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Comment by droro | Hacker News Reader

I had always wondered whether there was a missile that was spin stabilized with minimal control surfaces like this one. They mentioned it only had two control fins, but not whether they are independent. It seems possible to steer a spinning missile using a single actuator if the control pulses are precise enough.

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From https://secwww.jhuapl.edu/techdigest/Content/techdigest/pdf/... it seems that the two control fins are driven off a single shaft/motor - so they are not independent.

The RAM uses single plane tracking - basically imaging that it's only ever steering in pitch. But because it's constantly rolling, "pitch" will sweep over both axes and control to zero.

Yeah (and apologies if I get this wrong after 30 years) with a rolling airframe, you can have two fins on the same axis that "wiggle" back and forth at the same frequency as the rotation of the missile. Direction control comes from the phase of the movement, so the fins are pushed in the direction you want to go when it rotates to that point.

Vikhr also does this, and Starstreak and M1156 both use a clutch to transfer force from a freely spinning fin assembly at the nose to the rest of the projectile when appropriate, achieving guidance without any movable control surfaces! Missile designers are very clever.

>spin stabilized with minimal control surfaces

The modern RAM Block 2 has four independently-actuated fins for agility[0], so nowadays the spin stabilization isn't being used to reduce the number of control surfaces.

Spin stabilization is still quite valuable. If an actuator fails (or worse, jams hard over) the missile won't veer off course, it just has reduced control authority.

[0] https://en.wikipedia.org/wiki/RIM-116_Rolling_Airframe_Missi...

BTW That's a pretty common control scheme for Soviet and Soviet-derived ATGMs. https://thesovietarmourblog.blogspot.com/2021/07/soviet-atgm...

newer versions have 4 fins... better read the actual article, pretty good stuff there

Just curious....what else have you always wondered about?

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