Tad, your density calculation is nearly correct, but not quite. You forgot
to square the 2.54 value (core diameter in centimetres). The correct volume
of your motor is 463 cu.cm. As such, your propellant density is 1.74 g/cu.cm,
or 1.74/1.84 = 0.945 or 94.5% of theoretical. Now, a couple of things. This
assumes that there is no water lost during the cooking process. If there is
water lost, the theoretical density will be higher than 1.84 g/cc. Also, is
the 806 grams the amount of propellant prepared, or the actual amount in
the motor? Some propellant is always left in the pot and this amount can be
significant when it comes to the density check.
As someone suggested, you should determine the true density of your
propellant by casting a slug of known volume and mass.
Richard
On Fri, Nov 20, 2020 at 10:37 PM Redacted sender tadserralt for DMARC <
dmarc-noreply@xxxxxxxxxxxxx> wrote:
Ok, so it is further confirmed. Assuming I've done my calculations
correctly, the density of the J-Class motor was way off. 78% from the
ideal. Compared to the K-Class I successfully fired at 90% from ideal. I
hate to admit it but I just learned how to measure density. If you would
be so kind, please verify my math:
-Flexi-fuel (65%KNO3, 18%Sucrose(powdered), 17% Corn syrup):
0.65/2.109 + 0.18/1.5805 + 0.17/1.4 = .543518
1/.543518 = 1.839865 g/cm3 (Ideal Density of Flexi-fuel)
-12 inch long 2 inch diameter 1 inch core with total propellant mass of
806 grams:
3.14159/4 x (25.806cm - 2.54cm) x 30.48cm = 556.96281 Total Volume
806/556.96281 = 1.4471 g/cm3 (Actual Density of propellant in question)
That's ONLY 78.6% of the ideal.
I performed the same calculation for the K-Class 2"OD, 1"core, 21 inch
length and propellant mass of 1,623 grams and came up with a measured
density of 1.66515 g/cm3 which equates to 90% from ideal.
Did I do my math correctly??
If I did, then clearly, there must have been voids in the J-class motor
which caused a spike in thrust which caused the delay element to shoot
out. In hindsight, I'm thankful the delay element was there and not just
an end cap; because then, it would've definitely cato'd.
Lesson learned. Check density.
Cheers,
Tad
-----Original Message-----
From: Richard Nakka <richard.rocketry@xxxxxxxxx>
To: sugpro@xxxxxxxxxxxxx
Sent: Fri, Nov 20, 2020 11:34 am
Subject: [sugpro] Re: Near cato...what happened?
Tad,
I have not yet looked at the video, however, I do not think it was any
sort of nozzle blockage. You are correct in pointing out that the increased
thrust level would not have developed if there was a blockage.
I suspect a flaw in the grain, either a large void or possible disbonding
at the casing/propellant interface. Being flexifuel, I gather that the
latter is less likely.
Did you do a check of the grain density? This is an excellent means of
determining if your grain is 'sound'. If there is a void, it'll reveal
itself in a lower than expected grain density. Calculating the density is
not hard, you just need to know the empty and full weight of the motor and
measure the grain volume. I would never fire *any* motor without doing
this key 'quality' check.
Richard
On Fri, Nov 20, 2020 at 9:08 AM Redacted sender tadserralt for DMARC <
dmarc-noreply@xxxxxxxxxxxxx> wrote:
Hello everyone,
Below is a link to a video of the latest static test on a J-Class 2"
Diameter, 1" core, 12" single grain pvc motor with a .5625" nozzle. Kn
initial 171 and KN Max and Final at 278. I've had very good success with
this progressive single grain burn using Pollino's Flexi-fuel formula 65/18/17
KNO3/Su/Corn syrup. I've had nominal results with multiple 15 inch 1.25"
diameter single grain I-class motors and a recent successful 24 inch 2"
diameter 1" core single 24 inch grain K-Class PVC motor with a .8125"
nozzle (Kn initial: 155 Kn Max and Final: 279).
https://www.youtube.com/watch?v=h9SMcbwAi5U&feature=youtu.be
So what the hell happened here? This was half the size of the successful
K-Class and I maintained same formulation and a proper Kn profile. It
didn't cato. It just blew the forward closure containing the delay element
and burned right thru a 2 inch wood plank and toasted my scale. All in
less than .4 seconds. I considered Overpressurization due to a blockage
caused by the epoxy plug I used in the igniter on one side in order to
force the initial burn down the core. But even then, I would think that
over-pressurization due to some blockage would obviously increase the
pressure inside but not increase the thrust; and yet notice the pounds of
thrust. My K-Class had a max thrust of 256 lbs. This J-Class went to more
than 340 lbs before it scorched the scale!! And all of this in less than
.4 seconds!! Can a blockage do that? Or maybe a partial blockage causing
smaller throat diameter? Cracked grain increasing burn rate? I doubt it
because that's the whole point of flexi-fuel: very viscous which allows a
single grain pour. What exactly happened? I really would like to know
before proceeding any further. Hoping the footage helps in figuring it
out. Thank you for your kind attention.
Tad
