Robert Zubrin’s commentary on NASA’s Space Launch System [“NASA’s New Booster Program,” Oct. 17, page 19] missed the mark in a number of respects. He fails to recognize the current economic climate, as well as grant that the development of any launch vehicle program is not solely driven by mission requirements. It’s easy to Monday-morning quarterback choices made at the onset of any technology development program, but without acknowledgment that schedule and budget are the other two constraining arms of the program triad (mission requirements being the third), such speculation and criticism demonstrate a naiveté.

His first assertion regarding the Space Launch System (SLS) — that it is “in fact … the same in all important respects as the shuttle-derived Ares 5 heavy lifter” — is incorrect. The Ares 5 booster proposed to use a 10-meter-diameter core tank as well as yet-to-be-developed and flight-qualified five-segment solid rocket boosters. The SLS uses the same 8.4-meter-diameter core that the space shuttle used for the external tank, and initially will utilize four-segment solid rocket boosters that are not only flight-qualified but also man-rated. This is a very important distinction. By maintaining the shuttle’s original core tank diameter and initially choosing to employ the shuttle’s four-segment boosters, the existing handling fixtures, tooling and launch pad, as well as the trained Kennedy Space Center work force, can be utilized — reducing schedule, reducing costs by maximizing reuse of existing infrastructure and possibly saving jobs. Actually, the new SLS shares these two major design characteristics with Zubrin’s side-mounted power-pod Ares design.

Zubrin’s next argument is that the SLS budget of $3 billion per year will be able to support 30,000 workers at $100,000 apiece. He writes, “There’s no way a horde of this size … can be usefully employed to build a rocket. Rather, the intent seems to be to take the bulk of the budget for the now retired shuttle and continue to spend it among appropriate constituencies through another election cycle or two.” The cost of materials, facilities and fabrication seems to be missing in Zubrin’s estimate. But more importantly, there is something to be said for maintaining as many scientists and engineers as possible through downturns in individual aerospace market segments. The retention of personnel is frequently referred to derogatorily as “corporate welfare,” but building launch vehicles involves a fair degree of “tribal knowledge,” where the more seasoned engineers pass on lessons derived from experience to the next generations. It is far more time- and cost-effective to maintain the lineage of knowledge and expertise than to try to spin it up from scratch.

Zubrin also asserts that the Saturn 5 was developed in response to the requirements for the manned lunar missions. While it is true that there was a mission driving the development of the Saturn 5, had the booster waited for all of the requirements to be mission derived (including final lunar module ascent stage weight), then the Saturn 5would have grown considerably not only in capability but also in size, cost and program schedule. The lunar module was built under relentless weight constraints imposed by the Saturn 5’s capabilities. Similarly, weight restrictions imposed on Saturn’s S-2 second stage were driven by the first and third stages, already well ahead in development. The S-2’s clever common fuel/oxidizer bulkhead solution saved size and weight as well as cost, and helped keep the program on track for the 1969 manned landing.

While Zubrin correctly points out that the United States is the only country that has placed rovers on Mars and sent probes to all of the outer planets, he fails to acknowledge that these payloads were all launched by existing vehicles and that they were all built to the constraints of those launchers. It should also be noted that those launchers (Atlas, Delta and Titan) were subject to performance upgrades throughout their entire service lives, increasing their capabilities; they weren’t built specifically to meet any one program’s needs. Similarly, no performance limitations are built-in on the new SLS. Plans already exist to employ more advanced strap-on boosters at some point, as well as higher-performing upper stages.

His conclusion that NASA’s vendors are lobbying for their own projects and that the deliverables will result in an unusable launch vehicle is without merit. Certainly an aerospace company will want to deliver what it knows it is capable of, as well as maximize investment in its capital infrastructure to deliver goods (e.g., ATK producing solid rocket boosters). Having been involved in the engineering of a number of aerospace programs, including a new launch vehicle, I have always had to work within the capability constraints of the vendor. Zubrin’s presupposition that NASA (to whom he gives credit for a number of successes over the last several decades) cannot manage its major suppliers is unsubstantiated.

To quote from David Akin’s Laws of Spacecraft Design, “Capabilities drive requirements…” If a heavy-lift launch vehicle of any reasonable capability is built, rest assured that there will be missions designed to be executable within its constraints. While mission planning is high on the list of important elements that go into the decision-making process for a new launch vehicle, it shouldn’t prevent commencing to design and build. The budget and political-will windows to build a new heavy lifter appear to be open right now — let’s not delay further.

Greg Gosian
, N.Y.