Fast Sugar Propellant

[Greg O'Bryant]

I have been doing more experimentation with adding glycerin to sugar propellant. I have been trying to make a propellant that would be suitable in an end burner engine. I have found that the more glycerin you add the slower it makes the propellant burn and the harder it becomes to balance the proper amount of fuel to oxidizer. I think the ratio problem is from the glycerin evaporating out of the hot propellant. So I have been adding less and less glycerin until I have come up with this formula. It is so easy, just make the standard 32.5g to 17.5g ratio and replace 1.5 grams of the sugar with glycerin. I place 32.5g KNO3, 16g sucrose and 1g iron oxide in a frying pan with water and re-crystallize. Leave it in the pan at 250 degrees until it is completely dehydrated. Next turn the heat up to 300 and add the glycerin. It will take a little time to work in the glycerin. You will notice that the propellant will go from a hard chewy texture to more of a crumbly texture that packs together really well. This is the benefit of the glycerin it makes it less elastic and more pliable when it is hot. It requires a certain temp to have this effect and I think it has to do with when the sugar is at it's melting point. This is where I was having problems before, I wasn’t letting the propellant get hot enough for the glycerin to have it’s full effect. When it cools down it is rock hard. I don't know exactly how fast the burn rate is, but from observation it is pretty good. Last the fuel is made from simple sucrose and glycerin both things that you can get from the grocery store.

[Greg O'Bryant]

Does any body have any ideas on a way to bond rocket candy to the inside of PVC pipe? After practicing casting grains into case inhibitors I feel confident in my ability to go forward with an end burner configuration. I made some case inhibitors out of paper and wood good. I coated the inside of the inhibitor with glue and let it dry. When I cast the propellant the heat and residual moisture would activate the glue and bond the grain to the inhibitor. I'm thinking of coating the inside of the pvc pipe with cement and letting it get tacky, then just casting the propellant rite into the pipe. What do you guys think? If I don't hear back I will move on with my plans and let you guys know.

[NickC]

I thought I read something on Nakka's site about grain cracking? I dunno, you might want to research it further.

[Greg O'Bryant]

Grain cracking is a concern, but I don't think that it is a problem the way I want to make my engines. The grain cracks when the propellant is pushed beyond its flexibility, right? In a core burner the web thickness is a lot less than in an end burner so an end burner should be less susceptible to bending and cracking? Also I'm not planning on making engines that have a propellant diameter larger than one inch. I have made rocket candy engines in the past that were core burners with a grain diameter of one inch that worked just fine, case bonded in paper tubes. I have read that grain cracking becomes a bigger problem with larger engines. I am the most concerned with the grain detaching from the case wall and increasing the KN ratio to an unsafe level. So any ideas on how to bond rocket candy to PVC would be very helpful. Thanks

[NickC]

why don't you just use paper/tagboard case inhibitors?

BTW- what's your nozzle design for this motor?

[NickC]

So how's the endburner going? I'm very interested in the endburner since it would make casting extremely easy. BTW- on Nakka's site it states that PVC is a great inhibitor, but it's main flaw was that it didn't bond well to many propellants.

[Greg O'Bryant]

I won't have time to work on it today. I will keep you posted :)

[NickC]

after reviewing all the inhibitor methods on Nakka's site, what about the epoxy/cotton? You have to pack it though, so this would be very difficult, right? Probably some kind of tagboard would be your best bet, because PVC doesn't bond well and if it does come loose with an endburning motor there could be some bad crap.


What kind of Kn numbers do you plan on?

[Greg O'Bryant]

I made an engine today. It had a really fast burn time, and eroded the nozzle worse than my previous engines I have made with paper. I will leave the steel washer out of the nozzle until I am sure the grain is bonding properly. The erosion is kind of a back up for over pressurizing. My next plan is this. The 1-inch PVC pipe is actually larger than 1â€

[NickC]

high pressure also increases burn rate. So if it already burns twice as fast you would get ~256psi. Then you have more then you have ~250% the amount of pressure that is applied to the KN/Sucrose to get 100psi, so you may end up with 300-500psi. This is most definitely a geometric equation and not linear. I wish I could understand that nozzle theory page though.

[Ray]

You'll find that you can't really predict with any level of accuracy what your burn rate coefficient and burn rate exponant are without some sort of experimentation.

Even the pro's that I know can't predict these values.

If you stick with the formulas that Richard has provided, including things you may see as minor, like particle size, brand of KN, brand of sugar etc, you can have a reasonable assumption that the burn rate that he has provided is going to be what you get...

At the end of the day, you just have to try it. You won't have the same results as others, but you can control things in your world enough to have repeatable results. Careful control of the process etc, is key here.

OH, test, test, test...start at low KN and work up. You are doing PVC motors...the casings are cheap...make a bunch, start with a low KN, increase them incrementally until you blow one up, then back off a bit. This method isn't as good as measuring everything along the way, but with a bit more time, a bit more smoke, and a bit more risk, you arrive at your destination just the same.

REMEMBER to be SAFE along the way. If you suspect you'll CATO the motor, take precautions...if you DON'T suspect you'll CATO the motor, take the same precautions. Its when you are over confident and careless that you get hurt.

There are only two kinds of experimental motors....those that haven't CATO'd and those that WILL.

[NickC]

I guess a good goal would be to get it away from the experimental stages as soon as possible :) . With an end burner you can get longer burn rates with a slimmer casing than with core burners, and you don't get any web regression. I think that may be an option that the Sugar Shot to Space program could look at.

edit- read up more on the SS2S program and they want it to be an amateur rocket, not just an amateur propellant rocket and they must hae a ballistic coeffecient of 12lbs./m^2 so a slim design would be useless as they have to increase their width to meet these standards.

[NickC]

on the wood glue thing, will the wood glue reactivate once the propellant is packed into it?

[Ray]

depends on the wood glue.

I would just leave the inner wrap of the paper free of glue. It'll bond well to the melted sugar propellant without the glue. The paper will absorb some of the propellant bonding it nicely to the tubing.

James Yawn has a great technique. Use wood glue instead of the spray adhesive, the spray adhesive may burn...you don't want that.

[Greg O'Bryant]

Forget the wood glue. The case inhibitor was causing too many problems. Mainly because if it was going to fit, it would be too flimsy. I just tested another engine with great results. What I Did was make a casting sleeve out of paper and tape like on Jimmy Yawn's page. http://www.jamesyawn.com/skillet/index.html
To make the casting sleeve take a piece of plane paper and cover both sides of it with plastic packing tape. The propellant wont stick to this and it allows it to release from the mold. I lined a piece of PVC pipe with this sleeve, being careful not to overlap, and cast my grain in it. After the grain cooled a bit I removed the grain by gently tapping it out with a hammer and dowel. After I removed the sleeve I had a propellant grain that had just the right diameter to cement directly to the inside of the PVC case. I also cemented in a wooden nozzle (1/8 " throat) and a bulkhead the same way. I know wood is not a good nozzle material, and in the future I will use clay. When I fired the engine it started off with quite a bit of thrust, and as the nozzle eroded it calmed down. The good news is that cementing the propellant grain directly to the PVC case seems to have worked perfectly! It had a long and steady burn time, about 4 seconds. I would guess that if the pressure were maintained than it's burn time would be about 2 seconds. The grain weighed 50 grams and was approximately 2 1/2" long. I was thinking that this engine would be even better if there were a way to make a reusable nozzle and bulkhead from ordinary tools and parts. Any ideas?