Silicone rubber fuel

[ollie366]

I've been dabbling around trying new types of fuels including epoxy, polyester resin, urethanes, etc, after using BP and sorbitol sugar fuels. All have upsides and downsides. Last year I also tried making fuels with silicone rubber as the binder/fuel but had issues with CATOs. It makes for a very powerful fuel. I tried many different formulations but none as good as what I found this year- read below.

I did some additional research on the use of silicone rubber and found a formulation and approach to using it that works quite well. It's still not optimized but has a lot of potential for a high power, relatively low cost fuel that is easy to produce.

My first rocket this year using the fuel was a 3/4" ID x 7" long (paper parallel wound tube), nozzleless, with a 1/4" core 3.5" long. It took off great and went perhaps 300ft until the it reached the uncored part of the fuel, where it continued to burn but did not continue to climb. This said a nozzle was needed.

I made more fuel and packed it into a 1/2" ID paper tube with a pressed clay nozzle and a 5/32" orifice. I did not use a core- strictly an end burner. I tested it in my stand and it burned smoothly with a lot of " vigor" and no CATO. Progress.

I built some 3/4" ID units with 4" of fuel without a core, one with a pressed clay nozzle, the other with a hydraulic cement nozzle just to see if one material was better then the other. Both were drilled to have a 1/4" orifice (ratio from the 5/32 nozzle in the 1/2" tube). I tried these in the stand and once again, great smooth, vigorous burns for perhaps 5 sec. There was some slag on the outside of the nozzle but it did not close the orifice at all. Turns out the clay nozzle's orifice was still about 1/4" while the cement nozzle opened up slightly.

I then made a 1/2" ID rocket using the same procedure as before- end burn, 5/32" orifice, and wooden plug epoxied into the top of the tube. I used a 1/4" sq x 18" dowel as a stabilizer. The rocket took off quickly and reached 500ft+ and after reaching apogee, there was a pop, probably where the fuel reached the end of the tube and blew out the wooden plug.

The first fuels used "dark" aluminum but the latest one used 30 micron in the mix and other than a few sparklers, I dis not notice much difference. If I had the instrumentation I probably would have observed more power with the finer Al, but qualitatively, there wasn't much difference.

The latest fuel mixture is (wt %):

Ammonium perchlorate 65 (ground in a coffee grinder)

Aluminum 13 (30 micron)

Silicone rubber 22 (GE Silicone II, clear)

 

It takes a bit to mix it initially but after a few minutes, it forms a putty like ball that can be worked to mix it uniformly. After curing (overnight), the resultant fuel is a moderately dense material that is solid and tough. It seems to expand slightly, which works well when packing it into a tube- it's tight in the tube.

 

I'll be interested in comments and any info from others who have tried this type of fuel. I'll be making some 3/4" rockets for testing in the next week or so, but my results so far are promising. I don't have the tools/software to quantify the performance but in comparison to pressed cored BP or cored sugar rockets I've built and launched, this fuel seems to be a better performer, based on observations.

[NeighborJ]

I think you will find that most of the gas production is created by the AP burning. The Al burns hot enough to counteract the silicones natural tendency to cause strobing.

 

The blue strobe fuel which is virtually identical as far as fuel value, burns energetic enough to send the stars swimming thru the air. It really only uses the fuel contained within the AP along with a small ammount of PVC. The aluminum works mostly as a burn rate modifier to stabilize the burn and is likely the #1 cause of the slag buildup.

[ollie366]

Thanks- quite interesting and helpful info. I was thinking the slag came from the Si in the silicone forming SiO2 which has a somewhat lower melting point than aluminum oxide. Perhaps it's some aluminum silicate compound with a lower MP?

This fuel certainly generates a lot of gas!

[NeighborJ]

Makes a nice big cloud from strand tests. The blue strobe fuel leaves a very small amount of flaky tan residue. I wouldn't call it slaggy but it just blows away in a gentle breeze. Edited December 14, 2019 by NeighborJ

[SharkWhisperer]

Greetings Folks, and Ollie.

 

Before I comment/query on Ollie's silicone fuel experimentation, I just wanted to say that I've been following various topics in the APC forum for quite some time, though I only now have registered an account.

 

Anyways, Ollie, I'm curious how you're handling your fuel after formulation, and specifics of how you're loading your rocket tubes:

 

1) Are you simply loading tubes with the fluid silicone mixture and allowing it to dry inside the tubes (if so, no air bubbles forming/retained in polymerized fuel?)?

2) Are you allowing polymerization to complete and then loading tubes? Granulating or adding "solidified" fuel as large (preformed?) boluses?

3) Pressing fuel to consolidate? If so, what forces are you using.

4) Are you using spindled tooling for either cores or endburners, or are you drilling out cores/nozzles after fuel loading/consolidation? By hand with a drill bit or otherwise?

5) Any idea what mesh your AP is after coffee-grinder treatment?

6) Did you select AP over K perchlorate for any specific reason besides larger gas volume generation? Have you dabbled with Pot Perch/silicone formulations at all?

 

Thanks in advance!

[biffo24]

Hi Ollie please take care grinding Ap ,do it remotely if you have to.B

[NeighborJ]

WOAH! Missed that part. Yeah, AP cannot be "safely" milled. Buy the stuff in the proper mesh size. It can and will detonate using it's own ammonia as the fuel.

[ollie366]

Thanks for the tip on grinding the AP. I researched this quite extensively and never found a reference to an issue with grinding it but I'll certainly refrain from doing it. I bought 90 micron AP and it's quite fine but over time, it agglomerated, so the coffee grinder was used to break it up. I ran the grinder for perhaps 15 secs. No more!

 

WRT the questions raised by Shark, here are the answers next to the questions:

 

1) Are you simply loading tubes with the fluid silicone mixture and allowing it to dry inside the tubes (if so, no air bubbles forming/retained in polymerized fuel?)?

- after thorough mixing using a wooden " Popsicle stick" the fuel is granular and can easily be pressed together to form a putty. I put in about 1/2" of the putty into the tube (the clay or cement nozzle was in place) and pressed it by hand with a wood dowel, slightly smaller than the ID of the tube. It packs really well and seems to combine with the prior material with no voids. To test this, I used the same procedure but into a 1/2" ID teflon tube that is almost transparent. The dowel does a nice job of blending the segments together and there doesn't appear to be any air bubbles. The cured material from the teflon tube is like a pencil eraser in consistency. BTW, after curing, I loaded the grain into a 1/2" ID cardboard tube, using fast curing epoxy on the tube walls and on the outside of the grain to prevent bypass. This worked well, the small rocket went out of sight.

2) Are you allowing polymerization to complete and then loading tubes? Granulating or adding "solidified" fuel as large (preformed?) boluses?

- I load the just-mixed fuel into the cardboard tubes and let it cure in the tube- takes about 12 hours. The one in the teflon tube takes a lot longer to cure, so I push it out of the tube and let it cure on the bench- in about 4 hours.

3) Pressing fuel to consolidate? If so, what forces are you using.

- just using the wood dowel by hand, as above.

4) Are you using spindled tooling for either cores or endburners, or are you drilling out cores/nozzles after fuel loading/consolidation? By hand with a drill bit or otherwise?

- endburners only. My tests last year with cores resulted in CATOs most of the time- too much fuel surface area. I did use a nozzleless cored rocket but it's performance was anemic.

5) Any idea what mesh your AP is after coffee-grinder treatment?

- no but I started with 90 micron. The particles after grinding are very fine, no grittiness, but it's a qualitative assessment.

6) Did you select AP over K perchlorate for any specific reason besides larger gas volume generation? Have you dabbled with Pot Perch/silicone formulations at all?

- I tried a few different blends of KP with silicone/Al last year but as I recall, it was difficult to ignite and not very potent. The high gas volume from AP is a big advantage over KP. I did not optimize the KP blends -I'll give it another try, especially now I know about issue with grinding the AP.

Hope that helps. I would like to hear from others that have used the oxidizer/silicone/Al fuels with successes and failures.

[ollie366]

I did some further checking on grinding AP and found this thread that indicates it's OK to use a grinder for AP as long as you take precautions for contamination.

https://www.amateurpyro.com/forums/topic/3805-ammonium-perchlorate/

 

I use a coffee grinder that is exclusively for AP. I understand the interactions with other oxidizers so I elected to get a grinder just for AP, another for KN, another for fuels (sorbitol). I wash out the KN and dry it then use it for KP.

 

I do see significant corrosion of the AP grinder as mentioned in the article.

[NeighborJ]

Please don't be fooled into thinking it is safe to mill AP. Once it reaces a particle size of around 15micron it's sensitivity goes up drastically. Simply the action of the spinning blades poses a real risk of explosive ignition.

 

AP is made and sold as spherical particles because it is less sensitive than the angular particles created from milling. Even if you could ensure that none of your particles are below 15 micron its sensitivity has been altered to a much more sensitive particle shape.

 

AP rocket fuels get around this inability to mill by using catalysts such as copper chromite, ferrocene, potassium dichromate and a few others.

[ollie366]

Thanks for the response and info. Are you saying the use of one of the catalysts will enable one to use unmilled, 90 micron AP?

[NeighborJ]

Yes

Ferrocene is one i have a lot of experience with. Use less than .5%, any more and it becomes unstable.

 

Potassium dichromate is quite toxic but it works terriffic and can be used in quantities up to 5%.

 

I haven't had a need to try the copper chromite but it is quite expensive so I am waiting for a use where nothing else will work.

Edited December 20, 2019 by NeighborJ