How do I make a rocket that won't flip?

[Windspren]

So, every time I launch a rocket, it flips at 5 km and there's no way to regain control. So, what makes a rocket flip? This happenes with every rocket I build. I've tried adding control surfaces and moar boosters, but nothing works.

My rockets typically consist of a heavy first stage with some boosters, and the payload with a stage for orbiting. I've been using the first stage for circulation. My center of mass is usually in the center of the rocket.

All help is appreciated!

Edited August 3, 2015 by Laythe Dweller

[Harry Rhodan]

My rockets typically consist of a heavy first stage with some boosters

Bottom heavy with too much thrust. This thread has a few nice pictures explaining the basics quite well.

[InterCity]

Put some tail fins on the bottom of your rocket. Make sure your CoM is well above your CoL and, most importantly, start your gravity turn early, probably as low as 500 m AGL. If you follow your prograde during ascent, you should have no problems.

[Red Iron Crown]

If you can make the rocket with a single tank for the core stage that helps, too. CoM doesn't move downward as much as it drains compared to a multitank arrangement. I've also sometimes disabled an upper tank and enabled it manually after the lower one is emptied.

[MaxwellsDemon]

This is something I had trouble with-- this and my rocket "flexing" and often breaking up.

There doesn't seem to be one magic formula, but a few things I've observed so far:

1. The brawniest booster isn't necessarily the best booster. I found I was trying to muscle my way into orbit too quickly, where a less-powerful engine burning longer worked better.

2. Strategically-placed struts can help make your stack more rigid if you tend to build tall like I do.

3. Tail fins only really help in the lower atmosphere, but try larger ones if you think it might be a control issue.

4. Your mileage may vary, but I've had the most success with a "core" sustainer stage surrounded by solid boosters that (ideally) are expended right about the time I get to 300 m/s. Changing the number and size of solid boosters to aim at that mark seems to work for me.

[Streetwind]

4. Your mileage may vary, but I've had the most success with a "core" sustainer stage surrounded by solid boosters that (ideally) are expended right about the time I get to 300 m/s. Changing the number and size of solid boosters to aim at that mark seems to work for me.

It's a double-edged sword. You probably found that that is the most stable configuration, but it is so because it's inefficient.

Between roughly 325 and 400 m/s is the transsonic region, where you suddenly run into a "wall" of sharply increased drag (the literal "sound barrier"). When you push through past 400 m/s, it lets you go again and your drag is suddenly lower than ever. If your solid boosters cut out at 300 m/s, though, that means you just transitioned from a high TWR to a low TWR situation, and then you get wave drag on top of that. You'll spend a long time fighting against it because you just can't accelerate. If the launch boosters had carried you all the way to 400 m/s or more, they would have released your lower-TWR center stage into a smoothed-out supersonic airstream with little drag acting on it, so even with low TWR it will accelerate well.

Of course, pushing hard through the transsonic region not only saves fuel, but also makes the wave drag short and brutal. During IRL rocket launches, this is often called out as "max-Q", meaning "maximum dynamic pressure" - the point during the flight at which the aerodynamic forces acting on the rocket are the strongest. The harder you push, the higher your peak dynamic pressure gets. And if the rocket is not well stabilized, this is the point where it likes to flip the most.

Ideally you want to build a rocket that can push hard through the sound barrier without flipping. That's sometimes difficult, but in return, that's how you learn and build player skill

[Windspren]

Bottom heavy with too much thrust. This thread has a few nice pictures explaining the basics quite well.

Actually, my center of mass is near the top when the first stage is empty.

Put some tail fins on the bottom of your rocket. Make sure your CoM is well above your CoL and, most importantly, start your gravity turn early, probably as low as 500 m AGL. If you follow your prograde during ascent, you should have no problems.

I spammed fins and it actually worked! Thanks!

If you can make the rocket with a single tank for the core stage that helps, too. CoM doesn't move downward as much as it drains compared to a multitank arrangement. I've also sometimes disabled an upper tank and enabled it manually after the lower one is emptied.

I've been using a single orange tank, with another for heavier payloads

It's a double-edged sword. You probably found that that is the most stable configuration, but it is so because it's inefficient.

Between roughly 325 and 400 m/s is the transsonic region, where you suddenly run into a "wall" of sharply increased drag (the literal "sound barrier"). When you push through past 400 m/s, it lets you go again and your drag is suddenly lower than ever. If your solid boosters cut out at 300 m/s, though, that means you just transitioned from a high TWR to a low TWR situation, and then you get wave drag on top of that. You'll spend a long time fighting against it because you just can't accelerate. If the launch boosters had carried you all the way to 400 m/s or more, they would have released your lower-TWR center stage into a smoothed-out supersonic airstream with little drag acting on it, so even with low TWR it will accelerate well.

Of course, pushing hard through the transsonic region not only saves fuel, but also makes the wave drag short and brutal. During IRL rocket launches, this is often called out as "max-Q", meaning "maximum dynamic pressure" - the point during the flight at which the aerodynamic forces acting on the rocket are the strongest. The harder you push, the higher your peak dynamic pressure gets. And if the rocket is not well stabilized, this is the point where it likes to flip the most.

Ideally you want to build a rocket that can push hard through the sound barrier without flipping. That's sometimes difficult, but in return, that's how you learn and build player skill

The rocket does flip at ~300 m/s, and I always wondered why. Thanks!

So, by spamming fins and struts the problem has been solved! Thanks everyone!

[MaxwellsDemon]

Of course, pushing hard through the transsonic region not only saves fuel, but also makes the wave drag short and brutal. During IRL rocket launches, this is often called out as "max-Q", meaning "maximum dynamic pressure" - the point during the flight at which the aerodynamic forces acting on the rocket are the strongest. The harder you push, the higher your peak dynamic pressure gets. And if the rocket is not well stabilized, this is the point where it likes to flip the most.

Ideally you want to build a rocket that can push hard through the sound barrier without flipping. That's sometimes difficult, but in return, that's how you learn and build player skill

Actually, max Q is precisely why I prefer to lose my non-throttleable solid boosters at that point. I may be doing the wrong thing, of course, but it's not completely without cause.

[Laie]

[Sharpy]

"Some tail fins" is a fine advice.

SOME Tail Fins is a better advice.

Take example from Warzou's Cygnus. No flimsy little delta-deluxe or toy AV-R8. He just sticks full-sized delta wings on his rockets and they are very stable even with quite unbalanced payloads.

[Pecan]

...No flimsy little delta-deluxe or toy AV-R8...

Still using basic fins most of the time here - I hate structural mass ^^.

[GeneralVeers]

So, every time I launch a rocket, it flips at 5 km and there's no way to regain control. So, what makes a rocket flip?

Probably aerodynamic effects. You can verify with this test: next time a rocket flips, shut off the engines, turn off SAS, and just let the thing sail. It will likely do a 180 and then stabilize--flying backwards. Then, after it passes apoapsis, it will likely do another 180 and fly backwards as it's going DOWN. If it does that, it's definitely aerodynamic; your rocket is draggy in the front and wants the part with the most drag (usually the payload) to be in the back.

There's a few ways to fix this. Cover the payload in a fairing. Enough counterforce (thrusters or SAS) to beat the drag. Or put enough drag in the back of the vehicle to make IT the draggy part.

And my personal favorite--flip the payload and launch it upside down. Then it can't flip because it's already flipped! No, really. Not joking. Every time I try this, it works. Though the resulting rockets look like something out of a 70's Theodore Sturgeon sci-fi novel.....

Or.....if you hate aerodynamic shenanigans and just want to play the game: hit ALT-F12, go to the Physics/Drag tab, and uncheck "Apply Drag".

Edited August 5, 2015 by WedgeAntilles