Chair Force Engineer

Sunday, November 18, 2007

Marching to Mars

SpaceWorks Engineering has finally released the AIAA Space 2007 paper and presentation slides on manned flights to Mars. While NASA has focused on getting back to the moon, it's good that somebody is looking ahead to the next step.

The SpaceWorks study makes several assumptions; some are good, while others I feel are invalid. Here's a run-down of the key assumptions which drive the mission design.
--Use of Ares I & Ares V to launch the Mars mission: I think the chances of obtaining full funding for Ares V development are virtually nil in the current political climate. Then again, Ares V reduces the number of launches that need to be performed, which reduces loss-of-mission figures.
--Zero-boil-off systems for cryogenic storage: People I've spoken with on the subject believe that we've gotten pretty good at slowing the boil-off of cryogenic propellants, but we're not at a point where we can say that boil-off can be neglected from our studies.
--Use of aerobraking at Mars: this is a risky technology, but the associated mass savings it enables are enough to justify its development. I'd just like to see NASA make the investment. However, it is noted that SpaceWorks does not utilize aerocapture for the Earth Return Vehicle (ERV) when it arrives at Mars, and I'm wondering why they would choose propulsive capture for this element of their architecture.
--No In-Situ Resource Utilization (ISRU) at Mars: I'd lump ISRU in the same class as aerobraking. It's risky, but it's worth the cost of development. Besides, if we want to establish long-term bases on Mars, we will need ISRU to survive. It's best to develop it now than to leave it off. SpaceWorks neglects it in the study, and pays a mass penalty as a result.
--Chemical propulsion used throughout: the omission of Solar-Electric (ion) or nuclear (thermal or electric) propulsion seriously hinders any mission architecture (through higher mass and lower flexibility,) even though it saves development cost and schedule. I would personally like to see nuclear-thermal or VASIMR propulsion mature before we start sending humans to Mars.
--Surface power provided by Radioisotope Generators: My understanding of the SpaceWorks proposal's ASRG units is that they're essentially RTG's on steroids (using the heat from the radiation source to drive a stirling generator.) But if they are willing to take the political risk of flying radioactive sources on the mission, why not go all the way to a true fission reactor? The development costs of such a fission reactor might be the strike against flying it.
--No artificial gravity provided: This saves on mass and development costs, but I don't think enough is known about how well astronauts will be able to function on Mars after being subjected for 205 days of zero-G on the way to Mars.
--Crew size of three: this is because the study's authors wanted to keep the consumables budget to a minimum. I feel that three crew is too small to handle the workload for the mission. A crew of four to six would result in more science return from the Mars surface mission. To the study's detriment, no consumables budget is provided, so it's impossible to say what the mass impact of adding another crew member would be.

There's a lot I liked about this study, in spite of some assumptions I disagreed with.
--It shows that Ares I&V can support a Mars architecture, using four Ares V's and one Ares I (for crew launch) per mission. Previous NASA reference missions have used six launches of a "Magnum" launcher, with performance slightly less than Jupiter-232. The study counts the Ares V launch as a significant Loss-of-Mission driver, so four Ares V launches are probably more reliable than six Magnum launches.
--The use of drop-tanks on the In-Space Propulsion Stage for the ERV squeezes out more velocity by shedding unused structural mass. I wish the concept was applied to the Trans-Mars injection stage as well.
--Use of existing engines on in-space propulsion stages. The venerable RL-10B-2 is recycled from Delta III & IV to ensure an Isp of 464 seconds on the in-space propulsion stages.

The cost of SpaceWorks's Mars mission will probably make a lot of jaws drop. The plan calls for spending $96.8 Billion dollars between 2025 and 2040 for the first Mars mission. This figure is quoted in FY2007 dollars. It does seem reasonable when compared with a previous NASA estimate of $50 Billion, in FY 1993 dollars. While the costs are spread out, the inevitable sticker shock is going to motivate the plan's political opponents. It's worth noting that over 10% of the life cycle costs are associated with the rigid surface habitat, and I wonder if this cost can be slashed by adapting the TransHab inflatable technology for this application (it's being used anyway for the deep-space legs of the SpaceWorks Mars mission.)

More striking than the budget is the risk associated with the mission. There's a 38.5% chance the mission will be lost, and an 11.5% chance the crew will be lost on each Mars expedition (these numbers are given with 50% confidence.) While NASA will have no problem finding astronauts willing to fly in such conditions, will the American taxpayers be willing to fund such a risky venture?

Anyways, I'd highly recommend a read of the SpaceWorks study to all of this blog's readers. It's definitely worth wrapping your brain around, and it should serve as a starting point for future Mars planning.