Is there an optimal thrust to weight ratio or does it not matter at all?

  • $\begingroup$ “Optimal” according to what metric? Cost optimal is usually just enough TWR to get safely off the launch pad. $\endgroup$ – Russell Borogove Feb 25 '18 at 5:17
  • $\begingroup$ @RussellBorogove I think the OP would be flexible on metric. Is there any at all, or is the TWR of a rocket on the launch pad simply the result and consequence of other things that have been optimized? Probably nobody says "I need a launch vehicle with a TWR of 1.2 +/-0.05". For a weapon, there may indeed be a need for very rapid acceleration. But getting stuff to orbit, probably no, as long as its's not absurdly low, like 1.000000000001, or absurdly high like 10. $\endgroup$ – uhoh Feb 25 '18 at 6:27
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    $\begingroup$ Well I know it at least matters that TWR is above one. If you have a rocket with a billion tons of fuel, but a TWR that maxes out at 0.9, you will effectively have 0 delta-v. $\endgroup$ – Elijah Seed Arita Feb 25 '18 at 6:28
  • $\begingroup$ Probably the highest was Sprint, which accelerated at 100 g's. $\endgroup$ – Organic Marble Feb 25 '18 at 16:47

There are some limits:

  1. The thrust/weight ratio (TWR) has to be above 1 at liftoff.
  2. TWR has to be low enough to keep the acceleration below reasonable limits. Many rockets these days are designed so that the acceleration near the end of the first stage burn (when weight is lowest) is around 4G. Unmanned rockets can go a bit higher, but over 9G is rare for satellite launchers. Military missiles can use much higher accelerations.
  3. A high TWR means you build up lots of speed in the lower parts of the atmosphere, so you get lots of aerodynamic pressure and heating.
  4. A low TWR means it takes longer to reach orbital speed, so your gravity losses are higher.

In practice, TWR at liftoff is slightly over 1 for many orbital launchers.

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