How Can I Make a "Slow and Soft" Sugar Rocket?

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larry cottrill
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How Can I Make a "Slow and Soft" Sugar Rocket?

Post by larry cottrill » Thu Jan 26, 2006 2:10 pm

Here's a weird question for you guys. I'm asking because I would like a suitable rocket engine for U-control model aircraft !!!

I am only familiar with (and comfortable with) sugar and saltpetre as a fuel. Like most rocket fuels, it burns fast and develops fairly high pressure (at least the way most engine casings are designed). My question is, is there a way a "sugar rocket" fuel grain could be "moderated" so it will develop less thrust but burn over a much longer time frame?

Obviously, a casing with a fairly "wide open" (less restrictive) nozzle will provide lower operating pressure, so that's part of it. But I'm thinking that some inert substance blended in as a "moderator" is the real answer.

The requirements for a model airplane are very different from those for a rocket (where your only "lift" is thrust). An engine that would only run for a few seconds would be absurd for model airplane use. I want an engine and fuel design that will run for at least two minutes, even if the thrust is a figure that you guys would find totally unacceptable.

The idea is to be able to realistically model something like the Bell X-1. With suitable rocket power, even very small scale models would be feasible. But without a reasonable run time, it isn't worth trying.

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re: How Can I Make a "Slow and Soft" Sugar Rocket?

Post by Zippiot » Thu Jan 26, 2006 3:30 pm

i came across this site late last night.

http://www.jamesyawn.com/modelrocket/grains/index.html

he uses the rcandy as the rocket fuel and black powder as the ejection charge, soon there will be a comparison between the rcandy and an equal sized c motor. from wut i understand the rcandy is much less powerful but more stable and longer burning.
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re: How Can I Make a "Slow and Soft" Sugar Rocket?

Post by Ray » Thu Jan 26, 2006 4:33 pm

Larry, a two minute burn time for a model rocket is an eternity...you'll encounter numerous problems getting to burn that long. The most significant will be the heat produced. It'll melt the casing if you don't protect it some how.

You could easily get to a 15-20 second burn. Just make an end burning motor. You'll want to protect the casing from the heat, somehow. Multiple layers of paper can suffice...experimentation will be the key to finding out how much.

You could slow the propellent down with some chemicals, but in the Experimental community, it isn't common to try to slow down the burn...so I am not at all familiar with what you would add. You'd pby give up impulse along the way.

If you just want instant gratification, Aerotech makes a G12 motor specifically designed for rocket boosted gliders...you may find it suitable. 100NS total thrust, 12 N average=burn time around 8 seconds...

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re: How Can I Make a "Slow and Soft" Sugar Rocket?

Post by Zippiot » Thu Jan 26, 2006 7:27 pm

adding iron oxide boosts rocket impulse correct? wut about regular iron?

and wud copper powder make it shoot green?
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re: How Can I Make a "Slow and Soft" Sugar Rocket?

Post by Zippiot » Thu Jan 26, 2006 7:45 pm

i found it!!!

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

also check the page previous to that one.

he says that without the presence of iron oxide it burns very very slowly and loses sum thrust. im sure u have epoxy, so mix it up without the iron oxide or with very little of it and you should get a long light burn
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Re: re: How Can I Make a "Slow and Soft" Sugar Roc

Post by Ray » Thu Jan 26, 2006 7:51 pm

Zippiot wrote:adding iron oxide boosts rocket impulse correct? wut about regular iron?

and wud copper powder make it shoot green?
Iron Oxide doesn't boost impulse, but increases burn rate...sometimes dramatically. Burn rate doesn't equal impulse...

Copper makes a nice blue flame. Barium will give you green.

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Re: re: How Can I Make a "Slow and Soft" Sugar Roc

Post by Ray » Thu Jan 26, 2006 7:57 pm

Zippiot wrote:i found it!!!

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

also check the page previous to that one.

he says that without the presence of iron oxide it burns very very slowly and loses sum thrust. im sure u have epoxy, so mix it up without the iron oxide or with very little of it and you should get a long light burn
Take note, that long burn in the rocketry world (at least in amateur rocketry) is 15 seconds...well short of the 2 minutes that Larry is looking for.

The epoxy mix may not burn without the iron oxide...you'll have to test.

End burn configuration, slow propellant, large-ish diameter, low pressure...you can get to two minutes...but you'll burn up your liner, unless its really thick. If you have a propellent that burns 0.1"/second, then you'll need 12 inches of the stuff for a 2 minute burn...0.1"/second is REALLY slow.

Put a end burning 38mm grain in a 54mm type motor, fill the gap with something to protect the case, you might be able to get it to that 2 minute mark.

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re: How Can I Make a "Slow and Soft" Sugar Rocket?

Post by El-Kablooey » Thu Jan 26, 2006 8:18 pm

You can get Jetex fuel grains on Ebay, still in their original packages. Some of the old Jetex motors will run for 30 seconds or so. You could make your won motor that held a few more grains, possibly.
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re: How Can I Make a "Slow and Soft" Sugar Rocket?

Post by larry cottrill » Thu Jan 26, 2006 8:25 pm

Well, those are some great suggestions. While epoxy seems expensive, it is probably at least as easy to work with as melted sugar. It really sounds like an excellent material to try. Of course, getting the burn rate down that slow (thereby getting a reasonable compact engine) will be the trick.

An 8- or 10-second burn would be hardly more than a good takeoff run in the case of a U-control model plane of typical "sport scale" power-to-weight ratio.

Thanks very much!

L Cottrill

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re: How Can I Make a "Slow and Soft" Sugar Rocket?

Post by Zippiot » Thu Jan 26, 2006 8:40 pm

increasing the length of the motor will give a longer burn but towards the end the pressure would drop dramatically (is my edumacated guess). staging is wut i wud recomend, make a big fat one that burns quick to get you off the ground and a thin one to get you moving once airborne.

staging is really easy. cap the end of the motor with paper, and glu it in place. add another glob of propellant, cover with epoxy or hot glu, and use a drill bit (by hand) slowly cut away to get a small hole in the top where the propellant is exposed. then place the second motor on top of it, and make sure that the hole u drilled is the same size as the nozzle of the second stage. maybe roll up a piece of propellant and stick it up the nozzle of the second stage so it is more likely to catch.

the more propllant u add between them the longer the delay before the second stage takes off.

all this is based on wut i have read the last few days over the internet
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re: How Can I Make a "Slow and Soft" Sugar Rocket?

Post by Zippiot » Thu Jan 26, 2006 8:45 pm

here, buttloads more info on rockets than you cud ever wish to read. all very well written and extensive.

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

my question is some other recipies involving kno3, like the ratio of pvc to kno3 or pvc glu to kno3
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re: How Can I Make a "Slow and Soft" Sugar Rocket?

Post by Zippiot » Thu Jan 26, 2006 8:54 pm

im sorry im making so many posts, but according to the site i just found, epsom salts (magnesium sulfate i think) slows the burn to about 1.73 mm/sec, this is slower than adding water to the mix. charcoal makes it burn about 2 mm/sec, and some other ingredients speeds the burn to nearly 3 mm/sec

all this is open environment testing
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re: How Can I Make a "Slow and Soft" Sugar Rocket?

Post by MJD » Thu Jan 26, 2006 9:17 pm

Iron Oxide doesn't boost impulse, but increases burn rate...sometimes dramatically. Burn rate doesn't equal impulse...
Yes it does.. for the same grain design increasing burn rate increases impulse. Impulse is thrust. Total impulse is the integral of impulse over time, i.e. the total motive energy delivered by the motor.

To comment on the original question, if you are building end burners you are not going to have much of a burn rate problem, in fact you could need more. Control lines make a lot of drag so you need reasonable thrust. I built and flew a ukie in 1992, using Estes C6's CHAD staged together. The model was small (18" or so) and it flew at a normal speed, as in similar to .049 sheet balsa ukies.

With a cool burning propellant like saltpetre/sugar, insulating the case for that long a burn isn't too bad a task. I woud work up though. Try McMaster Carr for XX paper-phenolic (XX "Garolite") tubing in various sizes. I'd start with 1/8" to be conservative. 40 second burn hobby composite motors (end burners) have been built successfully in the past.[/quote]

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Re: re: How Can I Make a "Slow and Soft" Sugar Roc

Post by Ray » Thu Jan 26, 2006 9:28 pm

MJD wrote:
Iron Oxide doesn't boost impulse, but increases burn rate...sometimes dramatically. Burn rate doesn't equal impulse...
Yes it does.. for the same grain design increasing burn rate increases impulse. Impulse is thrust. Total impulse is the integral of impulse over time, i.e. the total motive energy delivered by the motor.
Yep, I stand corrected...I always hear impulse as total impulse in my mind...gets me all the time.

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re: How Can I Make a "Slow and Soft" Sugar Rocket?

Post by MJD » Thu Jan 26, 2006 10:36 pm

Ah, I was just nitpickin'. Don't mind me.

MJD

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