HTP/PLA/KMnO4 Hybrid Rocket Engine

[Jerry Fisher]

   I infused PLA with KMnO4 and used it as a fuel core for rocket grade hydrogen peroxide. The PLA fuel core can be segmented, printed on a desk top 3D printer, and infused with KMnO4 at high temperature and pressure. The infusion takes less than an hour and results in the KMnO4 being evenly distributed throughout the PLA. I put together a small rocket engine using a 1/4 inch stainless steel mist nozzle as the HTP injector, a Soda Stream bottle for an oxidizer tank, a low pressure 12 V DC solenoid opening valve, a plastic one way valve, a graphite nozzle, and some CPVC piping glued together with CPVC cement. Primitive, but enough to demonstrate the catalytic reaction of HTP with PLA/KMnO4 fuel core. As long as I don't go over 100 psi chamber pressure and run longer than 10 seconds, it holds together long enough for the demonstration. I've written a paper on the HTP and PLA/KMnO4 hybrid rocket engine. I licensed the paper under the creative commons share alike copyright to engage in a dialog on this concept, if there is interest. Also, I've updated my website at www.fisherspacesystems.com. The website has a link to the paper as well as links to some engine test.

[Jerry Fisher]

  To simplify (i.e. get rid of the igniter) 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 oxidizer was increased from 57 cm2 to 90 cm2. I used the same geometry for three separate 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 igniter. On the one that ignited, I did observe an even burn through the length of the fuel core. And, if my calculations are correct, the O/F ratio was very close to theoretical. Next month it's back to the glow wire igniter and more quantitative results on O/F ratio. Also, I'm adding another variable to the test.

[Jerry Fisher]

In the latest 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 base lined 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 mono-propellant for micro satellites (Ref). In the articles, the authors stated that ethanol was chosen because it was somewhat compatible 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 auto-ignition 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 (https://rumble.com/vkhm09-htp-blended-with-ethanol-using-a-mmo-catalyst.html), burn through occurred at the injector 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

[rubicondsrv]

   I infused PLA with KMnO4 and used it as a fuel core for rocket grade hydrogen peroxide. The PLA fuel core can be segmented, printed on a desk top 3D printer, and infused with KMnO4 at high temperature and pressure. The infusion takes less than an hour and results in the KMnO4 being evenly distributed throughout the PLA. I put together a small rocket engine using a 1/4 inch stainless steel mist nozzle as the HTP injector, a Soda Stream bottle for an oxidizer tank, a low pressure 12 V DC solenoid opening valve, a plastic one way valve, a graphite nozzle, and some CPVC piping glued together with CPVC cement. Primitive, but enough to demonstrate the catalytic reaction of HTP with PLA/KMnO4 fuel core. As long as I don't go over 100 psi chamber pressure and run longer than 10 seconds, it holds together long enough for the demonstration. I've written a paper on the HTP and PLA/KMnO4 hybrid rocket engine. I licensed the paper under the creative commons share alike copyright to engage in a dialog on this concept, if there is interest. Also, I've updated my website at www.fisherspacesystems.com. The website has a link to the paper as well as links to some engine test.

so this is basically a hypergolic hybrid motor using HTP and a permanganate filled plastic fuel grain?   

[Jerry Fisher]

   

so this is basically a hypergolic hybrid motor using HTP and a permanganate filled plastic fuel grain?   

Yes! It's as Simple as that. The infusion of potassium permanganate into polylactic acid, a thermoplastic polyester, makes the PLA hypergolic with HTP. Also, I surmise that KMnO₄ reduces the melting temperature of the composite thus increasing the regression rate of the material. Using the "star chamber" fuel grain geometry, I've found that the O/F ratio is ~3, close to the theoretical value from the NASA CEA code. I'm cautiously optimistic that the characteristic velocity will be close to theory as well. In the next series of experiments, I'm inserting a pressure probe into the combustion chamber. With the pressure reading, I'll determine c* and compare it to theory.

[rubicondsrv]

it looks like the fuel grain is getting melted to the chamber,  you may want to try wrapping the fuel in some paper that has been soaked in a borax solution to both slow down the gasses leaking between the segments and allow the grain to be removed from the tube easily. this may also reduce the burn through problems.

 

[Jerry Fisher]

you may want to try wrapping the fuel in some paper that has been soaked in a borax solution

Thanks! I'll put that in my tool box. I'm using PVC pond liner (14 mil) right now and it is working just fine. It's good to have backups.

[Jerry Fisher]

   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 (https://rumble.com/vlx4qr-08-24-2021-htpeplakmno4-chamber-pressure.html) 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 (2.5 theoretical) 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%. I still plan on launching a class I HTPE hybrid before the end of the year. Next month I'll lock down the thrust and begin building the flight system.

[rubicondsrv]

   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 (https://rumble.com/vlx4qr-08-24-2021-htpeplakmno4-chamber-pressure.html) 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 (2.5 theoretical) 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%. I still plan on launching a class I HTPE hybrid before the end of the year. Next month I'll lock down the thrust and begin building the flight system.

looks like you didn't have burn through problems this time either. did you have to add a heater for ignition this time?

[libra]

Just saying in passing - you should correct the thread title from HYBIRD to hybrid.

Interesting stuff, always been a fan of H2O2 as oxidizer.

[Jerry Fisher]

looks like you didn't have burn through problems this time either. did you have to add a heater for ignition this time?

No burn through. It was a nice even burn. I plan on doing a longitudinal cut down the fuel core to get a better look at the burn. I'll post some pictures. I used a high resistance wire to preheat the nozzle. I'm not sure I need it. I'll do some testing to see if I can get rid of the preheat. Also, I left the solenoid valve open after the burn to cool down the mixing chamber. There was little warping this time.

[Jerry Fisher]

you should correct the thread title from HYBIRD to hybrid.

Thanks! I must've looked at that thread 100 times and never caught that misspelling

[libra]

Glad to be of help ! It's a very minor issue, btw.

[Jerry Fisher]

   There were five test in September (four succeeded & one failed). I varied the length of the PLA/KMnO4 fuel core as follows: 16.5 cm, 15.0 cm, 13.5 cm, and 12.0 cm. All other parameters were the same. I used a blend of 55 ml of ~ 87% HTP and 2.1 ml of denatured ethanol (O/F = 26.2) as the oxidizer. The propellant tank was pressurized to 130 psig using CO2 gas as the pressurant. I used a 1/4" stainless steel mist nozzle with a 1.0 mm orifice as the injector and a graphite phenolic nozzle with an initial throat diameter of 5 mm. The objective was to determine what effect the length of the fuel core had on the operation of the engine and to select the best length to continue. The ignition oxidizer flux of ~14 gm/cm2-sec, the run-time oxidizer flux of ~6 gm/cm2-sec, the fuel core regression rate of ~0.2 mm/sec, and the characteristic velocity of ~1390 m/sec were consistent on three out of the four successful test. The deciding factor was the oxidizer to fuel ratio, the thrust, and the burn time. For the 15 cm fuel core the O/F ratio was 2.3, close to theoretical. Ignition occurred in one second and lasted for ~7 sec https://rumble.com/vnb0wz-htpepla-kmno4-class-i-engine.html. There was a net positive thrust of greater than 16.2 N at ignition and lasted throughout the burn. Based on these results, I've selected the 15 cm fuel core for the class I flight system.

[colbourne]

I find this thread very interesting . Some friends of mine built a H2O2 /liquiid parafin motor from the Montreal Solaire plans. Due to not having H2O2 of higher enough concentration thrust was not high enough.
I experimented by making very light weight rockets, using aluminium dust mixed with wax, as the solid fuel,  and a bag of KMnO4 as the catalyst. Ignition was  with a sparkler. I used low grade H2O2 as obtained from gardening shops of 60% so was not expecting good results. My results were very chaotic and would not be suitable for a serious rocket. I was happy with just getting the wax to ignite.

Try adding metal powder to your fuel and you might get better thrust.

[edzieba]

Adding metal powder would interfere with the FDM propellant grain fabrication method, as hard substances in the feedstock erode the inside of the extrusion nozzle.

[Jerry Fisher]

Adding metal powder would interfere with the FDM propellant grain fabrication method, as hard substances in the feedstock erode the inside of the extrusion nozzle.

I've used off the shelf PLA/Al and printed out fuel core segments. I tried to auto-ignite using my mixed metal oxide as a catalyst with HTP. I got a lot of hot gas but no ignition. I had to replace my nozzle. The PLA/Al must've eroded my nozzle and damaged it.

I have not tried PLA/Al infused with KMnO4. I did a CEA run (frozen composition) with 15% Al, 6% ethanol, 77% PLA, and 2% KMnO4 as fuel for 85% HTP. There was a 1.3% increase in c* (1485 vs 1504 m/sec) and the O/F ratio went from 2.75 to 2.5. So, theoretically, it may not be worth the trouble. However, I'm getting ready to change out my nozzle anyway so I'll make a batch and we'll see.

Others (ref) have used Al as a metal additive to increase the regression rate of PLA. However, the results were disappointing. The Al did not increase the regression rate over straight PLA.

Ref: Small-Scale Static Fire Tests of 3D Printing Hybrid Rocket Fuel Grains Produced from Dierent Materials, Mitchell McFarland and Elsa Antunes, College of Science and Engineering, James Cook University, 1 James Cook Drive, Townsville, QLD 4811, Australia

[colbourne]

The preferred catalyst if money is not a problem is using a silver coated material (with as much surface area as possible in a box), through which the HTP is forced.
If the KMnO4 is mixed with the fuel , I thought it would be to late to achieve efficient catalysation of the HTP.

[Jerry Fisher]

True! Silver screens are the preferred method for catalyzing HTP. However, the cost of this catalyst is only one of the disadvantages. Keep in mind, my immediate objective is to develop a hybrid rocket motor for a Class I (as defined by the FAA) rocket for which the total mass allotment is 1.5 kg (3.3 lb).

The silver (density = 10.5 gm/cm³) is deposited on screens made of brass, nickel, or stainless steel (density ~7.9 gm/cm³). The silver screens are alternated with stainless steel screens and loaded under high pressure in the catalyst bed. My engine would be ~1" in diameter and 3"-4" long. I haven't done the calculations, but you can imagine the weight of this catalyst.

Another disadvantage is the pressure drop across the bed. Bed pressure drops usually run from 75 to 125 psi. Since I currently pressurize my propellant tank to 130 psi, I would have to double my pressure to get the same operating conditions. The higher pressure requirement will impact system mass.

Also, to increase performance, I've added a small amount (O/F = 25) of ethanol to the HTP. This increased the decomposition temperature from 380 C to 1178 C. In a catalyst pack based on silver, the silver would melt away quickly (silver melts at ~961 C) and most likely, would not sustain the combustion (and a waste of good money).

Further more, Byeonguk A., et el, (ref) found that the PLA fuel inlet partially melted down in the gravity direction because the inlet surface was directly exposed to the hot decomposed gas (~741 C). Using HTPE, my inlet temperature will be ~1178 C. This could result in the PLA melting and plugging up the fuel core or nozzle throat leading to a catastrophic failure. This has happened once but, let's not talk about that.

Finally, a silver screen catalyst may sound simple but is actually quite complicated. There is a rigorous procedure to go through getting the silver screen activated, multiple acid baths and cleanings. Infusion of KMnO₄ into PLA is much simpler.

As to your last point on efficient HTP decomposition, the mixing chamber or post combustion chamber takes care of that. I've found that a characteristic length of ~33" works well. The mixing chamber is just a 4.6 cm long CPVC pipe 1" in diameter cemented into two 1" CPVC couplings.

Sorry about the long post in response to your comment but, once I got started, I couldn't stop.
 
Ref: Byeonguk Ahn, Vikas Khandu Ghosale, and Sejin Kwon, Three-Dimensionally Printed
Polylactic Acid as Solid Fuel for Hydrogen Peroxide Hybrid Rockets, Journal of Propulsion
and Power, Technical Notes, Downloaded on November 30, 2020, http://arc.aiaa.org

[colbourne]

The Solaire plans used dry ice to pressurize the fuel and HTP. It also used a bag of KMnO4 before the combustion chamber. It sounds like you are making good progress. Have you managed to have any flights with this set up yet ?