flipping rocket

[Joe Kerbal]

newbie question! 

dealing with some contracts that demand for puttin in orbit pretty heavy stuff I always stumble into a probem of this very big rockets I make that I can't control. Basically the problem is that as soon as the direction marker moves from zenith it just point down and the rocket start twisting and flipping. I think the problem is mainly the center of mass of the rocket but I could not figure out how to solve it. I've also made complicated structure to move down the heavy parts but that didn't work. What am I missing? is there a technique of missile buildings or some very basic strategy I'm missing here?

[Regor]

50 minutes ago, Joe Kerbal said:

newbie question! 

dealing with some contracts that demand for puttin in orbit pretty heavy stuff I always stumble into a probem of this very big rockets I make that I can't control. Basically the problem is that as soon as the direction marker moves from zenith it just point down and the rocket start twisting and flipping. I think the problem is mainly the center of mass of the rocket but I could not figure out how to solve it. I've also made complicated structure to move down the heavy parts but that didn't work. What am I missing? is there a technique of missile buildings or some very basic strategy I'm missing here?

try adding fins at the bottom for passive stability or gimballing for active control. You can do bath if you want.

[Superfluous J]

54 minutes ago, Joe Kerbal said:

I've also made complicated structure to move down the heavy parts but that didn't work. What am I missing?

You're going the wrong way.

You want more massive stiff in FRONT. At the top.

Think about arrows and darts. The heavy, metallic part is at the front, and the lighter, more draggy part is at the back.

Edited June 3, 2020 by Superfluous J

[Joe Kerbal]

5 minutes ago, Superfluous J said:

You want more massive stiff in FRONT. At the top.

I'll try it. You mean boosters too? I mean if you have a big rocket you have to put on orbit would you add boosters close to the tip of it and not at the bottom?

[Superfluous J]

3 minutes ago, Joe Kerbal said:

I'll try it. You mean boosters too? I mean if you have a big rocket you have to put on orbit would you add boosters close to the tip of it and not at the bottom?

No the explodey parts still go at the bottom, But they ARE heavy so you need to compensate for them by getting anything else you can up high.

The whole reason the Saturn 5 worked was because the first stage, as it emptied of fuel, got lighter and lighter but the upper stages still had fuel so remained heavy. Even though the engines were heavy, they were not AS heavy as the fuel in the upper stages.

So, if you have a 2-stage booster, you will naturally have the heavy part up front. Unless you have some huge, spherical shaped fairing around a very light payload. That'd be like trying to fire an arrow stuck into a basketball.

[AHHans]

In particular for beginners it is easier to have passive stability. That means having the heavy parts at the top and the draggy parts at the bottom, as @Superfluous J and @mabdi36 already said. I personally never felt that trying to mess around with the mass distribution of rockets was worthwhile, but adding fins at the bottom is easy.

But with more experience you can also get unstable rockets into orbit. The trick here is to always point directly into the wind, so that the air doesn't push you strongly in one direction and the corrective force from the engine gimbal can keep the rocket straight. So you can try getting your rocket into orbit while mostly pointing prograde. (There is also a discussion about how to do this in this thread.)

[Joe Kerbal]

Thank you all. Getting a bit of theory is always better as both learning and  puttin it in practice is part of the fun. I added actually tons of fins and all but that seemed not being enough. I'll care about weight distribution and have a look at the suggested thread

 

 

[jimmymcgoochie]

A picture would help here as without seeing the rocket in question it’s hard to give specific advice.

 I can suggest a few things that might help:

• MOAR REACTION WHEELS! More torque = more control, use the biggest wheels you can (surface attach some cubic struts and then add large reaction wheels if you can’t put them in the stack e.g. if you’re using a 3.75m rocket) and put them mostly in the first stage as it sounds like you’re having trouble shortly after leaving the pad and they will be overkill at high altitudes or in orbit. Don’t worry too much about power, unless you’re using an obscene number of reaction wheels the engine alternators will be more than enough to power them all.
• MOAR FINS! And the bigger, the better. Controllable fins like the AV-R8 winglet will provide additional control when lower in the atmosphere, but are heavier than fixed fins. Any aero surfaces add mass so become a hindrance at higher altitudes which is why you should only put fins on the lowest stages.
• MOAR GIMBALS! Use engines with a greater gimbal ability to keep your rocket under control. Add vernier engines like Thuds or Cubs if you have to and consider either adding a mod that gives SRBs some gimbal or using LF/Ox boosters on your first stage.
• MOAR- er, I mean, LESS DRAG! Try to make your fairing as tight fitting as possible to minimise its surface area and try to avoid a blunt tip which adds drag- a slightly heavier but pointier fairing might help you.
• MOAR STRUTS! With advanced tweakables enabled in the settings you can add auto-struts between different parts; unlike the strut part these autostruts have no drag and can help your rocket stay the right shape. Add some autostruts between the base of your rocket and the payload to keep it rigid and prevent it bending which will quickly cause a tumble; add some more struts between your boosters and core stage, with struts from the booster nose cones to the root part which will suppress their tendency to oscillate which can also cause instability. Use regular struts if necessary to hold your payload still inside the fairing as a payload that flops around inside the fairing can easily become unstable and if the payload includes your control point that will make the rocket very hard to control.

And if none of that works, try launching the payloads in smaller pieces and assembling in orbit.

[Joe Kerbal]

Thankyou jimmy. That list of suggestions were what I basically attempted myself. I discovered the biggest problem is actually the unmanned POD. Replacing it with a mk-1 with a pilot works... while using one of those cheap prod with the same exactly rocket shape and stages makes it becoming very unstable. I think I'll try add lot of reaction wheels.  

[jimmymcgoochie]

@Joe Kerbal probe cores have feeble reaction wheels in them, and certainly won’t be enough to control a rocket during launch. Even a Mk1 crew pod isn’t that great, you’ll save a lot of mass by using a dedicated reaction wheel (or wheels) in your rocket stack to control it instead of having to put crew in the rocket instead which is unnecessary payload mass and you have to get them back down again. Engine gimbal can help too, but it’s still better to use reaction wheels and fins/winglets on the first stage to keep your rockets pointy end up.

[Snark]

On 6/3/2020 at 8:39 AM, Joe Kerbal said:

Thankyou jimmy. That list of suggestions were what I basically attempted myself. I discovered the biggest problem is actually the unmanned POD. Replacing it with a mk-1 with a pilot works... while using one of those cheap prod with the same exactly rocket shape and stages makes it becoming very unstable. I think I'll try add lot of reaction wheels. 

Could you post a picture of your rocket?  A picture's worth a thousand words, and all that-- if we could see your rocket, we'd probably be able to give more constructive suggestions.