After years of flying single-stage high power rockets, I decided to make my first foray into the world of high-power staging with the Quantum Leap II kit sold by Public Missiles Ltd. Although I had built mid-power multi-stage rockets before, high-power staging would present some new challenges. First, there is the question of lighting the sustainer motor in flight; the motor would have to be lit with an e-match powered by only a 9V battery, and the timing of ignition would need to be controlled by an on-board computer. Secondly, the sustainer could potentially go too high up to see visually, so I would need to use a GPS tracking device to assist in recovery of the sustainer. This project would be a chance for me to gain new experience in both of these techniques.
For this project I chose to use PML's Quantum Leap II kit, however, the kit would require some modifications to suit my needs. I planned to fly the rocket on two J motors, and I wanted to make sure the fin can and airframe would be strong enough to withstand the high aerodynamic forces likely to be encountered during the flight. So I chose to substitute phenolic tubing for the traditional plastic tubing and then fiberglass the entire airframe. In addition, I had to design an electronics bay to hold the three different computers that the sustainer would carry: a PerfectFlite altimeter, a Transolve PK6 flight computer to deploy the parachute, and the GPS tracking device. In addition to these electronics, the booster would carry the PML AccuFire timer to light the sustainer motor. One final modification I chose to make to the kit was to increase the size of the parachute in the sustainer. The kit ships with a 48" chute, but I was concerned that if the sustainer motor did not light, the weight of the extra propellant would cause the rocket to descend too fast. This was especially a concern because I live in Colorado and would be launching the rocket at a relatively high altitude; one of the rocketry clubs I attend here has a launch site in the Rocky Mountains at just under 8,800 ft!
Figures 1A and 1B are photos of the electronics bay. I chose to build the electronics bay into the coupler at the base of the payload bay beneath the nosecone. Rather than epoxying the coupler into the airframe, I used removable plastic rivets to attach the coupler to the payload bay. Doing this allows the coupler to be removed for easy access to the electronics. In the photo you can also see the safety switch which is used to arm the Transolve computer once the rocket is on the launch pad and ready to fly. In addition to conserving precious battery voltage while the rocket is being prepped, this prevents the black powder deployment charge from accidentally being ignited during prep!
Figures 2A - 2D show the details of the electronics bay and the three computers it contains. The computers are mounted on a rectangular piece of bass wood with two threaded rods epoxied along the edges. The threaded rods slide through two holes in the bulk plate at the bottom of the coupler, where wing nuts are used to fasten the assembly into place. A terminal block located on the outside of the bulk plate allows the e-match for the ejection charge to be connected after the electronics bay is closed. The bulk plate also contains a small hole for the antenna of the GPS transmitter to pass through. During flight this hole is sealed with electrical tape to prevent ejection charge gases from passing through into the electronics bay.
Since the electronics bay is located above the parachute compartment, I chose to use an e-match with very long leads for the ejection charge. This allowed me to place the ejection charge at the bottom of the parachute compartment, beneath all the "laundry". For an ejection charge cannister I simply used a 1-inch long piece of 3/8 inch diameter cardboard tubing which I taped around the head of the match. Figure 3 shows the ejection charge after it has been taped over the match head and loaded with black powder.
For GPS tracking, I chose to use the BRB900 transmitter sold by BigRedBee. I ordered the device with a small wire-whip antenna because I did not have space in the parachute bay for a larger "rubber duck" antenna which would have increased the range of the transmitter. I also chose not to order the traditional LCD-screen receiver; instead I ordered a receiver station with a USB cord that could be connected to my laptop. Receiving the data on my laptop would allow me to write all the data into a text file so that I would have a record of every data point received from the GPS tracker. The BRB900 also stores its position data in internal memory which can be downloaded after the flight, but I wanted to have a record of the transmitted data in case the rocket was lost or destroyed.
The booster of the Quantum Leap II is designed for motor-ejection recovery. The booster is connected to the sustainer by an inter-stage coupler, which also houses the AccuFire timer for lighting the sustainer motor (Figure 4). The outside of the inter-stage coupler has a safety switch for arming the timer only when the rocket is on the launch pad. This is crucial for safety as the e-match has to be inserted into the sustainer motor during prep. The match runs from the timer out through a small hole in the upper bulk-plate of the inter-stage coupler and is then inserted into the sustainer motor. Because the interstage coupler must slide into the bottom of the sustainer's fin can with very little clearance, it is difficult to place any motor retention hardware on the sustainer. I wanted to use motor ejection as a backup in the sustainer, in case the Transolve computer failed to deploy the parachute. However, I was quite wary of using motor ejection without motor retention hardware. In the end, after some discussion with more experienced flyers on rocketryforum.com, I chose to go ahead with friction fitting the sustainer motor, and for added security I wrapped aluminum duct tape around the aft closure of the motor casing and the motor mount tube (Figure 5).
Figure 6 is a stitched image of the entire rocket as it would be assembled for flight.
I decided to launch the rocket on two J motors. In the booster, I used an Aerotech J-315R. I chose this motor because of the beautiful red flames produced by the "Redline" propellant. For the sustainer I needed a motor that would light quickly and easily with an e-match. While Aerotech motors can be lit with an e-match that has been dipped in pyrogen, they generally don't light as fast as Cessaroni motors. For this reason I chose to use a Cessaroni J-280 motor with "Smoky Sam" propellant. I hoped that the thick smoke produced by this propellant would make it easier to see the sustainer as it soared high into the sky. Before the launch I ran simulations using the RocSim 9 software which predicted that the sustainer could reach an altitude of just over 8,000 ft AGL. This would be a new altitude record for me if the flight was successful (my previous record being 6,800 ft attained on my Level 2 certification flight).
I launched the Quantum Leap II at the Northern Colorado Rocketry club launch on March 2nd, 2013. Prepping the rocket took a bit longer than I expected it would: over two hours in total for hooking up all the electronics and building the motors (Figures 7A - 7C). Finally I had the rocket on the launch pad at around 12:30 pm. Conditions were great, with very little wind. I held my breath during the countdown, thinking of all the things that could go wrong and destroy my months of hard work. Would the sustainer motor light? Would the parachutes deploy? Would I receive good data from the GPS tracker and be able to find my rocket? I think all of us who enjoy this hobby know the feeling you get right before you push the button!
The launch was perfect. The rocket lifted off shooting gorgeous red flames from the booster. Then, just after the booster motor burned out, the sustainer motor lit and the thick black smoke of the "Smoky Sam" propellant streaked up into the sky. Soon I was straining my eyes to even see the sustainer. The booster was already descending under parachute; a faint puff of smoke indicated that a deployment event had occurred with the sustainer, and then I could just barely make out the parachute. What a wonderful sight to see!
My elation soon turned to nervousness again though, as high-altitude winds quickly began to carry the sustainer far from the launch site. I realized that I would not be able to see where it landed. My friend and launch assistant, Hannah, was able to keep her eye on it for a little bit longer than me, but eventually even she lost sight of the rocket. I began to wonder if it had been a good idea to use the larger parachute. From the distance the rocket had been carried away I knew that my only reasonable hope of finding it rested with the GPS tracker. But for how long would I continue to receive the signal?
We walked out to recover the booster, which had landed in a thick snow patch close to the launch site. The AccuFire timer was beeping out an altitude of 1,712 ft. With the booster safely recovered my attention now turned to the problem of locating the sustainer. I opened up the text file where I had been recording the GPS data. The last signal received from the tracker indicated a point 3.2 km (2.0 mi) from the launch site! Fortunately I could see from the elevation of this position that the rocket was only ~100 m above the ground when I lost the signal. Surely this location must be close to the landing site; once we got there we could probably pick up the signal again, which would lead us right to the rocket.
The bigger problem, however, would be getting to the landing site. It was on public land but across the highway from the launch area, and we spent some time driving up and down the highway before we located a 4WD trail that we could use to get close to the location. Unfortunately, however, during the time it took to drive out there my laptop battery died, so I would not be able to pick up the signal again. I had foolishly left my laptop turned on during the lengthy pre-launch prep, a mistake which I will not make again.
We were able to drive to a spot 800 meters from the location of the last signal, and from there we started hiking. When we reached the location, my rocket was nowhere in sight. I began to worry that we wouldn't ever find it, but I also knew that the true landing site had to be somewhere down-wind from where we were standing. We agreed to split up; Hannah would walk directly down-wind, and I would hike up to the top of a nearby ridge and walk the ridge line, looking in both directions. When I reached the top of the ridge, my rocket was right there, lying in the mud (Figure 8). I whooped with delight and ran to retrieve it. The PerfectFlite altimeter had recorded an altitude of 7,303 ft - a new personal record!
Figures 9A and 9B are graphs of the altitude data recorded by the PerfectFlite altimeter. The staging event can be clearly distinguished in the data, occurring ~6 s into the flight. The average descent rate was 23 ft/s, which is a bit fast and means that I definitely made the right decision to use the 54" parachute.
Figure 10 is a Google Earth image with the data from the BRB900 GPS tracker plotted. The yellow line is the position data which the BRB900 stored in its internal memory. This data perfectly tracks the roads we drove on after recovering the rocket, and I therefore believe it to be quite accurate. However, unfortunately, there is a gap in the internally stored data corresponding to the time when the rocket was in flight. This results in the perfectly straight line between the launch site and the landing site, as there are no data points recorded in between. The blue line is the transmitted position data that I received from the tracker and recorded on my laptop, and it shows the actual path that the rocket took during flight. It is interesting to note that the wind direction changed significantly shortly before the rocket landed. This change in wind direction made finding the rocket more difficult, as we initially tried to search SE of the last received position, and in fact the rocket was NE of this point. The rocket was 127 m up in the air when I received the last signal from it, and the true landing site was 140 m from the position of the last signal. I would have had virtually no chance of finding my rocket without the GPS tracker, and I will definitely use this technology on all of my future high-altitude flights.
The rocket was in good condition after the flight, other than some minor damage to one fin, perhaps caused by the high descent rate. The motor casing was still securely friction fitted in the sustainer after flight, indicating that when done correctly this is a viable alternative to motor retention hardware even when using 54 mm motors. The interstage coupler was quite dirty after the launch (Figures 11A and 11B). Clearly it did get a little toasted when the sustainer motor lit, but there was no structural damage. I was able to clean off most of the soot with a damp sponge, and then I had to sand it a bit to get it clean enough to slide smoothly into the sustainer again.
In conclusion, I learned a lot from this project and had a great time building and flying it. Although high power staging takes a lot of work, it was well worth the effort. You can attain much higher velocity and altitude with staging than you could ever achieve on a single motor. To anyone out there who is thinking of making the "leap" from one stage to two stages, I definitely recommend giving it a try!
Questions or comments about this project? Please contact the author: firstname.lastname@example.org