1. You didn’t answer my question.

2. With the higher altitude and lower air pressure you have resultant lower reaction mass available for propulsion.

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You hit 2 balancing points:

1. Where you are at an altitude that there is sufficient reaction mass to generate sufficient thrust to overcome drag and accelerate.

2. The input air temperature is actually low enough to provide sufficient delta T that your heating mechanism is actually imparting energy into the incoming air flow.