# What is the time sequence for SRB burn-out and separation?

I understand that SRBs cannot be throttled or shut down, they need to "burn out". Since they are often used in pairs, I assume there is a statistical spread in the burn duration of the two SRBs. During any period between burn-outs, the off-axis thrust of the burning SRB would need to be promptly countered by the main engines, then again when the second SRB burns out. I assume everything needs to settle from this Texas Two-Step before separation?

Does this asynchronous burn-out actually happen? Does it need to be compensated for? What is the delay between burn-out and separation?

• Given that the shuttle program shut down years ago you should be asking if it ever did happen.
– GdD
Mar 31, 2022 at 16:35
• An easy solution would be to separate before burn-out, which is AFAIk how the Shuttle did it. Mar 31, 2022 at 18:28
• OTOH, the Delta carries them quite a while after shutdown in order to avoid dropping them on oil platforms. Mar 31, 2022 at 18:34

For the shuttle, the separation sequence started when the sensed chamber pressure of both SRBs dropped below 50 psi (hereafter referred to as "cue"). Here is a chart of reconstructed data from an early flight.

At the time of separation, each SRB was still producing ~ 30,000 lbf of thrust. So, a reasonable person might consider this "prior to burn-out", although that is a matter of definition. "Chamber pressure < 50" could also be considered burn-out.

The separation sequence went as follows

Cue + 0.8 seconds:

• Command SRB Thrust Vector Control to null position
• Transition Orbiter control logic to 2nd stage mode
• Reset the attitude reference to the current attitude, zeroing out errors, and hold this attitude

Cue + 2.5 seconds:

• Fire pyrotechnics and booster separation motors to effect physical separation

Cue + 6.5 seconds:

• Reset to normal attitude reference

References:

• @Woody Table 3.7 & figure 3.8 of ref#1 look useful for the time/thrust differential aspect of the question Mar 31, 2022 at 19:04
• @BrendanLuke15 an example of actual thrust differential vs spec is here imgur.com/VJUdrDg Notice how the allowable fans out at the end of web time. Mar 31, 2022 at 20:12
• @BrendanLuke15 ... and table 3.2 compares the performance of the 2 SRB's. The time of the 50 psi cue was the same down to 0.1seconds, so I guess there was no "2-step' to deal with. Mar 31, 2022 at 23:57
• @Woody the SRBs were cast in pairs using propellant mixed in the same batches. They tried to make them as similar to each other as possible. Apr 1, 2022 at 0:36
• @Woody that was done here too Apr 1, 2022 at 1:23