Not necessarily: this grain is a cylinder, a cone, and a finocyl, top to
bottom.
The cylinder maximizes volume, the finocyl offsets the progressivity of the
cylinder to produce a neutral burn overall.
Changing hardness might work against erosion but risks cracking.
Bill
On Sat, Apr 15, 2017 at 5:51 PM Wedge Oldham <wedgeoldham@xxxxxxxxx> wrote:
You mention sections? I thought a finocyl design was a single grain.
Sent from my iPhone
On Apr 14, 2017, at 7:19 AM, William Claybaugh <wclaybaugh2@xxxxxxxxx>
wrote:
Chuck:
Is there any reason the fins can't be at the fore end of the grain, thus
reducing the risk of erosion?
So doing would require some redesign of the core diameters in a way that
might lead to a more progressive grain (the current grain is neutral: the
cylinder's progression is exactly off-set by a regressive finocyl) but it
might could deal with any erosion issue in the finocyl if there is no other
reason for keeping the fins at the nozzle end.
Bill
On Wed, Apr 12, 2017 at 5:43 PM, Redacted sender crogers168 for DMARC <
dmarc-noreply@xxxxxxxxxxxxx> wrote:
<< I am using the port area / throat area greater than 1.36 criteria as
well as mass flow below 1.75 lbm/sec-sq. in. rule. >>
It's a great place to start. For Finocyl grains a great place to start
is the non-erosive Core Mach Number and non-erosive Core Mass Flux for a
cylindrical core, like you're doing, and then figure the Core Complexity
Factor for the Finocyl is going to make the erosive burning worse.
For the 2004 CSXT GoFast rocket we went a little aggressive on the Core
Mach Number and Core Mass Flux, again we were trading getting enough
propellant into the rocket to make it over 100 km without the motor failing
from erosive burning.
The tough part of course is that the Finocyl grain "fins" are at the end
of the grain, making the grain core complex at the end of the grain, right
where erosive burning will be worse. The cylindrical core front portion is
easier to calculate the core mass flux, and there is less (or no) erosive
burning up front in the motor core. The cylindrical core-based erosivity
design criteria are probably directly applicable to your conic core
section. It's the Finocyl "fin" section which is the issue.
What would be a great subscale test is Finocyl grains and cylindrical
core grains fired at the same Core Mach Number and same Core Mass Flux.
Cylindrical core grains can be used to develop the erosivity design
criteria, as I did, but it would be good to know that a Finocyl grain with
a Core Complexity Factor "X" would have erosive burning "Y%" higher than
the baseline cylindrical core grain.
Brute force may be the best way to go.
And congratulations on your 82% volumetric loading, that's a very good
volumetric loading.
Charles E. (Chuck) Rogers
CRogers168@xxxxxxx
-----Original Message-----
From: William Claybaugh <wclaybaugh2@xxxxxxxxx>
To: arocket <arocket@xxxxxxxxxxxxx>
Sent: Wed, Apr 12, 2017 4:18 pm
Subject: [AR] Re: Erosive burning rule of thumb?
Chuck:
I am using the port area / throat area greater than 1.36 criteria as well
as mass flow below 1.75 lbm/sec-sq. in. rule.
The issue is that I am working with a relatively complex core burning
grain that consists of a cylinder, a cone, and a finacyl where I could
really use a top-level rule of thumb to assist initial diameter and length
assignment for each section.
I have a design that meets the two criteria in each section and overall
but have little short of brut force to test whether it is the highest
loading that can be achieved w/o erosion. Will brut force the modeling if
necessary, but would love a rule of thumb that quickly eliminated designs
that are likely to erode.
Currently at 82% volumetric loading, btw.
Bill
On Wed, Apr 12, 2017 at 4:47 PM Redacted sender crogers168 for DMARC <
dmarc-noreply@xxxxxxxxxxxxx> wrote:
There's a technical article I authored on Erosive Burning Design Criteria
for Solid Rocket Motors which was published in High Power Rocketry Magazine
which can be downloaded from the RASAero web site from the Solid Rocket
Motor Technical Report Downloads page at:
http://www.rasaero.com/dl_solid_motor.htm
Scroll down to the bottom for the Erosive Burning Design Criteria for
High Power and Experimental/Amateur Solid Rocket Motors technical article.
There's a short technical summary available of the article, and the full
article for download.
The combined Core Mach Number/Core Mass Flux design criteria is what was
used to design the core of the 2004 CSXT GoFast rocket. The technical
article includes data for burn rate as a function of chamber pressure and
core mass flux for the Derek Deville D8 propellant which was used for the
2004 CSXT GoFast S motor. The core mass flux for the onset of erosive
burning can be seen in the D8 propellant burn rate experimental data.
Chuck Rogers
CRogers168@xxxxxxx
-----Original Message-----
From: William Claybaugh <wclaybaugh2@xxxxxxxxx>
To: arocket <arocket@xxxxxxxxxxxxx>
Sent: Wed, Apr 12, 2017 1:54 pm
Subject: [AR] Erosive burning rule of thumb?
Is there any experience-based rule for erosive burning in
cylinderical-type grains (Bates, Finocyl, etc.)? I am particularly looking
for experience as to how small the core diameter can be as a percentage of
grain diameter: 50% appears to assure no erosion at L/D of 10:1 or less,
what about 33%?
When does overall length begin to play? I know that around 15:1 will lead
to erosion with a 50% core diameter, for example. What is the "critical"
length at 33%? What about other core diameters?
Bill
