This was the first test of this motor design. Motor is designed to be operating at a maximum of 612 PSI, a pressure which my other motors have no problem with. In this test, however, the nozzle blew out. Without the pressure gauge and video I would assume that the concrete nozzle was not sufficient to hold the expected 612 PSI of this motor. Conveniently, I had a pressure gauge connected to the motor which was able to show in the video that the nozzle was not the cause of the failure. May not be the best test stand in the world but it surely beats nothing!
The gauge has a maximum reading of 1,000 PSI with a number at every 200 psi graduation. If you watch the video you will see where the gauge passes 1,000 PSI!! The nozzle held well up to 1,000 psi so it surely wasn't the cause of the failure.
I'm now thinking that the grains were not inhibited properly. I used 3 layers of posterboard paper as inhibitor. Comments are welcomed.
Update:
It was explained to me by a senior sugpro member that my high pressure may not be just a problem with inhibition, but rather the fact that I'm using sucrose and dextrose and not purely dextrose as the reducer. My motor design was based on the assumption of dextrose fuel, but the sucrose+dextrose mix may have caused the motor to behave more like a sucrose only mixture, or at least an average of the two. This means higher pressures than expected.
If you are flying anything over an I-class motor, then you are probably already using dual deployment in your recovery system. The problem with dual deployment is that they (usually) use 2 chambers for each parachute - one for the drogue, another for the main.
That was a problem for me with the ‘Hurricane’ rocket build. I didn’t have the space for two parachute chambers, and adding a second one simulated at a much lower altitude. What was the solution? A tether release dual deployment setup.
A tether release setup utilizes one chamber for both drogue and main parachute. When the rocket reaches apogee, everything is ejected (nose, drogue, main chute). The main is however restrained and not allowed to inflate until the appropriate altitude is met during descent. Once the rocket reaches the correct altitude, the charge for the main parachute cuts the retaining cord and allows the chute to inflate.
Some folks have drilled bullet casings in their setup. They drill the casings right through, running the retaining cord (fish line) through it, fill the shell up with black powder, and then seal it. The basic idea is to have the charge cut a string that was holding your main chute from inflating.
In the ‘Hurricane’, i basically utilized a drinking straw as bullet casings is illegal to use in my country. The straw is cut to about an inch and pierced right through the middle with a hot needle sewing needle. A fishing line is then fed through the holes and an igniter placed on the inside of the straw. The straw is then filled with BP and sealed with hot glue or crazy glue/superglue.
Next, I rolled the chute like normal – keeping the shroud lines on the inside. The rolled parachute is then placed in computer paper for protection – this is where some guys use nomex to make what they call a burrito for the chute. The fish line with the pyro-straw wraps around the chute really tight and tied into a knot and will hold until the pyrostraw is fired. For extra protection, a piece of aluminum tape is placed under the straw so that it doesn’t burn through the paper and burn the chute. A small square of poster board can be used here as well.
The most important part of this setup is the tether. The tether was simply a nylon cord where one end connects to the fishing line, and the other connects to the shock cord. The purpose of this is to keep the ejection forces off the parachute itself and prevents it from breaking the lead wire. I made a small video ato demonstrate what i did.
