24mm Performance Rocket Scratch Build

[WizardOfAZ]

Ok, so here's a plan, but I need advice. I have a bunch of 24mm motor tubes, as well as a long sleek nose I bought from a "spare parts" box at my hobby shop. I'm planning to use basswood for fins, and thats about it. It's going to be very long and sleek, with triple elliptical fins. The question is, what is the sweet spot for body length? Longer is faster, but at what point is too long too heavy and reduces performance? What would an ideal length be for something like this be?

[jpummil]

Josh,

Toss it in a sim program and tinker with fin shape,length, etc.

I guess it comes down to weight vs ballistic coefficient...

OR has an optimization function, but it's still kind of a work in progress I think...

quick bare bones calc using std tubing 18" long, balsa 5:1 nosecone and 3ea helical fins on an E30 shows 2388 ft and Mach 0.83. Extending to 24" gives 2292 ft at Mach 0.81.

[Peter Olivola]

There's a little gem hidden in Stine's "Handbook of Model Rocketry" on this subject. Bottom line is a diameter to length ratio in the 1:10-1:12 range. That puts a 24mm minimum diameter rocket in the 10" total length category. If you use a fairly narrow range of limits in OR to optimize for length use a range of between 7-14 inches.

Another thing to play with would be nose cone shape. For low mach number rockets, cones and ogives are not the best shape.

Another factor is weight. In this motor size range, the motor itself is above or close to optimum weight so anything you can do to reduce the weight of the rest of the rocket will pay dividends, but you can't get down to optimum weight, so don't spend time trying to optimize this. Work on the length and nose cone shape there and build as light as you can.

[JPVegh]

In my experience shorter is faster. The ideal length is as short as you can go and still maintain stability. Eliptical fins probably not the best choice for a speed machine, a delta is probably better suited. Have you ever seen a Vaughan Brothers Extreme? Something like that should get you there.

[New Ocean]

Since when is longer faster? Only if you fill the rocket with motor does it help to get longer and thinner (for a while.) For a set motor, the fastest rocket will be that motor with a nosecone stuck on it. You must have a stable rocket, sure, but nothing more. Excess mass and volume both sacrifice final velocity. The first one is obvious, the latter is a combo of parasitic drag, and displacing a larger volume of air. Base drag stays constant.

Now for altitude, you may want extra length to attain enough stability for a straight flight. But again, length is the enemy of fast for a given motor. Altitude rockets are only as long as they have to be. Optimum mass may allow more airframe, but it will still cost you in drag. If you need to add mass for altitude, add something dense to the nosecone that is basically attached to the motor.

[maxvelocity]

When I build my screamers I start by laying the longest motor I can fit on the table then a chute and cord wadded up as small as I can go then electronics then measure it all. Now I have my minimum length I throw it in rocksim and figure what the smallest fins I can use and still be plenty stable. Depending on diameter I might even cut my tube a little short for extra performance and a little challenge.

[GDJ]

I did fins like these a few times. This rocket is my Daughters.



It's not very long, but it's quick, fast, and very stable. Launch rod doesn't move a millimeter after launch. Honest.

It just begs for C6-7's every launch. Bloody near disappears everytime, but after 12 launches I guess It likes being home.

[Grimracer]

I have an upscaled Astron Sprint for 24mm.. The BT is BT60.

On an F35-11 it went to 2,615'.. SCREAMING! Around 7oz less motor..

Grim

[WizardOfAZ]

I see.

So, one day I built a Quest Astra, but graphted on a 24mm motor tube since the stock was a little thin. It looked exactly like a stock Astra, but with a 24mm motor in it. Maybe doing something like this again would be good. Unfortunately, that Astra was launched by some friends of mine, without approval, at night, after a few beverages, never to be seen again. Wonder how high that went.

I could customize the nose I have so a streamer pulls backwards out of it, keep the tube for motor only, then go for minimum weight. I'll try that...