The scenario:
I am in the process of designing and building a model
airship/dirigible I wish to make the gas bladders of the airship out
of 1mil polyethylene film, a sample technical spec can be found here:
http://www.boedeker.com/polye_p.htm
I have read the articles on the NASA's Jet
Propulsion Laboratory, Pasadena, California:
http://www.nasatech.com/Briefs/July03/NPO21139.html
And other pages like:
http://www.isas.ac.jp/home/Sci_Bal/engstudyofsuper.html
In the fourth paragraph of the NASA page it states:
“One corresponding meshed-pumpkin design calls for reinforcement of
the membrane with a 1-by-1-in. (2.54-by-2.54-cm) mesh of
polybenzoxazole scrim fiber of 25 denier (a lineal mass density of
about 2.8 mg/m). With this reinforcement, the complex composite
membrane could be replaced by a simple polyethylene film 0.5 mil (12.7
ìm) thick, reducing the mass of the balloon to <400 kg. The mesh would
provide a strength of 400 N/m, giving a factor of safety of 5,
relative to the strength required for a pumpkin balloon with a bulge
radius of 8 m.”
My questions concerning the above information are:
1: What are the correct formulas to use to calculate the required
(tensile) strength of the material used in the in the envelope? (Is it
P*R/t where P = pressure in psi, R is the radius of the sphere, and t
is the thickness of the envelope)
2: In the above paragraph it is said that the complex membrane could
be replaced by a .5 mil film and a mesh making the balloon both
stronger and lighter. Can you please step me through the
formulas/process of making these determinations? I am looking for how
to determine what strength the material needs to be and how strong the
threads of the net need to be. The purpose is to better understand why
a net is used instead of an entire sheet of cloth as the reinforcement
and how it was determined.
3: Just as a side curiosity would the reverse of the above situation
use the same formulas/process (i.e. the envelope is draped over a
frame and some of the air was removed from inside the envelope).
And no this is not a homework question, I'm trying to build a model
airship and want to learn how to select the correct materials. |
Request for Question Clarification by
hedgie-ga
on
21 Sep 2003 02:20 PDT
myxlplix
This is a decent price for a question, which however had
kept researchers away so far, due to it's complexity.
It may be impossible to do it justice an any price.
The following can be attempted at this price:
1) Find formulas which determine tensile strength required to
contain
gas in the inflated sphere, at given pressure P , inflated Radius
R, unstrained radius R0
2) formula for strength of mesh+film composite membranes
This would be tensile strength of a composite, as dependent
on properties of the film, mesh fibers and structure (thread
spacing)
3) is not clear. What is possible is to derive equation for P(R)
Pressure as function of 'inflated radius, from properties of the
membrane.
Is that what you are interested in?
It would be a 'best effort' - I did not do the search yet, but expect
to find at least references to printed resources (are these of
interest?)
Do you want me research the three items as I understand them,
as rephrased above?
hedgie
|
Clarification of Question by
myxlplix-ga
on
21 Sep 2003 09:09 PDT
1) Find formulas which determine tensile strength required to
contain gas in the inflated sphere, at given pressure P , inflated
Radius
R, unstrained radius R0
The answer to this question would be useful but cannot be a best
effort because I do know the formula's are out there and I just need
someone to confirm I have the correct ones since this is not my area
of expertise.
On another note I have been told there is a formula to design hot
air balloon envelopes that keep the tension off of the membrane and
transfers it to the dozen or so load straps that come down the sides
of the balloon.
2) formula for strength of mesh+film composite membranes
This would be tensile strength of a composite, as dependent on
properties of the film, mesh fibers and structure (thread
spacing)
This would be very useful if explained well. If I am understanding
your statement I will be able to use the formula to calculate the
tensile strength of the composite strength of the envelope and will be
able to determine the strength of the thread needed for a "net". I
have recently read a note where a "net" placed over an envelope
reduces the necessary strength of the envelope film (effectively
making the radius of the envelope the distance between each thread of
the envelope but no formulas were given). Again this needs to be more
than just best guess because I know the formulas are out there.
3)just ignore item 3) I'll find a beginners level explaination of
Euler Buckling.
If I can get clear explanation for 1) and 2) then I will be happy to
tip well. The reason I did not offer more is because I am told this is
is basic stuff in "statics" classes (classes of which I never took).
Let me know if I can be of any more help if you decide to take on this
less than easy task.
|
Request for Question Clarification by
hedgie-ga
on
22 Sep 2003 21:56 PDT
Hi myxlplix
I had a cursory look on the web and decided to pass.
It is indeed basic engineering stuff, but lot of that
is still in books only, not on the web.
You probably know
Balloon HQ presents: Balloon science 101
www.balloonhq.com/faq/science.html
beyond that level, I would suggest books, usenet groups
engineering consultation services...
P.S. The formula you mentioned is supported by equations
on page 5 of this report:
... as follows: TS = calculated radial tensile strength TS = pd/2t
Where: p = burst pressure
d ... While the inflated balloon dilates the afflicted area, it can
also ...
www.advpoly.com/NewsData/BalloonPaper.pdf
Factor 2 in 2t depends on the shape of the baloon, of course.
Book referenced as [1] is apparently this one:
http://www.amazon.com/exec/obidos/tg/detail/-/0072486732/
hedgie
|
Clarification of Question by
myxlplix-ga
on
23 Sep 2003 19:49 PDT
If you know of a way that I could at least tip you for the effort and
given a lead I would be happy to do so.
|
