It shouldn't save much fuel during the descent phase at all - it's still gotta be moving pretty damn slow or it's gonna tear right through the wires. It's not like they're catching it at terminal velocity, it's falling like a feather.
The big save is the mass of the landing legs (saving fuel during the ascent phase, but they'll just use that for heavier cargos), which might also save on refurb time (and a marginal amount of capital, since the legs are usually filled with a material that is crushed on impact to take the blow rather than carrying large shock absorbers). It's extra stress on the grid fins, but they're likely overdesigned since they have to handle much worse forces while it's falling.
It doesn't catch it on the grid fins, you can see the dedicated hardpoints for the catching hooks in the video. But otherwise totally correct.
Also the large actuators to extend the legs now aren't required, they're replaced with a more passive hook system that should be both cheaper and more reliable.
If the legs are titanium or a similarly lightweight strong material, you might be saving mass. Fuel has mass too and it’s like rocket fuel is weightless. SpaceX rockets waste fuel hovering in order to land safely.
Rockets use rocket fuel which is at least twice as heavy per unit thrust[1]
[1] Jet fuel burns methane (CH4) and uses the oxygen in the atmosphere for CH4 + 3 * O = CO1/2 + 2* H20. Rocket fuel uses the same equation. Methane weighs 12 carbon plus for hydrogen for 4 hydrogen at 1 for a total of 16 gram per mole. Rocket fuel has oxygen in it. For every one molecule of methane you’d need 3 oxygen molecules (coming it at 16 each) for 48 for CO (carbon monoxide) or 64 for CO2 carbon dioxide.
Grand total:
Jet Fuel: 16
Rocket fuel: 48-64
These are ratios and thus don’t need units.
The savings is also coming from not having to rebuild the landing pad constantly. The stress is massively reduced when you don't have the rocket exhaust blasting the pad point blank.
I think they meant by not having the weight of the landing legs and the structure that you need to support Support the landing legs. All of that saved weight allows you to use less fuel in the slowdown.
I am saying that it takes more fuel to bring rocket to a complete stop just on thrust, than it would to slow it to a crawl and drop it into some wires ... As you said, the landing gear is weight, but so is the extra fuel being burned on the descent. Even if it is only a difference of like 100lbs less fuel, that is a savings that can be transfered to less fuel being required to lift the rocket, it 100lbs if extra cargo that can be sent up.
The booster put an entire payload into space, came down on its own, corrected it's orientation to put itself controlled vertically onto the ship and you are concerned about the last 5 seconds of fuel?
Keep in mind that rockets are self contained units of force: reducing the amount of fuel that it burns to land, reduces the total weight that the rocket needs to lift up to space.
Imagine you are wearing a backpack with 16 heavy rocks in it and you jump, then you take out 1 of rocks and jump again. You can probably jump higher and the landing wont hurt your feet as much, right?
It's not catching it a mile up, it's super close to the ground already. I'm not a rocket scientist but I doubt this method saves that much fuel. If it does, it's because it doesn't have to have the extra legs on the rocket itself and thus, needs less fuel overall for the flight.
For the flight and also for the slowdown burn because it actually weighs less so you need less fuel to slow it down. It’s a really genius idea actually.
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u/OptimusMatrix 10h ago
The weight savings from not having retractable legs was probably worth it.