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ENGINEERING PROBLEMS IN THE DESIGN OF SUPERCONDUCTING SYSTEMS
R. Beuligmann, S. Ackerman, G. Danninger, D. Hackley, J. Parmer, R. Tatro
To cite this version:
R. Beuligmann, S. Ackerman, G. Danninger, D. Hackley, J. Parmer, et al.. ENGINEERING PROB-
LEMS IN THE DESIGN OF SUPERCONDUCTING SYSTEMS. Journal de Physique Colloques,
1984, 45 (C1), pp.C1-569-C1-573. �10.1051/jphyscol:19841115�. �jpa-00223584�
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Colloque C1, suppl6ment a u n o 1, Tome 45, janvier 1984 page Cl-569
ENGINEERING PROBLEMS IN THE DESIGN OF SUPERCONDUCTING SYSTEMS
R.F. Beuligmann, S.L. Ackerman, G.A. Danninger, D.S. Hackley, J . P . Parmer and R.E. T a t r o
Genera2 Dynamics Convair DivCsion, San Diego, California, U. S . A.
~ 6 s u m k - Depuis 1977, ~ i n e r a l Dynamics participe tris activement dans les programmes les plus impor- tants de la supraconductivitk appliquke aux USA. Ce mkmoire prksente quelques unes des expkriences les plus significatives et discute certains prob1;mes intiressants qui se sont posks au cours de ces programmes.
Abstract
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General Dynamics has been deeply involved since 1977 in several major U.S. superconducting magnet programs. This paper discusses some of the more significant experiences and problems that occur- red in these programs.Introduction
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In recent years, the United States Department of Energy has established a strong policy of encouraging and soliciting industry participation in its major fusion programs. Congress, as well as the past and current heads of the Department of Energy and the directors of government-sponsored laboratories, recognize the importance of applying industrial management and technical expertise and depth t o this rapid- ly evolving field. It is obvious that industry, particularly the aerospace segment, has experience and resources not usually available at government-sponsored or owned laboratory facilities. This paper discusses some of General Dynamics' experiences in this new field of large superconducting systems.Technology Transfer Results from Cooperation at All Levels
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The transition of detail design and fabrication from laboratory to industry has been proceeding well. Several current major superconducting magnet pro- grams attest to these facts. Mr. Sam L. Ackerman of General Dynamics will present a paper o n the super- conducting magnets of the Elmo Bumpy Torus Proof-of-Principle program later this afternoon. His paper reflects this outstanding example of technology transfer. The Mirror Fusion Test Facility-B (MFTF-B) mir- ror coils, which were conceived by Lawrence Livermore National Laboratory (LLNL), are also being analyzed, designed, and fabricated by General Dynamics Convair Division. Ten solenoid coils have been delivered to LLNL. Mr. J.W. Wohlwend of General Dynamics will also present a paper this afternoon on the MFTF-B magnet system. The Large Coil Program, initiated by the Department of Energy and im- plemented at the Oak Ridge National Laboratories by the Union Carbide Corporation-Nuclear Division is another outstanding example of technology transfer and industry participation, as already well documented by previous General Dynamics papers in various conference proceedings.National Laboratory Consultants Offer Valuable Experience, but Thorough Evaluation is Required
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We at General Dynamics evaluate technology ideas from the laboratories in the same manner as has been common with ideas generated by our own staff; i.e., by a thorough evaluation to determine if the recommendations d o apply and are cost-effective and workable in this particular application. This open system evaluation process has been an aerospace practice that was proven mandatory and successful in the man-in-space pro- gram. On several occasions, General Dynamics has received conflicting information from laboratory con- sultants. Recommendations offered were based on narrow bands of personal information and limited by the relatively short history and shallow depth of superconducting magnet technology. Frequently, the recom- mendations overlooked economical producibility aspects and relied on individual specialty skills and ex- perience.One such instance involved the Prototype Magnet System (PMS), which is an isotope-separation magnet that General Dynamics designed, manufactured, and delivered to TRW for use on a Department of Energy program. The helium stack was one of the critical design elements of this magnet. It was important Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19841115
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to provide the lowest heat leak possible for this component, commensurate with using the stack for helium storage but with a height limitation. Consultants from several laboratories offered possible solutions for this design; but, when the designs were laid out in detail, we found that the required closeout welding around the stack could not be done. After two more iterations with laboratory consultants, General Dynamics came up with a solution to the problem, which was a practical compromise between the best technical design and a design that could be readily produced. The laboratory background combined with in- dustry practicality solved the problem.
We do recognize the overall valuable experience that the laboratory personnel offer. The current cooperative team relationship is definitely beneficial to technology development - with laboratories in the management, monitoring, and advisory role, and industry in the "principal doer" role.
In addition to the technical contributions that industry can make, the aerospace industry in particular has a disciplined management system that will be required as new reactor systems are evolved. Rigid quality controls, project format, and scheduling techniques for administering large programs are already being ap- plied to subcontractors who support our superconducting magnet programs. In many cases, with the help of the aerospace prime contractors, these subcontractors have implemented in-house quality and process con- trols as well as financial and project management systems derived from the already tested and proven aerospace systems.
Don't Ask For More Than is Needed in a Specification - One basic principle that should be remembered when a specification for a magnet system is being prepared is "Don't ask for more than you need." The Large Coil Program (LCP) specification is an example of the application of this principle. ORNL has done an excellent job of specifying in great detail the true requirements for the LCP coils. However, one requirement of the LCP specification that has always been controversial is the "one coil out7' (one coil not energized) load case. This load case imposes extreme side loads on the two adjacent coils and has the greatest influence on design of the magnet load-bearing case structure. The magnitude of these loads was so great and the design problem so difficult that General Dynamics iterated the design of the LCP coil case three times before we successfully met these loads. The controversy centered about a situation that, at that time in tokamak con- ceptualization, was not clear; i.e., was "one coil out" a valid load case? Because it could not at that time be proven to be an invalid load case for the Large Coil task, it was treated as valid 2nd included in the specification. Now, several years later, the Fusion Engineering Design Center has determined the case to be invalid. Had this determination been possible at the start of LCP, millions of dollars and much schedule time could have been saved on the six LCP coils.
Limitation of Material Use Must be Understood -The type of material and its property limitations have a large effect on the producibility, performance, and economics of magnets. A few years ago, we chose to design an all-aluminum, circular-saddle, magnetohydrodynamic (MHD) magnet because we thought it would reduce system total weight and machining time. The results proved otherwise, and we eventually returned to a more conventional steel design. What happened was that, as we got into the details of the structural loads and mechanical design, we found that the high-strength aluminum alloys became brittle at 4.5K. The design became fatigue-limited, and some critical load paths had to be sized by crack growth rates. In other parts of the structure, low-cost producibility dictated welded joints; but we found that the weld heat zone lowered the mechanical strength t o the point where the desirable properties of the high-strength aluminum alloy were lost. When we substituted a low alloy aluminum that had weld properties equal to the parent metal, we gained some on factory machining time, but lost on joint design.
Electrical resistivity measurements of the chosen aluminum alloy versus steel showed aluminum to be 10 times more conductive. In some of the vessel shells that were at 4.5K, the use of aluminum allowed the quench eddy currents and their Lorentz forces to increase to the point where they were controlling the design. All of this is on the negative side for aluminum, but this is not to say steel does not have its own set of problems.
Two major material property problems of steel have been obtaining thick-section steel with the re- quired mechanical properties and designing for steel swelling in the fusion neutron radiation environment.
Some steel will swell as much as 309'0, given sufficient exposure. Problems of radiation embrittlement must also be considered, particularly in pulsed systems where the combination of embrittlement and cyclic loading will fatigue limit the magnet'life. Magnet life in fusion systems is also limited by nuclear radiation damage to the winding electrical insulation. At present, we use 2 x 10l0 rad as the maximum possible limit on polyimide electrical insulation. Further testing and development of radiation-resistant materials are needed to extend this limit to provide more reliable coil electrical protection.
Tokamak Toroidal Field Coils are Uniquely Space Constrained - In no other magnet design is space so con- strained as in the straight leg of the D-shaped T F coil. The LCP specification captured this problem exactly.
The lack o f space continually pressed the design and greatly contributed to cost and schedule growth, as space problems caused numerous iterations to optimize between various technical disciplines, caused com- plexity in the hardware, and especially caused difficulty in the ability to safely install and wire the 260 sen- sors and heaters required for the LCP test coil. In fact, when it was found during the General Dynamics LCP winding that cumulative as-built tolerances gave us room for an extra conductor turn per layer (which earlier could not be predicted with certainty), we sacrificed this extra turn in order to use the space to pro- vide extra protection for sensors and instrument wires.
Verification Tests Quantified Weld Shrinkage and Distortion - Early in the proposal cycle on LCP, we at General Dynamics recognized the need to understand, predict, and compensate for weld shrinkage and distortion. We ran tests early in the program that simulated our anticipated design. During design, it was necessary to repeat some of these tests to reflect the latest configuration so that we were more certain of the weld shrinkage and distortion. In many cases, the knowledge of how the material behaved during welding was used to improve the weld joint design. In all cases, the behavior was noted and parts were set up for welding in a position that compensated for the shrinkage or distortion, thus ensuring acceptability of the final product.
Conductor Process Line Testing Uncovered Cold Weld Metallurgy Problem
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During conductor manufacturing for LCP, we made continuous lengths from 150-foot-lengths of rectangular cross-sectioned, quarter-hard copper conductor. We joined these pieces by cold welding (molecular diffusion between two parts under great pressure). As we gathered experience in making and testing cold welds, we reached a point where we believed we had become very proficient at both making the weld and in having a sense of when a weld was not going to hold and needed to be rewelded. However, after a fairly long run of successful welds, we ran in- to several successive bad welds, which we were not able to explain immediately. (We were reminded of the previous similar occurrence at Lawrence Livermore National Laboratories on MFTF-A, in which several bad cold welds occurred in a single shift, defying all explanation.)Our metallurgists examined the failed LCP cold welds and discovered that occasionally a thin, brittle plane of full-hard copper developed in the weld-affected zone. The thinness of the plane made it difficult to find by examination. This brittle plane was the area where the welds failed. The solution found was to local- ly recrystallize this excessively hard copper to the dead soft condition. This, however, left a small zone dead soft instead of quarter-hard, as required by the design. To correct this problem, we locally stretched the copper, straining the dead soft zone back to the quarter-hard condition. This stretching also constituted a proof load test of each cold weld.
The Team Approach Works
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Engineering, Manufacturing and Quality-
The history of General Dynamics magnet fabrication programs attests to the value of early involvement of the manufacturing and quality engineering aspects of the program. Many fabrication problems were foreseen and avoided through this team effort; fabrication problems that could not be foreseen were quickly resolved by the engineering, manufacturing, and quality team.From the onset of a new program at General Dynamics, manufacturing producibility and quality engineering participate in all aspects of a new design. These technical disciplines are colocated with the magnet designers to be sure that, as the detail designs evolve, consideration is given to how the components of the system will be manufactured and inspected. Manufacturing sequence arid flow diagrams are prepared for proper assembly. Early in the design phase, we treat the choice of materials, select the method of fabrication, ascertain type of weld joints and welding equipment, and establish training aids and tools so that technicians and mechanics unfamiliar with magnet technology and manufacturing can be trained to perform the factory operations.
Prepare Procedures Early, Review Them Early, and Practice "First Time Tasks" Where Possible - It is common aerospace practice at General Dynamics to estabIish "procedures" for the process, assembly, or test, where this step is critical to the satisfactory performance of the product and/or is sufficiently complex to warrant this additional control.
Preparation of a procedure often uncovers problems that can be corrected by revising the design, changing the manufacturing approach, or by carefully devised procedures. Thus, if procedures are at least
"skeletonized" (expecting later revisions and improvements) during the design phase, the program can benefit from early identification of problems and act upon the solutions before hardware is committed.
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Another real benefit experienced at General Dynamics through early procedure preparation is to avoid the loss of ideas. The details of the approach and plan are recorded when they are first conceived; later they can be resurrected, reviewed, and updated when the work is first to be performed. Often the time between concurrence with the engineering design and performing the manufacturing task is measured in months or even years. Even if the same personnel are available, the good ideas on "how to d o it" are easily lost if not documented in the detail of a procedure.
Our experience has shown that, when significant tasks are undertaken for the first time, the "thinking through and early procedure preparation" may not be sufficient to avoid all the problems and learn the right way - it should be tried. Just as tool proofing is an accepted discipline in a n organization dedicated to rate manufacturing, procedure trial is a necessary discipline in our field of large component manufacture and assembly before starting an expensive and/or irreversible process for the first time.
Emphasize "On-The-Floor" Supervision and Engineering Presence - Shop supervision has the major respon- sibility to make the product and meet the schedule. Supervisors must control work being performed by their continuing presence, by their intimate and detailed knowledge of the task, by planning and scheduling the details of the task, and by constantly seeking improved approaches or methods. This leads to the need to
"check it out," which can be greatly facilitated by the proximity and routine presence of engineering sup- port in the shop. The benefits of this approach are more easily recognizable in fabricating a single product in a dedicated facility and are enhanced when a "program team" has the true spirit of working together to
"make it happen. "
An engineering design is not a "good design" unless it can be produced in a cost-effective manner.
When the time comes to turn the engineering into reality, it is often too late to establish how "good" the design is. Even some of our best designs, which carefully considered manufacturing details, still experienced unforeseeable producibility problems. T o minimize the impact of these problems and the manufacturing deviations that can occur, engineering must be kept involved throughout the manufacturing phase. Also, our experience underscores our strong belief that the best involvement is "on-the-spot" presence. This can provide early identification, solution, and corrective action of manufacturing problems.
Furthermore, "on-the-spot" engineering involvement can prevent problems through the engineer's observation and anticipation or, more likely, by the operator, technician, or inspector raising a question before the mistake has been made. "On-the-spot" engineering does not require permanent assignment to the shop floor. It requires only the occasional presence o f the engineer each day and, most important, his in- terest in the shop's problems. This interchange z u s t be encouraged by shop supertision to be effective.
Ensure That Tools and Equipment Get Engineering Design Review
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During the evolution of the product design, those who must produce it (manufacturing) perform a review for producibility and concurrently create the concepts for manufacturing. When the product design is complete, tool design must provide for manufac- turing and handling the product as needed for the manufacturing concept. At General Dynamics, the magnet engineers d o not have design responsibility for the tools because the tool designers in the manufac- turing organization are better suited to perform this function. However, we should not lose the reservoir of knowledge about the product that resides in engineering. For example, although engineering attempts to define all technical criteria, it may not; consequently, the tool design may inadvertently damage the pro- duct. We have minimized these possible problems by requiring that the magnet engineers review the tool design and concur that it will achieve the tool objective without detriment to the product.Funding Restraints and Discontinuities Cause Schedule Delays and Cost Growth
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The marked difference in con- tract and project performance between adequately and continuously funded programs versus those with yearly and changing funding allocations is well documented and demonstrated at General Dynamics.Unplanned, intermittent funding and stop-work directions cause havoc with the proper allocation of per- sonnel and facilities. Large overruns occur and valuable, trained skills are lost when work stoppages are as short as a two- or three-month period or when planned work must be postponed to stay within a funding limit.
An unfortunate example of this occurred on two major programs funded by the Department o f Energy. The first was o n the Large Coil Program. During the entire program, adequate funding was con- strained. Plans were altered on several occasions to live within the funding limits. Work was repeatedly started, stopped, and postponed to meet these limits; and the program manager had difficulty in maintain- ing trained program personnel. This was a significant contributing factor to the cost growth incurred on this program. Funding limitations and several false starts on the magnetohydrodynamic program (which was known as the Stanford Magnet) caused difficulties similar to those above for the short two-year life of this project.
An outstanding example of what can be accomplished when funding is consistent and adequate can be found in the MFTF-B magnet program now under way at General Dynamics. We have met every schedule date and earned incentive fees in performing this work. Part of the program has been redirected, but those items that are being fabricated are meeting the schedule and performance specification within the true fun- ding plan.
Contractors Should Be Allowed Adequate Cost Contingency
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Lack of adequate cost contingency can destroy a contractor's ability to react to and solve unforeseeable problems. The LCP contract allowed no contingency for original proposal costs, changes, or forecasts, requiring instead that contingency be suggested by the contractor and held by the customer. As a result, contractors were forced to return for additional funds to cover each unforeseeable problem. The time lag on scoping, costing, and approving the additional funds, combined with funding limitations, was a major factor in schedule problems. On one program, when the customer had no more funds within a given year, the General Dynamics program manager took steps to convince corporate management to advance funds to permit work to continue until the customer received funding for the next fiscal year. The entire process of lack of cost contingency places an undersirable and unnecessary strain on customer-contractor relationships, as well upon the contractor's internal relation- ships. The lesson learned is that "Government Procurement Regulations allow contractor cost contingency because it works to the overall benefit of the program."Conclusions
