I'm wondering if pressure is going to cause air voids to migrate laterally and
get pushed through a semipermiable casting tube, wouldn't it also be pushed up
along a nonpermiable casting tube and motor case especially if the propellant
doesn't like sticking to the case material? And if this migration takes place,
you would think this increases disbonding of the propellant from the motor case
since air is being forced between the two.
Rick
On Monday, November 23, 2020, 09:25:06 AM PST, Richard Nakka
<richard.rocketry@xxxxxxxxx> wrote:
You are correct, using compression may not help you much as your casting tube
is PVC.
A surge in thrust as you experienced is definitely consistent with a burning
area greater than design. This could be a result of a crack, void(s) or
disbonding. Another possibility is burn rate. You mentioned that you used a
modified formulation. Is it possible that this modified formulation has a
greater burn rate under pressure? This could also explain the higher than
expected pressure. And might this modified formulation be less flexible, such
that disbonding may have occurred? By this I mean that the propellant may not
adhere as well to the casing walls, and under pressure, as the PVC casing
expands, a tiny gap may have opened up between the grain and the casing walls.
Richard
On Mon, Nov 23, 2020 at 10:16 AM Redacted sender tadserralt for DMARC
<dmarc-noreply@xxxxxxxxxxxxx> wrote:
Oh, dang. Forgot to square it. Yep, Thanks Richard.
To answer your question, the 806 was the actual difference between the empty
and full motor weight.
As suggested, I will cast a slug of propellant with known weight and volume and
determine the true density of my propellant.
Curing under pressure will help in my bates grain setup where I use porous
casting tubes. But for this particular setup and based on what you discussed
with Rick, It won't help since it is a single grain pour into an impervious
material like pvc. Correct?
And lastly, based on the info we have now with my density of this static test,
do we still believe the surge in thrust and the compromise of the delay grain
was caused by a void/crack in the grain?
Thanks,Tad
-----Original Message-----
From: Richard Nakka <richard.rocketry@xxxxxxxxx>
To: sugpro@xxxxxxxxxxxxx
Sent: Mon, Nov 23, 2020 10:27 am
Subject: [sugpro] Re: Near cato...what happened?
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
