Off thread and just to be a pain
Imagine that you had an engine which offered constant thrust completely independent of speed or altitude. Such engines exist. Namely rockets.
From the formula you will see that a rockets thrust in a vacuum (space) will be greater than at sea level. As a rocket climbs the ambient pressure progressively reduces until you reach space where it becomes effectively zero (space is not a perfect vacuum).
Gross thrust = Fn = mVe+ Ae(Pe - Pamb)
where:
m = propellant flow (kg/s or lb/s)
Ve = jet velocity at nozzle exit plane (m/s or s)
Ae = flow area at nozzle exit plane (m2 or ft2)
Pe = static pressure at nozzle exit plane (Pa or lb/ft2)
Pamb = ambient (or atmospheric) pressure (Pa or lb/ft2)
