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
