Monday, December 20, 2010

Way out of the box...

A 3rd stage made of 3L soda bottles surrounding a 3U Cube sat.
3L soda bottles are good to 100PSI with a Mass ratio of > 40 when used with H2O2.

The "goodness" of normal rocket tanks are usually rated by the PV/mass. The best aerospace composite tanks, like the Scorpius pressurization tanks used by Armadillo in the 180sec super mod can have mass ratios of better than 9 at 2200 PSI working pressure. If you could scale this to a 200 PSI tank then you get a water filled tank mass ratio of >80. The fundamental problem is for the small sizes we are talking about you can never get that low. The tank wall would be paper thin. At small sizes the issues are minimum gage not minimum strength. (Hard to beat a soda bottle 0.25mm thick. )

As soon as you go up in size bigger than minimalist nano sat launcher then properly built aerospace quality tanks are a must.

I also suspect that when fighting minimum gage issues going up in pressure for a tiny launcher can have benefits in motors size, expansion ratio, flow path sizes etc....
A big puzzle with too many knobs.

Here is a link to the specs for high quality aerospace tanks...


George Katz said...

Hi Paul,

I was interested to read this post, that you are considering using soda bottles for the third stage. Is this a serious consideration or is it mainly a what-if scenario?

If it is a serious consideration, would you be joining the bottles mechanically at their bases
since you mentioned previously that you would like to avoid any type of glue coming in contact with the peroxide if you were to splice them together.

A 2L bottle will increase it's volume by about 7.5% when pressurised to 120psi and grow in circumference by about 8mm. It also stretches by about 6mm. I think this kind of expansion would need to be considered in the design of the frame that holds them, though you have probably considered that as well.

What would be your thoughts on temperature issues with PET tanks exceeding their glass transition temperature of ~67 degrees C either due to the rocket heating due to air friction at lower altitudes or above ~130km in the thermosphere (if the tanks are pressurised still at that altitude)?

Oh and Merry Christmas :)

- George

RM said...

I think the trick would be plumbing.... the plumbing could easily weigh more than the tanks.

Have you considered blow molding your own PET tanks? Soda bottle blanks are readily available. A mold wouldn't be too hard to fab, might open up the trade space a little

RM said...

Like these

Paul Breed said...

If you search you can find 360gm preforms designed for 20L bottles.
(MR = 56)

Paul Breed said...

Note that its really unlikely that a 20L PET bottle WOULD not have the pressure handling capability of a 3L soda bottle.

Paul Breed said...


I wonder if it can be "Reshaped"/Stretched into something that looks like the desired tank liner.

RM said...

Make your own preforms?

Tony Rusi said...

There was a guy from Canada reinforcing soda bottles with sparse wound carbon- fiber too.

Paul Breed said...

If you go back to the early blog 2006 early 2007 you will see that I made some spectra wound tanks, that the water rocket people have now copied.

Unknown said...


I like the out of the box thinking. Looking at the design, a little bit of thermal wrap and you'd be all set.

You never know what this might lead to.

gravityloss said...

There is probably some industrial use where intermediate low pressures in the 10 bar range are used in 10-1000 liter sizes.

There are plastic animal feed barrels for mink farms and pier pontoons that are 100-150 liters but they are flat bottomed so that they can stand on their own. One might be able to blow them or their preforms though. This has probably been hashed here before...

Monroe said...


Pet Blow Mold said...

really a very interesting and useful post for me. Thanks for sharing this useful post with us....

High Power Rocketry said...

What about exposure to very low temperatures in the tank, and does plastic burn when exposed to a strong oxidizer?

muebles en lugo said...

I believe one and all must look at it.

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