4 ms·
You have to take the slowing-down fuel with you on the speeding-up part of the trip though, which means it takes much more energy to speed up in the first place
by manicdee 8y ago
You have to take the slowing-down fuel with you on the speeding-up part of the trip though, which means it takes much more energy to speed up in the first place.
- twtw 8y agoYes. SpaceX surely knows this, and reckons that the cost is worth full reusability with reduced refurbishment cost.
- Retric 8y agoNo, they are using a heat shield which is unnecessary if you slow down before reentry. What confuses people is you need to bleed off a little energy to start decent, and another tiny amount to land. But, between those points 99% of the energy is bleed off via air.
- manicdee 8y agoYou can compare the weight of the heat shield required for atmospheric reentry braking to the weight of the fuel you need for propulsive reentry braking: if the fuel weighs more, use the heat shield instead. This could be the difference, for example, between 100t of fuel versus 15t of heatshield.
- ygra 8y agoIt eats into the payload, of course. That's why they have different landing trajectories (RTLS, barge in the ocean) and sometimes don't land at all, depending on the payload mass. The propellant itself doesn't cost much (Elon once noted that fully fueling a Falcon 9 is about 200k, which is nothing compared to the 62M that a launch costs).
- Retric 8y agoFalcon uses air to slow down most of the way, they even use parachutes to get as slow as possible, before turning on the engine for the past possible second.
- rbanffy 8y agoSpaceX opted to use fuel to decelerate over parachutes partly because a similar delta-v could be achieved with less mass in fuel than in parachutes, with better re-usability of the whole and much better control.
- Retric 8y agoThey are not slowing down from orbital velocity, the first stage of falcon 9 also benifits from air breaking. So it’s not much fuel specifically becase they don’t need to slow it down very much.