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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"?

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  • $\begingroup$ excellent question, you beat me to it! I was going to cite your 1.2 comment as an authoritative source ;-) $\endgroup$
    – uhoh
    Commented Jan 2, 2020 at 3:00
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    $\begingroup$ Good grief, never cite my comments as authoritative. $\endgroup$ Commented Jan 2, 2020 at 3:03
  • $\begingroup$ uhoh! space.stackexchange.com/a/17308/12102 $\endgroup$
    – uhoh
    Commented Jan 2, 2020 at 3:05
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    $\begingroup$ @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.) $\endgroup$ Commented Jan 2, 2020 at 16:50
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    $\begingroup$ 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) $\endgroup$
    – asdfex
    Commented Jan 2, 2020 at 18:20

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