alternate grain compositions

[MadMat]

Has anyone seen any comp.s or experimented with different rocket grains? My thought was either synthetic rubber compounds or plastics and an oxidizer; such as polystyrene or vinyl for the fuel. My thought would be to dissolve the fuel in a solvent add finely milled oxidizer and cast the grain. Any thoughts?

 

[lloyd]

"Synthetic rubber" is a staple goods fuel item in the AmRoc community. Look it up!

 

Lloyd

[lloyd]

PS... start with "amateur rocketry", and use the key-word "polybutadiene".

 

LS

[MadMat]

I've thought about this for a while now, thanks for pointing me in the right direction.

[Arthur]

Fireworks tend to use BP or whistle comps loaded by some form of pressure/impact. High power rocketry usually uses much more esoteric propellants, -sometimes mixtures, sometimes two parts combined in the motor, often the grain fits in a reloadable rocket body, sometimes the grain core has interesting shapes for thrust profiling. A deeper study of High Power Rocketry will bring up lots of interesting articles on the web, some blogs and likely a forum or ten.

 

Actually BP ceases to function well at high altitudes as it's burn rate is very pressure dependent, -high altitude means low pressure means that BP practically stops burning.

[stix]

What oxidizer are you thinking of using?

[MadMat]

I need to do more research before thinking about which oxidizer to use. I have seen that nitrous oxide is used with polybutadiene to create a hybrid motor. I was hoping for more of a two component solid fuel/oxidizer combination.

[lloyd]

AP/polybutadiene is a common 'cast' motor grain.

 

I can tell you from having made them, that they do not shrink when molded, but it's advisable to cast the grain outside the tube, then inhibit the outer surfaces before inserting it in the tube.

 

AP has the requirement of fairly high chamber pressure, or it won't burn smoothly. To that end, some folks who lean toward pyro add a bit of potassium perchlorate to the mixture.

 

I made a line of shotgun-launched fire-starting flares for the US Forestry service back in the late '90s. Those were ONLY KClO4-oxidized, with polybutadiene as the fuel, and a bit of iron oxide added as a burn-rate catalyst. Nothing else.

 

Lloyd

Edited April 8, 2017 by lloyd

[MadMat]

Lloyd, was your comp cooked, dry mixed and pressed or was a solvent used? Just curious because I figure that a fast-drying solvent such as acetone would be the least labor intensive way to cast it.

[lloyd]

Mat, it's apparent you're not familiar with composite fuels as made by the AmRoc guys. They often refer to these as "rubber grains".

 

The mixture is a BARELY 'pourable' thick liquid when made. It's CAST in a mold, not pressed. The polybutadiene rubber resin fluid is mixed with a curing catalyst just before incorporating with the oxidizer. It hardens (like an epoxy would) on its own to a rubbery solid. I baked mine at a low temperature to accelerate the cure, but some catalysts will work at room temperature. (takes about 24 hours with those)

 

Lloyd

[MadMat]

Oh ok, (brain fart) Yeah I did read about it and the curing catalyst (duh)

[lloyd]

<grin> We ALL miss details, sometimes. Me, most of all!

 

Lloyd

[stix]

As Lloyd suggests: AP/polybutadiene is a common 'cast' motor grain - plus a small percentage of Aluminium powder is often added.

 

Alternatively, Richard Nakka did some successful tests using KNO3 as the oxidizer:

http://www.nakka-rocketry.net/rnx_int.html

 

Cheers.

[Arthur]

Somewhere I have seen a mix instruction said to be NASA's SRB fuel -it must be on the web somewhere. IIRC it was Ammonium Perchlorate with a specific aluminium and HTB (hydroxy terminated butadiene rubber).

[stix]

Yes Arthur, that SRB fuel formula is pretty much as I've read up on in the past.

 

MadMat, If you want to get into APCP, then that is a path where it's best to get involved with HPR folk, who not only have the knowledge, but are able to have a legal framework to work within.

 

This is the main issue today (ie. these current times) facing the "experimental enthusiast".

[lloyd]

Yep, I was friends with John Rahkonen for a number of years before his death. He was (at least in part) responsible for the design and testing of the SRB formula.

 

In his later life, he stocked tens of thousands of pounds of surplus AP from Morton Thiokol that were perfectly suitable for use in AmRoc, but of the wrong particle sizes or crystalline structure for the SRB program. Had he not salvaged it, it would have been just dumped in piles outdoors for the weather to wash into the soil.... (Btw... that's where about 90% of the 'perchlorate contamination of the water table' out west came from.)

 

Lloyd

[MadMat]

I am thinking of trying polystyrene as a fuel and potassium perchlorate as the oxidizer. My idea would be to disslove the polystyrene (styrofoam) in a fast drying solvent, add in finely milled potassium perchlorate, cast it and allow it to dry. I have done initial calculations and figure the composition would be 87% KClO4, 13% polystyrene. Unfortunately, I'm afraid I might run into problems with brittle/crumbly grain.

[lloyd]

Mat,

I'd not be as afraid of a crumbly grain as I would of shrinkage. You'd need to use a VERY small amount of solvent to prevent that, and anything less just might cause the 'crumbly' issue you cite. But enough to properly glue the styrene will also swell it to the point where, as the solvent evaporates (if it EVER does, completely), the composition will shrink considerably.

 

That's one of the reasons AmRoc guys use two-part curing binder/fuels, instead of solvent-bound ones.

 

Lloyd

[MadMat]

I thought of that and.... it could be compensated for by simply making the mold for casting oversized. A little testing would be required, but if the same percentage of solvent is used everytime, the shrinkage should be consistent as well. (I used to work in the tool making field and silicone molds were made 2% oversized to allow for shrinkage of the cured silicone.)

Edited April 13, 2017 by MadMat

[lloyd]

Nope... it will 'funnel'. Mark my words!

 

Lloyd

[MadMat]

Oh well, it was an idea.

Edited April 13, 2017 by MadMat

[NeighborJ]

I've fallen in love with pressing or ramming fuel around a spindle. I can press a rocket in minutes and fly it immediately. Screen mixed and rammed sugar rockets are lacking in power and have size limitations compared to cooked fuel. I've seen a thread here last year about cooking then granulating r candy for pressing. I never tried it but it would seem to be the ticket for speeding up the fuel and overcoming the size limitations. It should make a perfect, strong, bubble free grain and be able to fly immediately.

[MadMat]

In the past I've always cooked my R-Candy, but recently I tried pressing a screen mixed sugar rocket. The performance was disappointing and absolutely required a core for flight. The rocket I made had a short core (approx. 20% the length of the grain). It flew well for the duration of the core (about 50 feet) then flight began to arc badly and fall, all while the fuel was still burning.

[MadMat]

Well I tried making a small 20g sample of polystyrene/potassium perchlorate comp using acetone as a solvent. The good news; the polystyrene didn't actually appear to dissolve in the acetone, but rather "melted". It stayed rather pliable even after most of the acetone had evaporated (pressable?). I think, at this point, shrinkage would be negligible. Test firing a small disk of it produced a bright white flame. The bad news; It was hard to light and burned very slowly. Unless, the burn rate increases dramatically under pressure, this stuff is a no-go.

 

I might also add that it produced white smoke and little, if any, residue. So at least my percentages for the comp are right.

Edited April 13, 2017 by MadMat