By Benjamin Wilkosz
Part 5, 28 April, 2006
- Terminal Velocity, part 1, 23 December, 2004
- Terminal Velocity, part 2, 16 February, 2005
- Terminal Velocity, part 3, 27 April, 2005
- Terminal Velocity, part 4, 1 March, 2006
- Terminal Velocity, part 6, 12 March, 2007
- Terminal Velocity, part 7, 9 October, 2007
- NLD23 launch, 28 April, 2006
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).
Terminal Velocity - Testing the Avalon 008 at NLD23
At NLD23 the power plant for the "Terminal Velocity" project was tested. After some months of designing and fabricating, all the necessary parts were ready to see if they could withstand a full thrust test run.
Casting the Avalon 008 reload was the first time I worked with a sorbitol propellant and I have to say that this propellant is very easy to handle. Although not all the grains where perfect, I decided that the quality was good enough for a first static test. The total grain mass was 2630g, the aim was 2700g. With the next cast I expect to increase the grain mass by taking the shrink effects of the propellant into account and to compensate it. The preparation of the propellant was a bit different than with the other motors I made. This time I dried the potassium nitrate, to ensure good mechanical properties, but didn't grind the oxidator. Because of the dimensions of the Avalon 008 the difference in combustion efficiency due to the smaller oxidizer particles would be negligible. And considering the amount of time needed to grind 2kg of potassium nitrate firmly, I decided to skip this phase. As with the Avalon 007 all the grains were casted with phenol using aluminium moulds to ensure the exact geometry.
The assembly of the Avalon 008 was very straight forward and took just over half an hour. After fitting the thermal liner (phenol tube), by winding it with A2 paper, all six grains were glued to each other in the liner. This was done to prevent a catastrophically burn-through between two grains, as I once experienced with one of the Avalon007 motors. A stainless steel disk was installed in front of the forward closure. Purpose of this disk is to protect the aluminium forward-closure from the high temperature inside of the combustion chamber. Not only can erosion be a hazard to the aluminium forward, but maybe more important, to the pressure safety mechanisms. The forward of the Avalon 008 has a weakest point, calculated to fail at 10Mpa to ensure the safety of the casing. Elevated temperatures could cause the forward to blow at a lower pressure. A central drill hole makes the propellant accessible for the flame from the pyrogen and ensures a controlled evacuation of the combustion chamber in case of an over pressurisation.
As with earlier motors a pyrogen was used to start the motor. To ensure a fast start the first grain was coated with nitrocellulose black powder.
Instead of two O-rings, the new nozzle was designed with just one O-ring to minimize the weight of the nozzle. It was obvious that the O-ring would have to withstand very high temperatures at a pressure of 3,0 Mpa. Therefore Viton™ O-rings were ordered. Unfortunately something went wrong with the order, and a conventional O-ring was used. As an extra protection and help for the O-ring the nozzle and O-ring were greased with Mastic Blue™, a high temperature sealant.
The test and the results
The static test was conducted on my hydraulic test bench. This system is accurate (also under dynamical conditions), easy to use and not expensive. For this test the bench was extended with an aluminium evacuation chamber underneath the motor (as can be seen on the pictures). The hydraulic cylinder is filled with oil and a small volume of air to optimise the dynamic behaviour of the load cell. Two phenol rings, greased with petroleum jelly (vaseline) functioned as a sliding bearing.
After pushing the ignition button the motor started very fast. The start was quite turbulent, due to the high amount of nitrocellulose black powder coating and the large pyrogen. A small fireball escaping from the nozzle could be seen. Shortly after that, the motor builds op pressure. Within 0,3 seconds the motor was ignited fully, delivering approximately 450N of thrust. The pressure started to build up and at 0,8 seconds after ignition the motor was running at full throttle, delivering 820N of thrust. It was astonishing to see how stable the motor operated. For the next 2,6 seconds the motor seemed to be in total equilibrium. An effect I hoped for, described in Terminal Velocity, part 4. I was shocked the first time I saw the video by what seemed to be a stagnation of the test bench. During the first run with this test bench to close tolerances of the sliding bearing caused the casing to jam due to the thermal expansion, as can be seen in Terminal Velocity, part 2. But a closer look at the video revealed that the needle was still moving and after 3,4 seconds the tail off set in. With a jammed test bench the load cell would be fixed for a few minutes till the casing is cooled. The tail off reveals the inner structure of the motor. To prevent too strong erosive effects the two grains closest to the nozzle have a 30mm shaft, instead of a 25mm shaft. After the burnout of these two last grains the motor pressure stabilises at 3,7 seconds for a very short time. At 4,1 seconds after ignitions almost al the propellant is used and the motor settles down...
As always this result must be seen as an approximation of the reality. Faulty readings (positive and negative), friction of the slide bearings and/or load cell (negative) and other factors can cause a failure in the thrust curve.
During the field inspection I discovered that the nozzle O-ring was heavily damaged! Under the high temperature and pressure the outside of the O-ring was liquefied and pressed trough the gap between the nozzle/snap ring and the casing, solidifying and carbonising on the hot outer surface of the nozzle. The inner side of the O-ring was totally carbonised! Luckily no signs of erosion in the casing could be found. Without the high temperature sealant the situation could be very different. This test showed that this thin walled nozzle MUST be provided with a high temperature O-ring!
Now it is time to prepare the Terminal Velocity for its first flight at NLD24! Now all the characteristics of the Avalon 008 are known, first calculations (approximations) can be made. With the Terminal Velocity weighing approximately 4,7kg the rocket will reach an altitude of 2400m and a maximum velocity of 260m/s (Mach 0.78).
At this point I would like to thank Mayfran Limburg B.V. for turning the snaring grooves into the casing, Hogeschool Zuyd for providing me with a lathe large enough to turn the casing, Robby van Sambeek for anodising the casing (great job!), Fred van Arkel and Rafael Wilkosz, who provided me with the images/photo's of the static test and last but not least Jolijn Harmsen for helping me to clean the motor!
Next part: Terminal Velocity, part 6, 12 March, 2007