May 2019 meeting

Dave Nordling, Secretary, RRS.ORG


The RRS met for our monthly meeting on Friday, May 10, 2019, at 7:30pm at the Ken Nakaoka Community Center in Gardena, CA. We were not as well attended this month, but we did have three new people join us at the meeting. With Mother’s Day weekend and graduation ceremonies happening at this time of year, many had other commitments.

The RRS welcomed David Minar and John Krell to the May 2019 meeting.

I must admit my error this week. I did realize too late that my monthly email to announce the meeting was not sent this week. Typically, I do send an email reminder to our active membership list (or anyone else who wants to know) on the Monday before meeting which also contains a copy of the agenda. Next month, I will not forget as this unintended experiment has shown that our reminder emails can be valuable to our membership.

The RRS has our monthly meetings always at 7:30pm on the 2nd Friday of each month. I usually mark my whole calendar each year with all of the 2nd Fridays to avoid schedule conflicts, but this is only a suggestion. Our meeting location is at the Ken Nakaoka Community Center in Gardena, unless otherwise announced in advance.

Also, in about the week prior to the meeting around the first of the month, I will gather up the agenda topics expected for the next month and post . All members are welcome to send their suggestions and ideas for agenda topics. We especially want short topics (5 minutes) on things related to rocketry, chemistry, payloads or just any kind of project you’re working on. The society is about sharing knowledge. Send an email to the RRS secretary or any of the executive council and we can put it on the agenda. Some of our members who aren’t in town have submitted things to be presented at meetings in the past which is also something the society can share in our meeting if the materials are clear enough and sent well in advance.

After calling the meeting to order and the reading of the treasury report, we began our agenda for May and covered most of the topics leaving some for June 14, 2019.

Waldo Stakes (left reclining) and new RRS member, Kent Schwitkis, a professor at Compton College. Kent joined us at the 2019 RRS symposium and we bid him welcome at the May 2019 meeting.

(1) Discussion of the 2019 RRS Symposium

The 2019 RRS Space & Rocketry Symposium held on Saturday, April 27th, was a success. We had a few different presenters this year in our lineup, welcomed a few new exhibitors and were very well attended over most of the day. According to Frank, we broke last year’s attendance by a small margin. RRS president, Osvaldo Tarditti, was very pleased with the amount of support we got from our membership in the night before and early morning of the symposium. Also, the RRS was very glad to have support we did at the end of the symposium in tearing down and cleaning up at the close of the day. These often overlooked simple details make running the symposium a real pleasure even at the end of a long day.

Frank Miuccio, RRS Vice President and Symposium Coordinator. The RRS thanks to Frank for all of his hard work in bringing together many people and many things to make this 2019 symposium another success.

We had our first opportunity to discuss the positive things at the symposium at the May meeting and this took up most of our meeting time. We were very happy to see so many groups come out including Spaceport LA, ROC and two organizations within the US Air Force (SMC at LA AFB and AFRL from Edwards AFB). The food truck vendor was also a great success. Many people enjoyed the pleasant weather and good food we had on site at lunch time. The outdoor exhibits were also a big hit. We are also thankful to LAPD CSP and the Los Angeles county sheriff’s department. The society will take notes to help with improving and expanding our next symposium still in planning for April 2021.

Some things that could have been better was sound quality. It was generally agreed that better speaker placement and the cloth barriers behind our audience rows was not sufficient to dampen out the noise from several running exhibits and the general foot traffic. It was my suggestion that we return to the 2017 format of having our presentations in the separate meeting room in the back and keep our exhibitors in the ballroom and at the main entrance.

We have also discovered that our exhibition seems to do well starting first thing in the morning and lasting until mid-afternoon. However, our audience attendance for our speakers tended to be better starting in the late morning and lasting all the way to the end of the day. The RRS is considering having fewer presenters but keeping the duration and range of content the same. Our audience seems to enjoy the topics that our speakers have been offering, so we will continue in this direction. The RRS will likely discuss more of our member and attendee impressions of the symposium at the next meeting on June 14, 2019.

(2) Terry Price’s presentation on composite materials used in rocketry – delayed to next month

Terry was unable to attend the May meeting, however, he did say that he could join us next month. Terry gave a great hour and a half presentation of composite materials at the EAA 96 monthly meeting in April. I hope he can bring some of the same excitement that he generated at the Compton Airport to our June 14 meeting in Gardena.

Terry Price, retired, formerly at Cerritos College and a nationally recognized expert in aerospace composite materials

(3) Upcoming events at the RRS Mojave Test Area

The rocket build event with Spaceport LA planned for early May was cancelled. We hope to reschedule a similar event with them very soon. The RRS is always interested in supporting these rocket build events with public and private groups if there is sufficient interest. Larry Hoffing is our events coordinator and the point of contact for setting up these kinds of events.

events@rrs.org – Larry Hoffing

UCLA will be having their rocket launch event at the end of the Spring Quarter 2019 on Saturday, June 1st. Several model rocket motors will be fired as part of Dr. Mitchell Spearrin’s undergraduate class. We have been glad to support UCLA for three years running in hosting this event. The RRS has also been glad to support their liquid and solid rocket motor teams over recent years.

At this same June 1st launch event, Osvaldo and I talked about securing the alpha thrust stand and having a series of static hot fires to generate more thrust curves for our micrograin alphas. From the one and only thrust curve we made, it appears that the RRS standard alpha qualifies as an “I” sized motor. We had a setback last year when the concrete slab to which the rocket and thrust stand was secured proved to be an insufficient foundation to react the swift impulse loading of an alpha. We have this footage of this defective hot-fire test on our Instagram account. It’s almost comical, but we must do better for the sake of safety and good science. Further, we need more data. The RRS does not intend to commercialize the standard alpha, but for the sake of future projects using this vehicle as a test bed, it would be good to have more recent motor performance measurements.

First of ten alphas right at liftoff

Some of our membership had indicated interest in building payloads to fly inside of the RRS standard alpha rockets we fly at each of these school events. Nearly all of these payload tubes are empty and are available to members able to build and supply their payloads before the event. Contact the RRS president for more details on alpha rocket payload tube specifications.

president@rrs.org

Frank has confirmed another rocket build event starting this summer in June through the LAPD CSP program. LAPD has been an ardent supporter and our rocket build programs are a big hit with the kids. The final part of the project is the launch event at the RRS Mojave Test Area (MTA). This will likely take place on Saturday, July 13th. We often have at least six alphas and sometimes as many as a dozen rockets at these events.

(X1) A brief word to potential users of the RRS MTA

Our membership is welcome to bring more tests and flights to the manifest. The only requirements the RRS has is that all participants download and submit a Standard Record Form available from the RRS.ORG forms library. This document provides a basic description of the project that members or other authorized attendees intend to execute at the RRS MTA. Supporting illustrations and documents are encouraged to help explain the task and operations for the supervising pyrotechnic operator. Approval of all activities at the RRS MTA are at the discretion of the supervising pyrotechnic operator and the RRS.

For those outside of our membership wishing to use the RRS MTA, the society requires submission of Standard Record Forms for all proposed activities. These must be submitted to the RRS president at least 14 calendar days in advance of the planned MTA event. Details of these tests can be held confidentially, but the RRS must have the opportunity to review, understand and approve all activities at the RRS MTA well in advance of the event. The RRS would prefer to have at least a month’s notice if not more. The RRS has been glad to assist more and more groups particularly with universities, however, we can not always support events particularly when we get little or no advance notice.

In the professional aerospace industry, it is a common practice to schedule a date with the testing site many months in advance and use this calendar deadline to help motivate the team to achieving meaningful results in time for the pre-set date. Scheduling a test date as an after-thought in executing a project often leads to disappointing results. Also, the society strongly recommends that users consider using times of the year other than at the end of semesters or quarters. If an event is planned well in advance, we will keep it on our calendar and can more easily have the personnel and resources ready.

The RRS must coordinate our activities with several parties and the better informed we are and the more notice we have will result in the RRS being better able to serve our prospective membership and clients. For any questions about RRS policy or practices, please contact the RRS president. The RRS is a volunteer organization and will always make our best effort to support rocket programs and projects.

(4) Pyrotechnic Operators for Rocketry in California

The RRS has been on a campaign to get more of our membership to apply and attain their pyrotechnic operators license. We’ve enjoyed a lot of support from many of our licensed membership in getting the necessary letters of recommendation for the application process. The RRS has also been glad to have the advice and assistance of the California Fire Marshal’s office. Osvaldo Tarditti, Larry Hoffing, Drew Cortopassi, Chris Lujan and myself have been in the application process for the rocketry pyrotechnic operators license. At the May 2019 meeting, Alastair Martin indicated his interest in becoming a pyrotechnic operator also. With more pyrotechnic operators on the state roster, there is more opportunity for more groups to conduct events throughout California. The RRS plans to remain active in our support to the public.

It’s with pleasure that I announce that I have attained my 3rd class rockets pyrotechnic operator’s license from CALFIRE this month. This annually renewable license allows me to buy high powered solid motors and supervise launch events within this class of solid motors. I hope to soon announce more pyrotechnic operators at the RRS very soon. The RRS is also very glad for the support of the licensed pyro-operators in our society and outside of our society at Friends of Amateur Rocketry (FAR). It is through the mutual support of all rocketry organizations that we expand our voice in the state and maintain a high standard for safety and the bold tradition of experimentation made possible in the Golden State.

RRS’s newest pyrotechnic operator, Dave Nordling

It’s my intention to upgrade my license to rockets 2nd class to be able to supervise and manage the unlimited category of solid rocket motors. The RRS is an organization that regularly conducts flights and testing in the unlimited class of solid motors. Our proud tradition of responsible experimentation in solid rocket technologies has one of our perrennial activities for over seven decades from micrograin to composite motors.

For more information on the licensing process for pyrotechnic operators as it pertains to rocketry, please contact CALFIRE directly.

(5) RRS Constitutional Committee

Now that the 2019 RRS symposium is complete, the RRS Constitutional Committee will begin their task of systematically revising and updating the RRS constitution. This task was approved by the society earlier this year and the committee comprised of two regular members (Larry Hoffing and Bill Janczewski) and one executive council member (Frank Miuccio) were appointed to begin this task over the summer.

The RRS Constitution has been updated periodically through amendments and notices over the years. The society has decided that a full review and incorporation of all changes be made in an update to the constitution to be voted and approved by our active voting membership near this year’s end. The main intent of the committee is to examine the constitution and recommend changes to better reflect how the society operates today. RRS president, Osvaldo Tarditti, did offer a word of caution that the Constitution should not be too specifically worded to over-regulate our operations. He has recommended that the committee take a minimalist approach and the committee will take this under advisement. Frank already has copies of the last edition of the RRS Constitution and amendments. These materials are available to all society members and some may already be on the RRS website.

This new revision will be known as the 2020 Constitution. It is a big effort and the society looks forward to the committee’s draft to be presented at the September 13, 2019, monthly meeting where suggestions and discussions will first commence.

(6) RRS Social Media Improvements – Recurring Topic

Alastair Martin was able to talk a little bit about RRS social media improvements and advertising of RRS events in general. One of his suggestions was that the society examine the use of EDDM (Every Door, Direct Mailers). These are the large postcard advertisements that organizations use to go into the mailboxes of targetted audiences throughout a region. Alastair has used these in the past with some of his events and had great success. The cost is substantial, but the results could be many more people getting notice to come to future symposiums and other large events with the RRS.

RRS media officer, Alastair Martin, discusses his ideas for marketing future RRS events at the May 2019 meeting

I had offered a suggestion that the RRS consider the use of more forms of electronic payment to help our ability to sell items at events or take membership dues and application fees. Venmo is a smartphone application that can be convenient for some to make direct payments to the RRS. The society seemed open to the idea, but it seemed to require further discussion and a vote to implement the change. There may be other means of electronic payment to consider however we must be cognizant of the fees involved and how practical each means would be for the society. We can bring this subject up again at the next meeting on June 14, 2019.

Currently, the RRS only has a PayPal account which is connected to our “DONATE” button on the RRS.ORG website. We encourage new applicants to use the “DONATE” button to tender their application fee ($40) if they desire associate membership at the RRS. Also, for our current membership, annual dues payments are much easier if made through PayPal or using our “DONATE” button. The only we ask for online donors is that they make a note of what the contribution is for and who we have to thank.

The RRS still accepts payments by direct mail to our post office box shown in our RRS.ORG website. The mailbox is not very frequently examined so it is strongly advised that all persons making payments to the RRS by direct mail contact the RRS president or RRS treasurer by email in advance to let them know to inspect the mailbox soon after the mail arrives. It is not uncommon for our direct mail to sit for a week or more.

Lastly, Alastair made mention of the next Rocket Talk Radio podcast which Richard Garcia and I will be a part of on Saturday, May 25th. The subject of this hour-long program will be Robert Truax. Truax was a key figure in 20th century rocketry and an important pioneer in the decades before the so-called “NewSpace” movement.

(7) RRS Participation with the CALFIRE committee on rocketry

The RRS was invited to attend CALFIRE’s review of the state’s regulations and laws on rocketry. Both the RRS and FAR have been reviewing regulations and preparing suggestions for improvements and clarifications to CALFIRE legislation governing the practice of amateur rocketry in California. The state has been very open to hearing from the amateur rocketry community about ideas and common sense changes that would make regulation better for all participants. Osvaldo has been involved with this activity and will soon schedule a separate meeting for RRS pyro-op’s to share and review our ideas. The CALFIRE committee will meet again in November 2019 and the RRS will offer our views on how the law can best serve the public.

(8) Topics for next meeting

Time ran out in the May meeting. Next month’s agenda will include further discussion about the RRS involvement with the base11 organization and the liquid rocket project (LR101) at Tomorrow’s Aeronautical Museum (TAM).

Richard Garcia has indicated his interest in getting the first prototype of the RRS standard liquid rocket motor built and tested.

RRS members have been helping different university groups on their liquid rocket projects and we expect to greatly expand our activities in this area as this year unfolds.

Lastly, the quarterly update for the SuperDosa project will have be made at the July 12, 2019, meeting.

Our next meeting of the RRS will be Friday, June 14, 2019. We will likely have something to share from the MTA launch event with UCLA by then. If there are any corrections or additions to be made to this monthly meeting report, please notify the RRS secretary.

secretary@rrs.org


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