Clear PVC Rocket Motors

Using 1-1/4th inch SCH 40 clear PVC tubing
From McMaster-Carr, stock # 49035K86
http://www.mcmaster.com/#49035K86

Several years ago one Bill Baldwin gave me a DVD promoting HyperTek hybrid motors.  It was a video demonstrating how they work.  
The demos used a clear motor, allowing us to see the the ignition system, etc.  inside the motor.  
A firing was done with this clear motor.  The burn was visible, and dramatic.  
So I knew it could be done.  At least with a hybrid.  

Searching the web, I found that PVC was available in a clear version, but I was discouraged by the price.  
Actually, the tube itself isn't too bad, about $3. per linear foot.  
But the clear PVC fittings are exhorbitant.  A single end-cap costs $15.00!  

Good news from the McMaster-Carr website:   This tubing is compatible with standard white PVC fittings.  
So I'm using these $0.50 end caps from Lowe's, and adding wooden spacers to keep the propellant grains in the clear area.

Spacers are 1-1/8th inch hardwood dowel.  Solid chunks at the head end, drilled-out sections at the nozzle end to let exhaust gasses get by.

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Test 1:   Uninhibited grain

Click for Video of this test (3 camera views, NTSC) Click for high-def camera view, 1/10th speed  (27 megs)

Test 1 7/2/11A Uninhibited grain Len:  3.1 inch OD:  1.226 inch Core:  0.5 inch Mass:  84.2g Nozzle, steel:  .3125 inch throat diameter Kn initial (and maximum):  245  est. pressure 850psi Kn final:  195    est. pressure 600 psi Burn time:  0.57 second

 

The fired casing appears opaque when front-lighted.  But put a bulb behind it and some of the light comes through.

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Test 2:   2 BATES grains, plain rcandy

Click for Video of this test Click for high-def 1/10th speed camera view (17 megs)

Test 2 7/3/11A 2 BATES grains, plain rcandy Len:  2.13 inch OD:  1.274 inch Core:  0.5 inch Total propellant mass:  125g Nozzle, steel:  .3125 inch throat diameter Kn initial:  141   est. pressure 350psi Kn max:  158   est pressure 425 psi Kn final:  138    est. pressure 350 psi Burn time:  1.13 second

Inhibitor is 3 coats 5-minute epoxy.  I want it to be clear so the grains will remain translucent.  

      
Apparently, it held up OK.  A bit charred, but somewhat translucent.  

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Test 3:   2 BATES grains with a little Ti

Click for Video of this test Click for high-def 1/10th speed camera view (18 megs)

Test 3 7/4/11B 2 BATES grains, rcandy with 0.33% fine Ti flake Grain Len:  2.1 inch OD:  1.274 inch (includes epoxy coat, 3 layers) Core:  0.49 inch Total propellant mass:  125g Nozzle, steel:  .3125 inch throat diameter Kn initial:  141   est. pressure 350psi Kn max:  158   est pressure 425 psi Kn final:  138    est. pressure 350 psi Burn time:  0.87 second

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Test 4:  Moonburner grain with a little Ti


Perhaps more of a C-slot, considering that the "moon" is already a crescent.  

Click for Video of this test Click for high-def 1/10th speed camera view (20 megs)

Test 4 7/6/11A One moonburner grain, rcandy with 0.33% fine Ti flake Grain Len:  4.261 inch OD:  1.319 inch (includes epoxy coat, 3 layers) Core:  0.501 inch Total propellant mass:  119g Nozzle, non-virgin RCS 1550:  .305 inch throat diameter Kn initial:    123      Kn Max:  123 Max   est. pressure 300psi Burn time:  1.43 seconds

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Test 5:  3 BATES grains with RIO


Use of RIO is to make the grains opaque.  It also happens to make them burn faster.  

New method of cutting grains:
 
OK, so maybe that wasn't the best idea I've ever had.  But of the three cuts I've made so far, two were pretty good.
It would help if the grains were perfetly round and smooth, but the epoxy coat is kinda lumpy.
It might help even more if I went about it slowly and carefully, but I didn't this time.  
The Snaggletooth grain was evened-up some before loading.  

The wooden spacer at the nozzle end has a 5/8ths inch hole drilled through its length to let the exhaust gasses through.  

Click for Video of this test Click for high-def 1/10th speed camera view (30 megs)

Test 5 7/12/11B 3 BATES grains containing 0.5% Red Iron Oxide (RIO) Grain Lenth average:   2.21 inch OD:  1.335 inch (includes epoxy coat, 3 layers) Core:  0.516 inch Total propellant mass:  200g Nozzle, non-virgin RCS 1550, drilled to .375 inch throat diameter Kn initial:  158    Kn Max:  177    Kn Final:  159         Max est. pressure 500psi Burn time:  1.27 seconds

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Test 6:   3 BATES grains with RIO and posterboard inhibitor


Propellant is from same batch as Test 5, containing RIO to make the grains opaque.   Plus, these grains are inhibited with 3 layers of posterboard, making them even more opaque, and possibly masking some of the burn between the grains.  

Grains were cut with tubing cutter, and came out pretty good.  The posterboard inhibitor helped, and going slow and easy helped even more.  

I started to put spacers between the grains, but then noticed that they were a snug fit in the tube.  Thus they stayed in place all by themselves, with a gap between the grains.  

The wooden dowel spacer at the nozzle end has a 7/8ths inch hole drilled its length to let exhaust gasses through.  

Click for Video of this test Click for high-def 1/10th speed camera view (23 megs)

Test 6 7/16/11A 3 BATES grains containing 0.5% Red Iron Oxide (RIO) Inhibitor:  3 layers posterboard, adhered with epoxy. Grain Lenth average:   2.32 inch OD:  1.251 inch - propellant only, not including inhibitor Core:  0.55 inch Total propellant mass:  194g Nozzle, non-virgin RCS 1550, drilled to .375 inch throat diameter Kn initial:  168    Kn Max:  189    Kn Final:  174         Max est. pressure 550psi Burn time:  1.00 second

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Test 7:   2 BATES grains, green AP propellant


This is a wild and perhaps unwise experiment.  Good thing I have some sturdy trees to hide behind.  Propellant is ancient.  After Tom Henderson did one of his excellent Thunderflame workshops at my place in 2007, I made a half-batch of the green AP mix from one of his kits.  This was the first AP propellant I had made outside of class.  It was made without the aid of a KitchenAid, and had sat around for about 4 years with little protection.  It was no longer dark grey on the outside, but a pinkish ivory color.  It did not look happy.    

But I cut into the stick, and the inside seemed OK.  It burned continuously when lit with a torch, so I cut two grains and drilled a 3/8ths inch core in them.  

The blue stuff between the grains is fuse paper.  Since the propellant required a bit of roasting with a torch to get it going, I thought some accessory pyrogen might be called for.   I made a little mistake, cutting the head-end wooden spacer a bit too long.  Being ADHD-compliant, I just squashed it all together tightly, seating the head-end cap well but compressing the grains end-to-end and eliminating much of the airspace between them.  

First ignitor did not set fire to the propellant.  A bit of the fuse paper burned, but to no avail.

Second ignitor had some Mg flakes in it.  The motor started and burned for a few seconds and went out.  I weighed the motor - about 12 grams of propellant had been consumed in this mini-burn.

Third ignitor was supplemented with a wad of fuse paper containing a mix of KNO3 and Mg flakes, rammed at the head end of the propellant column.  Because of this wad, the ignitor only went in as far as the juncture of the two grains.  You can see in the video that ignition starts in the middle and progresses forward.  

This igniter worked.  After a strong burn of about 2 seconds, the motor went into hibernation for a couple of seconds then started chuffing.  It chuffed like crazy for about 7 seconds then mercifully went out.  

My first guess about the chuffing is that the propellant had degraded on the outside, where it would have been exposed to air and moisture over the years.  The propellant was dark gray in the middle of the grain, whitish toward the outside.  

Click for Video of this test  (4 megs) Click for high-def 1/10th speed camera view (7 megs)

Test 7 7/17/11A 2 BATES grains, Thunderflame green propellant made in 2007 Inhibitor:  spiral cardboard tubing Grain Lenth average:   2.11 inch OD:  1.26 inch, including inhibitor Core:  0.375 inch Total grain weight:  132.2g Total propellant mass:  130g, approximately Nozzle, new RCS 1550L   0.290 diameter throat Kn initial:  144         Kn Max:  173      Kn final:  148 Burn time:  2.63 seconds (good burn only, not the chuffing part)

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Test 8:  3 BATES grains with RIO
New high-speed camera!



So I bit the hook of high-speed video and ordered a Casio FH100.  It arrived today.  This camera can record at a remarkable rate of 1000 frames per second, but at dismal resolution.  It can record at 420 fps at poor resolution, 240 fps at fairly bad resolution... up to 1280x720 at the usual 30fps.  
Audio is NOT recorded with the high speed movies, but that sets me free to do wacky stuff with music and sound effects.  Prepare to be annoyed!  

Here is my very first high-speed movie! (2.5 megs, 12 seconds, 420fps) ... and an even more dramatic shot, starring a famous, incredibly handsome actor.  You might recognize him.  (2.2megs, 21 seconds, 420 fps) I recorded the burn of a fuse-paper igniter at 1000fps  Low resolution, but lots of flame.  Looks like a special effect for a cheap war movie.  (.9 meg, 28 seconds, 1000fps) And here is a slug of propellant burning upright.  It contains fine Ti flake, makes a nice little display.   (31 megs, 46 seconds, 120 fps)

 
Again cutting the grains with the same tubing cutter I use for the PVC, this time with the coring rod still in place.  It works!  The grain ends are slightly uneven, and one of them had a few flakes fall off.  But they are perfectly usable, if not beautiful.  The long grain has 3 heavy coats of 5-minute epoxy on it to serve as an inhibitor.  

The high speed movie of the motor burn is a bit disappointing, but I think I can correct the worst errors next time by framing the shot better (It should have shown a little of the exhaust plume for reference) and providing better light so you can see something other than the grains burning.  This camera is supposed to work very well in low light - perhaps I did not have it set up right.  Maybe I'll even read the manual!

But you can see the grain end faces reced from the O-rings I placed between the grains.  Also I note that the aft grain burned out first, followed by the middle grain, then the head end grain.  Perhaps there is a mild "erosive burning" effect here?  Otherwise, I would expect the head end grain to burn out first because the pressure is a bit higher there... right?  

Click for Video of this test Click for high-speed camera view, 420fps, 320x240 resolution (6 megs)

Test 8 7/23/11B 3 BATES grains containing 1% Red Iron Oxide (RIO) Grain Lenth average:   2.23 inch OD:  1.26 inch (includes epoxy coat, 3 layers) Core:  0.527 inch Total propellant mass:  171.4g Nozzle, non-virgin RCS 1550, drilled to .375 inch throat diameter Kn initial:  155    Kn Max:  176    Kn Final:  163       Max est. pressure 500psi Burn time:  1.43 seconds

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Test 9:  3 BATES grains with RIO
Another try with the new high-speed camera.



Last night's high speed movie was very dark.  I decided to try it again at dusk, so there would be some ambient light but not enough to mask the propellant burn.  

No good.  This movie is even darker.  Plus, I had spaced out and forgotten that high speed modes crop the image, so I framed it wrong and inadvertently chopped off the head end grain and the nozzle.  Guess I need to pay some more dues with this camera.  

But I am getting better at cutting grains with a tubing cutter.  These came out pretty good.  Here is a video of one being cut.  (2.5 megs, 19 seconds, boring.)

Note that the grains are spaced out in the casing, with a gap of about 3/8ths inch between each grain.  They are held in place by untidy globs of epoxy.  I'm hoping that this space will allow recording of the flow of gasses between the grains.  

Click for Video of this test - NTSC from Sony camcorder (1 meg, 9 seconds) Same video rendered in HD (7 megs) Click for high-speed camera view, 420fps, 320x240 resolution (4.3 megs, 38 seconds)

Test 9 7/24/11A 3 BATES grains containing 1% Red Iron Oxide (RIO) Grain Lenth average:   2.234 inch OD:  1.3 inch (includes epoxy coat, 3 layers) Core:  0.543 inch Total propellant mass:  185.2g Nozzle, non-virgin RCS 1550, drilled to .375 inch throat diameter Kn initial:  163    Kn Max:  182    Kn Final:  165       Max est. pressure 550psi Burn time:  1.03 seconds

Back to Recrystallized Rocketry

Jimmy Yawn
7/24/11
jyawn@sfcc.net