I first flew model rockets when I was in elementary school, but didn’t have opportunity to advance to more powerful machines until much later in life.
When I joined the Tripoli Rocketry Association (TRA) thirty years later I was surprised and excited to learn how hobby rocketry had advanced during my hiatus. I began building small rockets which I’d built earlier, that typically weigh less than a pound and reach an apogee (maximum altitude) of a thousand feet or so above ground level (AGL), when they are flown on maximum power. I quickly started to design and build more powerful machines.
There is a limit to the power of a commercial rocket motor which amateurs can legally acquire and fly. Rocket motors begin with A-class, and double in strength for every letter: a B-class motor is twice the power of an A-class motor; a C motor is four times the power of an A motor, and so forth. The strongest motor I’d flown was a D. The lowest level of “high-power” rocketry, called Level-1 (or L1), involves flying H- and I-class motors. Obtaining and flying L1 motors requires the rocketeer is L1-certified by a national rocketry association such as TRA.
Once an L1 candidate has constructed a rocket presumably capable of flying an L1 motor without being obliterated by aerodynamic forces, it is necessary to obtain L1-certification before flying the rocket on a L1 motor. Certification requires passing a two-part multiple-choice exam evaluating rocketry technical and regulatory knowledge. The test is taken at the rocket field, and must be passed for the L1 certification process to continue.
To learn technical aspects of hobby rocketry, I read Harry Stein’s timeless Handbook of Model Rocketry. The regulation aspect involves memorizing fire statutes and codes, which is not my strong suit. Thankfully TRA provides a manual explaining the rules and regulations and providing many sample test questions. The actual L1 exam uses a subset of questions drawn from the TRA manual. I made a flash card for every sample question, and quizzed myself until I was able to answer each question within seconds.
I purchased a Public Missiles Ltd kit for my L1 flight. I selected their version of a fire-and-forget anti-aircraft missile called AMRAMM. Construction took me two weeks to complete, and another week to paint. Every aspect of a high-power rocket build is detailed and exacting.
Mine was a simple rocket that required no electronics to operate the recovery (parachute) system. Instead, seven seconds after the motor burned out a gunpowder charge produced pressure sufficient to dislodge the nosecone and release a parachute, bringing the rocket safely back to Earth. The “ejection delay” differs as a function of the strength of the motor: a weaker motor has a shorter delay than a stronger motor. The delay is intended to “pop the chute” at apogee, when aerodynamic forces acting on the rocket are minimal.
I photographed my AMRAAM (seen on the far right) next to lower-power members of my fleet, for comparison.
Simulation software indicated that the rocket would fly to apogee at 753 feet AGL (the height of a 75-story building) using the minimal L1 motor I’d selected to fly for my L1-cert attempt. To be sure that I could easily find the rocket when it landed I attached an electronic beeper to an elastic “shock cord” which came with the kit and was used to connect the nose cone to the inside of the rocket body. When the cone was ejected and pulled out the parachute, a trip wire initiated a loud electronic chirp until the beeper was turned off.
The flight and landing were perfect. However, the nose cone and beeper separated from the rocket body, because the ejection charge was too powerful for the elastic band connecting cone to body. I saw approximately where the rocket landed, 600 feet or so from the launch rail. I was sure I’d find the cone and beeper quickly. When I arrived at the location where I thought the rocket landed, I clearly heard the beeper chirping. When I walked toward the chirp it suddenly shifted direction—as did I. Then it happened again! And then again, and again, and again! I was walking in circles, wondering if I was hearing things which weren’t real. Then I found another rocket—with the same beeper which I used. And then another. It was obvious that there were many rockets using this chirper, landing in the same area as mine. So I waited for other rocketeers to find their rockets, until mine was the last one remaining: then it was easy to find.
But, the nose cone separation invalidated my flight, so I had to try again. Instead of using elastic band, I used much more durable woven strap. I flew the rocket again, and certified as Level-1.
Eight months later I flew the same rocket at the same field using a more powerful H motor. Simulation indicated apogee at 2,126 feet AGL. It flew perfectly, and I thought I knew where it landed. After hours of searching I had to return to Chicago: my rocket was lost. The next morning I called the park ranger station and described my plight, as well as where I believed the rocket likely landed, and my contact information. I called every day. On the fourth day I received a call from the ranger saying the rocket was found undamaged, and was available for pick-up.
I immediately went to the store and bought cake and sodas, and drove the hour to the ranger station. When I arrived four rangers brought the rocket out in a parade—each holding a section. We had a joyous party!
When I returned home I rebuilt the rocket to make it “dual-recovery.” When a “dual-deploy” rocket reaches apogee a charge is fired by an on-board altimeter, separating the booster from the payload (the nose cone remains attached). A small drogue parachute connected to the strap holding the two rocket sections together initiates a flat spin, bringing the rocket on a straight trajectory toward the ground. At a pre-programmed altitude AGL (in this case, 750 feet), a second charge dislodges the nose cone and brings out the primary parachute: a strategy called “close-proximity recovery.”
I also repainted the rocket booster to maximize its visibility on the way up: black is the last color seen before a rocket goes out-of-sight. And I painted the payload (containing electronics, parachute, and nose cone) Ferrari red: no fauna in the park are that color. The following photo shows the rebuilt rocket flying on an I-class motor to apogee at 3,180 feet AGL. The rocket landed close to the pad.
Many flights of dual-recovery rockets later I’d managed to lose a dozen rockets. Optimal data analysis (a statistical methodology I invented) revealed that I only began losing rockets if the underlying wind was 10 MPH or stronger. I never flew at the Bong Recreational Park in winds of 10 MPH or greater, but I discovered new ways to lose rockets.
Hobby rocketry is never completed, it is always work in progress…
The sky is no limit!
Paul R. Yarnold, Ph.D.
April 7, 2021