[SS-Propellant] efficiencies of KN sugar propellants

[Antoon Vyverman]

Hello Richard,

Some time ago I made an evaluation of the efficiency and performance of different type of KN sugar propellants. Tha calculations were based on the PROPEP values for each type of propellant and the measured thrust.To make the comparison possible, over and underexpansion of the fixed Ae/At for each case was taken into account. The deviation between theoretical  and measured performance yields the efficiency. In each of the cases only one rocket motor was evaluated. Following results were obtained:
- KNSU: 92,5%
- KNDX: 92.5 %
- KNSBSU: 90.5%
- KNSB: 88%
It strongly indicates that the usage of sorbitol (alone or in combination with sucrose) does decrease the efficiency.
As sorbitol melts at a much lower temperature and is also less viscous than sucrose or dextrose, it may become liquid and be transported over the propellent grain at ignition and shortly after, be deposed between segments, settle on the inner surface and even be expelled. This is probably the reason why it takes a while before the whole surface burns. In almost every case I know, it takes about 0.25 to even 0.5s to reach normal thrust, while according to my experience with KNSU it is generally less than 0.1s. KNDX seems to be situated in between. The delay in ignition of the whole grain surface seems to be related to the smelting temperature of the fuel (or the difference between decomposition and melting temperature?). Moreover sorbitol boils at 295°C, so before it completely decomposes. This may lead to a less complete chemical reaction with KNO3. Available volume may become important as in liquid motors. From about 11 pressure measurements made by J.Koster (NERO) on KNSB 60/40 the characteristic velocity was determined to be 797 m/s compared to a theoretical value of 880 m/s (mix between frozen en shifted), or an efficiecy of 90.4%. This may indicate that the losses are essentially due to the incompleteness of the chemical reaction. 
Hopefully the efficiency of KNSB will increase in larger motors, but this will only be during about half of the time.
If we take into account the density of the propellant, than KNDX is clearly the most performant. The difference with KNSB in terms of Isp*density is more than 7% !



Antoon (Tony) Vyverman
Middelweg 56
B-3001 LEUVEN
Tel: +32(0)16404800
fax:: +32(0)16406100

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