semi span/rocket length ratio?

[Matt]

hey guys
well... im building a rocket that is very long and has a very small semi span

the rocket is about 2.6m (8' 6") long and only has a semi span on it's fins is only 76mm (3") and a root cord of only 110mm (4.5").
so really small fins... having the smal fins even though rocsim says the rocket is stable (barrowman equations) static margin1.2
is this rocket actually stable..
it looks like if the nosecone disturbs the airflow over the fins it could distrupt the stability and having the small fins makes the static margin move quite dramatically with small changes in fin size

do you think I'll have any problems?

 

[DynaSoar]

Originally posted by Matt
hey guys
well... im building a rocket that is very long and has a very small semi span

the rocket is about 2.6m (8' 6") long and only has a semi span on it's fins is only 76mm (3") and a root cord of only 110mm (4.5").
so really small fins... having the smal fins even though rocsim says the rocket is stable (barrowman equations) static margin1.2
is this rocket actually stable..
it looks like if the nosecone disturbs the airflow over the fins it could distrupt the stability and having the small fins makes the static margin move quite dramatically with small changes in fin size

do you think I'll have any problems?

With that much body between the nose and fins you'll generate plenty of turbulence anyway, so what and how much the nose causes will be lost in the shuffle. Turbulence is not necessarily a problem, as long as it stay chaotic.

With a bird this long, and with fins this small I'm assuming it's thin, you could run up aganst a periodic vibration mode causing it to wobble. There's two ways to find out, and I'm pretty certain you don't have access to a wind tunnel.

Also with birds this long people tend to allow the recovery system to have as much of the internal space as it wants. This can be a bad thing in marginally stable rockets. If under G load during take off, if the recovery gear slides backwards a significant amount, it can change the CG suddenly while the bird is still getting up to speed. I prevent this in such designs by gluing a small length of coupler with a dowel glued across the top far enough down the tube for everything to fit but not much more. This does make it hard to load the wadding from the top, so I load it from the bottom, before the motor.

Also, keep an eye on guide length. Your CG is going to start a significant distance up the rod/rail.

 

[hokkyokusei]

You don't say what body diameter your rocket is, but I'm guessing it's quite small, producing a long thin rocket?

IIRC Barrowman makes certain assumptions about the layout of a rocket which mean that the one caliber stability rule doesn't always work.

Short, stubbby rockets may be stable with less than one caliber and long skinny rockets mya need more than one caliber. The Estes fat Boy and Mean Machine are examples that come to mind.

Here's an excellent article on the topic, by Robert Galejs:
https://www.argoshpr.ch/PDF/sentinel39-galejs.pdf

 

[Matt]

the body diamter is only 80mm(3.15")

 

[DynaSoar]

Originally posted by Matt
the body diamter is only 80mm(3.15")

That's not too bad a profile (32.5:1). The AT Mirage is slightly longer in proportion (33.5:1). With tailcone+Slimline mine's about 35:1. But then the fins are swept.

If you really need to keep the fins that small, making them swept might improve your stability margin. If not swept, then maybe set back a bit with a notch cut out of the inner trailing edge to keep clear of the exhaust. Compare the WAC Corporal clipped delta fins with the similar but notched Aerobee-Hi fins. On the Aerobee the trailing edge is actually behind the aft end of the body. Better for the CP.

Giving them a bit more chord would help too without causing as much extra drag as additional span would.

Yeah, I'm a little bit leery of your stability, not knowing the fin configuration.

 

[JRThro]

Originally posted by hokkyokusei
You don't say what body diameter your rocket is, but I'm guessing it's quite small, producing a long thin rocket?

IIRC Barrowman makes certain assumptions about the layout of a rocket which mean that the one caliber stability rule doesn't always work.

Short, stubbby rockets may be stable with less than one caliber and long skinny rockets mya need more than one caliber. The Estes fat Boy and Mean Machine are examples that come to mind.

Here's an excellent article on the topic, by Robert Galejs:
https://www.argoshpr.ch/PDF/sentinel39-galejs.pdf

Thanks for that link!

I have a 32.5" long by 0.787" BT by about 5" tip-to-tip fin span 4FNC rocket that I haven't flown yet. Barrowman (via VCP) gives it a stability margin of 10.18 with a C6-5 motor in it, but it almost sounds like I should swing test it anyway.

 

[Matt]

with a static margin of 10.18 the rocket will fly into the wind

 

[Damian]

Hey ho.

I built a rocket very similar to that.

Over 2 metres. 76mm fins, 3" diameter.

I used the body lift extension to the barrowman equations.
What that did tell me is that the effect of body lift was not as significant as I originally thought may be the case. ALthough I have to say that it was enough to force me into offering 2 calibres of safety vs. the usual 1 calibre.

It was a very heavy rocket (5.5kg aka 12.1 lbs) so I elected to launch in still conditions to ensure the AOA on rail departure was not beyond 5 degrees.

It flew as str8 as an arrow - over 2 miles up.
Having said that the fins DID look TOO SMALL and it did get the odd dissapproving glance!

Regards

Damian