5.5in Thumper Jr. Stability

[geof]

I'm building the Thumper Jr 5.5in kit from Polecat.

The instructions say to add nose weight until the CG is (<=) 24in from the nose. I'm currently examining stability in OpenRocket. I've been considering Bruce Levison's "aft cone" stability simulation adjustment he recommends for short fat rockets with a length/diameter ratio "much less than 10:1". (His example is the Estes Fat Boy with a ratio of 5:1). Here's a link to Levison's method. Thumper Jr. has a ratio of 7:1. I'm not sure if that is "much less" than 10:1.

Here are some results from OpenRocket:
Nose Weight.....Aft Cone Simulated.....Stability Margin.....CG from front
32oz................yes..........................1.7.....................22.7in
32oz................no...........................1.0......................22.7in
0oz.................yes..........................0.6.....................28.6in
0oz..................no..........................-0.1......................28.6in

These are loaded with a J415W, which is probably the largest motor I would fly with.

Based on this, it seems like I'll need nearly 2 pounds of nose weight. I'm a little surprised by this because this is about 30% of the total unloaded weight (i.e. the rocket without nose weight sims at 4.25 pounds). Also, the instructions are pretty casual about mentioning the CG. In fact, they don't even mention nose weight until after the step where the nose bulkhead is glued in. Attaching weight at the bulkhead at that point would move the weight aft, requiring even more weight than if I put it in the very tip before bulkhead attachment (which I would obviously do).

So, I guess my question is essentially whether anyone else has built this kit, and what did you do? Any advice is welcome. I'm attaching my OpenRocket file. I wrote Polecat asking for their file but got no reply.

Thanks for your thoughts.

Geof

PS. My file doesn't include a chute yet. But that's relatively lightweight and would be positioned near the CG so it's not going to matter much for these calculations.

View attachment thumperjr.ork

 

[markkoelsch]

I think this rocket qualifies for Levinson's method.

What I would do is as follows.
1) build the airframe (making sure to measure epoxy weight) and update the sim as you go to reflect this.
2) weigh all recovery component and update the sim.
3) start adjusting the sim to determine the mass required in the nose. I would use the Levinson tail cone technique.

A couple other thoughts. First, build the motor mount/fins as light as you can safely. Most rockets are over built.

The Levinson method was developed in Rocksim- not Openrocket. While both are similarly calculated, the numbers are not quite the same for Cp calculations even in an identically dimensioned rocket. Some work should likely be done to test Levinson's work around in Openrocket vs Rocksim to see if they are valid.

Secondly,

 

[flynfrog]

You probably wont need any nose weight. The thing makes a ton of base drag.

 

[AHansom]

The Thumper has been around for a long time and was probably designed and tested the old fashion trial and error way without using any simulations. I have a 4", 7.5" and a 10" build it per the instructions and it will fly just fine.

 

[geof]

That woman in the second photo looks a lot like Kari from Mythbusters. Could it be?

 

[AHansom]

That woman in the second photo looks a lot like Kari from Mythbusters. Could it be?

Yes it is. From a LDRS show a few years ago

 

[fyrwrxz]

Yes it is. From a LDRS show a few years ago

Polecat was sold to another company after years on the market. I volunteered to help Jack put kits together, but he bailed completely a few months later. My bet was any files did not follow along. I only added about 8-10 oz. (I really don't remember, but it was NOT 2 lbs!) Had a few miles on it and flew fine, but I cratered it after I forgot to add 'extra' BP for motor ejection. It's a big volume to pressurize and I just forgot because the sun was going down and I hurried... Don't succumb to 'last flightitis'! Straight smoke and good chutes, brother!