Why Does Anybody Use Engine Gimbal?

[Geschosskopf]

3 minutes ago, FullMetalMachinist said:

I don't think it's necessarily the fact that the engine gimbals that causes these issues. Don't get me wrong, I agree that there's a problem. I just don't think it's solely the gimbal that is the problem. It just looks that way because engine gimbal is so powerful. But if you take a craft that has loads of aero control surfaces (but is still just barely stable), the stock SAS will still wobble all over the place. 

Yes, there is a virtue in only using just the amount of control you need.  This is very hard to achieve with gimbals, however.

 

3 minutes ago, FullMetalMachinist said:

I think the real root cause is that the PID controller for SAS is very badly tuned. Sure, engine gimbal makes it stand out more due to the large forces and the "instant" nature of its motion, but it's not the root problem. Stock SAS is. 

Well, at least in 1.1.2 it's way better than it used to be.  For instance, it's now possible to use the stock "point-in-direction" (not to be confused with "proportional-integral-derivative" BTW) thing to change heading instead of merely hold it once you'd pointed that way manually.  Used to be that if you were pointing say prograde and hit the button for radial+ (or any other 90^ turn), the thing would overshoot wildly and repeatedly in both direction.  Now it pretty much just goes where you want and stops with only a minimum of feeling around for it.

[FancyMouse]

I need gimbal for landing/lifting off on a non-atmospheric planet/moon, especially large ships. That's a way better option than RCS/reaction wheel for a fast turn near the surface.

[Alshain]

1 hour ago, 5thHorseman said:

Aha. So your argument isn't that gimbaling engines are inherently bad, but that Kerbal Space Program's IMPLEMENTAITON of them is bad.

I'm on line with that.

The two main problems with gimbaling engines is that they're far from your command pod and that they are all-or-nothing. Those two things together create the wobble you are complaining about. If you put a probe core way down at the bottom of your ship and control from there, it's not even remotely as bad.

The engine gimbal is not all or nothing, your keyboard is.  If you have a flight stick, or even an XBox controller, it will scale.  It's no different than using ailerons with a keyboard really.  A keyboard is just an inferior way to simulate this sort of thing.  I understand not everyone owns a controller or stick, but the game does simulate gimbal correctly.

Edited May 17, 2016 by Alshain

[AeroGav]

I always disable gimbal on my spaceplanes for efficiency, I don't want engine thrust being diverted off axis to trim the nose up .

This means instead of engine thrust producing forward motion, some is being used to push the tail down, so that the nose stays up.

You pay for this twice, because the wings have to create additional lift to overcome this downforce as well as the plane's weight, creating more drag.

Better for canards to generate the nose up trim force on their own, at leas that's contributing to overall lift.   And if correctly sized, they'll be doing that at a lift drag ratio of 4 to 1 

I have reaction wheels for exo atmospheric control.

Gimbal on a rocket upper stage is ok imho

[Superfluous J]

46 minutes ago, Alshain said:

The engine gimbal is not all or nothing, your keyboard is.  If you have a flight stick, or even an XBox controller, it will scale.  It's no different than using ailerons with a keyboard really.  A keyboard is just an inferior way to simulate this sort of thing.  I understand not everyone owns a controller or stick, but the game does simulate gimbal correctly.

 

Is this a recent thing? I remember others complaining about it in the past. That makes me want to try a flight stick or at least a controller (which would not require a purchase)

I know firsthand that "hold prograde" and similar buttons flail about the gimbaling so much that it at least appears that they do full gimbaling.

[Alshain]

16 minutes ago, 5thHorseman said:

 

Is this a recent thing? I remember others complaining about it in the past. That makes me want to try a flight stick or at least a controller (which would not require a purchase)

I know firsthand that "hold prograde" and similar buttons flail about the gimbaling so much that it at least appears that they do full gimbaling.

I honestly have no idea how recent it is, I had to open the game to confirm it.

Now, yes SAS is all or nothing, I'm well aware of that one (SAS is really stupid), but the gimbal itself is not.

Here you go, made a video.  It is a modded install but I'm fairly confident I have no mod doing that.

EDIT: May be wrong on that, Claw's stock bug fix package does something to gimbal." I'll investigate further.  We'll get to the bottom of this.

EDIT2: Well I had Gimbal Plus in that mod disabled, but I'm gonna play it safe and kill all mods and try the test again.

FINAL EDIT: Confirmed, stock behavior.

 

Edited May 17, 2016 by Alshain

[Warzouz]

I use gimbal, but In usually tune it down to 10 to 30%

• Take off : I favor control surfaces at the bottom of my engine
• Mid flight : delicate part when you usually have no wings left. Gimbal is useful, but tuned down to 20% to avoid wobble
• Space maneuver : I favor reaction wheels. I don't like RCS because you need many additional parts and weight.
• LKO burns : I usually like Gimbal so it let me keep straight. I usually launch heavy stuff, and reactions wheels aren't designed to control all the ship.
• Landers : Usually pod reaction wheels are enough. Gimbal add usually too much control power.

I agree that gimbal is always too powerful by default. Even if not mandatory, it's an nice toggle control feature.

But again, the problem with gimbal and too powerful torque reaction wheel is that SAS is clumsy. It overcompensate, creates wobble a eats a lot of energy (usually EC/s)

 

 

[Red Iron Crown]

I generally don't like relying on the comically OP stock reaction wheels for control authority, so I don't.

I generally don't like using fins on rockets, as they're just extra mass and drag.

So I use gimbals to point my rockets during ascent. I could use RCS instead, but that's again more mass and part count and I already have a gimballing engine, so why not? It is very useful to limit the gimbal significantly to prevent overcontrol. It's much better if you use Ven's Stock Revamp or any other mod that adds gimbal response time (why this isn't turned on in stock is beyond me, it's a stock feature that is disabled).

[Sereneti]

21 hours ago, Kerbart said:

More efficient is not always more desirable. There's something else positive about wings at the nose, and that is "positive feedback loop" which is generally a Bad Thing (generally; not everyone wants to stay in control of their vehicle when launching it).  Controls at the nose? Pitch up, and your control surfaces' angle of attack increases. So now they generate more lift, resulting in more torque, resulting in a steeper pitch, increased angle of attack, more lift, etc.

Fins behind the COM will have a stabilizing effect; fins (and surfaces) in front of the COM will result in an unstable vehicle. Nimble perhaps, but frightingly unstable.

i had the controls at the nose...

https://en.wikipedia.org/wiki/Skylon_%28spacecraft%29

the skylon spaceplane had the controll surfaces on the nose, too...

engine gimbal are used in RL too
https://en.wikipedia.org/wiki/Thrust_vectoring

Edited May 17, 2016 by Sereneti

[awfulhumanbeing]

Gimbaling gives better stability during burnout if you don't use it too much. It gives you additional rotation that doesn't take any additional fuel, and a little electricity when needed and when not needed it can be turned off. Gimbals are useful during takeoff, orbital maneuvers and landing for better precision.

Anyways, gimbaling is just a tool in the hands of a player that is given to him with an option of either using it or not.

[RX2000]

I'm a pretty big KSP newb, so I still find it hard to get a decent sized rocket into orbit using non-gimbal engines. I put on fins but it only does so much with the rockets I build. I guess I'm just doing it wrong, but thats why I build with gimbaled engines.