ELI5. If the weight of rocket fuel is so important, why do SpaceX boosters waste a ton of fuel landing themselves?
In: Engineering
That’s the whole point of their boosters: they can be re-used. Historically they’ve just been ditched after their fuel tanks are empty and left to fall into the ocean, destroyed on impact for sure. SpaceX is trying to save money by making them land themselves for re-use rather than having to build them over and over again.
Yes, it means the rockets are a bit heavier for the capability and its own fuel requirements. I don’t know what the boosters cost, I’d bet a few million dollars easily. If they can be re-used it sounds like a win to me.
Since they’re empty at landing they use much less landing than launching but it is true they sacrifice total payload to be able to land. But that’s offset by being able to build one rocket to launch 20 times
Because the gimbaling first stage rocket engine costs millions of dollars apiece and it’s better to not smash it into the ocean at mach jesus. The weight of the fuel is important in the second stage because it also requires you to make a much bigger first stage. Putting more fuel in a lander means putting more fuel in the second stage and putting more fuel^2 in the first stage. Putting 5% more fuel into the first stage just means you have a 5% heavier first stage.
Being able to reuse the rocket itself instead of having to build a new one far exceeds the cost of all the extra fuel needed to do so.
Because the rocket equation is just a part of the “how much does it cost to put a kg into orbit” equation. Turns out it’s cheaper to carry around a few extra tons of fuel to save a rocket worth tens or hundreds of millions of dollars.
1. They don’t use much fuel at all to land. The atmosphere does the majority of the deceleration. For example, the falcon 9 would have to time the final landing burn perfectly because the *minimum* thrust of one engine was enough to make the booster stage climb again. Minimum power of one engine was literally too much thrust to hover.
2. The amount of money they save by reusing their boosters is VASTLY more cost effective than having extra fuel and slightly more powerful engines to carry that fuel.
Fuel is cheap. Getting it into orbit is expensive, and the rockets are expensive. These boosters don’t go to orbit, they give a push to get the ship off the ground and up to speed. It also doesn’t take much fuel comparatively to land a mostly empty booster, since most of the weight has either been ejected or has gone up to orbit.
It adds a bit of cost but that is very much outweighed by saving the boosters (if you can actually do it reliably).
because the cost of the booster is more than the cost of the extra fuel needed for them to land themselves.
one could maybe make an argument that the payload has to be smaller bc of the extra fuel but apparently the trade off is worth it
rocket fuel is only a fraction of the cost of a launch.
saving a booster > adding a bit more rocket fuel.
As opposed to say crashing?
Rocket fuel is important but the rocket is more expensive than the fuel.
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