A Question about Hoop Strength

MarkIX · Post by **MarkIX** » 2004-03-04 08:42pm

First some background
I like rockets I like thinking about rockets and designing rockets
the latest thing I have become interested in is the BDB (big Dumb Booster) concept which uses pressurised fuel tanks instead of turbopumps to supply the fuel at the cost of a greater weight penalty in heavier fuel tanks I have been trying to work out what wall thickness you would need for a particualr Sized tank of a particualr material and while googling madly I have come across the formula for hoop stress but I don't really understand it because most site expect you to know the underlying principles alread. It has occurred to me that I don't know the right questions to ask and I was wondering if any of the engineers here could help, I'm not looking to be given the answers I'm looking for questions I could ask or places I could go to find the answers.
Thank you for your time
And please no joke about wants big rocket must have small ....

Howedar · Post by **Howedar** » 2004-03-05 02:31am

Are you interested in an explaination so you can do the calculations yourself, or are you just idly curious about some numbers? I cannot help with either at this time, but the latter would probably be quicker and easier.

Post by **Darth Wong** » 2004-03-05 04:41am

Hoop strength is just tensile strength relative to the forces exerted in a circumferential direction around a pressure vessel. I don't remember the actual geometric derivation (I vaguely recall that it had something to do with cutting the pressure vessel in half and integrating for pressure vectors normal to the plane), but the formulas are fairly easy to get your hands on if you have a basic stress analysis textbook.

It's certainly not particularly complicated, anyway.

Rogue 9 · Post by **Rogue 9** » 2004-03-05 04:45am

I build rockets as a hobby. Lets see what I can come up with. Tomorrow. Now its late.

MarkIX · Post by **MarkIX** » 2004-03-05 04:58am

I guess I'm trying to get a handle on how the Formulas where derived I like to know how thing work, so I can use them better. I can find the formula and plug numbers into it. I get a result but to be honest I'm not sure what the result means I fell a bit foolish saying that.

As for building rockets well I'm not very good I've tried various stuff but I don't seem to be very good, all that talk about how dangerous Zinc Sulfur can be and mine was a fire retardent

Post by **Darth Wong** » 2004-03-05 05:02am

MarkIX wrote:I guess I'm trying to get a handle on how the Formulas where derived I like to know how thing work, so I can use them better.

Off the top of my head, I'm guessing that it was probably done by resolving the radial forces exerted by the gas pressure in the tube into X and Y vectors, and then using calculus to integrate around the circumference of a half-cylinder to find the total force being exerted in the X direction (normal to the cutting plane). Once you have that, you simply divide it by the cross-sectional area of the pressure vessel intersected with the cutting plane and you have hoop stress.

Crown · Post by **Crown** » 2004-03-05 06:12am

To imagine Hoop stress, think of an aircraft's fuselage ... for rockets it is mainly used when calculating propellant tanks and their dimensions and the pressures you store stuff in.

I am assuming that for your level (buying model rocket engines) you are trying to think of the rocket body overall which is really not needed ...

That help?

Also do a google for 'rocket project' and try RMIT or First Year Aerospace in the search as well (my course) which is just basically what I did way back when. It is an excellent page on a step-by-step for rocket (model) building and flying, aswell as some preliminary design calcs.

Crown · Post by **Crown** » 2004-03-05 06:13am

Also a good site for rocket engine design is; here

Basically the University's of Maryland course for rocket design.

EDIT :: And this link also has the relevant calculation on how to determine the fuel tank sizes and such, which you asked.

For a good reference book for Hoop stress in tanks (which the link sort of skips over), find an AIAA educational series book along the lines of 'Spacecraft design' which covers the fuel tanks more in line with Hoop stresses considerations (I am sorry but I don't have to book on me to tell you step by step, but you should be able to find at a local Uni Library). I used this book for my thesis and I highly recommend it.

Crown · Post by **Crown** » 2004-03-06 01:17am

Any of these books should have what you are looking for;

Brown CD. Elements of Spacecraft Design. AIAA Educational Series, 2002.

Brown CD. Spacecraft Propulsion. AIAA Educational Series, 1996.

Brown CD. Spacecraft Mission Design Second Edition. AIAA Educational Series, 1998.

The first one is the updated version of the other two combined, but I can't remember if it covered fuel tanks that much, whereas the second one delt with rocket design exclusively.

MarkIX · Post by **MarkIX** » 2004-03-07 07:53pm

Thank you all very Much
I would have replied earlier but I don't have internet on the weekend