January 11, 2003
The grains that were created on the Melt/Cast Grain Preparation
page match the dimensions of Aerotech commercial 38mm AP Grains. This allows the same motor
assembly method to be followed that is familiar to many already.
The motor that we will be assembling is calculated to
be an H180. It will have a total impulse of 215.5 N-sec.
Before beginning assembly, the nozzle must be drilled to the proper diameter. The diameter
should be 15/64 inches for the 3 grain sorbitol motor demonstrated here.
The motor liner should first be lightly coated with petroleum jelly (or even better, use
Radio Shack Lubricant with Teflon part no. 64-2326). The threads of the closure are also
lubricated and the O-rings are lightly lubricated per Aerotech instructions. Grains are
then inserted into the liner tube, and the liner tube inserted into the casing. A delay
insulator is then inserted into the forward end of the casing, followed by the Forward O-ring.
The Plugged Forward Closure is then screwed onto the casing. The Aft delay insulator is
inserted into the aft end of the casing, followed by the Aft O-ring. The nozzle is then
inserted inside the Aft O-ring, and the Aft closure screwed into place. The motor is now
ready for launch.
The demonstration motors were designed with Richard Nakka's SRM.XLS Excel spreadsheet.
A screen shot of the anticipated thrust curve is shown in the image above.
The 3 grain motor was then assembled and launched on a digital test stand to
verify the performance. This graph is a screen shot of the software
written to interface with a home-made digital test stand.
The actual result seems to follow the predicted performance very well,
differing only as expected in the initial pressure and fall off pressure
ends of the graph. The spreadsheet is a very handy piece of software for
designing these motors.
After the test, the motor components looked very similar to those of a typical
Aerotech reload. The liner was charred on the inside, but the outside remained
in good shape. The o-rings showed no visible signs of wear and might be usable
again. The insulator washers were burned on the inside edge extensively, but
held up well where in contact with the o-ring. The nozzle was eroded approximately
1/32 of an inch. No damage to the reusable hardware was observed.
The motors performed as designed and are ready for flight!
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