By Aerik Carlton
While working with private space industry clients on launch complex structures, we have found some notable structural challenges. This article is meant to share some of Nishkian Dean’s recent launch site design experience. Specifically, an atypical structural load consideration that was identified and requested to be evaluated by our client.
Recently, launch tower structures, designed by others, near one of our project sites had been found to present weld failures at circular structural member support connections. The design and construction team that completed the nearby tower believed the weld failures to be representative of “bad welds” during construction. This conclusion was drawn because there appeared to be random failure locations. The failed connections were rewelded, but the repaired welds were found to have failed at the same locations in a follow-up inspection.
The failure of these members was surprising, mostly because they had been designed for large pressures associated with launch activities and hurricane wind forces for these structures, which the structure had not experienced. The second weld failures also occurred in the absence of either launch activities or hurricane activity, and there were only low-wind-velocity days between weld repairs and the second failure. The determined cause was attributed to wind-induced flutter, or gallop, of the members that resulted during low-velocity-wind cyclical loading. The team’s solution was to install air foils for fluid flow disruption around the circular members that experienced weld failures.
Due to the large rocket thruster impingement loadings present during launch activities, circular structural members have been found to be advantageous. Circular cross-sections allow for the same structural section properties: moment of inertia, section modulus, etc. to be present regardless of the direction of load (i.e. as the rocket gains elevation and the angle of load from the engine thrust changes, the structural member maintains the same resistance without reduction due to the change in loading angle). Wind and rocket thrust loads can be reasonably reduced by 20-30% of the projected area due to the allowable shape factor for circular structural cross-section. Fluid shedding, due to the rounded shape, is what allows the reduction in projected area for pressure loadings. When we are talking about 8 psi (1152 psf) of thrust pressure on the structure, shape factor reduction to the projected area is significantly beneficial. The use of circular framing members presents a slightly different issue, however, when considering relatively light wind loading conditions as evident from the nearby tower weld failures our client highlighted.
Nishkian Dean was tasked with mitigating the same type of behavior for a new tower at a nearby launch site in conjunction with the structural design for the client’s new facilities; for crew to vehicle access and lightning protection. The client asked that we design these operations support structures to accommodate their current vehicles as well as allowing for larger, more powerful, and greater range vehicles planned for the future.
A circular cross-sectional shape can exhibit interesting behavior in calm, steady wind. The phenomena our neighboring tower experienced is called “Karman vortex street,” and it happens during steady state fluid flow (or steady non-gusting wind) around a cylinder. You can be forgiven for not having that term readily available for recall from your fluid dynamics course work. I was fully unaware of the term until researching a solution to our issue. Karman vortex street is when the laminar (steady, non-gusting) fluid flow (or calm steady wind) around the cross-section begins to “stick” to the back side (leeward side) of the member and results in an alternating, rhythmic pattern of pressure differences from either side of the shape. This alternating pattern of “stick” creates a turbulent flow condition behind the member, and similar to an airplane wing, induces lift on the member due to the pressure differences. Pairing the lifting force in each direction and the gravitation pull on the member’s self-weight with respect to time, we end up with a vortex-induced vibration (VIV) loading condition.
To further explain and give a visual example of Karman vortex street, let’s take a look at the photo below. In the image, we can see an atmospheric example of Karman vortex street in cloud formations (located at the top of the photo) at the leeward (downwind) side of an island in the northern Sea of Japan. The photo has a wind directionality, moving from the bottom of the photo to the top, and the island is located at the bottom of the photo. As the clouds pass around the island, they swirl into eddies which alternate from the left and right sides of the island. This phenomenon is known as Karmon vortex street and was determined to be the cause of our client’s experience with weld failures.
Rishiri-to island, Sea of Japan off the northwest coast of Hokkaido, Japan
Taken: between April 19 and May 1, 2001
Photo Credit: NASA STS-100 Shuttle Mission https://spaceflight.nasa.gov/gallery/images/shuttle/sts-100/html/sts100-710-182.html
Licensing Agreement: https://www.nasa.gov/audience/formedia/features/Advertising_Guidelines.html
Determining which members in a structure will present VIV is the first step in designing for the issue. As our client saw with the neighboring launch tower, the structural members to exhibit VIV failures appeared to be random at first glance. To understand the nature of the failure, we needed to discover that we are dealing with a vibrational load that is time-dependent, and as such, we need to calculate the natural resonant frequency of our individual structural members. This process is very similar to what we, as structural engineers, do for seismic design when establishing our earthquake lateral loads. The load is dependent on the natural period of the structure. Turning to Roark’s Formulas for stress and strain, we obtain equations for the natural frequency of our members based upon support condition, length, material, and cross-sectional properties.
We can then establish the wind loading environment to see at what wind velocities moving around our individual structural member sizes will elicit vortex shedding frequencies in the vicinity of our structural members’ resonant frequencies. The tricky part here is establishing what makes sense for applicable wind velocities at the site and what boundary conditions can be set. Because hurricane activity was a design issue already being evaluated, the lower- to mid-velocities associated with hurricane wind were the upper boundary (assuming gusting would break up the required laminar flow required for Karman vortex street to occur long enough to gain a vibrational response). Evidence to the issues on the neighboring tower was given, as some of our tower’s members had resonant frequencies close to vortex shedding frequencies at wind velocities between 5 and 10 mph.
At this point we identify the members in the structure that are at risk of VIV. Using an equation for harmonic vortex shedding lifting force from Blevins (2001), we can establish a time-history load function. The load is then applied to a single degree of freedom model for the member, and we find the deflection versus time plot for the member during VIV that accounts for stiffness and damping. From the deflections and member properties, the reaction loads to the support connections are determined. And, from there, we evaluate our connections for cyclical loading.
In our design, we were able to simply lengthen our welds on the members that exhibited VIV to add the required weld capacity to overcome cyclical fatigue. However, there are many ways of mitigating the negative response to vortex conditions. Blevins (2001) outlines several options, including: helical strakes, shrouds, axial slats, streamlined fairings, splitters, ribbons, pivoting guiding vanes, and flat spoiler plates. The neighboring tower took advantage of the streamlined fairing option, while we implemented helical strakes on our tower topping fiber reinforced plastic (FRP) mast similar to those used on exhaust chimneys in other industrial applications.
Please contact Nishkian Dean if your project has vibration analysis considerations – we would be happy to share more of our expertise on this subject.
Aerik Carlton is an Engineering Designer with Nishkian Dean. Aerik can be reached at firstname.lastname@example.org.
AISC. American Institute of Steel Construction (2011). Steel Construction Manual. 14th Ed. AISC. Chicago, IL.
Biggs, J. M. (1964). Introduction to Structural Dynamics. McGraw-Hill. New York.
Blevins, R. D. (2001). Flow-Induced Vibration. 2nd Ed. Krieger Publishing Company. Malabar, FL.
Young, W. C., Budynas, R. G. (2002). Roark’s Formulas for Stress and Strain. 7th Ed. McGraw Hill. New York.
Aerospace is one of the most remarkable market segments in which the Nishkian firms have been privileged to contribute their engineering expertise. Nishkian Dean VP and Managing Principal, Edwin T. Dean, designed rocket launch facility infrastructure prior to founding Nishkian Dean in 1999 and, since then, this small firm in Portland, Oregon, has designed or assisted in the construction of some major aerospace projects throughout the United States. During this same time, the aerospace industry has transformed from a large institutionally-focused bureaucratic process to one that has embraced commercial innovation.
In recent years, entrepreneurial disruptors like SpaceX, headed by Elon Musk of Tesla fame, and Blue Origin, headed by Amazon founder Jeff Bezos, have driven the industry to commercialize space forward in ways not imagined only a few years ago. But even prior to their entrepreneurial disruption of the space industry, there had been great movement toward commercialization by the United States Air Force and NASA. With the EELV[i] program, the US Air Force, being the largest single user of launch services, pushed the industry to reduce costs through innovation and commercialization, to which Boeing and Lockheed Martin responded. NASA has promoted commercialization through its support of the ISS[ii] with the end of the Shuttle era. This cultural shift within the industry created the opportunity for firms like ours to bring a fresh way of looking at these infrastructure projects. We relish the opportunities we have had so far to help foster this transformation.
As an engineering consultant, Nishkian Dean’s success working in the recent revolution of the aerospace industry is a direct result of our being able to infuse our commercial design mindset into this transformation. Our focus on responsive functional design and constructability has made us a valuable consultant in this industry. At their core, major launch facilities are driven largely by structural requirements, which aligns with our core discipline of structural design, but they are far more complex than that. This allowed us to create a strong team and collaborate with experienced consultants to help support our efforts. We recognized the opportunity that we had to champion and organize a team of consultants and expand our role beyond our core discipline. We had to accept the expertise of our consultants to handle systems that we had no prior experience with, such as cryogenic fuel systems dealing with super-cold liquid oxygen, liquid hydrogen, or, more exotically, hypergolics that fuel spacecraft. We brought in experts in electrical power, communications, including specialized GC3[iii] systems, environmental control, and specialized mechanical systems. With our cadre of consultants, we have taken on many adventurous projects.
In 2004, we had the opportunity to propose on a small portion of the structural design on the renovation of the existing SLC-3 launch complex at Vandenberg AFB for a new Atlas V. It was a small, 12-person structural consultant competing against the titans of the engineering industry, such as URS, Jacobs, and others, that won the confidence of the builder, Hensel Phelps, and the owner/operator, Lockheed Martin, to bring us on to eventually be the key design entity. In the end, our design team had committed over 50,000 man-hours in less than 16 months in supporting the incredible success of this project to meet the schedule and budget goals that it demanded. Our team helped the project meet its mission requirements and it helped establish ourselves as the team that can get this done.
We have had many continued successes following this project, both in the Western Range and at facilities at Cape Canaveral Air Force Station and Kennedy Space Center in Florida. When disaster struck on October 24, 2005, with Hurricane Wilma destroying the 280-foot-tall door on Lockheed Martin’s Vertical Integration Facility (VIF), it put NASA’s New Horizons[iv] mission to Pluto in jeopardy. We were there the very next day to assess the damage and to work with the team to design, fabricate, install, and test new doors in 7 weeks’ time, ultimately ensuring the mission’s success. New Horizons launched on January 19, 2006, and flew by Pluto 9 years later, on July 14, 2015.
We have also been called on to assist in erecting the 600-foot-tall lightning towers at Launch Complex 39B at the John F. Kennedy Space Center, and the erection of the nearly 380-foot-tall NASA Mobile Launcher for Constellation[v] and now for use with SLS[vi]. Additionally, we have worked on the complex installation of moveable platforms within the historic NASA Vertical Assembly Building (VAB) in support of NASA’s quest to send astronauts to Mars. In a further testament to commercialization, we are now working with private entrepreneurs’ facilities on their own quests to go to Mars.
Closer to Earth, one of our latest challenges has been to support restoring America’s ability to once again launch astronauts from US soil. Boeing and United Launch Alliance (ULA), as a part of their $4.2 billion CCtCap award, are developing infrastructure to support manned spaceflight launches from Launch Complex 41. And how do you get astronauts from the ground into a capsule 200 feet in the air on a “clean pad”? You build a 250-foot tall access tower less than 60 feet from the launch pad with an articulating arm so that they can safely walk over and climb in, naturally. Now substantially complete, this installation provides astronauts with a state-of-the-art means to reliably and safely begin their journey to the International Space Station. Restoring manned spaceflight to American soil through the design of this complex is something we are proud to have played a part in.
Launch systems are complex and often extremely large mechanical structural systems. Buildings weighing more than 8 million pounds that need to move, be jacked up, and driven by hydraulic power systems as fast as a person can walk; an 70,000-lb arm that must be able to swing in seconds while being remotely monitored and controlled; and precision-operable platforms powered by nitrogen motors are just a few examples of the challenges we have faced in our aerospace work. To develop these designs, we bring together a diverse group of engineers with broad technical backgrounds to address the structural-mechanical-electrical challenges.
These challenges make the aerospace market uniquely exciting for us to tackle. The projects involve the integration of a multi-disciplined design team with the suppliers, fabricators, and constructors who put it all together, and the testing engineers who, along with us, validate functionality. This teamwork creates an organization that has a far-reaching expertise beyond what is done in the conventional building market. Moving parts, intricate control systems, and the requirement that these systems function every single time that they are employed is a tall order that punctuates the care and coordination that goes into each design.
Unsurprisingly, the launch environment is very extreme. In conventional design, we may deal with a 100 PSF live load, or perhaps at maximum a 250 PSF live load to accommodate a truck on a sidewalk. In the launch environment, we may see loads as high as 250 PSI – that is pounds per square inch, or 144 times the sidewalk load example, or 36,000 PSF, with a total launch load in the millions of pounds. We also work with extreme temperatures from liquid oxygen or LOX at -350°F or even colder liquid hydrogen at -425°F.
Liquid oxygen, in addition to being extremely cold, is incredibly corrosive and like hydrogen is explosive in the presence of an ignition source. The fuel for solid rocket motors is largely composed of aluminum perchlorate, which when burned leaves a hydrogen chloride residue that combines with water to create a highly-destructive, corrosive byproduct of hydrochloric acid that coats the surfaces of the structures on the launch pad. If not properly cleaned and coated, these chemicals can literally melt steel structures.
Nishkian Dean has developed over many years a unique niche where we are able to provide specialized design services for rocket launch facility infrastructure. Leading a design team of consultants, or supporting the structural design of aerospace launch facilities, has proven to be a market in which we truly enjoy working. Playing a role in helping the US space program excel is an incredible legacy to be able to look back on.
Edwin T. Dean, PE, SE is Vice President and Managing Principal of Nishkian Dean a structural engineering consulting firm in Portland, Oregon.
[i] Evolved Expendable Launch Vehicle (EELV) is an expendable launch system program of the United States Air Force (USAF), intended to assure access to space for Department of Defense and other United States government payloads. The program, which began in the 1990s with the goal of making government space launches more affordable and reliable, resulted in the development of two launch systems, Delta IV and Atlas V. These were later joined by the Falcon 9. These launch systems are the primary methods for launching U.S. military satellites. The USAF plans to use the EELV family of launch vehicles until at least 2030. (Wikipedia)
[ii] The International Space Station (ISS) is a space station, or a habitable artificial satellite, in low Earth orbit. Its first component launched into orbit in 1998, and the ISS is now the largest man-made body in low Earth orbit and can often be seen with the naked eye from Earth. The ISS consists of pressurized modules, external trusses, solar arrays, and other components. ISS components have been launched by Russian Proton and Soyuz rockets, and American Space Shuttles. (Wikipedia)
[iii] Ground command, control and communications (GC3) systems used to communicate with flight vehicles and spacecraft systems.
[iv] New Horizons is an interplanetary space probe that was launched as a part of NASA’s New Frontiers program. Engineered by the Johns Hopkins University Applied Physics Laboratory (APL) and the Southwest Research Institute (SwRI), with a team led by S. Alan Stern, the spacecraft was launched in 2006 with the primary mission to perform a flyby study of the Pluto system in 2015, and a secondary mission to fly by and study one or more other Kuiper belt objects (KBOs) in the decade to follow. (Wikipedia)
[v] The Constellation Program (abbreviated CxP) was a manned spaceflight program developed by NASA, the space agency of the United States, from 2005 to 2009. The major goals of the program were “completion of the International Space Station” and a “return to the Moon no later than 2020″ with a crewed flight to the planet Mars as the ultimate goal. The program’s logo reflected the three stages of the program: the Earth (ISS), the Moon, and finally Mars—while the Mars goal also found expression in the name given to the program’s booster rockets: Ares (The Greek equivalent of the Roman god Mars). The technological aims of the program included the regaining of significant astronaut experience beyond low Earth orbit and the development of technologies necessary to enable sustained human presence on other planetary bodies. (Wikipedia)
[vi] The Space Launch System (SLS) is an American Space Shuttle-derived heavy expendable launch vehicle. It is part of NASA’s deep space exploration plans including a manned mission to Mars. SLS follows the cancellation of the Constellation program, and is to replace the retired Space Shuttle. The NASA Authorization Act of 2010 envisions the transformation of the Constellation program’s Ares I and Ares V vehicle designs into a single launch vehicle usable for both crew and cargo, similar to the Ares IV. The SLS is to be the most powerful rocket ever built with a total thrust greater than that of the Saturn V, putting the SLS into the super heavy-lift launch vehicle class of rockets. (Wikipedia)
VIF courtesy of Lockheed Martin
NASA 39B Lightning courtesy of NASA
NASA ML erection courtesy of NASA
CAT Astronaut courtesy of NASA
There are many things to take into account when designing and operating cryogenic systems. Possibly the most important consideration is the extreme temperature ranges involved. Two of the most commonly used propellants (rocket fuels) are liquid oxygen (LO2) and liquid hydrogen (LH2). The boiling point of LO2 is 297 degrees below zero Fahrenheit, while LH2 boils at -423 degrees Fahrenheit. Moving these super-cold liquids from Point A (likely, a storage tank) to Point B (a rocket or shuttle) requires a great deal of careful engineering.
For example, a 100 foot-long section of stainless steel pipe, exposed to super-cold LO2 temperatures, will shrink about four inches in length. Although this may not seem like much of a difference, it equates to thousands of pounds per square inch of stress on the piping. If the pipe section is rigidly restrained, the stress forces could cause pipe supports to fail, welds to crack, and rocket fuel to leak from the piping.
The solution is to design cryogenic systems with enough flexibility that the piping is allowed to contract and expand as necessary. This is commonly accomplished through the use of expansion loops (see Figure 1). These loops are essentially U-shaped pipe sections that allow for cryogenic piping to move and flex when necessary.
As 2015 began, the New Horizons spacecraft awoke from an extended hibernation period on its journey to Pluto and the Kuiper Belt. The spacecraft, currently in the ninth year of its trip, is nearing the outer fringe of our solar system and is documenting this previously unexplored area. It has flown over 3 billion miles so far and will conclude its planned mission after completing approximately 135 million more, sending images of Pluto and its’ moons 4.67 billion miles back to Earth. This monumental undertaking would have been postponed for years if not for the rapid response of support firms, including Nishkian Dean, in the late Fall of 2005.
On October 24, 2005, Hurricane Wilma, part of that year’s record breaking hurricane season, moved through the Yucatan Peninsula, into the Gulf of Mexico, eventually making first landfall on US soil in Cape Romano, Florida. The storm devastated the coast and blew through Cape Canaveral Air Force Station and SLC-41, causing catastrophic failure of a major potion of the 40’ wide by 280’ tall folding fabric door (Megadoor) on the vertical integration facility (VIF), which housed the launch vehicle for the New Horizons mission. This nearly resulted in an estimated three-year delay of NASA’s planned New Horizon’s launch.
The ending of the Shuttle program in 2011 meant that the US would no longer possess the capability to put men and women into space, making us completely reliant on other nations for transit to the International Space Station and destinations beyond. Although our team has the opportunity to work on many exciting projects every day, no challenge has been as exciting as our role in restoring US capabilities to support human space flight. On December 4, 2014, NASA’s Orion Exploration Flight Test 1 is scheduled to launch aboard a Delta IV Heavy off of Launch Complex 37 (LC-37) at Cape Canaveral Air Force Station. During the planning process for the upcoming launch, Orion’s preparations hit some complications that Nishkian Dean was excited to have a hand in repairing.
Orion is assembled in the Neil Armstrong, Operations and Checkout Building, commonly known as the O&C. Recently renamed after astronaut Neil Armstrong, the facilities heritage goes back to processing Apollo missions when it was first built in 1964. In 2012 the O&C was totally re-outfitted and restored to operational condition in order to support the assembly and future refurbishment of the Orion space capsule and Crew Service Module (CSM). Nishkian Dean took on an important role in the restoration when the Orion tooling and test fixtures proved to be far heavier than what the O&C floor was capable of supporting. Our team worked with Lockheed Martin Space Systems and Hensel Phelps to install an innovative pile underpinning system to permanently shore up the floor. The 140 piles were installed through the floor and bear on the heel of the basement retaining wall below, leaving only a 20-inch hole to patch in the clean-room environment of the O&C. The need to shore the floor was realized only late in the construction process so it was critical that a method be developed that wouldn’t damage existing construction, disrupt ongoing work, or contaminate the clean environment of the O&C. The solution proved to be efficient, keeping the O&C project on schedule, and Orion was successfully assembled and tested in the O&C then shipped out for the next step on its journey into space.