Friday, November 20, 2009

Ten Questions for HSF Committee - Question 7

Should the relative risks and rewards of Heavy lift and refueling be evaluated?

The Augustine Committee report states:

"Using a launch system with more than three critical launches begins to cause unacceptably low mission launch reliability. Therefore a prudent strategy would be to use launch vehicles that allow the completion of a lunar mission with no more than three launches without refueling. This would imply a launch mass to low-Earth orbit of at least 65 to 70 mt based on current NASA lunar plans. Vehicles in the range up to about 100 mt will require in-space refueling for more demanding missions. Vehicle above this launch capability will be enhanced by in-space refueling, but will not require it. When in-space refueling is developed, any of these launchers will become more capable."

Calling this a "prudent strategy" implies that developing in-space refueling is more risky than developing a large heavy lift vehicle. The idea is that with the HLV, we can at least do lunar missions, and more ambitious missions will be the bonus we get if refueling is developed. If we have a really big HLV, maybe we don't need refueling at all. Under inevitable budget pressures, such an outlook will likely result in refueling funds ultimately being diverted to the "critical path" HLV.

Why not look at this the other way around? What type of missions can we accomplish with existing rockets plus refueling? Could we have 3 "layers" of missions: easy missions that only require existing rockets, baseline exploration missions where we need refueling, and a "bonus" set of missions where we need both refueling and heavy lift? Then refueling will be on the critical path, but heavy lift won't. Even without refueling or heavy lift we will still accomplish something.
Of course it wouldn't be prudent to take this approach if refueling is riskier than HLV. However, is this the case?

We already know the Saturn rockets were ended after only a few flights. We already know Ares V development is so expensive that it causes havoc to even a less-constrained budget. Ares V development also causes extreme delays for any exploration that relies on it. This form of heavy lift obviously comes with serious budget and schedule risk, and there is some degree of technical risk as well. Developing, demonstrating, and operating refueling is not without risk, but is it really more risky than heavy lift development and operation? Perhaps the answer is complex, depending on the specific HLV and refueling technologies used.

Let's turn the tables and at least consider a prudent strategy baselining refueling in our exploration plans, while allowing heavy lift to give us greater exploration capabilities should that risky technology arrive.

Risk is not the only factor when deciding whether we should put heavy lift or refueling on the critical path. Potential benefits are also important, and can justify greater risk. For example, if the military has an important security satellite that is needed to defend the nation ready to launch that requires heavy lift, then developing heavy lift would be beneficial because it allows this national security mission to occur. At the moment, I don't see evidence that such benefits are really there for heavy lift, but that could change if some organization other than NASA exploration steps forward with plans and money to use heavy lift. I mentioned some benefits of refueling here; suffice it to say that they appear to be quite compelling both for exploration and for general benefits to the nation.

As an aside, I'm not sure why lunar missions sized by NASA's current plans are used as the baseline in this case. Considering that the exploration budget is a huge issue, why not use some easier destination, such as Earth orbit, Earth-Moon Lagrange points, and lunar orbit, as the baseline, and consider more difficult destinations like lunar surface missions as "bonuses" if refueling and other approaches like reusable landers and reusable spacecraft don't work out? This seems like a more prudent strategy in the case where very ambitious versions of heavy lift appear likely to use far too much of the available budget and schedule. Such as strategy looks even better if we use a smaller HLV that can later be upgraded if needed.

Interestingly, the 65 to 70 mt to LEO threshold discussed above contradicts more modest HLV capabilities described elsewhere in the report as the minimum needed for exploration.

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