4 ms·

I still find it hard to believe that it is energy-efficient to carry fuel all the way up and then all the way back down only to spend it in the last few meters

by po 13y ago

I still find it hard to believe that it is energy-efficient to carry fuel all the way up and then all the way back down only to spend it in the last few meters of the decent. I'm sure they've run the numbers and decided it is but it is the most surprising part of this approach to me. Does anyone have an idea of how much fuel is spent in the return phase?

jccooper 13y ago
Not all that much, really. An almost-empty rocket stage is very light, and it doesn't take much to slow it down from terminal velocity (which can be rather low if you fall right) to 0. In fact, one of their main problems seems to be throttling down an engine enough that it produces less thrust than the weight of the thing. I once did some rough numbers to guess that the landing propulsion will take 10-20s with 1-2 of the motors, using 5-10% of the fuel at launch. Hair-raising, but well within the capabilities of today's sensors, actuators, and computers. The extra fuel is not so bad on a first stage, which is not so weight sensitive as higher stages. And it may be about what it would re-enter with anyway, given reserves. In which case, it's a pretty good deal. A pinpoint 5mph dry landing is a lot nicer than a 30mph wet one, which it what you'd get with parachutes, and it only costs you the weight of legs, since all the other engine and guidance stuff has to be there already.
nawitus 13y ago
SpaceX is optimizing costs rather than energy-efficiency. Obviously you need less fuel if you're not doing a soft-landing back.
- danmaz74 13y ago
  Energy efficiency also needs to consider how much energy you need to build the vector. It looks quite likely to me that reusing the vector would be more energy efficient too.
  - nawitus 13y ago
    Energy efficiency means many different things. It's tricky to estimate if the total energy efficiency is smaller using a reusable rocket. Could be.

by po 13y ago

jccooper 13y ago

Not all that much, really. An almost-empty rocket stage is very light, and it doesn't take much to slow it down from terminal velocity (which can be rather low if you fall right) to 0. In fact, one of their main problems seems to be throttling down an engine enough that it produces less thrust than the weight of the thing. I once did some rough numbers to guess that the landing propulsion will take 10-20s with 1-2 of the motors, using 5-10% of the fuel at launch. Hair-raising, but well within the capabilities of today's sensors, actuators, and computers. The extra fuel is not so bad on a first stage, which is not so weight sensitive as higher stages. And it may be about what it would re-enter with anyway, given reserves. In which case, it's a pretty good deal. A pinpoint 5mph dry landing is a lot nicer than a 30mph wet one, which it what you'd get with parachutes, and it only costs you the weight of legs, since all the other engine and guidance stuff has to be there already.

nawitus 13y ago

SpaceX is optimizing costs rather than energy-efficiency. Obviously you need less fuel if you're not doing a soft-landing back.

danmaz74 13y ago
Energy efficiency also needs to consider how much energy you need to build the vector. It looks quite likely to me that reusing the vector would be more energy efficient too.
- nawitus 13y ago
  Energy efficiency means many different things. It's tricky to estimate if the total energy efficiency is smaller using a reusable rocket. Could be.