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.


Spaceport L.A. holds its Rocktoberfest event at Relativity Space

The Reaction Research Society (RRS) was very fortunate to attend an event sponsored by Spaceport L.A. This “Rocktoberfest” event held on November 15, 2018, was at the Relativity Space offices in Inglewood. This was a sold-out event with many people present from industry and the space-loving public.

A full house in attendance of the Spaceport L.A. panel session held at Relativity Space, 11-15-2018

Spaceport L.A. is an organization aiming to unite, build and support a space professional community in the thriving heart of Los Angeles. Through discussions, meetings and events held by Spaceport L.A., their aim is to propel further innovations and potentially foster breakthroughs to enhance space exploration. Members include technical professionals from all disciplines and other passionate individuals with the same aim to support the space community. The RRS naturally fits with these goals and we hope to have Spaceport L.A. participate at the 2019 RRS symposium at the Ken Nakaoka Community Center in Gardena in April (exact date to be confirmed soon).

Spaceport L.A. – main website

The event brought a good crowd with people from Virgin Orbit, SpaceX, Phase Four, Aerospace Corporation and other professional organizations. The pork schnitzel offered from the food truck outside and the beer served at the event made for a fine supper before the panel discussion that was to take place.

Contrary to the hopes of many, Relativity did not offer a tour of their facilities at the event, but several members of Relativity were in attendance including former University of Southern California (USC) Rocket Propulsion Laboratory (RPL) member and Chief Technical Officer (CTO) of Relativity, Jordan Noone. I had a chance to talk with Jordan and his new Director of Propulsion at Relativity, Nate Scholten.

Dave Nordling, Secretary, RRS, standing with Nate Scholten, Director of Propulsion at Relativity Space; in the background is the massive 3D printed aluminum upper stage tank prototype on display

Relativity is an orbital launch company founded in 2015 by Tim Ellis and Jordan Noone that is taking a fundamentally new approach to building and flying rockets. With its customized and proprietary 3D printing technologies, they plan to iteratively build rockets at a cost and speed that better enables humanity’s visions in space.

Relativity Space

Also, in attendance at the event was Mike Kwapisz, Vice President of Engineering at Phase Four (P4). Phase Four is an El Segundo based start-up company making a new RF-based electrode-less, electromagnetic-based propulsion system. The “Maxwell” engine is designed for high reliability, long operating life and is modular for smaller low-power satellites to larger spacecraft systems.

The panel discussion had many fine speakers representing a wide swath of newer aerospace industries. Tim Buzza, former SpaceX manager and now a director at Relativity Space, gave a very insightful talk about his early days at SpaceX with the Falcon 1 and the trying times they had in bringing a new launch vehicle to the market.

Dr. Greg Autry, also on the panel, is a founding director of the Southern California Commercial Spaceflight Initiative at the USC Marshall School of Business. He offered many great insights about how the commercial space market has grown from once a purely speculative concept to an expanding and thriving industry in the United States and abroad.

Spaceport L.A. panel discussion, Marco Villa (Tyvak), Tim Buzza (Relativity Space), James Behmer (Phase Four) and Gene Autry (USC) with Curtis Iwata moderating.

James Behmer of Phase Four was also on the panel and shared a bit of their history in bringing a new modular propulsion technology into the growing small satellite market.

Lastly, Marco Villa, Chief Operating Officer (COO) of Tyvak Nano-Satellite Systems, was on the panel sharing his insights from his previous days at SpaceX as the Director of Mission Operations. Dr. Villa also talked about how Irvine based, Tyvak is developing nano-satellites for the growing market. He also mentioned an interesting statistic that today there are over 100 companies world-wide that are endeavoring to offer access to space. In an age of consolidation, it is good to hear how the market is expanding in some respects.

Space[prt L.A.’s banner hangs below Relativity Space’s 3D printed aluminum upper stage tank on display

The panel discussion was fascinating and I hope Spaceport L.A. will sponsor another event soon. I am very thankful to Curtis Iwata and all of the volunteers at Spaceport L.A. for coordinating a great evening that fostered many great discussions with more to come in the future. I have left a link to the Spaceport L.A. website above and a few links below to the companies represented on the panel. Also, I am grateful to Relativity Space for opening their doors for this networking event through Spaceport L.A. It was a fascinating night.

Tyvak Nano-Satellite Systems Inc. – a Terran Orbital company

Relativity Space

Phase Four – advanced spacecraft propulsion

USC Marshall School of Business – Dr. Greg Autry

Please look for the next RRS event or just stop into one of our monthly meetings at the Ken Nakaoka Community Center in Gardena. Our next meeting will be Friday, December 14, 2018, at 7:30pm.

RRS visit to Additive Rocket Corporation

The Reaction Research Society (RRS) was glad to receive an invitation to an open house held by the Additive Rocket Corporation (ARC) in San Diego on November 13, 2018, which invited guests from academia and industry to have an exclusive look at the company’s business. The event was co-sponsored by the University of California San Diego (UCSD’s) Atkinson Hall Prototyping Laboratory. It was well worth the long drive through south-bound traffic from Los Angeles to arrive at UCSD by early evening.

Additive Rocket Corporation in San Diego, CA

Additive Rocket Corporation

The welcoming speech was given by Dr. Jeff Sundabrae showcasing the Atkinson Hall Prototyping Laboratory at UCSD and proud partnerships that they have cultivated.

Andy Kieatiwong, CEO of Additive Rocket Corporation, and Dave Nordling, Secretary, RRS

Andy Kieatiwong, CEO of ARC, gave the headline speech about his new company, founded in partnership with his friend and colleague, Kyle Adriany.

Kyle Adriany, CTO of Additive Rocket Corporation (ARC)

ARC uses a proprietary iterative design process that takes advantage of the freedom in design offered by additive manufacturing. ARC has developed their own software algorithms that can rapidly analyze and compare a wide range of design alternatives very quickly which ultimately leads to a few highly efficient designs that should enable a low cost, high performance rocket to be made.

ARC is a small start-up in San Diego. Several of their engineers and experts were in attendance at the event as ARC showed the audience their mission to produce high impulse, ultra-low cost additively manufactured engines. Many of the guests were fellow researchers at the Prototype Laboratory and UCSD. The most exciting part was getting a tour of the university laboratories and the shared space that ARC has with their EOS M290 large scale metallic 3D printer made by Electro Optical Systems (EOS). It is an amazing piece of technology to observe in action.

Electro Optical Systems (EOS), M290 industrial 3D metal printer

An uncooled thrust chamber prototype of a 125-lbf rocket motor, made by ARC’s 3D printing machine in Inconel 718

ARC has already built their first prototypes and is planning a series of hot fire testing of their “Nemesis” engine, hopefully at the RRS Mojave Test Area (MTA), in the very near future. With successful rounds of testing to anchor their design algorithms, ARC should be able to offer a powerful and elegant rocket at very competitive prices to a growing marketplace.

Additive Rocket Corporation’s “Nemesis” engine

I was glad to attend the event and greatly appreciate the hospitality of ARC and UCSD. Many found it very exciting to witness firsthand a remarkable material process that is slowly and surely changing the manufacturing marketplace.

The RRS is hopeful that ARC will attend and speak at our forthcoming 2019 RRS symposium in April. Stay tuned to our website for further updates from ARC and the 2019 RRS symposium at the Ken Nakaoka Community Center in Gardena.