Friday, October 24, 2014

Propellant testing (Dextrose, Sorbitol, Erythritol)

I've had a hard time finding time to post, but the good news is that I have been finding little bits of time here and there to continue working on rocket stuff.  I went through and organized my garage/work space a while back which was good.  I also when through my bins, range box, the stuff I haul out to a lunch and organized, took stock, and replenished on the things that I was out of.  I also recently rebuilt a laptop and geared it towards rocketry so that all of the design, drawing, spreadsheets, photos, etc are centrally located.  I have made a new motor case to use with my 2.375" motor to test Dextrose with.  The motor was originally designed as an 8 segment KNER  (erythritol based propellant).  That test cato'd half way through the burn.  I do still want to redo that test, but this time with a stepped core. After that test I reduced the segments to 6 and it was successfully flown, though the recovery left a little something to be desired.  The motor got so hot that the while floating down on the parachute the lower half of the aeroshell (made of heavy cardboard) caught fire.  It eventually fell away from the 'chute and came in ballistic.  Fortunatly that nozzle didn't sustain any damage.   Any way, the next time I manage to get out to FAR, I plan to fire this motor as a 4 segment and as a 6 segment motor, with some luck I may get the 8 segment motor ready as well. 

Saturday, October 18, 2014


I suspect that the switch from sorbitol to dextrose based propellant was the reason the TR-1 motor experienced a cato.  More specifically the fact that most Dextrose is the monohydrate form which contains molecularly bound water,   Even more specifically, the fact that I didn't remove all that water is the reason.  I believe that the extra water lead to grain slumping which was the ultimate demise of the TR-1 motor.  Fortunately I cast an extra an extra segment and as that one wasn't used by the end of the day of testing it had been exposed to the ambient temperature of the day which was in the upper 90s.  It had been in the same location as some similarly sized KNSB segements cast by Rick.  The KNSB segments looked completely fine while the KNDX segment had the consistency of taffy and the core had all but disappeared.  I had some concerns after casting the segments.  There were a number of observations made during the casting process that were likely a result of the water content. It melted quickly and poured easily,  but the most obvious and concerning indicator was that the burn rate couldn't be measured because burning was so unstable that it kept self extinguishing,  I assumed that wouldn't be the case when fired in the motor  burning under pressure.  I didn't consider at the time that the water content may effect the dimensional stability of the segment.
I assumed that most of the water would be driven off during casting but that was apparently not the case.  More on that in a bit.
As far as dextrose based propellant (KNDX) goes I decided I needed to get more experience with it. The first step was to remove the water from the dextrose and from anhydrous dextrose. This was done (as suggested by Richard Nakka's site) by dessicating the dextrose in an oven at 175 f  for two hours.  The sample weighed 901gr prior and 825gr after, approximately a 09% reduction.  
The dextrose came out of the oven caked together.  The  clumps were pretty tough; initially I thought I could push them through a sifter but ultimately had to use a coffee grinder to break them up.  I cast a small sample to measure the burn rate from.  Melting took much longer to commence and the resulting propellant was noticeably more viscous.   The resulting propellant sample burned well and the resulting burn rate was in the expected range. 

Back to the TR-1 grain segment.  Those segments we approximately 5lbs each and took about an hour to melt and cast.  One hour at a nearly twice the temperature used in the desicating process seems like enough to remove the water.  The problem though is that not all of the mixed dry ingredients were added at the same time.  Typically small amounts are added and once melted more dry mix is added.  The result is that the last amount added spend much less being heated.  I'm sure some water was driven of those initial TR-1 segments but clearly not all.

Monday, September 29, 2014

Dextrose Testing

So the TR-1 didn't go the way that I had hoped but it wasn't all bad.  It reinforced the idea that making even a small change to your plan should be very carefully considered.  A lesson worth reinforcement.  The damage was not nearly as bad as it could have been and was, for the most part,  limited to the casing, pressure gauge , and the bulkhead.  The bulkhead will need to be re-drilled and tapped but there is enough space left to do that.  The Casing is relatively easy to fabricate and I have already procured new stock.  The nozzle surprisingly was relatively unscathed and after removing a few bolts that had been sheared off in it, it's good to go.   I have continued going through all of my boxes of rocket and rocket related stuff in an effort to organize my workspace and found that I actually have enough sorbitol on hand to cast this motor.  I will take a more stepped approach geared towards a full scale test before proceeding with another full on motor test.  Hopefully this winter around December or January.  The first step is a smaller single grain test motor sized for the 4" diameter grain segment.

In addition to prepping the TR-1 motor for another test I will be doing some smaller scale testing utilizing Dextrose.   I plan to use the 2.375" motor that designed, tested, and flew back in 2009/10.

Originally this motor was designed for sorbitol based propellant.  Ultimately it was fired with KNSB based propellant and a 50/50 sorbitol/Erythitol mix based propellant.   I have constructed a four segment casing for this motor which would produce a low MEOP of 600psi with KNSB and 950psi with KNDX.
SRM screen grab


Monday, September 22, 2014

TR-1 Cato

Theo and I tested the TR-1 motor on Saturday and unfortunately suffered a cato.  It was an impressive sounding motor for that first few milliseconds though.  Apparently I should have rethought my decision to use Dextrose in place of Sorbitol.  On paper the change seemed fine but I had limited experience with Dextrose and never in something of this size.  The actual casting of the grain segments went fine but I experienced some slumping in two of segments that I cast.  At the time I decided that the slumping was due to having removed the segments in question too soon from the mould.  I believe that segment slump either contributed to or was the direct cause of the cato.  Fortunately I had cast an extra segment and at the end of the day, long after the firing, that segment which had been stored in the same conditions as some similarly sized Sorbitol segments that Rick had cast had slumped.  The sorbitol segments remained fine.  The dextrose segment had slumped so severely that the core was essentially gone.  I know of many folks that have use dextrose based propellants with no reports of slumping so I’m unclear of the reason, maybe the size of the segments?  For now I will switch back to Sorbitol in the TR-1 motor and save the dextrose for some much smaller scale motor testing.  We had a good time regardless.  The guys at FAR were very helpful, and the site had grown quite a bit since our last visit there some three years or so ago.  Rick Maschek also provided lots of assitance and camera work.  Rick also had a nice firing of a KNSB motor with a Double-D grain configuration.  The third photo is of Rick's firing.  The last photo is af an  AP motor firing that anomther FAR member (Erik, I think) conducted.


Thursday, September 18, 2014

Loki Booster

I got this Loki booster a while ago.  It's a little banged up (the fins are slightly bent) but overall it's in pretty good shape since it's probably around 60 years old.  This one was never fired or filled with propellant. The transition section doesn't have the roll pins installed and can be removed to revel the interior of the motor.  It is also possible to see the rubber liner that protects the thin aluminum motor casing from the heat of combustion.  The black spots seen at the base of the motor and on the nozzle retaining ring seem to be the same material as the liner.  I'm not sure what the pin on the top of the transition section is for; it's spring loaded and maybe it's meant to help ensure a clean dart separation. The pin on the side was, I believe, to keep the dart from spinning as the rocket was spin stabilized.  It's cool to think that this small booster was capable of getting a dart up to 34 miles...

Monday, September 15, 2014

TR-1 Test Fit

Tested the fit of the nozzle and bulkhead with o-rings installed.  Everything fit together nicely and no adjustments were made.

Thursday, September 11, 2014

TR-1 Casting Stuff

Delrin coring rods are cut to length and everything fits together nicely.