Missile Question

US System Seems Counter Intuitive

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Missile Question

#1  Postby Calilasseia » Jan 20, 2018 11:21 pm

I've just learned of the existence of a missile system called the RIM-116 Rolling Airframe Missile, and there's an aspect to this system that puzzles me.

Apparently, the missile rotates in flight about its longitudinal axis, in much the same manner as a bullet or a shell fired through a rifled gun barrel. Which strikes me as a pretty odd thing for a missile to do.

One of the reasons that rifling is used to make a ballistic projectile spin, is to increase stability, and increase resistance to external forces causing the projectile to deviate from its intended flight path. Which makes sense for a projectile that you cannot steer in flight.

But a missile is, by definition, a guided weapons system, with built in capability for in-flight manoeuvring. One of the advantages of a guided missile, is that it can turn to follow targets performing evasive manoeuvres. Adding extra moment of inertia about the longitudinal axis, which would increase the effort the manoeuvring system had to exert to turn the missile, seems to me in part at least to defeat the purpose of having a guided missile in the first place. So what's the point of this?
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Re: Missile Question

#2  Postby Shagz » Jan 21, 2018 12:51 am

Missiles and rockets often have fins for stability in flight; however, fins add weight to the system, and also increase surface area, which increases drag, which means more energy is required to reach the desired speed. Forgoing fins also might make the rocket and launcher more compact. So my guess is it might have something to do with that. I am no rocket expert, though, having only a "Kerbil Space Program" level of knowledge of this stuff.
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Re: Missile Question

#3  Postby Macdoc » Jan 21, 2018 7:25 am

Not sure missle and guided missle are interchangeable but that's a nit pick.

Sort of like a steadicam ....adding some inertia by way of the rolling airframe may make it easier to compute the path required and fewer over shoot adjustments.
Also definitely less subject to atmospheric influence.

Keeping the missile stable by rotation I suggest even saves guidance fuel at the expense of rapid change in direction.
Computing power is cheap these days. Getting rid of fins allows more to stored in a fighter and in munition stores.
I can see the advantages.
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Re: Missile Question

#4  Postby LucidFlight » Jan 21, 2018 10:22 am

I think Macdoc has the gist of it.

Ahem, ahem...
The main article states that the RIM-166 "rolls around the longitudinal axis to stabilize the flight path much like a bullet."

This is totally incorrect. In order for the missile to be spin stabilized, it would have to roll at a rate of several Kilohertz which can be verified by analysis of the gyroscopic motion. At this roll rate, the internal stresses would be so high, the missile would tear itself apart.

The primary reason for rolling the airframe is to save on cost of both the control actuator system and the seeker angle resolver capability. The single set of control surfaces are phased with deflection commands to achieve pitch and yaw motion. The rolling seeker head has two passive RF antennas that can determine angle via interferometry in pitch and yaw due to the roll. A good description of this weapon is found in the document: dtic.mil/cgi-bin/GetTRDoc?AD=ADA390349

https://en.wikipedia.org/wiki/Talk%3ARI ... me_Missile
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Re: Missile Question

#5  Postby Keep It Real » Jan 21, 2018 10:41 am

What a prodigious waste of resources making missiles at all in this day and age.
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