Assembling Motors
 
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.

Static Testing



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.

 

Conclusion:

The motors performed as designed and are ready for flight!

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