Sorbitol has gained great favor as a fuel in sugar motor composite propellants. This is primarily due to the low melting point of this sugar (~97°C). Given this low melting point and the amorphous nature (non-crystallinity) of the sugar, propellant melts can be worked with even after they have cooled enough to be safe to touch. Sorbitol is one of the sugar alcohols and as such, its melt viscosity is low and responds very well to the presence of surfactant to reduce the melt viscosity of a molten propellant formulation. The viscosity of a 65/35(w) KNO3 to sorbitol melt with 0.1% polystep B-1 surfactant included is low enough to easily pour into 18mm casting tubes, even with coring rod present. Sorbitol is also one of the lower priced sugar alcohols. Propellants using sorbitol as the fuel exhibit a "burn rate under pressure" plateau. When in the plateau region the burn rate does not increase much even though the pressure is increasing. The breadth of this plateau seems to be increased with the addition of surfactant. The burn rates under pressure observed with surfactant in the formulation are essentially the same as the burn rate of a burn strand measured at atmospheric pressure. This seems to occur in a region of chamber pressures between ~250 to 700 psi. In my test stand a 65/35 KNSO formulation with no surfactant burns for ~ 1.7 seconds in 38mm motors. This thrust duration increases to ~4.0 seconds with surfactant present and remains at around 4 seconds unless chamber pressures exceed 700 psi. This "plateau" phenomenon disappears when any other small particulate additive such as iron oxide is used in the formulation. A number of static engine test stand results are given below run in the <700 psi pressure range.
KNSO w/B-1
Burn time = 4.0 sec.
Propellant wt. = 0.270 lb
Nozzle Throat = 0.0178 in2
Max. Thrust = 11 lbs
Max Chamber Pressure = 412 psi
Ave Thrust = 7.7 lbs = 34.3 N
Total Impulse = 137 N sec. = 30.8 lb sec.
Isp = 114 sec.
Motor Class = G34
KNSO w/B-1
Burn Time = 3.52 sec.
Propellant wt. = 122.7g = 0.270lb
Nozzle Throat area = 0.01568 in2
Max. Thrust = 14 lbs
Max chamber pressure = 600 psi
Ave Thrust = 9.2 lbs = 40.9N
Total Impulse = 32.2 lb sec. = 143.3 N sec.
ISP = 119
Engine class = G41
338 KNSO B-1 with Phenolic liner, o-rings
Throat area = 0.021 in2
Propellant mass = 165.3 g = 0.364 lb
Thrust duration = 4.37 sec.
Max. Thrust = 13.2 lbs
Max. Chamber pressure = 450 psi
Ave. Thrust = 10.4 lbs = 46.4 N
Total impulse = 203 N sec. = 45.5 lb sec.
ISP = 125 sec.
Motor class = H46
429 KNSO + B-1
Thrust duration = 3.33 sec.
Nozzle throat area = 0.0183 in2
Propellant Mass = 117.8 g = 0.259 lb
Max thrust = 13.9 lbs
Max chamber pressure = 540 psi
AVE Thrust = 9.4 lbs = 41.8 N
Total impulse = 139 N sec. = 31.3 lb sec.
ISP = 121
Motor class = G42
Initial Kn ~ 540
43 mm grains w/ 0.25" cores
254 KNSO with B-1 concrete Nozzle
Throat area = 0.038 in2
Propellant mass = 433 9 g = 0.955 lb
Thrust duration = 6.64 sec.
Max Chamber pressure = 493 psi
Ave. Thrust = 18.4 lbs = 82 N
Total Impulse = 544 N sec. = 122 lb sec.
ISP = 128 sec.
Motor class = I 82
Kn ~ 500
54 mm 2 grain Aerotech motor hardware using a homemade nozzle prepared from Quikrete concrete mix (Lowe's)
and a metal nozzle insert prepared from 5/8" threaded bar stock. Motor liner was spiral wound paper/phenolic.
O-ring spacers employed for grain separation. No pyrogen used on grain faces or core. 1/2" core.
3 grain 54 mm motor using graphite nozzle
KNSO with B-1
Burn time = 7.65 sec.
Nozzle throat = 0.04667 in2 (1/4 ")
Propellant mass = 643.6g = 1.418 lb
Max Thrust = 24.8 lbs
Max Chamber Pressure = 480 psi
Ave. Thrust = 21.6 lb = 96.1 N
Total Impulse = 735 N sec. = 165 lb sec.
ISP = 117
Lower density propellant delivers Isp of only 106 vs ave. of 117 for higher density propellant. Nakka's SRM calculated burn time @ 500 psi is around 1.5 sec.. Burn rates using surfactant are significantly longer using sorbitol fuel with surfactant. It is assumed that the surfactant greatly broadens and defines the burn "plateau" observed by Nakka with the use as sorbitol as a fuel.
SORBITOL
