5/16in Bottle Rocket tubes breaking while pressing.

[Czx7]

Im working with some 5/16in tooling from woody’s and pressing whistle. I have had multiple burst while pressing and Im wondering if there is a better pressure to build these motors with whistle? Any tips would be much appreciated.

[Czx7]

I have not been using my force gauge as much on these little motors and have just been going till it feels nice and compacted. I may have to make a tube support for them but my main issue is the best pressure for these 5/16in motors.

Another question, would a arbor press be better for these motors?

Edited October 17 by Czx7
Addition.

[Arthur]

If you've only a narrow range between bursting the tube and being under pressed, you could look at the tubes and a support. Have you tested these? If there really is only a small window of pressure between fail and fail you need a pressure gauge to keep within the sweet spot. Otherwise look at thicker stronger tubes and/or a tube support. However a tube support commits you to one size of tube OD and ID for ever.

Smaller increments may make the compacting more uniform.

[Czx7]

Thanks for the info. The window has not been that small but, I have not been using my force gauge because I have not know what would be the best pressure for these smaller tubes. I tend to get the best results with whistle at 7000-7500psi on whistle but I don't totally know how that transfers to these smaller tubes.

[FrankRizzo]

Yes, an arbor press is much better suited for that loading pressure.

Edited October 17 by FrankRizzo

[Czx7]

Im thinking that I need to buy another force gauge because the one Im using is rated quite high for these rockets. What would you guys recommend for a pound rating on the gauge for the 5/16 whistle rockets.

[DavidF]

You can make up a cheap tube support for any length and not worry about OD variations with a piece of mylar 0.005" thick. There's a thread about it somewhere on here. It's not complicated. The mylar is rolled up so that the tube just slides in, with a friction fit. Rubber bands on the outside keep it under control. It's then tightened onto the tube by rolling it tight. The piece of mylar for that size rocket might be 4-5 feet long when rolled out. I can provide more details if you want or need. You can press a whistle rocket to 7000psi with it. I used to make them out of brass, but it's too springy, hard to work with, and expensive. The logic of how it works is in a video I made a long time ago. It details a brass support, before I tried mylar. Same idea though.

For small rockets like those an arbor press with a torque wrench to set the force is best.

[Czx7]

Thank you I will defiantly look into Mylar for these small motors. Do you think that I need to get a lower rated force gauge because I can work with an arbor press but I don’t have a torque wrench available to use.

Edited October 17 by Czx7

[cmjlab]

I second Dave F's tube support idea, I've used the brass ones and they worked for me - though I did buy a fancy tube support eventually.  

For the 5/16" tube's especially, I've poured fiberglass resin molds inside of a larger steel pipe (like 3/4" steel gas pipe, etc) as a makeshift tube support, they've also worked. To ensure the resin didnt slide out once set, i cut grooves inside the steel pipe for it to grab. To ensure a proper fit to the paper tube, I've used a sacrificial tube with a Vaseline coating as a release agent, and I've drilled them after pouring with the appropriate sized bit.

I bet you can get a cheap torque wrench from Walmart or Harbor Freight for much less than buying a pre-made ptof Guage, then you can use your Arbor Press to get pretty consistent pressures based on the torque wrench measurements, especially on 5/16" tubes.

[Czx7]

What would you guys recommend as a increment size for pressing these?

I was planning on just experimenting but I wanted to see if I could get a good place to start.

 

Edited October 18 by Czx7
Addition

[cmjlab]

I hadn't thought about that before.  I've been doing 1/4" increments since it works for spolettes, and may actually represent close to 2/3 ID, but I'm sure more experienced rocket makers can give you a better answer.

[DavidF]

Using an arbor press with a torque wrench is ideal for these small motors. I'd use the P to F gauge to set the torque wrench so that a slow pull registers about 540 pounds on the gauge when the torque wrench clicks. I'm pretty sure Caleb (Woody) says you can switch out the dial on his P to F to read with more accuracy at lower forces. Also, I think Ned Gorski has a YouTube video about modifying a small arbor press to do torque wrench pressing. I mounted a heavy pulley on the left side of mine to have better control of the ram.

[robbo]

Find a "rigid shaft coupling" with a 1/2" ID.  I have 2 types from two manufacturers.  Maybe one came from Grainger?  Not perfect (they don't completely surround the tube), but good enough for the low pressures with the small NEPT tubes.  I think about $20-30 a few years ago.