Above is a picture of the LW-1 motor. The premise behind the motor is to make a motor lighter than my previous motors. As such, I am using 6061 T-6 aluminum for the casing, and mild steel for the nozzle and upper bulkhead. While the nozzle and bulkhead are still steel, I will take care to remove as much excess material as possible, to make the motor lighter.
The high strength aluminum has a tensile strength about the same as the mild steel I have worked with in the past, in fact, it has the same casing thickness as well at .065". While strong, the aluminum is also fairly soft, and care must be taking in the handling and machining of the metal. The biggest drawback of the aluminum is it's lack of resistance to heat. Requiring the use of a thermal barrier in the combustion chamber. I am going to try using a heavy walled cardboard mailing tube for thermal protection.
One other problem I am going to try solving with this motor. The upper bulkhead will need to be retained, in the resent steel motors I have built, I welded the forward closure in place. That's not an option now, so I am going to try a new approach. Steel retaining pins will be used to retain the nozzle, with the pins inserted from the inside and flush with the outer casing surface.
Here is the nozzle and upper bulkhead, next to them are the steel rivets I intend to use to retain the upper bulkhead.
Here is the upper bulkhead in place, with the rivets inserted into the casing. I still need to grind each one of the rivets to the exact length. I'll use a drop of epoxy or thread lock to hold the pins in place when the motor is assembled. Once the motor comes to pressure, the force inside the case will hold the pins in place. I already made one mistake, I drilled through the upper bulkhead for the retaining pins without regard for the location of the o-ring grooves in the bulkhead. So I ended up drilling into the second o-ring groove. I was planning on using some high temp RTV at the bulkhead for thermal protection anyway. So that should give me a double seal even without the second o-ring. (I hope)
Here is side shot of the upper bulkhead.
Design Notes:
Casing is 2.25" Outside Diameter, 2.12" Inside Diameter
Casing Weight: 199.1 grams
Nozzle Weight: 245.8 grams
Nozzle Throat Diameter: .625"
Exit Cone Diameter: 1.45"
Upper Bulkhead Weight: 140.3 grams
Casing Overall: 15"
Internal Chamber Length: 12.71"
Bulkhead Depth: .915"
Cardboard Liner Thickness: .061"
Here's a graphic showing the layout of the bulkhead end.
The motor is ready for its first test, I wanted to get some of the prep work done in advance. So I started with the cardboard thermal liner. It is 2" mailing tube, with an internal diameter of 2", and an outside diameter that's just a little to big to fit into my casing. So first of all I peeled off the outer layer of slick paper wound around the tube, that left me with bare cardboard. It would now go into the casing, but it was still a tight fit so I sanded the tube lightly. Once done, it was a snug fit but slid in with modest force. I wanted complete thermal protection, I carefully measured the internal length of the casing at 12.71". The cardboard liner was then cut to 12.71", or as close as I could.
The thermal liner was slid into the case, then the upper bulkhead was inserted to it's retained position, pushing the thermal liner into position butting against the bulkhead. The bulkhead was then removed, and a thin bead of high temp RTV applied to the end edge of the thermal liner. The bulkhead was reinstalled and the retaining pins inserted. Looking through the open end of the casing, a small bead of RTV was rolled up around the thermal liner and onto the bulkhead. That should make for a perfect seal of the bulkhead, as well as complete thermal protection.
I wanted to do this the night before, to give the RTV a chance to cure before inserting the propellant grains. I don't want the grains sealing to the propellant, as combustion gases need to flow around the grain ends.
I also cast grains for the motor. With the thermal liner, I needed a casting tube just under 2". The casting stands I used for the MB-2 would work, but I needed a slightly smaller diameter grain. For the first set of 2 grains I used 2" (OD) PVC vacuum tubing inside my 2" PVC casting stand, with a layer of paper as a bonding layer for my inhibitor. I had used these in the past, and I knew they got real soft with hot propellant in them. But they were about the perfect size so I gave them a try.
The propellant is standard 65/35 sugar propellant with 2% propylene glycol to thin it for casting. After casting the grains, it was a bit of a pain removing them from the vacuum tubing, as expected, there was some deformation of the PVC due to the heat. The PVC is also thick enough that it reduced the grain OD more than needed, I wanted the grains with an OD of about 1.9", the PVC reduced them to an OD of 1.8". For the second batch, I used PVC vacuum tubing for one grain, and thin cardboard in the second. The thin cardboard actually works better, and resulted in the 1.9" grain diameter I wanted.
Grains:
#1) 249.6 grams 4.0"
#2) 230.9 grams 3.875"
#3) 249.8 grams 4.0625"
Core Diameter: .85"
Outside Diameter: 1.83" with one layer paper inhibitor.
Total propellant weight:725.3 grams; 1.597 pounds
This is a graphic of the predicted chamber pressure. From past experience, I'm predicting the pressure will be 20 to 25% higher, and the curve steeper with more tail off of pressure.
This is the predicted thrust profile. Again, I expect it to be steeper with a more gradual tapering off of the thrust.
Here is the time/thrust trace from the test.
The first static test has been completed.
Here's some data from the test:
Burn Time: 1.85 seconds
Maximum Thrust: 323.59 lb.
Maximum Chamber Pressure: 750 psi (approx.)
Isp: 124.7 seconds
Total Impulse: 199.229 lb. seconds; 885.57 Newton seconds (mid "J" class)
Here's the motor at full thrust. After the test I immediately went to the motor to see how hot the casing was. It was hot, it would sizzle a drop of water. But there was no apparent damage or blow by at either end.
Here's the motor as soon as I got home with it. The nozzle slid right out, the blackened nozzle was the only sign the motor had been fired.
Here's the thermal liner removed from the casing. I used water to rinse out the casing before removing the liner. The liner pulled apart some when I removed it. There is only one area, about 3 inches from the left that may have burned through. The inside of the casing was in perfect condition and showed no signs of heat or discoloration.
In summary. The motor performed well, and I'm pleased with the results. As expected, the maximum thrust and chamber pressure were higher than software predictions. With such a large nozzle throat at .625", I gave myself a huge margin of safety on the chamber pressure. The idea is to be able to use the same nozzle with a longer motor, still keeping the chamber pressure at safe levels. What I probably will do, is make shorter grains in the future, that should keep the thrust profile more neutral, as opposed to low initial thrust ramping up the entire burn.
A strange thing happened when I was analyzing the data from the test. I would go through the numbers and I kept getting an Isp of 144. While that would be nice, it didn't seem likely. That's higher than I got when using Mg in the propellant. When predicting the performance earlier, the software was telling me I should have total propellant weight of over 1.5 pounds. And my measured weight was only 1.37 pounds. Was my grain density that poor? Did I measure the diameter wrong?
In the end, I went back and looked at my grain weight numbers, a simple mistake in adding the weights was the problem, the total propellant weight was over 1.5 pounds, as it should have been, and the Isp then worked out to a reasonable 125.
If you've ever wondered why many of my web pages seem disjointed, numbers thrown in here and there. Sometimes switching from one thing to another. These are notes I'm making to myself, numbers and data I can go back to and look at in the future. That's really why I started the web site, so I would have an excuse/reason to get everything recorded. That fact that anyone else can see the information is an added benefit. I do try to add enough thoughts into the pages so they might just make sense to someone else though, so I am thinking of the fellow enthusiast too.
This motor is intended for the new fiberglass rocket I am building. With the motor as tested in this test, it would propel the rocket to about 4,665 feet. With a maximized grain size, the motor would propel the rocket to over 5,000'.
Update: 11-1-2004
I made a couple of changes to the LW-1 motor.
First I'm using EPDM rubber for insulation now, the EPDM is much easier to use than cardboard and seems to perform better. I used the LW-1 motor casing with the EPDM for a test firing of a composite propellant and it held up very well. Sugar propellants have a much lower combustion temperature and should hold up even better. By using the EPDM which is .045" thick I increase the useable chamber diameter over using cardboard .
The next change is going to 4 grains rather than three. Four grains simply provide for a more neutral Kn, which should result in a more neutral thrust profile as well.
I thought maybe I should revisit my casting tools again. Above is a picture showing the required components for casting two grains for the LW-1 motor.
The EPDM is cut to exactly fit inside a 3.75" length of 2" PVC schedule 40 pipe. The paper bonding layer is also cut to be a perfect fit inside the EPDM rubber. I should note, these EPDM rubber sheets are not used in the motor, they are simply spacers to leave the grain the desired diameter. A single large sheet of EPDM rubber is cut as a thermal insulating layer for the motor later.The paper sheets are pre curled by pulling them over a counter edge, if the paper is cut properly you almost can't see the seam, and there is no overlap of the paper. In the picture above the seam side is showing, and you really can't see it. I use a guillotine style paper cutter for nice true cuts.
The casting stand is a wood 2x4. The stand is drilled out to accept the end of the coring tool. As you can see in the picture the end of the coring tool was turned down in the lathe, when inserting the coring tool it's important to make sure the coring tool goes all the way into the hole in the base and the larger diameter at the bottom rests on the wood base. The 2" PVC sections slide into the retainers which are hot glued to the wood base. I simply cut a 2" PVC coupler in half to make the retainers.
Once the PVC pipe has the EPDM spacer and paper bonding layer in it, I lay a square of wax paper over the retainer on the base, the PVC pipe is then slid onto the base and holds the wax paper in place. The wax paper of course keeps the propellant from sticking to the wood base.
I have started using wax paper around the coring tool. It is possible not to use it. But the coring tool must be removed while the propellant is still warm, otherwise removal of the coring tool is difficult and may result in grain fracturing during the removal process. I do try to remove the coring tool while the propellant is still warm even with the wax paper, it generally slides out by hand, but may require a slight tap on the bottom end to break the grain free. All grains should always be carefully inspected for any cracks. Any grains found with a crack should be disposed of.
I generally fill the casting tube to within about an inch of the top, then insert the coring tool. As the coring tool is pushed into the propellant, the displaced propellant fills the tube to the top. Grains need to be overfilled somewhat, as the propellant contracts as it cools. Cutting, or trimming the end(s) of the grains has always been a bit nerve wracking for me. I use a power miter saw and cut very, very slowly. Even then, I sometimes see a spark fly off the cut surface of the grain. So use face protection and leather gloves, and do it outside away from combustibles.
Once the grains have cooled and been trimmed, I use foil tape around the outside of the grains as an inhibitor to keep the outside surface from burning. The single layer of paper inhibits burning too, but wouldn't be enough by itself. The paper creates a superior surface for the foil tape to bond to.
Remember, the grains must be a loose fit in the chamber diameter and length. A tight fitting grain can cause the pressure inside the grains core to increase to the point the grain cracks, creating a large surface burn area and almost certain motor overpressure. These grains with paper and foil tape have a maximum outside diameter of 1.93". While the motor chamber with the rubber insulation has an inside diameter of 2.03". Leaving .1" total of free space for gases to flow around the grain.
Attention to detail is important when casting grains. It really isn't all that difficult, just take your time. Make notes of what you do so you can repeat successes and not failures. Do a check of grain density to make sure there are no large voids in your propellant. Of course, if you're using propylene glycol, grain density shouldn't be a problem. With the propylene glycol, the heated propellant is poured into the casting tubes in a thin stream which pretty much eliminates any voids.