Chair Force Engineer

Monday, July 16, 2007

Stuck on the Pad

The comments section on Transterrestrial Musings brings out some interesting statements, especially on the post about Scott Horowitz's departure from NASA.

Interestingly, one commenter claims that United Launch Alliance is pitching a replacement for the 5-segment SRB, and it looks an awful lot like the Atlas V Phase 2 (5.4 meter core, 2x RD-180.) According to the commenter, this would eventually be developed into a flyback booster and used on the Ares V as well. Would NASA really be serious about developing a kerosene-fueled alternative to the shuttle SRB, and dealing a major blow to ATK's solid rocket business?

As pleasant as that fantasy sounds, it won't be happening. For one thing, the booster would be quite underpowered. It takes the thrust of three RD-180's to approximate the thrust of one Shuttle SRB. While I have no doubt that the booster described above (5.4 meter core + 2x RD-180, 5.5 meter upper stage + 1x J-2X) would work, it would fall short of the criteria that NASA used to design Ares I in the first place. Its thrust-to-weight would be too low to fly the depressed trajectories that NASA wants (to eliminate "black zones" where an abort would place unsurvivable stresses on a crew.)

Development of the hypothesized booster would be pricey. It would likely cost more than the 5-segment SRB, and it would dwarf the cost of completing the development and man-rating for the Atlas V Heavy. While the hypothesized booster would have commercial applications, I don't believe that taxpayer dollars should be used to subsidize a large portion of the booster's development costs (especially when we have a post-shuttle spaceflight gap that needs to be closed.)

The flyback booster idea has merit in the long term, but it's unlikely to be realized anytime soon. While flyback boosters have been on the drawing boards for the last 45 years, none have actually been built. Such a task is easier said than done. Development of a flyback booster should be conducted as an X-Plane program, jointly sponsored by NASA and the Air Force. Such a booster would have to pioneer deceleration and transition to a glide at altitudes around 100,000 feet and speeds between Mach 3 and 6. It would need a structure that can handle the massive thermal swings between cryogenic temperatures and hypersonic heating. It would need a turbine propulsion system that could survive the flight and then activate for the final approach to the runway, and it would need a reliable guidance system that could bring it back to base and touch down for a smooth landing in a variety of weather conditions. Flyback boosters are a high-risk, high-payoff technology that has been woefully underfunded over the years. If we want to be back on the moon by 2020, flyback boosters will not be part of the initial plan.

It would be nice to see the post-shuttle spaceflight gap reduced to just two or three years. The only way that will happen is by leveraging Shuttle, Atlas and Delta components that already have flight history. The Shuttle SRB, or a 5-segment derivative, will certainly be part of that future. The "back to basics" approach is why I support DIRECT. I could also get behind the Atlas V (with 3, 4 or 5 Aerojet SRB's) for crew launch to ISS, if the mass and size of the Orion capsule can be cut down to fall within the Atlas performance envelope.

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