How to prevent rocket tipping over after staging

[KhanFoil]

Screenshot

This is my rocket. I am early in the game and have built the best rocket I can to do science and try to hit orbit.

The problem is my rocket tips over after discarding the second SRB. Here's the parts list:

1. MK16 Parachute
2. MK1 Command Pod
3. SC-9001 Science, Jr
4. Experiment Storage Unit (Couldn't figure out how to open it, so I stuck thermometer, goo pod, and barometer on the outside. Also stuck two sets of batteries on it to power the SC-9001
5. Heat Shield 1.25 MM
6. TD-12 Decoupler (All Decouplers are this one)
7. FL-200 Fuel Tank
8. LV-T30 LRB
9. Decoupler
10. BACC "Thumper" SRB
11. Decoupler
12. RT-10 "Hammer" SRB

The Fuel Tank and the BACC Thumper both have four sets of Basic Fins on them.

First stage burns fine. I begin angling the rocket at 70 degrees. Separation and second stage also burns fine. Maintain attitude. When separating the second SRB, the rocket begins to tumble forward. Points down and can't get it to right itself.

I read about heavy-drag and moved the fins around some. It changes the behavior a bit, but doesn't fix the problem.

Ideas?

 

[amarillo114]

Try to attach the Srb boosters radially (I feel that your rocket is way too large) maybe it helps in some way (although i'm quite new at KSP). If you want to store your science tools you need to use the service bay. The experiment container unit can store the results of the experiments but not the tools. You can put the experiment container inside the service bay. Don't forget to add antennas and bateries for being able to transmit the experiment results from orbit to kerbin (or you will get the gathered science if you land with the experiment container still attached to the pod) Hope that helps! 

 

 

* noob advice,  surely there are better explanations and suggestions! 

[OHara]

20 minutes ago, KhanFoil said:

• SC-9001 Science, Jr
• Experiment Storage Unit (Couldn't figure out how to open it, so I stuck thermometer, goo pod, and barometer on the outside. Also stuck two sets of batteries on it to power the SC-9001.)
• Heat Shield 1.25 MM

The different sizes of these 3 parts along the stack leaves flat surfaces that are caught by the airflow.  (KSP doesn't simulate airflow in detail, but does estimate drag forces on the exposed surfaces of each part.) That drag force is about as strong to the forces on your fins, and has a bigger lever arm to flip the craft around.

As amarillo says, the 'Experiment Storage Unit' stores information, not physical parts.

If you have some kind of cargo bay available, you can put the Experiment Storage Unit inside a bay.  There is a 1.25-m 'Service Bay' that is the right size to give smooth airflow.

Alternatively, you could put the Experiment Storage Unit between the pod and parachute.

[KhanFoil]

New Screenshot

Thanks for the feedback.

It took two Storage Bays to hold the Experiment Storage Unit and wrap it completely to create no surfaces.

Launched. No difference.

Since the problem happens after all of the SRBs are expended and ejected, I don't think clustering them will help.

Still mystified.

Edited December 28, 2019 by KhanFoil
Add screenshot

[HvP]

I have a few observations.

The LV-T30 liquid fueled engine has no gimbal control, and the fins you are using are also not controllable. This means that your only source of directional control is the reaction wheel inside your capsule which could be very weak compared to the aerodynamic forces acting on your craft. A better choice, if you have access to it, is the LV-T45 engine. It has the same aspect and slightly lower thrust, but it does have a gimbal to allow the engine to pivot and reorient the craft in flight. You can probably even get away with using a LV-909 "Terrier". It's much lighter and more fuel efficient, but doesn't have nearly as much thrust, which may be just fine as a final stage.

I'm also guessing that, unless you have turned down the throttle on your second stage, that you are going VERY fast when you switch to your upper stage. The faster you ram through the air, the more it amplifies any small deviation from forward and tends to turn you around. If you look at your g-force meter on the right side of your navball, or the TWR (thrust-to-weight ratio) when you expand the stage column on the right, you really don't want to have a TWR/acceleration much higher than about 2 to 2.5 gees at most during your ascent.

Also, make sure that your Science Jr. materials bay is connected directly to the top of that service bay. In your new screenshot it looks like the bottom of the Science Jr. is clipping through the top of the service bay and stuck to the top of something inside it. This can trick the game into creating more drag than there should be because it doesn't see a smooth connection between those parts.

(P.S. - I also noticed that you're getting the bug that creates multiple copies of the delta-v icon. As far as I'm aware, this is happening on console when you switch engines in the VAB and it adds another delta-v app to recalculate the numbers for the new engine. Until there is a fix for this I think the best way to prevent this is to try and limit how often you replace engines while building in the editor. At least, until you exit and reenter.)

Edited December 28, 2019 by HvP

[Vanamonde]

HvP has it. Neither the engines nor the fins you are using help with steering. Early in a career, though, that's just something you have to work around because you don't have alternatives. You could mount the fins with a slight twist to impart rotation and gryoscopically stabilize the craft until it passes through most of the atmosphere, but it's inelegant and unreliable. Mostly, I just fly small ships and short missions until I get better parts. 

[VoidSquid]

Not much to add, @HvP and @Vanamonde already covered the main issues. What I see in addition: the fins at the upper stage are not helping at all, just the contrary, fins should be always placed as low as possible. Weight distribution: always try to have the center of mass as much away as possible from the center of thrust. If you have the parts available, best to use a SRB plus fins only for the first stage, next stage use a Swivel engine as that one has gimbaling (better: Reliant plus active fins, but you said those aren't available yet). I don't know if the functionality is available for consoles, but on PC, you can use F12 to show the aerodynamic forces, very useful to analyze why a rocket flips. Finally, I recommend you watch this video here:

 

[HvP]

Also, if you don't have access to any of the other engines and all else fails sometimes the simplest solutions are the best.

You might just wait several seconds after the second SRB burns out before you separate it to ignite your final stage. If you're moving too fast through the air it allows you time to get into the thinner layer of atmosphere and/or slow down a little before going to the next stage.

Just because the rocket flips in dense atmosphere doesn't mean it will still flip in more rarefied air. Depending on your ascent trajectory you might not even have to ignite the final stage until you have left the atmosphere. Leaving the SRB attached gives you the advantage of keeping those fins on the back to stabilize your craft until you are high enough that the reaction wheel in your capsule no longer has aero-shock to fight against.

Now this technique depends entirely on the steepness of your ascent, whether or not the second stage booster gave you enough momentum to get above the atmosphere, and how much speed you can afford to lose before igniting that last stage. It won't be particularly efficient, but it could possibly save the mission.

[OHara]

8 hours ago, KhanFoil said:

It took two Storage Bays to hold the Experiment Storage Unit and wrap it completely to create no surfaces.

Launched. No difference.

Aw, shucks. 

I copied your rocket from your screenshots.  The one with the exposed ExperimentStorageUnit also flipped for me when decoupling the last SRB, but then when I put it in the ServiceBays it stayed nicely stable for me.  The way you put the fins for passive stability worked fine for me.   

My rocket looks a little different, though, in exactly the aspect that HvP pointed out.

6 hours ago, HvP said:

Also, make sure that your Science Jr. materials bay is connected directly to the top of that service bay. In your new screenshot it looks like the bottom of the Science Jr. is clipping through the top of the service bay and stuck to the top of something inside it.

KSP looks at what part is connected to what other part, in the very literal computer-like way, and can think you meant to have two exposed flat ends of rocket sections really close to each other. 

(Also, KSP doesn't always notice that parts too large for one cargo bay, are properly contained in two, but that worked fine in my build of your rocket.)

On the way down, it was difficult to keep it going heat-shield first, and there I needed Vanamonde's spin-stabilization idea.  An alternative is to put the ExperimentStorageUnit on top of the capsule, so the heavy capsule is closer to the heat-shield end.

[RabarberMannen]

if it tips in the atmosphere it helps with fins. the fins  should be as low down on the  stage as possible. if you    press the three symbol letters symmetry button it comes up three balls. the blue should be under the yellow and the   purple should be straight under the yellow and point its   arrow  down. if this doesn't help then you need to rethink your rocket. or ad more reactionwheels 

[Zamolxes77]

To get into orbit in the beginning, all you need is capsule, parachutes, 14 small fuel tanks and a swivel engine. That's it ! No staging, no losses, you can recover everything !

Watch this: 

Works in 1.8.1.

Edited December 31, 2019 by Zamolxes77