So, for constant throttle setting equation to calculate thrust looks like

$ \ F = \dot{m}_\mathrm e V_\mathrm e + (p_\mathrm e - p_0) A_\mathrm e$

But if we are looking at flightclub.io simulation, the graph of throttle for first stage changes with time. What do we need to do then in the equation to get the right thrust?? Do we need to just multiply the resulted thrust by throttle coefficient in the current moment of flight?

(Stage 1 is the blue graph)

Flight - GPS III SV04
Rocket - Falcon 9 Block 5


1 Answer 1


For a first approximation, just reduce the mass flow in the equation by the % throttled down.

So if they reduce the throttle by 10%, cut down the mass flow by 10%.

An advantage of doing it this way versus just cutting the thrust down is that your simulation will reflect the reduced mass flow during the throttle-down, not just the reduced thrust.

  • $\begingroup$ Equations both are the same, arent they? $\endgroup$ Oct 7, 2020 at 18:37
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    $\begingroup$ But I just want to know (if its possible to answer without creating a new question), why does throttle on graph drops from 0.95 to 0,75 at 57th second? What causes that change? Or shoulld I create another question? $\endgroup$ Oct 7, 2020 at 18:44
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    $\begingroup$ @mad.redhead I don't know specifically for Falcon, but some vehicles throttle down as they pass through maximum dynamic pressure. Based on the time, I would guess that is what is going on here. See space.stackexchange.com/a/16963/6944 $\endgroup$ Oct 7, 2020 at 18:46
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    $\begingroup$ @mad.redhead also space.stackexchange.com/q/24252/6944 $\endgroup$ Oct 7, 2020 at 20:22
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    $\begingroup$ @mad.redhead Besides throttling down to limit max Q, some launchers also throttle down or shut down engines to reduce the maximum G-load on the upper structure and payload late in a stage burn. As propellant is consumed, the mass of the rocket decreases, and acceleration and thus G-load increase; throttling back reduces it. $\endgroup$ Oct 7, 2020 at 21:49

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