15
$\begingroup$

The relationship is not intuitively clear to me, would like an answer with references and/or graphs. The question arose while reading about Space Shuttle's SRBs.

$\endgroup$
5
  • $\begingroup$ You're asking about PMBT prior to launch, right? $\endgroup$
    – TildalWave
    Sep 16 '15 at 13:07
  • $\begingroup$ @TildalWave - yup. $\endgroup$ Sep 16 '15 at 13:10
  • 1
    $\begingroup$ Burn rate increases with PMBT, and this increases thrust. I am not sure about the effect on Isp, so posting as a comment. $\endgroup$ Sep 16 '15 at 16:58
  • $\begingroup$ There is a deleted answer which appears to contain some relevant information. Wondering why it was deleted. $\endgroup$
    – Anthony X
    Nov 22 '15 at 23:17
  • 1
    $\begingroup$ @AnthonyX - we don't condone copyright violations. The answer had a scan from a textbook (without any explanation or proper attribution). $\endgroup$ Nov 23 '15 at 9:11
10
$\begingroup$

Increasing Propellant Mean Bulk Temperature (PMBT) increases the burn rate which increases the thrust and causes the motor to burn out sooner. The opposite is true for decreasing PMBT.

This graph from Sutton, 4th edition, shows the effect for large changes in PMBT.

enter image description here

I don't have numbers for Isp but I doubt it affects it much, because both thrust and mass flowrate move in the same direction.

The space shuttle's solid rocket boosters were definitely subject to this effect. The motor contractor Thiokol regularly produced predictions of what the PMBT would be at liftoff, for use in preflight simulations and trajectory design.

I found a few examples in my notes for shuttle mission STS-114

  • March 6 / 63 deg F
  • May 4 / 69 deg F
  • Jul 18 / 83 deg F

Sample Trajectory Design Data Package change notice

enter image description here

$\endgroup$
2
  • 4
    $\begingroup$ So the candle that burns twice as bright really does burn half as long? $\endgroup$
    – DrSheldon
    Jun 16 at 14:07
  • 2
    $\begingroup$ Excellent, thanks for this! $\endgroup$
    – DylanSp
    Jun 16 at 18:41

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.