Does Gimbal Waste Delta-V?

[spikeyhat09]

the question: when your engines have gimbal enabled, are you wasting Delta-V since the thrust is sometimes vectored at an angle rather than parallel with your ship? if it happens IRL, is it also modeled in KSP?

Edited July 20, 2013 by spikeyhat09

[stupid_chris]

In a way, yes, instead of having a single vector for thrust, you end up with a smaller than usual prograde vector and a small perpendicular vector. Basically, part of your thrust is used to turn your ship, so yeah, some of your fuel is wasted in attitude control.

[Kimberly]

Yes. Your ship will veer off course because the thrust was at an angle, and then ASAS will make the gimbal thrust the other way to cancel out that movement and keep the ship stable. This constant back-and-forth is a waste of fuel.

[spikeyhat09]

In a way, yes, instead of having a single vector for thrust, you end up with a smaller than usual prograde vector and a small perpendicular vector. Basically, part of your thrust is used to turn your ship, so yeah, some of your fuel is wasted in attitude control.

so this does in fact happen IRL, but what about KSP? are the physics complicated/realistic enough or whatever such that this happens ingame as well?

[stupid_chris]

so this does in fact happen IRL, but what about KSP? are the physics complicated/realistic enough or whatever such that this happens ingame as well?

The physics in KSP on what concerns rocket flight are pretty darn accurate. When gimbal is applied, it doesn't just tell the game "make the ship go left because REASONS", it really changes the orientation of the thrust vector, so the effects on your ship are the same as those you would normally expect, and the delta V requirement is the same. Though, this is a ridiculously low number, you probably lose just a few m/s per launch.

[Maddmatts]

Yes, any thrust being used to counter instability is a loss in the game as well. And it's only a feature by coincidence since the game is already doing the math for vectors dynamically.

[sojourner]

There is also loss of delta-V from the engine not pointing exactly on the line of travel. The term I see used for this on space related forums is "cosine losses".

[PakledHostage]

The term I see used for this on space related forums is "cosine losses".

I seem to remember seeing "steering losses" listed in MechJeb's delta-V UI. I haven't ever used it though. Maybe someone else can confirm?

[astropapi1]

Funny enough, I hadn't thought of that.

IIRC, steering error the angle from prograde or retrograde at which you're burning or something like that. I may be (and probably am) wrong. Can somebody check?

[Coneshot]

Without the constant smaller course corrections, a larger single course correction might just be even more inefficient.. (not to mention correcting for "Yay, pinwheel time!" ascents)

[Kosmo-not]

I believe steering losses in mechjeb refer to the amount of delta-V expended to change the direction of your velocity vector. Any angle other than prograde will incur steering losses.

As for engine gimbal, you are wasting hardly any delta-V due to the cosine effect. Gimbal range for stock parts are typically 1°, with the highest being 3°. These would incur delta-V losses (assuming full deflection at all times) of 0.0152% and 0.137% respectively. So don't worry about engine gimbal causing a significant loss of delta-V.

Edited July 20, 2013 by Kosmo-not

[bsalis]

What Kosmo-not said, as usual. Efficiency loss from vectoring engines is negligible.

I don't know why I keep seeing threads asking about efficiency when from many pics i see it seems that efficiency is least concern. If you want efficient, think LT-Ns, lots of tanks and little else except staging.

[Brotoro]

Sure you lose a tiny amount of delta-V from gimbaling motors... But you lose a lot more delta-V if your ship spins out of control because you forgot to include gimbaling motors to control it.

[vidboi]

I don't know why I keep seeing threads asking about efficiency when from many pics i see it seems that efficiency is least concern. If you want efficient, think LT-Ns, lots of tanks and little else except staging.

That's not efficiency, that's just your amount delta-v. Your efficiency is how much delta-v you use for a given manoeuvre, which is what this concerns. Given that gimballing engines help your rockets staying pointing in the right direction (i.e. towards the prograde direction) I'm sure the losses are more than made up for, and the new 0.21 ASAS should eliminate gimbal oscillations.

[arq]

Technically, yes. But if the gimbal is limited to 1deg then the loss is limited to

1 - cos(1 * pi/180) == 0.00015 == 0.015%

Of course, this is assuming a lack of significant wobble in the rocket. Still, if the rocket wobbles 3deg (a lot!) and your gimbal is still going crazy then your inefficiency is still limited to 0.0024, or 0.24%. These should be upper bounds, so your actual loss should be less.

And both of these issues will be fixed (at least mostly) with 0.21 and the new ASAS.