Category Archives: Presenting

Burst discs in liquid rocketry

Posted on by

by Dave Nordling, Secretary, Reaction Research Society

I was recently asked for advice on the installation of a burst disk in a university liquid rocket project. As any pressure relief device is an important subject to consider carefully, I wanted to present a summary of my thoughts to our broader readership.

The Reaction Research Society (RRS) is happy to offer advice, but my first recommendation to any university team would be to talk with your university professors, professional advisers and mentors that are involved with your project. A burst disk is an important component and its function can be critical for safety and preserving your vehicle in any over-pressurization scenario. The subject of your rocket system pressurization, venting and relief devices is extremely important to study well and thoroughly understand before proceeding with any component selection or testing.  Your university is the best place to start.

For those who are doing a liquid rocket project outside of a university program, I would also recommend to consult with experts and reputable manufacturers and distributors of pressure relief devices including burst disks.

Burst discs (the spelling “disk” or “disc” is interchangeable) are one simple form of a pressure relief device or valve that is designed to prevent over-pressurization of a pressure vessel and potential catastrophe.  Burst disks are also sometimes called “rupture disks” which clearly describe their function.

https://en.wikipedia.org/wiki/Rupture_disc

In liquid rocket system designs, burst disks are often placed not only at the pressurant bottle to protect the higher pressure part of the system, but also at the lower pressure end of the regulator which protects the propellant tanks being pressurized. In the event of pressure regulator failure, the burst disk can protect the propellant tank.

Burst disks are usually in the form of a dead-ended pressure fitting that is adapted to directly connect into the pressure vessel either directly into the pressure vessel volume boundary itself or by a tube connection that is also directly connected into the pressure vessel volume boundary. These fittings have a frangible or breakable membrane that is designed to fail when the pressure reaches a specific design point.

An illustration of the burst disk fitting concept

A burst disk is a “one-time use” device and can not be reset after they have “actuated”. As a pressure relief device, the burst disk is often chosen for its compact size and simplicity. They are in common usage in many industries and can fulfill their relief function very well if they are sized and located properly.

They must be securely and directly connected into the volume of the pressure vessel and have no valves or other hardware which would isolate, block, impinge or constrain the relief function in any way. The American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel (B&PV) code does have some general advice on this subject and this is a good place to start your study.

These devices are simple to understand but fairly complex to size properly. Beyond the design of the burst disk, you must also consider where these devices will physically fit on your vehicle, where are they located and what is the environment doing around your relief device

The burst disk body and membrane can be subject to corrosion or physical damage that could reduce it’s effective bursting pressure. It’s important to consider the material compatibility of all body, seals and membrane materials that are exposed or “wetted” to the gases inside. Also, its important to avoid getting gouges, nicks or marks on the membrane that would form stress concentrations and weaken the membrane. Even when being cautious, don’t leave your burst disk covered when it needs to be ready to perform. Careful handling is good advice at all points in the project.

There are three things to consider when locating and installing a burst disk:
(1) relief (set) pressure, (2) minimum flow rate required, (3) where is your burst disk pointed?

(1) Set pressure of the relief device

Any relief device must be set to actuate (or in the case of a burst disk, to rupture) at a pressure above all of your nominal conditions, but also adequately below any and all failure modes.  In some pressure vessel or relief device codes, there are rules of thumb about the set pressure must be a specific percentage (%) above the maximum expected operating pressure (MEOP) or maximum allowable working pressure (MAWP). The thorough examination of all operating conditions and hardware limitations is essential of finding the right set pressure for the relief device. 

ASME also has codes for sizing relief valves in process piping, but the rocket industry doesn’t have a particular specification. The aerospace industry does often draft their own specifications and requirements which follow good industrial practices and always include careful design and testing as part of proving the designs to be sufficient.

Another consideration beyond the static pressure in your pressure vessel is the temperature environment of the gases inside. Beyond the fact that higher temperatures from a thermodynamic standpoint create higher pressures, a burst disk relies on the material strength of the membrane and the yield and ultimate strength can weaken under higher temperatures. Some materials (examples are low quality steels) can also become weaker under cold temperatures. Always consider the full range of temperature environments in every application. It’s important to size each burst disk individually and resist the temptation to assume that one device will suit all environments.

There’s a big tolerance on a burst disk set pressure, so be aware of that imprecision. Burst disks are compact but getting a membrane to burst at an exact pressure is not really practical and thus these devices are not very precise.  Ask the manufacturer about the expected tolerance on any relief device. It’s also wise to test a few of these devices to measure the actual burst pressure. Make sure you are recording data because failure happens suddenly and you are unlikely to visually see the last pressure reading before burst. If you blink, you can miss the most important data point. Therefore, use a data acquisition system when testing your pressure relief devices.

(2) Minimum flow rate required

Any pressure relief device when activated must be able to drop pressure fast enough to avoid over-pressurizing and failing the pressure vessel. This is a less commonly evaluated situation but its equally important to recognize any scenarios where the transient pressure rise would challenge the relief flow rate needed to keep the pressure below a safe level at all times. Steam pressure systems have this problem and so do cryogenic vessels.  Most designers just choose a fitting similar in size to the lines being used, but this isn’t always accurate. 

Relief devices are nearly always sized relative to their flow rate afforded.  This is sometimes called the “capacity” of the relief valve or burst disk. You’ll need to know your gas and upstream conditions. With this, you’ll need to know the open area when the valve is opened and make this is the smallest restriction in the entire flow path. The open area can be expressed as either the discharge area (Cd A) or the valve coefficient “Cv” value. With each device in each specific location, you must select a burst disk capable of venting enough flow to cover the whole range of expected conditions. This is crucial to finding the right burst disk or relief valve. A device that does not have a large enough capacity will not protect your fluid system.

Another consideration for your relief device is if you have any flow path that is smaller than the area of your relief device. One example of poor design is having your pressure relief device located at the end of a long skinny tube. Even if the open area of the tubing is larger than the pressure relief valve opening, the length of the line can accumulate enough flow friction in the tubing that can unintentionally add up enough pressure drop to pose a significant restriction to your relief flow. This is to say nothing of someone accidentally denting or kinking the tubing which would create a severe blockage of the relief flow. It’s always smart to have your pressure relief device very closely coupled to the pressure vessel volume that you are protecting. This means keeping the distance as short as possible. Always know all of your flow path areas and line lengths!

Another classic mistake in fluid system design is putting a valve or any other restriction device in-between the pressure boundary volume and the pressure relief device that is protecting it. Careful consideration of all valve placements and their positions in all operating modes and under all possible operating scenarios. Put simply: “Do NOT EVER create a situation where the pressure relief device can be isolated or impeded in its operation at any time for any reason, even temporarily. Some piping codes absolutely forbid this. Careful peer-review of your pressure and instrument diagrams (P&ID’s) must look for this situation and avoid it. More than reviewing the paper schematics, one should physically trace all flow paths to be sure the builder hasn’t made such a mistake. The physical hardware must always match the P&ID.

(3) Watch where your burst disk is pointed! 

When your burst disk goes off, any foreign object debris (FOD) near the discharging outlet can be thrown out at high speed causing injury or damage to nearby hardware and structures. Even without particulates or FOD, the impinging high-speed sonic jet of gas is very dangerous.  No one should be standing near a fluid system while any part of it is pressurized anyway, but you should always consider what might happen when your burst disk goes off. You won’t always know when the device will go off. Be prepared at all times.

Make sure all hardware is also secure enough to take the sudden thrust from the burst disk relieving itself. This can be a sudden and powerful force that breaks hardware or knocks things over. The rocket thrust equation also applies in this case. To calculate this thrust value, you do this in two parts: (1) You consider the choked flow pressure differential multiplied by the discharge area and (2) add in the product of the mass flow rate of the gas escaping multiplied by the sonic velocity of the upstream gas conditions.

Calculation of the thrust load from a discharging relief device such as a burst disk

As a design note, for nearly all gases, if the upstream pressure is more than double that of the downstream pressure, the flow velocity through any flow path restriction(s) or “orifice area” is sonic or at the speed of sound as computed by the upstream gas pressure and temperature conditions. This is called “choked” flow.

One potential fix to the jet thrust problem out of relief device is to divert and diffuse the discharging outlet flow in opposing or evenly distributed directions as long as the combined discharge flow areas are sufficiently large and balanced.

An illustration of a burst disk device with balanced venting

Another consideration to be made with a burst disk or pressure relief device is to consider the downstream environment where your burst disk is discharging.

Is the relieving gas or gas mixture going to create a flammable or toxic environment? If so, you need to consider how and where you are diverting the hazardous gases being relieved. Some burst disk fittings have threaded ends on both ends which allows the discharging flow to be routed to a safe location, if this is a necessary feature.

Screw-type burst disk fittings made by Zook in three basic types

Are you creating a dangerous environment (reduced oxygen) within a confined space? The subject of confined space safety is very important and worthy of a separate article in itself. Most testing will be done outdoors and in a very well ventilated environment, but the rocket business is full of horror stories of people who have become injured or asphyxiated simply from improper consideration of confined space safety.

A less often considered scenario is whether the space where the burst disk or relief valve is discharging into is fully open to the environment or not. It is possible to overly restrict or “back up” a burst disk or relief valve if the interstage volume in your rocket isn’t very large or isn’t adequately vented to the outside. Sometimes your discharge space simply isn’t big enough. It is very important to know your vehicle hardware geometry very well, measure your volumes and consider all flow areas out of all assemblies.

Find a reputable burst disk manufacturer and distributor

There are a few reputable manufacturers of burst disks. Fike is one that comes to mind, but they tend to be for very large piping sizes used in facility plants. Fike has been providing reliable products for many years to many industries including oil/gas and the aerospace industry. Swagelok has access to a lot of fluid component manufacturers which may be more suitable.

Zook is another manufacturer of burst disk fittings. These in-line devices come as a holder fitting and replaceable disk. The screw-type fittings are two-piece assemblies and have standard pipe thread ends. The disks come in a range of nominal set pressures.

Screw-type burst disk fittings by Zook

zookdisk.com

There are certainly other manufacturers and all of them should be able to provide you with good advice or transfer you to a distributor company to help you with selecting an appropriate device. Before you call or email, you must have already taken the time to understand your pressure environment, capacity and design requirements first. A good component distributor is one that is willing to work with you to find the right part for your application and educate you in making the best choice. Literature is easy to find online and always consider more than one manufacturer to get a good price.

A few last words of caution

Burst disk devices can be manufactured from scratch and other amateur rocketry hobbyists have attempted to do so. This is not a good idea. There are a lot of considerations to make in building a reliable burst disk from scratch not to mention the time and materials to adequately prove the design. To make a burst disk from scratch would become every bit as expensive as simply going to a reputable manufacturer and using their product.

As much as your group may want to save money, pressure relief devices are a critical part of your fluid system to which lives may be at stake.  Don’t be cheap. Find a quality manufacturer, select the right product and test them.  Ebay is not the place to find quality products.

If anyone has anything to add to this subject, please contact the RRS secretary or the RRS director of research.

secretary@rrs.org

research@rrs.org

Rockets in the Projects

Posted on by

by Larry Hoffing, Educational Outreach Coordinator, Reaction Research Society

On a dusty, old blackboard at the Jordan Downs Community Center, it had a chalk tray but no chalk. I don’t think it had been used since the whiteboard had come of age. Juan, our local Los Angeles Police Department (LAPD) Community Safety Partnership (CSP) officer, saw my predicament, disappeared into his police sports utility vehicle and returned with a piece of white chalk he “outlined bodies with on the sidewalk.” He has a wry sense of humor and not without reason. These officers are sometimes called out to respond to a nearby situation on the street. Peace and war are very close neighbors in this part of town.

CSP arose in 2011 out of a program in the city of Los Angeles founded by Connie Rice, a civil rights leader, and the Urban Peace Institute. The idea was to pair the Los Angeles Housing Authority with the LAPD in an effort to improve relations with citizens in the public housing projects of Jordan Downs, Nickerson Gardens, Imperial Courts and Ramona Gardens. The Watts Bears youth football program is just one outgrowth of this program focusing on improving the quality of life and supporting the community.  Spending time with kids and financial investment pays off in the long term in lifetime earnings and with higher graduation percentages.

I’ve been teaching rocketry since the early 1970’s when model rockets (and more particularly the model rocket motors) became legal in California. The Los Angeles Unified School District (LAUSD) instituted a summer playground Youth Services program that I spearheaded- federal monies helped to expand it city-wide.  The Los Angeles Fire Department, following a demonstration flight, permitted some of the city’s first launch sites at Pierce and Valley community colleges. In 2017, the LAPD came to my organization, the Reaction Research Society (RRS), to pitch the idea to start a Science, Technology, Engineering, Arts and Mathematics (STEAM) program that would come to be called “Rockets In The Projects.” The idea was simple — reach out to at risk students with grand ideas about reaching for the stars and a practical series of learning experiences to help show them the way.

Founded in 1943 in Glendale, California, RRS is the oldest continuously operating amateur rocket organization in the United States. The organization pre-dates NASA by 15 years. Some of our early members went on to help launch the space race. We are uniquely positioned to partner with the LAPD to bring space flight and rocketry to south Los Angeles youth with our organization having an FAA-approved launch site north of Edwards Air Force Base in the Mojave Desert and licensed pyrotechnic operators.

Our founder, George James, presents at the 2018 RRS symposium last year.

In this program, after learning the basic laws of motion, some chemistry and the principles of propulsion and aerodynamics, we build and fly experimental rockets. It’s the first time most students, mainly from 4th to 6th grade, hold an Allen (hex) wrench, use fasteners (button head machine screws) and assemble parts with O-rings. (The O-rings on the Challenger didn’t get tested properly, it’s a simple but critical part). They learn about reducers, payload sections, and an ogive nose cone and what’s best suited for subsonic  vs. supersonic flights (the answer: it depends).

This is an equal opportunity team building program using the “A” for Art in STEAM. Each team makes it unique by painting the outside surfaces. Unicorns, polka dots, and stars are popular rocket paint schemes alongside video game and rock group names. 

5 of the 6 alphas sit in the rack; Osvaldo’s alpha with a parachute sits to the left.

These micrograin rockets develop around 300 pounds of thrust, leaving the launch rail in a fraction of a second and travel over a mile at velocities approaching the speed of sound. UCLA, USC, Cal Poly Pomona and other universities’ rocket labs test their latest designs at our site. It’s the real deal.

A fish-eyed lens view of an RRS standard alpha streaking up into the blue Mojave sky. This is our standard teaching tool on the raw power of a rocket.

This program is a chance for many students, most of whom have never been to the California desert, to reach beyond their four square city blocks. We introduce them to desert safety, launch protocols, and some of the local flora and fauna. There is a cool white rock found in the desert, also called “fire rock”, that is a type of quartz, that when struck together, creates sparks. 

Live demonstration of micrograin propellant at the MTA

We demonstrate the burning of various rocket fuels, including one used in the Space Shuttle, similar to a hard rubber eraser. A parent might ask, “What is the difference in fuels?” A really good question: gasoline is fine for getting around in a car, but it doesn’t have the “oomph” needed to escape the bonds of the Earth at about 25,000 miles per hour.

I drink from a bottle of water and say, “This rocket fuel tastes great!”. I ask for a show of hands and two believers raise theirs, hesitantly, agreeing that I’ve just drunk rocket fuel. “Don’t believe me?” I say. “Apply electric current to water (but do not try this at home!) and you can break off the hydrogen from the oxygen by a process called hydrolysis.” I clearly remember the day this experiment was run in my Van Nuys High School science class almost 50 years ago. 

Liquefied hydrogen is a fuel of choice for space exploration, along with liquid oxygen. If you are going to explore space, you need to bring an oxidizer with the fuel as there is neither above the boundary of space at the von Karman line (the boundary between the earth’s atmosphere and space, estimated at from 50 to 62 miles above sea level by various agencies).

Orientation for the launch event at the George Dosa building at the RRS MTA, 2019-09-21

There is no doubt that these rocketeers with the CSP program will remember “launch day” for a long time. One student was so overcome, he told us this was the best thing that had happened in his life. These young students are at an age when some day in the future they might not only be able to buy a ticket for interplanetary trips with the stars beckoning, but make it happen. Pressing a red launch button is life changing. Part of it has to do with the spirit of flight, the satisfaction of teamwork and building something yourself, something comes to life the moment the red button is pushed.

The RRS encourages the sciences and engineering, but it doesn’t really matter what career path they choose. The students accomplish something amazing in our program. We are proud of the work we do with the community and LAPD and we will have more classes to come.

The students of Boyle Heights with the officers of LAPD CSP and the RRS pose for a group photo at the RRS MTA, 2019-09-21

EDITOR’S NOTE: Larry Hoffing is the Educational Outreach Coordinator for the Reaction Research Society (RRS.ORG), a 501(c)3 educational non-profit organization. He started flying rockets in junior high in the 1960’s. He is also a licensed rockets pyrotechnic operator in the state of California.

Contact Larry Hoffing at “events@rrs.org”

A Tribute to Mr. George Dosa

Posted on by

by David Crisalli, Reaction Research Society

Some time in October of 1966, I had hitched a ride and gone down to an RRS meeting in Gardena. I was 13 and still in the 8th grade. At that meeting, I met Mr. Dosa for the first time. I met several other RRS members that evening, but Mr. Dosa was the most memorable. He was warmly welcoming, very enthusiastic about rocketry as a field of study, and also excited about having new students like me join the Society. 

As I attended more meetings and began to get involved in designing and building rockets, Mr. Dosa was always ready to offer help of all kinds from the loan of technical documents to the manufacturing of parts on the lathe and other tools he had in his garage. I spent many an enjoyable hour with him making steel nozzles, aluminum adapters, and fiberglass nose cones.

At one particular meeting in 1967, Mr. Richard Butterfield showed a 16 mm film of a hydrogen peroxide liquid mono-propellant rocket built and launched by RRS members David Elliot and Lee Rosenthal some 15 years before. I was completely captivated as I watched the two high school students in the film machine parts, fabricate sheet metal components, static test a liquid rocket motor in Mint Canyon, and then successfully launch the rocket in the Mojave Desert. Mr. Dosa saw my interest and enthusiasm and talked to me at some length about liquid rockets after the film. Then he asked if I would like to see the one he was working on. I jumped at the chance. 

The RRS meetings in those years were held in an old, small, wooden building on an isolated piece of property owned by a division of Pratt & Whitney in Gardena. It was really a shed but the RRS had been given permission to hold its monthly evening meetings there and store some of its equipment there. On the same piece of property, some 50 or so feet away, was a slightly larger wooden structure. Although larger, it was more of an empty garage and was not as suitable for meetings as the smaller building. When I told Mr. Dosa I would love to see the liquid rocket he was working on, he led me out of the meeting building and across the dark space between the buildings. It was probable nearly 10 PM by this time and there were no lights in the areas around either building. 

As Mr. Dosa opened the door into the very dark second building, he told me to wait there until he could turn on the light. “The light” was a single low wattage bulb hanging on a wire from the high ceiling. When the light came on, even in that dim glow from a single bulb, what I saw took my breath away. There, lying horizontally on a plywood table, was a bi-propellant liquid fueled rocket with the upper half of the skin removed. All of the tanks, plumbing, bulkheads, stringers, and longerons were precisely made and beautifully assembled. The rocket was more than 15 feet long and about eight inches in diameter. It was designed, Mr. Dosa explained, to run on 90% hydrogen peroxide and ethyl alcohol. I marveled as each piece of the structure and propellant plumbing was explained to me. The design was also unique in that Mr. Dosa had made the fuselage octagonal rather than round. This left him “corners” inside the rocket skin that he had used to run plumbing and wiring. The beautifully made fiberglass nose cone and boat tail were both round and the structure smoothly transitioned from octagonal to round at both ends. Mr. Dosa, a master at many fabrication techniques, had fashioned incredibly precise sheet aluminum sections that perfectly mated with the octagonal structure on one end and the perfectly round nose and boat tail on the other.

I could have stayed and talked to Mr. Dosa for hours, but it was very late now and my ride was leaving. Needless to say, I was completely stunned by what I had seen that evening and over the next several months and years, I must have made quite a pest of myself often keeping Mr. Dosa on the phone for long periods asking questions and listening to his patient explanations. From our first meeting in 1966 until I left for the Naval Academy in 1972, I met and worked with Mr. Dosa at RRS meetings and at rocket firings in the desert many, many times. Each and every time, it was a great joy to see him, talk to him, and learn from him. 

When I left for the Navy in the summer of 1972, “George” as he now had me call him, told me that he had been in the U.S. Navy during World War 2. He had met his lovely wife, Ann, overseas and brought her back home after the war. He wished me the best of luck in the Navy and asked me to stop by and see him whenever I got back to southern California.

After being gone for 18 years, I did find my way back to an RRS meeting and renewed my old acquaintance with George. In the intervening almost two decades, he had changed very little and was still as welcoming, enthusiastic, and as patient an instructor as ever. In the early 1990’s, I volunteered to restart publication of the long dormant RRS News. George was more than a little excited as he was always a huge proponent of documenting all of the projects that RRS members undertook. We began a very enjoyable and several year collaboration writing, editing, and publishing the RRS News more or less, once a quarter. 

During that same time frame, a few members of the RRS and I had started teaching a solid propellant class. As part of that class, several of us had written a course handbook. At the beginning of that course book, Niels Anderson and I had written a dedication to George because of his long, tireless mentoring of so many students and RRS members over the years. I include it here because I believe it captures the essence of who George was within the Society…

“Since the days of Dr. Robert Goddard, the United States has always had its share of rocket enthusiasts and experimentalists. In 1943, even before the end of the Second World War, the young students who founded the Reaction Research Society were hard at work experimenting with propulsion systems. As the “Space Age” dawned, the imaginations of millions were fired with the possibility of flight beyond the atmosphere of Earth. But to members of the many amateur rocketry groups forming during those days, flights of the imagination were not enough. Those with the interest, drive, and courage to try, designed and built fantastic rockets that exploded out of their launch towers on towering pillars of fire and smoke. These were not cardboard models with minuscule motors producing ounces of thrust. These were thundering metal machines, many feet long, producing thousands of pounds of thrust, and flying into the clear desert skies at unbelievable speeds. 

It was a great time of advancement, adventure, and experimentation. Some of those who built these great, unforgiving machines also became the mentors for hundreds of others who followed. These special few not only pursued their own projects, but stopped to share what they had learned with others. Guiding, advising, encouraging, they were tireless in their belief that there was much to be learned in the pursuit of amateur rocketry and they helped all who came and asked. Amateur rocketry, as a whole, owes a debt of gratitude to the few who trained and directed those of us too young and full of wild enthusiasm for our own good. They taught us many things, fed our enthusiasm for learning, encouraged us through failures, and kept us safe all the while with their knowledge and experience. 

This course is dedicated to one such man, Mr. George Dosa. George has been an active rocket propulsion experimentalist for many years. In many ways, he can truly be considered one of the founding fathers of experimental rocketry. George Dosa was the state of California’s first licensed solid propellant rocket pyrotechnic operator. He has been the back-bone of the Reaction Research Society for the last 38 years and still serves today as the Director of Research for the RRS. 

George has dedicated his life to the continuance, advancement and testing of experimental rocket propulsion systems. He represents the very essence of the golden years of experimental rocketry and has crusaded to preserve the right of new experimenters to follow this fascinating and technical hobby. Giving generously of his own time, he has contributed greatly to the education and encouragement of others. As a consequence, the Reaction Research Society would like to thank George by dedicating this first in a series of amateur rocketry propulsion classes to him personally and to his efforts in behalf of amateur rocketry over the years. ” 

Niels Anderson and David Crisalli, March 1996

George told me once that he had been born 30 years too early…he would have liked to have been that much younger when the age of rocketry began to blossom in the 1950’s and 1960’s. From my standpoint, George was born at exactly the right time. Had he been born later, we might not have met and worked together as we did. George lived for nearly a century and all through that time he was a kind, patient, and enthusiastic teacher, a gentle man with dreams of exploring the heavens. I will miss him greatly and I will say farewell (for now) with an old nautical expression….I wish you fair winds and a following sea, George. In a twinkling of God’s eye, we will meet again.

Most sincerely, 

David E. Crisalli, August 2019

David Crisalli is a lifetime member and former President of the RRS. He also is the owner of Polaris, Inc. in Simi Valley, California, a rocket propulsion testing and consulting company.