Ship becomes unstable

[ashenfang]

I have a ship built using two liquid fueled upper stages and a long burn solid booster first stage. Ship flies fine until hitting about 4-500 m/s @ 10k meters altitude when all of the sudden it veers off course. I am using stability assist. What is causing this? 

[eddiew]

Too many variables. Pics or can't help.

(Most likely your centre of lift is in front of your centre of mass. Do you have fins at the bottom and weight the top? What about after staging?)

[ashenfang]

No fins. But weight at the top.

[FrostedShoe]

Add 8 fins to the bottom and try again, if it works or veers slowly then you have too much drag at the front and too much thrust at the back (extreme kerbal mullet). You could also limit the trust of the first stage in the VAB to keep your speed down or go liquid fuel on all stages and reduce the throttle. 

[Urses]

You hit the typical behaivor of a "Mushroom" design in supersonic range.

You have some ways to come around it.

Kerbal way slap many fins on the truster end of the rocket.

The "normal" way streamline the front. Add some fins backwards and go sure for fuel flow to use fuel from bottom to the top. I can't explain the math, but explained as a i see it. In thinner atmo at highspeeds you need higher drag to provide stability, think of a dart arrow. This is best provided by lifting Surfaces like cannards or tail fins as they give you steering abilitys in lower atmo layers where the mass is to high for reaction wheels alone. The second aspect is how your rocket is build. A chubby one needs way more draggy equip backwards as a slic one. My personal rule of thumb is as long i need thrust below 30km i need fins on the thrusting stage.

[Harry Rhodan]

11 hours ago, ashenfang said:

No fins. But weight at the top.

Screenshots.

[Snark]

17 hours ago, ashenfang said:

What is causing this?

As everyone's been suggesting:  screenshots please.  By just saying "my ship is unstable, what's wrong?", there's really nothing we can help with.

It's kind of like walking into an auto mechanic's shop-- without your car-- and saying to them, "My car's not working, what should I do?"  They're not going to be able to give you any helpful answers without looking at your car. 

If you've never posted screenshots in the forum before:  it's pretty straightforward.  Instructions in spoiler.

Spoiler

1. Take a screenshot in-game by pressing F1.  It gets saved as a .PNG file in the "Screenshots" folder of your KSP install.
2. Go to imgur.com.  It's simple, free, and doesn't require setting up an account.  (Or you could use another image-hosting site if you like. I just suggest imgur because it's really easy and is what most people here use.)
3. Click the green "create new post" button, then when it pops up the "drop files here" page, just drag your screenshot into the relevant box and wait a few seconds while it makes your post.
4. Now you're looking at the image on an imgur.com page.  Right-click on the image and choose "Copy Image Location".
5. Now go to your forum post and paste that URL.  Presto!  It will get automagically converted into an in-line image.
   

In general, with any instability problem you will fix it with some combination of the following:

• CoM as far forward as possible.
• Fins on the back, as far behind the CoM as possible.
• Aerodynamic front end.
• Stick to during ascent.

...but specific advice will need to wait until you can give us some screenshots.

 

17 hours ago, ashenfang said:

Ship flies fine until hitting about 4-500 m/s @ 10k meters altitude when all of the sudden it veers off course.

If it seems to start okay and then flips, there are basically two possibilities for what's going on:

• Possibility 1:  Your ship is fundamentally aerodynamically unstable, even on the launch pad, and your stabilization mechanism doesn't scale with aero forces (e.g. you're trying to rely on reaction wheels and/or engine gimbal for control).  In that case, it could be that you do fine in the early, slow part of flight (because your control mechanisms are stronger than the aero instability that wants to flip you), but when you get going really fast, the aero forces grow so big that they overwhelm your control.
• Possibility 2:  Your ship is actually aerodynamically unstable on the pad, but as you burn fuel, you shift the CoM lower and become unstable.

I'm guessing that #1 is probably more likely than #2, mainly because usually ships' CoM moves up rather than down as they burn fuel, so #2 seems somewhat unlikely.  I also note that you comment that you don't have fins, which also makes #1 sound more likely.

But, again-- a screenshot would settle both matters. 

[ashenfang]

Excellent suggestions, the help is appreciated! It appears the problem was shifting center of mass. As someone said above, as the SRB consumed fuel, the ship became top heavy with too much thrust "whipping" the front end. I corrected this by testing the lower stage using 2 smaller SRB's. They produced less thrust and I could dump the weight quicker keep the center of mass central. 

[Plusck]

3 hours ago, ashenfang said:

Excellent suggestions, the help is appreciated! It appears the problem was shifting center of mass. As someone said above, as the SRB consumed fuel, the ship became top heavy with too much thrust "whipping" the front end. I corrected this by testing the lower stage using 2 smaller SRB's. They produced less thrust and I could dump the weight quicker keep the center of mass central. 

Actually, "top heavy" is exactly what you want on a rocket. Aero forces on the back of the rocket tend to push it back towards prograde, while forces at the front tend to push it away. With the mass centred at the top, there's a long lever arm for those "good" forces and a short lever arm for the "bad" ones.

However, if there is any sort of "whipping" effect as you say, it surely means that your thrust isn't perfectly centred. If your thrust is off-centre and you don't have gimbal, then yes that long lever arm starts becoming a problem.