And, duh. It occurs to me you may want to release this pressurant
inside the rocket body to propel the payload away at some velocity as
soon as the four pistons have released? So just shooting a hole in the
burst disc probably isn't practical, as unlike what I'd envisioned, it's
not opening to the exterior. The rocket shooting itself is even less
acceptable. Oh well.
A small linear solenoid with the burst pin on the end of the shaft? My
limited experience with puncture-actuating thin pressurized burst discs
was that they don't require all that much impact if the tip hitting them
is both razor-sharp and such as to initiate a tear rather than a
circular puncture - in that case, a commercial hunting arrow tip.
Henry
On 10/25/2020 9:41 PM, Henry Vanderbilt wrote:
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