All;
Thanks for these thoughtful comments,
I did look at HPR CO2 systems but was not able to make the pressure /
volume work out: even the biggest cylinders don’t add enough pressure to
the system volume to burst the minimum burst disc. Also, the one I have on
hand vents hot gas to the payload compartment, which is a no no.
(The minimum burst disc thickness is driven by worst case 50g
accelerations; that means a nominal 125 psia system pressure requires at
least 200 psia to burst. I’d prefer going a little higher (250 psia at
burst) to assure the disc doesn’t become a failure point.
I take Henry’s point—so to say—about using a four barb arrowhead for
piercing. I’m scoring the burst discs in that pattern to get them to tear
along the score lines. Depth of score TBD by testing after the valve is
finished.
I’m not particularly weight constrained (I may need to carry ballast to
keep the upper stage stable at maximum velocity.) but I am given to
avoiding complexity. The current plan has one moving part and I’m not much
inclined to add any more.
I’m am aware of pressure issues with NC; that part of the valve is sealed
at atmospheric pressure.
Bill
On Sun, Oct 25, 2020 at 10:54 PM Henry Vanderbilt <
hvanderbilt@xxxxxxxxxxxxxx> wrote:
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> 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>
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] *On Behalf Of *William Claybaugh
*Sent:* Sunday, 25 October 2020 8:10 AM
*To:* 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>
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>
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