undefined | Better HN

0 pointsjordan0day12y ago0 comments

I imagine there's a lot less fuel required to land than is required to take off. That is, on launch, the rocket is probably (eventually) reaching speeds of several thousand miles per hour.

Upon landing, the atmosphere does most of the slowing-down for you, so you only need enough fuel to reduce the speed from terminal velocity (not sure what this would be for a rocket, probably several hundred miles an hour, at least) to 0. So, I'm sure it's not an insubstantial amount of fuel, but maybe less than you'd think?

0 comments

JshWright12y ago

Not to mention that the giant flying fuel tank (er... 'rocket') is _substantially_ lighter now that you've burned most of the fuel off.

InclinedPlane12y ago

And separated from the 2nd stage and the payload, of course.

Laremere12y ago

Two other points to consider:

Atmosphere is slowing you down as you go up as well, so lift off requires fighting that as well. Therefore the energy you've expended is not entirely stored in potential energy. (Fun fact: If it weren't for the atmosphere, rockets would actually take off almost horizontally. They go up at first to get out of the thickest air before going sideways.)

The craft is much lighter, because it no longer has the payload, and also has used almost all of its fuel.

Florin_Andrei12y ago

> I imagine there's a lot less fuel required to land than is required to take off.

Right.

At take off, the delta vee is the escape velocity PLUS all the losses due to friction, which are tremendous.

At landing, the delta vee is only equal to terminal velocity. EDIT: Okay, plus some flying time on top of the landing point.

Someone12y ago

Another way to state this: at launch, air resistance is your enemy. When landing, it's your friend.

j / k navigate · click thread line to collapse