Unstable Heavy Lift Vehicle

[shadragon]

I have an issue with my latest heavy lift rocket. Within 5 seconds after take off, the vehicle starts drifting to the left or right and my command inputs do little. I've checked COG and that looks good. I've put on the largest fins I have, but nothing I do lets me get it past 10,000M without tumbling wildly. 

I'm using the middle sized reaction wheel (I don't have the big one yet in the tree), SAS is on and I'm full throttle on the liquid fuel rockets. Any suggs what I should check?  

[Waseemq1235]

Does it want to flip and face downwards? If so, then try lowering the center of mass down, because having the center of mass far at the top causes it to want to flip.

Hope this helps.

[Harry Rhodan]

16 minutes ago, shadragon said:

Any suggs what I should check?  

You should check to add a screenshot if you wan't a detailed explanation what went wrong.

14 minutes ago, Waseemq1235 said:

Does it want to flip and face downwards? If so, then try lowering the center of mass down, because having the center of mass far at the top causes it to want to flip.

No. Definitely not. A low CoM will be even less stable. Top heavy and bottom draggy is good, bottom heavy and top draggy bad.

[Waseemq1235]

 

4 minutes ago, Harry Rhodan said:

No. Definitely not. A low CoM will be even less stable. Top heavy and bottom draggy is good, bottom heavy and top draggy bad.

Huh, I should test that, but I am certain that a top heavy rocket will tend to flip, you can test that with a hammer, holding the hammer with 2 of your fingers will cause the hammer to flip

[DD_bwest]

its hard to tell what could be happening without a picture of the rocket.  any number of things could be causing it.   

[swjr-swis]

18 minutes ago, Waseemq1235 said:

Does it want to flip and face downwards? If so, then try lowering the center of mass down, because having the center of mass far at the top causes it to want to flip.

Actually it's exactly the other way around: try keep the heaviest part of the rocket as high up a possible (not easy because fuel drains from the top first), and the draggiest parts (fins) as far down as possible. This keeps the rocket pointing to space.

If you already checked the CoM indicator in the VAB, with full tanks and empty tanks (!), then you may be having an issue with a bug in the game that sometimes causes engines placed in radial symmetry to not all provide full thrust. Are you using one of the adapters that go from one to multiple attachment nodes? If yes, this may be your issue, and I don't know that there is a solution to it currently, other than not using those adapters.

A picture or craft file may allow people to help better.

[Rocket In My Pocket]

2 minutes ago, Waseemq1235 said:

Huh, I should test that, but I am certain that a top heavy rocket will tend to flip, you can test that with a hammer, holding the hammer with 2 of your fingers will cause the hammer to flip

Think of it more like an arrow flying towards it's target. Fins at the back, heavy piece of metal at the front.

Edited September 7, 2016 by Rocket In My Pocket

[Waseemq1235]

2 minutes ago, Rocket In My Pocket said:

Think of it more like an arrow flying towards it's target. Fins at the back, heavy piece of metal at the front.

Well, I gotta experiment with that, my apologies.

[DD_bwest]

6 minutes ago, Waseemq1235 said:

 

Huh, I should test that, but I am certain that a top heavy rocket will tend to flip, you can test that with a hammer, holding the hammer with 2 of your fingers will cause the hammer to flip

if you've got a large Pop bottle (soda)    fill it 1/3 of the way up and hold it upside down on 2 fingers.   Then fill it full and repeat.

[Rocket In My Pocket]

Just now, Waseemq1235 said:

Well, I gotta experiment with that, my apologies.

No need to apologize, its very unintuitive from a common sense stand point. Even real life rocket engineers struggled with this concept.

https://en.wikipedia.org/wiki/Pendulum_rocket_fallacy

[Waseemq1235]

Just now, Rocket In My Pocket said:

No need to apologize, its very unintuitive from a common sense stand point. Even real life rocket engineers struggled with this concept.

https://en.wikipedia.org/wiki/Pendulum_rocket_fallacy

Well, but, I wasn't trying to understand, I was actually trying to incorrectly answer a question. So... I do have to apologize..

[Aegolius13]

A screenshot will be the most helpful aid for diagnosis.  But here are a few ideas:

What is your TWR at launch?  If it's barely above 1, your rocket might have trouble fighting the pull of gravity on the nose.  If it's humongous, you could end up going too fast while still in thick atmosphere (though this is very unlikely to be the problem 5 seconds out).  

Which liquid fueled engine(s) are you using?  If it's the Reliant or Dart, those engines do not gimbal so your SAS will be pretty weak early on. 

Are you using SRBs?  They can do weird things even you if have a gimbaling LF engine to help.  

You say "liquid fueled rockets" (plural).  How are they set up?  Could your engines be off-center somehow?  You could try looking at torque with RCS Build Aid, or reattaching the engines.

If you have multiple fuel tanks in the first stage, try manually moving fuel to the forward tank to keep the center of mass in front.  But again, this should not cause flipping 5 seconds after launch.  

You could also try rebuilding the rocket from scratch in case there was  one-time bug. 

 

[steuben]

You might need more reaction wheels, maybe more control surfaces, and/or maybe more gimbal.

I'm going to list a couple of silly things to check:
- you're not running out of electric charge part way up
- if you push the craft into the turn it does respond

You probably already have checked those... but I've spent hours beating up a failure to start issues, in other areas of RL, that could have been resolved with a restart or making sure it is plugged in.

 

[shadragon]

Hi all, thanks for the quick answers. I'm at work so screenshots will have to wait til later

@steuben I did stack three Advanced Inline Stabilizers and saw no improvement. It happens right off the pad and I have a huge battery in the lander so I doubt that is electricity starvation, but I will check tonight. 

@aegolius It's a two-stage center (I think it is on a Twin Boar and three tanks) with lander / rover up top in a fairing. Second stage has single fuel tank and liquid fuel motor (Can't remember the name). On each side of the center are liquid fuel tanks and (I think) Skipper motors with fuel lines piping into the center tank.   

@swjr-swis I designed it as you stated. The exact design (One center two-stage with outlying tanks and motors) worked flawlessly when I used smaller rockets. The lander / rover is about 5 tons so it is heavy at the nose. I don't believe I'm using an adapter as you describe. Just the fairing around the lander.  

Will put together a craft file and screenshots tonight. 

[Aegolius13]

4 minutes ago, shadragon said:

with lander / rover up top in a fairing.

It's possible this has a center of mass that's slightly off center - especially if you're carrying the rover upright (i.e, front of the rover points straight up).  If you have RCS Build Aid, I'd suggest moving your payload around horizontally, and seeing if you can get the "engine torque" number down.  But I am skeptical this is a big enough issue to overpower the thrust vectoring on those big engines. 

If your fairing is very wide and/or blunt, that could be the problem.  Draggy, light things like to go the back of the rocket, and a big, mostly empty fairing can be both.  You could try to get rid of the fairing altogether, narrow it, or make it more pointy in front.  

Another concern might be floppiness in the payload area.  Have you tried MOAR STRUTS? 

[swjr-swis]

1 hour ago, shadragon said:

The lander / rover is about 5 tons so it is heavy at the nose.

That implies wheels and/or landing legs. Be advised that either of those can currently cause phantom forces, especially if there is any clipping, and there's not a whole lot one can do about it (dev notes tell us we can expect fixes in 1.2).

 

56 minutes ago, Aegolius13 said:

Another concern might be floppiness in the payload area.  Have you tried MOAR STRUTS?

So much truth, it deserves repeating. Payload in a fairing is not automatically secure. Try attaching a few cubic octagonal struts radially to the fairing base, offset them up along the inside edge of the fairing, then strut from those to the payload. Just keep in mind that the cubic struts attached this way will remain perfectly rigid in their position relative to the fairing base, so keep enough clearance for later separation.

[Aegolius13]

OK, one more idea.  Are you sure you're controlling the craft from an upward facing control point?  If the ship is trying to fly from a sideways or upside-down probe core on, say, the rover, SAS can do some very weird things.  This, along with a misaligned center of mass, are the two things that have caused my rockets to crash super-quickly at launch like you're describing.  

[DD_bwest]

1 hour ago, shadragon said:

Hi all, thanks for the quick answers. I'm at work so screenshots will have to wait til later

@steuben I did stack three Advanced Inline Stabilizers and saw no improvement. It happens right off the pad and I have a huge battery in the lander so I doubt that is electricity starvation, but I will check tonight. 

@aegolius It's a two-stage center (I think it is on a Twin Boar and three tanks) with lander / rover up top in a fairing. Second stage has single fuel tank and liquid fuel motor (Can't remember the name). On each side of the center are liquid fuel tanks and (I think) Skipper motors with fuel lines piping into the center tank.   

@swjr-swis I designed it as you stated. The exact design (One center two-stage with outlying tanks and motors) worked flawlessly when I used smaller rockets. The lander / rover is about 5 tons so it is heavy at the nose. I don't believe I'm using an adapter as you describe. Just the fairing around the lander.  

Will put together a craft file and screenshots tonight. 

as mentioned above, if your control part is upside down itll instantly try to right itself on launch. it probably does the same if its sideways.    you can tell if its doing this if your navball starts in the brown side, rather than the normal blue.

its a mistake ive made a few times lol

[Stoney3K]

25 minutes ago, Aegolius13 said:

OK, one more idea.  Are you sure you're controlling the craft from an upward facing control point?  If the ship is trying to fly from a sideways or upside-down probe core on, say, the rover, SAS can do some very weird things.  This, along with a misaligned center of mass, are the two things that have caused my rockets to crash super-quickly at launch like you're describing.  

Not just upwards-facing, but also in line with the rocket's center of thrust.

If your control point is offset from the craft's center of thrust it will keep trying to gimbal the engines to fly sideways if you want to follow a prograde vector. ('Stability Assist' mode shouldn't be affected, though.)

A lopsided payload mass is less important if you have a huge, massive booster stacked underneath it. The payload moving around in the fairing will cause a big mess quickly.

You can strut directly from a fairing base now so you don't need to mess around with cubic struts.  

[Plusck]

4 hours ago, shadragon said:

I'm using the middle sized reaction wheel (I don't have the big one yet in the tree), SAS is on and I'm full throttle on the liquid fuel rockets. Any suggs what I should check?  

That's actually a good thing, often. The large reaction wheel is very powerful, perhaps a touch too powerful for its own joints, so it does tend to weaken the rocket wherever you place it. I find the medium ones far less of a problem.

Stacking them, however, adds a joint for each one in the stack, which can cause problems.

 

However, I'm guessing that the real problem is you're trying to "control from" something in the fairing payload.

As others have said, strutting is really necessary inside fairings.

I would add that even though it may be a waste of cash, you should probably add a probe core to the lifter section and "control from" that part until you get into orbit. That means the probe core is solidly attached to the engines and control surfaces. The payload can wobble around a touch and it won't affect it, and it won't try to overcorrect for that wobble making the rocket shake apart at its natural frequency.

[shadragon]

Here's the craft file. Don't laugh too hard. I made a lot of compromises in this design. 

https://kerbalx.com/shadragon/Mun-or-Bust

[DD_bwest]

25 minutes ago, shadragon said:

Here's the craft file. Don't laugh too hard. I made a lot of compromises in this design. 

https://kerbalx.com/shadragon/Mun-or-Bust

is that top probe core your control point?   cause its upside down.   If your controlling from it, your rocket will instantly try to flip itself and crash lol

so it would be your problem if it is

edit: also,  is your lander also upside down?  cause your landing gear is facing up so it partly emplies that it is.  so if thats also a controlling point and also upside down, it would also cause the same problem

edit 2:   I loaded up ksp cause i forgot if the blue notch on the can was up or down. (brain fart)  your lander can is upside down,  as well as that probe core.  so its the fact that your controlling it from one of those 2 upside down that is causing the problem.

Edited September 7, 2016 by DD_bwest

[Aegolius13]

Yup, I tried flying it and the navball was a field of brown - the active control point is upside down. My suggestion: flip the HECS to face upright and use that as your initial control point.  Once your lander deploys and you want it to face the right way, select the lander can as the control point.  Other than navball orientation and SAS direction, it should make no difference for any other functionality.    

You can always stick another probe core somewhere along your stack, though floppiness may ensue unless you put it in a cargo bay.  You could also try just using the basic SAS instead of "hold prograde".  Or, I suppose, you could use "hold retrograde," I guess?  Though the very prospect terrifies me.  

Edited September 7, 2016 by Aegolius13
Clarified flipping the HECS

[DD_bwest]

the easiest is probably just to add a tiny probe core to the top.  its weight would be essentially nothing, so wouldnt change much about how the original design would have flown.  

or if your lander/rover are attached via a docking port, you can select one facing up and control from it.

[Aegolius13]

Actually yeah, subbing in a docking port instead of the decoupler inside your fairing might the easiest way to add an up-facing control point.  You can right click it in the VAB to enable staged release, like a decoupler, or just right click it on the ship when you want to release.