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Viper Rocket Trike

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Updated Aug 2021
08-24-2021: Of the last five test in August 2021, the test on August 24 was the best. I increased the throat diameter to 5 mm, decreased the characteristic length to 33 in, increased the oxidizer tank pressure to 130 psig, increased the length of the fuel core to 16.5 cm, and added a pressure probe to the mixing chamber. Ignition occurred in 1.5 to 2.0 sec. The chamber pressure rose to ~93 psig in 2.0 sec and was steady throughout the ignition. Burn time was ~5 sec. The video shows a net positive thrust greater than 14 N (3.2 lb) at ignition and held throughout the burn time. Shut down was instantaneous. The oxidizer to fuel ratio was ~2.3 and total mass flow rate was ~13.4 gm/sec resulting in a characteristic velocity of 1,163 m/sec with a c* efficiency of ~77%. Read more in the August EOM report.

07-29-2021: In the lastest series of test, to shorten the ignition time, I blended ~85% hydrogen peroxide with ethanol using oxidizer to fuel ratios of 30 and 20. The first test baselined the series using just HTP/PLA/KMnO4 hybrid motor. The next test used an HTPE blend with a 30:1 ratio (i.e 1 ml of ethanol added to 30 ml of HTP). The third test used an HTPE blend with a 20:1 ratio (i.e. 1.5 ml of ethanol added to 30 ml of HTP). The HTP oxidizer was at 85% and the fuel grain, PLA infused with KMnO4, core was 2.3 cm in diameter, 15 cm long, and in the star configuration for all three test. Why would I add ethanol to 85% hydrogen peroxide?

Welllll! Several months ago I read a couple of articles on blended HTP as a monopropellant for micro satellites (Ref). In the articles, the authors stated that ethanol was chosen because it was somewhat compatable with HTP and significantly increased the specific impulse. In theory (using the NASA CEA program), adding a small amount (O/F = 30) of ethanol not only doubled the specific impulse but, also doubled the combustion chamber temperature (from 380 C to 1178 C). This necessitates the use of a more robust catalyst. In a catalyst pack based on silver, the silver would melt away quickly (silver melts at ~961 C) and in a catalysts pack based on platinum, the platinum would wash out (platinum melts at ~1770 C). And they cost a lot, platinum more so than silver.

When I used the NASA CEA code to check on HTPE blend with the PLA/KMnO4 fuel, there was only a small increase (<1%) in specific impulse and combustion temperature. I was basically adding a hydrocarbon fuel to another hydrocarbon fuel. So, in theory, it didn't make sense. But, in practice, the HTPE blend showed significant melt of the PLA/KMnO4 over the straight HTP oxidizer. However, I didn't get an ignition. I may keep the HTPE blend at O/F=30 just because it may improve the O/F ratio of the HTP/PLA/KMnO4 hybrid rocket motor.

The HTPE blend got me thinking. What if I used an O/F ratio of 10 (i.e 3 ml of ethanol to 30 ml of HTP) with my mixed metal oxide (MMO) I've been working on for the last SEVEN YEARS (first batch was 07/25/2014) but couldn't get up to the autoignition temperature of ethanol? So, I pulled my MMO off of the shelf, used it as the catalyst in a small rocket engine, and the results were awesome! On the first test, ignition occurred in about 9 sec and ran for another 9 sec at an initial flow rate of 1.7 ml/sec at 120 psia.

I used a 1/4" stainless steel mist nozzle with a 0.5 mm orifice as the injector, four porous ceramic cylinders (3 1/8" x 7/8") infused with my MMO and sintered at 600 C, a 1" CPVC pipe 1 3/4" long combustion chamber, and a graphite nozzle with a throat diameter of 3.4 mm (L*=101"). As shown in the video, burn through at the injector occurred toward the end of the run. There was significant charring in the combustion chamber suggesting ignition but it was not visible on the video. I suspect the L* was to great (it was a miscalculation on my part) and will have to redo the experiment with a smaller L*. Three of the four cylinders were recovered and were still reactive to ~85% HTP. In a followup test, I reused the cylinders with a stainless steel mist nozzle with a 0.8 mm orifice (flow rate 5.4 ml/sec at 120 psia) but got no ignition. I've bounded the flow rate on the test and look forward to further experiments with my MMO.

I now have two rocket engines to experiment with, HTP/PLA/KMnO4 hybrid rocket engine and HTPE rocket engine :)

Ref: J Lee and S Kwon, 2013, Evaluation of ethanol-blended hydrogen peroxide monopropellant on a 10 N class thruster, J. Propul. Power. AIAA Early Edition and J Huh, J Lee, D Seo, S Kang, and S Kwon, 2013, Fabrication of ethanol blended hydrogen peroxide 50 mN class MEMS thruster, PowerMEMS 2013, Journal of Physics: Conference Series 476 (2013) 012124

06-30-2021: To simplify (i.e. get rid of the ignitor) and to explore different fuel grain geometries, I changed the PLA fuel core from an annular design with a 20% infill to a five point star configuration. The typical "star chamber" fuel grain geometry has a rapid rise in thrust, levels off to an even thrust, and then decreases rapidly. With this fuel grain geometry, the surface area of the PLA/KMnO4 exposed to the HTP oxidizer was increased from 57 cm2 to 90 cm2.

   I used the same geometry for three seperate test. The only parameter I changed was the flow rate. Of the three test, only one ignited and that took ~11 seconds. So, it looks like it's back to the ignitor.

   On the one that ignited, I did observe an even burn through the length of the fuel core. This is different from most solid and hybrid fuel grains that typically have a conical burn pattern from inlet to exit. And, if my calculations are correct, the O/F ratio was very close to theoretical.

   Next month, it's back to the glow wire ignitor and more quantitative results on O/F ratio. Also, I'm adding another variable to the test.

04-08-2021: This is a repeat of the 01-27-2021 test but with a 1/4" stainless steel mist injector with a 1.0 mm diameter orifice, a 3.0 mm fuel core wall thickness, L/D=12.5 (vs 2.0 mm, L/D=10.5 in 01-27 test), and 85% HTP (vs 90%). Combustion instabilities seem to be under control due to a higher pressure drop across the injector. The net positive thrust occurs at the end of the video corresponding to the erosion of the graphite nozzle throat from 3.6 mm to 5.2 mm. The increase in throat diameter reduces chamber pressure and increases mass flow rate resulting in a net positive thrust. Net positive thrust occurred for ~2 sec at a thrust of greater than 12 N. PLA fuel core segments were infused with potassium permanganate at high temperature and pressure.

01/27/2021: This is a test of 3, 3D printed poly lactic acid (PLA) fuel core segments with 20% hexagon infill infused with potassium permanganate (KMnO4) at high pressure and temperature. Each segment is 5 cm long and 1.7 cm in diameter making the total length 15 cm. The mixing chamber is preheated for about 10 sec using a high resistance glow wire. Ninety percent hydrogen peroxide (HTP) is introduced into the fuel core under a pressure of 120 psi by opening a normally closed solenoid valve. The KMnO4 catalyzes the HTP producing the initial temperature and pressure for the thermal environment. Once the high temperature environment is established, thermal and catalytic decomposition of HTP continues.
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"Simplicity is the ultimate sophistication."
- Leonardo da Vinci
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Copyright (c) 2021 Jerry F Fisher
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