heat resistant motor casings

I've been looking to test a homemade rocket I made with a sugar motor. Problem is I've always used commercially bought motor casings which is shared among a group of us. I've used PVC/ABS as motor casings before for smaller rockets but now I'm looking at 38 and 55mm engines. My worry (without testing) is that it will melt the nozzle, and possibly ruining my rocket. I'll be testing this weekend to see if I can use PVC or not but I was wondering if anyone else had experience with custom motor casings on a budget? Maybe some kind of high heat coating the inside of the PVC or just order a custom aluminum casing with threading?

PVC won't be allowed at rocket launches due to safety concerns.

Gerald

Spend the money on Loki Research hardware, or Mike Fishers Binder EX hardware, then you have commercially available casting tubes and liner options. They both make outstanding quality snap ring style hardware, and they are much safer than pvc casings.

Honestly, if you are on a budget in this hobby, buy the correct tool that you can afford that will do that job, or wait and save until you can afford it. Altimeters, parachutes, etc. etc. and especially motor casings, rule applies for all.
Failure is not an option, it happens, and this is one of ways to mitigate. Correct for motor casings are aluminium, fiberglass, carbon fiber, phenolic, etc. For re-usability, go for good brand aluminium cases.
I have experienced failure because of 'cutting corners to save $$' and won't do it again.

The Loki and other commercial casings like Aerotech or CTI feature phenolic or cotton fiber type based liners for thermal protection of the aluminum casing. If you found a sufficient thick walled 6061 extruded aluminum tube you could design your own casing with some math as long as you have a liner that thermally protects it. If you don’t feel comfortable designing a pressure vessel with a safety factor then save up and buy some Loki hardware because it’s not worth getting hurt. People don’t like pvc because it when it fails as all plastics fail they shatter and are brittle. Metals elastically deform permanently in a predictable manner in theory. Some universities have made their own casings when they couldn’t afford quality name brand hardware. The problem with making a casing in design is you need all this experimental data to really know how thick of walls to use or how to thread it or how thick or closures or size of nozzle. If you don’t know the pressures then just buy a name brand casing. Plus if a name brand casing ruptures you might get it replaced. You gotta look at the time to manufacture a casing plus tooling like lathes or drill presses. And steel tubing isn’t encouraged for amateur rockets. The nozzle has supersonic flow at throat and I’ve had a compressible gas dynamics class but I wouldn’t exactly call sizing a area throat to exit trivial. You gotta have some experience practically and know how to get the pressure transducer or a load cell setup to collect data on a motor reload to size it correctly. Making a nozzle on a lathe takes another skill. Loki could make a nozzle specifically for your sizing needs. The research forum would likely have a lot of help to you. You don’t need to be an engineer but it’s not going to be easy to design a casing and you may need multiple design revisions when it explodes or ruptures experimentally due to variations in tests or propellant. I’d guess a seamless extruded aluminum tube of high quality. A lot of people encourage a commercial casing because it’s proven and the easier route. You may want to try some HPR commercial reloads with casings as money allows. I still have an H motor I need to go fly sometime when work allows. I skimped on a shock cord once not knowing a quality difference and never again will I skimp. When the liner fails the commercial aluminum casings seem to fail thermally. So liner defects are a big deal.

And I don’t know a whole lot and am still inexperienced compared to many of this forum. They are trying to save you time and money also recommending a safer route by a commercial flight proven product. The commercial casing that flys with a high pressure APCP load would easily withstand a sugar motor with appropriate nozzle sizing.

When I first got into research motors I made some single use PVC cased motors with built in delay and ejection charge. I used phenolic liners and water putty nozzles with stainless steel washers in the throat. Max pressure was around 200 psi. Then TRA banned PVC cases so that was the end of that. Never had a cato with one either. For liner, you can use phenolic or coat the inside of the tube with EPDM. But you can't fly it at a TRA launch, so you may as well just purchase the right aluminum hardware from me.

I'm a machinist, and have made my own motor hardware for Rx use. It's not tough to do. But if you don't already have it, what you will spend on tooling alone will buy you a whole lot of commercially available motor cases, complete with nozzles, closures, o-rings, and snap rings, that take commercially available liners/casting tubes designed for your safe, intelligent, and mentored Rx activities.

Please see the post directly above this one, and stay safe.

Or you could but this Aluminum sugar motor.........45.00

https://www.rocketryforum.com/threads/real-sugar-motor-naaka-designed.146599/

cherokeej said:

I'm a machinist, and have made my own motor hardware for Rx use. It's not tough to do. But if you don't already have it, what you will spend on tooling alone will buy you a whole lot of commercially available motor cases, complete with nozzles, closures, o-rings, and snap rings, that take commercially available liners/casting tubes designed for your safe, intelligent, and mentored Rx activities.

Please see the post directly above this one, and stay safe.

Click to expand...

How do you mill such thin wall casings?

Bob Clark

RGClark said:

How do you mill such thin wall casings?

Bob Clark

Click to expand...

Boring and turning, especially most grades of aluminium, are not particularly difficult until you get to very very thin wall thicknesses. One method, depending on final tolerances needed, is to turn the OD above finish size, while leaving some wall thickness for support. Then bore to size (or under and hone finish if required), then turn the OD to finish size using a mandrel to support the ID. Often the mandrel might be plastic such as pvc, just to provide a little extra rigidity. I've even seen sand packed in thin wall tubes during grinding to prevent the work piece chattering.

There are also boring heads that machine and burnish bores, leaving a polished finished with very fine tolerances. Not usually practical to buy for a one off but if you're pushing the limits then it's going to start to cost.

Speeds, feeds and choice of machining inserts play a huge role in being able to hold fine tolerances. Often there are roll or extrusion processes that will get thinner wall sections much quicker, but the pay off is you pay for it in tooling to begin with.

Moved to propulsion.