Chair Force Engineer

Wednesday, October 01, 2008

"Stages to Saturn" and Lessons from Saturn

I recently finished reading Stages to Saturn, Roger Bilstein's authoritative tome on the development of the Saturn rockets. The book was first written in 1979, with the perspective that Saturn-like rockets would soon be replaced with reusable launchers like the upcoming Shuttle. Perhaps Mr. Bilstein was surprised to see that, by the 2003 edition, his book could again be referenced by the designers of a new generation of launchers.

Philosopher George Santayana has been memorialized for his famous aphorism, "Those who fail to learn from history are doomed to repeat it." Naturally this begs the question, "How has today's NASA learned from history?" The major lessons from Apollo can be debatable, and NASA's implementation of those lessons is even more open to controversy.

When the Saturn A-I was still on the drawing boards in late 1959, there was debate as to what the upper stage would be. The cluster-tank first stage was already set, but various ICBM's were being eyed to form the second stage. While the Titan I first stage was preferred, it was rejected for numerous reasons (including structural limitations.) In Spring 1960, the Silverstein Committee convinced Wernher von Braun and his team to bite the fiscal bullet and develop an all-new, hydrogen-fueled upper stage.

During the Saturn program, NASA faced opportunities where existing hardware could have been used in a sub-optimal manner, but they instead invested in high-risk, high-payoff technologies that made Saturn a success (particularly the massive F-1 engine and the J-2 engine, which was a major impetus for the development of hydrogen as a rocket fuel.) The Ares rockets are shuttle-flavored, but not exactly shuttle-derived. The existing Shuttle SRB and Space Shuttle Main Engine were rejected because they couldn't meet mission requirements. But it's debatable whether a solid fuel first stage is necessary to begin with.

The Saturn rockets became astounding successes because they had ample development time and budgets. The Saturn program had the luxury of investing in new engines like F-1 and J-2 and all the challenges that accompanied them. By contrast, Ares I is being developed on a shoestring budget until the Shuttle is retired in 2010. The conflicting demand is that Ares I be operational as soon as possible, to minimize the loss of space launch capability between shuttle retirement and Ares I first flight.

Another reason for Saturn's spectacular success was the inherent design conservatism of the team at Marshall who developed the moon rockets. Wernher von Braun was skeptical of the low mass estimates that were given for the Apollo spacecraft and lunar module at the start of the program. He discreetly had his engineers design to much higher performance margins. Sure enough, von Braun was vindicated as spacecraft mass grew, particularly in the lunar module. The structural margins in all of the stages were such that mass could be removed from the S-II stage in order to increase the Saturn V's payload.

In designing Ares I, NASA quickly forgot the guidance of von Braun. The mass budgets for Ares I are so tight that Lockheed Martin was forced to scale back Orion to a "zero base" vehicle and add redundancy back in as vehicle performance improved. While there's still some performance margin, the margin between vehicle performance and payload mass should be much higher at such an early stage in the program's life (currently past Preliminary Design Review, with delta-PDR's remaining on issues like thrust oscillation damping.)

One lesson from the Saturn staging process could have averted the failure of Falcon I Flight 3. During a Saturn launch, the vehicle was allowed to coast for a period of time following stage burnout. When the stages separated, a series of retrorockets would create additional spacing between the upper and lower stages before the upper stage ignited. My guess is that Falcon I's designers omitted the retrorockets to squeeze extra payload mass into the design; the consequence is that some reliability is sacrificed.

The use of common bulkheads was key to Saturn's mass savings on stages 2&3. In both cases, the design of the bulkheads gave the contractors significant manufacturing challenges. Common bulkheads were rejected for the S-IC because of the density difference between liquid oxygen and kerosene (although this didn't stop its use in Atlas.) Because of the temperature difference between hydrogen and oxygen propellants, the bulkhead consisted of two thin skins separated by a precisely-shaped layer of insulation. The segments of the skins had to be welded together from individual gores. NASA and Boeing will hopefully be able to apply the same techniques when manufacturing the common bulkhead for the Ares I upper stages.

A massive program like Saturn cannot survive without effective management. This became an issue during North American Aviation's manufacturing of the S-II stage, and it appeared that NAA was overextended between the S-II and Apollo spacecraft. This becomes a bigger problem for Project Constellation because there are fewer aerospace contractors left who can manage the production of a major component, like the spacecraft or a rocket stage. Will LockMart or Boeing be able to produce the Ares V core in addition to the Orion Spacecraft or Ares I upper stage? What about production of the Earth Departure Stage or Altair lander?

While Project Constellation is fairly young, logistics is a challenge that deserves serious consideration at this stage of the program. Apollo neglected logistics longer than it should have, but innovative solutions were found through the use of river barges and "Pregnant Guppies" for transport of large rocket stages. The logistics problem for Constellation takes on an added dimension because the Ares V core will be the largest rocket stage ever produced. I don't know if NASA has seriously looked into what it will take to produce the Ares V core and transport it from Michoud to the Cape. It's an open question of whether the existing facilities and vehicles are up to the challenge.

Most importantly, the Saturn rockets succeeded because of ample supplies of genius and luck. People like George Low, Werner von Braun, Sam Phillips, and George Mueller were instrumental to the program's success. One of von Braun's biggest virtues was not his own ideas, but his ability to support other people's ideas when they conflicted with his own (ideas like all-up testing and lunar orbit rendezvous.) Many of the leaders in the Apollo effort took large gambles; the biggest gamble of all was all-up testing for Saturn V. The fact that all three stages of Saturn V worked properly on the first launch (Apollo 4) is testament to the geniuses and methodical managers and meticulous technicians who got it correct on the first try. The same could be said about Apollo 8, launched around the moon prior to an earth-orbit test of the Apollo spacecraft aboard the Saturn V. Only time will tell if the same genius is currently at play with NASA.

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