The space shuttle, like most launch vehicles, was a multistage vehicle (albeit one where the various stages were mostly stuck to the sides of each other rather than being stacked vertically), reducing the amount of mass and weight needing to be carried all the way into orbit:

  • The first stage was the two SRBs, which burned from liftoff until fuel exhaustion about two minutes later, at which point they fell off the sides of the shuttle stack, were pushed away by sepratrons at their head and tail ends, and parachuted into the ocean for recovery and reuse.
  • The second stage was the three ET-fed RS-25 SSMEs on the orbiter’s rump, which burned from several seconds prior to liftoff until shutting down approximately eight and a half minutes after liftoff; following engine shutdown, the ET fell off the orbiter’s belly, reentered the earth’s atmosphere, and despawned over the South Pacific.
  • The third stage was the two AJ10s in the OMS pods on the orbiter’s upper rump, which burned twice: first for about one-and-three-quarters minutes, starting a few seconds after SRB separation, and then again for between thirty seconds and a minute, starting about eighteen to twenty minutes after liftoff, to perform the final orbital insertion. (In the event of a severe underspeed, another burn could be inserted, this one up to several minutes long, starting ten to eleven minutes after liftoff.)

Although the SRBs and ET separated properly every time they were required to do so throughout the duration of the shuttle fleet’s entire service life, failures-to-stage (both with side-mounted strap-ons and with vertically-stacked core stages) have occurred many times with expendable launchers,1 and there is no reason to assume that they couldn’t have happened with the shuttles; yet, the space shuttle flightcrew operations manual contains no information on what to expect or what actions to take in the event of an SRB or ET separation failure, not even in its otherwise-extremely-comprehensive “Emergency Procedures” section.

Given that the shuttle stack would already have been very high in the atmosphere by the time of SRB burnout, and would have been well and truly in space by the time ET jettison was commanded, the aerodynamic forces on the vehicle stack would have been low to negligible even with a retained SRB(s) or ET, and, thus, I’m assuming that an SRB or ET separation failure would not have led to an immediate loss of control. The main effect of a separation failure would, presumably, have been the weight and mass penalty of the non-separated components:

  • In the event of an SRB separation failure, the added deadweight would, most likely, render the shuttle totally incapable of reaching orbit, probably requiring either an RTLS or TAL abort (which one would probably depend on the mission’s trajectory, gross weight, and Δv margin); a TAL would probably be preferable, if at all possible, because a) a TAL was a much simpler and more forgiving abort mode than an RTLS, keeping the vehicle much further away from the edges of its flight envelope than an RTLS would, and b) the aerodynamics during the late stages of RTLS powered flight (with the vehicle at relatively low altitude, moving back towards the launch site, and powering out of a momentarily-vertical freefall), and during the extended mated-coast phase used for an RTLS, might be rather iffy with one or both SRBs still attached to the ET, potentially risking a loss of longitudinal or directional control or a collision with the jettisoned ET-cum-SRB(s). In either case, the ET (with SRB[s] still attached) would presumably be jettisoned at the normal time for whatever abort mode was chosen.
  • In the event of an ET separation failure, the vehicle - already quite close to full orbital speed - might (depending on the mass of payload carried) have been capable of reaching at least a low ATO orbit with a set of extended OMS burns (possibly using the orbiter’s aft RCS jets to assist); this answer, concerning earlier plans to bring an ET all the way into orbit for use as a space station or whatever, indicates that a shuttle could potentially have reached orbit without jettisoning the ET, at least under the right conditions (light payload, no other performance-reducing occurrences during launch).2

What were the procedures, if any, for responding to an SRB or ET separation failure?

1: Including on at least one manned mission.

2: Pressing uphill to some kind of orbit (even a low one), and then either waiting for rescue or attempting an emergency EVA to manually free the orbiter from the ET, would pretty much have been the only even-potentially-survivable option for an ET separation failure, as reentering with the ET still attached would have (not necessarily in this order):3

  • Led to a loss of control, due to the aerodynamic forces on the ET.
  • Caused the ET to explode from shock heating (via the boiling and thermal expansion of the residual propellants in the ET), likely damaging the orbiter (especially its lower surface, home to important parts of the orbiter’s thermal-protection system) in the process.
  • Likely blocked the orbiter’s landing gear from being lowered.

All of these would probably have been bad.

3 As a result, if (say) the ET were to fail to separate during a TAL, the astronauts would basically be screwed.

  • 1
    $\begingroup$ "Despawned over the South Pacific" ? $\endgroup$ Commented Jun 2, 2020 at 21:19
  • $\begingroup$ Die. The only option is death. It is not clear this would be survivable. $\endgroup$
    – geoffc
    Commented Jun 2, 2020 at 21:36
  • 1
    $\begingroup$ For an SRB sep failure (especially a one-sided one), I suspect that the likely main concern is that the resulting center of mass and center of pressure may be outside of the SSME gimbal limits, making it impossible to eliminate the resulting moments due to off-axis thrust and causing a loss of attitude control resulting in tumbling and LOCV. $\endgroup$
    – TooTea
    Commented Jun 4, 2020 at 9:28
  • 1
    $\begingroup$ I assume that under the "third stage" bullet you meant to say "after ET separation" instead of "after SRB separation". Is that so, or was there really a reason to fire the OMS with the SSMEs still going full blast? $\endgroup$
    – TooTea
    Commented Jun 4, 2020 at 9:31
  • 1

1 Answer 1


Referencing the Ascent Checklist Ascent Cue Card:

enter image description here

There were no procedures for failed SRB separation, and nothing the crew or ground could do about it. 1

The ET separation sequence2 could be halted by either excessive vehicle rotation rates, or by failure of the shutoff valves in the main propulsion system feedlines to close (or indicate closed). The crew would be informed of this by a ET SEP INH message.

Refering to the procedure, we see that the crew would check that the tank was in fact still attached, and then take the ET SEP switch to manual.


If the rotation rates were high, they would stop the Main Propulsion System dump (in case it was causing the rates), null the rates, then press the ET SEP manual pushbutton. After the vehicle maneuvered away from the tank, the crew would restart the dump.

If the rates were nominal, they are directed to assume that the failure is due to to a valve problem. This involves simply waiting a while to allow the tank to blow down, then manually separating the tank in the Post OMS 1 timeframe. See the ET PHOTO MANEUVER/MPS DUMP actions in the checklist there. The reference to VI < 25760 is asking if the OMS-1 burn is going to be required due to an underspeed at Main Engine Cutoff; if this is the case, it's handled on page 2-8 in the OMS 1 BURN procedures and just involves waiting as long as you can before you must start the burn.

For RTLS and TAL the ET sep's were not inhibited.

There was one other malfunction procedure related to ET sep. Certain combinations of failures could prevent the flight software from being able to determine the position of the ET SEP switch. If this happened on a non-RTLS ascent, the software would assume that the switch was in MAN. (On an RTLS, the sofware assumed it was in AUTO so that the sequence would go ahead). The crew would get a message ET SEP MAN and after Main Engine Cutoff, would override the position of the switch in the software by making entries on a flight software screen. (Ascent/Entry Systems Procedures)

1 I remember fixing a problem in the Shuttle Mission Simulator Master Events Controller (MEC) model where some enthusiastic malfunction entries + the coding error prevented the MEC from firing enough pyros to separate the SRBs. I don't remember the details of the resulting trajectory but I'm pretty sure "they did not go to space that day". That's a lot of dead weight to haul around. (I just checked my logbook and found notes about this, but not what the trajectory impacts were, sadly.)

2 Review of the ET sep sequence:

After the SSMEs shut down, the ET separation sequence began. First the valves on each side of the fluid connections closed. Three explosive bolts on each umbilical plate fired to free the interface. Then triple redundant hydraulic actuators pulled the orbiter side umbilical plates into the Orbiter below the outer mold line. The system then checked to ensure that all valves were closed. If not, a waiting period began to let the systems blow down through the open valves and a warning was issued to the crew. Finally pyrotechnics blew the structural interface apart and the Orbiter fired down-firing jets to fly away from the ET. Then the pilot flipped switches to release the centerline latches on the doors, close the doors, and latch them closed.

From https://space.stackexchange.com/a/18667/6944

This schematic from the Ascent Pocket Checklist shows the feedline disconnect valves (annotation mine). Four total, two on either side of the separation plane.

enter image description here

  • $\begingroup$ Are you sure you couldn't TAL abort with a SRB separation failure? If not, you probably could return to launch site. A three-engine RTLS wasn't considered all that hard. $\endgroup$
    – Joshua
    Commented Oct 10, 2020 at 3:35
  • 2
    $\begingroup$ Not sure where you're getting your info. Sources would be good. "A three-engine RTLS wasn't considered all that hard." Sure. $\endgroup$ Commented Oct 10, 2020 at 4:28

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