Comment by marze

3 days ago

Historic rockets around 1.2, including Shuttle, check your numbers. Starship is not a "historic rocket". It and Falcon 9 fly a trajectory that is optimized for something else, not max payload for a set first stage thrust.

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marze

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idlewords  3 days ago

The shuttle stack weighted 4.5 million lbs at takeoff and had a total thrust of about 6.8 million lbf, giving a thrust to weight ratio of 1.5.

  • marze  2 days ago

    That is incorrect. Shuttle liftoff thrust was 5.7 million pounds, giving a ratio of 1.25.

    At liftoff, each SRB produces 2.65 million pounds thrust [1]. Using Wikipedia numbers instead (2.8 million pounds for SRB thrust at liftoff [2]) gives a liftoff thrust to weight ratio of 1.33. The 3xSME produce 0.4 million pounds thrust at liftoff. The SRB thrust did increase as the burn progressed, then ramped down for max Q.

    People are accustomed to SpaceX rockets rapid acceleration, since they conduct most launches these days, but historic rockets (and New Glenn) accelerated more slowly off the pad.

    [1] https://www.nasa.gov/reference/the-space-shuttle/

    [2] https://en.wikipedia.org/wiki/Space_Shuttle_Solid_Rocket_Boo...

    • idlewords  1 day ago

      Your math is wrong (you left out two of the main engines):

      1) SSME thrust is 418k lbf. The shuttle has three engines, so 1.25M lbf

      2) SRB is 2.65 M lbf. Two boosters give 5.3M lbf.

      3) That sums up to 6.6M lbf

      4) Shuttle weighs 4.4 million lbs.

      5) That works out to a 1.5 thrust-to-weight ratio at sea level launch.

      If you still don't believe, watch a Shuttle launch and time how long it takes to clear the tower. You'll get the same answer (vertical acceleration at about 1.5g)

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