Around 10 years ago, DARK’s standard solid propellant was a simple 2-component mixture (oxidizer/polymeric binder) which delivered a specific impulse under real-world conditions in the range of 170 s - 180 s in small rocket motors. This propellant was easy to cast in segments up to a few kg and since it had a moderate flame temperature, it could in practice be used with nozzles machined from stainless steel without cooling as long as the burn was kept to below 5 seconds or so.
It was decided to develop a propellant with a higher performance and an approach was taken where polymeric binder was changed in order to allow a higher solids loading and also it was decided to add a small amount of energetic metal in order to raise flame temperature (and hence specific impulse). However, in order to keep the flame temperature moderate the initial propellant candidate formulation for a new “standard DARK propellant” kept the metals loading to just 6% which is well below common professional propellant formulations (which typically use 12% - 25% metals loading).
The resulting propellant turned out to have comparatively good rheological properties during mixing and casting, good ballistic performance and consistent pressure-burnrate behavior. Using a stainless steel nozzle, this propellant is limited to a low chamber pressures - even for short burn duration. At a chamber pressure above roughly 20 bar a graphite insert becomes necessary, since a stainless nozzle throat melts away. The propellant has not only been tested in DARK’s test motor using ca. 600 g segments, but also in a small flight motor and has delivered a specific impulse well above 200 s.
The problem with the new propellant, however, is that the flame temperature is only about 200 degrees above the melting point of the metal oxide formed form the 6% metal loading. This makes it possible for the oxide to condense out on the surface of the nozzle throat in a thin layer. This phenomenon of “negative throat erosion” has been observed in practice and since it is not reproducible it is impossible to design the motor and fuel grain to take advantage of this “feature”.
Given that a graphite insert is now, in reality, necessary in any case, the remedy seems straight forward: Simply increase the metals loading substantially to bring the flame temperature well above the oxide melting point and (hopefully) eliminate oxide condensation in the nozzle. DARK’s static tests at Stold in July 2016 were the first test of a new propellant candidate with a 16% metals loading. At the same time, the tests were the perfect opportunity to test DARK’s new graphite insert nozzles for the test motor.
The two first tests firings of the propellant may be seen youtube: https://www.youtube.com/watch?v=CZwzZJLyCwM These tests covered a pressure range of 15 bar to 60 bar and the range of klemmung was 230 to around 460.
First test (TM0116): Although the average chamber pressure is quite low (ca 25 bar) the apparent specific impulse is just under 200 s. Second test (TM0216): At a moderate average chamber pressure, the specific impulse exceeds 200 s. This is a highly encouraging result for such a small motor (just 540 g of propellant) with significant heat losses.
When another version of the propellant with a 2% copper phthalocyanine catalyst was tested at an unusually low klemmung of just 60 the resulting burn exhibited strong oscillations at 11 Hz; never reaching a chamber pressure above 3 bar before finally self-extinguishing with most of the propellant remaining in the motor! Plot of recorded chamber pressure in bar for the (30 second) duration of the low klemmung “burn” - and a zoom in of the first 5 seconds (including the recorded thrust in Newton scaled by 1/20)
It is currently unclear what causes the new (and catalyzed) propellant’s inability to burn at a stable rate with a small klemmung. It burns very well in open air. We plan to re-test the propellant at a larger klemmung at a later time.