NEFAR Launch
March 11, 2006

Paper nose cone gets a real test
Or two...


Last month's little "accident" presented me the "opportunity" to rebuild the lower section of the Sugar Rush, as well as an "opportunity" to buy another 54mm motor casing.  The upper section of the airframe was not badly damaged, indicating that the new nose cone can handle a bit of trauma.  That's a good sign. Anything that can't take some abuse should not be hanging out with me.  But the lower section was trashed, the only surviving parts were the fins and motor retainer.  

So I ordered a bunch of stuff from Public Missiles, including a pre-slotted 36 inch section of body tube.  

And with the VasterCard still warm, I ordered the 4-grain Loki 54mm motor casing this time.  

I had been wanting a 4-grain motor almost from the start.  The Sudden Rush had flown very well on its first flights with the 3-grain 54mm motor, but was very hard to track for three reasons.

1.  It took off fast.
2.  It went high.
3.  It left no trail.
4.  It was blue.

I like some of these reasons, #1 and #2 in particular.  But the rocket was almost lost because it vanished upon takeoff, ejected the main parachute at apogee, and drifted a long ways on the last launch of the day.  

Last things first.  That's my philosophy.  You see, at work I have a semi-administrative position.  But I'm not very good at being a manager, I'd rather be a mule.  So one of my favorite strategies is to take on the most recent problem first.  Highly distractable by nature, I'm a natural for emergencies.  In fact, I am so good at responding to emergencies that I've developed habits which tend to create them.  This lets me ignore long-standing issues, which are more trouble.  People get invested in the problem, and are threatened when there is a chance it will be solved.  So one must be careful to avoid the appearance of effectiveness in the face of embedded problems.  The solutions are often simple, but the social risks are high. "Simple" isn't necessarily "easy."  Rockets, for example.

Speaking of rockets, problem #4 had a short history, and since several people had already advised me of the "blue rocket/blue sky" problem, the social winds were favoring a real solution.  So I traded good taste for high visibility and painted it dayglow pink.  Re-christened the Sugar Rush, it's hard to look at, easy to see.  

Solving problem #3 also had an effect on #1 and #2.  I used some of the propellant space for an end-burning smoke grain.  Problem with this solution is that KN/sucrose burns pretty fast, so I needed a long smoke grain to keep burning long enough for the rocket to reach apogee.  In fact, it took most of one grain-length, so in effect I had turned my 3-grain motor into a 2-grain motor.  This was adequate to propel the rocket upwards to 3800 feet with a good smoke trail.  I tried some novel smoke grain designs intended to squeeze 20 seconds of smoke into a 1-inch grain.  They were a lot of trouble, not completely reliable, and did not result in substantially higher altitudes.  But some of them worked - a few examples were at the NEFAR launch last May.  

So I longed to relive the 4800 feet I got on the first flight.  A 4-grain motor casing would let me use one grain length for smoke, and still have 3 grains for propulsion.  The original fin section of the Sudden Rush would only accommodate a 3-grain 54mm motor, but now that I must rebuild it I can make the new fin section longer.  What a deal!  And all from one little CATO.

Launch 1

Designation:   3/11/06A
Motor casing: Loki 1600 4-grain 54mm.
Propellant: Recrystallized KN/sucrose made with electric skillet method
Grains:   3 BATES grains, average length 3.5 inches, propellant diameter 1.77 inches
Grain core: 0.63 inch
Web Thickness: ((1.77 - 0.63) / 2) = 0.57 inches
"Smoke" grain at head end, 2 inches long, inhibited at top end with disk of 3/32 inch thick basswood, covered with epoxy, cardstock, and 1 layer Nashua 322 foil tape.  Depth of smoke composition is 1.91 inches.  
Nozzle: New Loki nozzle, 0.435 inch throat
Kn ratios:   Initial:  227   Maximum:  281     Final:  266
Max pressure:   952 psi (BurnSim)
Ignitor:   Bridge wire wrapped in fuse paper.  Tiny bit of black powder rolled next to the bridge wire to ensure ignition with the 12 volt system, and a little Ti sponge sprinkled on the paper for extra heat.  Wrapped with masking tape.
Total propellant mass:   664 grams (assuming header grain burns as intended, 751g if my suspicions are founded)
Total mass at liftoff: 154.4 oz. (=  4.377 kg = 9.65 lbs)
Airframe: 3 inch diameter PML Quantum Tube, body tube 54 inches long.  
Conical nose cone made from paper, PVC fittings and fiberglass, and containing altimeter.  
610 gram sandbag ballast added to payload section to ensure stability.
Coefficient of Drag: 0.26 at 300 fps (RocCAD)
Total liftoff weight: 154.4 oz ( = 9.65 lbs = 4.377 kg )
Ejection:   Using PML Co-Pilot, adapted to fit in the nose cone.  Set to fire in redundant mode, both matches to fire at apogee 1 second apart.
Ejection charge:   2.6 grams of Red Dot sealed in a cardboard pouch with 2 Christmas-tree e-match heads.  

Videos of this launch:
Short, boost only, 3 meg .wmv file. 25 seconds of video
Another short video of the boost taken by Jon Barnhart, 2 meg .wmv file, 9 seconds if video
Longer, Boost to touchdown with a little drama, 8 meg .wmv file, 97 seconds of video

Nice launch, don't you think?  But a little mystery.  Upon recovery, the altimeter beeped out its reading:  4939 feet.  A record!  My highest previous flight was the first with this bird, 4864 feet.  So despite concerns that a true-cone nose cone is a "bad" design, it does not prevent good performance..  

But there was a minor injury to the airframe:


I wondered about this odd ding, What on earth could it have hit that hard?  And it was falling fins-down.  Perhaps the nose cone slammed into it?  But there was no commensurate ding on the forward section.  First rocket geek I showed it to said "looks like you almost had a zipper there."  Bing!  It was the shock cord.  What else could have made this indention?  But that means the rocket was going pretty fast when ejection occurred.  I have had some scary long delays with this altimeter, the rocket falling a hundred feet or more before the 'chute pops.  Guess it could have happened here....

But wait!  I check out the videos and hear some stuff.  Here are the events, best as I can gather them:

Event   Time
First ignition smoke  0
Liftoff    0.734 second
Motor burnout   1.8 seconds
Smoke burnout   5.4 seconds
Ejection sound heard   11.3 seconds

So 10.5 seconds from liftoff to ejection.  Hmmm.  That's a bit too quick.  RocCAD indicates that his rocket would reach apogee at 16.4 seconds.  

Oh, oh, oh... I forgot something.  How about the time it took that ejection "pop" to reach our ears?  If the rocket indeed went to 4939 feet, then it should have taken 4.5 seconds for the sound to travel to the ground.  That leaves barely over 6 seconds flight time.  Rocket would have been going quite fast at that time, so no wonder there is a ding.  The wonder is that there was no significant damage - at 6 seconds the rocket would have been going several hundred feet per second, where ejection should have caused at least a full zipper, possibly shock cord breakage, eyebolts pulled out, parachute torn.....  

Then again, the altimeter reading of nearly 5000 feet.   How could it possibly.....?

Something is amiss here.  Maybe the altimeter gave a wrong reading.  Perhaps the new nose cone altimeter bay arrangement interferes with proper readings somehow.  Maybe the sound we heard was not the ejection but something else, and the true ejection pop was obscured by wind or other noise.  

Smoke burnout occurred way too early, suggesting that most of the smoke grain burned as propellant during the boost phase.  If that were so, it could explain the altitude difference, but not if ejection occurred only 6 seconds into the flight.... that borders on the impossible.  

If you have a clue on this one, please feel free to fill me in.

Launch 2

Designation:  3/11/06B

Specs?  Just look at the last load.  I made them as much the same as possible.  The only difference is that the nozzle has now been used once.  Measurements made afterward suggest that the throat diameter is a little larger for this flight, perhaps 0.441 inch.

I was delighted to meet Josh Chatham (left) and Jon Barnhart, both AE students at Embry-Riddle who helped me get my rocket on the rail after setting up theirs.  Besides being handsome and intelligent, they are very pleasant and like rockets!  Hmmm.... I wonder if ERAU needs a career counselor?

Videos of this launch:
Short, boost only, 2 meg .wmv file, 18 seconds of video
Another short video of the boost taken by Jon, 1 meg .wmv file, 6 seconds of video
Long, Boost to touchdown, 7 meg .wmv file, 85 seconds of video

Flight is nominal, altimeter reported altitude of 5048 feet.  Here is a graphic of it's cute little beeps, captured from the video file:

On this flight, the smoke trail persisted for about 10 seconds, closer to what I had in mind but still a bit short.  

Sound from this launch was louder than usual.  Even my bad ears picked up on it.  Greg Lukach asked if I had added something to the propellant to make it louder.  (I hadn't, but it's a thought!)  Watching the video frame-by-frame, I notice an odd cloud of smoke jetting upwards just before liftoff, and the flight starts with  "Pow!" sound.  First thought is that the ignitor wire bunched up and plugged the nozzle, building up pressure before (fortunately!) being ejected and off she goes.  But notice the beverage can under that pretty tailcone.  The can was donated by Dominik Psutka, one of the ERAU students who helped me get this rocket set up (thanks!)  The can is there to hold the rocket up and away from the rail attachment bolts, which were binding on the airframe when I let it all the way down.  The opening of the can is at the top, right under the nozzle.  When going back to set up the third launch I found this can, blown wide open!  It looked like someone had put a large firecracker in it.  (I used to do this as a kid - exploded cans often make interesting shapes.  Mom called it "pop art.")  Now I'm thinking that the pop at liftoff might have been the can bursting.  Wish I had thought to take a picture of the can.  Also wish I had looked at the ignitor wire to see if it was badly mangled, but I didn't.  Oh well... If wishes were horses I would be up to my armpits in manure.  

So my question to experienced rocketeers out there:  Is this likely to happen with a "normal" motor?  I am accustomed to seeing holes burned into these sodacan standoffs, sometimes to the point that there is little left.  Some might completely disappear - it's hard to find cans out there at the pads, I might have to start taking my own trash to the launch.  

Also while doing cleanup after the launch, I noticed a good bit of black stuff inside the tailcone and lower part of the motor mount, more than usual.  I suspect that the can may have focused the early exhaust back up toward the motor before it popped.  Just a guess.

Launch 3 

This time using two moonburner grains, salvaged from last month's launch.  I had made three loads with the "clever" smoke grain, now suspected of causing the motor to CATO.  Now I cut off the lower part of each grain to get rid of the funny smoke grain, and put the two sections together in the motor casing, careful to line up the cores.  

But I cut them a bit too short by mistake, thus the header grain needed to be longer I had planned.  So I made it long, and pressed an offset core 1 inch deep.  When loading, I ran a short piece of black match in this core and stuffed it in with about 2 square inches of rich fuse paper, with one end of the match dangling out.  I hoped this would convey flame to the forward end of the core quickly.  Apparently it did.  

Designation:   3/11/06C
Motor casing: Loki 1600 4-grain 54mm.
Propellant: Recrystallized KN/sucrose made with electric skillet method
Grains:   2 moonburner grains, average length 4.3 inches
Grain core: 0.63 inch, flared to 1 inch at nozzle end
Web Thickness: (1.77-.63) = 1.14 inches
"Smoke" grain at head end also provides some thrust.  It is 4 inches long, with 1 inch deep moonburner core, aligned with the cores of the primary boost grains.  The header grain adds 60g to the propellant load, plus another 39g for the forward burn into the header during the boost phase.  Thus it is assumed that 2.14 inches of this grain will be consumed during the high-pressure "boost" phase of motor operation.  the remaining 1.86 inches will burn at the 1atm speed of 11 seconds per inch.  That should provide a smoke trail for 20 seconds after motor burnout, enough to get it to apogee and then some.  We shall see....
Nozzle: Almost-new Loki nozzle, estimated to have 0.447 inch throat*
Initial Kn:   194  (highly regressive thrust curve, tapering off to nothing in 2.5 seconds)
Max pressure:   555 psi (BurnSim)
Ignitor:   Bridge wire with fuse paper, BP and Ti, as above.  In addition, a 2 inch long strand of black match inserted in the head-end grain's short core, and 2 square inches of rich fuse paper packed in to hold it in place.  This is to ensure that the far end of that core gets ignited right away.
Total propellant mass:   603 grams (assuming header grain burns as intended, 755g if my suspicions are founded)
Total mass at liftoff: 154.4 oz. (=  4.377 kg = 9.65 lbs)

* Throat of brand-new nozzle measured before any firings at 0.435 inch.  After three firings it measures 0.453 inch, averaging an increase of 0.006 inch each firing.  

Videos of this launch:
Short, Boost only, 2 meg .wmv file  17 seconds of video
Long, Boost to touchdown, 5 meg .wmv file, 62 seconds of video

I notice a little "tail wagging" upon liftoff and shortly thereafter.  I've seen this a lot with moonburner motors, and attribute it to the off-axis thrust generated by the off-center core.

Funny thing.... there was a nice smoke trail for about 3 seconds after liftoff, then none.  Now it could have been obscured by the clouds behind, but I don't think so.... My guess is that the header grain's inhibitor "leaked" and allowed it to burn as propellant during the boost phase.  

Let's compare that to what else we know, or can find out.

I will use Richard Nakka's excellent EZ-Alt spreadsheet to "reverse engineer" this flight.

Altitude, as reported by CPR-3000 altimeter:  4189 feet.

Reverse-engineering EZAlt to generate this altitude, I enter:  

Motor average thrust:  65 lb *
Motor total impulse:  167.5 lb-sec (= 745 Ns)  (here, I entered numbers repeatedly to converge on the Peak Altitude reported by my altimeter)
Motor propellant weight:  1.330 lb ( = 0.603 kg)
Rocket dead weight:  7.93 lb (not including propellant)
Rocket diameter (max) 3.0 inches
Rocket drag coefficient:  0.26 (simulated with RocCAD)

Peak altitude = 4191 feet
Time to peak altitude = 16.7 seconds
Max velocity = 537 feet per second (= 366 mph)
Burnout altitude = 897 feet

* Simulated with BurnSim, and close to the average thrust from a similar motor static tested:  3-20-05A

So now the sanity check, calculating delivered ISP:

(745 Ns thrust / .603 kg propellant ) / 9.8 (acceleration of gravity) = 126

This is within the realm of possibility, but on the high side considering the low Kn value and the very regressive thrust curve of the moonburner grain.  Especially when compared with 3-20-05A, a very similar motor which had a delivered ISP of only 99.  Thus it seems likely that more of the header grain burned during boost phase than was intended.  

So let's figure that one out.  Total propellant, including the intended smoke grain is 755 grams.  

(745 Ns / 0.755 kg) / 9.8 = ISP 101

Bing!  Now we are in the ballpark with my static-tested motor!  This strongly suggests inhibitor failure of the head-end grain, such that it burned completely during the boost phase burn.  

I will examine the contents of the case liners to see what can be determined.  Fortunately, I brought them home.  Unfortunately, I used a wooden dowel to ram the nozzle out for quick reloading in the field, so the contents have been distrubed by an ape.  I'll look anyway.

Autopsy:  Guess which liner tube was used with the moonburner grains?  


Looks like this liner material is marginal for a moonburner at this level.  I've fired a bunch of moonburners in the 3-grain casing with no significant burnthrough, so am guessing that the higher intensity of the 4-grain casing takes it just past the edge.  Motor case seems undamaged despite this burn-through.

The top two are the BATES grain loads.  They show evidence of a little gas leakage at the head end.  This could be significant, in that the gasses would have flowed around the end-burning smoke grain, possibly igniting the head end if it's inhibitor were somehow compromised.  Or the outside inhibitor might have somehow failed.  I really need to develop a slow-burning smoke element so I can use that recess in the head end closure.  Perhaps I'll start on that tomorrow....

Cutting the liner tubes open to examine the contents.  Well, this doesn't tell me much, except that the inside of a burned motor smells bad.  On the first (left) motor, the wood disk that inhibited the head-end of the smoke grain looks like it could have burned through, but my sleuthing is compromised by my klutziness - I'd used a dowel to ram the nozzle out from the top end, crushing the stuff in between.   On the second and third shots, I got smart enough to push the nozzle up to break everything loose, then a spare section of liner tube can be used to push everything out the other way without damaging the contents of the liner.  But I see no compelling evidence of anything here.

Executive summary:  The new Loki casing works fine.  The rebuilt Sugar Rush flies well.  The conical nose cone isn't so bad as to prevent a good flight.  The smoke grain need some work.  On the average, life is good.

Jimmy Yawn