sugar rocket nozzle opening sizes

I have some questions regarding the ratio of motor diameter, core diameter and nozzle choke diameter.
i have seen ratios such as 1" id tube, 3/8" core, 3/16" nozzle
first is this a good formula? second if i wanted to make a 2" diameter motor, would i then just double the other dimentions?
I have been researching sugar rockets for a while, and i want to be as well informed as possible before i invest time and money into the hobby
lastly, how important is it that the KNO3 be finely powdered when adding it to the hot sugar mixture?
I can get fertilizer grade KNO3 for like $15 for 50# just wondering if this would work for propellant comps.
thanks, brian
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Brian,
Check out Richard Nakka's site. He covers sugar motors very well.
You can also check out James Yawn's site. Different way of processing the propellant.
Google them both.
The ratios you ask about are important. Especially the Kn ratio. This is the ratio between burn surface area over nozzle throat area. Believe me, you really want to understand the relationships here BEFORE you start stuffing tubes with any propellant. Too low, and it chuffs. Too high, and it goes bang!
James

well
it
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Brian, be very careful at that juncture--I almost made those same mistakes myself, assuming that scaling up would solve the problem. Unfortunately,(or fortunately, I should say,) it was pointed out to me that the properties of the fuel do not change commensurately with the physical dimensions of the hardware. As for the grade of your KNO3, just remember, the more surface area of the materials exposed, the higher your burn rate. Like James says in his post, the two single most thorough 'sugar-motor' resources on the web are Nakka's site, as well as James Yawn's. Good Luck, and don't ferget the math! Bob352
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so since powdered sugar has more surface area than granulated, woud you say powdered sugar is better?

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That's going to be dependent on a few variable factors, Brian. In my own personal experience, I've only utilized granular, but have milled it down to refine it a bit.(using a mortar/pestil, BTW) Now the KNO3 I always work into as fine a subtance as possible. Just bear in mind, however, that when making 'R-candy' for propellant, the combined ingredients are heated together, so the additional exposed surface area of powdered sugar would tend to benefit the mixing process, more so than assist in achieving higher burn rates. In the short time that I've been experimenting, I've found that core geometry serves as a better venue for performance alteration, and also seems to be a lot safer than veering off of established procedure when actually formulating the propellant. As always, be safe!
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