Casting Session today

[Ray]

Today I am going to a friends house and we will be casting 48" of 54mm rocket propellent, around 3 Kg. Ground testing of a 6x1.9" grain motor and a 5x1.9" motor too. The grains are set up for a 38mm casing and 1.9" long. Simulations indicate around 650 NS for the 6 grain casing and 550 for the 5 grain.

Formula is an Ammonium Perchlorate, R45M rubber, 1% aluminum, 0.1% Iron Oxide. Some other supporting crosslinkers, and chemical cross binders.

Ground testing will be performed at a remote site, KN of around 330, combustion chamber pressure around 1000 psi. ISP of this propellent at that pressure is around 240 seconds. This propellent is a little low on ISP, but it is a nice slow burning propellent, each motor burns for about 2 seconds under pressure. Additional Aluminum would increase ISP and more Iron would increase burn rate.

We'll see how the session goes.

New Equipment for this cast are a Heavy duty Kitchen aid Mixer, 4.5 qt. and a 15 micron vacuum pump.

Previous densities for the propellent I have made are around 92% of theoretical, the vacuum pump should raise that somewhat. Hoping for at least 95% of theoretical density.

I have ground tested (min 2 times) a 4 grain 38mm motor, and a 3 grain 38mm motor, both at a KN of 250, corresponding to a chamber pressure of around 700 psi.

There has been one flight of the 4 grain motor in a minimum diameter (just big enough for the motor) rocket, using dual deployment for recovery. The flight was around 4500'. I attempted to fly the 3 grain motor, but the motor lit from the aft end and never came up to pressure. The rocket and motor just sat on the pad for about 30 seconds with a lazy flame coming out of the nozzle.

I'll post with the results of the casting session and the ground testing by the end the weekend.

[Hank]

Hello- Well, good luck and results with this one.
What grain form do you use?
Hank

[Ray]

Todays casting session used Bates Grain configuration. Next session will include at least one moon burner grain.

The session went very well. Found that casting 54mm grains was much easier to do than 38mm. There is a lot more room to get the mix in past the coring rod.

The mixer worked very well, no more sore arms for me.

The vacuum pump worked very well. The first degassing would have easily tripled the volume of the mix. Space constraints in the mixing bowl necessitated applying vacuum, then releasing to get the mix to settle. I had to do this at least a dozen times before I could allow the pump to pull to full vacuum. The gauge read in excess of 28" of mercury.

The grains are curing as I write this...tomorrow I will remove the coring rods and weigh the grains to determine the density. True success will be densities in excess of 95%.

I'll post more when I get the weights.

[Hank]

Hello- Ray, I am interested in the vacuum setup you are using to pump fuel for casting. Could you please describe?
Yeah, beware the void.....

Hank

[Ray]

I am not actually using the pump to move the propellant. I am just using it during the process to remove the gas from the mix. I leave the mix in the mixing bowl, place a 1/2" lexan lid on it and pull a vacuum for about an hour. That gets much of the gas out. The pump is a 6CFM Yellow Jacket brand, you can find info on it here.

I got density measurements on the grains I cast...around 92%. Not what I had hoped for, but still burnable.

I static tested two motors that same evening. A 5 grain 38mm motor. Kn around 300, chamber pressure around 800 psi. The motor burned fine. At the end of the burn, one of the casting liners came out of the nozzle and popped quite loudly. The 6 grain 38mm motor burned well too. Kn around 340, chamber pressure around 850 psi. No pops, and very little tail off of thrust. Good motors, both of them.

Next casting session, I'll be sure to dry the chemicals (humid day last time). I am going to mix all the wet goods except the curative and the dry goods the evening before I cast. The day of the casting, I'll mix in the crurative and cast. I use Tepanol in the mix to bind the AP and the R45M together, the reaction produces ammonia gas...I think the reaction is still happening when I pack the grains...adding gas to the mix.

The curative is Isocyanate based, so any moisture in the chemicals, will react and produce Carbon Dioxide...again, adding gas to the mix.

[Hank]

Hello, Ray- Gladdened to read of success.
I understand your use of the pump. It is to creat a low-pressure enviornment to draw off gasses produced by the constituent chemicals of your fuel.
I just researched Tetranol, never having used the stuff, myself. The recommended amount is .2% to .05%. What does this do to the cure time?
Rocket power is going to be incorporated in my next flying model. I've learned how to build a swept wing and wish to see how my work will stand up to a supersonic enviornment, as well as investigate whether the tailess design will remain stable in flight. More thrust than a big rubber band.
What do you use for casings? I wish to try PVC tubing.
Thanks for the information. Hank

[Ray]

Not Tetranol, Tepanol aka HX-878. It participates in the crosslinking reaction and without it you need more curative. I did a batch without it one time just to see what would happen, it took weeks to cure to a bubble gum state...and the Ammonium Perchlorate could easily be rubbed out of the grain.

I use aluminum cases, graphite nozzles and a phenolic impregnated paper as a heat liner to protect the case.

I haven't heard of anybody trying PVC with AP and metals, but I suppose it could be done. I have seen lots of Potasium nitrate/sugar motors done in PVC and some Ammonium Nitrate/Mg motor done in PVC too. There are some design charactaristics you should be aware of. Protect the PVC with a good liner. Don't let the pressures exceed about 350 psi (~24 bar). Use the caps that go on the OUTSIDE of the pipe, and be very consistent in your mixing technique. Richard Nakka is the accepted authority on the subject of Sugar motors and has some step by step instructions on how to do it. If you follow them carefully, you'll have a good motor.

I like AP because the ISP is much higher than sugar and AN motors and the mixing is about the same. I wouldn't use anything other than Aluminum tubing to make AP motors. Graphite nozzles, while expensive, last a long time (multiple burns). I bought the casings complete from this company. Don't exceed ~1100 psi on these cases.

[Hank]

Hello, Ray- Yes, Tepranol.
Thanks for the links. I need to upgrade my knowledge. I haven't mixed fuels for five years.
I have Mg and phase stable AN that will be bound in HTPB. The curing agent has gone stale on the shelf. I used Silicon gasket adhesive and bicycle inner tubes in the past and will probably try this in bits of PVC tubing. Have you ever been to the Hell Fire launches?
Durham's Water Putty was used for nozzles. I have some Graphite that will never be used for EDM tooling. I'll give that stuff a go.
Hank

[Hank]

Hello- Tepanol.
Attached is a sketch I drew of the airframe to work out build plans.

Hank

[marksteamnz]

Hello, Ray- Yes, Tepranol.
Thanks for the links. I need to upgrade my knowledge. I haven't mixed fuels for five years.
I have Mg and phase stable AN that will be bound in HTPB. The curing agent has gone stale on the shelf. I used Silicon gasket adhesive and bicycle inner tubes in the past and will probably try this in bits of PVC tubing. Have you ever been to the Hell Fire launches?
Durham's Water Putty was used for nozzles. I have some Graphite that will never be used for EDM tooling. I'll give that stuff a go.
Hank

Hank I supply cases, bulkheads and CNCed graphite nozzles. They are not all polished and anodized but they do the job and in the critical areas such as nozzle contour are better than some of the others avaliable. You need to make/obtain the liner, ignitor and fuel grain.
For your project they will give you a better margin of safety.
The web site is due for an update but to give you an idea of my capabilities I have supplied a 1100mm by 75mm dia case for testing recently.

[Hank]

Mark- Thank You for your offer but part of my doing things is being able to do them. I have a machine shop here and a Pennsylvania colleague with a barn full of supplies.
What is the amateur rocketry scene like in NZ?
Hank

[marksteamnz]

[SNIP
What is the amateur rocketry scene like in NZ?
Hank[/quote]

Considering we are a small population of only 4 million pretty good.
http://www.rockets.co.nz/nzra/nzra.htm
http://www.nzex.tk/
The last site is being rebuilt but is run by 3 16 year olds all called Matt, who I help out with nozzles etc. Nice guys with big plans and parents that are suportive.

[Tom]

Mark,
Nice stuff, at a very nice price. Ever considered sticking in a middle bulk head, to get something like a Ratt Works hybrid? Estimated cost?

Tom

[Ray]

I went out to my friends house yesterday to cast some more propellent and to static test some of the grains I made in the previous casting session.

Casting went well. We tried to do some vacuum mixing of the propellent, but the system wasn't that good, so a minor failure there. We were prepared and proceeded to use the Kitchen aide mixer. De gassing went very well and we were able to get down to 1mm absolute vacuum during the process. It is amazing how much the mix swelled when the vacuum was first applied, more than 3x its size. Mixing under vacuum shows great promise, so I'll need to come up with a way to do that, just to see if its going to improve the process.

Static testing didn't go as well. We started with a "commercial" EX case, purchased from a new guy to the business. He told us that the liner would need to be sanded...actually the case was easily undersize by 0.01" - 0.015". Thats a lot of sanding to do on the liner. The snap rings were too big, but only slightly, so a bit of sanding to the ends allowed us to *JUST* get them into place. Then the o-ring groove on the graphite nozzle wasn't deep enough and required the o-ring to have to compress around 0.050", too much. The snap ring groove didn't have a chamfered edge and coupled with too much o-ring compression managed to shear parts of the o-ring off. At this point we decided that the gods didn't want us to test this motor and aborted the static test. Too many things working marginally for my comfort. We'll fix the issues and test another day. Future purchases of motor hardware will not be through this guy.

[marksteamnz]

Ray there are some tricks to getting an O ring over a circlip groove. The size of the O ring and the recess makes a difference. I've actually modified design to take a 5mm diameter o ring as it is lees prone to sheering little bits of rubber off. The danger with small circlip grooves is if the edges are to rounded the circlip suport is reduced. No excuse for the circlip not fitting this should be test fitted to make sure by the supplier, I'd give them a call and ask them to fix the casing.
If you want my notes and hints sheet on o ring installing let me know.

Tom sorry missed your earlier question. No problem to fit a second bulkhead but if I were to offer hybrids I'd need to supply all the extra's and as I rely on feed back from NZEX it would be a major project time wise. For example there was no hipower launch this month as the two possible dates were rained out.