Is sorbitol really better than sugar?

[Zwalter]

As far as I am concerned, the reason why people use sorbitol instead of sugar is because it has a low melting point and its more safe when caramelizing it. But sorbitol is more expensive than sugar and harder to find here in the Philippines. Why not just pre heat in high temperature the sugar to caramelize it then turn the heat to low then add the potassium nitrate? Wouldn't that make much more sense than using sorbitol? Can anyone else tell me if there are other advantages to sorbitol aside from having a low melting point. Thanks!!! 

[Silavite]

2 hours ago, Zwalter said:

As far as I am concerned, the reason why people use sorbitol instead of sugar is because it has a low melting point and its more safe when caramelizing it. But sorbitol is more expensive than sugar and harder to find here in the Philippines. Why not just pre heat in high temperature the sugar to caramelize it then turn the heat to low then add the potassium nitrate? Wouldn't that make much more sense than using sorbitol? Can anyone else tell me if there are other advantages to sorbitol aside from having a low melting point. Thanks!!! 

I am something of a neophyte when it comes to making sugar (or similar) rockets, but I was under the impression that when using sucrose (table sugar), you're not supposed to let it caramelize. In the words of Richard Nakka,

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Sucrose tends to decompose (caramelize) at this temperature. Caramelization has been found to be a function of both temperature and time. Since caramelization is detrimental to the performance of KNSU, keeping the temperature as low as possible is important.

Sorbitol, by contrast, does not caramelize.

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Testing of the Sorbitol-based propellant confirmed that the melting point is significantly lower than sucrose-based, and, significantly, no caramelization (decomposition) occurs as a result of normal heating.

I'll quote the final section of the sorbitol/KNO₃ page here, since it seems relevant. It's worth noting that Nakka refers to sorbitol/KNO₃ as KNSB and sucrose/KNO₃ as KNSU.

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The KNSB propellant has a big advantage over the sucrose-based KNSU propellant with regard to casting, and has certain advantages over dextrose-based KNDX propellant, as well. The temperature of the melted mixture is significantly lower than KNSU, which results in an improved safety margin with regard to overheating. Lower casting temperature also leads to the significant advantage of less rapid freeze-up of the melted slurry, which results in a better cast product with fewer voids or air bubbles. Since KNSB propellant does not caramelize, the pot life is essentially unlimited. This makes it very suitable for casting very large propellant grains. The largest KNSB powered motor that has been successfully fired to date is a 300 mm static test motor built by Rick Maschek which contained two grain segments, each having a propellant mass of 55 kg (121 lbs).

The mechanical properties of cast KNSB propellant are similar to sucrose-based KNSU and dextrose-based KNDX propellants, that is, rigid and brittle. The tensile strength, however, is appreciable. As such, the structural integrity of a grain made from this material is substantial, and if reasonable care is taken in the design and support of the grain in the motor chamber, large grains are capable of withstanding the high acceleration loads imparted by flight.

The hygroscopic tendency to absorb moisture from the air is significantly less than that of the sucrose-based KNSU propellant and is slightly better in this regard than KNDX. It appears that there is a relative humidity threshold that determines whether or not a KNSB grain will be hygroscopic. I have observed that when the humidity is low (less than 50%) that KNSB propellant left in the open air will remain bone dry.

The performance of KNSB propellant is theoretically similar to that of both the KNSU and KNDX propellants. It is however important to coat KNSB burning surfaces with ignition primer and to provide adequate inter-segment spacing of the grain segments in a motor. This is a consequence of bare KNSB being hesitant to ignite. With this approach, KNSB delivered performance is consistent with performance predictions and comparable to that of KNSU and KNDX.