SSI External Tank Report
VI – Miscellaneous Applications
VI. ET Project – Miscellaneous Applications
This section details a variety of external tank applications that are not readily matched with the previous categories. These applications are in no way less important or feasible. Indeed, the military and scientific applications may very well be the first use of the ET or the ET/ACC combination on orbit. The majority of the science applications were suggested in two workshops held at the California Space Institute in 1982 – 1983 (56, 95).
The External Tank has been proposed as the heart of a variety of telescopes for observations of the earth and space across the majority of the electromagnetic spectrum. These include: The use of the volume of the tank and the ET/ACC carrier as a way to carry large mirrors into orbit; the use of the tank itself as a strongback for the construction and stabilization of large structures; and the use of the interior of the tank itself as a telescope for high energy astronomy.
One of the early uses of the tank could be to carry mirror segments in the ACC for on orbit construction into a LDR (35, 81). This telescope is primarily intended for research in the infrared and submillimeter wavelengths. Its basic design would also serve nicely for a large diameter optical reflector. The basic plan is to carry seven panels composed of seven hexagonal segments stacked on top of one another in the ACC. Each panel is proposed to be 24.5 feet across. The panels are removed from the ACC and assembled on orbit into a rigid structure about 66 feet across. There are 18 edges to be connected on orbit in this plan. An alternate method would be to carry the smaller segments in the cargo bay and assemble them into the LDR on-orbit. This alternate method is not as attractive because it requires several more edges to be joined together. This leads to additional problems associated with pointing and mirror adjustment of 49 total segments.
The astronomical advantages of this type telescope in the optical, infrared, and submillimeter wavelengths is enormous when compared with current and planned ground and space based telescopes. In the optical wavelengths, the 65 foot diameter compares nicely with the 200 inch (about 17 feet) Hale telescope and the 400 inch (over 33 feet) proposed Texas multiple mirror reflector. In the infrared, the pure dimensions of this telescope alone have the potential to improve upon the IRAS data by several orders of magnitude. The advantages gained by the use of the ET and the ET/ACC are gained by the ability to launch a few large panels for construction rather than having to connect 49 small segments to one another during a 7 – 10 day shuttle sortie. The ET also can provide a very large gravity gradient stabilized base or an anchor to which the telescope can be attached to for operation.
The ET also has potential as the base for cosmic ray, gamma ray, and X-ray observations (40, 56, 91). A tank that is outfitted with proper detectors and stabilized in a gravity gradient mode could be turned into a gamma ray telescope as pictured below. The interior of the hydrogen tank could be turned into large orbiting Ion or Cerenkov chambers for detection of cosmic and gamma rays. The cost of these would be low enough due to the use of a tank, so that there is the possibility of flying more than one high-energy observatory. This leads to the possibility of long baseline detections of activity and thus better, more comprehensive data. The tank is also large enough to mount large arrays of proportional counter systems to conduct X-ray astronomy of the heavens.
As was mentioned with the LDR, there are advantages in using the ET in observing the longer wavelengths. The ACC itself is large enough to accommodate a very large deployable antenna. The diameter of the mirror can be increased by nesting panels and attaching them together in orbit. The observation of the earth and planets by Synthetic Aperture Radar involve large antennas. The ET can serve as the structural base for a large antenna used for this purpose. It even has the capability of being cut in half lengthwise and turned into a long trough antenna for these purposes. Once again, size, mass, and structural strength are all advantages.
The ET can serve to fill several other observational science needs (56, 95). It can be used as a space-based occultor for ground and space based observations. Two tanks tethered together can be used as a low-altitude gravity gradient detector. The tank can be used to study the interactions of large bodies and the upper atmosphere, plasmas, and magnetospheric effects of charged and uncharged bodies in space. Most of these are studies that are possible with large inexpensive bodies in orbit that would not be possible on near this scale in an operation that does not use the ET.
The first class of application is to use the interior of either tank as a waste management facility (56, 95). If the waste is non-biological, the interior of either tank is sufficient to store scraps and debris that would become a hazard if left in orbit. This debris could later be used as reaction mass or melted to serve structural needs. The second class of waste is biological (95). Any system containing living components will generate these wastes. In order to make a space based operation as self-sufficient as possible, it is in the interest of planners to recycle the maximum amount of consumables. This typically involves some sort of waste management. The tank can be turned into a very large microbiology lab that could conduct large experiments on waste recycling. The tank is large enough to partition into several portions and run several experiments. The advantage of large size is biological inertia. This means that any unwanted changes could be identified and dealt with before the experiment fails.
A large pressurized volume can be used to conduct experiments and development of those strains of plants and animals best suited to zero gravity. This would serve to develop living things that could be used to support living systems from within the space station. A large inflatable could be attached to the exterior of the ET and used as a farm as detailed below. The isolation of the tank in orbit could allow genetic experiments to be conducted in space that would not be possible on the surface (25, 56, 95).
The ET could also be used as an active or passive lifeboat (95). The oxygen or hydrogen tank that is being used for an experiment in waste management or farming would be pressurized. This provides a lifeboat available for use to a stranded crew or the crew of a station that is unable to provide normal life support due to problems or an emergency. The hydrogen tank is large enough to provide several man-months of activity before the CO2 levels become dangerous (95). This is a passive life support system. Several tanks in several orbits could provide an excellent safety margin for a large space-based operation. All that is required is to fill the tanks with oxygen or a nitrogen – oxygen mixture at 14.7 psi and release them into the desired orbit.
Additionally, the SOFI can provide some radiation protection. The aluminum shell also provides some protection from radiation (95). An additional possible ET life support contribution is gravity. According to NASA scientists, the human body does not tolerate long periods of weightlessness without serious deterioration of bones and muscles (55). Two ETs can be attached by a tether and swung around each other to create gravity for crews living inside. They can also be attached end to end in the form of a torus and spun up to provide gravity. The ET is structurally strong enough to tolerate the loads imposed by both of these proposals (48). It is even stronger if it is pressurized. The only requirement is to make the rotating facility large enough that coriolis effects are minimized. A rotation rate of 1 – 3 revolutions per minute seems tolerable at this time but would require experimentation with a crew to find out. This rotation rate also requires either a long tether or a very large diameter torus.
There are also significant military applications possible with the ET and the ET/ACC combination (25, 56). Like the applications mentioned previously, these take advantage of the low cost, large size, and large mass available with an ET. There will be no attempt made here to sell or hide possible military applications. Like any other space-based activity, the use of the ET is available for both civilian and military uses. Both activities will likely take place in the near future.
Several advantages exist in the use of the ET as a military space station. The primary advantages being large size, easy expandability, and low cost. The same capabilities (mentioned in previous sections) of low cost commercial space stations apply equally well to military stations. These stations can serve as separate bases from which DOD satellites can be launched, retrieved, repaired, or deactivated. The previously mentioned possible on-orbit storage of spares also allows a military operator to quickly replace a satellite that has stopped functioning for whatever reason. This response time advantage will be a military advantage in future conflicts. The possible use of the ET as a hangar or a battle station allows concealment and surprise in the use of military assets on orbit. The ACC also provides a space for the launching of large diameter mirrors which could be used as space based high energy weapons (25, 56). These would also take advantage of the ease of launching an ACC payload and therefore be more responsive to future needs.
There are other possible military uses for an ET based space station. The first one is a space-borne equivalent of the Coast Guard. The search and rescue aspects of the coast guard mission are well known and similar type role can be filled by a military station. An important consideration may be based on international military activities. The Soviet Union and the United States are involved in the SARSAT program that includes five active spacecraft so far. There is no reason why this could not be extended to manned military (or civilian) systems as well. The second concerns on-orbit inspection and deactivation of other satellites. There has been a proposed spaceplane that can serve this mission (25). The spaceplane could be based out of an ET adapted station. It makes far more sense to do this with a small manned spacecraft than it does with an orbiter. Another use of a manned station could be the military conduct of orbital surveillance, photography, communications, and new systems experimentation and development. For example, the military roles and missions currently practiced on the Salyut can be practiced on an ET based military station.
The ET could also be used for a number of additional orbital tests, missions, and roles (25). For example, a number of identically configured empty tanks could serve as orbital decoys. A tank could be used as a ‘Junk ASAT’. The ET could be maneuvered into the flightpath of a target satellite and destroy it on impact. The tank could be melted and foamed on-orbit. This metallic foam could serve as a blast and impact shield. The ET can also be used as a target for SDI R&D experiments or demonstrations. The idea here is to deorbit the ET and shoot it on the way down. The tank is large enough to take more damage than conventional satellites and thus may be an advantage to the military use of space. The real advantage in using the ET as the basis of a military space station is that it keeps military activities separate from the civilian activities. This is politically a very important advantage.
An ET based OTV could clean out each individual orbit of space junk and debris over a period of time (25). This could be done by two methods. The first one would use a soft rendezvous with a minimal Delta-V and could use tethers or a very precise rendezvous to clear the required altitudes of debris. The retrieval would be slow and controlled and likely very fuel intensive. The second could be somewhat larger and more massive and be used as a hard target with the appropriate use of foamed metals, netting, or nested cylinders and capture the debris by impact. This is an idea worth consideration that has not had anything more than very preliminary work done on it.
The ET is also a potential resource for a wide variety of presently unknown, wildly speculative, and unanticipated applications. As has been demonstrated by a number of other space based activities in the past, the present discussions of ET and ET/ACC applications will likely serve as a starting point and nothing more. There have been several less than mundane or ‘far out’ proposals for the ET that may be of great importance in the near or not so near future. One proposal is to use the STS or an STS derived vehicle to eliminate nuclear waste by launching it from the surface and storing it in deep space or sending it out of the solar system. The author then suggests using the jettisoned tanks as a starting structural base for setting up large industrial operations in orbit (43). Another author proposes deorbiting the tanks and selling the remaining aluminum at the going rates for scrap aluminum (39). A third author proposes using tanks as landing vehicles for large cargoes to be deposited on the moon (or possibly Mars). The cargo is carried in one of the tanks and the landing (or controlled crash) is conducted in such a way that the impact and crushing of the tank absorbs the excess energy in landing and thus cuts the energy requirements necessary for a soft landing of cargo.
The point in this discussion is this. The ET presents an enormous possibility in the expansion of operations into space. The ET is an item that is very flexible to a broad range of user applications (25). It is simply capable of an enormous range of possible uses, most likely yet to be conceived. We do not yet know what is the best way to use the tank. What we do know is that the more investigators look at the tank and tank applications, the more possible applications are proposed.
Two important decisions need to be made quickly. The first decision that needs to be made is the early insertion and on-orbit storage of the ET. The second decision concerns ownership and identification of possible markets for the tank. We are in the position at this time to start planning for the future of this nation in space. If this resource is inserted into orbit and made available for sale to interested parties either before or after launch, the possibilities for the future of this nation in space are far greater for far less money.
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