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According to a Teslerati article linked from another QA:

The most critical component for the booster, according to Musk, is that there is “a lot of force pushing up.” He states that to launch Starship, the Super Heavy booster would ideally produce roughly 7500 tons of thrust – about twice the thrust of that of a Saturn V rocket. For a reusable rocket design – such as with the Super Heavy booster – a high thrust to weight ratio (about 1.5:1) is a necessity for efficient operation.

Most modern launchers have an initial thrust-to-weight ratio off the pad of about 1.2:1, with some outliers.

The excess above 1:1 TWR determines how long it takes to clear the launch tower, so less than around 1.2 isn't desirable. Above that minimum TWR, my understanding has been that carrying more fuel in the first stage is generally better than not; the dry mass penalty of slightly larger tanks is outweighed by the acceleration potential from the additional carried fuel.

What is it about reusable rockets that makes a higher TWR "a necessity for efficient operation"?

Russell Borogove
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  • excellent question, you beat me to it! I was going to cite your 1.2 comment as an authoritative source ;-) – uhoh Jan 02 '20 at 03:00
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    Good grief, never cite my comments as authoritative. – Russell Borogove Jan 02 '20 at 03:03
  • uhoh! https://space.stackexchange.com/a/17308/12102 – uhoh Jan 02 '20 at 03:05
  • Isn't a higher TWR beneficial for all boosters? I mean if you can get higher TWR with the same engine mass, you have less gravity losses and thus need less fuel... problem is that more thrust usually means bigger engines, which means more mass... – Polygnome Jan 02 '20 at 13:35
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    @Polygnome No. Consider your high TWR booster with fuel mass X. If you instead carry fuel mass X+1, then you can burn 1 unit of fuel to gain a nonzero amount of vertical height and speed before becoming a high TWR booster with fuel mass X. (Tankage mass messes with this analysis somewhat, of course, but the general principle holds.) – Russell Borogove Jan 02 '20 at 16:50
  • @RussellBorogove Sure, you're right, but you get a high TWR booster with a worse mass/fuel ratio. And fuel needed for landing is larger, adding an other penalty on the amount of fuel to be used for the launch itself. – asdfex Jan 02 '20 at 17:31
  • @asdfex The mass ratio is worse, but the increase in total impulse available outweighs that hit. I don't think the landing reserve gets any worse as a fraction of launch mass. – Russell Borogove Jan 02 '20 at 17:57
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    More impulse (also) means more speed to cancel during landing. High TWR on the other hand means short burn time and less height at MECO which is favorable for landing. I think we have to compare boosters with identical performance here, i.e. same payload, same dv. (NB, I'm not arguing here, just collecting things to take into account) – asdfex Jan 02 '20 at 18:20
  • You're correct that high TWR gives you a lower and slower first stage cutoff, which is one of a few possibilities I can think of as an answer to the question. – Russell Borogove Jan 02 '20 at 18:24
  • Speed should be larger with high TWR - the booster spent the same dv, but in less time so gravity losses are lower. – asdfex Jan 03 '20 at 13:04
  • @asdfex The booster spent less dv because it didn’t bring enough fuel. Think of two identical stages, one with a full fuel load, one with an 80% fuel load. One comes off the pad at 1.2:1, the other at 1.5:1. Which one gets farther? – Russell Borogove Jan 03 '20 at 15:23
  • We're talking about different things. I'm talking about two identical rockets, one with a modified engine that has more power. You scale the tanks, I scale the engine. – asdfex Jan 03 '20 at 15:32
  • Let us continue this discussion in chat. – Russell Borogove Jan 03 '20 at 15:54

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