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Kangroozeeh

What's wrong with my satelite?

Question

So I've built this satelite and I've watched like a dozen videos on youtube about rocket design because mine are always either flipping or so stable, that they just go straight upwards which makes creating an efficient orbit impossible.
Now I have this contract where I have to builx a satelite and set it at a polar orbit around Kerbin, but around 5,3km my rocket starts to flip without a reason. The center of lift is well below the center of mass  just as all those youtube videos said, but it won't do the trick. Any advice?

Link to rocket


Also, if I build small rockets, I can easily control them, but as they gain mass it becomes almost impossible to steer them in any way. Again, the youtubers seem to do just fine. Even if I place the center of lift very near the center of mass, it's still so cloggy to controll that getting into orbit becomes a real difficulty with a limited amount of fuel. I'd really like to enjoy this game, but apparently there's something about rocket design that I'm missing out on guys.

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4 answers to this question

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  • It's light and draggy in the front
  • It's using vertically stacked tanks
  • It doesn't have any torque authority

Fix those three things and you're good.  :)

Light and draggy in front:  You want your CoM to be as high as possible, and you want to be streamlined in the front.  That means you really want to avoid any big, low-mass parts at the top of the rocket.  For example, you have those paired conical adapters on there.  I assume you're doing that because they "look cool"?  (I can't tell that they have any functional purpose.)  Those things are killing you; they're big, they have a lot of drag, and they're very very light.  So just get rid of them.  As it stands right now, trying to keep your rocket nose-first is like trying to throw a dart backwards.

Vertically stacked tanks:  The problem with having fuel tanks stacked on top of each other is that the engine drains the top tank first, which causes your CoM to be catastrophically lowered and screws up your stability.  The ideal way to fix this is to replace the stack-of-little-tanks with a single taller tank.  I'm guessing that the reason you're not doing that is that this is a career game and you haven't unlocked the taller tanks yet?  In that case, here's a workaround that ought to help you:  in the VAB, disable all the tanks except the bottom one.  This will force the rocket to drain the bottom tank first (since it's the only usable one), which will improve your CoM rather than making it worse.  It does mean that your rocket will conk out as soon as it drains that tank, so you'll have to watch the tank and enable the one above it as soon as it's almost empty, so that you won't have any interruption of thrust.

 

No torque authority:  There are only three ways to turn a rocket in KSP.  1. Gimbaled engines, 2. Aerodynamic steering with control surfaces, 3. Reaction wheels.  You don't have any of those.  Your liquid-fueled engine is a Reliant (no gimbal), and the SRBs have no gimbal either.  You have static fins that can't steer.  And you don't have any reaction wheels.

Suggest replacing the Reliant with a Swivel (it has gimbal), and add a reaction wheel (the 1.25m one should be fine).  Those by themselves may or may not be enough-- aero forces tend to dominate during the high-speed phase of ascent.  So you may need to replace your fins with steerable ones, such as the AV-R8 that @Stone Blue suggests.

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You're running into a problem with the way that KSP displays the CoL marker. It only displays lift given by certain parts, and assumes 0 degrees angle of attack (if the rocket is straight up in the VAB, or straight forward in the SPH). It doesn't take into account drag forces, which is what's killing you. 

Those rockomax adapters are light and big, making them very draggy. They're also waaaay out on the front of your rocket, giving them a big lever arm to swing around on. You also only have the basic fins on. While those are better than nothing, they're not helping much either. I would use the AV-R8 winglets instead, or any other fins as long as they're controllable. 

I can't tell clearly tell from the pictures which LFO engine you're using, but you should be using the "swivel" because it has gimbal, which will help with control (the "reliant" has no gimbal). 

Without taking a super close look at your rocket, my gut feeling is that it should make orbit if you follow my advice above, and be very careful on your flight path. Start your gravity turn almost right away, only 100-500 meters up, tilting about 5 degrees. After that, stick very close to the prograde marker, try not to let your heading leave the little circle in the middle of the marker. 

Edited by FullMetalMachinist

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I agree... Using just the one Reliant, (no gimbal), and 4 boosters (which also do not gimbal), set lower than the nozzle on your Reliant, AND using those small solid fins (no adjustable control), AND that they are set so high up on the boosters, I would try AV-R8s also, set nearer the bottom of the boosters... If you havent unlocked the AV-R8s yet, at least try a little bit bigger fins...

But yeah, you do not have any gimballing, or maneuvering ability... Also, I dont know how much stability they provide in atmo, especially on a rocket, but you could maybe try adding an SAS or reaction wheel module right below your 1st stage decoupler...

Also, this is from the KSP wiki for the Stayputnik you have:
 

"Usage

Generally placed at the top of the rocket, the Stayputnik Mk. 1 provides full command module functionality. Due to the extremely low internal torque, rockets using a Stayputnik Mk. 1 module tend to require vectored thrust, RCS thrusters or reaction wheels."

 

Edited by Stone Blue

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