PR991030-1       Date: 30 October 1999


KN-Sorbitol Propellant Over-heating Experiment


To determine the effects of over-heating the Potassium Nitrate- Sorbitol mixture.

In particular, the physical effects of over-heating the mixture is to be determined,

as well as the auto-ignition temperature. Both objectives are important

and relate to the safety aspect of casting this propellant, the former to provide

physical signs that over-heating is occurring, and the latter, to know how much

of a "safety margin" exists between normal casting temperatures and the

hazardous auto-ignition temperature.


A 16 gram sample of KN-Sorbitol powdered mixture (65/35 O/F ratio; prepared

by the standard procedure) was placed in a small (3 inch) aluminum foil pan.

In the middle of the sample, a thermocouple (type K) was placed, attached to a

metal ring such that the bead was positioned about 3 mm. above the pan bottom

(Fig.1). The pan was placed in a holding rack positioned a certain height above

the tip of an electric heat gun (1400 watt; 1100 deg.F max. rating). A DVM was

used to measure the emf of the thermocouple. Heating was commenced and

the physical appearance and thermocouple emf monitored every minute.

The setup for this experiment is shown in Figure 2.


Shortly after heating began, it was clear that the rate of heating was insufficient.

The height between gun tip and pan was reduced from 13 cm. to 6 cm., then

heating was continued. The mixture was largely melted after about 8 minutes.

The edges of the sample showed signs of decomposition (yellowing) but the

material at the middle remained milky. The sample was then stirred to make

it more homogenous and to make the heating more uniform. After 12 minutes,

it was decided to raise the heat gun tip to within 3.5 cm of the pan, as the

rate of temperature rise was minimal at this point. After an additional 2

minutes, puffs of smoke appeared, together with some light bubbling.

A strong odour of burning caramel was detected. After 19 minutes, it was

decided to terminate heating, in order to closely examine the decomposed

sample. The sample was found to have severe charring below the surface.

The entire bottom layer was blackened (Fig.3). Some pieces of the sample

were ignited, and burned normally. Details of the results are given in Table 1

and in Figure 4.


The maximum temperature of the sample, just prior to termination of heating,

was just over 300 C. The sample was severely decomposed at this point, in

particular, the material in contact with the heated bottom surface of the pan.

As such, it would appear unlikely that accidental ignition would occur during

a normal casting operation, given the unmistakable indicators of overheating,

such as marked colour change, smoke emanation, strong caramel odour,

and if heated sufficiently, charring.

T/C sensor
Figure 1 -- Detail of T/C sensor in sample pan

Figure 2 -- Setup for experiment

Figure 3 -- Remains of sample after test (with charred bottom layer exposed)

Results table
Results graph
Figure 4 -- Heating curve