When you melt them together (very cautiously, in a double boiler), KNO3 and
sugar makes a fairly low-specific-impulse rocket fuel. Actually, it's the
sugar that melts and becomes the fuel component. I don't believe the KNO3
goes into solution very well in molten sugar. It winds up acting as the
oxidizer that's intimately (and hopefully, evenly) dispersed throughout the
sugar once it cools and hardens. The KNO3 needs to be in relatively fine
powder form before it's added to the molten sugar, so it won't wind up
providing oxidizer-rich pockets within the hardened sugar mass.
For fairly obvious reasons, this stuff is referred to as "cotton candy".
KNO3/sugar is something that model rocketeers used (among other things) in
the old days, before the advent of factory-made engines. It's about the
"lowest powered" material that one could put in a rocket and expect to get
some thrust out of. Metal powder/perclorate/asphalt is a lot more efficient
as a solid rocket propellant, albeit less stable and far more dangerous for
at-home compounding. Many modern SRB systems use this latter composition, or
modified versions of it.
Adding a finely-powdered reactive metal to the "cotton candy's" mix, such as
aluminum or magnesium in the proper proportions, will significantly improve
its specific thrust. However, it does so at the expense of requiring the
motor to have a better casing to handle the increased temp/pressure, as well
as requiring a nozzle that's made of a more ablation-resistant material than
you might use for simple KNO3/sugar mixes. (Added Al or Mg dust also
provides the rocket with a spectacularly bright exhaust plume and beautiful
white exhaust trails due to the additional reaction of the metal with the
oxygen in the KNO3.)
I don't believe that you could confine this stuff well enough to have it
detonate in the same manner that a high explosive would under similar
circumstances. It will certainly burst an inadequately-designed motor tube
but that's not really classified as a detonation. I do recall reading about
some of the old-time basement bombers inadvertently setting off an engine
they'd just poured/cast into a motor tube, as they were screwing in the
exhaust nozzle. This was due to their allowing some of the molten mixture to
get into the threads of where the nozzle screwed into the motor body.
Once the nozzle threads grind into the fuel spillage on the motor tube
threads, they'll crush it and may provide adequate heat from the friction to
light it off. At that point. it's ...pfft...WHOOSH! Without any warning,
you get hit in the face by the 6000 deg main exhaust plume of the main
charge and perhaps the nozzle if it wasn't screwed in very far before it set
off the main charge.
There have been a few similar accidents that have happened in munitions
plants. Friction-sensitive explosives (such as picrate-based compounds)
wound up getting onto the threads of the ports of mortar or artillery
shells, where they're filled by glugging in pourable explosives. I think the
Japanese had some problems with this late in WW2, when a lot of their
munitions were being made by conscripted grade-school kids, under emergency
conditions, in bombed-out factories.
So, please be careful if you feel that you must play with any of this stuff.
I don't recommend anyone do it but I suppose that cooking up some "cotton
candy" is safer than trying to make high explosives in the bathroom.
Geoff
I heard that a mixture of KNO3 and sugar
can be explosive. Does anyone know in
what ratio this mixture is explosive?
Thanks!
It's not an explosive, period. If you put it in a strong casing, it can generate enough pressure to burst the casing and go "boom" but there is no explosive shock wave, no Munroe effect.
Also, sugar doesn't melt at 100°C, so a double boiler is right out.
Finally, adding powdered magnesium to hot solution of sugar and nitrate just might get you a date with the local burn ward.