Gaseous Oxygen and Propane Rocket Engine Machining and Test

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.

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.

www.rocketmotorparts.com (site no longer available)

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.

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.

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.

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

December 2018 meeting

The Reaction Research Society (RRS) held its final monthly meeting of our 75th anniversary year, 2018, on our usual 2nd Friday, December 14th.

We were glad to be visited by Waldo Stakes, Curator of the Saxon Museum in Boron, California, and well-known rocket car maker. Wilbur Owens was kind enough to bring Waldo to the meeting.

Wilbur Owens and Waldo Stakes at the December 2018 RRS meeting in Gardena, newly elected RRS treasurer, Chris Lujan looks on

The agenda for December was kept short as the RRS wanted to take some time to celebrate our accomplishments this year.

After coming to order and the reading of the treasury report, we began the shortened agenda.

[1]
The results of the elections were announced by email from our election chairman, Larry Hoffing, who could not attend. The full slate of officer candidates were voted unanimously with no write-in’s.

The RRS officers for 2019 are as follows:

President – Osvaldo Tarditti
Vice President – Frank Miuccio
Secretary – Dave Nordling
Treasurer – Chris Lujan

We thank our appointed election chairman, Larry, for his service.

The newly elected officers will start their new terms at the beginning of the calendar year. All of our appointed positions at the RRS remain the same as the executive council has voted to retain them. We are thankful to all of our new and continuing membership for making this an important milestone year for the society.

[2]
The next topic was to discuss the UCLA hot fire event at the MTA in November. Osvaldo was very impressed with the UCLA team in how well they communicated and their preparation for a safe hot-fire test. Despite having an earlier setback, they recovered to have a hot-fire in the early night hours that gave them valuable data for their next design iteration. The RRS is glad to support university groups with our testing site. It is great when everyone puts safety and organization first.

Osvaldo reads off the list of events we conducted at the RRS over this anniversary year, 2018; Michael Lunny looks on

The RRS has had other university groups express interest in using our Mojave Test Area (MTA). To any groups wanting to use the RRS MTA, please contact the RRS president by email.

president@rrs.org

We also appreciate each group filling out our RRS standard record form to help explain the basics of what they would like to accomplish. The standard record form can be found under “Forms” on the RRS website.

[3]
The last of the scheduled agenda topics was the announcement of the official date of the 2019 RRS symposium. The symposium will be held on Saturday, April 27, 2019, at the Ken Nakaoka Community Center in Gardena. As before, we will have entire first floor and outdoor courtyard.

The 2019 RRS symposium will be held on Saturday, April 27, 2019

The RRS has had great success in getting a range of speakers from industry, academia and government agencies. We plan to invite the Navy China Lake, Air Force Space and Missile Center, NASA Goddard Spaceflight Center, Northrop Grumman, Relativity and the Additive Rocket Corporation just to name a few, as well as several of our local and regional university rocketry groups such as UCLA, USC, Cal Poly Pomona, CSULB and LMU just to name a few. In the new year as invitations go out and we get confirmation, we will update our agenda for the April symposium. We hope to have an even larger turnout both on our stage, at the exhibition and of course our public audience.

The RRS will be holding a 2-hour panel session as the final event in our speaker series. The RRS is working on building this panel from a good cross-section of industry, government and academia to look at the state of affairs in space today and looking ahead to what may come next as the industry expands.

To those who stayed to the end last year, we were provided a rare treat by former member Bill Claybaugh as he presented his assessment of the current aerospace industry. We would strongly encourage everyone to plan to stay the full day as we will be bringing out more special things as the day goes on.

[9]
This topic was on the original agenda, but we thought we wouldn’t have time. RRS member, Alastair Martin, through his company, Production Tribe, is starting a pod-cast called Rocket Talk Radio. RRS director of research, Richard Garcia, and the RRS secretary, Dave Nordling, have agreed to be the first guests on this pod-cast that will talk about current topics in space and rocketry. As the first few podcasts are made, he’ll look to have other guests, to talk for an hour over a range of questions posed by Alastair and the participating audience.

[X1]
Osvaldo went over the long list of events we held at the RRS starting with the Aerospace Corporation launch test of their prototype liquid-hybrid rocket motor, the three launch events we held through the LAPD Community Safety Partnership (CSP) program where we were able to engage 112 kids in the at-risk communities of Watts in Los Angeles. Both USC and UCLA had a lot of success in their respective rocket programs.

Although USC did have a major setback in their Traveller III flight, they are well positioned for a very successful flight on their long journey to be the first university group to break the von Karman line into space with their boosted dart. The RRS is glad to be recurring partner with USC by assisting with their ground testing needs.

UCLA has also had a successful year with their liquid rocket programs and the RRS is glad to continue our support.

New RRS member, Dmitri Timohovich, and myself journeyed out to the MTA on December 2 for a site improvement project to mount the new road sign we had made. As I was feeling very under the weather on that afternoon, Dmitri did a stellar job in helping the society put a visible marker at our big iron gate leading the way into the RRS Mojave Test Area (MTA). The RRS thanks Dmitri for his hard work for the society.

Dmitri Timohovich makes short work of digging post holes with his motorized auger machine.

The new RRS sign at the first iron gate to the Mojave Test Area ready to greet the next visitors

[X2]
RRS member, Jack Oswald, presented his current progress with his large solid motor building project. His team have been working hard this year having some successes and failures, but always learning. His new ballistic evaluation motor design will correct some of the prior deficiencies in the earlier design and is expected to produce high quality results that should finalize his motor design. Jack and his team from the former Chaminade rocketry group have made a lot of progress and have also helped me with my smaller BEM design. Based on our conversation at the December meeting, I think I will have to re-visit some of my assumptions. Many thanks to Jack and his team’s hard work in supporting the society with their ambitious goals.

[X3]
RRS director of research, Richard Garcia, was not able to attend the December meeting, but he did provide a progress update for the RRS standard liquid project. His injector and chamber design is nearly finished and seems to have all the right features to proceed with build. The injector design is based on a previous design used by the RRS in past liquid rocket projects. He is also re-using his ablative chamber liner feature for this smaller 125-lbf LOX-ethanol engine design. His analytical model of the propellant blowdown system seem to anchor his predictions for what will hopefully be a successful hot fire test in this coming new year, 2019. The RRS is working hard to anchor a reliable and simple to build liquid rocket engine system that university groups can use for liquid rocket competitions in the future without having to start completely from scratch. We will provide updates as this project advances.

[X4]
Richard also brought up the idea of building a rocket sculpture to the right of our big iron gate at the MTA. The society welcomed the idea and we hope to bring this concept to life sometime very soon.

Richard Garcia’s rocket sculpture concept; soon to be seen at the RRS MTA

[X5]
Lastly, Frank was presented with a Community Service award by the Los Angeles Women Police Officers and Associates (LAWPOA) to honor the continuing work of the Reaction Research Society with the LAPD CSP. It is with great pleasure that the RRS accepts this recognition and reaffirms our commitment to this strong program that has done a lot of good for the young minds we hope to inspire.

Bill Janczewski holds up the LAWPOA Community Service award presented to the Reaction Research Society; I think I caught Frank a little off-guard in this photo? He was very pleased.

Our next monthly meeting will be on Friday, January 11, 2019. For an update on our intended agenda for each meeting, check the RRS.ORG website under the “Forum” section.

We look forward to starting the new year with preparations for the 2019 symposium and we will need our full membership to help make this event an even greater success.

Also, the RRS is talking about holding one of our monthly meetings at the Tomorrow’s Museum at the Compton Airport. Also, Jack Oswald has been working on getting tours of The Boring Company in Hawthorne and the Point Mugu Naval Base sometime in the new year.

The RRS wishes everyone happy holidays and a happy new year.

MTA event, 2018-11-17

The Reaction Research Society (RRS) was glad to offer our Mojave Test Area (MTA) to UCLA for a series of tests of their liquid rocket. This was a private event, but Osvaldo and Elisa were there to witness a successful hot-fire series.

UCLA has been working on liquid rockets and this event was to test the improved version of their 650 lbf thrust LOX/ethanol engine. After validating minor modifications to the plumbing and an improved mechanism for their pneumatic valve actuators, UCLA expected good performance from this test with an expected burn time of 13.8 seconds and an expected total impulse of 9000 lbf-sec.

UCLA makes preparations on their liquid rocket, 11-17-2018 at the MTA

Other improvements include collecting better data. Data collection has been a challenge for many teams over the years. Tank, manifold and chamber pressure measurements were successful combined with thermocouples on the LOX lines for a better estimate of density and on the engine outer surface to anchor heat transfer assumptions. This temperature data has helped to better anchor their estimates of characteristic velocity (C*) and specific impulse (Isp). UCLA was not making direct flow rate measurements in this test, but has planned to do so in another forthcoming test.

UCLA’s liquid rocket in position

UCLA has also been giving their newer student team members opportunities on this project by passing knowledge gained from the more experienced members as turnover is a necessity with graduation.

UCLA liquid rocket hot fire way after sunset, 11-17-2018

Results from the hot-fire seemed to show that UCLA’s computational models were fairly close to actual performance. Total impulse was less than predicted at 8174 lbf-sec, average thrust at 467 lbf and peak thrust at 550 lbf, but a longer than predicted burn duration of 17.0 seconds.

These are good results but improvements can be made, particularly in getting direct propellant flow rate measurements. Both C* and Isp can be directly measured from propellant flow rate.

Further refinement of their assumptions based on this new hard data will help them in their next hot-fire planned for January 2019. The RRS is glad to assist UCLA and other universities with their liquid rocket projects at our Mojave Test Area (MTA). The RRS is ready to help UCLA take their next step in the new year.

We will surely discuss the results of this and the upcoming test of UCLA’s liquid rocket at the next RRS meeting, Friday, December 14th, 7:30pm, at the Ken Nakaoka Community Center in Gardena.