This month's Challenge Question: Specs & Techs from IHS Engineering360:
The
SpaceX Falcon 9 v1.1 uses nine Merlin 1D rocket engines in its first stage. The
thrust of a Merlin 1D engine in a vacuum is 720 kN, about 10% more than its 650
kN thrust at sea level. Why does the thrust of the Merlin 1D increase with
increasing altitude?
And the answer is:
The thrust of a rocket engine is equal to:
The first term is the mass
flow rate of the exhaust gas in kg/s times the exhaust gas velocity. The second
term is the product of the crosssectional area of the nozzle exhaust exit and
the difference between the exhaust gas pressure at nozzle exit and the external
ambient pressure.
Essentially, sea level atmospheric pressure distorts the
exhaust gas flow, lowering its cross sectional area, and making the rocket
engine less efficient, thus lowering thrust.
