Just Another thing on TWR

[Spaced Out]

If you have a rocket, do you basically figure out a way to stuff fuel in it until the TWR would cause more gravity losses than the delta v gained? And I also have been told that it is advantageous to have a low TWR due to stresses on the vehicle when accelerating very fast so you can build a lighter craft. Is a lower thrust to weight ratio better for these reasons, and is there any more I haven't mentioned? Also, what would be a good TWR for a rocket to maximize delta v and minimize gravity losses?

[Bill Phil]

Well, as you burn your fuel your TWR increases, and your loads increase.

Most real TWR limitations come from people. The NASA maximum recommendation for long term acceleration is four gees. 

[Pand5461]

It's not as easy. High TWR certainly reduces gravity losses if you just launch rocket vertically to a suborbital trajectory. On the other hand, you need to either steer more aggressively to get a gravity turn or spend more time at high pith angles if you have too large TWR, increasing losses in both cases.

Also note that gravity losses scale approximately as g / (a - g) = 1 / (TWR - 1), so going from 1.1 to 1.5 is vast improvement, from 2 to 3 - not so much.

That said, it's best to have maximum possible TWR at liftoff but probably not higher than 3 in the maxQ region (mainly to limit aero stresses, drag losses are usually negligible IRL, contrary to KSP). After rocket passes dense atmosphere, it needs to steer, so high TWR leads to high steering losses here. Therefore, something between 1 and 2 is the choice (sometimes even lower, like Saturn V with 0.8 at second stage). For circularization, TWR does not matter much so IRL it's usually low because smaller engines are cheaper.

Ironically, the closer you are to the ground, the higher acceleration you actually need, which is absolute opposite to what really happens at rocket launch.

Edited August 29, 2017 by Pand5461