First the thrust of a rocket engine is dependent upon two parameters. The flow out of the nozzle and the pressure force at the exit plane.
For most launch vehicles the pressure thrust is zero at some altitude. American rockets are traditionally tuned for optimum expansion (i.e., exit plane pressure equal to ambient pressure) at sea level while Russian rockets are traditionally tuned for optimum expansion at about two minutes into the initial boost. N.B. this is for launch vehicles that are designed to operate within an atmosphere and thus to adapt to changing ambient pressures.
For the LM at landing, about half the thrust is the pressure force and half the force is due to the flow.
That's actually higher than I would have estimated. I would have put it around 10-20 percent of total thrust.
Optimum expansion is not practically achievable for a vacuum. You can never have a nozzle in which the average static exit plane pressure is zero. But because the nozzle doesn't have to accommodate changes in atmospheric pressure as it climbs, it can be optimized as far as possible. Oops, I think I see my mistake: I haven't factored in the throttle. At 25% throttle you would have much less than optimal expansion.
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