Spool dimensions

[iter]

I'm curious about stable proportions for spool rockets. The ones I've seen (and the one I've flown) have a height that's roughly equivalent to diameter. I wonder how much taller can one make a spool and still have is stay stable on the way up.

Ari.

[chadrog]

I can't give you an answer, but I'm very curious of this as well. Got a couple new spools yesterday.

[awseiger]

I can't give you an answer, but I'm very curious of this as well. Got a couple new spools yesterday.

Me too! A guy at work gave me a giant CAT5 spool today, it should make a great HPR rocket. Possibly NAR L2 material?

From what I've heard through the grapevine, the CP of a flat disk is the disk's diameter in front of the airflow. I know that people have flown spools without the top disk, so this seems to be accurate. But, this has absolutely no mathematical basis and could be completely wrong.

[iter]

CP of a flat disk is the disk's diameter in front of the airflow.

CP must be behind the disk, or it would be unstable.

Ari.

[awseiger]

But a disk isn't stable?

[adrian]

But a disk isn't stable?

Ordinarily, no. But if you get it going fast enough before it leaves the rod, there's a lot of base drag which brings the CP some way behind the disc and makes it stable, which is why saucers can make stable flights. Once the motor burns out the disc loses velocity very soon due to that drag, making it unstable, so it tumbles down.

A spool without the top disc is basically a saucer with a tube sticking out of the front; the CP will be as far back as for the basic saucer but the CG may be further forward, if that tube contains something nice and heavy such as the front half of a motor casing. I'd guess that a spool without the bottom disc will be in trouble...

I have no idea of the maths behind all this so I can't say how fast a disc needs to go to be stable, let alone how far two discs (i.e. a spool) can be apart and still be stable. But since a spool rocket is (a) never going to move very fast and (b) doesn't have a hard pointy bit, it's relatively safe to mess about with. So some experiments involving a spool with a telescoping shaft or a series of spools with varying shaft lengths but otherwise identical might be in order.