[SS-Propellant] efficiencies of KN sugar propellants

[Rich Nakka]

Hi Tony,
Thanks for the interesting comments....

Antoon Vyverman wrote:

> 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,

A couple of things:
1st, yes, that is of particular interest, the fact the the decomposition 
mechanism of sorbitol is significantly different than either sucrose or 
dextrose. Sorbitol does *not* caramelize. It would be  undoubtedly 
beneficial for someone to research this aspect of the sugar properties, 
and see how this plays a role in the overall picture.

2nd. The low melting point of sorbitol may *not* play an important role 
in its odd behaviour. I have experimented with fructose (KNFR) and found 
it to behave essentially identical to KNDX.
And, here's the kicker, the melting point of fructose is lower than 
sorbitol, 103C versus 110C.
Interestingly, fructose decomposes (caramelizes) similar to dextrose.

> 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% !

Part of the reason why the efficiency of KNSB is lower is due to the 
lower average chamber pressure.
http://members.aol.com/ricbnakk/pix/ispcht3b.gif

I agree with you , Tony, KNDX would definitely be the better choice of 
propellant, with regard to motor performance.
The practical problem of using KNDX is twofold, however. One, the 
potlife if less, and two, KNDX hardens as it cools down. The first issue 
*may* give us problems when casting large 34 kg grain segments. The 2nd 
issue tends to cause big problems with disbonding at the 
propellant/casting tube interface.
If we can overcome these two issues, KNDX would definitely be a 
contender for SS2S.

Richard

>  
>  
>  
> Antoon (Tony) Vyverman
> Middelweg 56
> B-3001 LEUVEN
> Tel: +32(0)16404800
> fax:: +32(0)16406100
>  
>  
>
>------------------------------------------------------------------------
>
>_______________________________________________
>SS-Propellant mailing list
>SS-Propellant at exrocketry.net
>http://www.exrocketry.net/mailman/listinfo/ss-propellant
>  
>

-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://www.exrocketry.net/pipermail/ss-propellant/attachments/20051119/c75e9614/attachment-0001.htm