This September at BALLS 30 me (Andrej Vrbec) and my friend Denis Banovic flew a two stage project. Since we are from Europe we packed all the rocket parts, tools and supplies in a big box and sent it across the ocean with cargo freight. We arrived in Reno one week before BALLS and rented a garage there. In the next three days we processed more than 180lbs of propellant and casted four motors- two boosters and two sustainers. Booster motor is a P motor, is 150mm in diameter and has 30kg (66lbs) of propellant. Sustainer motor is an O motor, is 100mm in diameter and has 11kg (24lbs) of propellant. All the motors are case bonded, monolithic grain type. Booster motor has a six-point star geometry and a faster burning propellant, while the sustainer is a simple core burner with a slow burning propellant. Thermal insulation/inhibitor was spin-cast in the casing a few hours before propellant was poured in. Since the flame zone gets in touch with the casing wall only at the very end, the casing insulation is relatively thin- 1mm for the booster and 0,5mm for the sustainer. Propellant was designed so that it can be processed on the field, with as little equipment as possible. Ridgid paddle mixer from Home Depot equiped with a custom made mixing paddle was used for mayority of mixing. After curative was added the propellant was mixed in a compact vacuum mixer that I built myself. The same Ridgid mixer was also used to turn the spin casting setup for thermal insulation. Off course having a propellant that is pourable and easy to work with, also means it can't have very high solids loading. That took some toll on the Isp, but in the end it worked out pretty good. Booster propellant had 80% solids loading and the sustainer 78%. My biggest concern was that AP that we bought in the US will have different particle size distribution than my regular AP, which could have a big impact on viscosity of uncured propellant. I prepared buckets (ingredients in each bucket were already pre-weighed for each batch) of propellant premix which contained everything except the AP and the curative. The only thing we had to do was empty the bucket in the mixer, add the AP, mix, add the curative, vacuum mix and pour. AP that we got was very good quality, but it had slighty higher fraction of fine particles which resulted in somewhat thicker propellant slurry that I was used too. I corrected for that by using somewhat higher processing temperature. After the motors were poured, they were cured for three days. Cores were cut out of styrofoam on a hot wire CNC machine. When the propellant was cured, cores were simply dissolved with a mixture of acetone and xylene. After a week of hard work in Reno we finally moved to Black Rock. One thing that was one of my main concerns was the launch pad. We didn't ship our launch pad because of the weight and the shipping was already expensive enough without it. To the rescue came Tony Alcocer who kindly borrowed us his launch pad and basically saved us, as we couldn't fly our project without the pad. The pad was very sturdy and worked perfectly for our rocket. Since the rocket weighed about 180lbs we decided to fly from Unistrut rail. I got the dimensions for Unistrut launch lugs from Aerocon website and I turned them from Delrin, hoping they will fit on a rail I have never seen before. Luckily the fit was perfect and as you can see in the video, the launch lugs got partially melted during flight from the air friction. We built two identical rockets. Most of the rocket is made from various aluminum alloys except the nosecone which houses the electronics and is made from fiberglass. Nose tip is machined from a phenolic composite and the same material is also used in the nozzles. One interesting feature is the interstage. It's of conical design and the cone that is bolted to the booster fits perfectly in the sustainer nozzle. Great care was taken when machining, making sure that the angle of the interstage cone is exactly the same as the sustainer nozzle divergent angle. Beauty of this design is that when the booster is pushing hard the connection between stages is very rigid and strong, but once the stages start to drag separate only slight movement is enough to break the stages apart.
First flight was on saturday. Basically everything worked as planned (motors worked, stage separation worked, high altitude ignition worked, rocket was stable for the entire flight, structurally it was strong enough to survive Mach 5 flight) except the telemetry. We got the telemetry for 4 minutes and then it stoped working. Based on data from telemetry flight went like this:
- Booster burned out and separated at an altitude of 3,5km and Mach 2,4
- Sustainer then coasted for 11 seconds and reached an altitude of 10km
- Sustainer motor ignited and burned out at an altitude of 20,5km and Mach 4,9
- Apogee charge was fired after 163 seconds
- After 4 minutes of flight telemetry was lost
We were using Telemega with an added RF amplifier, Raven 4 and a Featherweight tracker, plus a GoPro camera.
Second flight was almost identical except that the electronics ignited the sustainer at the booster burnout, without any delay. Why that happened is a mystery as both units were programmed at the same time and were double checked, so that the programming was identical on both. Because the sustainer fired without any delay the maximum speed was even higher, toping at Mach 5,4. Despite this the sustainer flew straight and true, but again telemetry from Telemega was lost a few minutes into the flight. After several minutes we got a data point from Featherweight tracker which showed that sustainer is 2km above ground and 26km away in the mountains east of Gerlach. We got there after some interesting off-roading and searched the area extensively but didn't find it.
All in all this was quite an adventure. I think we came very close in pursuit of our dream to fly to space. Based on time of flight to apogee, data we got from telemetry and comparing that to simulations I'm certain that at least the first one flew to space (over 100km). However we don't have the video or high resolution data from the flight computers. So the only option now is to work harder and try again.
First flight was on saturday. Basically everything worked as planned (motors worked, stage separation worked, high altitude ignition worked, rocket was stable for the entire flight, structurally it was strong enough to survive Mach 5 flight) except the telemetry. We got the telemetry for 4 minutes and then it stoped working. Based on data from telemetry flight went like this:
- Booster burned out and separated at an altitude of 3,5km and Mach 2,4
- Sustainer then coasted for 11 seconds and reached an altitude of 10km
- Sustainer motor ignited and burned out at an altitude of 20,5km and Mach 4,9
- Apogee charge was fired after 163 seconds
- After 4 minutes of flight telemetry was lost
We were using Telemega with an added RF amplifier, Raven 4 and a Featherweight tracker, plus a GoPro camera.
Second flight was almost identical except that the electronics ignited the sustainer at the booster burnout, without any delay. Why that happened is a mystery as both units were programmed at the same time and were double checked, so that the programming was identical on both. Because the sustainer fired without any delay the maximum speed was even higher, toping at Mach 5,4. Despite this the sustainer flew straight and true, but again telemetry from Telemega was lost a few minutes into the flight. After several minutes we got a data point from Featherweight tracker which showed that sustainer is 2km above ground and 26km away in the mountains east of Gerlach. We got there after some interesting off-roading and searched the area extensively but didn't find it.
All in all this was quite an adventure. I think we came very close in pursuit of our dream to fly to space. Based on time of flight to apogee, data we got from telemetry and comparing that to simulations I'm certain that at least the first one flew to space (over 100km). However we don't have the video or high resolution data from the flight computers. So the only option now is to work harder and try again.
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