I am reading some course material about the 'Thrust load ratio'. It states the following:

The thrust load ratio is: $Psi = {T \over Mg}$

$T$ being the effective thrust, equal to the actual (constant) thrust minus the drag.

Given that with increasing velocity, the aerodynamic resistance increases and the rocket mass decreases, it is acceptable to assume that the effective thrust is constant, as an approximation. This is only valid in the more dense layers of the atmosphere.

This strikes me as rather odd because assuming that the throttle of the engines doesn't change the thrust of the engines doesn't change but the velocity does. So the aerodynamic drag increases but the thrust remains the same hence the effective thrust changes.

The only reasonable explanation that I can think of is that it is a typo and that it should be 'as velocity increases, drag increases but while the velocity is increasing the height of the rocket increases as well and atmospheric density decreases which decreases drag'. Which would be two effects that cancel each other. Or am I missing something?

EDIT: I did not mention the source because it is course material given at the University of Delft and it is not published online. So I think I also can't do so. However below you can find a snapshot of the specific paragraph:


  • 2
    $\begingroup$ Can you please give the source where you read this passage. Thanks. $\endgroup$
    – Star Man
    Aug 17 '19 at 17:08
  • $\begingroup$ See the edit above (coulnd't link as it is no online source) $\endgroup$
    – ThaNoob
    Aug 17 '19 at 18:11
  • 2
    $\begingroup$ I think they meant "the effective specific thrust is constant" (i.e. effective thrust per unit mass). This would be at the point when effective thrust (real thrust minus drag) is decreasing at the same rate that mass is decreasing due to fuel consumption. $\endgroup$ Aug 18 '19 at 1:19
  • $\begingroup$ @RussellBorogove sounds like that's the answer. $\endgroup$ Aug 18 '19 at 1:58
  • $\begingroup$ Allright great thanks guys! What do I do with this question? Because I think it's only going to confuse future people. Do I delete it? Or do I adapt it to 'Can specific thrust ... ' $\endgroup$
    – ThaNoob
    Aug 18 '19 at 6:42

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