Candy rocket rules of thumb

[madmike]

Can anyone shed some info on the design of candy rockets? ; I don't mean the fuel reciepes, but the design of the basic diameter and depth of the hollow core inside of the fuel in relationship to the motor's diameter and length.

[steve]

good question, I haven't been able to find that info on jimmy yawn's site which is pretty much the standard for this sort of thing. for my engine (which I just test fired for the first time this morning!) I made the hollow section in the grains the same as the nozzle throat (3/16). I'm pretty sure that the grains should be as tall as they are wide (eg: 1 inch tall for 1 inch dia) for the bates design to function properly.

this seemed to work fairly well in my engine, although you should ignite it at the grain furthest from the nozzle to ensure a strong initial burn. I didn't do this which is why my engine took so long to come up to pressure (I think)

[steve]

Found it!
http://www.jamesyawn.com/modelrocket/ba ... index.html
http://www.jamesyawn.com/modelrocket/grains/index.html
that should help.

[Ray]

Can anyone shed some info on the design of candy rockets? ; I don't mean the fuel reciepes, but the design of the basic diameter and depth of the hollow core inside of the fuel in relationship to the motor's diameter and length.

You want your grains length to be about 1.6 times the diameter for a proper bates grain. The ends should not be inhibited, the outside of the grain should be inhibited. The nozzle throat should be around 50% of the area of your core to prevent erosive burning. This doesn't mean you should just run off and make a motor using these basic values...you'll want to understand a bit more of what you are doing first.

Depending on the material you use for the casing, you should run around 800-1000psi. Do the material strength calculations to determine the actual maximum pressure.

A better source for rocket motor theory is Richard Nakka's page. He focuses very heavily on Sugar rocket motors. You'll need to learn about burn rate exponent, burn rate coefficient, KN ratio, ISP*, density, and many more items to properly design a motor.

A great program to predict the performance of a rocket motor is BurnSim. It'll cost you 25USD but the money you save in hardware will more than make up for the cost of the program. To model a sugar motor you'll have to know ISP*, Burn rate exponant, burn rate coefficient, specific heat ratio (although you can use 1.25 as a starting point) and density of the propellent you want to use. A few propellents are included in the basic release, not formulas, but the values mentioned above.

[Greg O'Bryant]

what size of rocket do you want to make? If you are just getting started with making rocket motors a lot of this might seem to complicated, and in fact may be over kill if you are planning on making rockets that are realatively small, and not great performers. Also you will have to do some experimenting to get things just right. I used to make engines that would probably be in the D-E range by following almost the exact same demensions that you would find for a core burning black powder rocket. The casings were made out of cardboard and they had a clay nozzle, they would still go at least a thousand feet or so. I didn't use any special grains, impulse data or calculations. What I would do is have a safe way to test them before trying to fly them. for example I would light them upside down in an iron pipe that was buried in the ground. You want the pipe to be larger in diameter than the engine so that it doesn't add confinement when an engine may blow up, which some will! Anyway start with a shallow core depth, keeping everything else the same, start increasing the core depth on your engines until you start to notice some failures. Keep good notes and then simply back off to a safe working depth. Next a good way to fly these engines if you are still not shure how they will behaive, and you don't want to sacrifec a modle, is to simply strap on a wooden dowel so effectively you have a giant bottle rocket. A few demesions for starting off may be something like the following, mind you I am just pulling theses out of my head and I can't promise anything, but these demensions are roughly what I have used before.
3/4" inside case diameter 3/8" case wall thickness
1" clay nozle depth
2-3" core depth past the nozzle
3/16-1/4" in core or nozzle throat diameter
1 1/4 " of propellant past the end of the core depth
Remeber consistency is key you must keep good notes.
Also I would recomend reading a few books on the subject,if you haven't, like "Building Your Own Rocket Motors" by the teleflite corporation.
Make certain that you are legal thanks Bush you now need a low explosives permit just to make them and even more permits to store your propellant and the finished engines. I hope that this has helped and have a merry Christmas!