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Any system using solid propellant rockets always has single-point catastrophic failure modes associated with those rockets, precisely because you cannot turn them off. The hybrid was supposed to address that, in a design simpler than standard liquids, and intermediate in performance between solids and liquids. Although, theoretically, LOX-rubber looks just about as good as LOX-hydrocarbon. NASA was never big on hybrids, although some others are, notably Rutan. I think that was kinda stupid of NASA, and smart on Rutan's part. Nice abortable, restartable, throttleable rocket motor.
Man-rating has less to do with probabilities of failure, and more to do with redundancies and escape modes, by NASA's own standards. The Shuttle failed on both counts, and the part-solid Ares designs retain the same basic SRB as the Shuttle, that proved so unrecoverable with Challenger back in Jan 86. However, at least the heat shield vulnerability that killed Columbia has been alleviated by eliminating the side-cluster geometry and the fragile ceramic tiles / carbon-carbon LE pieces. (They didn't really need to eliminate both of those features in the Ares designs, it was having both features together in the Shuttle design that caused the single-point failure path. The long "stick" configuration in Ares-1 is causing serious problems of its own.)
People keep claiming that the commercial launchers will be less safe than Ares/Orion, but I am very unconvinced that their assertion is true. About the safest thing you can build is a capsule atop a liquid or hybrid booster, with an escape tower or its equivalent. I dunno much about the Orbital Sciences design, but what Spacex wants to man-rate in its Falcon-9/Dragon is exactly what I just described as the "safest approach possible". Such a design with liquids is just inherently safer than anything with solids. It is always possible to fall short in your design, but the basic approach must have that potential, or it can never really be "safe" in practice.
Besides, that triple-O-ring joint NASA went to after Challenger, on the basic SRB they are upgrading for Ares, was exactly the wrong thing to do on a solid motor. Thiokol knew that, but NASA insisted, in its great "experiential wisdom" with solids, and NASA held the checkbook. Remember, the real "smarts" lies not in the government, but in the contractors! That's why they hire contractors, precisely because they cannot do it for themselves in the labs.
3 O-rings are worse than 2, in turn worse than 1. In terms of verification leak-check in a motor environment that is filthy with very hot aerosol solids, the one-O-ring joint is by far the most reliable. What is the point of "redundancy" if it lowers the overall probability of success? How else do you think we built missile motors with 40+ year shelf lives, good from -65F to 145 F soakouts, and proof against quite a bit of mechanical abuse without any field verification of integrity? We understood the risks and how to address them, our customer (the government labs) did not. (They never built a successful production article. Not one. We built 10's-100's of 1000's of them at a time.)
Don't get me wrong: men can fly quite safely with solid rockets. What you have to do is deliberately design around the failure modes and make them survivable. In part, that's why I put podded ramjet nacelles out on wings in my airbreather-assisted stage rocket study. An integral booster failure there might cause loss of the engine pod from the wing, but not so much the wing itself, if you allow for it in your original design. (And that, in part, is why I kept yammering about 40% structural fractions.) That way, the stage is still functional, even if a solid booster malfunctions.
GW Johnson
McGregor, Texas
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