For those of you that saw the flight of Scientific Method at Mile High Mayhem, here is the process of how I cast the fuel grains.
Since hybrids are a mystery to most people out there I thought I'd document how I cast my motors. I'll be adding pictures and descriptions as I get the grain done. The motor is an M-class, 7500Ns total impulse. I use 3 pounds of nitrous oxide.
First I cut the liner to rough length on my chop saw. The liner is 2.5" SCH 80 PVC pipe. I then took it to the lathe and faced and turned the injector recess into it. I cut it to length and then faced the end. After this I cleaned it with alcohol to get all the grease and gunk off of it.
Liner on the bench, finished. I weighed the liner so I could calculate the fuel density later. The liner weighs 1094.7 grams, and after firing should weigh about 500 grams.
Liner vertical on bench.
Injector end of the liner. It has a recess for the forward closure. It also has some shallow threads turned to help bond the fuel to the casing there. This part of the fuel burns out first and I don't want it stripping off easily.
This is the nozzle end. It is simply faced and the nozzle recesses in here. There is also a six inch long post combustion chamber before the nozzle to help efficiency.
These are the internal parts that shape the grain geometry. There is a stepped top cap that goes in the injector end. The other parts form the post combustion chamber so that the fuel doesn't ooze into it.
I've found the simplest method to keep things from bonding is a barrier method. Forget those fancy sprays and waxes. I used sandwich bags and multiple layers to keep the fuel off the internals. I also use tape to keep everything neat. The cord makes it easy to pull the stop out later.
Here I've put the cord into the spacer that is a smaller diameter than the inside of the grain. Again I've used multiple bags to keep the fuel from the parts.
Here is the parts that make sure we have a clean post combustion chamber in the nozzle end of the grain.
I put one last spacer on top to account for the nozzle overlap and now it's done.
Here is the cap on the end of the grain. I'll later use a clamp on this end to keep everything tight while curing.
Here is the grain cap taped to the liner. The tape does not seal, just to keep everything together when casting the grain.
I'll post more pictures of casting the fuel later - have to get the temperatures up in the workshop.
Edward
Tonight I cast the fuel grain. The formula called for 800 grams and I used my Kitchen Aid 6qt mixer to mix in everything except the curative. For that I used my Armstrong mixer. The formula is 70% solids, yet is not pourable. It is like damp sand once everything is mixed. Makes it very nice to be able to scoop into the casting tube.
One trick I've picked up from other research folks is to pour to within 10 grams on your liquids, then use a syringe like the one pictured to get the last bit. It sure makes it easier and you don't have over pour.
Here are some of the liquids and the aluminum on top. The aluminum is very fine and doesn't sink down into the liquids. You have to mix it with the beater for a bit by hand to get it incorporated, then you can turn the mixer on so you don't have a cloud of dust.
Here is the aluminum mixed with some of the liquids. I just mix this until it is wetted and then continue.
The first mystery component added. I mix each one for about a minute then scrape down and mix for another minute.
Second mystery component added. Again I mix, scrape and mix again.
Third mystery component. Mix, scrape and mix. The mixture is starting to get drier so I am more vigilant on getting it out of the beater and off of the sides of the bowl.
Four mystery component. After this I will mix for about 5 minutes, scrape down, mix another five minutes, scrape down and then mix for 3 minutes on a higher speed. The mixture starts out fairly dry but it gets wetter the more you mix it. Also, during scrape downs you can use the fuel to scrape clean the bowl and suck up all of the liquids.
800 grams is about as much as one single person can mix and cast by themselves. After the last mix and scrape I added the remaining liquids and mixed in the bowl with my Armstrong mixer. I had the casting tube prepared and the top cap already in sandwich baggies. I scooped the formula into the tube and then put on the top cap. I then clamped it with a bar clamp to make sure I didn't spill any. I could have taped it but a bar clamp is much easier. After about an hour the mixture will be fully cured.
One thing that makes it much easier to clean up after mixing (or building) is that on my workbench I have old 3' x 4' plans that I don't need on the table. You can create a very nice mess and just tear a single sheet off of it and you have a brand new surface. I really wish I had discovered this sooner as I have many old sets of useless plans around. It also is nice to be able to sketch on them and write things down. I had the formula written on the sheet and kept track of the components as I went. You really have to clean up well after mixing - if it sets up you are stuck with it on whatever it touches. I've found acetone to be the only thing to take it off and you have a 15 minute window to clean after the final liquids are added. I have in the past had some cure in the bowl and I had to use thermal methods enhanced with oxygen to remove it. I also have had to use a wire grinding wheel to get it all off. You can see the bowl is beat up.
Now, this wouldn't be a complete thread without the obligatory post cleanup scrap burning!
Edward
Today I finished the grain - it is ready for firing. I just had to machine the various recesses and the core. After it was complete I did a complete wipe down with acetone and then wrote the specifications on the grain. After firing they will still be there.
Here is a view of the injector side of the grain. There is a recess in the PVC for the injector to sit on. The largest recess is for the injector to fit into. The next recess is for the Pyrodex Pellets to sit into. Then it is drilled smaller than the pellets so that they don't fall down the core and then you can see the core of the motor.
Here are the specifications on the grain. I generally write these on the side so anyone that has questions I can answer them and so I have a reference. The density turned out a little low, but within tolerances.
Here is a view from the nozzle side. You can see the bare PVC acting as a post combustion chamber and the single central port on the fuel grain. When we step up to the 114mm N motor the geometry will have to be difference. We cannot use the same simple center bore.
I will also be posting pictures as I get the motor refurbished and ready to fly.
Edward
Thanks for the instructions. I have found them fascinating, another thing to add to my bucket list. I presume this will be launched on Sunday or Monday?
Weather permitting it will be Sunday. Last flight was 9,170' at mach. This flight should do slightly more in altitude.
Edward
Tonight I assembled the motor for a test fit and wrote down the specifications in my notebook. The liner is a tiny bit - about .050" short - but will work fine. Better that than not fit between the snap rings.
Here is the motor completely assembled. It is 39" long. Quite the portly motor at 12.5 pounds empty. It will be 15.5 pounds when the tank is filled with 3 pounds of nitrous.
I'll be getting all the o-rings packaged up and ready to go as well as filling the nitrous tank.
Edward
Tonight on the National Geographic Channel they had multiple episodes of the "Rocket City Rednecks" on. Our main character works at Huntsville. His daddy, who is one of the members of the crew was a machinist making parts for Apollo 11, so there is some history. One of the things they did tonight was build a rocket that used moonshine for fuel (Everclear?) and nitrous oxide. It got to something close to 750' if I remember correctly. The rocket was fibreglass. The dual deploy did not work correctly and it did a lawndart. Your tank on top reminds me of it. I am planning on being out at the range on Sunday, so if I can assist in anyway, please let me know and I will do what I can or whatever you will allow me to do.
BEAR
Edward,
Looks like quality craftsmanship to me, looking forward to the flight. Last one was a beauty. Awesome smoke!
Fingers crossed on getting her lit.
There won't be smoke on this one. Tweaked the fuel to get rid of that. It should light as long as we have good weather.
Edward
This is great looking and quite exciting. What is the bird that it will power?
I hope I can be there to see this baby fly. Well done and very well documented Ed.
The rocket is 5.5" diameter and 69" tall (including nosecone). I took a 48" piece of 5.5" tube and added a nosecone. The motor takes up 38" of space and the shoulder of the nosecone takes 5" of space so it leaves 5" for the parachute. It is a very compact rocket and I like how it flies.
Edward
Latest Post: Hello World Our newest member: gloriaz485793 Recent Posts Unread Posts
