Burst discs in liquid rocketry

by Dave Nordling, Secretary, Reaction Research Society


I was recently asked for advice on the installation of a burst disk device 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 that I would make 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

They 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. This 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.

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.

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.

Burst disk devices can be manufactured from scratch and other amateur rocketry hobbyists have attempted to do so. I would not advise this option as there are a lot of considerations to make and to adequately test and safely prove such a design from scratch would become every bit as expensive as simply going to a reputable manufacturer and using their product.

One last word of caution

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 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


November 2019 meeting

by Dave Nordling, Secretary, Reaction Research Society


The Reaction Research Society (RRS) held it’s monthly meeting on November 8, 2019 at the Ken Nakaoka Community Center in Gardena, California. The society had a full agenda plus our annual nominations for the executive council. Frank Miuccio attempted to establish a teleconference at the meeting to connect our director of research, Richard Garcia, and an outside organization that wanted to address the society. This teleconferencing was successful and the society will consider having more of these to help bring in more participants on special topics.

[1] Treasurer’s report on membership and dues status

The RRS treasurer is conducting a review of our membership roster to not only update our records with the many new members that have joined us this year, but also to determine the dues status for each. Like in all non-profit organizations, regular annual dues payment is essential to keeping the society funded for the many projects we do and are planning. Upgrades at the MTA are also impacted if our membership does not keep their dues payments current.

Chris’s report was not ready at this month’s meeting, but he will be soon notifying some of our delinquent members that they need to keep their dues paid to remain in active status. It is the duty of all RRS members to keep their contact information current with the RRS treasurer. The society can not be responsible for missing communications if our members do not do their part by making communication possible. Also, members who are not current in their dues payment risk losing their active status with the society.

treasurer@rrs.org

I have always paid my dues to the society on January 1st of each new year. This greatly simplifies the process and I need no reminder to do so. Membership dues ($40 USD per year) to the society can be paid through the “Donate” button on the RRS.ORG website which links to Paypal.

We remind all of our donors and those paying dues in this manner to include your name in the “Notes” section along with the purpose of your donation. Without including your name, the RRS can not tell who has paid their dues.

The RRS.ORG website has more information on this subject. For any questions, please contact the RRS treasurer.

[2] Update on the next RRS MTA launch event with LAPD CSP and 99th Street Elementary School

Frank, Larry and Osvaldo are in the middle of another class, this time with 99th Street Elementary in partnership with the Los Angeles Police Department (LAPD) Community Safety Partnership (CSP). The class is going well and the final launch event is still planned for Saturday, December 7, 2019.

At this same launch event, we are also planning to host the University of Southern California’s (USC) Rocket Propulsion Laboratory (RPL) with the launch of their latest solid motor powered rocket. USC has been making continuing progress even after their landmark flight to be the first university-built rocket to break the von Karman line into space.

[3] Preparations for the 2020 RRS symposium

With the society approving the symposium for our fourth year in a row, Frank is working with the Ken Nakaoka Community Center to establish the date. Tentatively, the 2020 RRS symposium will be held Saturday, March 28, 2020. The society has decided to try to hold the symposium earlier in the year to avoid the onset of the summer heat which makes the event very uncomfortable in the absence of climate control at the Ken Nakaoka Community Center.

Future site of the 2020 RRS symposium

More information on this subject will be posted as it develops. Our symposium coordinator for the 2020 event will again be our society vice president, Frank Miuccio.

vicepresident@rrs.org

[4] RRS solid propellant making classes at the MTA

The RRS has been approached by an outside organization about conducting solid propellant motor making classes. Many years ago, the RRS held a few of these events which became very popular. The RRS has not yet decided if we will restart these classes, but a group is examining the possibility and will report back to the society on the viability of such a project.

composite grain, before and after

[5] 2020 Constitutional Committee progress report

pending… carried over from October 2019 meeting report

[6] Annual elections for the RRS executive council

As required by our Constitution, the RRS appoints an election chairman to oversee and execute the process of nominations and balloting for each of the four executive council offices for new terms starting in the new calendar year. Larry Hoffing, again, agreed to be our election chairman for this cycle.

Nominations were held and were open to our administrative membership. Nominations were received and our election chairman will be sending out ballots by email. This is another good example of why all members should keep their contact information current and remain in active status with the society. Balloting will be closed prior to the next monthly meeting in December and the results announced at that meeting.

[7] CSFM committee on amateur rocketry

Last month, the RRS and Mark Holthaus of the Friends of Amateur Rocketry (FAR) met to discuss a list of proposed changes to the California State Fire Marshal’s (CSFM) definitions that govern amateur rocketry. This small group was intended to be made from active amateur rocketry groups around California to help advise the CSFM subcommittee on changes that would help improve regulation of amateur rocketry and make needed clarifications to help all groups continue to operate safely and legally.

from left to right, Mark Holthaus (Treasurer of FAR), Osvaldo Tarditti (RRS president), Larry Hoffing (RRS events coordinator), Dave Nordling (RRS secretary) meet to discuss proposed definition changes to CSFM laws governing amateur rocketry, 10/15/2019

The RRS and FAR held a second meeting at the Ken Nakaoka Community Center which included David Reese of the Rocketry Organization of California (ROC). ROC members, Chris Kobel and David Reese have been very helpful in providing helpful improvements to how the certain classes of rocketry are defined.

This amateur rocketry committee will be presenting their collective suggestions to the Fire Marshal in early December 2019. The RRS, FAR, ROC and the rest of the amateur rocketry groups in California are glad to assist the CSFM office in making these suggested updates.

[8] Social media updates

There was no report from Alastair Martin and/or Bill Janczewski this month for social media improvements for the Reaction Research Society. We hope to have more to discuss in the next month on this regular topic. As always, members are welcome to offer their advice and proposals to either or both of our media coordinators.

Alastair and I did have a conversation about expanding our following on Instagram. We will continue to show the highlights of our events and the people involved, but I hope to bring more technical content which seems to be our primary source of interest.

See the RRS on Instagram: reactionresearchsociety

Alastair and his production company, Production Tribe LLC, has created yet another podcast in the “Before SpaceX” series with special guest, rocket propulsion expert, author and RRS member, James R. “Jim” French. RRS secretary, Dave Nordling, and RRS director of research, Richard Garcia, supported this excellent discussion about the American Rocket Company (AMROC), a space start-up company in the 1980’s and 1990’s. The show is still in editing and will be posted very soon his website.

https://podcasts.apple.com/us/podcast/rocket-talk-radio/id1474556513

[9] Compton Comet STEM club formation and program

Several students from the Compton College STEM club attended the November meeting of the RRS along with their advisor and fellow RRS member, Kent Schwitkis. Jamie Alvarez, the STEM club president was in attendance. The Compton Comet is the project name for the liquid rocket projects that the Compton College team is working on. There are about 20 students in the group and the RRS is glad to support this team and the other university teams looking to compete or at least expand their range of practical skills.

The Compton Comet team has been holding meetings at Tomorrow’s Aeronautical Museum (TAM) at the Compton/Woodley airport. They are planning a few trips to inspect launch and additional hardware assembly sites and make a report back to the RRS at the December 2019 meeting. RRS member, Waldo Stakes, has also been an important part of this program.

The STEM club is having another Estes rocket competition later this month. The first of these events was very successful. Part of the experience is getting practice with the simpler rockets and using OpenRocket simulation software to make and verify predictions.

IN CLOSING

RRS members, Frank Miuccio, Alastair Martin and Kent Schwitkis contributed to this report. The next RRS meeting will be December 13, 2019. If there are any corrections or additions to make for the monthly report, please contact the RRS secretary.

secretary@rrs.org


In Memoriam: Brandy (Robert “Bob”) Bruce-Sharp (1953-2019)

by Larry Hoffing, Events Coordinator, Reaction Research Society and Korey Kline (contributing)


In January 2019, Brandy (Robert “Bob”) Bruce-Sharp passed away. As reported by Mark Clark and Tripoli, Brandy went quickly from non-Hodgkin’s lymphoma. At the BALLS 28 launch this past September, Brandy and wife Abbie’s ashes were launched and spread in an Aerotech M1939 rocket.

from left to right; Robin Meredith, Jim Bornwell, Jane (Brandy’s sister), Mark Clark, Gary Rosenfield

Bob was my friend and fellow student at Los Angeles (Robert H. Goddard) Pierce College rocket club that I founded in the early 1970’s. Around this time we met a high school kid named Korey Kline, he was only fifteen at the time but already a veteran shop mechanic. His high school rocket club, inspired by the Pierce club, found a Korean War five-inch diameter HVAR rocket at a vacant military recruiting office.

Bob Bruce, Michael Gill and Larry Hoffing of the Robert H. Goddard Rocket Society of Pierce College in Los Angeles
Bob Bruce and Larry Hoffing stand at the launch rail for a row of model rockets in the 1970’s

We proceeded to convert it to a zinc-sulfur rocket which required bolting some ports of the multiport nozzle shut, adding fins, and a payload section- I think Bob’s mother sewed the parachute. I did most of the machining in the school’s metal shop while some of the welding students added the fins. I remember hauling the thing, which weighed about 40 lbs unloaded, for a show-and-tell presentation at the Pacific Rocket Society (PRS) which met at the (now defunct) Northrop Institute of Technology in El Segundo.  B. J. Humphreys was the PRS president at that time.

B.J. Humphreys, past president of the Pacific Rocket Society and builder/pilot of the first rocket-powered wheelchair
Bob Bruce and Larry Hoffing in September 1972

Bob named the rocket “Bifrost” (pronounced “BIF-roast”) which is the old Norse term for the rainbow bridge to Asgard.  Our mission was to fly Bifrost at the Mojave Test Area (MTA) near the town of Cantil, CA, where the RRS, FAR and Polaris Inc. (under the direction of Dave Crisalli) currently test. In those days the bunker was covered with telephone poles that had been trucked out by RRS member John Mariano and his cousin in the 1960’s.  There’s a pile of them still laying at the MTA to this day! Access to the site was by dune buggy. The yellow blockhouse with its ballistic glass windows still stands at the RRS MTA.

This sets the scene for the launch of the missile on a wooden home-built Bruce rack – 3 aluminum fins had been welded onto a cowling over the casing as we hadn’t realized the launch racks were built for four-fin rockets (Hint: let your new members know about the launch rack configuration ahead of time). The PRS had most of the pyro-op’s at that time.  The PRS pyro-op in charge was afraid we’d blow up the rocket and surrounding area so he made us fill the casing half way with sand. It must have weighed 80 lbs.

5, 4, 3, 2, 1, ignition! The rocket lifted off, flew a short distance, and crashed. The failure to go the distance was attributed to the multi nozzle ports, we didn’t bolt enough of them shut to build pressure. There is a photo of it flying, I gave it to my machine shop professor, and unfortunately don’t have a copy.

Bob was a consummate modeler and draftsman. He won top prize from Estes in 1972 for his remote control Space Shuttle which was a thing of beauty.

Bob Bruce wins first prize in the Estes rocket modelling competition for his space shuttle design

Bob started a rocket kit company with Korey in the mid 1970’s called California Model Rockets, a precursor to large/high power rocketry. One of my biggest regrets in life is not joining them in this endeavor. Bob and I had previously invented the largest model rocket in the world we called “The Wopper” . The California State Fire Marshal rules at the time was that model rockets had a weight limit of 1 lbm including the motor, so designing it was no easy task. We got the brilliant idea one day of enclosing foam rings and horizontal balsa slats with construction paper to create giant tubes. The biggest F-sized motor at the time was F-100’s made by Flight Systems Inc. The large model rocket flew spectacularly to about 300 feet.

Bob relocated to Arizona sometime in the 1980’s to pursue drafting, and afterward I lost touch with him. However, my memories are vivid of us mixing and testing “Blue Knight” candy fuel (sugar motors), and launching model rockets at Half (Hof) Mile Square in Fountain Valley, CA (a former air ship site) with the Westchester YMCA Rocket Club. Hof Mile was a trip. Wheeled sail cars raced around on the landing strip as we launched rockets. We’d even have to pick up the gate keys from the local base commander. It was here while looking for a rocket in the tumbleweed that I stumbled upon a huge, beautiful red fox when the area was still wild back then.

Korey remembers Bob as his earliest mentor for rocketry. “By example he taught me to think outside the box!” Korey says he was only fifteen when he met us and we (and his mother) had to drive him to the rocket club meetings. Bob also introduced Korey to B. J. Humphreys of the PRS and Gary Rosenfield at the RRS.

One project we all worked on together was building the Hang Loose Bi-Plane, a one-man glider made of bamboo, wood, wire, & plastic sheeting. We cut and bent bamboo spars for the airfoil wings at Korey’s house over his mom’s gas stove. The glider had around a 14-foot wingspan and a 12-foot rudder.  It was Korey that drew the short straw to fly first. With Korey hanging in the center and the two of us at the wing tips we took off running from the top of a hill in Granada Hills. Korey lifted off about 10 feet in the air and started sailing down towards a school fence at the bottom where he bailed out before hitting.

Another thing we did that I can mention now was flying model rockets out of Korey’s in ground swimming pool. We sealed the motor and electric igniter with wax and lowered the rocket and launch pad to the bottom of the pool with the controller on deck. All I can say is that a sea launched rocket is pretty cool when it breaks the surface!

There were many interesting things we did with Bob and we remember him dearly. He loved his muscle car too, I remember other drivers on the 405 Freeway coming along side trying to race us, but Bob wasn’t a speeder, just a tinkerer. I often wonder what became of that car.

Mark Clark further reports on the Tripoli members forum:

“Brandy [aka Bob] started flying rockets in the 1960’s and at Miles Square Park and very early Lucerne launches. Getting into high power in the late 1980’s, he had moved to the Phoenix area and was a founding member of Arizona High Power Rocketry Association (AHPRA).

Brandy started Sonic Systems that locally sold reloads and nationally 7 1/2″ sized mosquito-type nosecones. Those who saw the ads in High Powered Rocketry (HPR) magazine will remember them. Brandy was also involved with the BALLS launch for the 18 years AHPRA was involved and a frequent poster to these forums. Brandy was a great friend for nearly 30 years.”

www.ahpra.com

www.ballslaunch.com

Brandy “Bob” Bruce-Sharp at the BALLS 25 launch with his up-scaled Centuri Enerjet 2650 rocket