By Benjamin Wilkosz
Part 3, 27 April, 2005
- Terminal Velocity, part 1, 23 December, 2004
- Terminal Velocity, part 2, 16 February, 2005
- Terminal Velocity, part 4, 1 March, 2006
- Terminal Velocity, part 5, 28 April, 2006
- Terminal Velocity, part 6, 12 March, 2007
- Terminal Velocity, part 7, 9 October, 2007
Disclaimer: all liability waved! The contents of this page is presented for informational purposes only. Do not try to recreate any experiments presented in this page. The NAVRO and the author of this article cannot assume responsibility for any use readers make of this information. In The Netherlands it is forbidden by law to own this type of propellant if you do not have an exemption of the "Wet Explosieven Civiel Gebruik" (WECG).
Unfortunately disaster struck during the second test firing of the Avalon007c. After a short countdown the motor slowly came to live. One second later the thrust climbed exponentially to a max of approximately 4000N ending the thrust phase after just one second. Disappointed with the results I went home to clean the motor. During the disassembly of the motor I removed the thermal liner and I was alarmed. The liner was in a perfect condition except for one small hole, approximately 3mm in diameter. Around the hole signs of high temperature gas flow could be seen. Terrified I looked in my casing and discovered that the casing was damaged by the gas flow. Within a split second the gases had eroded a volume the size of a droplet... small but unacceptable. The flight of the Terminal Velocity had to be cancelled. Frustrated by the setback I came to the conclusion that I had to develop an motor based on a propellant other than KNO3/dextrose, at least for an L-class motor. The great advantage of a low melting point during casting turns into a disadvantage when used in lager grains, since the grain regression isn't linear/predictable anymore. Also is it very difficult to protect all the aluminium surfaces from the very aggressive gasses produced by this fast burning propellant.
At this point I started to think about composite propellants. The NAVRO was developing a new propellant, called Kalinitrox +. Instead of researching all the aspects of my new motor on my own, I could use the knowledge obtained by NAVRO's tests. The propellant is very promising with a specific impulse of 150s (approximation of the expected practical impulse). Also the propellant doesn't show the very aggressive burn characteristic and erosion effects of the KNO3/dextrose propellant. Combined with the fact that the grain can be drilled and turned safely makes Kalinitrox + a perfect amateur rocket propellant.
At NLD21 I discussed the details with several NAVRO members. Gerben-Jan Lighthart (president of the NAVRO) proposed to develop a whole range of rocket motors based on Kalinitrox +. This way more members could fly Kalinitrox + motors. Drawings of a 60mm and a 80mm motor were made using CAD to get a sense of how the motor should be. But before we could start developing we had to get a better grip on the propellant and its characteristics. The static tests to determine the throat diameter showed that the "burning surface to throat area ratio" should be increased from a 450 (test ratio) to an estimated 550. A very positive result was the regression, which proved to be very predictable, as in the earlier tests.
Concerning my motor, I planned to build a 3500Ns 70mm motor. As with the Avalon007, I aim for a maximum efficiency, reliability and a high safety factor. To lower the costs and workload for the rocket motor I decided to use conventional C45 steel for the nozzle and thin ALMgSi 0.5 tube for the casing. I chose C45 for the nozzle for its good mechanical properties (shear strength, melting point and hardness). The only disadvantage is the manufacturing of the material. But I think of this more as a challenge than a problem. The geometry of the nozzle will be optimised to deal with thermal loads and maximize nozzle efficiency. The thin aluminium tube doesn't need to be turned to the right outer diameter, as with the Avalon007 casings. This will save huge amounts of time, which comes in handy when more casings need to be produced. The forward closure will be made of aluminium to save weight. A pyrogen system will ensure a quick ignition of the motor. Both the nozzle and the forward closure will be fixed using snap rings, improving "field reload-ability" and lowering mechanical loads during use.
Hopefully the Terminal Velocity can fly on a Kalinitrox+ motor in the near future.
Next part: Terminal Velocity, part 4, 1 March, 2006