Monday, October 04, 2010

Tanks the other hard part

The Rocket Equation has a term that is the rocket motor efficiency and a term that is the mass ratio. In a pressure fed system the biggest component of that mass ratio is the tank.
If I could make a tank identical to a 2L soda bottle only 5 to 10 times bigger that would be almost ideal. I could probably find a mechanism, to join a bunch 2L soda bottles and I may pursue that, but I'm also going to try my hand at composite tanks. Toward that end I made mold forms for a 6" diameter cylindrical test tank mold. I heated it up and waxed it with mold release and setup the vacuum bag etc.. got ready for my first composite test part.

I have something non-rocket to do tonight, but tomorrow I hope to try making my first composite part since the solar plane project.

The first tanks will have glued Mylar liners that are not H2O2 safe, but the flight tanks will have mylar/PET liners with no glue. The jury is still out if the tanks will be seamed via vacuum heat, sonic welding, or RF welding.In addition to the liner I also want the tank to be chemically resistant to the H2O2. I want any liner failures to not be immediately catastrophic.

I'm getting a bunch of samples and setting up to do compatibility testing next weekend out at FAR. I'm testing glass, kevlar and carbon fibers with normal epoxy, vinyl ester and special chemically resistant vinyl ester resins. My gut says that plain E glass with the chem compatible resin will be fine, I'm almost certain that carbon with epoxy won't be. I hope the Kevlar and chem resistant resin also work as the kevlar is a bunch stronger than the glass.

Its a multi part test, start with 30% and work your way up to heated 85 or 90%. Anything past 50% really has to be done remotely. I'm planning to soak samples in PET soda bottle bottoms. sitting in hot water on a hot plate. The sample will have a string and pulley so I can pull the sample out of the solution remotely and shoot it with the 243. I could set up some kind of hammer and anvil test, but I worry that if it goes bang, the hammer becomes a projectile.

I figure I'll get the preliminary screening done this weekend and the samples that pass will get the hotplate test in two weeks.

14 comments:

QuantumG said...

Can we please have video of the tests? :)

Anonymous said...

Have you considered trying the plastic tanks you see on office water coolers? They should be widely available and reasonably durable.

Dan said...

How difficult would making your own PET blow mold be? I would think that forming the blank would be the hardest part.
It seems like a 4"- 5" ID pipe could be sawed in half. Two would probably be necessary to make up the kerf of the blade, but with two halves, a vacuum pump, some heater bands, and hot air blower; it seems do-able. Well all that and a decent release compound.
I am not sure what I would do to make hemispherical ends though.

What kind of fitting would you want the outlet to be? The blank could be molded on that.

~DanielW

Dan said...

Upon further review.. there are a few factors that could make consistent parts a nightmare for a garage operation. But the professional plastics world would think it remarkably simple to accomplish. Unfortunately you and I can't afford $50,000 for the "cheap" tooling costs.

Monroe said...

Find a steel tank the size you want and use it for the mold for your PET tank. Stretch blow molding should work fine. I could do some test tanks fairly easily.

Paul Breed said...

The 750 gram preform for PET 5 Gal water tanks is about the best your going to do.... the preform os an off the shelf part. I called a bunch of custom bottle people to see what custom bottles would cost . If you aren't doing 10M units they don't answer your e-mail or phone messages... unless they are a U.S front for a custom house from China then they answer and I can't use them ;-( ITAR.

Dan said...

Looks like you already researched it thoroughly. Do those 750g preforms have the gigantic neck? I was thinking of trying my hand at a differentially throttled, hovering water rocket. Something that would fly for only a few seconds, just to see if I can handle the circuits and programming part.

I am interested to see what you come up with for tanking. Though I won't require the same aspect ratio.

Anonymous said...

Making the mold wouldn't be all that difficult. Still expensive though. I worked at a factory that made PET bottles for their canola and sunflower oil and one ofs is something you can forget about. The molds are preheated and then take a while to get up to temperature with the operator keeping an eye one things and adjusting the parameters such as closing and opening times of the molds. It took heaps of preforms before that machine was running steady. And it used a lot of energy. 3 phases at 220v and the currentmeter was steady at, iirc, around 800 amps. But once it was going it kept spitting out bottles 24 hours through out the week.
Long time ago:) (lousy job)

Roland

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