AN ALUMINUM SALVAGE STATION FOR THE EXTERNAL TANK (ASSET)
AFIT / GSE / ENY / 90D-02
Thesis Presented to the Faculty of the School of Engineering of the Air Force Institute of Technology, Air University, Wright-Patterson AFB, Dayton, Ohio, In Partial Fulfillment of the Requirements for the Degree of Masters of Science in Systems Engineering December, 1990
James M. Haislip, Jr., Capt, USAF
Roger E. Linscott, Capt, USAF
William C. Raynes, Jr., Capt, USA
Michael A. Skinner, Capt, USAF
David L. Van Matre, Capt, USAF
Approved for public release, distribution unlimited
Summary Provided by Jim Haislip. Written 1991
Intro: A design team at the Air Force Institute of Technology (AFIT) recently investigated using the external tank as a source of raw stock for on-orbit construction of space-based facilities. Their study, dubbed ASSET(Aluminum Salvage Station for External Tanks )1, documented the techniques and hardware needed to extract aluminum from the barrel sections of the LH2 tank. The barrel sections are manufactured in a way that id conducive to using simple, automated cutting techniques to sequentially remove strips of aluminum from each barrel section. These strips, upon further processing, produce rather large quantities of 1.25″ x 1.25″ thick (.10″ web) I-beams and 163 225″ long x 9.5″ wide by .125″ thick aluminum plate.
Salvage Concept: Although the AFIT design team investigated salvaging aluminum from all three major sections of the tank, stripping aluminum from the LO2 tank and Intertank proved to be rather complex and manpower intensive. As a result, the AFIT team focused primarily on the extraction of aluminum strips from the LH2 tank barrel sections.
The LH2 tank consists of forward and aft domes, four cylindrical barrel sections, and five major ring frames used to connect the barrel sections to each other and to the domes. Eight skin panels are fusion butt-welded to make up the walls of each barrel section. The skin panels are integrally stiffened by machining them from plate stock, producing a 0.125″ thick skin and the 1.25″ tall T-stringers that run down the longitudinal axis of the ET as indicated in Figure 1. Each barrel section consists of 96 stringers (with the exception of the aft barrel section, which has only 90). The cylindrical symmetry of the barrel section coupled with the uniform spacing of the longitudinal stiffeners, make possible an automated technique for cutting the LH2 tank from the inside out.
The stringers provide a handy “track” upon which a self-propelled cutting platform “rides” during a cutting pass. By making longitudinal cuts of the skin along the edges of the T-stringers and then making circumferential cuts at both ends of the barrel section, each pass produces a single 225″ long section of crude stock (see Figure 2). These sections are then transported to a work-station designed to strip the SOFI from the stock and separate the T-stringer from the skin, producing I-beam and slightly curved plate stock. The “cutting” is performed by an electron beam welder, whose power output is adjusted to cut, rather than weld aluminum.
The ASSET study discusses how a single ET could be outfitted with the appropriate hardware to provide for on-board electrical systems and cutting tools; with an orbital maneuvering and station keeping capability; and with several preflight tank modifications used to simplify the overall facility design. The facility then doubles as a docking platform for assembly line approach to salvaging additional ETs.
End Products: The ASSET design team found that over 13,000 lbs of aerospace grade aluminum can be salvaged from a given tank in a way that is cost competitive with delivering the same material to LEO using the payload bay of an STS orbiter. Table 1 summarizes the product yield from stripping a single tank. Notice that over 6,100 linear feet of I-beam and 4,800 square feet of plate are salvaged.
|Plate||2219 Al||9082 lbs||81 pcs 163″ long|
|89 pcs 225.7″ long|
|178 pcs 225.95″ long|
|I-beam||2219 Al||3123 lbs||81 pcs 163″ long|
|89 pcs 225.7″ long|
|178 pcs 225.95″ long|
An additional 1,000 lbs of aluminum (of various sizes and shapes) is recovered as a byproduct of the salvage operation.
Motivation: Several motives exist for pursuing this concept further. Beyond the obvious use of the raw stock, the ASSET facility could be readily used as a long-term microgravity experiment platform. By virtue of the facility’s infrastructure, ASSET could also make an ideal space-based construction outpost. Also, a substantial amount of on-orbit operational experience will accumulate during the deployment and operation of ASSET. Other spin-off benefits can be realized as well.
Raw Stock Usage: I-Beam. Of the two products identified in Table 1, the I-beams require no additional processing and can be used immediately as construction material. One application immediately comes to mind: Making use of the I-beam to manufacture truss elements for large space structures (e.g., space stations, solar power platforms, deep space staging nodes, research platforms, construction bays, etc.). Current plans for large space structures are, of necessity, subject to rigorous mass / volume constraints, which in turn require resource intense design efforts. Making use of a readily available aluminum stock (of known physical properties) at LEO could relieve some of these mass / volume constraints. Large space structure could be deployed by making use of automated welding or riveting techniques to produce truss / structural elements, while the physical properties of the beams would reduce the need to design around flexibility effects in these types of structures.
Plate. Making immediate use of the plate stock would be a little less straightforward since the plate is actually a long, narrow strip (9.5″ wide) of a rather large cylinder with a radius of curvature of 166.5″. If one were to place one of these strips on a flat surface, the edges of the plate would curve aup above the flat surface by approximately 0.068″ (or about the thickness of the plate). Although the plate is not exactly flat, it very nearly is. Consequently, this plate material could be used in an assortment of ways. Some of these include being used as mounting surfaces for solar cells / solar blankets, for microgravity experiments, and for instrumentation platforms. The plate stock could potentially be used to help construct the walls of a space hangar, refueling bay, or other such facility. Finally, given the appropriate machinery, the plate material could be processed further using techniques discussed elsewhere in this document.
ASSET Facility: One could make supporting arguments for making use of the entire ASSET facility for several well-suited applications. For instance, mission planning could be tailored to maximize the benefits derived from using ASSET as a long term microgravity experiment of DDT&E platform. Much of the infrastructure required to operate these types of platforms is already in place on ASSET. In fact, ASSET would make an ideal space-based construction outpost. In the discussion on raw stock usage, an assumption was made concerning the existence of an operations base for constructing the truss elements. The ASSET facility, as envisioned int eh AFIT study, would require only minor modifications to fill this need.
Technology Driver: Due to the EVA intensive nature of ASSET operations, several innovations would be required of current manned spaceflight technologies. In order to increase the efficiencies of the man-machine interface in the hostile space environment, current spacesuit designs must be enhanced to allow for greater EVA mobility. A call for high pressure (hard) suits would inevitably precede ASSET deployment. High pressure suits would benefit not only ASSET, but nearly all EVA intensive space operations. An evolutionary step beyond the hard suit, the so-called “man-in-the-can” concept, could prove invaluable in any space based construction project. ASSET deployment could spawn many such innovations.
Further Study: One aspect of the ASSET thesis which could prove quite fruitful, but requires further study, is an analysis of LO2 tank applications. A cursory investigation into cutting the LO2 tank for additional structural aluminum met with only moderate success. However, one particular area of LO2 tank applications holds great promise. The ASSET LO2 tank could, given sufficient resources and effort, be converted into a habitation module. This dovetails nicely with the space-based construction outpost concept. A rather large construction crew could be housed in the 19,5863 cubic feet of space afforded by the LO2 tank. Instead of deorbiting subsequent tanks once the I-beam and plate has been salvaged, each follow-on ET would have its LO2 tank removed. These tanks could be outfitted with the appropriate hardware to independently house temporarily and / or permanent biological, astronomical, or industrial research teams as well as space-based construction crews. Modules could be interconnected using the salvaged I-beams and plate to make a rather large space station. An LO2-based space station would make an ideal “dirty platform” for spacecraft refueling operations and other “dirty” industrial activities. Although this type of ET application would require greater resources to implement, the payoff could prove to be much greater than the value of the raw stock alone.
Conclusion: The ASSET concept should be pursued for a variety of reasons. The on-orbit operations experience gained, combined with the potential uses of the raw stock and the promise of space-based construction / research outposts makes for an appealing long term business venture. Serious analysis must be carried out prior to moving beyond the conceptual design phase. Arguably the most useful analysis at this stage would be the development of a 5-10 year business plan. A business plan would provide the framework needed to identify areas requiring further research, identify funding and revenue sources, and establish useful cost estimates, DDT&E milestones, and O&M requirements. We must undertake projects similar to ASSET, or run the risk of failing at perhaps the most important strategic interest of any nation: space commerce.
Bibliography: Haislip, J.N., and others, AN ALUMINUM SALVAGE STATION FOR THE EXTERNAL TANK (ASSET), Master’s Thesis, AFIT/GSE/ENY/90D-02, Air Force Institute of Technology, Wright-Patterson AFB, OH, Dec 1990.