Do g-forces significantly effect thrust-to-weight ratio?

[GibLit]

Thanks for the replies/info guys. Makes more sense now.

[Jason Patterson]

here are a few questions on this topic.....

i heard TWR at launch is most efficent at 2.2. is that correct?

what should my TWR be once in space?

what should my TWR be at different altitudes?

It's said on here that ascending at terminal velocity is the most efficient way possible. Going faster increases aerodynamic drag and going slower increases gravity drag with a balance of the two being most effective. I haven't done the math myself to demonstrate that the precise balance between the two is most efficient, but it's not unreasonable. In order to make this happen you would want your thrust to weight ratio to always be 2 in an atmosphere. Half of the thrust opposes gravity while the other half accelerates the ship up to terminal velocity. Practically speaking, anything from 1.8 to 2.2 does pretty darned well, and the TWR varies somewhat as the fuel in the boosters burns out.

In space your TWR doesn't really matter, as long as it is high enough to get your ship moving in a timely fashion. Something like 0.2 works just fine for interplanetary burns on most missions. Isp is much much more important.

When your ascent speed is limited by aerodynamic drag, you want a TWR of roughly 2. Once you are above an altitude where that is the case, you can have a TWR as high as you like (as long as it doesn't bring you up above terminal velocity, anyway.) On Kerbin you can pretty much open up the throttle above 15km or so, since it's not likely you'll be ascending at terminal velocity at that altitude on a normal rocket launch.

[M5000]

What are you people talking about using a TWR of 1 or 2 in space? Jesus you either have a ton of engines or you're using super low Isp engines.. I'm entirely content on a TWR of like .02....

[Jason Patterson]

Jesus you either have a ton of engines or you're using super low Isp engines.. I'm entirely content on a TWR of like .02....

If your TWR is too low you just can't make an accurate ejection burn (at least not without a whole lot of additional planning.) It also leads to additional gravity drag (since you're burning upward) and a loss of the Oberth effect (since your velocity will be decreasing as you climb during the burn, or at the very least it will be increasing very slowly.) Great Isp can definitely overcome the losses in the long run, but if you can spend a fraction of the delta-v and save yourself a couple of hours, why not do it?

[Dunbaratu]

The only reason for wanting a high thrust once you're in orbit is to cause your burns to be closer to pinpoint accurate. The maneuver nodes are calculated based on the assumption that you can provide all the delta-V you want in a single instant at a single location on your flight path, which isn't entirely true but is often close enough to true when doing very large orbital movements where you're still close to the same point in your elliptical geometry after several minutes of thrusting. But the downside to having high thrust is that it makes it hard to "STOP" your burn at exactly the point you wanted to. It starts to become hard to control when even at the lowest throttle setting the difference between burning 1 second versus burning 1.5 seconds makes the difference between hitting or missing your target. So it's a tradeoff how high you want your thrust to be.

I find that I prefer low thrust, with the caveat that I have to remember to "straddle" the maneuver point time mark because of how slow it is. i.e. if it predicts that I will be burning for 2 minutes, then I want to start my burn at T minus 1 minute, rather than start it at T zero, so that the *middle* of the burn time is at T zero, rather than the start of the burn time.

Also, when doing a slow burn at Apopsis or Periopsis where I know my maneuver node is entirely prograde or retrograde without the other two axis "knobs" having been twiddled, I find that I tend to ignore the blue maneuver marker and instead just burn using the retrograde and prograde marks themselves because I know the blue mark is only correct at one pinpoint time during the burn. I start my 4 minute burn and then keep a careful watch on the prograde mark and slightly rotate to follow it as it moves while I burn, so I'm never burning "up" or "down" during the slow burn.