Although obviously not an efficient part of a trajectory, the ability to hover may be useful. It can be shown that a vehicle hovering with rocket propulsion that has a mass ratio of 1/e (= 1/2.718, -0.367) can do so for a period in seconds equal to the specific impulse, which is also usually expressed in seconds. This neat result is a vivid demonstration of the otherwise unobvious dimensions of specific impulse. Stable hover requires throttlable propulsion. This can be achieved either

Combined techniques - system engineering

with very careful nozzle and valve design (e.g. on the Apollo Lunar Module), or a more modern technique is to pulse-modulate thrusters having fixed thrust values (e.g. on the Phoenix lander).

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