Moonburner Grains Longer Burn Time Delayed Titanium

The best ideas come in my sleep.  Don't remember my dreams, or else I would be innundated with a zillion fabulous ideas all at once and could not cope.  Certainly many people suffer sleep disorders because they remember and thus fear.

But when awakened before the crack of noon, my sleep patterns are disturbed and new thoughts escape.  

Like last Sunday, I became vaguely conscious of the words:  "...a smudge is a smidgen that's been smeared.  A sniggle is a smile with a wiggle...."

(Am I now documentably alliterate?  Maybe I should apply for a grant.  Seek a diagnosis.  Get medicated.)

I awoke again at a more normal hour with my left side asleep.  Hmmm.  What I mean is that my left arm and leg were all tingly with that "asleep" feeling that comes from too much pressure on one side.  

de grain

A few months ago there was some discussion on the SugPro discussion list as to how one might create D-grains.  

The elegant answer was provided by Bill Colburn.  He suggested sealing the ends of a nearly-filled inhibitor tube and laying it on its side so that the soft propellant would settle to the bottom and cure there, leaving a "D" shaped hole at the top.  Simple, brilliant.

As I lay in the Chapel of St. Mattress, tingly stuff was settling to the lower side of the blanketary inhibitor tube.  


Call me old-fashioned, but I love to hear the Doppler shift as a long-burning rocket motor streaks skyward.  I get precious little of that with my candy propellant.  Yes, they streak skyward.  But the burn time is short, leaving little audible effect.  There isn't much echo at my flying field.  

So I did a few tests of moon-burner grains to lengthen the burn time.  The web thickness is essentially doubled, so instead of 0.8 to 1 second, I was getting burn times of 1.7 to 2 seconds from my 38mm casings.  I should be able to get 3 to 4 seconds from my 54mm casing.  Of course, there will be a reduction in average thrust, but with the longer burn time should have roughly the same total thrust.  Thus these motors should be OK for light-bodied airframes.


de fire

At the February NEFAR launch, I fired a couple of 38 mm loads in my Ariel, one with short-burning Bates grains, the other with even-shorter-burning uninhibited grain.  The primary purpose was to flight-test the delay grains I had developed late last year.  The delays had worked well in static tests but had not yet flown.  The propellant was enhanced with titanium, a great crowd-pleaser.  And much as I hate to admit it, I am into crowd-pleasing whenever there is a crowd I wish to please, like the rocketry club and its visitors.  

But one of my motors somehow managed to set fire to a patch of dry grass a little ways from the launcher.  No big deal, several rocket geeks promptly converged on the spot and stomped it out.  Another arrived a moment later with the fire extinguisher, just dying to use it.  And it was a small, isolated patch of grass so the fire wouldn't have gone anywhere anyway.  

This event reminded me of a request on the SugPro list about a year ago to address this issue:  What about making a load that fires plain propellant first, then transitions to Ti propellant once the airframe is well underway?  Not only would this reduce the fire hazard, it might provide a nice effect.

I did a few tests placing Ti-rich propellant in the smoke grain, an end-burner.  The Ti propellant was hidden under a web-thickness layer of plain propellant.  In worked OK once in three tests, demonstrating that the concept is viable.  But it fired the Ti early in most tests, demonstrating that it needed some work.

So here I lay, "tingly stuff" like titanium sparkles and thinking of Bill's D-grain idea.  And sniggling.  Hey!  How about making a skinny D-grain of Ti propellant, and packing plain propellant around it, and placing the open core on the opposite side, huh?  That way the burn would have to progress through a thick layer of plain propellant to get at the sparkly stuff, right?



Since I am used to making round cores (love to twist that mandrel!) I decided to continue with the moonburner design.  I'll try D-grains sometime, but for now it seems that the difference between the moon and a D-grain would not be large.  

First attempt failed.  The grain was kludged together, and I had my doubts as to whether I should fire it.  Did it anyway, a low-Kn burn to be safe.  The motor worked OK, but there were so many compromises in the grain structure that it fired the Ti right away, no delay.  I won't bother you with details on that one, unless you want another example of what not to do.

Hot Dog!  Second attempt, first one worth illustrating.  
 

I took a cylindrical slug of warm propellant with 1% RIO and 10% Ti, and pressed it into a larger slug of plain propellant.   The dual slug was rolled into a rough cylinder, reheated to soften, and packed into a 38mm inhibitor tube which had been coated on the inside with 5-minute epoxy.   A 3/8ths inch wooden dowel was pressed down the side of the mold opposite the Ti/RIO propellant to create the "core."

Test 3-26-05A

Click Here to see an analysis and video of this test

This technique worked OK, but there was very little Ti delay.  I think because the Ti slug comes too close to the ends.   It became very challenging to make longer grains.  The warm slug of propellant wants to stretch, even at this size.  The thought of sniggling a bigger one into a 38-360 inhibitor tube did not appeal to me.

I tried this technique a couple more times, but instead of just rolling the "hot dog," I molded it inside a 1-inch PVC mold to get it nice and cylindrical.  It was then reheated, dropped into an epoxied inhibitor tube and pressed down firmly.  This worked OK, but was difficult with a 38-360 tube and nearly impossible with a 38-480.  The long slinky of soft propellant stretched, crinkled, and collapsed in an uncontrolled manner, no doubt leaving air pockets.  So I sought a better way...

A Better Way:  The ferTile crescent

My goal:  To make a 54mm moon-burning grain with delayed Ti entry. Two flavors of propellant are heating up.  The left blob is about 300g plain rcandy The smaller red blob on the right is 50g of the same stuff, but with 10 percent fine Ti flakes and 1 percent Fe2O3 added.   The iron oxide is there primarily for show, so that the difference between the propellants will show up well in the photos.  But it may serve another purpose, to make the tail-off a little quicker and thus the last part of the burn more efficient. Whether it does so is yet to be determined.

The inhibitor tube is painted with 5-minute epoxy.  This is to ensure that the propellant sticks to the inhibitor.  By itself, rcandy tends to stick where you don't want it, and not stick where you do.  The glue helps us counter Murphy's law. Here I have cut a section of 54mm inhibitor tubing from Loki, and it is 5 inches in length Why such a short grain?  My Loki 54mm motor has about 9.5 inches of propellant space.  The "normal" nozzle has a throat of 0.43 inch, which is way too big.  The smaller 54mm nozzle has a throat 0.25 inches in diameter.  Way too small.  Reducing the length to 5 inches will give me an initial Kn ratio of 286 and thus a pressure of perhaps 1000 psi.  To use the whole length of 9.5 inches would create a Kn of 466 and a pressure over 2000 psi.  At some point in the future, I will drill this nozzle out a bit so I can use the full propellant capacity at a sane Kn.

Tube is stood upright on a piece of cardboard with plastic tape on it.  Another way to evade Murphy, as neither epoxy nor rcandy will stick to this plastic.   But first, I measure it to get the length about right.  There should be a web-thickness gap at either end of the tube, so I make this slug 3 inches long. Hmmm.  Now that I think about it, that's a little too short.  I'll use it anyway.

The slug is slid inside the tube, and pressed down firmly with a section of PVC pipe.   I've used wooden dowels for this purpose too.   It occurs to me that a flat presser might be good, to make a shallow "D" grain here.   The propellant is pushed back from the ends of the tube a little, to ensure a web-thickness gap.

Stood on end again, I now notice that the red wedge is too close to the end of the tube.   Oh well, It's history now... this grain might start spewing a small amount of Ti early in the burn.  We shall see. A glob of plain propellant is dropped into the tube and pressed down to the bottom with a dowel.

Two more globs are two are pressed in, so let's take a look-see... The propellant is about halfway up the tube.   Time to start the core.

The coring rod is just a section of hardwood dowel which has been sanded and coated with vaseline.   It is pressed down through the hot rcandy until it finds the bottom.  This process also helps to compress the candy into crevices, I think. More rcandy is added and pressed around the coring rod.

One final chunk is added to finish off the top end.   It is pressed by hand.  I love this non-toxic propellant!   As it cools, it gets less sticky and easier to handle.

Another wooden dowel is used to make an angular channel from mid-grain to the coring rod.   This is to facilitate the ignitor going in, and hopefully coming out!

The coring rod is twisted occasionally to make sure it doesn't freeze up.   But that is mostly habit - since I have started coating the wood  with Vaseline, it never sticks anymore.

Nice, clean hole on the head end, odd-looking crevice on the nozzle end but that is intentional.

Since I can't fire this one right away, I will seal it with aluminum-foil tape.   Hopefully it will still be dry when I finally get to use it.

Static Test

Here is one just like it I fired the other day. The photos below are at 1/2 second intervals.   Click here for analysis and video of this test.

Titanium burn was delayed - there were a few early sparks, but a big rush came on about halfway through the burn.  


Two Moons
Load for 38-720 casing

NEFAR launch coming soon, opportunity to flight-test.  So I set about making a flyable motor.  

Problem:  my altimeter is still acting funny so I can't launch the Sugar Rush.  Solution:  I quickly rebuilt my Ariel airframe, injured in last month's little accident where the nose cone popped off but parachute stayed in the tube.  It landed hard and broke a coupling.  After replacing the mid-body tube, I painted it flourescent yellow to cheer it up some.  

The Ariel has no electronics, it requires motor ejection.  My delay grains take up a bit of the propellant space, so I want to use my 38-720 casing in order to get some altitude.  

Making a single grain eleven inches long seemed absurd.  So I made two grains.  The upper grain has a 3/8ths inch core and contains a wedgie of titanium propellant.  The shorter one, which goes to the nozzle end, has a 1/2 inch core and contains only plain propellant.  A dowel is inserted through both cores to hold them in alignment while being taped.  They are secured with a covering of al-foil tape.  

 
 

Static Test

Click here for specifications and a static test of the same motor.


So how about a launch already?
NEFAR launch, 4/9/05
    
Click Here to visit the web page documenting these flights.

Unfortunately, the video did not pick up the titanium sparkle very well.  It was visible to the eye, and it did begin after the rocket had left the pad.  

Good news is that any vectored thrust effects were insignificant, and the long-burn moon burner grain had plenty of thrust to get the Ariel off the pad.  
Both of these were fired with fuse-paper ignitors, which have proven reliable so far.

Simulation predicted that this rocket would go to about 1900 feet, and it looked like they did just about that.  The first one used somewhat faster propellant in the 2-inch delay grain and popped the chute a bit early (7 seconds per inch at 1 atmosphere.)  The second used 9-second propellant, and fired just past apogee.  So I need to mix up some 8-second propellant, or maybe shorten the delay grain a tad.  The airframe came back dirty but undamaged.  


I have been pleased and a bit surprised at how well the moon-burner grains have worked so far.  There are some issues to be resolved, like how to get the ignitor into the core, offset from the tiny nozzle throat.  In some cases, I had to install the ignitor while assembling the motor - a no-no at most launches - so a skinnier ignitor might be needed.  Another thought is to have the nozzle-end grain with an angled core, centered at the nozzle, angled to meet the moon-burn on the second grain.  Haven't tried that one yet, it's just a fantasy at this point.



Jimmy Yawn
jyawn@sfcc.net
4/24/05
Recrystallized Rocketry