stock parts - Throttle to fuel consumption ratio?

[draeath]

With the stock parts, does the fuel consumption scale with throttle linearly?

Eg, outside of atmosphere is there any advantage to a slower but longer burn?

[Nemoricus]

As far as I can tell, consumption is linearly proportional to throttle, yes.

As for the second question, no, there is not. And there are actually some disadvantages to doing so, since a long, slow burn is less efficient at putting you on any given orbit than a fast, high power burn.

That said, a lower throttle does give you more fine control over your acceleration.

[phoenix_ca]

A longer but less powerful burn with an engine of much higher vacIsp will be more efficient though. The only note about that is it's more important to start your burns at appropriate times; that is, start them early. A 30s burn starting 15s before periapsis will have a similar effect to an instantaneous delta-v change at periapsis, or at least, it spreads-out the unwanted effects. NERVA engines can be annoying like this, and I'd suggest against using MechJeb for large orbital corrections for that reason (MechJeb always starts a correction burn on the assumption that the total delta-v required can be delivered instantaneously; to do otherwise requires more calculations based on TWR, the burn's vector, Isp of the engine, etc...it's...quite a bit more math).

[draeath]

OK - so for initial launch - at the start, as long as you are gaining altitude it's OK to leave the throttle low? While still near the ground like that I assume the slowest you can ascend the better, because of drag loss. Or is it best to try and ride terminal velocity? Seems to me the closer to terminal velocity you are the more drag loss you would experience.

I'm thinking of, for example, the super-slow Delta-V ascent.

[John Bedlam]

There's a sweet spot between drag and gravity. If you throttle too low (or the TWR of your ship is too low), you're just throwing most of your fuel against the ground to stay as high as you are. Mechjeb seems to try not to exceed 200m/s in lower atmosphere.

[CombatWombat]

You can get an intuitive grasp of gravity drag vs aero drag by pushing the situation to its logical extremes. IE: Logically, the slowest ascent rate we could have would be 0m/s. In which case, you are effectively 0% efficient since you're just burning fuel to hover, not gaining any ground. On the other extreme (high velocity), thrust that would normally accelerate you, is being thrown away (into the wind, as it were). So clearly there is some optimum place, between the two extremes, and that will vary by ship design, atmosphere, etc. I guess you could make a pretty good guess of this optimum rate with some fancy spreadsheet antics.

[karolus10]

In general, rocket engines in 100% throttle are their optimal and most efficient working parameters, you can throttle down when you need lower thrust (in lower atmosphere too much acceleration aren't good because of higher drag loss), but throttling down aren't more efficient than full thrust.

In some engines, MAX throttle settings are much higher than most efficient working parameters, so you can get insane amount of thrust in exchange of lower efficiency - like in escape engines of planned dragon capsule, where you can burn all capsule fuel in just few seconds (or less), during abort sequence.

Edited September 27, 2012 by karolus10

[souza]

There's a sweet spot between drag and gravity.

I never thought of it that way I usualy thrust 100%. I have been experimenting with burning main engines and boosters stages together, or one after the other(this option I found was usualy better). Now I'm going to experiment on different thrust levels too

[phoenix_ca]

In some engines, MAX throttle settings are much higher than most efficient working parameters, so you can get insane amount of thrust in exchange of lower efficiency - like in escape engines of planned dragon capsule, where you can burn all capsule fuel in just few seconds (or less), during abort sequence.

Are you talking about real-world engines or those in KSP? If the latter, what data are you basing this on?