SSI External Tank Report
IV – Structures
This section will discuss the structural advantages in an ET based operation. The tank can be partially to completely disassembled in orbit without excessive difficulty. The SOFI can be stripped and discarded or used for other needs. The tank can be cut into pieces and those pieces either used for construction or melted or powdered for other needs. In addition to applications mentioned in other sections, the ET itself can be used as a structural member, workbench, or platform for the construction or deployment of large structures. It also can be used to conduct experiments on large structures as a testbed. The advantage of the strongback / testbed concept is that the ET is fully suited for future growth and expansion with the addition of more ETs, ET/ACC combinations, ACCs, or other modules.
Any operation which proposes the structural use of the ET needs to remove the SOFI from the ET exterior. The SOFI is sprayed on to the ET over an adhesive base. The adhesive is extremely effective, so much so that the removal of the SOFI from tank sections on the ground is done with fiberglass scrapers in a very dirty, labor intensive job (48). This type operation will be unacceptable in flight. Work on the subject by the California Space Institute has proposed several methods, which should work well in orbit (15, 56, 95).
The best choice for removal appears to be the space equivalent of a hot wire ‘cheese slicer’. Tests indicate cutting rates better than 33 feet per hour (15, 56). The problem with this technique is that there remains a thin layer of SOFI after the operation. The tradeoff here is how much contamination can the cutting, melting, or construction operation tolerate (92)? If the layer needs to be removed, there are several removal methods. It can be stripped over the space of a month or two by molecular oxygen impact (14). It can be removed by a ‘Weed Eater’ type rotary device or wire brush. This operation would tend to dirty up the vicinity of the station quickly with small bits of debris. It can also be removed thermally by use of a solar furnace or an electron beam gun (56).
Partial disassembly of the ET is a relatively simple repetitive operation (25). The major operation is the removal of the bolts that connect the Oxygen tank to the Intertank and the Intertank to the Hydrogen tank. Once taken apart, the interiors of the oxygen tank and the intertank sections become far more accessible for future use. The intertank also carries an extremely strong compression member called the SRB Beam, which can be removed for use on-orbit. The tank domes can also be removed from the respective tanks for use as reflectors, hatch covers, antennas, or similar items.
In addition to the major parts mentioned above, there are several minor parts that can be removed for orbital use (63). There is the SRB and Orbiter attach hardware. These are large strong parts carried on the SRB Beam and the aft end of the ET. This attach hardware can be removed and used as attach hardware for other constructs in orbit. Another type of part is the feedline. The feedlines are insulated, inspected, and tested 17-inch diameter lines which can be used in other ways on a space station when removed. There are electrical lines, electrical connections, sensors, minor hardware, and range safety devices also mounted on the tank for use during launch. All of these can be removed and utilized in a properly designed space operation (25).
The partially disassembled ET sections can be reassembled into extremely rigid structures of almost any size. Taylor has proposed a rigid torus constructed out of the ET and segments carried aloft as part of an ACC (82). The concept is to partially disassemble the ET and reconnect the hydrogen tank sections into an eight-sided torus which would be spun for artificial gravity. The angled sections are either cut from intertank sections or carried aloft as ACC payloads. The torus is about 300 feet in diameter and can house up to 200 people comfortably.
This torus concept was proposed as the basis for a manned mission to Mars (82). The ship was constructed out of ET hydrogen tanks, fueled, and launched out of earth orbit with the help of OTVs. The tanks were to be used on site as a permanent manned space station in orbit around Mars after the mission was complete. The large size and available ‘elbow room’ of this concept are very attractive to anybody faced with the prospect of spending several years in a VERY small room during a manned planetary mission. The torus would also be a way to fly the recently proposed ‘Space Castle’ concept of large interplanetary stations.
In addition to the proposed torus, the partially disassembled ET can be reconnected into almost any size and shape appropriate to the mission at hand. Several hydrogen tank barrels can be attached lengthwise into a long tube, which can be used for an electrical catapult (25). A pressurized hydrogen tank could be used as a strength member due to its increased strength under pressure. Several ET domes can be attached in an array to provide solar thermal power. ET domes (properly mirrored and shaped) can be used to concentrate sunlight on Sterling cycle heat engines for onboard power. As with any other ET application discussed in this report, the only limitations of partial disassembly and reconstruction are in the mind of the planner.
There are several plans for cutting, sectioning, and welding of the ET. One proposal uses conventional cutting and welding techniques under space conditions (20, 92). An important point to this is that conventional cutting and welding are known technologies involving known equipment being conducted under space conditions. We are very familiar with cutting and welding. There have been experiments involving welding and cutting conducted in both the US and Soviet space programs. The experiments have been successful and have warranted further study. There are welding studies being conducted in association with the construction and operation of the space station. If you can cut and weld a space station, you can also cut and weld any desired structure out of the ET. We are very familiar with the performance of aluminum being welded and cut on the earth. There is no obstacle today that prevents conventional cutting and welding in space (92).
There are two other techniques for cutting the ET. The first is the use of an Electron Gun and the second is the use of reflected sunlight by a small solar furnace (56). Both methods work by heating the aluminum until it melts. The solar furnace does not require additional power for operation however. Both of these techniques are not as familiar to metal processing as conventional techniques. This will involve additional experimentation in space before being selected for use.
The cutting of the ET into segments is attractive for several reasons. The first is that the cutting of the hydrogen tank into strips can supply many pieces of aerospace grade weldable 2219 aluminum measuring 5 feet wide by 60 – 80 feet long. We also have all the engineering drawings for these strips (20). Almost any space structure is possible if the technology of cutting and welding is proven in space. The point in the cutting and welding process is that we are very familiar with the behavior of these pieces in ground based applications and can transfer this experience to orbital operations. Another reason for the orbital cutting is the isolation for future use of several aluminum alloys that make up the overall tank. The behavior of the alloys by themselves is well known. The different alloys that make up particular portions of the ET can be isolated from one another by carefully planned and executed cutting for future welding, melting, and forming into the desired structures (92).
In a space-based operation, the cutting and welding of structures will be an early desired (if not required) activity. Because of the use of known technologies, the cutting and welding of the ET sections will be a valuable addition to the space construction ‘bag of tricks’. It is not particularly complex and does not require a particularly expensive R&D investment. The challenge is what to do with an essentially unlimited supply of potential structural segments at the rate of 30 – 35,000 pounds per delivered ET. Clearly, some sort of market study needs to be made in this regard (20).
Another structural technique is the complete melting of the ET and the on orbit fabrication of structural members such as thin shells, beams, rods, channel sections, or other controlled extrusion processes. There are a large number of proposed facilities for the melting of the ET in orbit and the future use of the melted aluminum from the melt. This is a somewhat more complex activity than the cutting and welding and will require additional R&D and training. The fabrication of structural members will require the equivalent of a factory to be flown in orbit. The energy for this factory will likely be from a solar reflector (15). Other sources require additional mass to be launched to the factory and are therefore more expensive.
There are several proposed facilities for the melting and processing of the ET. Three different types will be described in the following sections. They are a tether based facility proposed by Dr. Joe Carroll of CALSPACE (15), a combination ET storage rack and melting facility proposed by Tom Taylor (25), and the materials processing facility proposed by Dave Christensen of Wyle Labs (17).
The Carroll facility sections the ET by the use of an Electron beam gun and then uses a 30 – 40 meter (about 115 feet) diameter solar mirror to melt them into a graphite crucible for storage or future use (14, 15). The A-Frame is used for structural strength of the facility. The tether is used for tank storage and the stabilization of the crucible. If the melt (on the order of five tons) is heated sufficiently in the graphite crucible and stored with either a layer of slag or a lid, keeping it molten will not be a large problem. The way that this particular system works is that once the melt is up to temperature, the total energy contained in the melt is so high and the surface area is so small that the losses due to radiation are not significant. The liquid can be drawn off and fabricated to any desired form such as metal flakes or powder for propulsion, metal vapor deposition for thin reflective surfaces, metal for extrusions, metals for metal crystal growth experiments, and metal rope or braided metal rope for tether or construction purposes (56). The Carroll facility is very attractive in that it is relatively simple to set up and operate and does not require a large amount of heavy industrial equipment to be flown.
A second type facility would be the combination ET storage rack and melting facility (25). The illustration above proposes a large diameter thin mirror working on an ET. The concept is to melt the entire ET into a molten ball. This would enhance the drag characteristics of the ET on-orbit. For example, 36 ETs could be stored in a ball of about 27 feet in diameter (25). This ball would have two layers. The outer layer would be a crust of slag, which includes the remaining SOFI and the miscellaneous metallic portions. The interior would be almost pure aluminum alloy. The primary forces on this body would be the surface tension of the slag and aluminum. A possible application of this molten ball would be to introduce low-pressure gas into the center of the molten ball and inflate it like a balloon. A non-rotating blob of metal could be inflated and cooled to yield thin walled metallic shells. The thickness of the shells would be dependent on the purity of the aluminum being worked, the rate of inflation, and the ability to control the cooling rate. Rotating the blobs would produce complex curved shells with controlled thicknesses. It would also be possible to construct thermally sensitive aluminum springs out of this type rotating melt and cooling operation.
In addition to the structural shells, there is also the possibility of making foamed metal for use as lightweight structural members (25). This would work like nondirectional honeycomb in high performance aircraft wings. Additional uses would be to form powdered or foamed metal into structural members for the construction of large space platforms and perhaps Solar Power Satellites (73).
The third example of an orbital facility was conceived by Dave Christensen of Wyle Labs (17). It consists of two ETs connected end to end with associated ACC shrouds and payloads. The mirror on the end is the proposed Carroll 115 foot diameter solar collector capable of generating up to 300 kilowatts (kw) thermal or 100 kw electrical energy. This facility would conduct materials processing, energy generation, ET melting, and related missions. Excess electricity generated by the reflector would be transmitted to the sister space station. The materials processing conducted would be to melt the ET and prepare the aluminum for future storage or use. Once again, this is a utilization of the orbital resources for future needs. Early missions of this facility would be to conduct orbital testing of large structures. This is an example of an actual private facility that is being planned for flight with corporate funding that will utilize ETs as components, products, and working media.
Another device that would be of interest in the handling of molten aluminum is the Induction Furnace. If an induction furnace is asymmetrically wound with heating coils, it is possible to feed in stock to be melted on one end and force the melt out the other end (92). It may be possible to use this furnace to form extrusions directly without the use of a press.
This sort of furnace would solve some of the mass problems inherent in handling liquid metals in a weightless environment. Dr. Andrew Cutler of CALSPACE feels that this type facility can be flyable for less than 15,000 pounds mass (18, 20, 92). This furnace could use gravity gradient stabilization to help stabilize long slender extrusions.
Early studies of chemical processing of the ET indicate that it is more expensive in terms of energy (30 – 40 times) and launch mass than when using a thermal (reflected solar) furnace (56). For this reason, operations that discuss chemical manipulation of the SOFI and other ET minor parts have been addressed only in a minor way in this report. Undoubtedly, this will happen in the future. However, thermal materials processing will be the starting choice of near future operations due to lower energy and launch mass requirements.
The barrel of the ET has also been proposed as the base for a solar furnace (56). It is pictured below. Film deposition on the ET exterior may by used to retard SOFI outgassing. Finally, the ET itself could be used as a very large diameter spool for extruded wire or cable.
The use of the ET as a strongback takes advantage of the strength built into it as a base for construction. This has been referred to as a bedplate, a strongback, a base, or a station itself (56, 95). This concept provides mass and rigidity for operations such as space stations, satellite retrieval and repair, space antenna and reflector construction and flight, and pilot plants for goods and services. The recent evolution of the space station structural design from the Power Tower to the Dual Keel shows that structural strength and safety are very important to future planners (8). Once again, the ET can provide a very large strong object on-orbit for a minimal cost.
An early proposal by General Dynamics / Convair recommends the use of the Enterprise and an ET/ACC as an initial relatively inexpensive space station (56). The rationale here is that the Enterprise will never fly without massive reconstruction. In order to get some operational use out of it, the proposal is to remove the wings and other reentry and landing systems, stretch the cargo bay, mount it permanently on an ET and launch it directly into orbit. The station could be visited for startup or it could be launched manned with another orbiter ready for a quick servicing or resupply. The advantages are shuttle compatible hardware throughout, standard shuttle in orbit, the addition of the ET and ACC as part of the station, and a base for a RMS arm to operate. Once again, this is an ET based system that utilizes already purchased equipment for uses other than what was originally envisioned. This would be somewhat more expensive than launching and flying the basic ET/ACC station, but the potential for orbiter based operations should be extremely attractive to a buyer.
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