In general, burning while you descend is a terrible waste of fuel, because you have to generate 1G of acceleration just to keep your speed constant. (See http:/

In general, burning while you descend is a terrible waste of fuel, because you have to generate 1G of acceleration just to keep your speed constant. (See http://en.wikipedia.org/wiki/Gravity_drag. http://en.wikipedia.org/wiki/Gravity_drag.) That's fine if you're not fuel limited (like the grasshopper tests) but if you want to land with minimal use of propellants, you want to use as high acceleration as possible. So the idea is to let the rocket drop freely and then light the engines just early enough that you can stop before hitting the ground.