Bill,
Explanation appreciated. Especially since if I'd been paying more
attention I'd already know what you needed to accomplish here; I now
recall you explaining the four-piston payload retention system a while back.
So, if I understand correctly now, the burst disc is to release pressure
from the volume that's statically holding those pistons (and thus the
payload) in place? And you're looking at using a (dual-redundant)
nitrocellulose (smokeless powder, more or less) pyro to actuate some
sort of captive puncture-pin that would initiate tearing of the burst disc?
An electrically fired .22 CB cap in a ~3/4" barrel with the end in
contact with the burst disc just popped into my head. The small amount
of powder residue would be headed outward anyway, and would be further
swept from the system by the outrushing pressurant.
Yes, I know, a rocket that can shoot you may not be acceptable
safety-wise, but there is a certain symmetry to it.
Henry
On 10/25/2020 6:12 PM, William Claybaugh wrote:
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