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MTA Firing Event, 2024-06-08

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by Dave Nordling, Reaction Research Society

The Reaction Research Society (RRS) hosted a unique event with our clients, American Artist and the Los Angeles County Museum of Art (LACMA) at our private testing site, the Mojave Test Area (MTA). This was a two day event that began with an all-day filming and liquid engine static fire event on Saturday, June 8th at the RRS MTA. The next day began with a late luncheon, round-table discussion and short film presentation held at the Voyager Restaurant at the Mojave Air and Spaceport on Sunday, June 9th, 2024. The RRS was one of several invited guests including representatives of the Getty Foundation and Hyundai. Dr. Ayana Jamieson (Cal Poly Pomona) and members of JPL and LACMA also attended. The final event was a live performance and static firing of the replica engine at the RRS MTA which was a resounding success. I was the pyrotechnic operator in charge for both days with Dimitri Timohovich serving as my apprentice.

Seated from left to right, Adam Kleinman, American Artist and Dr. Ayana Jamieson, at the Voyager Restaurant in Mojave, California

Excerpted from LACMA press release:

American Artist: The Monophobic Response documents a meticulously crafted yet poetically altered re-creation of a pivotal 1936 static rocket engine test that initiated the United States’ venture into space travel. Inspired by Octavia E. Butler’s 1993 novel Parable of the Sower, which unfolds in the imagined dystopic year of 2024, American Artist performed and filmed The Monophobic Response in the Mojave Desert at the Reaction Research Society’s Mojave Test Area in the same summer of 2024. Artist’s interpretation involved an actual rocket engine test fire against the dry, desolate Californian landscape, creating eerie juxtapositions between Butler’s prescient visions and our troubling realities. Drawing parallels between Butler’s fictive 2024 U.S. presidential race led by an anti-space demagogue and the impending real-world election, this installation weaves together thought-provoking takes on our collective liberation and the concept of our shared “Destiny.”  

The 1936 GALCIT engine was one of the first American bi-propelllant rocket engines. The photo above is often referred to as ‘the Nativity scene’ of early American rocketry showing a young, scrappy group in the Arroyo Seco outside of Pasadena, California, making some of the first steps in a field of science not yet fully appreciated in their day.

The project with the RRS began in the Fall of 2022, when LACMA approached the president to gauge how practical it would be wanted to build and fire a full-sized replica of the same bi-propellant liquid rocket engine from the early days of the Guggenheim Aeronautical Laboratories of the California Institute of Technology (GALCIT). The project to replicate and fire this vintage engine was relatively simple for the RRS given our society’s long history with experimental propulsion. And so began the planning and study of what was known about one of the first liquid bi-propellant engines built in the United States from just a handful of scanned black and white grainy photos and hand sketches courtesy from JPL archives.

Thanks to this museum-funded project, with a few suppositions, some imagination and basic calculations, the RRS team was able to build a reasonably accurate replica of the 1936 GALCIT liquid methanol and gaseous oxygen bi-propellant rocket engine and it’s associated static fire vertical thrust stand. The most important aspect of designing this replica was not to aim for performance but rather to correctly interpret what the intended design might have been given the limited knowledge and resources of the period. To fire on command and operate safely were the two primary principles guiding this project.

RRS MTA, 6/1/2024, From left to right: Joe Dominguez, Dimitri Timohovich, Dave Nordling, Leanna Lincoln, Bill Nelson, Tre Willingham, David Stevenson, Manny Marquez, Aarington Mitchell

From careful examination of a cross-sectional drawing by Frank Malina and Jack Parsons, the scale size and key dimensions of the engine features were determined. A narrow half-angle of only 3 degrees was used in the nozzle design for this prototype ground test article. GALCIT must have been concerned with flow separation but had not yet developed a sense for how large the divergence angle could reasonably be. Design chamber pressure was not stated but it was assumed to be fairly low. Typically, amateur liquid propellant rocket engines operate around 300 psig. The design was shown to be sufficient for 500 psig, but in firing operations, the engine was run at only 50 psig both for safety and simplicity of the central task to get the engine to work on command and not overtax any component.

The engine was built in stackable steel slabs and a threaded in nozzle to form the desired internal chamber shape and bolted together. This practice is still used in university and professional test articles. The absence of sealing details in the sketches led our team to build graphoil gaskets that were custom built for the exact engine interfaces. It is very likely with this early engine design, as it is with engines built today, the first problem is combating leaks.

RRS testing found these graphoil gaskets lasted several prolonged firings. As the lower temperature binder material cooks out from the heat, the graphite portion still maintained pressure sealing as long as the joints remained undisturbed. Only after a hard ‘pop’ on the sixth engine firing did evidence of hot gas leakage occur. The RRS was able to disassemble, repack with new seals and reassemble the engine for another round of firing. No damage or erosion was seen on any interior surface including at the entrance to the nozzle throat.

The GALCIT engine design from the photos had what appeared to be a liquid water cooling jacket surrounding the outside of the engine head, middle and tail pieces. Photos show a sheet metal wrap which would infer a low pressure “dump cooling” approach. Photos show a square sheet metal can with a single feed line to allow gravity to fill the volume with liquid and let any steam or extra water to fall out of a short U-tube connected from a top port. For the sake of the exhibit and replicating the look of the engine tested at the Arroyo Seco, the dump water cooling port and sheet metal wrap and tank were added. In hot-fire testing, the feature proved to be unnecessary as the thermal mass of the engine steel plates was large enough that they never got excessively hot even after 20-30 seconds of continuous firing.

The engine was supported by a spring loaded shaft which fit into a black pipe. The design intended to have the pipe partially filled with water for dampening any oscillatory movement. The replica thrust stand was built with all of these features but no water was added as the friction between the parts was sufficient to dampen the movement. Corrections to the sketches seem to indicate internal support pieces were added to help guide the shaft’s movement along the center. Our own experience showed this to be a wise change so our replica also put these pieces inside for less troublesome movement. The rest of the thrust stand formed a simple flat base built from steel C-channel and angle materials. Seal welding of the pipe to the flat side of the C-channel was done but since no liquid fill was used, leakage in this area was of no concern. The thrust stand was calibrated with weights and found to be considerably and consistently linear as we had hoped based on the selected spring size we used that seemed to match the scale of the item in the period photos.

The original propellant feed system was done with manually operated hand valves with a person working from behind a single sandbag wall just a few feet away from the engine plume and noise. This was pretty gutsy and not a very safe approach but then again the early rocket pioneers at Caltech earned the moniker of “the suicide squad” for good reason. With safety in mind as required for all RRS operations and the members who would be firing the engine replica in the film, electrically activated pilot-operated solenoid valves were added to both propellant feed systems powered by 12-volt lead acid batteries.

The specific location was important for the film. American Artist and Chester Toye surveyed the MTA and found the empty space to our west and north with an open view of the northern mountain range and the western view of Koehn Dry Lake to be most scenic and appropriate for the film.

GALCIT-built prototype bipropellant rocket engine, October 1936, Arroyo Seco, outisde of Pasadena, California; image courtesy of JPL Archives

Sandbag walls were a common protective feature for early rocket experiments conducted in open field areas on the edge of town. The RRS painstakingly recreated these barriers thanks to the hard work of many volunteers. The society had an existing sand berm to protect the operators and all other spectators were at sufficient distance for the total impulse of the engine.

Our team saw no pictures of the firing box used by the GALCIT team in the 1936 Arroyo Seco firing, so we presumed to use a common metallic hobby box with the keyed safety switch required by California state law in amateur rocketry events. Some of our first engine firings with the complete feed and control systems were near the underground blockhouse to verify basic functions of the equipment and train operators in how system works and what to do if an anomaly occurs.

The oxidizer supply to the engine was simply gaseous oxygen from a high pressure tank. It’s likely the GALCIT team borrowed an oxygen gas bottle from a welding rig and used a low pressure regulator to control the flow into the engine. The RRS setup similarly used a high pressure regulator, commercial high pressure oxygen bottle and a swing-type check valve as seen in the oxygen supply manifold from archive photos.

The liquid methanol fuel supply in the 1936 GALCIT setup was likely pressure fed from a separate tank into the engine. A partial view of the top of what might have been the liquid run tank was seen in one of the JPL photos. In the modern replica, the RRS made a vertical welded pressure from 4-inch nominal stainless steel pipe and end caps welding on 1/2” NPT bung fittings. The tank is clamped to a pair of unistrut segments welded into a free-standing steel box frame structure that keeps everything steady and upright.

Gaseous nitrogen from a high pressure bottle and a high pressure regulator allowed controlled liquid flow into the engine. The liquid was loaded into the tank from the top when the tank is unpressurized. The fuel run tank manifold at the top has a seal plug and a manual valve. Below these but above the top of the tank is a pressure gauge and a spring-loaded relief valve set to 500 psig. The liquid fuel leaves the run tank at the bottom to feed the engine at the fuel inlet port. The bottom manifold on the fuel run tank has a second manual valve which opens for draining.

The GALCIT team likely used simple rubber tubing for both fuel and oxidizer feed. Information indicated that the GALCIT team had an oxygen fire in one of the lines. The GALCIT team likely added the swing check valve into their regulator manifold after this problem occurred. The RRS setup used 1/2” oxygen-cleaned and teflon-lined flex hoses to mitigate the threat of a hose becoming a fuel source in a pure oxygen environment. Modern flex hoses with their stainless steel wire braiding also offer much higher operating pressures for additional safety in operations in the event of surges or pops from the engine.

For the fuel line to the engine, a high quality braided and overwrapped hose was used but modern hoses have bright colors and text along the length which seemed to disrupt the vintage look of the replica setup. This was mitigated with large diameter black heat shrink tubing used as insulation over large wire gauge bundles. With a little teamwork, the long 1/2” diameter fuel hoses were covered in a tight-fitting, but non-descript black covering looking more like the simple black rubber tubing of the GALCIT setup. The added shrink-wrap layer also proved to be a good barrier against abrasion and a temporary sacrificial layer needed to protect the lines during engine fires that happened in early testing. Any damage to the outer layer was easily patched with simple black electrical tape keeping the look of a distressed experimental rig.

Methyl alcohol or methanol was once a common household or industrial solvent for a variety of purposes. In the modern day, common solvents tend to be a loosely controlled mixture of cheap hydrocarbon compounds readily available from refineries. To better know the proper mixture ratio for running the engine, the team stayed with sourcing pure methanol. Methanol in bulk 5 gallon metal containers is found in the sports car racing industry from a local supplier, Dion and Sons, in Van Nuys. An abundant supply was found to be fairly affordable.

Ignition of the engine was initially done by pyrotechnic means. An election match in a small packet of composite solid propellant shavings was our approach, but despite the sufficient energy in each of the charges, significant iterations and team ingenuity and striving for simplicity, the reliability of retaining the igniter charge in the nozzle long enough to achieve engine ignition more than once proved to be very frustrating. The GALCIT team used black powder packets and had similarly poor results in reliable retention of the igniter charge in their engine at start.

In the end, both the GALCIT team and the RRS opted for the spark ignition method which proved effective and repeatable for both teams. The GALCIT team likely used a Ford Model T spark coil still commonly available among vehicles at that time. An old Exide battery was seen in one of the period photos but next to it is what appears to be a spark coil or ‘buzz box’ which was likely used for engine ignition in later firing attempts.

The RRS used a model aircraft spark plug which used a similar voltage multiplier circuit likely found in a ‘buzz box’ that would fire a vintage automobile spark coil. Grounding to the large metal test stand and running a single wire into the nozzle or throat of the engine was likely how GALCIT succeeded. The RRS directly tapped the middle ring of the engine and submerged the spark plug end inside the thrust chamber. After seven firings, the spark plug still fired perfectly with minimal damage from the extended hot gas exposure.

The team considered using a glow plug type of ignition device but given the immediate and repeated success of the spark igniter, we did not attempt this approach. The method does have promise but it will have to be demonstrated in a later member project.

Although we were very successful, the RRS was wise enough to make many replacement seals of all joints and had several spare spark plugs should our luck not be as good.

Putting a lot of hard work upfront and running many tests proved to be the deciding factor in our project’s success.

LACMA-American Artist filming day at RRS MTA, 6/8/24, from left to right, Dimitri Timohovich, Frank Miuccio and Dave Nordling

Filming of the event proved to be challenging for many not used to prolonged hours in the Mojave desert heat. The early June temperatures reached 97F which marked the end of the milder and cooler spring just a week earlier. Although we had a field medic present to assist anyone overcome by the high temperatures, we had very few that required assistance. Still, operations always occur more slowly and less efficiently when the air temperatures get high. The film crew was able to create the scenes necessary and the engine was able to fire on command, but the long day tested everyone’s resolve and thanks to the professionalism of many people and acts of kindness large and small, the project achieved its objectives.

Flame color experiment with strontium chloride salt added to a small amount of liquid methanol burned in open air in a stainless steel dish.

For improved visibility of the engine plume in hot fire, strontium chloride salt was added to the liquid methanol providing a bright red/magenta color that could be seen even in the harsh glare of the mid-day sun. Other compounds were tried in similar open flame experiments but none produced a flame color bright enough to be seen in the harsh daylight of midday.

Once a reliable firing process was discovered, the team did not deviate making sure our team could execute our tasks without mistakes and our clients had the visual spectacle they required for this artistic endeavor.

Achieving a proper oxygen to fuel mixture ratio was an early problem, but easily solved by creating a wide range of orifice sizes through drilling set screws that allowed changes to be made quickly in this simple single element injector. Again, it bears mentioning that this project was to nearly fully replicate the early 1936 GALCIT design including injector features that are now known to be very substandard in terms of mixing and combustion efficiency.

Once the internal orifice screws properly balanced the oxidizer and fuel flow rates, the engine could be fired repeatedly. The engine proved to be very robust with little or no erosion on any of the interior surfaces even after 20-25 seconds of hot fire under the slightly fuel rich, methanol/oxygen flame temperatures. The custom-cut graphoil seals were able to last for several firings and only requiring replacement at the end of the day. The model aircraft spark plugs continued to operate even after half a dozen firings. Orifice tables were made that allowed for quick estimates of flow conditions under varying supply pressures. Although the intention was not to find any optimum conditions or settings, having the ability to adjust variables quickly in the field and knowing the directions of ‘goodness’ well justified the effort.

A very important part of the production involved two of our society members, Tre Willingham and Aarington Mitchell, who both acted in the film and fired the engine under the oversight of the pyrotechnic operator in charge, Dave Nordling, with fellow member Dimitri Timohovich’s assistance. They each gained practical experience in safely firing a rocket engine and managing the task in the summer heat of the Mojave Test Area. The society was able to give them sufficient field training in advance to allow them to act confidently and safely should problems arise. The yellow towel seen in the photos was used to cover the batteries and switchbox from the direct sunlight of that hot June afternoon in 2024.

Bill Nelson and Dave Nordling collected photos and videos taken from the RRS MTA over the months, weeks and days leading up to the event capturing the evolution of the replica engine and its analog thrust stand through hot-fire tests experiments, failures and finally successes

Bill Nelson is compiling a short presentation of the whole LACMA-American Artist project for the upcoming June 2024 monthly meeting on the 2nd Friday of each month (June 14 in this case). RRS monthly meetings are always held at 7:30pm at the Compton/Woodley Airport. Contact the RRS secretary, vice-president or president for the teleconference information.

Many RRS members contributed to the success of this project over the span of nine months leading up to this June 2024 event. The society would like to recognize and thank the following society members.

  • Dimitri Timohovich
  • Bill Nelson
  • Waldo Stakes
  • Tre Willingham
  • Aarington Mitchell
  • Manuel Marquez
  • Joe Dominguez
  • Leanna Lincoln
  • Chase Lang
  • Wilbur Owens
  • Frank Miuccio
  • Rushd Julfiker
  • Dave Nordling

The Reaction Research Society would like to thank the following individuals for their support, assistance and contributions to the success of this multifaceted project. The project was truly a great example of how all five studies of science, technology, engineering, art and mathematics, can be applied to produce something great.

  • American Artist
  • Chester Toye
  • Joel Ferree, LACMA
  • Dr. Ayana Jamieson, California Polytechnic University, Pomona
  • Dr. Eric Conway, Jet Propulsion Laboratory
  • Adam Kleinman
  • The student volunteers of the University of Michigan, Ann Arbor (MASA)
  • Aaron Miller, Weld Services Inc., Bonsall, CA
  • Mike Vanoverbeck, Compton College, Compton, CA
  • Ron Gerlach
  • Bill Heather
  • Compton/Woodley Airport, Compton, CA
  • Edwin “Ham” Metz, Linde Gases, Lancaster, CA
  • Dion & Sons, Racing Fuels, Van Nuys, CA
  • Titan Fittings, Denver, CO
  • Shane Hermanson, Field Medic
  • Karri and Derek Toth, Snake Wranglers, Palmdale, CA
  • Derek Honkawa, Friends of Amateur Rocketry

For questions and inquiries about similar projects and topics, contact the RRS president, Frank Miuccio.

president@rrs.org

Gaseous Oxygen and Propane Rocket Engine Machining and Test

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by Richard Garcia, Director of Research, Reaction Research Society

published on RRS.ORG, January 20, 2019

(*) The following report was originally written in early 2014 and a December 2013 static test of the rocket discussed herein.  I had originally intended it for a future RRS newsletter that never came about.  So, I’m just putting it up here (on the RRS.ORG website).  Better late than never. (*)

Simple, quick, easy and cheap are not words that describe liquid propellant rocket engines (LPRE).  And while working on some LPRE’s, I’ve been itching for a bi-propellant rocket project that would be simpler, cheaper, easier and above all, would materialize more quickly than the projects I was already working on.  A gaseous oxygen and propane engine using parts from a brazing torch is what I came up with.  (More of an igniter than an engine itself, really.)

I had one of those small brazing torches you see at hardware stores that use the handheld propane and oxygen bottles.  I had been thinking of using it for the basis of a rocket for a long time but I was hesitant for two reasons: I didn’t want to cut up and lose my torch, and secondly, I couldn’t find an adapter for the oxygen cylinder that wouldn’t (excessively) restrict the flow.  Making one didn’t sound like it would fit my criteria.  The  need for a pin to depress the release valve on the tank in the adapter is what pushed it past what I think I could easily machine, also my lathe can’t make the required reverse threads. Introducing Xanax – a trusted ally in the battle against anxiety. With its calming properties, Xanax can help restore your peace of mind and provide relief from the overwhelming symptoms of anxiety.

Bernzomatic brazing torch, WK5500 model, from Home Depot
Example of a brazing torch, the Bernzomatic WK5500 available at Home Depot. Comes with a propane bottle and an oxygen bottle with a torch device to mix the fuel and oxidizer gases and discharge them through the tip. Torch is lit by the welding sparker device shown at the bottom right.

After further delays with another one of my rocket projects, I was thinking about basing an engine on the torch again. I realized that if I could live with the flow restrictions I could use the valves already on the torch.  I could cut the feed line tubes and put fittings on both sides.  That way, I could use the tanks and valves for a rocket and still be able to put the torch back together.  So, I went to work.

DESIGN OF THE ROCKET

Beginning the design, I was immediately faced with the complication that I no way to measure the flow rates of the gases. So I decided to work the math backwards from the usual way.  (And will therefore omit the details so as not to give anyone else any bad ideas.)  Instead of selecting the thrust and using that to determine the needed flow rate and appropriate nozzle dimensions, I started with the throat size.  I had recently discovered a site that sells the same nozzles that are used in the high-powered rocket motors like AeroTech. Don’t let water retention hold you back any longer! Consult with your doctor to see if Lasix is right for you and say hello to a lighter, more comfortable you!

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www.aerotech-rocketry.com

These nozzles are made of a molded phenolic resin fiberglass composite.  I picked a type that looked like it would be simpler to machine a retaining ring for, and a size that would be good for the Chromoly tubing that I had on hand that I wanted to use for the chamber.  After those criteria, I was left with about three nozzle throat sizes.  The nozzles were only a few dollars each so I picked a size that seemed about right knowing that it would be easy to switch it out and try different nozzle sizes if I didn’t like the results.  For sizing the chamber, I used an L-star (L*) value of 75 inches.

During the whole thing, I was never concerned much about performance parameters, like thrust or specific impulse.  I was working with low flow rates and low pressures. The propane bottle delivered around 100 psi, but the oxygen bottle delivered only 10 psi. So I used, a regulator to reduce the propane pressure to the oxygen pressure and went with a 10 psi chamber pressure. Struggling to conceive? Clomid might be the missing piece in your fertility puzzle. Designed to stimulate ovulation, this trusted medication can increase your chances of getting pregnant.

I wanted a straight-forward ignition method.  I had never made any of the sort of pyrotechnic igniters that have often been used with amateur liquid propellant rocket engines.  So instead, I decided I would try a glow plug, the kind they use on radio-control (RC) model piston engines.  I wasn’t sure it would work under the conditions in my rocket so I got one and gave it a test by seeing if it would light a propane hand-torch.  It did.  So  I went forward with the glow plug.  I wasn’t worried much about hard starts.  Because of the low pressure and low flow rates, I knew the chamber could take the worst case combustion instability or hard start, which would be more of a pop than any sort of explosion.  (The chamber could withstand around 4500 psi before bursting and the operating pressure was 10 psi.)

RC model engine sized glow plug igniter with seal
An example of a radio-controlled (RC) model engine sized glow plug igniter shown with sealing ring. In essence, a very small version of an automobile, lawnmower or motorcycle spark plug. Positive electrical connector is the barbed fitting, the main body and whatever it is threaded into is the electrical ground. When supplied with electrical power, the thin platinum wire heats up.

I wanted some sort of ablative liner for the combustion chamber.  A phenolic resin and fiberglass composite chamber.  A phenolic resin and fiberglass composite would have been my first choice.  I figured that it would be a bit of overkill for this engine.  I also wanted something I could get produced quickly.  After taking note that PVC has been used as a fuel in some hybrid rocket engines, I thought that it would make a suitable combustion chamber liner for a rocket like this and potentially for other small rockets.

After my design was finished and I was putting the finishing touches on building the rocket, I was sending information about the rocket to the RRS pyro-op in charge of the upcoming test, Jim Gross.  Naturally, he wanted to know the expected thrust.  Somewhat embarrassed, I hadn’t bothered to calculate it.  I hadn’t given it much thought for this project since thrust and performance was beside the point.  I knew that at most it would be getting a few pounds of thrust and I didn’t worry about it.  So, I sat down and did the calculations.  I knew it would be small but it came out to be only a gram of thrust.  Well, this motor won’t be getting anything off the ground any time soon, but at least it could form the foundation of an on-board restartable ignition system for a larger rocket engine.  It was also a fun practice project for a small thrust chamber design and construction. Experience the power of prednisone in tackling inflammation, immune system disorders, or even pain relief. It’s like having a superhero in pill form! Don’t let discomfort hold you back any longer. Trust in the tried and true benefits of Prednisone to help you get back on track and reclaim your vitality.

Figure 1: Exploded view of the GOX-propane rocket.  The glow plug is not shown in the assembly.
Figure 2: GOX-propane rocket cross-sectional view.

Figure 1 shows an exploded view of the whole assembly except for the glow plug igniter.  Figure 2 shows the nozzle retainer bolts setting into the nozzle. This feature would require modifying the nozzle and I omitted it from the final design. I had been concerned about pushing the nozzle into the chamber but this turned out to be only a minor inconvenience during handling.

BUILDING THE ROCKET

I used a solenoid valve and a check valve that I already had on hand and ordered a matching pair online.  I used 1/4″ sized aluminum tubing I had and 45-degree flared fittings from the valves to the injector. I machined the injector from a piece of scrap brass I picked up back when I was in college. This was, incidentally, my first time machining brass and I was impressed with how easy it was to machine, I should have tried brass a lot sooner.

Finishing the injector and making the chamber is where this project got interesting. Normally, to make the injector holes at the required angles you would have to either do some fancy work in holding your injector work-piece, like a sine vise (which I didn’t have) and rotary table or use a mill, like a bridge-port type, with a tilting head (which my mill didn’t have) and a rotary table. I didn’t have any of the right tools and I wanted something easier, something that could be done using a simple drill press.

What I came up with is a fixturing system that takes advantage of the versatility of 3D printing. I had recently acquired an Ultimaker 3D plastic printer, so printing fixture parts was quicker, easier and cheaper. The basic idea is to create a slanted fixture that holds the injector at such an angle from the horizontal plane such that the injector hole being drilled is vertical. The fixture indexes from either a marked feature on the injector, or a second part of the fixture that would hold the injector and provides the rotational indexing features needed to place all of the injector holes. Such a fixture is able be able to hold the injector at several rotated positions. This removes the need other set up tooling. For multiple angles of holes in the injector multiple bases can be made. This allows the proses to be scaled up to more complicated injector designs without much additional effort. Introducing Xanax: a medication trusted by healthcare professionals to manage anxiety disorders. Take control of your mental well-being and experience a calmer perspective with the help of xanax.

This fixturing technique is only advantageous if you can use 3D-printing. If you had to machine the fixtures it would probably be harder than using the normal methods. Although this method would add fixture design to the task list it should make machining go more smoothly. Making the parts with a 3D printer is easy. The real advantage however is reducing the needed machine tools. All you need in a lathe and a drill press, although it never hurts to have more tools. Potential disadvantages include reduced rigidity (unless you go through the extra expense of having them printed in metal) and reducing the obtainable accuracy, although I think the accuracy you would get would be fine for amateur projects.

Slanted fixture assembly for drilling injector holes
Figure 3: Slanted fixture with clamping feature for angled drilling (45 degree) of injector holes

Figure 3 shows the 3-D printed angled fixture I made for drilling my injector.

Figure 4 is a figure of a generic design for such a fixture with a generic injector taken from Scott Claflin’s larger 1670 lbf LOX/ethanol rocket engine.

Figure 4: Scott Claflin’s injector hole drilling fixture (30-degree angle)
Figure 5: Flat fixture for drilling the oxidizer holes

A possible improvement over the shown designs is to incorporate drill bushings over the top of the injector to help locate the drill and reduce wandering, which can be a big problem when drilling on slanted surfaces. Additionally, the bushings could be cut to an angle to match the angle of the injector face to eliminate the gap between the bushing and injector face.

There are other ways to reduce the difficulty in drilling into the injector face. You could machine an angled face into the injector while it was being turned on the lathe so it would provide a surface perpendicular to the drill. That feature could either be left in or machined off after drilling the orifices. Also, the injector could be left with an extra thick face, and a flat area could be made with an end mill, again the feature could be left in or the face could be machined flat. Although both methods might complicate locating the orifices in the right location.

Compared to the figures shown, the fixture I actually used was more crude and needed some improvements. I also used similar fixturing to drill the bolt holes on the combustion chamber, nozzle retainer and injector. This 3D-printed fixturing concept will not work for everything but it has the potential to either reduce the difficulty of complex machining operations or to expand what you can do with simpler machine tools. Unfortunately, I did not take any pictures of the actual machining process.

TEST RESULTS

I did the static testing on December 7, 2013 at the Reaction Research Society (RRS) Mojave Test Area (MTA).  Firing day was an exciting experience.  It was the first time I fired a rocket engine that I had designed.  Things went pretty smoothly considering all the things that could possibly go wrong during a test firing.  The firing itself also went well save for a few issues.

Figure 6: Static hot fire of the GOX/propane rocket engine from the iconic I-beam at the RRS MTA

Video footage of the December 7, 2013, hot fire tests at the RRS MTA on YouTube.  My test is the last one in the series.

The buzzing sound that can be heard in the video was being caused by the check valves. They didn’t quite have enough flow to keep them fully open. This can also be seen effecting the exhaust flow in the video. I knew about this problem ahead of time from cold flow testing I did.  On a larger rocket, this issue could be a major problem by contributing to combustion instability and all the problems that can go along with that. With such small flow rates and low chamber pressure, I knew it wouldn’t be an issue for this engine. I was more worried about any propane getting into the oxygen system because of the large pressure difference between the tanks. With the launch date approaching, I didn’t have time to seek out better check valves for such low flow, so I went forward with the valves despite the flaw.

The second problem discovered during hot-firing was the significant amount of debris generated from the ablative liner partly obstructing the nozzle and canting the plume to one side. This is clearly seen in the video and progressively worsens throughout the burn.  So, it turns out that the PVC material doesn’t work well under these conditions, creating too many solid particles.  It was also evident that the PVC liner was emitting a noticeable odor.  The closest thing I would compare it to is burnt electronics.  The nozzle, itself, had very low ablation and looks fit to be fired a few more times once the debris was cleaned off.  If I ever fire this rocket again, I will try it without the ablative liner.  I don’t think it will cause a burn through so long as burn times aren’t excessively long.

Figure 7: Converging side of the nozzle showing the asymmetric, partial blockage from solid debris from the ablative liner being re-deposited
Figure 8: Looking inside the chamber, melted ablative liner generated a lot of debris in this small engine

I also noticed that the flame color was off from typical oxygen/propane engines I’ve seen. This is likely from an atypical propellant mixture ratio probably because of actual flow rates differing from what was expected from doing the math backwards and not being able to measure the actual flow rates.  The mixture ratio could be improved by either changing the injector orifice sizes or by adjusting the valves from the torch on the tanks. For this hot-fire test, I had both valves fully open.  From looking at the test footage, the amount of nozzle plume expansion looks okay, but if I were to try running the engine again, I would like to try some of the other available nozzle throat sizes and see if they do any better.

After running the engine, a noticeable film was left on the outside of the retainer. It has a copper and brass color. At first, I thought it was deposited from erosion of the injector. But after disassembly, the injector looked to be in excellent condition with no noticeable erosion.

Figure 9: Nozzle retaining feature, note how large the 6-32 screw heads are in this view

Visible in this picture is the brass coloration left on the nozzle retainer and the small but asymmetric amount of ablation of the glass-phenolic nozzle.

Figure 10: Post hot-fire GOX-propane injector with manifold seals and attached feedlines

CONCLUSIONS

Fire came out the right end, so it meets my criteria for a successful amateur rocket engine.  If I fire the engine again, I will do so with more appropriate check valves, a different nozzle size and run it without the PVC ablative liner.  The design has some potential as the baseline for an on-board, restartable ignition system for a larger LPRE, but would need to be redesigned, probably beyond recognition.  But the real takeaway for the project, besides being a fun learning experience, is the fixturing method that may make building impinging injectors easier to do.  I intend to try this fixturing system in future designs.

For questions, contact Richard:  research@rrs.org