Minimum Diameter Rocket for Rattworks I90

I figured it was about time I started a build thread on TRF 2.0!

Anyway, I've had this I90 hybrid motor for a couple of years now but not gotten around to building a rocket for it. At nearly a meter long and 29mm diameter, it's not something you can just shove in your average LOC/PML kit.

So I'm going for a minimum diameter design. Home made fiberglass tubing, glassed foam Von Karman nosecone, glassed balsa fins. Dual deploy using a Perfectflite MiniAlt and a Beeline tracker. I'm hoping it'll break 10,000ft, which should be enough to get me the UK I altitude record as well.







And just for fun, a couple of the 'glass tubes and the motor itself. The white stuff on the motor is talcum powder from being used as the mandrel for the tubes. I'll probably make another length of tube this week, as the outside of one came out slightly wrinkly and it'll be handy to have some spare.





Because the length of any section of tube is limited by using the motor as a mandrel, and the fact I have to keep things short so they're easy to transport by train to launches without looking too suspicious, I'm going to have to split the tube somewhere along the motor. The plan is to have a lower section with the fins completely loose, and use the motor as a coupler. The forward closure has a threaded hole in the top, so I'm going to screw that into a bolt in a bulkhead in the upper section, which retains the motor and joins the two bits of tube nicely, with the thrust ring stopping the fin section flying off the back. At least that's how I see it in my head...

One thing I'm not certain about is the fin size. It's way overstable at the moment, and I did have the fins a fair bit smaller, but then I read an article in the Apogee newsletter about stability of long skinny rockets, and that recommended going for fins a bit larger than were necessary to make it stable in Rocksim, to account for the forces on the rocket body when flying at a non-zero angle of attack. Does anyone else have any thoughts?

Hopefully I can get the Beeline ordered sometime this week and actually get on with some building!

Edit: Rough sim file attached.

Cheers,
Phil

View attachment 29mm min diam 2.rkt

A nice, mild, low-performance rocket, I see.

Your idea for splitting the tubes should work well.

And yes, you have had that motor for a few years. I wonder who handed it to you?

-Kevin

troj said:

And yes, you have had that motor for a few years. I wonder who handed it to you?

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Some strange bald guy in a field in Argonia, I think. I don't think I've talked to him since.

Wow. Interesting build.

Are you planning on doing tip to tip on the fiberglassed balsa fins? How many wraps of fiberglass did you use on the tubes?

Fins for long skinny rockets can be tricky. If you make this rocket overstable it will weathercock into the wind and you will lose altitude, and gain a long walk. If you make the fins to small, the rocket will cone. This means the back end of the rocket will start to make circles and the nose will stay strait up. This effect gets worse the longer the motor burns. You also lose altitude if the rocket does this. I've heard a good rule of thumb is that the fins should stick out (the semi-span) 1.5 times the body tube diameter. This should stop it from coneing. You can then shorten the root length until you get the CP where you want it.

On long skinny rockets like this stability is a little tricky. You can also keep the nose end light/back end a little heavier to bring the CG back. Like using plywood instead of balsa for the fins and fiberglassing them. Adding a bit of weight shouldn't be a problem for this build. What is the optimum weight? You might end up having to add a bit of weight to get it to go higher.

Keep us posted.

Sam

So where do the used train tickets come into it?

cydermaster said:

So where do the used train tickets come into it?

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He should wrap those around the airframe, and put a thin layer of fiberglass over them, so they're still visible....

-Kevin

rocket999 said:

Are you planning on doing tip to tip on the fiberglassed balsa fins? How many wraps of fiberglass did you use on the tubes?

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I'd like to avoid tip to tip on the fins if I can. Rockets not going to be going all that fast, so all it's likely to do is add weight and use up yet more epoxy.

rocket999 said:

Like using plywood instead of balsa for the fins and fiberglassing them. Adding a bit of weight shouldn't be a problem for this build. What is the optimum weight? You might end up having to add a bit of weight to get it to go higher.

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Thanks for the advice. Rocksim says the optimal mass is around 400g (14oz). I'm not sure if that weight includes the motor casing or not? I hope not, as that weighs about 300g on it's own! Either way, I'll be aiming to build light...

Cheers,
Phil

troj said:

He should wrap those around the airframe, and put a thin layer of fiberglass over them, so they're still visible....

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Erk, no thanks! They're mostly receipts, and so have the last 4 numbers of my debit card on them.

I'd guess that you'll get up to near mach, so I'd personally put a layer of glass over the fins. That's just a WAG though - I might be way off in my speed estimate.