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


MTA launch event, 2019-09-21

Larry Hoffing, Events Coordinator, RRS

Photo credits: Osvaldo Tarditti

The Reaction Research Society held another launch event with the LAPD CSP. This event was with the students of Boyle Heights. We had ten standard alphas launched into the blue Mojave sky that day including some model rockets made by Russell Hoffing and my grandson.

A lot of different groups came out for this launch event at the RRS MTA, 09-21-2019

We had students from the CSU Long Beach liquid rocket team come out to make some measurements for sub-system testing that they are planning at the RRS MTA this year.

CSU Long Beach inspects the vertical mounting structure at the RRS MTA
Model rockets in the George Dosa building undergoing preparation for launch
Students from Boyle Heights get their orientation instructions at the RRS MTA
RRS events coordinator, Larry Hoffing; RRS VP, Frank Miuccio and RRS president, Osvaldo Tarditi

Returning RRS member, John Krell, had worked up two prototype instrumentation packages for flights in the ninth and tenth RRS standard alphas on that day. Both rockets were recovered and the results were impressive.

John Krell arms his payload integrated into one of two RRS standard alphas, 09-21-2019
One of John’s prototypes survived the flight and is laid out on the table in the Dosa Bldg.
John Krell examines the recovered SD memory chips to determine the flight profile from the RRS alpha. Over 100 G’s of acceleration at take-off max’d out the sensor!

Materials have been acquired to repair the adjustable rail launcher that was damaged in early August this year. Osvaldo has been busy at work on the repairs. The RRS has several facility improvement projects in the works and we hope to bring this rail launcher back to service soon for larger rockets (4 to 6 inches).

John will hopefully have a full report and an overview of his design. Both Bill Behenna and Brian Johnson are also working on their own instrumentation designs for the RRS standard alpha. With this recent progress, this should help our others members take better data.

Still photo from RRS alpha #3 of 10 launched at the MTA on 09-21-2019.

We’ll discuss more of the results of this event in detail at the next RRS meeting on October 11, 2019. The RRS meets every 2nd Friday at the Ken Nakaoka Community Center in Gardena, California. Stop in and see how things went.


September 2019 meeting

by Dave Nordling, Secretary, RRS.ORG


The Reaction Research Society (RRS) met on Friday, September 13, 2019. We had several new people come out to visit including the CSU Long Beach liquid rocket team. They were coming to learn more about the society and our resources at the Mojave Test Area (MTA).

The RRS had a special occasion to celebrate at the meeting which we did with pizza. The RRS now has three new licensed pyro-ops, Osvaldo Tarditti, Larry Hoffing and Dave Nordling. This will help us a lot in holding more events at the MTA.

Abel, Wally, Tustin and Hunter from the CSULB Beach Launch Team, enjoyed our celebration and stayed for the meeting.

Since we had so many new people coming to the meeting, we decided to make introductions and share some of the stories and latest projects before getting to the meeting agenda topics.

Wolfram Blume, new member to the RRS, discussed his plans to static fire a gasoline fueled subsonic ramjet

New RRS member, Wolfram Blume, came to the meeting tonight to discuss his plans to build, test and ultimately fly a gasoline fuel ram-jet called the Gas Guzzler project. He’s been working on this project since 2011 and he presented the RRS president with his test request to conduct a static fire test along with many details of his initial designs. The RRS has not tested a ramjet in many years and this will be a very interesting project as it develops.

Waldo Stakes, RRS member, explains his latest progress with the steam rocket he’s been working.

Waldo Stakes came to the meeting to share with the society his latest progress with a steam rocket he’s been working on for Mad Mike Hughes. Waldo’s projects are always fascinating as he’s worked with a lot of different groups over the years in racing and in rocketry. He’s also been working with Compton College on the planning of their large liquid rocket. The RRS is also glad to be a part of Compton College’s ambitions to build a liquid rocket.

Mario and Oscar of Compton College with RRS member, Kent Schwitkis listen to Bill Behenna present his latest avoinics payload project to be built for the RRS alpha.

RRS member, Kent Schwitkis and a couple of his students from Compton College came to the society meeting. There are many bright students at Compton College interested in working with the RRS and we have already began to assist each student with tasks specific to projects their working at the college.

Bill Behenna shows his latest prototype of an alpha payload to measure acceleration and barometric pressure.

We decided to showcase our membership project first before beginning our agenda which was a very good idea. Bill Behenna has been hard at work on his avionics payload to be built to fly in the many RRS standard alphas we have at most of our launch events.

After calling the meeting to order and the reading of the treasury report, the RRS began our September meeting agenda.

(1) Next Launch Event at the RRS MTA with LAPD CSP and Boyle Heights

The RRS has finished with the last classroom presentation of the series. The students have painted their rockets and are ready for the final launch day, next Saturday, September 21, 2019. After propellant loading, the RRS will be ready to receive our next group to watch their handiwork take flight in the desert.

(2) RRS facility improvements

Osvaldo has been leading the task of evaluating facility improvements to the RRS. The main improvements under consideration are (1) improving our restroom facilities at the MTA and (2) replacing the old blockhouse at the MTA. Osvaldo has made some drawings of the new restroom facilities and is discussing the details with a vendor to get a quote.

In early August, our large adjustable rail launcher was damaged in a failed launch attempt of a large solid motor. Osvaldo began repairs and hopes to have the box rail system restored soon.

The RRS MTA site was also the victim of theft of many things from the George Dosa building. Security at our test site is difficult given its remote location. Several suggestions were made including adding cameras, improving our locks and doors, making opaque window inserts for the building, and simply being present at the site more often. The RRS has been the victim of theft before, but it is something that is never easy to recover.

(3) 2020 Constitutional Committee report

The committee was not able to make their report this month. Several factors have contributed to this delay over the summer. The committee will make its presentation to the society at the next meeting in October.

(4) Society votes on holding the 2020 RRS Symposium

After some discussion last month, the society decided that we will in fact hold the next RRS symposium in the Spring of 2020. Given the increasingly successful events we’ve had since 2017, and the many people who have encouraged us to keep this annual event, the administrative membership voted in favor. Frank Miuccio will again be our symposium coordinator and the RRS will be reaching out to presenters and exhibitors very soon. Our next order of business will be setting the date which is likely to be in the month of April.

(5) RRS to present at the CATIE conference at Antelope Valley College

Dr. Khalil Dajani of CSU Long Beach has invited the RRS to be one of the presenters at the Space Responsiveness Workshop and Exhibit at Antelope Valley College at the Hellenic Center in Lancaster, California. The 2019 California Aerospace Technologies Institute of Excellence will be held on Wednesday, September 18th where members from industry and the government will hear our presentation introducing the society and our capabilities at the Mojave Test Area. We hope to make some great contacts at this event and begin some new partnerships..

https://www.avc.edu/news/2019/Sept/space_responsiveness_workshop

(6) RRS social activities in planning

The RRS has focused a lot on educational and project activities, but we don’t often plan simple gatherings for fun. Larry had talked about having the RRS visit Mt. Wilson as a private group. At the meeting, we also talked about having a simple barbecue at the MTA as was done in times past. We plan to revisit this discussion again. Other members are welcome to share their ideas.

(7) RRS history project – Garboden archives

Lifetime member, George Garboden, has many boxes of papers and reports from the RRS in his possession that he would like to pass back to the society for archival. In support of the RRS history project, the society is always glad to get articles, clippings and any kind of archival materials and make them more available to our membership. Frank, Larry and I have been working on the logistics of getting a new storage location, but the most important step is finding the time to carefully make quality scans.

(8) Social media improvements

Alastair Martin announced the next pending episode of his podcast, Rocket Talk Radio. Other fellow RRS members, Dave Nordling and Richard Garcia, will take part in the next installment of the “Before SpaceX” series on September 28th. In this episode, we will be interviewing Jim French. Jim has had a long and interesting career as a rocket development engineer for the H-1 and F-1 engines at Rocketdyne in the 1950’s and later at TRW in the 1960’s with the Lunar Descent engine during the heyday of Apollo. His book “Firing a Rocket Engine” is available on Amazon.

“Firing a Rocket” by James French

Jim French also worked for a startup company called the American Rocket Company (AMROC) in Camarillo in the late 1980’s and early 1990’s. We hope to have a great conversation and learn a lot about his experiences at this commercial space company.

(9) Memories of George Dosa

As our last order of business, we shared with the society that we lost one of our oldest and most beloved members of our society. In our long history, George Dosa, had a profound impact on the society and many of our past and present members.


The RRS adjourned our meeting after a long series of very interesting discussions. We are thankful to all for coming and we will be holding our next monthly meeting, October 11, 2019. If there are any changes or additions to make to this monthly report, please notify the RRS secretary.

secretary@rrs.org