Liquid Rocket Components: Pyrotechnic Valves

by Tom Mueller


Editor’s Note: This is a reprinting of the original article written by RRS member, Tom Mueller on the subject of pyrotechnic actuated valves around 1995 (?). He mentions the build of two different rockets (the XLR-50 and the Condor) and a hypergolic rocket he intended to build after this article was written. We hope to gather more photos and details about these rockets and display them in future improvements to this posting. For now, please enjoy the subject matter as the information is very relevant today to amateur builders of liquid rockets. The RRS has been very active lately in re-exploring liquid rockets. The society thought this would be a timely and interesting subject to share with our readers.

For any questions, please contact the RRS secretary, secretary@rrs.org


For an amateur rocketeer seeking to build a liquid rocket, one of the most difficult components to obtain or build are remotely operated valves. A liquid rocket will require at least one valve to start the flow of propellants to the combustion chamber. In the two small liquid rockets I have flown in the last year or so, both used a pyrotechnic fire valve located between the pressurant tank and the propellant tanks. The propellants were held in the tanks by burst disks (or equivalent) in the propellant run lines. When the fire valve was actuated, the sudden pressure rise in the propellant tanks blew the burst disks, allowing propellant to flow to the injector. This method of controlling the flow to the rocket allows the use of only one valve, and eliminates liquid valves.

In the case of the first rocket, the XLR-50 which flew in October 1993, elimination of the liquid valve was important because the oxidizer was liquid oxygen, and a small cryogenic compatible valve is very difficult to construct.

For the second rocket, which flew in October 1994, the small size prevented the use of liquid valves. In fact, the single pyro valve I used was barely able to fit in the 1.5 inch rocket diameter. In this article I will describe the design of the valves that were used on these two vehicles, and variations of them that have been used in other rocket applications.

FIGURE 1: XLR-50 pyro-technic “fire” valve

The valve shown in Figure 1 consisted of a stainless steel body with a 0.375 inch diameter piston. The O-rings were Viton (material) and the squib charge was contained in a Delrin plastic cap. The Delrin was used to prevent shorting of the nichrome wire, and also to provide a frangible fuse in case the squib charge proved to be a little too energetic. In practice, I’ve never had the Delrin cap fracture.

The inlet and outlet lines to the tanks were silver brazed to the valve body. The valve was tested many times at inlet pressures of up to 1000 psi without any problems, other than the O-rings would need replaced after several firings due to minor nicks from the ports. To help alleviate this problem, the edges of the ports were rounded to help prevent the O-ring from getting pinched as the piston translates. This was accomplished using a small strip of emery cloth that was secured in a loop in one end of a short length of 0.020-inch stainless steel wire. The other end of the wire was clamped in a pin vise which in turn was chucked in a hand drill. As the wire was rotated by the drill, the emery was pulled snugly into the port, where it deformed into the shape of the inlet, and rounded the sharp edge. I used WD-40 as a lubricant for this operation, allowing the emery to wear out until it would finally pull through the port. I repeated this process a few times for each port until the piston would slide through the bore without the O-rings snagging the ports.

Another requirement is to lubricate the O-rings with a little Krytox grease. This helps the piston move freely and greatly reduces the problem of nicked O-rings.

FIGURE 2: Fire valve for a micro-rocket

The pyro valve I used in the 25 lbf thrust micro-rocket that was launched in October of 1994 is shown in Figure 2. This valve was identical in operation to the XLR-50 valve, with the major difference being its integration into the vehicle body. The valve body was a 1.5 inch diameter aluminum bulkhead that separated the nitrogen pressurant tank and the oxidizer tank. Because of the very small diameter of the rocket, the clearances between ports and O-rings were minimized, just allowing the valve to fit. The fuel outlet port was located at the vehicle center, providing pressure to the fuel tank by the central stand pipe that passed axially down the oxidizer tank. The piston stop was a piece of heat-treated alloy steel that was attached to the valve body by a screw. This stop was originally made from aluminum, but was bent by the impact of the piston in initial tests of the valve. The black powder charge in the Delrin cap was reduced and the black powder was changed from FFFg grade to a courser FFg powder, but the problem persisted. The stop was re-made from oil hardening steel and the problem was solved. In this application, the port diameters were only 1/16 inch so only a small amount of rounding was required to prevent the O-rings from getting pinched in the ports. The valve operated with a nitrogen lock-up pressure of 1000 psi.

FIGURE 3: Fire valve for Mark Ventura’s peroxide rocket

A more challenging application of the same basic valve design was used for the fire valve of Mark Ventura’s peroxide hybrid, as shown in Figure 3. This was the first application of this valve where liquid was the fluid being controlled, rather than gas. In this case the liquid was 85% hydrogen peroxide. The second difficulty was the fact that the ports were required to be 0.20 inch in diameter in order to handle the required flow rate. The valve was somewhat simpler than the previous valves in that only a single inlet and outlet were required. The valve body was made from a piece of 1.5-inch diameter 6061 aluminum, in which a 1/2-inch piston bore was drilled. The piston was also 6061 with Viton O-rings, which are peroxide compatible. The ports were 1/4-inch NPT pipe threads tapped into the aluminum body. The excess material on the sides of the valve was milled off, so that the valve was only about 3/4 of an inch thick, and weighed only 4 ounces. Even though the piston size was 1/2 inch, the same charge volume used in the 3/8 inch valves was sufficient to actuate the piston.

In testing the valve with water at a lock-up pressure of 800 psi, I was pleased to find that even with the large ports, O-ring pinching was not a problem. One saving factor was that the larger size of the ports made it easier to round the entrances on the bore side. The valve was tested with water several times successfully before giving it to Mark for the static test of his hybrid.

The only problem that occurred during the static test of hybrid rocket was that the leads to the nichrome wire kept shorting against the valve body. Three attempts were made before the squib was finally ignited and the engine ran beautifully. I have since been able to solve this problem by soldering insulated 32-gauge copper wire to the nichrome wire leads inside the Delrin cap. In this way, I can provide long leads to the valve with reliable ignition.

My next liquid rocket is a 650 lbf design that burns LOX and propane at 500 psia. This engine uses a Condor ablative chamber obtained from a surplus yard. For this reason, I call it the Condor rocket. This rocket uses a scuba tank with 3000 psi helium for the pressurant. I decided to build a high pressure version of my valve as the helium isolation valve for this rocket. When firing this rocket, just prior to the 10 second count, this valve will be fired, pressurizing the propellant tanks to 600 psi. I assumed going in to this design that the O-rings slipping past a port simply wasn’t going to work at 3000 psi.

At these pressures, the O-ring would extrude into the port. In order to get around this problem I came up with the design shown in Figure 4.

FIGURE 4: High pressure helium valve for Condor rocket

For this valve, the O-ring groves were moved from the piston to the cylinder bore of the valve body, so the O-rings do not move relative to the ports. The piston is made from stainless steel with a smooth surface finish and generous radii on all of the corners. The clearance between the piston and the bore was kept very small to prevent extrusion of the O-rings. The valve operation is similar to the one shown in Figure 3, and the valve body is made in the same way except female AN ports were used rather than NPT ports. When the valve is fired, the piston travels from the position shown in Figure 4a to that shown in Figure 4b. During this travel, the inlet pressure on the second O-ring will cause it to “blow out” as the piston major diameter translates past the O-ring groove. The O-ring is retained around the piston, causing no obstruction or other problems. This valve has been tested at 2400 psi inlet pressure with helium and works fine. It will be tested at 3000 psi prior to the first hot fire tests of the Condor rocket next spring.

As a side note, essentially an identical valve design as the one used on the Condor and Mark’s valve is a design shown in NASA publication SP-8080, “Liquid Rocket Pressure Regulators, Relief Valves, Check Valves, Burst Disks and Explosive Valves”.

A second pyro valve is used on the Condor system as shown in Figure 5. This valve is used to vent the LOX tank in the event of a failure to open the fire valve to the engine.

FIGURE 5: Emergency vent valve for LOX tank, Condor rocket

When the propellant tanks are pressurized by the helium pyro valve, the LOX tank auto vent valve (shown in Figure 6) closes. If the engine is not fired after a reasonable amount of time, the LOX will warm up, building pressure until something gives (probably the LOX tank). The pyro valve shown in Figure 5 is used as the emergency tank vent if the engine cannot be fired. The valve body is stainless steel with a stainless tube stub welded on for connection to the LOX tank. This valve has been tested to 800 psi with helium and works fine. In this case, some ‘nicking’ of the O-rings can be tolerated because the O-rings are not required to seal after the valve is fired. The ports in the bore are still rounded, however, to prevent the O-rings from getting nicked or pinched during assembly of the valve.

Even though it is not a pyro valve, I have shown the LOX auto-vent valve in Figure 6 because this design has proven to be very useful for venting cryogenic propellant tanks without requiring a separately actuated valve or control circuit. The valve uses a Teflon slider that is kept in the vent position as shown in Figure 6a.

This allows the tank to vent to the atmosphere, keeping the propellant at its normal boiling point. When the helium system is activated, the pressurant pushes the slider closed against the vent port, sealing off the LOX tank, as shown in Figure 6b. An O-ring is used around the slider to give it a friction fit so the aspiration of the LOX tank does not “suck” the slider to the closed position. This problem happened to David Crisalli (fellow RRS member) when he scaled this design up for use on his 1000 lbf rocket system. I have used this design on the LOX tank of my XLR-50 rocket, which used a 1/4-inch diameter slider, and on the Condor LOX tank, which uses a 1/2 inch slider. In both cases the vent valve worked perfectly.

FIGURE 6: Automatic LOX tank vent valve

The main fire valve on the Condor rocket is a pair of ball valves that are chained together to a single lever so that both the fuel and oxidizer can be actuated simultaneously for smooth engine startup. For static testing of the rocket, I will use a double-acting air cylinder to actuate the valves. For flight, however, I plan to use a pin that is removed by an explosive squib to hold the valve in the closed position. When the squib is ignited, the pin is pulled by the action of the charge on a piston, allowing the valves to be pulled to the open position by a spring. This method may not be very elegant, but it is simple, light, and packages well on the vehicle. David Crisalli has successfully employed this technique on his large rocket.

That covers the extent of the pyro valves I have built or plan to build so far. In the next newsletter, I will present the design and flight of the small hypergolic propellant rocket that used the valve shown in Figure 2.


75th anniversary symposium was a success!

The Reaction Research Society (RRS.ORG) was happy to celebrate its 75th anniversary as the country’s oldest continuously operating amateur rocketry society on April 21, 2018. At the Ken Nakaoka Community Center in Gardena, California, we shared this special occasion with over 300 people from the Los Angeles and San Diego area and welcomed several guests from places further away.

(left to right) Osvaldo Tarditti, Bill Janczewski, Dave Nordling, Jim Gross, Frank Miuccio, Larry Hoffing, Alastair Martin, Richard Garcia, Bill Claybaugh, Drew Cortopassi, Chris Lujan

RRS member, Michael Lunny mans the front desk at the 2018 RRS symposium

RRS members, Jim Gross and John Mariano at the 2018 symposium

Osvaldo Tarditti, George James, George Dosa and Jerry Irvine at the RRS symposium

Bill Claybaugh and RRS founder, George James, at the 2018 RRS symposium

The RRS had a display of some of our society projects past and present. Also, some of our members had their projects on display including Richard Garcia’s liquid rocket and Bill Claybaugh’s massive two-stage rocket.

Richard Garcia discusses his liquid rocket vehicle at the 2018 symposium

An early liquid rocket test motor from George Dosa, furfuryl alcohol and nitric acid

RRS micrograin rockets on display with historical notes

Bill Claybaugh, Osvaldo Tarditti and Bill Janczewski stand before Claybaugh’s two-stage solid rocket on display

Photo montage of micrograin rocket launches

All thirteen RRS mail flights from 1947 – 1990

We had copies of the special 75th anniversary edition of the RRS Astrojet newsletter available for sale at the symposium. Thank you to Bill Janczewski for his hard work in making this high quality newsletter and the bright sign on the column for everyone to see as they came in. The Astrojet can still be purchased through our RRS.ORG website at our PayPal button if you write a note for “Astrojet, (X) copies” and send your mailing address.

Or just simply contact the RRS by email.
secretary@rrs.org

75th anniversary issue of the Astrojet newsletter on sale

We shared our exhibition space with the Los Angeles Air Force Base’s (LA AFB) Space and Missile Command (SMC) as they presented the long history of SMC. Our thanks to Lt. Col. Porter and his team for having a huge display of the Air Force’s contributions to space, national security and improvements to our daily lives. Also, the air-driven rocket launcher demonstration in the courtyard was a big hit.

Karen Austin, Director of SMC History at the 2018 RRS symposium

LA AFB SMC history on display at the 2018 RRS symposium

Lt. Col. Porter speaks at the SMC history exhibit

Also, just outside the Ken Nakaoka Community Center in Gardena, was our colleagues at the Los Angeles Police Department (LAPD) Community Safety Partnership (CSP). Officers who have supported and participated in the rocket build classes we’ve had with Watts and Compton area schools were on hand to answer questions and show off the fun we’ve had over this last year.

LAPD CSP at the 2018 RRS symposium

We had several universities exhibiting and presenting at the RRS symposium including University of California Los Angeles (UCLA), University of Southern California (USC) and California State University Long Beach (CSULB). All of them had impressive work to show with flights pending in just a few weeks before the semester or quarter ends.

CSU Long Beach exhibits and presents at the 2018 RRS symposium

The Additive Rocket Corporation of San Diego exhibited and presented their unique technology.

The Additive Rocket Corporation of San Diego exhibits and presents at the RRS symposium

Other exhibitors at the RRS symposium was our fellow amateur rocketry group, Rocketry Organization of California (ROC).
Rocketry Organization of California

ROC on display at the 2018 RRS symposium

The Notre Dame Academy was also present at our symposium.
Notre Dame Academy – WIkipedia

Notre Dame Academy at the 2018 RRS symposium

Our friends at the China Lake Museum also had a display to show the Navy’s contributions to rocketry and the national defense.
China Lake Museum

China Lake Museum on display at 2018 RRS symposium

U.S. Rockets was also exhibiting at the RRS symposium.
U.S. Rockets – Jerry Irvine

U.S. Rockets exhibiting at the RRS symposium

We had several speakers presenting on current and historical topics of professional and amateur rocketry including Jacky Calvignac of Northrop Grumman, our founder George James of his organization, The Rocket Research Institute (RRI), John Steinmeyer of Orbital-ATK and David Krause of NASA Goddard Spaceflight Center’s Wallops Island Flight Facility in Virginia who called in by Skype.

George James, founder of the RRS, presents on behalf of the RRI

Jacky Calvignac shows the propulsion programs at Northrop Grumman

High School Rocket Propulsion Lab and new RRS members present their test firings from the RRS MTA

Aerospace Corporation’s presentation on additive manufactured propellant grains

We thank all of our attendees, presenters, exhibitors and just everyone who stopped in and had a good time with us. The RRS would like to especially thank Tony Richards for his photography taken at the RRS symposium.

The RRS will discuss at our next monthly meeting on May 11th if we’ll have another symposium next year in 2019. Based on the overwhelming response, this is very likely.

April 2018 meeting

The RRS held its monthly meeting on April 13, 2018 at the Ken Nakaoka Community Center in Gardena. We had a full agenda with the most important item being the 75th anniversary RRS symposium that is coming in only 8 days. We had a prompt start at 7:30pm with Osvaldo calling the meeting to order and giving the reading of the treasury report. We had a full attendance including our new RRS members, Jack, Dylan, Connor, Cooper and Byron from the former Chaminade High School rocketry club. They have been very busy with their solid rocket project. We didn’t have time to add their progress to our agenda, but we hope they’ll give an update at the May meeting.

Bill Janczewski and Chris Lujan just before the April 2018 meeting

The first agenda item discussed the work done by the Aerospace Corporation of El Segundo, California, on March 26th and 27th at the RRS MTA site. RRS member and Aerospace Corporation employee, Drew Cortopassi gave an excellent summary of the work. Aerospace Corporation’s experimental work was a success and we hope that Aerospace may return to use our site to advance their designs. Aerospace Corporation is one of our exhibitors and presenters at the meeting. A fuller discussion of their rocket testing at the RRS MTA will be given at the RRS’s 75th anniversary symposium, next Saturday, April 21st.

Aerospace Corporation tests an experimental solid motor design at the RRS MTA, 3/26/2018.

The next agenda topic was discussing the results from the launch event held last weekend with Florence Joyner Elementary School with the LAPD CSP program. Frank gave an excellent summary and Osvaldo discussed a theory explaining the odd bending of each alpha rocket found at the event. Most alpha rockets come down nearly vertically and burying themselves straight into the hard dry lake bed. At the launch event of April 7th, nearly all of the rocket propellant tubes were bent. Typically, this only happens if the rocket strikes a rock beneath the surface, but it is quite uncommon. Osvaldo’s invention of the Rockextractor proved to be a swift tool for reclaiming found alpha rockets.

Osvaldo’s newest invention, the Rockextractor

As a side note, Frank had mentioned that the USC short film “Rockets in the Projects” covering the November launch event with Grape Street Elementary class and the LAPD CSP program will be screened with other short films at Annenberg Hall on the campus of USC on Wednesday, April 18th. Seating is very limited so those interested in seeing the film should act quickly.

The third agenda topic was about the RRS expanding its roster of licensed pyrotechnic operators (pyro-op’s). We have great support from our current pyro-op’s but the society would benefit from having more. The RRS will be building a training manual that includes the materials mandated by the California Fire Marshal’s office to attain a license in rocketry. This tool will be an effective study tool for members looking to become licensed pyro-ops. At the very least, the training helps spread safe practices in the society. Given the limited time we had at this month’s meeting, we agreed to discuss this topic further at the next month’s meeting in May.

RRS pyro-op manual and training guide

The fourth agenda topic was something that Larry Hoffing discovered. The 2-bit Circus Foundation is a non-profit educational organization dedicated to the next generation of inventors to advance environmental stewardship and spur community engagement. The 2-bit circus has a futuristic arcade in downtown Los Angeles and has reached out to the RRS to be an exhibitor at their next event in Hawthorne, California. The RRS was supportive of this idea and once more details become available we will find a few members to attend the exhibition to help us reach more people at this event for STEAM (Science, Technology, Engineering, Art and Mathematics) education which our shared passion.

Two Bit Circus Foundation – STEAM education

The fifth agenda topic briefly discussed an idea that I have developed a second-stage design that fits with the RRS standard alpha payload tube. With a short segment of 1-1/4″ PVC tubing turned down to fit inside the 1.75″ OD payload tube and machined PVC end cap, a second stage motor with a graphite nozzle can be fired atop of the micrograin booster. A short length of Type-1 PVC round stock was donated to the RRS by Industrial Plastic Supply Inc. of Anaheim, California. There are many plastic suppliers, but not many that offer such a range of plastics in small quantities better suited for experimenters and hobbyists with modest personal budgets. I happily recommend them to all.

Industrial Plastic Supply Inc. – Anaheim, California

An interstage piece is necessary to trigger the second stage after a set time delay. I have designed an umbilical connector piece that uses a 3.5mm audio plug and panel-mount jack as a switch.

3.5 mm audio jack, panel mount

To internally mount the switch at a shallow angle for easy extraction of the plug as the alpha rocket lifts away from the launch rack required a unique plastic piece that Richard Garcia was able to 3D print for me in plastic. The fit check was a success so now I have to get the connector wired and mounted.

two of the umbilical jack mounts, plastic nozzle puck in the foreground

The solid motor grain itself was thought to be simple rocket candy, but other solid propellant types could be tried. Chris Lujan offered to pour a second stage grain for my design. Richard Garcia also offered to make his next motor grain to fit in this standard payload tube size. I have designed most pieces, but some aspects of this design need more work and testing. The RRS would like to encourage our members and our partner organizations to design and fly payloads with our standard alpha rockets which are easy to produce. Time was short so this topic was also tabled for the next month’s meeting as this work evolves.

RRS standard alpha, second stage assembly (work in progress)

The sixth agenda topic similarly had no real time to get into the details. The quarterly progress report of the SuperDosa project had only the RRS ballistic evaluation motor (BEM) to discuss. This workhorse tool will help the RRS accurately determine burn rate with variable nozzle puck sizes. Richard Garcia did turn out a basic set of graphite puck nozzles for the BEM.

RRS BEM graphite nozzle pucks, courtesy of Richard Garcia

The cylinder piece is with Osvaldo once he can find time to machine the bore and pressure ports. The top and bottom plates will be made soon by CNC Specialty Machining of Huntington Beach. This is the same machine shop that did a quality job with milling the S-type load cell adapter blocks for the RRS horizontal thrust stand. Thanks to Matt Moffitt of CNC Specialty Machining which is soon to relocate their business in Huntington Beach next month.

RRS ballistic evaluation motor design concept

The last agenda topic was the last preparations necessary for the RRS symposium coming next Saturday, April 21st. This event will easily be larger than last year’s event with over 400 Eventbrite reservations made just before the meeting. We have a longer list of speakers including new participants such as NASA Goddard Spaceflight Center’s (GSFC) Wallops Island Flight Facility (WFF) in Virginia. We will need all of our members to help us support this event. Please spread the word!

There will be a lot of work in setting up the night before. Members are encouraged to come help set things up at 7PM on the Friday night beforehand, April 20th. Contact Frank Miuccio who is our symposium coordinator if you would like to help.

vicepresident@rrs.org

The RRS meeting went out to the exhibition hall of the Ken Nakaoka Community Center in Gardena. The RRS will have the whole center for our public event and we hope everyone can come. Frank will send me the final agenda with the speaker list and the presentation times. This is an all-day event, but consult the RRS.ORG website for updated information. I plan to make a posting for the RRS symposium soon.

The meeting adjourned as the Ken Nakaoka Community Center closed. Osvaldo did some work for our new members with a clamping assembly to hold their rocket casing in their own horizontal thrust stand to test their full-sized boosted dart motor. Although independently conceived, this work nicely fits with the SuperDosa project and the RRS is glad to have another project to help advance the work of the society and our members.

RRS members stand outside of the Ken Nakaoka Community Center after the meeting with the horizontal thrust stand

If there is anything I have missed or misstated, please email me:
secretary@rrs.org

Our next meeting will be Friday, May 11, 2018. Please come as we will have much to discuss after the symposium and plenty to do in this summer of our 75th anniversary as a society.