NEFAR launch, April 8, 2006

I was delighted to have a visit from  Jonathan Carter this week, culminating in an opportunity to launch at the Bunnell site on Saturday.  

Jonathan had built an absolutely beautiful 3-grain 54mm motor with an extra-long smoke grain well for use with candy propellant.  This is to allow smoke to apogee with a grain burning at the rate of 10 seconds per linear inch.  The motor is perfectly anodized, a bright fuschia color that almost matches my pink airframe.  Too bad you can't see the motor when it's installed.  

 

Here  Jonathan is preparing the ignitor for the first static test of his motor.  Click here for documentation and video of this test.

Jonathan  prepares "Like a Woman Scorned" for launch.  This rocket is very nicely made, and light for its size.  The dark marks are residue from black powder ejection, as the body is unpainted fiberglass, and thus translucent.  

Like a Woman Scorned launch video (17 meg .wmv file)

The Sugar Rush was fired with a 4-grain Loki motor using Pixy Stix rcandy.  It had  Jonathan's extra-long head end closure, with a 2.3 inch long rcandy smoke grain.

Sims indicated that it would fly to about 5100 feet, and it has flown to similar altitudes with this load before.  But the altimeter only read 2510 feet.  I assume altimeter error, it was apparent that the rocket went higher than that.

Sugar Rush launch video (13 meg .wmv file)

Some other anomalies

Note the titanium sparkles, and the short section of dark smoke.  I believe this is where the smoke grain was compromised and it's inhibitor failed.  The only Ti in this motor was in the smoke grain, and it was covered with a layer of KN/Epoxy/RIO to prevent it from igniting too quickly.  There should not have been any Ti burning at this low altitude, so I know the smoke grain went bad somehow.

I believe the dark smoke is the epoxy grain burning.  The case was damaged, the head end flared just past the c-clip.  Also the nozzle washer was distorted and the nozzle cracked around its base.  Clear signs of overpressure, fortunately not as severe as experienced at the February launch, although by a similar cause.  

Listening closely to the sound from  Jonathan's video, I hear a "clunk" sound 0.61 seconds after liftoff, corresponding to the peak indicated on the wave graph.  Looking at the video, I find some "curlycues" in the smoke trail which would have occurred at just about that instand.  They look to me like a disturbance in the exhaust outflow.  


(Blue area is thrust time)

Running the BatesCalc spreadsheet, I find that the core of the grains would have reached the diameter of the smoke grain at about the point that the clunk was recorded and the exhaust disturbance was pictured.  Upon disassembling the fired motor, I discovered the aluminum tube from the smoke grain pressed against the nozzle, and forming a slight indentation where it rested, largely blocking the nozzle throat.  An O-ring was still there on the Al tube at the nozzle end, badly burned but intact.  It was forming  "seal" just before the nozzle throat.  

From this I assume that the smoke grain came loose at liftoff or shortly thereafter, possibly from inertia as the rocket was accelerated at 14g's or so.  Dislodged from its safe spot, the lightly-inhibited head-end of the smoke grain would have ignited and the grain would have burned from both ends, thus rather quickly.  Smoke did not persist after propellant grain burnout, so it is assumed that the smoke grain burned along with the propellant.  As such, it would have contributed to thrust.  Minus, of course, any inefficiencies caused by the smoke grain being in the nozzle area.

The smoke grain would have stayed at the head end as long as it were larger than the core of the propellant grains, but once the cores reached the diameter of the smoke grain tube, it would have been free to roam.  

I am surprised that this motor did not CATO.  It came close, but apparently there was enough leakage around the smoke grain tube (or through it) that pressure did not burst the casing.  

Well back to the drawing board.  Guess I could use a whole bunch of O-rings to secure the smoke grain, and just might try that next time.  Or perhaps cut a groove in the long smoke grain well and install a snap ring to provide positive retention.  

More likely, I'll give up being an apologist* and go back to Plan A.  That plan was to load the 4-grain case with 3 propellant grains, and use the remaining space for a large smoke grain.  Or maybe I really need a 5-grain case, fix the altimeter, and join the mile-high club...

* obessed with the logistics of apogee.

Flash forward to January 2007.  I sent the head end closure to Jonathan, who cut a snap-ring groove in it.  Since then I've been placing a washer over the smoke grain, then a snap-ring to lock it in much the way the nozzle is secured at the other end.  This works fine.  Smoke grains have stayed in place for all of my 11 launches so far this year (5/07).

Jimmy Yawn
www.jamesyawn.com
jyawn@sfcc.net
4/14/06