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Rifling RPM Question

Post questions and info about combustion (flammable vapor) powered cannons here. This includes discussion about fuels, ratios, ignition systems, safety, and anything else relevant.
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Rifling RPM Question

Unread postAuthor: Solar » Thu Nov 19, 2009 5:19 am

How many turns per second makes for good spin stabilization once the round is airborne? I thought I saw a thread about this recently, but did not see an answer. I would guess that it depends on the diameter of the round as well as many other factors, but how do you run numbers for rpm from taking the twist and velocity?
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Unread postAuthor: psycix » Thu Nov 19, 2009 6:56 am

http://en.wikipedia.org/wiki/Rifling#Twist_rate
You can use the Greenhill formula to get a twist rate and use the twist rate in combination with the velocity to calculate the RPM.

The results from the Greenhill formula will yield an amount within the acceptable range of spinning speeds.
The formula is just a rule of thumb, and note that this formula is made for firearms firing lead bullets, so it may be a bit off. Its more of an estimation, but it does show you what kind of values you are dealing with.


Incoming Ragnarok with an excessive amount of information in 3, 2, 1...
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Unread postAuthor: JDP12 » Thu Nov 19, 2009 7:16 am

I was expecting him to be the first one on this.... Disappointed
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Unread postAuthor: Ragnarok » Thu Nov 19, 2009 8:17 am

ilovetoblowthingsup wrote:I was expecting him to be the first one on this.... Disappointed

Ragnarok has to sleep, eat, and learn at some point during the day, and is currently simultaneously sculpting 5 different 1:30 scale models.

~~~~~

I tend to use a slightly more modern version of the Greenhill formula, which includes more emphasis on velocity and makes the equation more usable for our low velocities. It's not as advanced as other modern methods of determining twist rate, but it works well enough for spudding purposes.

2 * SQRT(Velocity * Density) * Diameter <sup>2</sup> / Length

Units are metric (m/s, g/cc, centimetres) - output is cm per turn. (Alternatively, length/diameter can be in metres, and output will be in metres per turn)
You can convert to RPM from here by dividing velocity by twist rate (appropriate units, convert first), and multiplying by 60.

As a general rule, you can sway from this twist rate value 25% or so either way without under-stability or "over-stability".

If you need the same barrel to stabilise projectiles with several varying "ideal twist rates", go for a rate that's about the same as the steepest (shortest) "ideal rate", but no more.
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Unread postAuthor: psycix » Fri Nov 20, 2009 6:14 am

Ragnarok wrote:Ragnarok has to sleep, eat, and learn at some point during the day, and is currently simultaneously sculpting 5 different 1:30 scale models.
Two with his hands, two with his feet, and one with his teeth! :lol:
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Unread postAuthor: Ragnarok » Fri Nov 20, 2009 8:17 am

Not that simultaneously... :roll:
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