Hot Cast Fuel Grains

[Arkangel]

Having just read some more of Richard Nakka's site I'm interested in casting rocket comps, especially KN Sorbitol or KN Sucrose. He suggests having a bath of hot vaseline to put your melting pot in, which is quite an useful principle.

The KN Sorbitol is an interesting propellant, but the casting procedure could end up messy and slow if you do one motor at a time, especially if they are relatively small motors. Also I reckon you have a good chance of getting bubbles in the grain, which would give some nasty pressure spikes.

An idea I'm toying with at the moment is to make a load of rocket bodies, with the upper end plug already epoxied in place. I'd then get maybe 10 bodies and wire them together like a honeycomb, all facing the same way. You could then fill them with a dry mix of propellant and drop them into the vaseline bath at 135 dgerees C. If you left them there for an hour or two, by the time you came back, the propellant should be molten and you can then pop in your coring mandrel (plus a bit of extra fuel if it's not full enough). Once the fuel had fully cooled, you could glue in a pre-cast and engineered nozzle plug and the motor is complete.

The outside of the motor may be impregnated with vaseline, but in some app's, that could be an advantage - waterproof etc. It just strikes me as a better way to mass produce a higher quality fuel grain.

Anyone got thoughts on this or any experience of a similar technique?

[xyz]

I don't think that that would work, because the fuel tends to shrink quite a lot when melted down (all the air spaces in the powder disappear and it becomes just pure fuel). When you prepare an amount of powdered fuel that occupies about 500mL of space, it shrinks to occupying about 150-200mL when you have melted it.

Pressing the melted fuel while it is still semi-liquid gets rid of any air bubbles. I have made rockets with melted propellant before and it is not really worth going to the trouble of melting the propellant unless the rocket is over 1" or so in diameter. Pressing or ramming the fuel is a better choice with smaller engines as it is a lot less time-consuming.

Also, In my experience a Vaseline, Oil bath, or other temperature regulated heat source is not necessary so long as you heat the fuel slowly and carefully with constant stirring. You can tell which bits are starting to melt when the white powder starts to turn an orangey yellow colour similar to toffee.

<small>[ March 20, 2003, 06:24 AM: Message edited by: xyz ]</small>

[frogfot]

Want to point out that vaseline would also get into propellant (if body is of paper), this would reduce it's burnrate.
I found that there are no problems with bubbles as long as you don't mix it vigorously during melting.. Also, after i fill bodytube i move around the propellant with long thin rod, this usually removes big air pockets.

Would also discourage using sucrose, cause it hardens very fast (faster then my brain is used to think). Dextrose and sorbitol would be much better for massproduction, since they don't caramelise, so one could melt 1kg of it and fill one engine at a time.. this would reduce time.
I'm still having troubles to find right coretool, so, no massproduction for me. <img border="0" title="" alt="[Frown]" src="frown.gif" />

[Mr Cool]

I often use KNO₃/Sorbitol, and have never had any problems with bubbles. The molten fuel is quite liquid, and careful pouring and then slightly shaking/tapping the motor will get them all to the surface. Prodding the molten fuel in the casing with a greased stick also helps them to rise - the fuel is a bit like tomato ketchup, in that it gets more liquid if you wobble it.
I use a bath of cooking oil for the heating, it's cheaper than vaseline and you don't get funny looks when buying it. Sorbitol's mp is around 130*C IIRC, cooking oil can easily get hot enough.
I wouldn't melt the fuel in the casings, they'll get horribly greasy (might cause problems when glueing the nozzle in place..?) and you'll need to top it up too like xyz said. Melting and pouring is quick, not too messy unless you try to do it while drunk, and gives good, consistent results.

P.S. - if you're drilling the cores in afterwards, definitely use sorbitol rather than sucrose. The grain will be softer and less brittle, so it's less likely to crack --&gt; CATO. Also the drill bit melts the fuel around the hole, giving a nice clean, smooth finish which looks very neat and proffesional :) .

<small>[ March 20, 2003, 09:50 AM: Message edited by: Mr Cool ]</small>

[kvitekrist]

me and my friend use oil bath with an metal bowl filled with kno3/sorbitol an then we just fill the motors, holding the core in place..
<img src="http://home.no.net/gjalring/sp/motor4.JPG" alt=" - " />

<img src="http://home.no.net/gjalring/sp/core1.jpg" alt=" - " />

<img src="http://home.no.net/gjalring/sp/core2.jpg" alt=" - " />

<small>[ March 22, 2003, 10:42 AM: Message edited by: kvitekrist ]</small>

[DaveTheShit]

How effective is just holding the core in place? I have always used a pre-built coring tool, but it broke just last week and I don't want to have to replace it.

[mr.evil]

i melt the KNO3/Sorbitol on my magnetic stirrer/hotplate until it gets sticky and looks like clay or something. Than i press it in the cardboard tubes, and after a hour or so when it's hardened i drill an core in it.

Never had any problems so far.

[Bigfoot]

Sorry for the stupid question, but where does a fellow purchase sorbitol? I understand it's a chem sweetener used in candy, but where does Joe Sixpack get it?

[mr.evil]

try reform stores (where it always smells funny and they sell those herbal pills) and such stores :)

the sorbitol i bought was €4,99 per 500grams.

by the way, don't use this stuff in your food! you may shit yourself empty than :D

<small>[ April 02, 2003, 12:07 PM: Message edited by: mr.evil ]</small>

[Mr Cool]

<a href="http://www.sugarlessshop.com" target="_blank">www.sugarlessshop.com,</a> $3.79/lb.
Also xylitol, H(CHOH)₅H. More expensive, but can be nitrated to a useful product similar to MHN but probably slightly more stable.

Edit: fixed link.

<small>[ April 03, 2003, 09:10 AM: Message edited by: Mr Cool ]</small>

[Arkangel]

You need to take the comma off the back of that link folks.

By the way, there are a lot of sweeteners available in supermarkets, but they contain another compound - the name escapes me - had a sachet of stuff so I could ask this question, but I appear to have lost it.

Anyone know if this can be used - stuff like Canderel?

[Guerilla]

I just tested burning some sweetener (contains "maltodextrin", don't know the correct equivalent for it) with KNO3. 6 parts of the nitrate and 4 of the sweetener.

I think it caramelised at even lower temperatures than sucrose, it didn't burn very smoothly or reliably either..

<small>[ April 04, 2003, 02:34 PM: Message edited by: Guerilla ]</small>

[Mr Cool]

I think artificial sweeteners are mainly aspartame based,
<img src="http://www.chm.bris.ac.uk/webprojects2000/srogers/imageast.gif" alt=" - " />
I don't think it'd be useful for much, it'd probably decide to decompose and go all black and smelly if you tried to melt cast a fuel with it in. Better to stick with sugar!