Model Rocket Fuel

[Zippidydoodah]

Don, thinking back to childhood reading rocket books, didn't they also use potassium nitrate and sugar as propellant?

[donhuff]

Yes they did ....and still do.

There is a group of guys that are trying to send an amateur rocket to space (100k or 62 miles) , using sugar/KNO3 propellant. http://sugarshot.org/

They have had a good many problems with it though. the biggest problems with the sugar is it absorbs water, and is very brittle when cured, which makes casting and using the large propellant "grains" not an easy thing to do.

When I first got into it, I tried making some KNO3 propellant, but used epoxy as the binder/fuel. It does OK but the KNO3 is so weak. The first time you use some ammonium perchlorate, you'll never touch the KNO3 again.

The most widely used propellants use a mix of ammonium perchlorate, aluminum, and a rubber based binder that also acts as a fuel. Then to that, there are all kinds of things that we add to make a special effect, ie... red iron oxide for a faster burn. Copper oxide for a blue flame. Strontium for reds. Titanium for sparks.

[Archer]

and they are STILL having problems with cracking.
The crack increases the burn interface which means the burn isn't controlled correctly raising pressure potentially outside the profile.

Was watching some of the private commercial burns from off site last year and they were getting about 1/3rd to 2/3rds the expected burn times before having to shut down.

[donhuff]

"before having to shut down"???????????????????????

I wish I knew how to do that. What I'm really saying is, there aint NO WAY to shut down a solid fueled motor once it is "lit". It stops, when there is no more propellant left to consume, or the pressure vessel no longer, holds pressure!

And, USUALLY, when there is even a single crack in the propellant, the motor explodes catastrophically....we call that a CATO.

[Archer]

Don,
You're correct, the solid propellant includes the oxidizer and the fuel.

However I know there were runs that were supposed to go 1.5-2 minutes came up short and seemed as if they were stopped or inerted somehow at ~40 seconds apparently without the earth shattering Ka-BOOM.

I don't know how they did that but I know word was out how long the test was SUPPOSED to run in the general public rumor mill.
It IS possible that the burn ran fast from whatever problem and perhaps was vented on purpose or through natural failure but the plume appeared more or less unchanged throughout the burn with no bursts or flares one would expect from increased burn rate or pressure. Not sure what we'd have seen had it let loose but I suspect we'd have noticed something even at the rather extreme range.

[donhuff]

Uh. I know we are way off subject. Should we move this to somewhere else? I think the original question had pretty much died out, so maybe we are ok.

I have read about some test where a LARGE hose and nozzles were placed up into the core, to shut down a burn. My fear of that would be that the if the water did not extinguish the fire, it would surely raise the pressure by restricting the nozzle. And some propellant mixes will not go out even when thrown into a bucket of water. It could be that they had a way to vent the casing some how, and that put the fire out. Some propellants will not sustain combustion without the pressure, but most will.

I'll bet the sugar motors are pretty easy to extinguish with water.


40 SECONDS???? that must have been a big motor? My largest motors at 4 inch diameter cases, have a typical burn time of 1.5 to 5.5 seconds and 1400 to 400 lbs. of peak thrust.

[Archer]

As for topicality, yeah we are way off and could start another thread or something but I'm not sure I know that much more to need another topic. The topic question has been addressed so I don't think we are hijacking anything exactly. I rechecked the rules and there's nothing in them that I saw about staying on topic. I'm used to rather wide ranging discussions on the other board or two I frequent. I can understand splitting topics off in order to maybe find them later but other than that I think we should be ok.

Commercial R&D test as I understood it and yes big honking motor. Couldn't name the actual project/vehicle as there have been several that MIGHT use the site in question. I can't tell you how far away we were but I'd guess maybe a mile or so and the plume was clearly visible to the naked eye VERY visible using binocs.

I guess water could be done but I'm not certain it would stop the reaction although it MIGHT damp it enough to lower the pressure maybe if it didn't decrease have the opposite effect through reduction in volume.

I've heard rumors at least one of the projects in question was abandoning the fuel mix they were using about that same time period and are testing other options now.

[donhuff]

Yeap. The mix or rather recipe, is the really important part. With manned flights, they tend to use a more fuel rich, proven stable and safe mixture. In missiles, they can try different stuff.