Ah yes, good point - I was implicitly assuming the liftoff weight was maxed out, by thinking that any upper stage weight increase would have to come from the 1st stage fuel load.
If I remember correctly, if they used a maximum burn right before impact they would only need about 1 tonne of fuel to take the first stage from orbit to the ground. Of course, that would require some crazy control systems, but it's a useful lower bound.
On the first stage every last little bit of weight counts since it only weighs the rocket down. They already lower their payload to orbit by about 30–40 % when they land the first stage. That stage has to fight most of Earth's gravity and atmosphere so accelerating at all takes a lot of effort. So adding weight to the part that is only needed for accelerating the rocket through the atmosphere lowers the total payload considerably. It's bad on the second stage too, but not that much (but even then, they shed the payload fairings or the Dragon nose cone when they're no longer needed).
Adding another (different) engine also complicates things. Currently they only have a single engine type for the whole rocket (plus thrusters), the one on the second stage having a different nozzle. This greatly reduces complexity and thus cost and risk.
It's not technically 1:1 on the first stage. It varies a bit from launcher to launcher, but it's in the ballpark of 4:1 (you lose a pound of useful payload for every four pounds you add to the first stage).
Your point is spot on though. Don't waste mass to account for failure modes that you can just avoid in the first place by landing upright.
But all that extra mass does eat into the payload capacity.
A craft that is 100 tonnes dry mass and delivers 20 tonnes payload needs enough fuel to boost 120 tonnes in the last part of the flight. A centaur upper stage by comparison has a mass of just 2.2 tonnes. So for most of the flight you need much more fuel to boost the same payload.
I still don't understand the point of your comment then. Yes, the rocket's (potential) TWR will be above 1 at all points of the flight.
What does that have to do with the comment you're replying to? The Falcon 9 has 9 engines. The combined thrust of all of them has to be greater than the weight of the fully loaded rocket. That doesn't mean a single engine will always have more thrust than the weight of the entire rocket.
Sort of true. The rocket equation doesn't account for the added weight of lifting the rocket itself, just the added fuel. Multiple launches can reduce the practical overhead, even if not changing the overhead in the limit of 0% rocket mass 100% fuel mass.
I don't think they fueled it up all the way. But I do thing the booster and maybe also starship itself is a little heavier than they plan to have it at the end.
*update* if you check the video just before launch (39:54), you can see that the fuel bars for both the booster and the ship is not 100% full: https://twitter.com/i/broadcasts/1OwxWYzDXjWGQ
Seems like they would want to launch higher than 35000 if they can since that would require less fuel for the rocket which seems like that is at least part of the point.
That would be a lot of extra launch weight just to return the first stage, not to mention the danger of having filled boosters on the rocket the whole launch.
Won't the extra weight require a change in earth re-entry fuel calculation etc. ? Without knowing the exact change in weight, they are going to be firing in the dark, as it were.
Thanks, fixed. I meant Falcon Heavy. And it's actually a little more than that to GTO, but then the thing has to get itself into GEO, so it's not all payload:
Falcon Heavy has the second highest lift capability of any operational rocket, with a payload of 63,800 kg (140,700 lb) to low Earth orbit, 26,700 kg (58,900 lb) to Geostationary Transfer Orbit, and 16,800 kg (37,000 lb) to trans-Mars injection.
You want to test this stuff as realistically as possible. Changing the amount of fuel means changing launch weight which means (presumably) changes in throttle profile etc. The control systems are also probably designed with a certain weight to reach orbit so it's possible that an unusually light rocket would be unstable.
I'm not sure whether this would actually make a difference, but if you know it works at nominal fuel weight then you have some confidence it would work during a real launch.
If I remember correctly, they carry more fuel than they need in the first stage as a safety reserve, and the plan is to use that reserve fuel to land the stage. So they actually don't pay an extra weight cost, they simply make use of margin they were including anyway. If they end up having to use their safety margin, then the rocket can't land, but in the majority of flights where things run smoothly, they get to land it.
The total fuel required is the same in both cases, but to do it in a single trip the upper stage would have to be nearly twice as large and the lower stage exactly twice as large. This is already far larger than anything that has been done before, so making it twice the size would be a significant problem.
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