Spinburner:  The Naked Grain Caseless, nozzleless, self-stabilized rockets.

Richard Creamer and Steve Ghioto came over last weekend to do some static tests.   I was demonstrating the good texture and very fast burn rate of propellant made with KNO3 from pvconly.com and texturized with a little vinegar.  I warmed up a remnant of a batch and made a test strand.  It burned at 6.5 seconds per linear inch, which is faster than uncatalyzed propellant made with any other KNO3 in my experience.  

Thus we started talking about making end-burner motors, and the excellent (if questionable) results I had obtained back in February.  So I added 1% Fe2O3 to the remnant of propellant, and it burned at 5 seconds per inch.  I assembled a couple of end-burning motors for later testing.  There was some propellant left over, a dangerous thing, so we had to burn it up.  In curious and interesting ways, of course.  One of our test strands took off by itself, leading to some irresponsible experimentation.  

We formed a few "rockets" by hand out of lumps of propellant, poking a core into them with a bamboo skewer and inserting a fuse.  Fired from a tube, they went pretty well, but where they would go was anybody's guess.  We each selected a tree to duck behind.  The most notable rocket exited the tube, took a 90 degree right-hand turn and flew 100 feet straight into the woods before burning out.  We followed it to make sure there was no fire.  It's kinda spooky when these things vanish, and all that's left is smoke drifting away.

Flashback to a conversation with Greg Peebles.  His students at FIT had to make their rocket spin, but not too much.  Tilting the fins would generate low spin off the launcher and high spin once the rocket was up to airspeed.  Thus it would lack stability when it was needed most, and get too much of it when it was needed least, along with high rotational speed and centrifugal forces that could disrupt their experiments.  Placing angled ridges in the nozzle was tried, and with ablatives so that they would only be present for a short time.  I understand that a lot of experimentation has been done with this approach so far.

But after our conversation, I started thinking about the possibilities of "rifling" the propellant grain itself.  Make something like a star grain, but twisted.  This would impart some spin on the propellant gasses, and perhaps spin the rocket itself.  The core would burn itself round and produce no more spin after a short time.  The amount of spin could be regulated by the depth of the star and the "twist" of the "rifling."  

Since then I have wondered about what happens when a rotating gas column hits a nozzle.  I can imagine several things, most of them bad.  But I don't know which would actually happen.  Experimentation is needed!

But that is at a larger scale, and beyond the scope of the present project.  So back to our little chunk-rockets.  What about making spiral cores in them?  Would they rotate?  Would they fly straight?  Too bad I don't have any spiral coring tools!  I've thought about making such things, but there are two problems:  1)  I don't have a lathe or milling machine.  2)  If I did, it would take me years to figure out how to do something like that.  Maybe never.  

Then a flash.... I DO have a spiral coring tool!  



Being a fervent fan of finely fermented fruit, I have several of them.
(Now you know my secret:  I am alliterate!)

There are two types of corkscrews.  I mean the screw itself, the thing that goes in the cork and grabs it by the innards.  Not the apparatus that twists it into the cork and wrenches the cork out of the bottle.  

There is the "snake" type, which is a wire wound around a mandrel.  It snakes itself into a spiral.  This is NOT the kind I'm using here.  They work OK for wine bottle corks, but don't seem very useful for this purpose.

The other kind is the spike.  It is like a long wood screw with very deep threads and a very steep pitch.  I guess it IS a wood screw.  Cork is wood, right?  Comes from an oak tree, right?  Did you know that the genus name of the oak family is "quercus?"  "Quercus suber" is the cork oak, "Quercus nigra" is water oak, "Quercus alba" is white oak.... There's lots of quercii out there.  I think the word "cork" came from "quercus."  Does that sound reasonable to you?  Sorta like "quirky," as in a twisted branch.  I was a bent twig once myself.  And I suspect that the first keyboards were made from oak, that's why we spend so much time tapping at the temple of QWERTYCUS, the ancient god of keyboarding.  

Let's use the one in mid-picture.  It is a spike with wide flangey threads.  I am NOT using the one I'm holding, not until dinnertime.

So I gave Rich the red corkscrew and he made the first spincaster.  He took about 8 grams of propellant and molded it by hand around the end of the screw.  It ended up about 1-1/2 inches long and 1/2 inch in diameter.  The screw went maybe an inch into the lump, and came out easily.  A fuse was inserted in the core and the slug-rocket inserted into a metal launch tube about two feet long.  Upon ignition, the little rocket flew absolutely straight out of the tube, making a qurious noise and burning up in midair about 100 feet away.

Later that night I set about trying to replicate this test.  Using a homemade model-rocket motor tube as a mold, I made a cylinder of propellant 2.75 inches long, 1/2 inch in diameter, and drove the screw into it 2 inches deep.  Fired from a 2-foot tube at a 75 degree angle, the rocket exited fast, spun off some "threads" at about 100 feet, and burned out while descending about 400 feet away.  "Fire hazard" I said, and vowed to do no more such tests until daylight.

         

Click Here for a movie of three firings.  (5 meg .wmv file, )

The last of these has a minimal casing.  It is wrapped with 1 layer of Nashua 322 aluminum foil duct tape.  Twisted at the head end, open at the bottom end.  It did not make such a dramatic noise, but it went a lot farther than the first two nudies.

So the corkscrew works, and makes for an interesting sound.  But I suspect that the high twist is sub-optimal, and the wide grooves cause them to fall apart prematurely.  A more gradual twist is needed.  

So I went to Home Deep Hole the other day to get some copper fittings.  Got a new faucet for the tub, and the old copper tubing is so beat-up that it is hard to get a solder joint that doesn't leak.  On the way out, I passed by the power tools section and noticed some interesting spirals among their wares.... drill bits!  Why didn't I think of that before!  I recalled that I have a collection of bits already, in a wide range of sizes.  So I scurried home.

 

Hey!  Where's my 3/16ths inch bit?  Oh, right.  I'm using it to make a rocket.

I added  a "cone" to the front end of this one, primarily because the bit went in too deep so I needed to extend it.  This is the first one fired in the movie below.

    

Warmed up a bit of propellant, catalyzed it with 1% RIO to speed the burn rate up to 7 seconds per inch at 1 atmosphere, and commenced.  

Molding tube is 2 inches long and 1/2 inch inside diameter.  It is lined with one turn of taped paper, which serves as a non-stick mold liner.

A wad of propellant is rammed into the tube.  And another, and another until it is almost full, about 1/8th inch from the end.

Then I turn it over so as to be working from the cooler end, and gently insert the 3/16ths inch drill bit, allowing it to twist its way in.

A strip of blue masking tape is wrapped around the drill bit at the 1-1/2 inch mark to tell me where to stop.  

Warm propellant likes to stick to the drill bit.  When that happens, removing the bit damages the spiral.  So I have tried coating it with WD-40, which helped a little, paraffin, which helped a little, but the best thing is cooling the bit and keeping it clean.  A vial of water and a towel are kept nearby.  I insert the bit about 1/4 inch then withdraw, stick it in the water, then wipe it dry.  Re-insert a little deeper, wiggle it loose, twist it out, dip it in water and wipe it dry.  Repeat until it has gone to full depth.  

Here is a good one.  A well-centered core with a 'bow tie' profile.   This one is molded in a 2-inch section of 1/2 inch copper pipe.  It sure absorbs heat well!   I have to wear a glove to hold it when packing the propellant.   These make a tight fit in the coppper tubing and do not launch well from it.  Instead, I use 1/2 inch EMT, which is a little larger ID.

To help them stay dry in this humid climate, I dip the grains in melted paraffin.  Being careful to keep the wax out of the core... that might make for poor ignition. The wax should also help lubricate the grain as it passes through the launch tube.  At least that's the theory.

Movies of drill-bit-core flights. Click here for a few pretty flights (7 meg .wmv file, 1 minute of video) Click here for a longer movie with some successes, some marginals, and a few clear failures (10 meg .wmv file, 2 minutes of video)

Yes, a few of these have a bit of Ti sponge in the propellant to make sparkles.  Unfortunatley, it also enhances the incendiary effect.

Now that I compare them, the corkscrew motors had a cooler sound and seemed more stable.  They also tore themselves up readily, so perhaps an in-between twist should be tried.  Maybe I do need to visit the hardware store again.  Or cut off the carbide end of a masonry bit.

Most of these were fired with a bare ignitor and 110VAC house current.  This was very successful in the first tests with SVRC KNO3/RIO burning at 5 seconds per inch.  But when I tried it with slower propellant, a "mere" 7 seconds per inch, there were a number of misfires.  No big deal, just stick in another ignitor, but for reliable ignition a bit of pyrogen might be useful.

I got these to ignite by dusting the core with meal powder and shaking out any excess.  

Other failures seemed to stem from the launch tube.  I molded some in a short section of copper pipe, then tried firing them from the same kind of pipe.  The  rockets were slightly smaller because I had used a turn of taped paper as a mold liner, but they were made larger by dipping in wax.  This was too tight a fit, most of these did not work well.  Also, the first section of EMT tubing was rusty inside, and that seemed to have a negative effect.  I found a cleaner piece, and it worked fine.  

The rockets made with the corkscrew generally threw off some "threads" shortly after emerging from the tube.  Less-deep threads of the drill bit pretty much eliminated this effect.  Also, in the one test where I wrapped the grain in foil tape, the threads seemed to stay put.  

Off-center cores seem to increase the failure rate, not surprisingly.  A jig for centering them well would be helpful

These little slugs are a lot of fun.  Quick and easy to make, entertaining to fire.  But I now wonder about larger motors.  What, indeed, happens when a spinning gas column has to pass through a nozzle throat?  Vortex?  Choke?  I would love to hear any opinions on the matter, or better yet, test results.

I hope to do some tests of this myself one day soon.

Jimmy Yawn
jyawn@sfccc.net
Recrystallized Rocketry
5/9/06