FEATURE: Brazing in Space: Enabling the Next Frontier
Brazing in Space: Enabling the Next Frontier
Fig. 1 — A truss support structure is envisioned for large antennas (A) and large telescopes (B) with 50-meter class apertures.
Many NASA exploration and space science missions envisioned for the 2010 to 2020 time period will use very large structures and complex platforms — Fig. 1. Delivery of large assets into space presents tremendous technical and economic challenges. It would require a new launch vehicle with enormous lift and cargo capacity to keep the cost of mass-to-orbit at an affordable level. It would also require the spaceflight community to adopt higher failure margin acceptance and risk tolerance standards. In the current economic and political environment, the loss of a large payload would result in a major setback to the space program. As an example, additional work on the International Space Station (ISS) has been put on hold following the tragic Columbia accident.
An alternative approach to developing large space infrastructures is to assemble them in space. Consequently, an in-space assembling technology has been identified as a critical or enabling technology. The Russian station Mir and the ISS represent the current state of the art of structures for in-space assembly. The ISS is based on modular assembly and mechanical joining, which required multiple shuttle launches, docking events, and quite intensive extravehicular activity.
The cost of the mission, risk tolerance, and failure margins can be better mitigated by building the structure in space using small and relatively inexpensive truss elements. These elements can be prefabricated on Earth and delivered to the “construction” site in a pallet or other container using an expendable launch vehicle. The key to successful in-space construction is a versatile, reliable, cost-
effective automated joining technology that is easy to use.
NASA materials scientists and engineers historically have been interested in the metallurgical aspects of melting and joining metals in space. A number of experiments have been performed in space on Skylab, Soyuz, and Salyut spacecrafts to study the interaction between solid and liquid metals. Some of these experiments are listed in Table 1.
The table shows that exothermic brazing has been a popular method to study with regard to formation of brazed joints in a zero-gravity (0-g) environment. The experiments exhibited no metallurgical problems while forming metallurgically sound brazed joints in space.
Fig. 2 — Russian cosmonaut Svetlana Savitzkaya working with the electron beam UHT in space during the Salyut 7 mission in July 1984.
In the past, NASA has sponsored several studies in an effort to identify the most promising joining technology for in-space assembly and repair of metallic structures (Refs. 1–4). These studies, conducted by the industry and universities, identified electron beam welding and brazing as very promising for in-space applications and well worth pursuing. However, NASA’s interest in the development of in-space joining technologies has not reached a level of effort that resulted in a flight demonstration of welding or brazing for construction in space. Mechanical joining (docking and fastening) was the only process that realized extensive flight experimentation, culminating in the assembly of the current ISS structure.
Fig. 3 — A neutral buoyancy test of the flight-ready UHT at the Marshall Space Flight Center.
Private industry made several attempts to address the need for additional in-space assembly and repair techniques. A big motivator for this was the energy crisis in the mid-1970s that led engineers to look at alternative energy sources. One such source was the concept of a Solar Power Satellite (SPS). Grumman and Boeing were among the companies that looked into the possibility of constructing a large SPS truss structure in space to serve as a platform for giant solar panels and microwave transmitters to beam power back to Earth. Grumman even designed a space fabrication demonstration system, named the Beam Builder (Ref. 3), for an automated assembly of the truss structure in space. The Beam Builder used resistance spot welding to join thin aluminum struts.
The former Soviet Union achieved the greatest accomplishment to date in developing a space-based joining capability. In the early 1960s, the Paton Welding Institute (PWI) in Kiev, Ukraine, initiated the development of a universal tool capable of heating, welding, brazing, cutting, and vapor deposition in space. The heart of that tool was an electron beam (EB) gun.
Electron Beam Is the Joining Heat Source of Choice
Fig. 4 — A schematic of EB brazing of the tube-to-fitting joint. The beam is deflected to sweep the inside surface of the braze joint. The preplaced filler metal melts and joins the strut to the fitting.
The Ukrainian researchers selected the electron beam as the most versatile energy source highly compatible with a space environment. Their effort was very successful and resulted in several flight demonstrations of the Universal Hand Tool (UHT) in the space environment on the Salyut spacecraft in the mid 1980s — Fig. 2. In early 1990, NASA initiated a joint program with PWI with the objective of testing the UHT welding and brazing capabilities in space in the shuttle cargo bay. The UHT tool was delivered to NASA Marshall where it successfully passed rigorous qualification and functional testing — Fig. 3. Unfortunately, NASA cancelled this program in 1997 to accommodate a busy shuttle schedule almost totally dedicated to the assembly of ISS.
Fig. 5 — A schematic representation of the strut-to-fitting joint. The tubular strut is inserted into the clamp-shaped fitting and snaps in place, hence the name “snap-n-braze.”
Electron beam processing is widely used in terrestrial applications. The main limitation of the process, in a terrestrial environment, is the need for vacuum chambers equipped with the various workpiece manipulators. The size of the chamber determines the maximum size of the workpiece. In the vacuum of space, there is no need for a vacuum chamber and workpiece size is virtually unlimited.
Electron beam processing is ideally suited for a space environment. The life of the cathode in the EB gun depends greatly on the vacuum level. In space, the vacuum is on the order of 10–9 to 10–10 torr. This is much higher than a typical vacuum chamber can provide. Generation of an EB is a mature technology, relatively simple, and does not require any mechanisms. The beam size and its position can be controlled electronically, which totally eliminates the need for moving parts —something that engineers always want to minimize when it comes to operations in space. An EB is an efficient energy source: about 77% of the energy required to generate an EB is delivered into the workpiece and only 8% is released into the gun itself (Ref. 5). Space flight experiments, performed by the Russian cosmonauts, demonstrated that a 0.6-kW EB gun was sufficient to perform welding and brazing in space. That is less power than is required to operate a typical hair dryer.
Vacuum Brazing Looks Promising
Fig. 6 — An artist’s view of the EB brazing process showing the beam (red) aiming at the inside surface of strut-to-fitting braze joint. The strut is shown semitransparent for clarity. Green lines represent preplaced filler metal.
Mechanical fastening, welding, brazing, and adhesive bonding are the four major joining processes used for terrestrial structures. Just as on Earth, there is no one single joining technology that can satisfy all our construction needs in space. For example, an assembly of several large modules is more suited to mechanical joining, similar to what is being used for the construction of ISS. A quick repair of pressurized modules, on the other hand, may be best accomplished by using adhesive patches.
In addition to the requirements of various applications, the space environment itself may influence our choice of joining processes. Such factors as zero gravity and the vacuum can present certain challenges when trying to adapt conventional joining methods for construction in space.
Fig. 7 — A simplified concept of the six-way node shown in the previous figure. Each fitting contains a preplaced filler metal (green). This open node architecture allows for a direct “line of sight” into each fitting. Consequently, an electron beam can be pointed into the braze joint from the open end of the fitting without interference from other struts connected to the same node.
There is, however, a terrestrial joining process that seems to be ideally suited for use in space. It is called vacuum brazing. Vacuum brazing is a mature technology used in a wide range of aerospace, aircraft, medical, nuclear, and other applications. It is based on the fact that the oxide films on the surfaces of metals and alloys break down when heated to high temperature in vacuum. This allows the molten brazing filler metals to wet the surfaces of the base materials and form high-quality, leak-free brazed joints. The main limitation to using vacuum brazing in an Earth environment is that it requires a vacuum chamber. In space, however, we already have an instant, cost free, and limitless vacuum chamber, with no maintenance. Another feature, common to brazing and soldering, is capillary action. The molten filler metal is drawn into the braze joint by the capillary forces that depend on joint clearance geometry and surface tension. On Earth, depending on the application and the joint design, the capillary force may have to overcome gravity to draw the molten filler metal into the joint and achieve complete penetration. In space, the orientation of the braze joint does not matter, as the capillary force has no “resistance” from gravity. Consequently, even very wide joint clearances can be filled with the filler metal. Another advantage of vacuum brazing is that it allows the joining of very thin sections of metallic components. Properly selected filler metal, combined with short brazing time, minimizes base metal erosion by the molten metal. Finally, vacuum brazing can accommodate a variety of complicated joint designs with intricate geometries, 3-D configurations, etc., that cannot be processed using conventional welding techniques. It is worth mentioning that brazing along with welding was recommended as one of the repair methods for the ISS(Ref. 6).
Current Effort at NASA Goddard Space Flight Center
In 2005, NASA Goddard initiated an internally funded program to develop a simple, cost-effective joining technology for robotic assembly of large truss structures in space.
This technology combines several key elements:
• Electron beam brazing
• “Snap-n-braze” strut-to-node joint design
• “Open node” truss design
• Beam Builder
• Low-temperature filler metals
Fig. 8 — A conceptual sketch of a triangular truss structure with the open node architecture allowing for EB brazing from the open ends of each fitting. Also shown is an assembly fixture holding the strut during the “snap-n-braze” process.
The EB brazing process being developed at Goddard can be described asfollows:
The beam impinges on the inside surfaces of the braze joint containing pre-placed filler metal, as shown in Fig. 4. In the early stages of this development effort, the beam will be stationary and the braze joint will be allowed to rotate to distribute the heat throughout the braze area. The ultimate goal, however, is to use a gun with beam-deflecting capability that will be able to follow the profile of the braze joint on stationary parts. Initially, EB brazing will be performed using a slightly modified, commercially available EB gun manufactured by Kimball Physics. The gun is capable of generating a beam of approximately 50 mA at 10 ke-V. The EB brazing process has been successfully proven in space by the Russian cosmonauts (Ref. 7).
‘Snap-n-Braze’ Joint Design
The strut-to-node joint will be designed using the “snap-n-braze” concept, schematically shown in Fig. 5. As one can see, the joint is not continuous due to the opening in the fitting for the insertion of the tubular strut. The fitting contains the filler metal preplaced in the internal groove prior to assembly and brazing — Fig. 6. This design uses the simple principle of capture and retention between the strut and the fitting so that a robotic arm can easily perform the strut-to-node joint assembly.
‘Open Node’ Truss Design
Fig. 9 — An artist’s view of the Beam Builder performing an assembly of a triangular truss structure in space. A robotic arm is attached to the strut manipulator equipped with two EB guns (red) for brazing the struts to fittings.
The nodes will be the “see-through” type allowing a direct line of sight for the EB to sweep the inside surface of each fitting in the location of the braze joint — Fig. 7. Consequently, the angles between the struts connected to the same node cannot exceed 90 deg. An example of such a truss is shown in Fig. 8. This truss consists of triangular sections. The material of choice for the construction of the truss structure is ultrathin-wall titanium tubing. Using titanium struts and nodes will allow almost 2.5 times weight saving compared with aluminum alloys.
The assembly of the truss structure will be accomplished using a Beam Builder concept, developed by Grumman Corp. in 1979. Northrop Grumman is partnering with Goddard to modify the original concept to incorporate EB brazing and the “snap-n-braze” assembly process for the robotic construction of the truss structure in space — Fig. 9.
Low-Temperature Filler Metals
Various experimental filler metals, with the brazing temperatures around 600°C, will be tested and compared against more traditional silver-based filler metals available commercially. The advantage of using as low a brazing temperature as possible is obvious — less energy will be required to make the braze joint.u
The assistance of James Smith, Goddard Space Flight Center, in creating the design concepts and the artwork is greatly appreciated.
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YURY FLOM (firstname.lastname@example.org) is with the Materials Engineering Branch, NASA Goddard Space Flight Center, Greenbelt, Md.
Based on a paper presented at the 35th International Brazing and Soldering Symposium held April 26, 2005, during the AWS Annual Convention, Dallas, Tex.