Chair Force Engineer

Wednesday, September 06, 2006

Aerobraking and aerocapture

In a recent post on Selenian Boondocks, Jon Goff comments on the essential development of aerobraking if we are to build a spacefairing society. His analysis is excellent, and I wanted to build upon what he said, in terms of how it applies to Mars exploration and why NASA should attempt it.

Part of Robert Zubrin's Mars Direct plan was that you'd save a large mass of propellant by aerobraking into a parking orbit around Mars. The friction between the spacecraft and the Martian atmosphere was expected to provide the complete velocity change needed to change from the interplanetary transfer to Mars orbit. This is a very daunting problem. Too much velocity change means you fall out of Martian orbit, while not enough will send you on an unpleasant trip back into interplanetary space.

NASA has made strikes in multi-pass aerobraking at Mars, but hasn't committed to doing it all in one pass. Mars Global Surveyor, Mars Odyssey and Mars Recon Orbiter have all used on-board propulsion to capture into highly elliptical orbits, followed by use of the solar arrays to aerobrake and lower the apoapsis of the orbit. The landers (Pathfinder and the two Mars Exploration Rovers, plus the ill-fated Polar Lander) have all made direct entries at Mars instead of braking into orbit before deorbiting.

To be fair, aerobraking on a single pass does present a lot of challenges. One must have an accurate atmospheric density profile at the time of atmospheric entry, a control system that can maintain a predetermined path leading to the correct velocity change and trajectory, and a structure that can withstand the thermal and atmospheric loads. I would liken it to bringing the shuttle back through the atmosphere, except that on Mars there's no mission control to guide you back down in real-time.

I am aware of no NASA effort to develop single-pass aerobraking, and previous Mars Design Reference Missions have used either propulsive capture or direct entry. (Propulsive capture wasn't as big of an issue for the plans that used nuclear thermal rockets, which offer almost double the Isp of LOX-LH2.) Clearly, somebody needs to step up to the plate and aerobrake in a single pass if we want a robust architecture for interplanetary travel.