By Timothy Raneyâ€¦Bald Engineer Guy with Glasses
This time, weâ€™ll discuss motor mount tube assembly, the shock cord and its anchor and the recovery parachute. Gosh, does it get any better?Â
Motor Mount Tube Assembly
Well gee; I completed assembling the motor mount tube with its motor retainer. This entire assembly includes the retainer, two centering rings and the motor tube. I followed the instructions for mounting the Highly Adaptive Motor Retainer (HAMR) made by Public Missiles, Ltd. This particular retainer was designed for 38mm motors.
After sanding the inside of retainer and the motor tube lightly with 220-grit abrasive paper, I applied two-part epoxy (30-minute type) and smoothed the fillets initially with a dental spatula. I also epoxied the centering rings at this time. The aft ring butted against the retainer – I slid the forward ring 1/2â€ in from tubeâ€™s end. Afterwards, I mounted the assembled motor tube on the coil winder â€“ the one I rebuilt for the electromagnet project last year. Itâ€™s always nice to build something that works equally well for more than one application, donâ€™t you think? With the motor tube rotating constantly, the epoxy fillets become very uniform during the curing process.
Shock Cord and its Anchor
The next task was to determine the location and install shock cord anchor. In doing so, I accounted for the maximum length of representative motors and the stainless steel wadding. When using the term â€œrepresentative motorsâ€, it meant length since I had decided on 38mm diameter â€œHâ€ motors earlier. So, I wanted to ensure there was enough space for the comparatively large selection of motor lengths up to ~203mm. Allowing enough room for the wadding is important too. If itâ€™s too tight, it might not allow sufficient expulsion gases to expel the parachute. Additionally, you run the risk of ejecting the motor instead given the pressures involved. I had to determine the shock cord length too. I searched rocketry forums and two references. I found many instances where the supporting data was largely empirical for shock cord lengths and other aspects of rocket design. The forum consensus was generally 3X the rocket length, in agreement with an Apogee Components Peak of Flight Newsletter (issue #120, 15 Feb. 2004). Another rule-of-thumb was 4X to 5X the rocket length in the NAR Member Guidebook (issue #8 â€“ 2010-2011 edition.).
Determining the parachutesâ€™ diameter correctly depends on knowing the rocketâ€™s launch mass. As I mentioned earlier, I bought a 24â€ diameter recovery parachute given an approximation of the rocketâ€™s mass. Well, since I had all the parts now, I decided to weigh them again to verify my previous estimate. The rocket airframe mass (tubes, fins, nose cone) was 655 grams (g). I then added 96g for the shock cord, 31g for parachute, 25g for stainless steel wadding and 19g for hardware and the fire-resistant fabric wadding.
Lastly, I added 125g nominal for prospective motor (H125-12A). Though the motor mass is just the propellant, not the casing. So, the total was a whopping 951g (0.951kg) or 2.07-lbs. Wow, almost a kilogram! Isnâ€™t that answer convenient? Though this result was very close the early estimate. Dare I say close enough for now? Well, given the mass is ~2-lbs.; we can expect a 20 foot per second nominal descent rate given the table in the Handbook of Model Rocketry. Â Letâ€™s go a little further, shall we? Is this descent rate satisfactory? How do we know? Great questions for you to study while waiting breathlessly for Part-5. Then you can tell me since I havenâ€™t figured out the answers yet!
Yes, I promise we will discuss motor selection, more about the nylon strap shock cord and installing its mount in Part Five (5).
 G. H. Stine & G.W. Stine, Handbook of Model Rocketry (7th Ed.), John Wiley & Sons, Inc., Hoboken, NJ, 2004. See figure 12-12, Descent Rate Chart, pg. 186