By Hein Olthof, Team leader solid propulsion group.

Figure 3: screen capture for highspeed footage.

After a period of silence, the solid propulsion group started a new testing period. The goal was to further refine the propellant composition ALAN-7, and to characterize it. Three types of tests were performed: atmospheric burn tests, pressurized burn tests and shear strength tests. For the shear strength tests and the pressurized burn experiments, new test devices were developed and built. Several compositions were tested, the only difference between them being the binder components, while the total binder content in each composition was kept at 20%, which is the minimum required for good cohesion. From the shear strength tests and the atmospheric burn tests, it was found that increasing the polyester content in the binder combination will increase the shear strength, but generally yields a propellant with a less favorable burning behavior. A graph of the shear strength versus the polyester percentage can be found in Figure 1. A more elaborate analysis of the results is needed to judge which binder combination is most suitable for our needs. A very good result is the fact that each of the propellant samples ignited well. The problems we had with ignition in the past seem to be resolved.

Shear strength versus the polyester percentage

For the pressurized tests, only one propellant was considered: ALAN 7.0. This composition was already flown in the Flaming Tiger rocket, but further characterization was needed in order to make a good motor design. Therefore, samples of this propellant were tested under pressure, in so-called Pressure Test Devices (PTD’s), with different values for the klemmung (burning area divided by the nozzle throat area). A high klemmung results in a high pressure, while a low klemmung gives a low pressure. Two types of grain geometry were tested. The end burner, which is a cylindrical grain, only burns on the front face. The core burner, which has a cylindrical hole in the middle, burns from the inside out, as well as on its front face. The first geometry gives a constant burning area, which eases calculations later on. However, to obtain a large klemmung value, the throat diameter needs to be very small, which increases the risk of clogging. The core burner has more burning surface and hence reduces this problem.
Some problems arose during the tests. Six PTD’s were prepared, of which only four ignited. The two that did not ignite had the same casing. During assembly, the ignition wires broke both times. This was caused by a flaw in the design, and will be solved later. One of the PTD’s did ignite, but suffered from severe leakage after the pressure sensor assembly came apart. The thrust that was delivered through the so created hole was enough to bend the 8mm steel tubing. The ignition was very fast, almost instantaneously, which is a good result. However, the pressure data have value only to the point where leakage started. One interesting observation was the occurrence of Mach diamonds in the exhaust for a short period of time. This gives nice photographs for presentations (See Figure 2)

figure 2: Mach diamonds in exaust

Then, we had another good ignition, from a core-burning PTD with low klemmung. Due to this low klemmung, startup was slow, but a good stable burn resulted in a chamber pressure of 12 Bars.

The next test was an end burner, with a very small throat. Whether this was the cause or not: the pressure inside the PTD increased so much, that the solid, 3mm steel plates could not support the load on the endcaps and bended. This ruined the PTD, and reduced it to a nice museum piece. It has to be mentioned that this was the only PTD of the old design, with tension rods instead of threads.

Finally, another core burner with high klemmung ignited very fast, resulting in a steep pressure curve. The chamber pressure quickly reached 80 Bars, after which the safety valve was activated. Because of this, no full pressure curve was obtained, but the high pressure that occurred is very promising. Next to that, the burn was very stable, as was observed from the High-speed camera footage. A photo of the PTD at full pressure is given in Figure 3.

Over all, we look back at a successful test campaign, with more than enough data to analyze in the coming weeks and a lot of lessons learnt. And of course with some nice new pieces for the DARE museum: a charred burn test stand, some pieces of bent tubing and an destroyed PTD. This propellant clearly has potential!