how can you understand best trade off between Dv and TWR?

[EtherDragon]

It's pretty complicated, and with KSP, there isn't perfect math to calculate the trade-off between TWR and Delta-V for various mission goals.

For lift-off to orbit, your TWR needs to be high enough to accelerate to near-terminal-velocity, which means you have to have >1 (or your not getting off the ground). TWR of 1.2 seems to be just about right (again, no solids maths to prove it either way) as higher TWR means you are carrying around extra rocket mass that ultimately goes unused, reducing your Delta-V.

Once in Orbit - it's a whole different game; the more Delta-V, the better. Generally TWR has little bearing (other than convenience...) which is precisely why you need multiple stages, so you can switch over to rocket motors that are better for each phase of launch to mission.

[WafflesToo]

blah blah, lithosphere braking maneuver

This made me giggle.

Likewise

Funny this comes up right now, been banging my head against the wall all week trying to work out some way of at least analyzing the effect TWR has on gravity drag during ascent. This is what I've worked out so far (I make no guarantees I've done this right)

Hohmann transfer from the surface to an 80km parking orbit requires 2255 m/s (including Kerbin's rotation of 175m/s).

Next I calculate how long it takes to get to 10km (and that is equal to SQRT (20000/(a-g)) in seconds). Multiply that by 1.707 and again by 9.81. That should be a rough guesstimate of your gravity drag. Add those two values together then add 1360 to account for vanilla air drag. And that should be roughly how much dV the rocket will need.

Disclaimer: this has not been tested very thouroughly yet, but the results seem to be close based on experience.

Edited January 18, 2014 by WafflesToo
fixed derpy math error

[rkman]

In principal there is no trade-off between TWR and delta-v.

Assuming the point is to complete a specific mission you really need to meet the range requirement (delta-v) and you really need to meet twr requirements for various phases of the mission (e.g. need to be able to launch, need to be able to do orbital insertion before leaving SOI). Any more than that is a waste of money (eventually will be in ksp).

Trading off one for the other would mean you are unable to complete the mission. Due to limited choice in engines and fuel amounts in ksp this means you will have more delta-v and more twr than really needed.

Edited January 18, 2014 by rkman