By Timothy Raneyâ€¦Bald Engineer Guy with Glasses
This time, we really will discuss motor selection is some detail, more about the nylon strap shock cord and installing its mount. Wow!
I did not decide the exact motor to use initially. I knew I would use an â€œHâ€ motor for my level-1 certification. This was not arbitrary choice since this certification is required for anyone wanting to fly a rocket powered by an â€œHâ€ motor. And I wanted to determine the exact motor to use based on a scientific methodology. Though it was tempting to go the other route: â€œGee, Iâ€™ll use the same motor Floyd usedâ€. So, I researched this topic further. It turns out thereâ€™s a relatively simple way of determining the correct motor. By â€œcorrect,â€ I mean one with enough thrust to propel the rocket safely to Alpha Centauri or perhaps a slightly closer destination. Yes, itâ€™s time to talk about the â€œthrust to weight ratioâ€ method.
Thrust to Weight Ratio
Well, I used the thrust to weight ratio chart above and the 5:1 ratio rule to size the motor for the ~2-lb. rocketâ€™s mass at launch. The thrust to weight ratio is an efficiency factor for rocket propulsion. Much more information is available via physics texts and many good internet sources. Try NASA, they know their rocket science. By the way, this very useful chart is available via Rocketry Info Central (www.info-central.org). Below is an assembly drawing for a 38/360 rocket motor - downloaded from Aerotech Consumer Aerospace - www.aerotech-rocketry.com. I used the motor dimensions to determine the motor mount tube size and length. With this data and the chart, checking the National Association of Rocketry (NAR) certified motors list was next. The NAR Standards and Testing Committee (S&T) publishes this list annually â€“ it includes all rocket motors certified for use at NAR launches. Just go the NAR website (www.nar.org) and download the list. So, reviewing the â€œHâ€ motors listed, I selected a 38mm H73-10 motor with a 185.6 Newton-seconds (N-sec) total impulse. The H73 motor is a lower total impulse choice in this family. I wanted to ensure the rocket could fly in a stable manner using a lower power motor given potential launch site restrictions due to weather or proximity to local structures and llamas.
So, keeping the 5:1 ratio in mind, an H73 motor can propel the rocket safely since it can develop up to a ~73N (~16-lb.) force to overcome the rocketâ€™s inertia at launch due to its mass. Though at this point, weâ€™ll ignore any static/dynamic friction between the launch rail and the rocketâ€™s launch lugs. This result now introduces us to thrust curves for a particular rocket motor and how we can use the curves for refining our motor selection.Â
Rocket Motor Thrust Curves
In discussing thrust curves, another exceedingly useful resource is the â€œRocket Motor Guideâ€ available via the ThrustCurve website (www.thrustcurve.org). This site has manufacturer specifications, certified performance data and other data on commercial model/high-power rocket motors. Itâ€™s a great source of information since we can download thrust curves for refining the input when using rocket flight simulation programs. Though some of these programs are preloaded with this data. This site has data on the rocket motors certified by NAR, Tripoli Rocketry Association (TRA) and the Canadian Association of Rocketry (CAR). Go visit the ThrustCurve website â€“ it has much to offer. Thus, with the thrust curve graph for the H73 motor at left, we can see the maximum impulse, but it also shows the average impulse over time. This data is importance since it shows the ~52.5N average thrust is still enough to sustain the rocketâ€™s flight and maintains the 5:1 thrust-to-weight ratio. Before we leave this topic, if you forget everything else, remember to check the resources cited above â€“ they do a great job of conveying this information.
Nylon Strap Shock Cord and Installing its Mount
Weâ€™ll wrap-up this episode by discussing the shock cord and its mount briefly again. I decided on a 3/8â€ wide by 27-foot nylon strap shock cord - 5X the 64â€ rocket length. The approximate breaking point for this cord is in the 1000-lb. range. Strong enough for this application with a very healthy margin. I also determined the shock cord anchor location â€“ about the midpoint of the aft body tube.Â This process consisted of merely aligning the internal components with the aft body tube as shown below â€“ you saw this image in part-4.
So, with the shock cord folded into a 10â€ length, parachute, shock cord mount and stainless steel wadding, I could see how everything would fit. This spacing allowed for sufficient room for the parts. At this point, I had to become a little creative for once when I applied the 2-part epoxy about 17â€ from the aft endâ€¦.without dripping epoxy anywhere between. Any globs of epoxy would prevent installing the motor mount tube later. So, I used some thin plastic material as a sleeve within the body tube to protect it from any dripping epoxy.
And I made a ~24â€ long spatula made from a wood dowel and tongue depressor as an epoxy applicator. If I were a doctor, I could stand two feet from you and check your throat. Say, â€œaahh.â€ What an invention! Yes, this scheme worked, but only to a point. The plastic inside the body tube got in the way and I removed it. I did drip a little epoxy that interfered with inserting the motor tube. However, it was not a big job removing the epoxy with a long chisel and then sanding those two areas. I then rolled 320-grit abrasive paper on a thick rubber tube on the end of metal rod for the sanding the areas smooth. This method worked very well. Afterwards, the motor mount tube slid into place nicely. What a save!
Next time, weâ€™ll actually install the shock cord anchor, the stainless steel wadding and the motor mount assembly. Other topics will include applying epoxy and installing the fins. Double wow - Weâ€™re on a roll, now!