Bill:
Generally as I understand it the "fins" of the Finocyl grain are at the nozzle
end because by their nature they increase the port cross-sectional area
compared to the cylindrical port. All of the "fin" "slots" add up to a lot of
port cross-sectional area.
So even though the Finocyl with a high Core Complexity Factor puts the core
complexity at the worst place where the erosive burning is worst, it also adds
port cross-sectional area where it helps lower erosive burning. You may lose
more propellant by having the cylindrical port last, it may have to be opened
up to raise the core cross sectional area on the nozzle end of the grain, to
reduce the core Mach number and core mass flux. This is probably why the
"fins" of the Finocyl grain are at the nozzle end.
Beyond that I don't see any fundamental reason why the "fins" couldn't be at
the head end of the grain. The Space Shuttle SRB's of course had a star grain
as the top grain in the motor. It added burning surface area during the early
portion of the burn to raise the early part of the thrust curve, when the star
had partially burned this caused a dip in the thrust curve, to go along with
the SSME throttle-back at max Q. Thus the SRB's also throttled back at max Q,
although on a pre-set schedule, i.e., the fixed (with temperature variations)
thrust curve of the motor. I believe the star grain at the front also helped
ignition.
Charles E. (Chuck) Rogers
CRogers168@xxxxxxx
-----Original Message-----
From: William Claybaugh <wclaybaugh2@xxxxxxxxx>
To: arocket <arocket@xxxxxxxxxxxxx>
Sent: Fri, Apr 14, 2017 7:20 am
Subject: [AR] Re: Erosive burning rule of thumb?
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
