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.
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asked Sep 16, 2015 at 11:36
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$\begingroup$ You're asking about PMBT prior to launch, right? $\endgroup$– TildalWaveSep 16, 2015 at 13:07
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$\begingroup$ @TildalWave - yup. $\endgroup$– Deer HunterSep 16, 2015 at 13:10
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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. $\endgroup$– Organic MarbleSep 16, 2015 at 16:58
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$\begingroup$ There is a deleted answer which appears to contain some relevant information. Wondering why it was deleted. $\endgroup$– Anthony XNov 22, 2015 at 23:17
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1$\begingroup$ @AnthonyX - we don't condone copyright violations. The answer had a scan from a textbook (without any explanation or proper attribution). $\endgroup$– Deer HunterNov 23, 2015 at 9:11
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1 Answer
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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.
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
answered Jun 16, 2021 at 13:52
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4$\begingroup$ So the candle that burns twice as bright really does burn half as long? $\endgroup$ Jun 16, 2021 at 14:07