Do a barrel roll!

[zeppelinmage]

...okay, more like a 360-degree pitch-over.

It seems whenever I build a heavy rocket, especially ones that require asparagus staging, I have problems with my gravity turn. Namely, stopping it. I'll get to 10km, then begin the pitch. But I try to slow/stop it, but the rocket keeps going. I usually end up killing my engines and just let it completely turn over. Then I can stop it at my desired pitch angle and restart my engines. Generally I can completely recover the flight, but it's a bit of a pain.

Does this happen to y'all? And what do you usually do to remedy it?

I have reaction wheels in the rocket - a large-diameter one roughly in the center. My engines are Mainsails, and my payload has RCS.

Edited January 6, 2014 by zeppelinmage

[Depth]

A few extra control surfaces near the bottom of the rocket is usually a good remedy, but I have experienced scrapping a design because I was unable to rectify it

I usually end up killing my engines and just let it completely turn over. Then I can stop it at my desired pitch angle and restart my engines. Generally I can completely recover the flight

I would LOVE to see that in real life

[Vanamonde]

What does the ship look like? Also, moving this to gameplay questions, since it's about ship designing advice.

[zeppelinmage]

I would LOVE to see that in real life

That would scare the @#!$ out of me in real life!

[zeppelinmage]

What does the ship look like? Also, moving this to gameplay questions, since it's about ship designing advice.

Thanks, wasn't sure exactly where to put it.

I've had it paused while in flight while I bounce back and forth, so here's an action pic:

The payload is two habitation modules for my station, those have the RCS. This is after popping my first asparagus stage, so there's an additional asparagus layer to go. This is what it looked like at the gravity turn.

[zeppelinmage]

I'll take a VAB shot of it after I complete the mission, if it would help.

[Vanamonde]

That boom up front is quite a bit of mass to start swinging and then stop it where you want it to be. Try putting some control surfaces on it, for one thing. And/or, make your turns in smaller steps so that it doesn't build up as much momentum.

[zeppelinmage]

It's going on my station, so I want to avoid any extraneous control surfaces. I'll try smaller steps for sure though.

[zeppelinmage]

Okay, that didn't work. I tried feathering the controls, with control surfaces on the lower stage... and nope. Still spins out of control at the slightest movement.

[AletzN1]

You should build the bottom stages around the grey tanks in the middle.

And put some more tanks in them, i think you are having too much weight with so many engines that dont even last that long.

Oh and 2.5 Reaction wheels. Lots of 'em

[zeppelinmage]

The grey stage is my "orbital" stage. There's an orange tank below it.

I found a solution - I switched out the mainsails for 4x LV-T45s. I think the mainsail gimbals were overloading or something. The LVs seem to work better. (Maybe because there's literally four times the number?)

That said, I would like to know how you all get these heavy lifters to work with mainsails.

Edited January 7, 2014 by Specialist290
Merging sequential posts

[Vanamonde]

Control surfaces toward the bottom will not have the leverage to hold that heavy nose up. If you don't want to *keep* control surfaces there, you could attach them to decouplers and eject them when they're no longer needed.

[Kasuha]

Control surfaces toward the bottom will not have the leverage to hold that heavy nose up. If you don't want to *keep* control surfaces there, you could attach them to decouplers and eject them when they're no longer needed.

Pendulum fallacy. Weight of the nose is irrelevant, what is relevant is drag and balance. If the ship's nose has higher drag than bottom, it will have tendency to turn over in atmosphere. Cupola module's drag is 0.4. And that's the problem.

The solution is to perform correct gravity turn, i.e. burn towards surface prograde (within the circle, i.e. +/- 5 degrees) until you get out of the thick atmosphere, i.e. somewhere around 30 km. It definitely means no sharp turns. And it may mean you need to time the maneuver right.

Control surfaces of course help on either end. I'd put them on boosters.

[zeppelinmage]

Pendulum fallacy. Weight of the nose is irrelevant, what is relevant is drag and balance. If the ship's nose has higher drag than bottom, it will have tendency to turn over in atmosphere. Cupola module's drag is 0.4. And that's the problem.

The solution is to perform correct gravity turn, i.e. burn towards surface prograde (within the circle, i.e. +/- 5 degrees) until you get out of the thick atmosphere, i.e. somewhere around 30 km. It definitely means no sharp turns. And it may mean you need to time the maneuver right.

Control surfaces of course help on either end. I'd put them on boosters.

Aha, it was the cupola's fault. If I was paying attention to the drag coefficients, I would have mounted the modules in reverse.

I'll definitely focus my burn towards prograde in my next launches. (I still have three more to do to complete my station.) Thanks!

[Vanamonde]

Pendulum fallacy.

I'm not talking about general design principles, but this specific situation. I believe the problem OP is having is that when he starts a rotation, that big mass upfront has so much momentum that it just keeps swinging, dragging the rest of the ship with it. In which case, since most of his steering authority is already at the back in the form of the vectoring engines, adding more control there is unlikely to fix it. Applying control surface leverage right to the problem area, though, seems to stand a better chance of keeping that momentum from getting out of hand in the first place.

[Kasuha]

I believe the problem OP is having is that when he starts a rotation, that big mass upfront has so much momentum that it just keeps swinging

But it has to get that momentum somewhere in the first place. And without external forces, stopping the rotation would take exactly the same time as getting it to rotate, not the full turn.

[BlackBicycle]

Seing that others are already engaged in a rather serious discussion, how about a bit of light-hearted break for all of you?

Go Google. In the search string, type "do a barrel roll". Without quotes.

: D

Edited January 7, 2014 by BlackBicycle

[Vanamonde]

Once the thing starts rotating, it isn't a simple matter of cancelling out the exact force that was used to start it rotating. The ship is in atmosphere, under gravity, thrusting along a certain axis which is itself now rotating, with the SAS flipping control surfaces and spinning reaction wheels. That means the weight is shifting and may no longer be over the center of thrust, air flow is exerting pressure from different directions, etc. The degree of control authority required to keep the thing flying straight is much less than the force that may need to be applied to stop a tumble once its begun, and the ship is simply exceeding its ability to hold attitude. OP, just *try* sticking some fairly large fins with control surfaces on that upper assembly and see whether it helps or not. I'm betting it will.

[Kasuha]

The ship is in atmosphere

In KSP physics, atmosphere only affects ship rotation if parts have different drag coefficient or lift. There are no parts with lift in the discussed design.

under gravity

No. The ship is effectively in zero G.

thrusting along a certain axis which is itself now rotating

Thrusting towards the ship's forward direction with only deviation being driven by controlled engine gimbal.

with the SAS flipping control surfaces and spinning reaction wheels

I don't see any control surfaces on that ship. And all SAS actions are towards stopping the rotation.

The ONLY important effects here are eventual mass disbalance in direction of the ship's thrust, and drag disbalance in direction of ship's movement through atmosphere. And since the ship is very likely mass-balanced, drag is the only thing that remains.

All the rest of effects you mentioned fall under pendulum fallacy.

[BigFatStupidHead]

Additionally, you mentioned a reaction wheel located in the centre of your ship? Reaction wheels work better with more leverage, so get them as far from your centre of mass as you can.

[Vanamonde]

The ONLY important effects here are eventual mass disbalance in direction of the ship's thrust

Fine. It won't work. It can't work. Ships accelerating in atmosphere near a large world should behave exactly like ships in vacuum and zero G, and engine gimbal still works just dandy even after the ship has rotated beyond the engine's gimbal arc, and SAS *never* over-corrects or gets confused about the direction it should be torqueing even while the ship is turning 360s.

OP will still lose nothing by trying it, and I'm still betting it will help, if not fix the problem.

[Markus Reese]

It is less a roll, but more of a capsizing. The main problem is that you have a mass far above COG as well. Without drawing force vector diagrams, you have an upwards inertial momentum, and a downward gravity force, then applied a rotation. Throw in the drag of the air, and it flips. Remedy is quite easy.

1. Have some fins. They do wonders in your low atmosphere.

2. Secondary high thrust gimbal engines. LV-45s are what I usually use but try making your inner engines skippers instead of mainsails. I think those should act to the same effect. Sides, if your turn is right, you don't need the heavy and high thrust mainsails during final staged. I usually am running off just LV 45 or skippers once I reach 45km

[Kasuha]

Ships accelerating in atmosphere near a large world should behave exactly like ships in vacuum and zero G,

Speaking about angular momentum, KSP physics and ships built of parts with the same drag coefficient, yes. That's exactly the case.

and engine gimbal still works just dandy even after the ship has rotated beyond the engine's gimbal arc

Exactly.

and SAS *never* over-corrects or gets confused about the direction it should be torqueing even while the ship is turning 360s.

I still have to see it correcting in the wrong direction. So yes. Besides, when going for orbit you usually override SAS output by manual steering, especially when things don't go the way they should. That makes SAS kinda irrelevant.

OP will still lose nothing by trying it, and I'm still betting it will help, if not fix the problem.

Of course that making random changes or changes that "worked before" may lead to a working solution. Life on earth is result of random changes and attempts at what will work and what won't done by Nature itself. But understanding what's wrong and dealing just with that usually costs less time and effort.