How about piercing something like a 12 or 16g CO2 cartridge for your pneumatic
drive? You can actuate that via pyro that’s completely isolated via a driving
piston? Use a suitable spring or compressible/breakable standoff to inhibit
premature piercing. Yes, your flow rate is limited through the pierced orifice,
but (provided it’s pyro actuated) the HPR vendors supplying CO2 recovery
devices use this technique to substitute pyros for complete rocket separation
for quite sizable (in some instances) rockets.
Troy
Troy
From: arocket-bounce@xxxxxxxxxxxxx [mailto:arocket-bounce@xxxxxxxxxxxxx] On ;
Behalf Of William Claybaugh
Sent: Monday, 26 October 2020 12:13 PM
To: arocket@xxxxxxxxxxxxx
Subject: [AR] Re: Burst disc thickness
Henry:
The pressurized air actuates four pistons that bind the payload to the rocket;
those pistons have to be released by releasing the pressure in order to
separate the payload.
It is in principle possible to have a closed pressure system with a burst disk
that is actuated by a pyro charge over pressuring a “calibrated” burst disk.
That is simpler than using a plunger to break the disk but means that the whole
system will be contaminated by the residuals; it also means that venting will
have to be overboard to avoid contamination of the internal volume of the
payload compartment. Finally, redundant charges are required to assure
operation which exacerbates all the above issues.
I am thus inclined to separate the hot actuating gas from the pressurizing
system.
Bill
On Sun, Oct 25, 2020 at 6:30 PM Henry Vanderbilt <hvanderbilt@xxxxxxxxxxxxxx
<mailto:hvanderbilt@xxxxxxxxxxxxxx> > wrote:
Bill,
Are you using the actuating gas for anything else on the vehicle? If not, as
long as you're thinking about using a pyro actuator anyway, might it be simpler
to modify the pyro device to generate the volume of gas you need directly, and
delete the cold gas supply? When possible, simplify. (Though I recognize that
there are any number of possible factors I may not be aware of rendering this
simplification impractical.)
Possible off-the-shelf source for such a precision-actuated pyro gas source
might be an auto airbag actuator.
Henry
On 10/25/2020 4:39 PM, William Claybaugh wrote:
Troy:
That’s an ad hoc genius idea.
My concern would be that we have two different burst failure problems: the low
pressure side had to fail toward low pressure, the high pressure side has to
fight the pressure on the other side of that disc. The charge required for the
latter is obviously greater than that required for the former. And in both
cases we are venting hot gas into our system....
Bill
On Sun, Oct 25, 2020 at 5:31 PM Troy Prideaux <troy@xxxxxxxxxxxxxxxxxxxxx
<mailto:troy@xxxxxxxxxxxxxxxxxxxxx> > wrote:
I helped a guy with PSANCP research for his master’s thesis years ago. He built
a pretty snazzy static testing rig c/w burst discs. He pretty much standardised
on cutting all the discs from Al-Alloy soda cans whose material specs are
apparently all within a pretty narrow tolerance. Being experimental PSANCPs and
being tested a across a broad spectrum of pressure ranges, the burst discs got
to experience multiple situations of “relieving”. He tested/characterised them
with various seat designs from sharp edge (sharp shearing of the BD) to large
radius that provided more of a typical burst. The sharp edge seats would
obviously fail at much lower pressures but provide a full open conduit when
they did fail.
Maybe 1 possibility might be use 2 burst discs with the pyro between them.
Arrange the BD seating so there’s a sharp shearing failure in the direction of
the pyro pressure (against the pressure source) so the failure point is both
lower in that direction, but also provides a full flow when open. This is one
of my ad-hoc/on the fly ideas without much thought put into it, so add
appropriate portions of salt.
Troy
From: arocket-bounce@xxxxxxxxxxxxx <mailto:arocket-bounce@xxxxxxxxxxxxx>
[mailto:arocket-bounce@xxxxxxxxxxxxx ;<mailto:arocket-bounce@xxxxxxxxxxxxx> ] On
Behalf Of William Claybaugh
Sent: Sunday, 25 October 2020 8:10 AM
To: arocket@xxxxxxxxxxxxx <mailto:arocket@xxxxxxxxxxxxx>
Subject: [AR] Re: Burst disc thickness
Steven:
Thanks, that conforms with my thinking.
I’m testing the initiators this next week: two forms of nitrocellulose (cotton
and cord) to get a working unit with an initial cut on the amount of gas
generated, I am assuming the cord will prove easier as it can be cut to a
desired length vs. the guncotton which will need to be weighed.
I’m having disks made in three thicknesses in accord with the circular flat
plate model and will test each to burst in the valve assembly once those are
finished.
Final testing will then use the initiators to punch through the disk under
pressure...the trick there will be to find an amount of actuating gas that does
the job with one charge working but does not over pressurize with two charges
igniting. (I’m using dual redundant initiation In accord with standard practice
for pyro actuated devices.)
Bill
On Sat, Oct 24, 2020 at 8:36 AM Steven Berg <skyshredder9488@xxxxxxxxx
<mailto:skyshredder9488@xxxxxxxxx> > wrote:
From my experience in using the flat plate model to design burst disks in a few
actuated systems, we used a circular flat plate to go into testing. The overall
thickness was typically all that needed to be varied. Adhesives and clamping in
the design were also the other first points of design review.
On Fri, Oct 23, 2020, 2:27 PM William Claybaugh <wclaybaugh2@xxxxxxxxx
<mailto:wclaybaugh2@xxxxxxxxx> > wrote:
Having inhaled deeply of the arcana of burst disc sizing; I find that the
entirety of the available math is about sizing the plumbing, not about design
of the burst disc.
For my current actuated burst disc design I have a 1.5" diameter disk that is
clamped at 1.25" diameter. Operating conditions are air at (probably something
better than) -50 degree F (cold soak on the way to 200 Km) and at an original
125 psia. Using the standard flat plate model I get this to be 0.050" thick
disc of 6061-T6 providing a safety factor of 1.43.
If I am going to manufacture my own disk (an assumption worthy of review), then
is it correct to just assume a circular flat plate for initial design, pending
testing?
I will be testing prototype initiators next week; for now I am using an e-match
with a very small bit of nitrocellulose to generate the pressuring gas. The
nitrocellulose is in both "guncotton" and cord form. I expect the former will
be faster but the latter possibly easier to control since cord length can be
directly measured.
All testing will be at a local energetic materials facility...
Bill