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. 

 

 

Dextrose

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 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.

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

 

 



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.

 

 

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...

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.

TR-1 Casting Stuff

 

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

Dextrose

The Dextrose from Lucky Vitamin has arrived.  I want to do a little testing with it since I don't have a lot of experience with Dextrose.  At least not nearly as much as with Sorbitol or Sucrose. 

TR-1 Motor Update

McMaster-Carr order arrived yesterday and included the o-rings, Delrin rod, screws, and pressure gauge.  I didn't get a chance to assemble the motor for a final test fit, but now that I have the o-rings that's next on the list of things to do.  The Delrin rod for casting the core in the propellant segments was a perfect fit in the casting base, so I'll just need to cut to length.  Delrin would be an ideal choice for the casting bases too, but the stock I need would be around $60 for two bases, so I'll stick with wood for now. 

TR-1 Motor Update

This past weekend I finished a few more tasks, although it's starting to look like I may not be quite ready to test fire the motor by the 20th of this month like I was hoping to do.  I was hoping to get o-rings by Saturday so I could do a final test fit of the nozzle and bulkhead but I didn't get them on Saturday like I had hoped, but I can do that this week as it shouldn't take long.  I did some minor machining on the nozzle to get the two pieces to mate together perfectly.  I also took the time to go back and adjust the inlet a bit by machining the inlet at decreasing inlet angles (30, 15, and 7.5 degrees) and then grinding and polishing smooth so the inlet is MUCH smoother and gradual transition to the throat.  No change to the throat diameter was made.  

 

I also made two casting bases out of scrap redwood that I had lying around.  The casting tubes are held in place securely with a friction fit.  In the past I have only used a 1/4" recess but these are a full 1/2"  deep so the tubes seem pretty snug.  The surfaces of the wood that comes into contact with the tubes/propellant were coated with epoxy.  Once cured, they were machined to the final dimensions.  This seals the surface from the grease used as a release agent and helps maintain dimensional stability.

 

Supplies from McMaster-Carr

I placed an order for the last of the things I'll need for this motor test.  Addiitonally, I need to make some bases for the casting tubes.  I'm planing to machine them from wood so it should be quick.  I've used wooden casting bases in the past.   ("painted" with epoxy for strength and so they don't absorb grease used to keep everything from sticking together)

Supplies ordered for the TR-1 motor test:

O-rings Dash 242 (McMaster-Carr 9452K192)

Screws  low-profile alloy steel socket head cap screw  (McMaster-Carr 92220A182)

1 1/4" Delrin rod  2'   (McMaster-Carr 8572K23)

1500psi Pressure Gauge (McMaster-Carr 3845K1)

Unrestricted Motor

I wrote this a long time ago and just noticed it sitting in the post section as a draft.  This launch was ten years ago in the spring of 2004 and apparently I was thinking about boosted darts back then as well.  I posted some pictures in 2009...I don't recall why this account wasn't posted at that time.  I also noticed that I mention the atlimeter was reporting the peak altitude but apparently I didn't record that info as its not mentioned. 



UNRESTRICTED L-CLASS MOTOR

Launch of L-class unrestricted motor

Unrestricted (burning on all surfaces) propellant grain

I designed this motor with several ideas in my mind. First was to have a motor capable of lifting heavier and larger payloads than I’ve done in the past. Secondly, I wanted a larger motor that didn’t require casting multiple grains, and finally I wanted a motor that could serve as a booster for a future boosted dart project. The motor design uses a single unrestricted burning grain, producing a relatively high thrust short duration burn profile. The motor serves as the casting mold reducing the amount of hardware needed for the casting process. A well-lubricated, thin paper liner is inserted into the motor before the propellant is cast. After the grain has cured, the liner is removed and discarded. This ensures that the grain has a slightly loose fit in the motor. KNO3/Sorbitol is the propellant, which, with its lower melting temperature, eases casting on cool, windy nights at the MTA. Finally, the motor is constructed entirely of steel which eliminates the need for any insulation.
The nozzle and bulkhead are fabricated from 1018 cold roll steel. The nozzle has a 45 degree convergence angle, and a 12 degree divergence angle, and has an area expansion ratio of 4. Both are retained in the casing by eight 8-32, ¼ inch low head cap screws and have a single Nitrile O-ring. Both the nozzle and bulkhead retaining screws are tightened against the nozzle or bulkhead flange. As such, the casing holes are sized to the head diameter of the fasteners. Load transfer is thus accomplished through bearing of the fastener head against the casing wall. The casing is made of mild steel. It has a 3” OD and a .065” wall thickness. The entire motor weighs a total of 4.19 lbs (1900 grams) which breaks down as follows; nozzle 1.04 lbs (472 grams), bulkhead 0.39lbs(176grams), casing 2.73 lbs. (1239 grams), retaining screws and O-rings .03 lbs (0.15 grams).
I had been planning on static testing this motor at the launch in March. Then, later it would be used as the booster on a two-stage airframe that Peter had been constructing. However, several days before the launch, we decided that we could fly the two-stage as a single stage with no motor in the upper portion of the rocket. Without the added weight of the motor in the upper part of the rocket we knew that it would be somewhat over stable. With the relatively low expected altitude, if it veered into the wind we wouldn’t be looking at too long of a walk.
We got to the MTA late Friday night and cast the grain before turning in for the night. In the morning, I had to remove the grain from the motor case so that I could peel the liner off of the outside of the grain. It didn’t take me long to realize that I had forgotten to grease the inside of the motor case the night before. It took about a half an hour of hammering with a rubber mallet (note second photo above) before we finally got the grain out. Once out of the case, the liner peeled away from the grain with no problem. The grain was still not completely cured as I could indent it with my fingernail. The end that we had hammered looked like it had been hammered on as it appeared slightly deformed but had no visible cracks in it. I think that if it had been fully cured it would have likely fractured from the hammering.

Once we got the motor ready, we turned our attention to the airframe and recovery system. This launch was as much a test of the airframe material as of the motor. The airframe was made of 4inch ABS pipe with ABS sheet used for the fins. The upper fins were attached only with ABS cement. The lower fins used the ABS cement and were additionally reinforced with a layer of epoxy and fiberglass. The entire airframe went together quickly and for minimal cost.
The total weight of the rocket as it went to the tower was just less than thirty-one pounds. After a few last minute details, we were ready to launch. The motor came up to pressure quickly, and the rocket roared out of the tower. After an initial turn into the wind as it cleared the tower, the rocket flew very straight considering its rough construction. It continued to look great up thru apogee. Shortly reaching apogee, the main charge ignited. The lower section, with the motor in it, came loose and began a ballistic decent. It was at that moment we realized that we had forgotten to install a set of screws meant to hold the two stages together when the airframe was flown in the single stage configuration. The upper portion of the rocket descended under the drogue chute until the secondary charge ignited at 1000 feet and blew that section apart. The upper part then dropped a thousand feet, and the nose cone landed gently under the main chute. Fortunately we recovered everything. Amazingly, all but one fin were still securely attached to the airframe, and the motor was in good condition. The casing did have a small deformation near the bulkhead end, which may have been attributable to the fact that we had to hammer the grain out of the casing, or may have been from the impact. Fortunately a new casing is easy to make. The altimeter was reporting the max altitude, even after falling from 1000 feet. The altimeters pressure sensor was ruptured by the excessively large secondary charge. The manufacturer (Perfectflite) replaced it and tested out the unit for a minimum charge.
Over all we were very pleased with the launch. All of the problems we had should be easily solved with a prelaunch checklist and a little less guesswork about the proper amount of BP for ejection charges. The ABS material turned out to be very versatile, easy to work with and exhibited enough strength for our purpose. The ABS sheet probably doesn’t have enough rigidity for a high-speed rocket fins, but worked fine at slower speeds. We reached a speed of around 380 mph with this launch. Considering the cost, we will probably use it again in the future.

TR-1 SRM update

I ran the motor through SRM a few times playing around with some of the numbers mostly because of  the switch from a Sorbitol based propellant to Dextrose based.  Ultimately the only change I'm making is the change in sugar type.  This has a few effects on the motor, all relatively minor.  The overall burn time of the motor is reduced by about 1/2 a second, average thrust goes up a bit, and the MEOP goes up to 1200 psi from 1050 psi.  The retention screws are designed to sheer at 1800 psi so the design safety factor is still a fairly conservative 1.5.

Static Test

I spent some time taking stock and organizing the garage workspace a bit this past weekend.  Not the most exciting thing, but useful.  Theo and I came across some old Estes motors and we decided to do a little impromptu motor test.  This motor was a D 12-5. 

 Here are the manufacturer’s stats on this motor:
Total Impulse: 16.84 newton-seconds
Peak thrust: 29.73 newtons
Burn Time: 1.65 seconds
Average Thrust: 10.21 newtons

The second image in the sequence is the motor under full thrust.  The third and fourth images are of the delay charge going off.  I drilled some vent holes in the wooden base, but they weren't enough to prevent the motor from ejecting itself from the "test stand", in this case a wooden block.  The motor only managed to go a few feet in the air.  Theo, "Whoa, we launched it!"

Loki

Ben pointed me to this PDF in the comments section of an earlier post (Thanks Ben!)  I hadn't actually seen this before.  This is a description of the Super Loki system that uses a larger 4" diameter booster and 1.65" dart.  The earliest Loki system used a 3" booster and the dart seems to have varied from 1.375-1.5" in diameter.  Another interesting difference, from what I can tell, is that the earlier version of the dart didn't utilize a boat tail.  That must have resulted in significantly more drag, and yet the performance of the dart was impressive.  This is a photo of one of those early versions that shows the connector/coupler that presumably fits into a recess at the base of the dart.

 

In later versions of the booster (and as seen in the PDF) you can see that the connector/coupler was hollow to accommodate the boattail of the dart.

TR-1 Motor update

We finished the bulkhead and nozzle. The nozzle has been done for a while but still needed the holes for the retaining screws drilled and tapped.

We also ordered some casting tubes from Uline. Four inch ID tubes with a wall thickness of 0.080".  I also intended to order some sorbitol and looked around online for the brand that I use to use, Now Foods, but I was surprised that it appears to have been discontinued. I'm sure there are other sources but am hesitant to try a new brand but I've noticed a significant difference in casting with other brands. After giving it some thought I decided to switch to Dextrose.  It's a bit of a change but propellant produced with Dextrose versus Sorbitol has a slightly higher burn rate which is good for a boosted dart.  It's also about half the cost which doesn't hurt.  I don't need to change the motor design as it was a bit conservative to begin with. The dextrose will result in a slightly higher chamber pressure but still well within safe limits. I bought the dextrose from Lucky Vitamin.  It's the monohydrate form of dextrose so 9% of its mass is water.  This isn't an issue as most, if not all, can be driven off during casting.  It is important to account for this when weighing out to assure the correct ratio.

TR-1 Rocket Motor Casing

Completed Motor Casing:

 

With the casing now completed I will focus on completeing the bulkhead and with that I will have all of the motor componets.  Then I'll turn my attention to making some simple casting tools for the 4" diamter grain segments.  I'm also gathering a list of supplies that I'm hoping to order soon including screws, o-rings, Sorbitol, casting tubes, and a few other misc. bits.

Boosted Darts

I've been a fan of the boosted dart concept for a long time.  My first introduction to the concept was the RRS's boosted dart project that was launched in 1996. I hadn't begun my journey into the world of amateur rocketry yet but just six years after that launch and a year or so of building my own motors, I found myself at the RRS site in the Mojave desert with what I thought was a big rocket, a scratch built rocket that was powered by my version of Chuck Knights J-class sugar motor fabricated from PVC pipe.  That rocket flew and was recovered successfully. Later that same day I witnessed a firing of a motor that dwarfed anything I had done or even thought was in the realm of what an amateur could do. It was a slightly larger version of the booster that the RRS guys had launched at Blackrock.  It was a spectacular firing that unfortunately cato'd.  It made a HUGE impact on me never the less and as I began researching the old sounding rockets of the 50's and 60's I came across the Loki Dart and became even more intrigued by the boosted dart concept.  The amazing thing about the boosted dart concept is that the dart typically coasts most of the way to apogee.  In the case of the RRS, the booster reached an apogee of around 3 miles while the dart coasted to approximately 50 miles.  It's kind of like hitting a baseball. Think about it, a baseball completely changes direction when the ball is hit by the bat and yet within those few milliseconds that it is contact with the bat it gets everything it needs to "coast" into the stands. I've been slowly getting back to rocketry lately and have been thinking about a high altitude attempt. The boosted dart concept seems like the way to go.  I don't currently have the ability to build something the size of  the RRS boosted dart project, but that's where the little Loki dart comes in. The Loki dart from the 50's had a max altitude around 34 miles. The booster for the Loki is a relatively tiny 3" in diameter and produced a bit over 2000lbs of thrust for a little under 2 seconds.  Now those numbers seem much more doable.  I should be able to fabricate a motor up to 7" in diameter, which is good since I'm working with a lower ISP propellant I'll need a larger booster than the Loki used to get similar performance.  The TR-1 motor that Theo and I are currently constructing has the same total thrust as the Loki booster. That motor isn't currently geared towards being a booster but with some relatively minor design changes it could be a decent booster for a boosted dart. I'm planning to fly this motor as a boosted dart to get some experience. If that goes well, the plan is to build a  larger 6" diameter optimized booster and see just how high we can go.

You can find a PDF of the RRS booster dart project on the RASAero site here.

TR-1 Rocket Motor Update

Started work on the after what has turned out to be far too long of a layoff...work and such getting in the way.  We got the motor casing cut to length, ends trued, and screw locations drilled.  Theo and I are hoping to fire this motor mid September and fly before the end of the year.  Pending a successful static firing the plan is to use this motor as a booster for a boosted dart configuration.  More on that later...