3 replies to this topic

[BPBR]

The most common I saw is 66% to 34% and 60% to 40%,it may work but for curiosity I also searched which reactions occurs.
5 C12H22O11 + 48 KNO3 = 36 CO2 + 24 N2 + 24 K2CO3 + 55 H2O

Doing the math,means you had to use 4848g of KNO3 and 1710g of sucrose,or 73,9% of KNO3,not very close of the mix most people use,any reason to that?

PS:Sorry english mistakes

[TheChemist]

There are several good reasons for not using the stoichiometric mixture (given by the math) for this reaction.
First of all this mixture is considered an explosive!
Second; the main areas of (peaceful) use are:

Smoke screens:
Incomplete combustion will give more smoke.

Solid state rocket fuel:
The fuel should be easy to cast and you do not want the fuel to damage the rocket by exploding or burning with too high a temperature.

From Richard Nakka's Experimental Rocketry Web Site: http://members.aol.c...on/sucrose.html

"The 65/35 (sucrose/nitrate) ratio has proven to give the best overall performance combined with acceptable casting qualities. Theoretically, the highest specific impulse is delivered at a 66/34 ratio, although the standard 65/35 ratio tends to be used by most experimentalists. There are three reasons for this:

The propellant characterization data has been obtained mainly for the 65/35 ratio
The performance difference is slight (about 1%).
The combustion temperature rises sharply with increased O/F ratio. At the 65/35 ratio, steel nozzles suffer no erosion, as there is an adequate margin between the theoretical flame temperature (1450C) and the melting point of steel (approx. 1500C). At higher O/F ratios, this margin is reduced such that a small error in weighing during preparation could result in a heat damaged nozzle."

[pyrotrev]

Also it may be that slightly incomplete combustion gives the greatest specific impulse?? I remember that the German V2 rockets used an 80/20 alcohol/water mix as fuel for this reason, the extra volume of gas gave more overall power even though the combustion temperature was lower than with pure alcohol. (hic )

[BPBR]

Thanks for replying.I though that using less KNO3 in smoke b**b would make sense because of what you said,but I didn't know that as a propellant more sugar could be too powerful/hot to the rocket.