Using solid rocket boosters on a manned launch comes with some additional risks because they can't be turned off or throttled down once lit. For the Shuttle this created some additional failures modes that would destroy the stack and kill the crew. Three that I know of:

  1. If one didn't light at launch.
  2. If one didn't jettison at the separation.
  3. If launched at temperatures that were too cold for O-Rings to function properly.

It's my understanding the Shuttle could not be man-rated today. How does the SLS deal with these failure modes in order to be man-rated? Are its SRBs improved in ways that can help deal with them (esp. low temperature launches), or is it solely because the SLS will have a crew escape system?

  • 2
    $\begingroup$ 1: It is possible to use several independant igniters for launch. 2: Several independant detonators may be used for the explosive charges used for separation. 3: An electrical heater for the O-Rings may be added to keep them at minimum rated temperature. But they even used two O-rings in parallel, but one of them had no contact to both surfaces when the hull of the SRB was vibrating and bending in flight. $\endgroup$
    – Uwe
    Apr 2, 2017 at 13:50
  • 3
    $\begingroup$ 2: Failed SRB jettison just meant you were hauling along dead mass, impairing your ability to attain orbit, but in the case of STS something like an AOA or even ATO abort probably would have be possible even with dead SRBs clinging to the spacecraft. 3: Compare Did the Challenger SRBs fail due to design for reuse? $\endgroup$
    – user
    Apr 3, 2017 at 14:49
  • $\begingroup$ The shuttle was man rated $\endgroup$ Mar 21, 2023 at 22:56
  • $\begingroup$ "the Shuttle could not be man-rated today" $\endgroup$ Mar 21, 2023 at 23:48

1 Answer 1


Human certification has little to do with a concept and more to do with the Factor of Safety in mission-critical design aspects. SRBs are very reliable and predictable in ignition and burn (if you look at their history, more so than very-complicated liquid-fueled engines), but you have seen some GEM boosters fail in the past because their factors of safety for the casting were so close to 1. When your FOS is close to 1, there is little margin for flight excursions such as wind shear, or manufacturing excursions such as air bubbles in the epoxy casing.

Human rating also considers backups for mission-critical components as well. The SSMEs had redundant flight computers, so that in the event one fails there are backups to carry on without mission interruption (which has happened before, on STS-93, when two SSME computers failed due to an electrical short and their backups prevented loss of the vehicle and crew). There may also be mechanical backups, such as redundant ignition pyrotechnics for engines.

NASA's declared requirements though aren't a straight FOS number -- it's a probability of failure number. This is determined differently for different technologies though, but almost always through a combination of simulation and ground testing.

Also of note: crew escape system is an important component of human-rating a "stack". But be aware, you can human-rate a subsystem (such as a liquid-fueled engine or solid rocket motor), but that does not human-rate the entire rocket stack and crew vehicle.


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