ELI5. If the weight of rocket fuel is so important, why do SpaceX boosters waste a ton of fuel landing themselves?
In: Engineering
The most expensive part of a rocket is the rocket, not the fuel.
Rocket A can’t be reused. It’s slightly smaller and slightly cheaper.
Rocket B is designed to be reused. It’s bigger and more expensive to accommodate the fuel needed to recover itself. It will also cost money to recover and refurbish Rocket B into launch condition.
But the calculus is that if Rocket B+2x B-amount of fuel+Recovery&refurbishing costs for Rocket B is cheaper than 2xRocket A+2xA-amount of Fuel, then two launches with Rocket B will be more efficient from an economical standpoint.
The value of Rocket B will degrade somewhat with each launch, but maybe it can handle 5-10 launches with lower priority cargo each run (nobody wants to launch an expensive satellite, or people for that matter, with a potentially unreliable rocket. But if the major cost of the launch is just getting that amount of weight into space then it’s fine to gamble on whether it will make it or not).
Weight of fuel is an upper limit factor for how big you want to build a rocket. There’s still a ton of room to operate under the upper limits though, and SpaceX is simply building under the upper size limits. This is saving a lot of money reusing equipment even with higher fuel consumption cost.
Not only does it decrease payload capacity, but what people are missing is the maintenance inspection/part/labor/transportation cost on these boosters (after experiencing significant thermal and stress loading from launch/reentry). It’s like if you got in an accident and totaled your car but it still drives. Would you spend more money to repair your car, or just buy a new one? Luckily if you’re SpaceX you can afford to eat the cost of the repair.
Being re-usable doesn’t preclude the rocket from being expendable if they have a very heavy lift or longer than normally possible burn in the future. It’s just higher cost..
Imagine if you had to buy a new car each time you went somewhere. That’s many money
Now, imagine if the gas tank was refillable and you could just put more gas in each trip. That’s less monies. Still expensive, but less than buying (or building from scratch) a new car
Adding say 5% extra fuel margin is cheaper than building a new rocket for each launch.
Rockets are built in stages.
The first stage is 80% of the cost AND the mass penalty is much smaller (because it doesn’t go all the way to space), so tail landing reuse is a no brainier (once you get it to work).
Second stage reuse has a higher mass penalty, for a smaller part of the rocket so reuse is ‘debatable’.
~95-97% of the rocket **to orbit** is fuel. The primary reason is in order to use fuel at a higher delta V you also had to burn energy to get the fuel there in the first place. So the rocket effectively has to burn fuel to raise it higher in the orbit so its available at that higher orbit to burn. The math barely works with chemical fuels which is why we STAGE our rockets and build them as light as possible.
For SpaceX roughly ~70% of the cost of the rocket is in that first stage (the most engines, biggest tanks), but this segment separates in the launch at a much lower delta V (lower altitude, slower speed). The penalty for having extra fuel in this stage is not nearly as devastating as having extra fuel in the 2nd stage (one of the reasons SpaceX gave up on that with Falcon 9).
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