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,

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.

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 overlooked.

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:,126697.html

Or a gas sample cylinder such as this one from Swagelok

For questions, contact Richard at

December 2018 meeting

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

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

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

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

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

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

The RRS officers for 2019 are as follows:

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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