The op-ed by Owen Garriott and Alan Stern, “Commercial Human Spaceflight Safer, Much Needed” [March 7, page 19], presents the case for commercial human spaceflight as a safer alternative to the current space shuttle. While certainly the shuttle risk is likely to eventually be considerably reduced by any credible system with an abort system, the example given by the authors, as well as other commercial advocates, displays a misunderstanding of the depth of the problem of flying crew on a developmental or relatively new launch vehicle. This lack of understanding is displayed in three strategic areas:
1. The fundamental nature of the changes required to convert a payload launcher into a crew launcher, as exhibited by the statement as to the modifications being “relatively modest,” despite the history of the Gemini-Titan program and Apollo program, and Ares 1 abort system studies to the contrary.
2. The significant reliability growth issues related to developmental launchers. A recent study by NASA Johnson Space Center on shuttle risk progression, consistent with the history of other new launchers, has indicated that the shuttle, despite all the effort focused on its initial safety, exhibited a 1-in-9 launch risk early on and did not achieve the 1-in-50 risk cited in the op-ed piece until well after the 100th launch.
3. A confusion between the reliability of an abort system — that is, the probability that it will activate when called upon — and the abort effectiveness — that is, the probability that it will safely mitigate the impact of the initial insult that caused its activation and will return the crew safely. The former could, perhaps optimistically, be as high as the 1-in-100 cited in the article, but the latter has been shown, by physical simulations against fragmentation, pressure wave, and radiation insults, to be far worse, about 80 percent in realistic cases.
For these reasons, while the implied formula in the op-ed piece is correct, the crew safety number of 1-in-5,000 is off by more than an order of magnitude for most vehicle options until they mature. Commercial crew options do offer eventual safety benefits over the current shuttle, but these benefits will take time to accrue and should not be overstated.
Joseph R. Fragola, Rockville Centre, N.Y.