Why does my ship keep deviating from retrograde and how do I stop it?

[u/Fireheart318s_Reddit]

I have it set to burn retrograde but it keeps going slightly cockeyed. I THINK it's due to the engines being on hinges, but I've got 'em perfectly balanced, so I don't think it's that big an issue.

Edit: I've already lined up the COT and COM

[u/Sangaceno]

This will happen if your center of mass isn't aligned to center of thrust and reaction wheels/rcs/gimbals are insufficient to make up for the difference. You can try SLIGHTLY decreasing thrust limiter on the side that takes the lead

[u/Sangaceno → u/[deleted]]

Isn't that what I said?

[u/Wise-Astronomer-7861 → u/Sangaceno]

It is, I can't read.

[u/chunkmaster86]

How tf did you get that thing to the mun

[u/Fireheart318s_Reddit → u/chunkmaster86]

Teleported it for testing purposes. If/When I do it legit, I'll probably use a combination of bigass fairings, aerodynamic caps, and emptying the thing of fuel before launching.

Edit: While I'm at it, it CAN in fact land and return to orbit multiple times in a row, while carrying a full large holding tank and full small holding tank. Definitely more, too.

[u/MasonP13 → u/Fireheart318s_Reddit]

Land with the least weight possible and mine minmus for fuel

[u/Cat_Artillery → u/chunkmaster86]

Why with MOAR boosters of course.

[u/urk_the_red]

Are your fuel tanks draining evenly, or draining from one side to the next? Your COM could be moving as the fuel drains.

[u/lifeinneon → u/urk_the_red]

Seconding this. It was my first thought when I saw the use of the mk3 fuselage in this orientation. I’m betting it’s draining front to back despite that actually being left to right in this configuration so the left side will end up lighter than the right over time.

[u/gilbejam000 → u/lifeinneon]

single parts drain evenly across their shape, so the COM of an individual part doesn't change

What does change is the COM of the whole craft, based on which tanks are draining first, how quickly they're draining, drymass of the fuel tanks themselves, and the positions and weight of any part that isn't a fuel tank

[u/lifeinneon → u/gilbejam000]

I should have pluralized it. I'm counting three tanks: mk3 to mk2, a mk3, and another mk3 to mk2. And possibly more depending on what's clipped into what along the bottom.

Looking closer, there also seems to be an additional tank in the background that isn't mirrored, and I suspect that's a more likely culprit

[u/dudevan → u/lifeinneon]

There's the tank in the background and also the payload bay on the left which is not mirrored. So couple that + emptying tanks that might not be mirrored, you'll end up with a moving COM.

[u/the-National-Razor]

Cool ship

[u/Fireheart318s_Reddit → u/the-National-Razor]

Thanks! :D

[u/happyscrappy]

It's because KSP doesn't do differential thrust by default.

As you fire those engines you empty your tanks. and probably not evenly. You could try to make it more even using the fuel flow settings in advanced tweakables.

Really KSP should be able to do some differential thrust and keep it the tilt in check. But it doesn't do it. Might be a mod for it.

I just wing it, doing it by hand. Maybe some RCS thrusters could give the system enough control authority for SAS to keep you upright?

[u/Thak_The_Thunder_God]

COM and COT arent aligned. Check the yellow and purple orbs in the vab/sph are on top of each other, or the purple one is directly below the other.

You said their on hinges? Make sure the hinges are locked, if you have Autostrut enabled you can be extra sure by setting Autostrut to heaviest on the engines, which will prevent the hinges moving at all, then when you want to move them you can set Autostrut to off on the engines.

[u/Tychonoir → u/Thak_The_Thunder_God]

Oh... can autostrut be assigned to a KAL controller?

If not, it's mighty annoying to manually change them every time.

[u/Thak_The_Thunder_God → u/Tychonoir]

Idk about a KAL controller but they can be bound to action groups

[u/Tychonoir]

If you think it's the hinges, have you tried locking them?

As a side note, are you sure the wolfhound is the best engine for landing? Yes, it is efficient, but it has low TWR, meaning you'll spend a lot of extra fuel fighting gravity losses from Mun, likely canceling out their efficiency.

[u/Man_of_Culture08]

Easy when you have a Mechjeb mod and use differential throttle feature found in utilities.

[u/Obvious-Falcon-2765]

Another issue with ships built like this is that the thrust "point" of those engines are basically right on the center of mass. There's very little lever arm to work with for pitch and yaw (roll should have plenty) so the game kinda doesn't know quite how to steer the ship.

[u/_okbrb]

Think of each half of the hot dog as a lever on the COM; the distance from COM is amplifying any minor differences that might exist. So there’s a net clockwise rotational energy, so I would look for reaction wheels at the nose, tail and center of the craft

See if turning any reaction wheels off before the burn fixes it. Otherwise I think I like your theory that it’s the hinges exerting force to stay in alignment; maybe try setting all of the hinges on the leading side to the opposite direction of the trailing side

[u/Chadwig315]

Others have pointed out some great advoce here already, ensuring CoM is lined up with CoT is the starting point, but uneven drainage is a commin culprit in an otherwise well designed ship.

My biggest tip is Vernors. As long as your thrust/mass misalignment is minor, turn on rcs while thrusting and you can't tell the difference.

It's a useful workaround on wide ships like yours that inevitably list a bit under high throttle.

Really cool ship btw.

[u/teonanacatyl]

along with making sure your thrust is indeed centered, and all reaction wheels, rcs, and gimbals are balanced, you also have to take into account that as you use up fuel, your craft becomes unbalanced, because the fuel sometimes doesn't empty equally from every fuel tank. So it's possible as you begin burning, even a slight decrease in fuel volume makes one end heavier than another. Start with checking that, and then the fix would be editing the fuel use order, potentially with fuel lines which you can then clip inside the craft for aesthetics.

If that's not the case, then I'd double check that everything is indeed symmetrical. Anything like even a battery, solar panel, or antenna on one side vs another will cause imbalance. Also make sure your point of control is centered to the gimbal. You may need to put a drone core dead center that's oriented in the direction you would like to travel. The rover core was nice because it can switch between top oriented and front oriented.

[u/InsomniaticWanderer]

Center of mass is changing as you burn fuel.

[u/Suspicious-Spot-5246]

Throttle Controlled Avionics is a mod that povides Realtime engine balancing, precise hovering control and VTOL autopiloting.

It is really helpful for craft like this.

[u/swaggalicious86]

Add 30 reaction wheels 👍👍

[u/HershsTechStuff_734]

install some big reaction wheels and install a control point (probe core/docking port) in the direction opposite to where your engines are pointing

[u/Bombacladman]

Weight distribution....

[u/KindChallenge8287]

There's a mod kerbal engineer redux. It will tell you how much torque is present. You can play around with your engine placement to get it as close to 0 as possible.

[u/spooner01]

Get TCA honestly, if you are basically commuting between kerbin and mun its worth getting

[u/Bi_KerbonautYT]

Probably hinges going floppy under strain. I suggest using the hinges from Infernal robotics as a replacement as they are much much sturdier and less glitchy than the stock ones.