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If Superheavy fails during launch (or even fails to launch in an unsafe way) the Starship itself might well be able to just light its engines and fly a suitable suborbital trajectory

https://space.stackexchange.com/a/46080/36625

Starship with a full payload weighs 1450 tons. (1200 tons fuel + 100 tons dry mass + 150 tons payload). It has a maximum thrust of 1500 tons.

That maximum cannot come at sea level, because neither the Raptor Vacuum nor the normal version is optimized for sea level. The latter has a vacuum ISP of 350 seconds, versus sea level ISP of 327 seconds, or just 93%. And I do not know the sea level performance of the former.

Even if we assume that all the engines were normal Raptors, we still get a sea level thrust of 1401 tons, or less than the weight. Obviously it's much worse for the Vacuum version.

Am I correct?

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Yes. It's not unusual for a second stage to have a TWR less than 1:1 at ignition; when it stages, it will have plenty of upward velocity imparted by the first stage, and as fuel is consumed the TWR will soon reach 1:1.

For example, the Saturn V second stage produces about 0.82:1 TWR at ignition. Because the rocket is pitched over nearly 70 degrees at staging, even once the second stage reaches 1:1 TWR, the vertical rate is still decreasing! The maximum vertical speed of the Saturn V on its initial ascent to orbit is in fact reached at first-stage cutoff.

Note that Starship can lift off and fly on its own by reducing fuel load and payload to achieve a better than 1:1 TWR at ignition.

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    $\begingroup$ Add to that, if staging is sufficiently late, gravitational losses will be already reduced - the transition between "need >1 TWR not to fall" (on the ground, or in vertical climb) and "no need for thrust not to fall" (in orbit) is gradual, so after a while TWR < 1 is still enough to keep climbing and approaching orbit. $\endgroup$
    – SF.
    Commented Apr 24, 2023 at 22:01
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    $\begingroup$ The transition is gradual in a sense, but it's very nonlinear (quadratic, I think?), so it's not until you're quite close to orbital speed that you're getting significant gains from Earth's curvature. $\endgroup$ Commented Apr 24, 2023 at 22:24
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    $\begingroup$ @RussellBorogove 5.5km/s to get to 0.5g. $\endgroup$ Commented Apr 25, 2023 at 4:03
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    $\begingroup$ At which point the rocket is pitched at about 75 degrees, so you'd need 2g acceleration to get 0.5g vertical. $\endgroup$ Commented Apr 25, 2023 at 4:34
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    $\begingroup$ @Joshua Due to the unusually high chamber pressure of the Raptor engine, it is actually possible to run the vacuum version at sea level without plume separation. See this test for an example [youtube.com/watch?v=oTQymcS8ApE ]. I also recall hearing that later starship upper stages will have 9 engines - 6 vacuum, 3 sea level. This should give enough thrust to lift off even while fully loaded. $\endgroup$ Commented Apr 25, 2023 at 22:13

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