynamic data that the Russians have accumulated over the years. Without full unfettered access, there will be a significant hurdle to reusing the Soyuz shape.

The last viable option is the shape pioneered on Mercury and used again for Gemini. A favorite of the armchair engineers is the proposed Big Gemini, which seems to meet most of the Orion requirements and weighs in under 16 tonnes. While American firms have access to the data generated on these missions and during the development of these capsules, there's no real-world data for how these capsules behave when returning from lunar trajectories at the velocities these trajectories dictate. Perhaps McDonnell did this analysis during the 60's, but nothing beats real flight-test data. Additionally, I'd be interested to see how Big G compared to Apollo in terms of crew volume available to each astronaut. While Apollo was cramped, it was still quite an improvement over Gemini where each astronaut was prettymuch confined to a seat for up to two weeks.

Interestingly, it would appear that the SpaceX Dragon stays true to the basic geometry of the Gemini spacecraft. I don't have exact figures to see if the cone angle on Dragon is the same as on Mercury/Gemini, but they both capture the "tall cone" profile. Dragon also corrects a big problem that I saw with Big G: the awkward docking system. Since Dragon is designed from the ground up and doesn't incorporate any Gemini legacy systems, it is free to contain a docking tunnel in the "nose" of the spacecraft. Big G would have required an additional set of aft windows for docking, and introduces the risk that hot plasma could enter through the heat shield hatch during re-entry.

One last shape worth considering was developed during the CORONA program. Again, this would need to be subjected to rigorous analysis before it was ready for a lunar flight program.

It's not so easy to qualify a new shape for a re-entry capsule, especially if it's going to be returning from the moon. Re-inventing the Apollo capsule might not be the optimal solution, but it's fast one that gives the space program a proven result.