I designed an orbital rocket, with a liquid-fueled first stage, and a solid-fueled second stage. I want to have the second stage to be spin stabilized. Because the second stage will be still in the atmosphere when spin-up starts, I will have to activate the engine before spin-up ends. The Sepratrons that spin up the rocket are at the stage's center of mass. However, I do not want to lose stability or velocity during spin-up and second stage ignition. How do I solve this rather annoying issue?
Kerbal Space Program – How to Time the Rocket’s Second Stage Spin-Up
kerbal-space-program
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Best Answer
You can totally do spin stabilization in KSP.
Usually you won't really want to, though, at least not in a Career / Science game, because by the time the game gives you access to any good ways to spin up your rocket (such as Sepratrons or Breaking Ground DLC robotics) you will have long since unlocked other, more practical ways of stabilization. But if you want to do it for a self-imposed challenge — say, to get into orbit using only solid rockets and no reaction wheels — then you certainly can.
In any case, the thing about spin stabilization is that once your rocket is spinning on its axis, it's going to stay pointed the way it's pointing, at least on average. Any pitch or yaw torque is only going to make the rocket wobble a bit, but the axis of rotation will stay mostly fixed. What that means, in practice, is that you'll want to have your rocket pointing the way you want it to accelerate before you spin it up. And, for an upper stage, that typically means horizontally.
Anyway, assuming that your upper stage is aerodynamically stable while the burned-out lower stage is still attached to it (which it should be, unless your payload is super draggy, since with the lower stage empty of fuel your center of mass should be pretty far front), what I'd recommend is something like the following flight plan:
No, this won't generally be the most efficient possible launch trajectory, and you won't have much fine control over the exact orbit you'll end up in. But that's the price you pay for doing it the hard way.