Revisiting an age old Problem (Cartwheeling Launch Vehicles)

[Turk_WLF]

First of all I would like to say, I understand that the Center of Gravity (CoG) changes as the Launch Vehicle’s fuel level decreases. I have tried to launched a few Satellites when the first stage drops off & the second stage ignites the Launch Vehicle begin to cartwheel, I know it’s because that my Launch Vehicle is “top heavy” & that throws off the CoG.

What I don’t understand is how to fix the problem could you please explain it in plain English.

 

Thanks,

[Superfluous J]

You just need to make sure that each stage that will be in atmosphere is aerodynamically stable forward. Take your first stage (the stage that, when you drop it, your ship starts flipping out) off the ship, and turn on the COM marker. Then find ways to make that marker as high as possible. Also, small fins on this stage can help get you that extra bit of stability, but may harm stability of the prior stage.

One obvious way to shift mass forward is to have another stage. It may even make your rocket more efficient, depending on the design.

Without a concrete example of what you're facing though (pictures work wonders) I can't really give any more specific advice.

Edited December 2, 2015 by 5thHorseman
I accidentally said "Make that marker as low as possible." I meat the exact opposite of that. Sorry!

[GoSlash27]

Best advice I can give is this: Use the (aerodynamically stable) booster stage to get your transstage to at least 30 km. That's about 1800 m/sec DV on a gravity turn.

 If you can get your upper stage that high before seperation, it won't tumble because the air's too thin.

tl/dr:

 If your second stage tumbles, you need more DV in the first stage and less DV in the second stage.

Best,

-Slashy

 

[Snark]

By the way-- Make sure you're actually following the correct design principle.  I noticed you used the term "top heavy" as if it's a bad thing; you may have got your thinking backwards.

For aero stability, top-heavy is good.  You want your vehicle as top-heavy as possible, with your mass towards the front (top) end, so that it will be in front of your center of drag.  What causes flipping ships is when they're bottom heavy.

I only mention it because this seems to be a frequent source of confusion, possibly because it's the exact opposite of what you want when you're designing a lander (which you want to build as bottom-heavy as possible so it won't tip over after landing).

[MircoMars]

I somewhat got used to the "salto suborbitale". if the rocket is stiff enough i can save it 2 out of 3 times. I think of it as an abort to orbit, with limited mission capabilities.