A Quick Word on Dip Tubes

by Richard Garcia, Director of Research, Reaction Research Society

I’ve had the opportunity to talk a lot with many amateur rocketeers getting into building liquid rockets. Because many commercial pressure vessels only come with one port, you often have to choose between making or modifying a pressure vessel. I always see a third option, dip tubes, being over looked.

I would not recommend using a dip tube on a flight vehicle, but they are quick to put together for a test stand. A dip tube can be made from two parts: a tee fitting and a modified compression fitting.

Compression fittings do not normally have enough room to send a tube all the way through them. Most compression fittings purposefully have a stop to bottom out the tubing at a specific depth within the fitting. Usually these compression fittings have enough meat on them to drill through and open their internal diameter to fit the tube straight through them.

Cross-section of a compression tube fitting, before and after drilling
Typical compression fitting before and after modification to allow the tubing to fully penetrate the fitting. The stop feature is common which makes tubing joints repeatable and accurate. This internal diameter is opened up drilling to allow the tube to pass completely through. Note that the compression fittings shown have a tapered pipe connection (NPT, typically). Other pipe or tubing connections are possible depending on the fitting manufacturer.

Putting this modified compression fitting along the straight path of the tee fitting will allow you to pass a tube through both fittings and into the tank itself. The tee connects on to the tank’s port and the tube goes down to just above the bottom of the tank (if connecting at the tank’s top). The branch of the tee feeds the tank ullage space with pressurant gas and the tube picks up fluid from the bottom of the tank as long as the liquid line is above the tube opening.

Dip tubes on top and bottom of tanks
Dip tubes shown in both top and bottom mounted configurations. Note that the fluid passes from the annular opening on the tank bottom mounting configuration. Note that the pressurant gas flows through the annular gap in the tank top mounting configuration. The tee fitting shown has tapered pipe fittings (NPT, typically).
Illustration of a dip tube mounted on top of a liquid filled tank
Illustration of a top mounted dip tube on a liquid filled tank. Pressurant gas is supplied at the branch connection of the tee fitting and flows into the ullage space through the annular flow path. The liquid is driven up the central tube and out of the tank if the gas pressure is sufficient to overcome the liquid head and tube friction.

This can be flipped upside down, pressurizing through the tube and flowing the liquids out of the tee branch connection. It is always important to consider the amount of flow area in the tubing you are using as well as the annular flow area created by this combination of fitting as it is likely to be the area of minimum restriction to flow.

illustration of dip tube used in pressurized expulsion of liquid from a tank
Illustration of a dip tube mounted on the bottom of a liquid filled tank. Pressurant gas flows up to the top into the ullage space of the tank expelling the liquid from the annular flow path in the tee fitting and out of the branch connection.

Also, instead of a tubing connection, one could put a thermocouple through the compression fitting of the proper size to make an internal fluid temperature measurement in the tank either immersed in the liquid below or measuring the ullage gas temperature in the top of the tank. This is a convenient way of mounting a temperature sensor.

P.S. Here are two kinds of tanks with ports on each end that are readily available. Try looking for automobile air tanks like this one from Speedway Motors:

https://www.speedwaymotors.com/Speedway-24-Inch-Aluminum-Air-Suspension-Tank-4-Gallon,126697.html

Or a gas sample cylinder such as this one from Swagelok

https://www.swagelok.com/en/catalog/Product/Detail?part=304L-HDF4-1000

For questions, contact Richard at

research@rrs.org

February 2019 meeting

Inside the EAA 96 hangar at the Compton Airport. The meeting ran very late as you can see by the clock.
EAA 96 Hangar at the Compton Airport, 1017 W. Alondra Blvd., Compton, CA, 90220

The RRS met for our February monthly meeting at the EAA 96 hangar at the Compton Airport on Friday night, February 8, at 730pm. The Experimental Aircraft Association (EAA), Local Chapter 96, was gracious enough to offer their main office boardroom. RRS members, Xavier Marshall and Wilbur Owens were kind enough to even provide food and drinks for our membership and guests. After reading of last month’s treasury report, we agreed to get an update next week as our president (doing the duties of the treasurer) was not able to attend this meeting. After the customary introductions of some of our visiting guests, we began the agenda.

Bill and Wyatt Janczewski at the February 2019 meeting at the EAA 96 hangar

(1) Discussion and task assignments for the 2019 RRS Symposium

The 2019 symposium will take place on Saturday, April 27, 2019. We have confirmed 8 of our 13 speaker slots and are working on building the panel discussion that will happen at the end of the symposium. We will likely soon fill all of our 13 speaker slots for the fourteen 30-minute sessions throughout the day. We have already confirmed several of our past speakers such as Northrop-Grumman and some new presenters such as the Air Force Space and Missile Command.

Frank said that the most important thing that all of our membership and friends can do is to spread the word and circulate our flyers as soon and as much as possible. Having an on-site food provider is in the works and John Mariano has offered to provide his brand, Celebrity Coffee, at the symposium. Based on the rate of (free) ticket sales through Eventbrite, we are on track to have a great symposium. We have 13 exhibitors confirmed and hope to have well over 21 exhibitors (last year’s total) by the time the symposium arrives. We are trying to pace events throughout the day to have a steady stream of participation from morning to afternoon.

Frank will soon be holding regular meetings to get as much of our membership involved with the myriad of tasks necessary to make the event fruitful and exciting. We ask all of our membership to do as much as they can. Ideas are always welcome, but people that can take action are appreciated even more. The RRS will not hold another symposium until 2021, so we would like to put as much of ourselves into this event as we can. Our symposium chairman is Frank Miuccio, please contact him or any of the Executive Council at any time.

vicepresident@rrs.org

president@rrs.org

secretary@rrs.org

treasurer@rrs.org

(2) Improvements to the RRS social media presence

In the months leading up to the 2019 RRS symposium, the RRS should look at improving our social media presence. One of the things we will do is post different advertising flyer designs on our Instagram and Facebook accounts.

Alastair and Bill have had fruitful discussions on this subject and would like to have recurring monthly spot on the meeting agenda as the RRS social media presence will always remain important in our bid to reach new and old members.

(3) Formation of the 2020 RRS Constitutional Committee

The RRS is an organization that has persisted for a very long time, but periodically, the way we operate has changed over the decades. The last time a review of the RRS Constitution was done was in the 1990’s. Although some amendments have occurred to update our organization, it has been observed that many improvements, clarifications or simple corrections need to be made to reflect how we operate today.

Alastair Martin, Wilbur Owens, Larry Hoffing and Frank Miuccio at the February 2019 meeting at the EAA 96 hangar at the Compton Airport; a Rocketdyne LR-101 vernier motor sits on the table

The RRS voted and approved the formation of a three-person Constitutional Committee consisting of one Executive Council member and two people from our regular membership. After a solicitation of our membership at the February meeting, the following people will form this temporary 2020 Constitutional Committee:

Frank Miuccio, Vice President

Larry Hoffing

Bill Janczewski

This 2020 committee will first gather up all known copies and amendments to the RRS constitution. It is important to best establish where we stand before proceeding with the editing process. Frank has much of these records and with this collection of information, the committee will create a new draft of the Constitution. The intent is not to make many (if any) changes so much as to make clarifications of roles and responsibilities in areas that have been vague or entirely absent.

After a great deal of effort, the RRS has updated our membership roster as best as we have been able to do so. We continuously call upon our past and present membership to pay their dues and remain active during this important time. Please contact the RRS treasurer, Chris Lujan, or make your payment to the RRS president, Osvaldo Tarditti.

treasurer@rrs.org

president@rrs.org

With the 2019 Symposium around the corner, our priority is to execute the April 27 symposium. Therefore, the 2020 committee will not have to report back to the RRS until our September 13, 2019, meeting. Having the whole of the summer of 2019 should allow the committee to perform the laborious duty of researching and retyping the Constitution in such as manner that makes it clearer. The new draft or 2020 Constitution will then be reviewed numbered paragraph by numbered paragraph to assure a thorough review to approve portions that make sense and discuss others that may require adjustment.

Concerns were expressed about maintaining the requirements of our 501(c)3 educational non-profit organization when it was formed. These are important concerns which will be addressed. The committee will likely need to seek advice from our membership and they certainly will reach out as necessary throughout their working period this year.

This 2020 committee will then present their draft at the September meeting taking specific feedback and returning their final draft at the November 8 meeting. The new “2020” Constitution will then be put to a vote by our active membership. By our articles, this must be approved by a 2/3rd’s majority which may take some time to do. As any changes will largely serve for clarification, this draft of the Constitution, paragraph by paragraph should be able to be approved by the vote taken across our active membership with a deadline of the end of the year, December 31, 2019. Once the 2020 update to the RRS Constitution is approved, all prior drafts will be voided and the committee dissolved. Further, to avoid a permanent state of churn, it was agreed that after approval of the 2020 Constitution, no further amendments will be made for at least one year to allow the society to operate long enough to see where the problems are. A Constitution is a living document, but changes are purposefully not easy to make without a significant consensus of our active membership.

If there are any questions (which I am virtually certain that there will be), please direct them to the RRS Constitutional Committee chairman, Frank Miuccio

vicepresident@rrs.org

(4) Rocket Talk Radio

Alastair Martin has started a pod-cast called “Rocket Talk Radio” which is an hourly program that will talk about selected topics in the rocket business. These topics will be very relevant to the increasingly active world of space exploration. Alastair’s company, Production Tribe LLC, is producing the show to which RRS members, Dave Nordling and Richard Garcia, have agreed to be regular guests on the show. At the first show, we had Waldo Stakes as our first guest. As the show continues, Alastair will have other guests on the show to explore the many number of exciting topics happening today and in the near tomorrow.

Richard Garcia waves hello to the studio at the first podcast of Rocket Talk Radio

Alastair Martin’s company, Production Tribe LLC, will be producing more shows soon and we hope to provide links on our website, RRS.ORG, from time to time. For those seeking ROCKET TALK RADIO, please go to Alastair’s website WATCHHOLLYWOOD.TV at the link below.

www.watchhollywood.tv

The next program is expected to be next week where ROCKET TALK RADIO will discuss the growing market of small launchers.

(5) Paintball Tanks and Regulators Used in Amateur Rocketry

Cameron Harrington is both a student at California Polytechnic State University in Pomona and a sponsored competitive paintball sportsman. After having some very interesting discussions about these commercially available, robust and mass-produced high pressure tanks and regulators, it is clear that they can be useful in building a simple pressure-fed liquid rocket. Ninja is one popular brand of these tanks and regulators used in paintball guns. The 4500 psi composite-overwrapped pressure vessels hold a finite volume (e.g. 77 cubic inches) of compressed nitrogen gas better suited for pressurizing fuel tanks. The Ninja Pro V2 regulator is adjustable by internal shims to allow a finite range of discharge pressures (350, 450, 550 psi etc) which work in small liquid rocket engines.

Ninja Pro V2 paintball regulator, adjustable outlet pressure by shims
Cal Poly Pomona students visit the EAA 96 hangar machine shop at the February 2019 meeting of the RRS

Cameron gave the society a brief overview of his experience with this hobby sport equipment and his experience in building a liquid rocket system. The society is considering buying a few of these devices for liquid rocket prototypes that will ultimately lead to a standardized design that the society can use and offer to other universities seeking a common-sense plan to flying a liquid rocket.

A simplified diagram of a pressure-fed, bi-propellant liquid rocket; valves and regulators have been omitted

(6) Ramiro Rodriguez, Deputy State Fire Marshal, CAL FIRE

The RRS was happy to be visited by Ramiro Rodriguez, Deputy State Fire Marshall with the California State Fire Marshal’s office (CAL FIRE). Deputy Rodriguez has been with CAL FIRE for over 19 years and largely supports Fireworks and the Motion Picture industry. He is glad to visit with amateur rocketry groups to see what our concerns and needs are. CAL FIRE has been busy streamlining and examining their processes to better serve the public and groups such as ours who benefit from CAL FIRE..

RRS members Drew Cortopassi and Chris Lujan sit on opposite sides of our special guest, Deputy State Fire Marshal Ramiro Rodriguez of CAL FIRE

Amateur rocketry, much like with hobby rocketry, is governed by the state laws and regulations concerning fireworks. The four primary duties of CAL FIRE are prevention, engineering, education and enforcement. They train fire departments and fire service professionals. They also are responsible for resource management in the state of California such as forestry and watershed projects. They are the licensing authority for all 12 classes of pyrotechnic operators including the 3 classes of rocketry pyro-op’s. Ramiro answered questions by our membership and attendees.

Pyro-op’s must be 21 years old, have a clean criminal record and must submit an application to the state with five letters of recommendation from active pyro-op’s at or above the class level that they are applying. CAL FIRE is willing to accept expired pyro-op licenses as long as that license hasn’t lapsed more than a year. This is a common problem in many groups that pyro-ops allow their licenses to lapse out of financial necessity or simple neglect. The RRS is very active in our goals to acquire and advance more pyro-ops not only for our society, but for the amateur rocketry community at large.

Ramiro read some statistics from CAL FIRE’s database, that there are only 10 active first-class rocketry pyro-ops in the entire state of California. Only 10 active second-class rocketry pyro-ops and 34 active third-class pyro-ops remain throughout the large expanse of the Golden State. Concern has been raised by the amateur rocketry community about the difficulty in acquiring five active and relevant signatories when pyro-op’s want to advance their level. CAL FIRE is considering ways of making this process easier to do as they would like to see an increase in the number of rocketry pyro-ops in the state of California. The obvious solution is to require a lesser number of signatories for applicants, but CAL FIRE has not made a decision on exactly what they intend to do.

Concern was also expressed regarding the necessity of the two-year waiting period between achieving rocketry classes. Some applicants have a large amount of experience either professionally or in activities with their society. Ramiro had said that CAL FIRE does have some discretionary authority to recognize significant experience in proving an applicant suitable to advance to the next level, but he underscored the importance of log sheets and the responsibility of all pyro-ops and trainees to take accurate clear records of the work that they do. Put simply, the more familiar CAL FIRE is with your activities, the easier it becomes for them to evaluate you.

This is a rich subject which many more had other questions, but given the late hour, we concluded by appreciating Ramiro’s time and was happy to make his acquaintance. The RRS and CAL FIRE have had a long, positive relationship and hope to continue to do so. The RRS has extended an invitation to Ramiro or another deputy from CAL FIRE to come visit our private testing site when we will hold another event on April 6th with the student of Crenshaw Elementary with the LAPD CSP.

first design of the 2019 RRS symposium flyer, Jan 2019

We also gave CAL FIRE an electronic file of our 2019 RRS Symposium flyer and have invited CAL FIRE to be a presenter and/or exhibitor at the Symposium.

(X1) Experimental Aircraft Association, Local Chapter 96

The RRS was happy to have our February 2019 meeting hosted by the Experimental Aircraft Association (EAA) Chapter 96 at the Compton Airport. Xavier Marshall is both an RRS member and the vice president of the EAA 96. The EAA 96 is encouraging hobbyists such as those in the RRS to become members as we have many areas of common interest. Aircraft and rockets require hands-on machining skills which the EAA 96 is willing to share with new members. To become a member of the EAA 96, you must join both the national and local chapter. Right now (but discount offer soon to expire) they are offering 3-years of membership for only $99 which covers both the local and national membership.

Presses and sheet metal working tools at the EAA 96 hangar at the Compton Airport

Xavier gave the RRS and visiting students from Cal Poly Pomona a tour of their machine shop which has a large lathe, a horizontal and vertical mill. The hangar is accessible to members 24/7 and the EAA has many members happy to help those needing to learn practical machining skills. This is a great opportunity for many of the RRS who do not have regular access to machining. The RRS is largely about making our own custom parts and the EAA 96 is an excellent resource to help.

Xavier Marshall leans against a large sheet metal brake in the EAA 96 machine shop

For questions about joining the EAA 96, please contact Xavier Marshall or Wilbur Owens

xavier.marshall@gmail.com

wil.owens@cox.net

Vertical mill at the EAA 96 machine shop

(X2) Visit to the Rocket Lab at Tomorrow’s Aeronautical Museum

Given the late hour, we were unable to take our society membership and visitors on a tour of the Rocket Lab at Tomorrow’s Aeronautical Museum. Waldo Stakes has been very active in this project to bring a small group of Compton locals to build a small liquid rocket of their own. On display at the meeting was a Rocketdyne NA-LR-101 liquid vernier rocket motor that they hope to static fire at the RRS MTA. This 1000-lbf kerosene/liquid oxygen rocket has been commonly used in past amateur rocketry projects due to its robust design, however, as these surplus motors are becoming more scarce, it is important to appreciate having such an asset for learning. The RRS is happy to help the Compton group with their goals in flying this motor in a future design.

It was suggested that the RRS hold their March 8th meeting at Tomorrow’s Aeronautical Museum at the Rocket Lab. Although this is a fine suggestion, the RRS had planned to return to our regular location at the Ken Nakaoka Community Center in Gardena. That being said, the RRS would like to schedule an event at the Rocket Lab very soon. The RRS will let our membership know when this visit to the Rocket Lab can be scheduled.

(X3) Upcoming testing events at the RRS MTA

As was mentioned a little earlier, the RRS has set a new class with the students of Crenshaw Elementary School through the LAPD CSP. The first class will start on Friday, March 1, and run each Friday until the launch event we will hold at the RRS Mojave Test Area (MTA) on April 6th.

The RRS classes continue to be very popular and we are glad to share our hobby and passion for rocketry and learning.

RRS member, Michael Lunny, has been working with his local high school, Redondo Union, were they intend to enter rocketry competition to launch a rocket payload with 3 eggs and subsequently land it by parachute. We hope they can come visit the RRS at the next meeting on March 8th. Redondo has expressed interest in launching at the MTA in late March.

(X4) Groups wanting to test at the RRS MTA

For all groups interested in working with the RRS or with testing or launching from our Mojave Test Area, please download and fill out our Standard Record Form from the RRS.ORG website under “Membership” tab, then under “Forms”. All requests must be filled out with a complete set of contact information and a full description of the testing. The most important thing is to declare your test date and hold to this date as resources have to be scheduled. All requests must be submitted to the RRS president for the society to review.

president@rrs.org

IN CLOSING

Our next meeting will be on Friday, March 8th, at our usual meeting location at the Ken Nakaoka Community Center in Gardena. If there are any questions, please let the RRS secretary know:

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