An approach I've wondered about is stacking burst disks. Have a burst
disk rated to pressure X for a tank pressure ~X*1.5. Two burst disks
are in series. You pressurize the tank to 0.8x, then pressurize the
space between the disks to 0.75x, then pressurize the tank to 1.5x.
Initiation is to dump the pressure from between the burst disks by
whatever approach is appealing, or let gas from the tank around the
first disk. The burst disk can't hold the full pressure so it bursts
them in series.
On Tue, Oct 27, 2020 at 12:49 AM 1bcjolly <1bcjolly@xxxxxxxxxxxxx> wrote:
Bill,
Any way a hot gas valve would work in your application? I'm thinking along
the lines of using a pressure regulator in the place of a burst disc. You
would be able to set the desired pressure, hook the nitrocellulose cartridge
to the input of the regulator, when the input pressure exceeded the set
pressure you would have gas available to do your work. Might not need more
than a simple regulator if only a one shot device is necessary. Just a
thought. Sorry if I have wasted your time.
Barry
Sent from my Sprint Tablet.
-------- Original message --------
From: Kevin Ho <rebelwithoutajob@xxxxxxxxxxx>
Date: 10/26/20 16:56 (GMT-05:00)
To: arocket@xxxxxxxxxxxxx
Subject: [AR] Re: Burst disc thickness
The benefit of a striker or hammer design vs direct pressure by the actuator
is significantly greater pressure at the point of impact (thus reliabiity)
and a much smaller actuator and battery. The cost is some greater complexity.
If you want to generate a puncture pressure with an actuator, you need an
actuator that can deliver that pressure, which may be reasonably large.
The lighter solution is for a much smaller actuator to push back and then
release a spring loaded mass, which then falls forward and strikes/punctures
the CO2 cylinder. Now you get the benefit of all the energy stored during the
entire stroke of the actuator.
A further benefit is the ability to easily add a mechanical safety, like a
transfer bar. You can use the motion of the actuator to raise a transfer bar,
so that unless the actuator is stroked, the hammer/striker hits nothing but
air. That would prevent inadvertent separation.
https://www.youtube.com/watch?v=S9ixC34PaKk
The equivalent rocket system is obviously much simpler, as it only has to go
bang once.
On Oct 25, 2020, at 10:40 PM, Troy Prideaux <troy@xxxxxxxxxxxxxxxxxxxxx>
wrote:
I’m not familiar with what mechanical systems there are out there. Of course
that would be a preferred solution to pyro *if* there wasn’t too much
mass/volume penalty to drive the puncture. Typically (for HPR separation
systems) all-mechanical puncture systems have been developed (maybe based on
aircraft lifejacket inflation systems?), but rely on clever design to achieve
the required force to achieve the required puncture orifice area for *that*
application. Apparently the force required to puncture out a 0.09” hole in a
16g CO2 canister is about 100lbs – remember the sealing diaphragm needs to be
strong enough to hold… well… probably upwards of 1200psi for really hot
conditions.
Now, I did emphasize “that” application which requires a fair bit of
throughput capacity as it’s both acting on a large volume and needs to
compensate (to some degrees) for leakage within the mating pieces, vent holes
and whatever else.
Bill’s application, on the other hand, is likely specifically driving
pistons with much more confined volumes and can probably expect no leakage to
compensate for, which does allow a much smaller orifice/puncture as driving
pressure is probably more important than pneumatic power.
Troy
From: arocket-bounce@xxxxxxxxxxxxx [mailto:arocket-bounce@xxxxxxxxxxxxx] On ;
Behalf Of Kevin Ho
Sent: Monday, 26 October 2020 2:59 PM
To: arocket@xxxxxxxxxxxxx
Subject: [AR] Re: Burst disc thickness
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>
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