SSI External Tank Report
II – Tin Can Uses
I. ET Project – Tin Can Uses
This section considers what is blithely referred to as Tin Can Uses. In spite of being less than technical, this title is the best overall classification of these External Tank Applications. The following section will discuss uses of the ET as the basis of orbital habitats, storage facilities, hangars and other ‘can’ type uses. This class of tank applications is likely where the ET can be put to the first use and possibly best advantage by the future space program.
As mentioned previously, the External Tank contains two separate tanks designed to hold liquid hydrogen and oxygen before and during STS launch. The respective capacities of 53,000 and 19,500 cubic feet are important in this context. NASA has a need for storage of some sort of volatiles on orbit to fuel Orbital Transfer Vehicles (OTVs), supply Space Station needs and refuel existing and planned satellites. The current (Sept 1985) requirement for volatile storage does not yet require a large storage facility on orbit. There is also a NASA projected requirement to supply 2.5 million pounds of cryogenic propellant in a ten year period that a scenario which utilizes the External Tank can fill far quicker and cheaper than any other plan (69).
On-orbit storage of cryogenics in the ET is possible. The tank can be wrapped with mylar reflective blankets to retard boiloff. This technique should be quite effective giving untended storage times of both residuals and additional stored cryogenics on the order of months (46). A single ET can provide all the volume required for storage of volatiles on orbit for a space station or an OTV servicing operation. The tank can be stabilized in a gravity gradient mode either free or attached to a tether to keep the liquid on the ‘bottom’ available for use (30). Cryogenics are also useful in providing backup power. A set of fuel cells designed for emergency operation can use residual or stored cryogenics as a separate backup power supply for the station. Fuel cells are mature technology with which we have extensive experience in space. The problem with the on orbit storage of cryogenics in the ET is that there is simply too much volume available in the ET oxygen and hydrogen tanks for currently planned operations. However, if the expansion of scientific, commercial and military operations occurs, the capability to store vast quantities of cryogenics will become very important.
An additional application of the cryogenic storage capability of the ET would be to partially disassemble the manned mission to the planets. The tradeoff here is a slightly heavier than required tank already in space versus a specially designed lightweight fuel tank brought to orbit (82). As is currently flown, the ET is too heavy to use as a fuel tank in this application (51). However, if the tank is partially disassembled and only the hydrogen tank used, this may be a possible application. The program cost savings will once again be in launch and developmental costs for specialized equipment.
A cryogenic storage facility also has military applications (25). These would include a refueling base for a small earth vehicle, a refueling base for military OTVs designed to fly classified payloads, and an ET based battle station for SDI purposes. The battle station could use liquid hydrogen stored in the ET with NERVA engine based generator flown in an ACC to power any desired high energy weapon.
The ET can also be used to store gases and water on orbit. The ground testing of both tanks provide a more than sufficient safety margin for 14.7 psi (one atmosphere) pressurization (48). Gasses can be stored in the ET indefinitely on orbit (95). Due to its structural design, a hydrogen tank can also be used as a structural member if pressurized.
Water is also an important item in LEO and can be stored indefinitely in the tank. The temperature can easily controlled by both external and internal techniques. The available storage volumes of 147,000 and 405,000 gallons in the oxygen and hydrogen tanks are far larger than required for planned future operations. For example, a single oxygen tank can store the water equivalent of 50 – 150 shuttle visits of scavenged residual cryogenics (95). Water also becomes an energy storage medium. Fuel cells in the station can convert the scavenged cryogenics into water. This excess electrical energy can be used to electrically raise the orbit of the station. This would save fuel and shuttle visits by allowing orbital maintenance without the use of propellant. The water can then be used aboard the station as desired or stored for future use. One future use would be to separate the water back into hydrogen and oxygen, liquefy it, and use it for the cryogenic uses mentioned earlier. This could be done during times of low energy use onboard the station.
One of the significant problems growing in LEO is debris. Every launch dumps something into orbit that could later become a threat to future operations. The ET can be partially disassembled (oxygen tank and intertank removed) and used as an orbital garbage can (25). The large volume of the hydrogen tank can be used to advantage in keeping a particular orbit clean.
An additional use would be orbital storage of recovered inoperative satellites or captured debris. The satellites could be disassembled for spare parts melted to provide raw materials for manufacturing, or ground up into powder to provide reaction mass for propulsion systems. The tank diameter is large enough to store a large number of recovered items. Artificial gravity can be induced by use of two tanks connected with a tether. The tanks can either be rotating about one another or ‘hanging’ in a gravity gradient stabilized mode depending on the gravity level desired.
A tank that is part of a space station can be used to store spares in orbit. These satellites can be serviced, checked out, and outfitted for launch as needed, and stored indefinitely for future use. When the spare is required, it can be serviced, checked out, and outfitted for launch with an extremely short notice. This will solve the problems inherent in a short notice change to shuttle launch manifest. The spare can be launched at leisure, stored until needed, and quickly flown. Program savings here are tied to manifest changes, bumping payloads, training requirements, and money lost due to not having the services provided by the broken satellite until it is replaced. An additional advantage of on orbit storage or assets where a crew can get to them is the ability to do preventative maintenance in space before launch. For example, an operational satellite shows problems after a year or so of operation. The problem is solved on the ground so that the repair can be made in space to the spare. This is a large advantage to space operations.
The ET can also be used to enhance the space station. The uses are limited only by the imagination of the respective planner and/or buyer. On the surface, it may seem that the recycling of ‘thrown away’ space vehicles is less than a desirable practice in manned space flight. This is simply not the case. The use of leftover parts of launch vehicles is something that the American space program is historically very good at doing as mentioned previously. Orbital construction and repair of satellites, space stations and other structures is a required skill in which astronauts are becoming proficient. Recent experience in satellite capture and repair by shuttle astronauts and major repairs on space stations conducted by Salyut and Skylab astronauts show that on orbit construction is not only possible, but the most cost effective choice for many applications. Indeed, the US is planning to construct the space station in orbit starting in 1992 (8).
As describe previously, the Aft Cargo Carrier (ACC) is a relatively inexpensive enhancement of the ET which allows a myriad of shuttle and space station enhancements to be flown. One of the attractive enhancements of the ACC is the capability to construct a fully functioning self contained space station module capable of supporting a crew up to seven, attaching it to the end of a tank, and launching it into orbit (4, 25). Possible missions of a manned ACC module include a workshop, corporate research facility, corporate space station, space station module, space segment, attachment segment, initial living quarters for a future expandable station, etc. Literally, anything that a space station module can offer, the ACC base module can provide more for a lower overall price (4, 25, 31, 77).
There are several very attractive advantages that a space effort can get out of flying a manned ACC. The first advantage is that it becomes possible to launch a small (in crew size – large in actual volume) manned space station that can be greatly expanded in the future with a single shuttle flight. The program savings in launch costs alone over conventional designs are staggering. The ACC can be constructed to contain all the life support, living and initial working space. The External Tank attached above provides gravity gradient stabilization, an average of 15,000 pounds of residual cryogenics available for immediate use or scavenging, structural strength of a 69,000 pound pressure vessel, and additional space of 53,500 cubic feet above the ACC inside the hydrogen tank that can be easily entered and turned into work areas, living quarters, hangars (pressurized or unpressurized), or storage facilities with a minimum of effort (25). The orbiter payload bay can carry the additional equipment necessary for station setup such as tools, solar panels, and radiator arrays. This concept lowers the cost of flying a manned space platform from the current cost reachable only by governments or government agencies to a cost that corporation can reasonably meet. It also gives the United States the capability to launch and fly a large space station with a single launch – something that we have not had since the Saturn V was operational.
The manned ACC is attractive for several other reasons. It provides a differently shaped volume than the payload bay – wide as opposed to long. This volume shape is more akin to that which people are used to living and working in on the earth and may possibly ease adaptation to space by visiting crews. A size comparison of the ACC based station shows that an ACC module is almost a third larger than the current volume available in the shuttle payload bay – 13,000 versus 10,000 cubic feet (4, 25). You get a major increase in shuttle capability with minimal modifications. The ACC, as mentioned previously, can be configured to any mission and any role. In addition to the habitation module, it can be configured to a multiple docking module, a lifeboat, a ‘storm shelter’ for solar flares, a spare, an on orbit farm, etc. Carrying the space station module outside the payload bay frees up the remaining payload bay volume and/or mass capability for additional paying customers. Conversely, the ACC can be used to launch other payloads with minimal impact if the payload to orbit capability. The ACC based module also can be designed to attach to anything that any other space facility module can be attached to.
As mentioned earlier, the hydrogen tank is also available for future expansion in an ET based space station concept. Entry can be made into the tank by a variety of means with entry diameters ranging from 3 feet (through the inspection manhole) all the way up to 27.5 feet if the dome is removed. The manhole is large enough for an astronaut with a spacesuit to fit through. The tank is clean, has a minimum of protrusions, and is ready for immediate habitation or ready with a minimum of effort (25). One of the concepts for internal modifications of the hydrogen tank on orbit is to use a large inflatable bag to provide the interior of the tank. This uses the tank primarily for micrometeorite protection and insulation. Setup is quick, clean, and very simple. Additional partitions can be inflated, erected, or unfolded in the tank to fit the needs and requirements inside. As an aside, inflatables also have a long successful history in the American space program. These were the Echo I and II reflective communications satellites flown in the early 1960s.
The hydrogen tank will provide a huge space – over 53,500 cubic feet – which is five to ten times larger than any proposed space station module that is known of at this writing (25). Once again, the advantage to the use of the ET is large size and availability for use after a single launch. The program savings here are mainly in launch and orbital construction costs.
In addition to the Hydrogen tank, the liquid oxygen tank is also available for use as a habitat. The early studies on External Tank applications by Marshall Space Flight Center proposed the adaptation of the oxygen tank to a manned mission as a first step (12). There are a few disadvantages to its use, which are useful to mention. These include a slightly more difficult access problem due to the location of the manholes and intertank section, more internal hardware to consider due to the slosh baffles, and a shape that is less than optimum for the use of the entire tank for habitation. These disadvantages make the early use of the oxygen tank less attractive than the hydrogen tank and the ET/ACC combination. However, it may serve very nicely as a water storage facility, a lifeboat, a passive life support habitat, a cryogenics storage facility, a biological experimentation station, or a farm in conjunction with an ET/ACC based space station (25,56,95). It also has a very large volume – comparable with Skylab (70) – which will be useful in future applications in orbit.
The Hydrogen Tank can also be turned into a hangar for satellite and vehicle servicing and repair. The top dome of the tank can be removed completely, swung aside, or removed and converted into a hangar door. The tank can also be left open as an unpressurized servicing facility. The Space Operations Center (SOC) studies of the late 1970s suggested a hangar and / or berthing facility be developed for this purpose. As a hangar, the ET can serve this purpose quite nicely with a minimum of modifications. If a pressurized work volume is desired, the tank can fill this need also by the addition of a hinge and a pressure seal. Entry diameter can be made anywhere from 3 feet to the 27.5 feet diameter of the tank itself. This will allow the servicing, testing, and construction of large space systems.
The SOC studies of the late 1970s suggested on-orbit construction of a hangar facility based on the size and support structure for payloads in the orbiter payload bay. The hydrogen tank interior can be easily adapted to this role for a far lower cost than what was envisioned by the SOC studies (25). Once again, the program savings are primarily launch costs. Additional savings are realized by on-orbit modification of an existing structure rather than developing, purchasing, and launching a new module for the same purpose.
As the United States works toward a continuous manned presence in space, interest in a similar presence by industry becomes stronger. Using the ET or the ET/ACC combination as a Corporate Space Station will provide a way for various companies to make this happen. As mentioned in the manned ACC section, the launch of a fully functional manned space station in a single shuttle flight is possible for a total cost of two to three generic communications satellites $200 – 300 million. And is something far more attractive to corporations than the $8 billion planned cost of the Space Station. The scenario including a corporate space station would do several things:
1. Promote industrial interest and investment in space by keeping costs down. This would enhance the current space station by keeping the industrial activities separate from the scientific and research activities aboard the space station.
2. Allow industrial users to keep their own on-orbit work a corporate secret. There is better security in the use of separate facilities in space.
3. Allows DOD or DOE to conduct tests, experimentation, and evaluation in orbit with sensitive systems that will not attract the attention that a secret shuttle launch from the Cape or Vandenberg will. As the SDI expands, the space-based requirements of currently proposed systems will require on-orbit testing. This will make an ET based station attractive to other governmental agencies. It also allows the space station to be a platform dedicated to civilian purposes in its entirety. This is an attractive political option.
4. Allows companies to conduct environmentally sensitive experimentation in space as needed. Issues, which are very sensitive to the general public become far less sensitive when the experimentation is a few hundred miles in space and those conducting the experiment, have to live with their experiments for months at a time. The advantage here will be to remove a potential problem from the surface and make those involved responsive to safety concerns.
5. Allows countries which are interested in space operations to purchase from the US an autonomous manned space station for the same cost that a U.S. company can buy one (75). Those countries that are interested in the space station would also be interested in the ET. If we remember the lesson of foreign sales of the F-20, foreign customers want to be actual owners and operators of the best equipment available. They will likely want a space station of their own and the ET/ACC combination is an excellent choice for their needs. This will also make the ET and ET based facilities an export item which will in turn improve the balance of trade. The ET as an export item is potentially a very attractive possibility.
6. Enhances the space station by getting a significantly wider range of customers involved in flying and operating in orbit. The space station could be a central point in a fleet of space platforms which will provide services to that fleet of domestic and foreign customers. It will provide an impetus for the permanent manned presence in space to become more permanent.
Another adaptation of the corporate space station concept was presented at the 7th Princeton Conference on Space Manufacturing as a way decrease the start-up costs of the High Frontier scenario proposed by Dr. G.K. O’Neill (33, 52). A version of the ET based corporate space station was proposed which would be useful in two ways. The first is the generation of revenues for future R&D toward the goal of Solar Power Satellite construction out of lunar soils. The second is a place where orbital testing, development, and work on actual hands-on space construction could be conducted. The basic idea is to initially rent floor space and support services to customers to generate cash flow (72). The funds are to be reinvested to expand the facility and to conduct the expensive R&D associated with setting up a construction base on the moon and in high lunar orbit. This is an example of a way that the ET can act as a catalyst for large-scale activities that would otherwise rely on the whims of governmental funding.
The majority of effort in the ‘Tin Can’ use of the ET has been spent in the area of habitable structures as discussed above. There are additional concepts to mention which include the use of the ET with inflatables, the ET as reentry and landing modules, and wake shields.
One of the proposed type applications involves inflatable structures (25, 95). As mentioned in another section, it has a large potential in a space operation. It is attractive to use inflatables with the ET/ACC because an inflatable is typically a low mass, large volume item. These structures could be used in conjunction with an ET to provide the interior of an ET based space station. They could be used to provide an orbital farm or growing facility. If it proves feasible to grow food in space, the work could be traded off against the $2,000 per pound launch costs for resupply from earth. This serves as sort of an orbital truck farm (95). The last proposed inflatable is the flexible docking tunnel concept (25). If there is a flexible docking tunnel attached to the space facility, this would save payload bay space in the orbiter in each trip to the station. The orbiter could conduct the mission for the paying customers and visit the station last. There would not need to be payload bay volume or lift capability used in carrying the docking adapter. Overall savings are in volume and lift costs. These have been projected at $180 million over a ten-year period (25). They may be far greater, especially if the additional volume can be used to support paying customers.
There have been two papers proposing the ET in various configurations as a reentry module (39). The direct use of the ET as a reentry module is not structurally possible according to the manufacturer (48). There are however, variations in this concept, which involve indirect ET use that may be possible. It may be possible to use the SOFI removed from the ET in structural processing as an ablative material (56). It may also be possible to form melted aluminum from the ET into an aerodynamic shape, cover it with ablative materials, and recover the aluminum itself after reentry for salvage and sales. The other class of applications includes the use of the ET as a landing module. This either crashes the ET as a raw materials carrier on the Lunar or Martian surfaces or drops it in a controlled soft landing using the crushing of a portion of the ET as a large shock absorber in the landing.
Any structure that can be used as a space station can be used as a large spacecraft (43, 82). The ET and ET based structures are no exception. The advantages are available volume and structural strength. As mentioned previously in this section, the primary problem is excess mass. The ET as it flies, may be a bit too massive to fly manned missions to the planets without partial disassembly in orbit (51, 80). The oxygen tank and intertank can be removed and the hydrogen tank can be used for habitats and fuel tanks. However, the use of a space station as a spacecraft is well within the realm of possibility if the designers are careful about mass and propulsion problems (82).
This is the use of the ET as a large windshield in orbit (17). On the lee (or downwind) side of an orbiting body, it is possible to achieve vacuum enhancements orders of magnitude better than ambient. The ET can be cut lengthwise, opened, and flown to provide this advantage for materials processing or experimentation in orbit. This application is highly drag intensive in that it aggravates the orbital maintenance problem of the ET. It also might not work out as desired due to the tendency of vehicles to collect orbital plasmas (48).