by Larry Hoffing, Educational Outreach Coordinator, Reaction Research Society
The Reaction Research Society (RRS.ORG) is glad to be a part of an upcoming event with Spaceport L.A. The “Rocket Workshop with the RRS” is an excellent opportunity for anyone who wants to get directly acquainted with rocketry. This event is meant for both professionals and non-professionals alike. From younger students to university students of all fields, to adults, this event is meant to give people the experience of assembling and flying your own rocket.
The event will begin on Saturday, May 4, 2019 with a subsequent launch of the rockets from the RRS Mojave Test Area (MTA) on Saturday, May 18, 2019. At this event, you can learn about the fundamentals and more practical knowledge of rocketry.
The event will be held at the HexLab MakerSpace in Van Nuys. This is laser-cutting service in the Los Angeles area. Check the Spaceport L.A. website for the details and updates.
The RRS standard alpha rocket is a very old, but reliable design still used in the society. Although micrograin propellant is not used anywhere else but at the RRS (where it was initially discovered in the 1940’s), it is a simple and powerful propellant combination that makes for an impressive show of raw power. I have attached an earlier description of the RRS standard alpha rocket below.
After the first session, on the second event, you can go out to the RRS’s private Mojave Test Area (MTA) at Koehn Dry Lake, east of Cantil, California in the high desert. The RRS will handle the propellants, you can see the impressive results from the safety of our observation bunker.
Amateur rocketry is our passion and purpose and the society is glad to hold this event with Spaceport L.A. and the public.
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, firstname.lastname@example.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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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: