Callisto Rocket Project

Q Class KNER Sugar Propellant

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The Callisto project comes about as a reemergence of my work in sugar propellants, namely large KNER propellant rockets. My work with hybrids and biprops will be put on hold for the foreseeable future and I'll concentrate my efforts and resources on a multi stage, multi year project starting with the Callisto "Q" class rocket. This will be a joint venture of the core members of the Iowa Amateur Rocketry Group, consisting of John Heithus, Steve Mason, Craig Shore and myself; Scott Fintel.

The first step will be to static test a 6 inch diameter "Q" class KNER motor. I've always felt a KNER motor could be pushed beyond the 12 grains I used in the Defiance "O" class motor. We'll find out soon enough if that is the case, as a static test is scheduled for late July or early August 2010. We discussed the option of building and testing smaller motors with similar diameter to length ratios as we intend with this motor, but in the end we decided to just go for it all on our first attempt. If this motor fails, we should have good data to give us an idea of what we need to do to correct the problem. My biggest fear be would erosive burning increasing chamber pressure to failure, but sugar propellants are very forgiving in terms of erosive burning, likely due to the high shear strength of the propellant.

Here are the Bates Grain Burn Rate Calculator results of the projected motor parameters.

12 July, 2010:

We held a meeting at the last IARG launch in June to finalize our plans, decide on the rocket name and make sure the group was all on the same page in terms of goals and expectations. The name Callisto was chosen in homage to the Ganymede rocket, of which certain keys parts will be reused on this new rocket. Ganymede is Jupiter's largest moon, Callisto is next largest, so the name just fell in place from that context. I had the unused 15' long N2O tank section of aluminum tube, a forward bulkhead, the graphite nozzle and the nose cone from the Ganymede to commit to the new rocket. So a large part of what would be the Callisto was already on hand.

After our June meeting I started ordering some of the basics to get us started. Aluminum for a nozzle retainer/expansion cone, custom casting tubes and a Delrin rod for coring propellant. I'll order a few more odds and ends this week that will allow for final motor assembly. Today the Delrin rod came in so I made a new casting stand. I managed to clean up the 2 melting pots I had used for the past couple of years melting asphalt, it wasn't an easy task but I decided it might be some time before I cast asphalt again and I might as well use what I have, rather than buy new melting pots. I used 3 melting pots for the Defiance grains, these are larger grains and I wanted to keep the melt load per pot down to a reasonable level. Steve is bringing his melting pot to give us 4 total. That should bring the load in each pot down to about 3 pounds of propellant. If that seems like too much, I may even add a fifth melting pot.

I also built a vibratory table today, it's just a motor with an offset weight mounted between 2 pieces of plywood with springs to isolate the vibration. I also made a speed control for the motor, this will allow frequency control of the vibration. This vibratory table will be used to help de-gas and settle the propellant once cast in the tubes. In the past I'd used a palm sander for this purpose, and it worked, but I think something dedicated to the job with a frequency control will do a better job, and help us attain a better propellant density.

Last Saturday John, Steve and I did some work on an old building we will use as a static test site. The building was a corn crib, designed to store ear corn in years gone by. While the building has past it's useful life for its intended purpose, the concrete it was built on should serve us well. There is a concrete channel about 2' square running down the middle of the building, we'll use this channel (called a drag way) to mount a test stand. Last Saturday we drilled into the concrete and installed 3) 3/4" rebars in the drag way, then poured a 6" thick back stop in the channel. This back stop will have to handle the force of the motor, expected to be over 3,200 pounds force, but needs to handle at least twice that load in the event of an unexpected accelerated burn.

13 July, 2010:

Here's a picture of the vibratory table and casting tube with ends and coring rod.

The motor for the vibratory table is a cheap little bench grinder. I just removed the wheels and added a steel plate with a bolt and nut to create the "off balance" needed. The blue box behind the table is the speed control, just a 600 watt light dimmer wired to an outlet in the box. I pulled out my data acquisition hardware the other day too, I have a 10,000 pound load cell but it's never been used. I managed to get it wired up and tested, at least tested to make sure it works. I'll have to make some sort of stand to pile weight on to calibrate the cell. I tested my pressure transducer as well, it has a built in calibration shunt which makes it easy to calibrate, the pressure transducer was right on the money in terms of calibration.

Here's a look at the forward bulkhead.

In the pictures above you see the bulkhead with a hose threaded into the pressure port, laying on top is a device for installing a forward end igniter. This device is a 1/4" NPT nipple with one end threaded for a 1/8" NPT nipple and a cap on the other end. The cap is drilled to allow igniter wires to pass through. Once the igniter is installed the entire inside will be potted with epoxy. This will allow us to install the igniter at the launch site for safety purposes, or replace a faulty igniter on site without having to do major disassembly of the rocket. The parts shown were just for test fitting, for actual use I'll use high pressure steel fittings with a hex cap to make installing easier. Head end ignition is almost a must on a rocket this size, trying to thread an igniter wire through 13' of propellant could be tricky. There is also a risk of the wire balling up on ignition and causing a pressure spike. Better to just use a simple head end ignition device to eliminate those possible problems.

16 July, 2010:

As you can see from the table I've cast the first grain. I used 4) melting pots set to 275-300 degrees F. Each pot held 1,250 grams of propellant, just slightly more than needed for a single grain pour, taking into account I over fill the casting tube by about an inch, then cut the grain to final size. I probably spent about 2 hours melting the propellant, stirring, degassing and vibrating the melting pots. After the propellant was melted I did an initial vibration of each pot on the vibrating table, I'm not sure how effective that was at this point in the process. Then each pot was pulled under vacuum and released 3 times, stirred and allow to sit for about 30 minutes, then pulled to vacuum 2 more times. Prior to pouring the propellant was allowed to rest about 30 minutes again, stirred gently and then poured. After all 4 melting pots were emptied into the casting tube I ran the vibratory table for several minutes.

The density on the first grain was very good, at least as good as the best density of a single grain on the Defiance rocket. With that in mind, I do hope to improve the density even slightly more. This first pour used some left over propellant from previous propellant pours, this reused propellant tends to be a little less fluid than fresh melts, so density should improve just from that fact alone. After I cast the first grain I checked on it every hour, after 4 hours the casting tube was still too hot to handle, so I left it to cool overnight. In the morning the grain was cool, when I picked up the grain the coring rod fell right out. This first pour went very well and I don't expect any problems down the road.

I tried to cast some dimples, or small islands on the bottom of the grain to serve as additional support for the casting tubes. See the graphic below:

The idea is to take some of strain off the casting tube during the high g loading of flight. I milled the 3 holes into the lower plate on the casting stand, but these propellant islands broke off. I'm guessing the thermal expansion then contraction sheared them off during cooling. What I may have to do is cast a tube of propellant, peel off the casting tube, then cut and glue the islands to the surface of the grain. I've calculated the load capacity of the casting tubes, while they are more than adequate to handle the load, it is possible they could be slightly off center from each other, that would greatly reduce the load capacity. So some sort of additional support may be required and I'd rather be safe than sorry.

24 July, 2010:

After being sick for almost a week I got back at casting grains last night. I've now got 5 grains cast and hope to get a grain a day cast until I'm done. John and I stopped out to the test site today and pulled the forms off the concrete we poured a couple of weeks ago.

Here's the old building foundation we'll use to anchor our test stand to.

Here you can see the concrete drag way and the back stop we installed.

A little closer shot of the back stop. It has one 3/4" rebar drilled into the floor of the crib (top of back stop) and two rebars drilled into the floor of the drag way. There is one more rebar going diagonally in front of the 3 anchored into the old concrete. Should be more than strong enough for what we are doing.

27 July 2010:

Casting propellant continues, but I also need to make progress on the motor itself. A few days ago I was planning to drill and tap the case and nozzle retainer, but one thing I needed to make sure of, the end of the case needs to be square. Usually I get a factory cut that's very square, this time I had one end cut on a band saw that left the cut slightly angled, but the factory end was also out of square. This is a big deal, the nozzle retainer needs to hold the nozzle perfectly perpendicular to the long axis of the rocket, if not, the thrust will be vectored slightly. The nozzle retainer itself was turned on my lathe and known to be true, so now the question is, how do you square the ends of a 15' long 6" diameter tube. Think about it for a while... Yeah, Ok, a lathe with a 20' bed would do the trick, but since that's not an option...

I decided to cut the end square in my milling machine, easy enough, but you need something to hold and rotate the tube. The only thing I had heavy enough to be stable was my lathe, so I pulled my lathe away from the wall and angled it towards the milling machine. The chuck in the lathe was too high to just chuck up one end in the lathe, so I needed to make a holder to hold my rotary table at the correct height. I also needed a way to bolt the lathe chuck to the milling table.

Craig came the rescue with a 15" diameter by 1.25" thick aluminum plate. I drilled and countersunk machined holes to bolt the plate to my rotary table.

Then precisely centered the chuck on plate and transfer punched the chuck bolt pattern on the plate. Then I drilled and tapped those holes to attach the chuck.

Here's everything coming together. Notice the bracket I made to hold the rotary table to the lathe.

The bracket that holds the rotary table is bolted to the bottom half of the lathe tailstock. I had to mill out the bottom of the square tube to conform to the bottom of the tailstock. The rotary table/aluminum plate/chuck is heavy, well over 100 pounds just in that part of this odd conflagration of parts and equipment.

Here's the milling machine end of things. I made another bracket to hold the tube at this end. I may have to mount bearings on this bracket, but I'm going to try covering the bracket legs in plastic and using a nylon strap to hold it first. There shouldn't be too much pressure on the tube since we will rotate it very slowly.

Here's a shot of everything connected. Good thing I have a long shop!

I'm going to need an extra set of hands to do this, John is going to come over later this week and give me a hand. Hopefully this set up will result in a square tube end, it's been a lot of work setting up.]

1 August 2010:

Yesterday John and I worked at milling the ends of the tube square. I had hoped with everything set up it would only take a few minutes, but that wasn't the case. After the first full turn on the rotary table I could see the end mill was still taking off material, and it shouldn't have been. So we looked at the tube in the chuck and it appeared it was pulling out of the chuck. We tried tightening the chuck but on the next revolution it still pulled out, it was pulling out about .009" each turn. So we tried re-aligning the lathe to the mill, still no good. We tried pulling the tube into the chuck with a nylon cargo strap, still no luck. I tried changing the rotation on the mill but that left a rough edge. In the end what worked was I changed the end mill to a woodruff cutter and moved to a different set of chuck jaws. Once we had it working we quickly had the other end squared.

Here's the result of all that set up and jig making work. A perfectly square tube end and mating surface for the nozzle retainer.

While I had the rotary table/mill set up I went ahead with drilling and tapping the nozzle retainer. There are 16) 1/4" bolts that will retain the nozzle.

10 August 2010:

The final 2 grains have been cast! All that's left to do with the propellant is trim to length and sodium silicate coat the final 2 casting tubes. I've been working on setting up the data acquisition hardware, I thought I had the load cell calibrated last weekend, but when I went to test it my hydraulic press, I clipped the data at about 2,700 pounds. That means the amplifier was set with too much gain, so I worked tonight dialing in the gain to get me as close to optimum as I could. I want the load cell to record up to about twice what I expect in pounds of thrust, that means I need somewhere around 7,000 pounds on the load cell. After much fiddling around I came up with a setting that topped out the load cell at 7,738 pounds, close enough... That gives me a resolution of about 15 pounds, reasonable considering the size of the motor. It would be nice to get down to the 1 pound level, but with a 10 bit ADC and a 20 volt range that just isn't going to happen.

We set a test date of August 22nd. We were going to try to test a week earlier, but the weather has been so hot, rainy and humid I was hoping for a little better weather to finish setting up the test stand. It sounds like the weather will be better this weekend, so we should be able to get the stand finished at least.

17 August 2010:

Here's all 15 grains (142.5 pounds of propellant) trimmed and the casting tubes coated with sodium silicate.

As testing day draws near, I've been working on the little odds and ends needed to prepare for the static test. Last Sunday I did the final work at the test site mounting the U beam and load cell bracket. Today I'm working on mixing up a big batch of ignition primer to paint on the cores and ends of the grains. I'm not looking forward to cutting the thermal liner, but that's something that needs to be done as well. Cutting thermal liners for motors is an exacting job, given the size of this motor it presents just that much more of a challenge.

I mixed up a batch of primer, consisting of 10 grams of Red Dot smokeless powder (for a binder) dissolved in a few ounces of acetone, 95 grams of black powder green meal, 5 grams of magnesium shavings (sieved at about 15 mesh), then added acetone to bring the liquid volume to 8 fluid ounces. I painted the surface of 2 grain ends that were trimmed from a full grain, one was a left over from the 5" Defiance rocket and the other was a freshly cut grain from the Callisto. Once the primer had dried I did a burn test.

In the picture/video, the lower grain is the Callisto grain end and the PIP at the top is the Defiance grain end. The old Defiance grain end was about twice as thick as the Callisto grain end. Click Here for the video.

This video portrays a fairly typical KNER burn in the open air, nothing too impressive. The primer mix did what it was supposed too, got the surface burning, although I've seen better starts than this. I probably should have let the primer dry a little longer and it likely would have performed better, but even as is, it should be adequate. One of reasons I cut the grain ends off, that would be the top end of the grain, is because the erythritol seems to pool just a little at the top before the grain sets. That would give the top of the grain a lower concentration of KNO3 and a poorer burn. Cutting the top off also removes the most porous part of the grain as any bubbles rise to the top. The last reason for cutting the tops off is simply to get consistent density throughout the grain, which leads to better volumetric loading of the motor.

I debated painting all the burning surfaces on the Callisto grains or just the upper cores and all the ends. I've done it both ways, if you get the upper end of the combustion chamber burning, the lower grains will ignite quickly as the hot gas rushes past them. This primer mix is perhaps a little thinner than some that I've used, so I decided to paint pretty much all the propellant burning surfaces. I paid special attention to the top 8 grains to make sure they were well coated. In the end I'm looking at maybe 90 grams of primer mix in the motor, certainly not enough to cause a huge pressure spike but enough to insure good ignition. Of course that's another good reason to static test, if the amount of primer is too little, the motor will come up to pressure a little slower and I can adjust the primer accordingly for the flight.