Big Sugar
Earlier this year, Richard Creamer proposed building a large motor using 4 inch diameter PVC pipe.  He formed a nozzle from "Rock-Ite," a concrete patching cement, with a one inch throat diameter.  I made propellant grains.  It would burn two Bates grains, each 5 inches long by 3.5 inch diameter with a 1.25 inch core diameter.  Rich, Steve Ghioto and I static-tested it at my place.  Since we didn't have a good way to attach it to a test stand, we dug a hole and fired it in the ground.  The test was a success!  It didn't blow up!  It made a loud thrusty sound and spewed a lot of smoke into the air.  No data, but it was loud!

There are four PVC parts to this motor casing.  A section of 4-inch Schedule 40 tube is cut 11 inches long.  A cap is placed on one end, a 4 inch to 2 inch reducer on the other, and a 2 inch to 1-1/4 inch reducer placed in that.  

These are just the parts, assembled temporarily for photographic purposes.  It does not yet have the graphite throat, set in Rock-Ite.  It is also missing a ring of PVC that will be placed around the nozzle end which serves as a thrust ring.  

The first motor had a nozzle made entirely of Rock-Ite.  It worked, but eroded quite a bit.  So for subsequent tests, Steve turned a nozzle throat of graphite and embedded it in the Rock-Ite.

Static Test 2

We tested it again at the NEFAR amateur launch on July 9th.  This time Steve had rigged up a mount to put it on the bath scale test stand so as to record the data using his audio/digital recorder.  This clever device records voltages as frequencies and stores them for later playback, capture by computer, and analysis.  While Steve did some other tests, I went about gluing the grains into the PVC motor.  As often happens when I am faced with a simple but critical task, I became an idiot.  The PVC glue had dried up and was very thick.  Instead of bravely cancelling the test or wisely going to a hardware store and getting a new can, I made do by slathering on a lot of it and mashing the coupling together best I could.

Upon ignition, the motor spat its nozzle unit high into the air and dumped the propellant grains on the ground near the launcher.  Burning grains roasted some of Steve's electronics, and Bill Baldwin lost a shoe sole stamping out the burning rubbish.  It was one of the finest smoke bombs I have ever made.  

If you can stand a 9 meg download for two minutes of badly-shot video, click here for the comedy act.  Lotsa feet, no faces.  

Static Test 3
 So a third test was called for.  On August 7th, we fired it again at my place.  This time we splurged and bought a brand-new bottle of PVC cement.   Nothing is too good for this motor!   Not yet aware that the bath scale had suffered damage, in the 7/9 CATO, we mounted it, turned on all our recording devices and fired.  It burned nicely, and the thrust was recorded.  After a few scary spikes, the motor settled into a beautiful Bates curve, thrust averaging 176 lbf for 3.28 seconds.  Total thrust of 2567 N-seconds places it at the low end of the "L" category, and suggests that it might loft an airframe to some altitude.  In the video, one can see the motor shifting slightly as thrust starts.  I am convinced that the spikes are artifacts, that the motor was "settling in" to the test stand, causing some bounce.

Flight Test 1

After our successful static test, Richard set about making an airframe to work with this motor, Steve made another motor casing, and I made the propellant grains.  

Mindful of the incendiary nature of burning metals, but desperately wanting to see some of the Ti mill chip in this flight, I made one grain a Ti delay type.  200g or propellant is mixed with 5g mill flake, and pressed into a crescent in the epoxy-coated inhibitor tube.  

Once the propellant is cool and the epoxy hard, I coat the remainder of the tube with more epoxy, place it on my core-centering jig, and pack in more propellant until full.  

Each grain contains 1150 grams of propellant.  Core diameter is 1.25 inches, length 5 inches.  Inhibitor/casting tube is made from a 28 inch-long strip of posterboard, sprayed with Scotch Super 77 spray adhesive and wrapped around a section of 3-inch Sch 40 pvc pipe, which is 3.5 inch OD.  I make these tubes a little longer than needed, then cut them to proper length on the chop-saw.

As soon as the grain is cooled, I wrap it all over with Nashua 324 aluminum foil duct tape, cover then ends, and press it down well all over to seal any leaks.  When well-sealed, this tape allows this hygroscopic propellant to keep indefinately.  Except for the little bit of Ti in one grain, this motor will be identical to the one we static tested on 8/7/05 - please see that page for details.


Loading the Motor Casing

Steve and I arrived first and began assembling the stuff.  Rick was not certain he would even be able to attend, and had most generously given Steve the airframe to assemble, fly, and report back.  Fortunately, Rick was able to join us before the launch, help to get it all together, and operate the ejection controller.  

The foil tape on the ends has been removed to expose the propellant.  The grains are wrapped with three strips of poster board, to serve as a case liner.  Each strip is 28 inches long.  Since the grains are 3.75 inches outside diameter, that would make for ...  (hmmm.  where is my calculator? Oh, there's one....)  it would make about 7 layers of posterboard between the grains and the casing.  This seems to be enough:  the casing has not been seriously compromised in any of our static tests.  (In fact, the same casing was used in all three static tests, and if there were room for another coupler, we could probably use it a fourth time.)



   

Richard's airframe is made from 6 inch diameter paper-phenolic tube with plywood fins and a plastic nose cone.  Richard also built the ejection mechanism using remote-control aircraft electronics.  One channel is set to fire the drogue at apogee, another to fire the main at lower altitude.  He wisely programmed-in a "save the bacon" routine so that both ejection charges would fire if the receiver lost contact with the transmitter for five seconds.

After struggling with the launch lugs and a rod that wouldn't quite go in, Greg Peebles saved the day by offering us a couple of rail buttons.  We gratefully accepted.  They went on easily and worked very well.  I have decided that I like rail buttons, and will order a bunch of them soon..

Here the assembled motor, ready for insertion into the airframe.  Duct tape is used in the classic manner to make it fit the airframe tube tightly.  Some small screws will be inserted through the airframe body tube into the forward PVC cap to ensure motor retention.  We don't want this thing falling on anyone's head from 4800 feet, do we?  

The ring toward the nozzle is a cut section of PVC coupling, split, and attached with screws.  This is the thrust ring, which prevents the motor from moving forward in the airframe.  That gap in the ring is fortunate.  When we first tried to mount the rocket on the launch rail, the retaining ring got in the way.  But we rotated the motor so that the gap aligned with the launch rail, and it fit perfectly.  You'da thought it was designed that way.  

The motor is fitted with a smallish fuse-paper ignitor contianing some fine Ti flake.  

The Flight 

Launched at NEFAR - Bunnell Blast 10/8/05

Richard Creamer, airframe and electronics
Steve Ghioto, motor casing
Jimmy Yawn, propellant grains

Click Here for 5 meg, 30 second video, .wmv format.

Upon ignition, there was a pop, a puff, and a long pause......   More smoke starts billowing, then it's on it's way!  

Good acceleration off the pad... nice, long burn... ejection a little early.  Rocket lost its main parachute upon deployment, perhaps because of high airspeed.

There is an interesting anomaly.  Notice how long the motor burns.  Looking at the video, the time between liftoff and last visible smoke is 6.3 seconds.  Burn time in our static test was 3.3 seconds.  



It's falling pretty fast.  Notice that the hollow plastic nose cone is up there cavorting with the drogue.  

This is the picture from which the above clip was clipped.  



I strongly suspect that the large cloud sucked up our rocket and spat it into the palm tree.  I felt a few drops.  



Thar it is.  Dangling by the shock cord.  I know the feeling well.  Nose cone and upper body are hanging over the top of the palm.


Rich, Steve, and Stephen Kiss assemble a 30' foot pole of EMT tubing to try to knock the rocket down.  Didn't quite work.  Eventually, we clamped a pocket knife to the pole and cut the shock cord to get the lower section down.  The upper section and nose cone remained comfortably seated in the palm of the palm.

So I went back on Sunday with an extension ladder, a pole saw, and a bunch of rope.  There's hardly anything you can't do if you have enough rope.  Who invented rope anyway?  Best thing since Socrates invented the sock, and Plato invented the plate.  Those old guys sure were smart.
 
One thing rope is good for is tying the top of the ladder up in the tree so it doesn't slip off. And for tying myself up in the tree so I don't have to be competent to stay alive.

After some hacking and whacking, the upper section is down along with a few parts of palm.  Nose cone is already down on the other side of the tree.  Now we can all go home.

So Big Sugar lives to fly again!  Current plan is to launch it at the November NEFAR launch, this time with a camcorder in it.

Wish us luck!  

Jimmy Yawn
jyawn@sfcc.net
Recrystallized Rocketry