Wednesday, December 29, 2010

PET third stage COTS tank





Betterbottle makes 5 and 6 gal PET water bottles. I could only find a source for the 5 Gal, so that is what I tested.

Empty 674 gm
MR with water of 29 with peroxide 39!
Burst at 125 PSI.
Long period of plastic deformation at around 100 psi
It burst in the thin center with the ends remaining substantially thicker.
This means one could probably prevent the banana deformation and increase the pressure with a center overwrap.


Chris Jefferies said...

I wonder how plastics will perform long term in vacuum. This might be OK for short exposure of a few days, but there are at least three factors to consider.

- Loss of plasticiser. The plasticisers that prevent plastics becoming brittle are volatile. They are slowly lost even at atmospheric pressure so plastics become more brittle with age. This process might be much faster in a vacuum.

- Effects of high and low temperatures. Plastics weaken at high temperature and become brittle at low temperature, for use in space it would be essential for them to be maintained near room temperature.

- Effects of radiation. Space is full of radiation. It would be easy to protect from visible and UV wavelengths but what about X-rays, cosmic rays etc? Plastics suffer radiation damage.

Paul Breed said...

Third stage will be in space for at most a few hours. Its job is to drop off the payload and deorbit itself.

Unknown said...


Are you thinking of using a carbon fiber sleeve over the center section for strength and then maybe covering the entire apparatus with beta cloth to deal with heating issues? It might be as simple as that to negate the major issues.

Anonymous said...

I don't know if true, premade COTS bottles are really going to work. It might take going a custom shape (thicker middle) in a PET bottle former using COTS preforms.

how COTS would it be to get say a 10L preform and have it used to make a 5L bottle? Thicker bottle walls might lower the initial MR but perhaps it gives you enough pressure so you don't need to add anything else.

After you started talking PET bottles I went and dig some light googling - looks like you can get a forming machine and maybe make your own shapes from the preforms. Not easy COTS, but an interesting idea if you wanted it in production quantities.

Michael Antoniewicz II said...

Why 'Hi-Tech' carbon fiber? What about wrapping it with a layer or three of fiberglass?

Paul Breed said...

Raw carbon fiber is not expensive in the context of ANY flying rocket vehicle. If your going to the trouble to do a wrap use the best materials.

Anonymous said...

If you decide to use a preform, then why not go to a sphere (better resistance)

Anonymous said...

How did you get such a good seal on the bottle?

Paul Breed said...

I machined an aluminum fitting with o-ring grove, the inside of the bottle neck was very smooth and uniform in diameter.

I then stuck it on the rotary table and drilled and tapped 12 8-32 retainer screws.
Then I made a backing plate to go on the outside and drilled that, then split it on the band saw so it would go over the neck below a support lip.

I used the backing plate to drill holes in the neck above the level of the o-ring seal.

You can see all of this on the two pictures I took. After I burst the tank I cut off the neck above my fitting and that weighed 33 gms, so a little bit more weight can come out. I did not weigh my fitting as it was crude and quick, and not flight weight.

All in all the described fitting fabrication took two lathe setups and two mill setups and about 2 hours total.

Both the lath and MILL are cnc, but for something this simple its quicker to just type the G-code directly into the machine. Doing a drawing and CAD model would take loner.

Anonymous said...

I am curious how the bottle responds when it has 3-5G's applied to it.
Without a lot of support and the
inability to pressurize much,
The strain would rupture the container.

Paul Breed said...

At most it will hold 60 lbs of propellant

at 10g that's 600lbs.

At 12" across and 100 PSI that's
11309 lbs length wise and at least double that in the other direction.

It won't notice the payload if its pressurized at all.

Anonymous said...

It's like the original Atlas that was pressurized.

Anonymous said...

If the container is filled with water
At 8lb/gal
and its a 5gal container at 5g you
would exceed the 125 psi burst strength by over 50%.

Adding pressure will keep the sides
from deforming the same as the sidewalls of a tire ,but this will not increase the burst strength.

Anonymous said...

Silver plate the entire damn engine.

Float a bladder in the tank. Fill it with H2O2. Surround it with vinyl chloride gas- the 25*C pressure is twice that of propane, no more. There's the pressure. No cryo, and the Delta V is about 40% better than LOX/Kerosene.

Paul Breed said...

Its 125 PSI, that's Pounds per Square Inch, so the force on a 12 inch diameter end cap is

113 square inches times 125 psi
= 14 thousand pounds!

ASt 8lbs/gal times 5 g * 5 gal we have 200 lbs, so 14,000 from pressure makes the 200 from content trivial.

The forces generated with pressure are way beyond what your intuition says.

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Anonymous said...

Forgive me if i misinterpret the 'During' photo, but is that a gas line going to a bottle half full of water? If so then please beware - it's /really/ dangerous to do hydrostatic burst testing with a gas phase present. Please save yourself the injury and do any future burst testing with water (or another incompressible fluid) only.

125psi = approx 140psi absolute, so there is 140/14.7 = x9.5 gas expansion at burst - you might get away with only cuts if a broken piece hits you. If you are angling for a 1000psi burst pressure on the wrapped tank that's x69 gas expansion ratio at burst - probably enough to kill, or at least send bits of broken bottle flying a considerable distance!!

But enough nagging - i'm sure you know this already (right?)...

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