Depending on the altitude/pressure the system is working at, it may be worth
skipping the pyro altogether and using mechanical striker on a spring, similar
to a Glock. It doesn’t take that much force to puncture a CO2 cartridge, and
you may be able to use actual Glock parts for it, which are cheap as dirt and
commonly available.
It’s mechanically well-trodden ground, and one great thing about it is making a
drop-safe hammer/trigger system is well known, and that can prevent an
accidental discharge.
On Oct 25, 2020, at 8:22 PM, Troy Prideaux <troy@xxxxxxxxxxxxxxxxxxxxx> wrote:
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