SSI External Tank Report – Introduction
This report was written under a grant from the Space Studies Institute of Princeton, N.J. The scope of the project is to research all the proposed on-orbit applications of the External Tank (ET) and write a report detailing the results of the research. The report is intended first for presentation to the National Commission on Space and second to provide an overview of current and past External Tank applications studies. The list of references in the back of the report, with the listing of those contacted during the research, will hopefully provide a source of additional information to those interested in the ET and its applications in space. I wish to thank all those listed on the pages following for their time, help, patience and comments in completing this work.
The ET is the only portion of the Space Transportation System (STS) currently expended on each flight. A typical launch will retain the ET for the SSME burn of over eight minutes and then jettison it for a controlled reentry in either the Indian or Pacific Ocean. An alternate launch trajectory called a direct injection can allow a shuttle to take an ET, an average of 15,000 pounds of residual cryogenics (16), and up to 2,000 pounds additional payload into a typical space station orbit (68). In other words, it costs nothing to deliver a 69,000 pound, factory tested, aluminum pressure vessel into low earth orbit (LEO). In an era of launch costs running about $2,000 per pound to LEO (69), this is clearly a resource worth utilizing in future space based operations. Each and every shuttle launch can deliver a tank to orbit. This can amount to several hundred tanks over a ten year period. the economic analyses of tank utilization on-orbit typically compares costs between a ground built and launched structure and a similar structure built on orbit out of the ET or parts supplied by the ET. These studies alone make ET applications on orbit extremely attractive. There are additional benefits that the ET can provide in orbit which can not be provided by a ground based item. These make the ET a better choice for almost every manned space operation currently envisioned. Why do we need the ET ? We need it because it will provide the leverage necessary to make future space based operations, industry and science far cheaper and easier than would be possible without the ET.
There are several groups interested in possible orbit applications of the External Tank. The interest spans the entire spectrum of all those interested in doing anything in space from hard science to DoD to private industry. The interest in the tank is also international, with papers proposing ET based space stations written by British and Czechoslovakian authors over the last six years (43, 70). Aeritalia has retained the services of a consultant working on the possibilities of ET applications on-orbit for a few years (75). A typical scientific interest is the Smithsonian Observatory, which is looking into the possibilities of constructing a variety of ET, based telescopes (32). DoD is interested in the ET due to its capability to carry large diameter payloads and serve as targets for SDI experiments. NASA is interested in the ET because they have to answer the question “Why don’t you use the External Tank in space ?” posed by members of congress frequently. They are also interested in it as a way to get more “bang for the buck” out of money spent in space. In other words, the ET is a way to cheaply conduct a very large expansion of a space-based operation with minimal cost to the public. Commercial interests are looking at the tank because it provides a way to conduct possible future manned private space operations at about the same cost as two or three generic communications satellites (17, 57, 58). It becomes possible for a company to fly their own large platform cheaply. This increases national interest in space and involves a larger segment of industry in space. Companies such as 3-M, Johnson & Johnson, Wyle Labs (17), Ford Aerospace (88), and Martin Marietta (25) all either have a present interest or possible future interest in the ET on orbit. The Space Studies Institute is interested in the tank as a way to make the construction of Solar Power Satellites (SPS) and space based habitats come about. There are additional private groups and consortiums interested in flying the tank. The interest should continue to grow as long as launch costs remain high and payloads remain constrained by the dimensions imposed by the shuttle cargo bay.
Interest in the spent stages of launch vehicles has been a part of the manned space program since the days of Gemini. The USAF proposed the Manned Orbiting Lab (MOL) program in the early 1960s as a DoD space station. This utilized the upper stage of a Titan booster and a modified Gemini capsule as the space platform. This program had crews selected before it was canceled. The Skylab program following Apollo utilized the Saturn V third stage as the heart of a space station. The program used Apollo hardware intended for lunar flight as a space platform. In the planning stage, there was serious consideration given to actual on orbit construction of the Skylab. This did not happen because a Saturn V booster came available to launch a ground constructed platform and because there was concern that hydrogen leaking out of the spent third stage insulation (inside the tank – not outside, like the ET) would pose a fire hazard. The American space program is historically very good at adapting hardware from the intended to a new use. On orbit use of the ET is nothing new from this perspective.
As mentioned previously, the ET is attractive primarily due to launch costs and capabilities that do not exist in other launch systems. For example, the scavenging of residual cryogenics from the ET alone can give a program saving s of $3.5 billion when compared to the cost of launching the same cryogenics in the payload bay to support an Orbital Transfer Vehicle (OTV) operation (29, 32, 63). A partially disassembly of the ET on orbit and construction of a hangar out of the hydrogen tank will eliminate the need to design and launch a maintenance structure in the orbiter. The savings here is the difference between on orbit construction using the ET and the design, launching and on orbit construction of a specialized structure.
The capabilities that do not exist with current or planned operations that the ET can provide are limited only by the imagination. These have been broken down into the following areas:
1. External Tank Description – What properties of the ET can we take advantage of ? How is it put together ?
2. Tin Can Uses – What can you use a 33 ton, 150 feet long, 27.5 feet diameter, pressure tested, aerospace grade, aluminum can for ?
3. ET as a Fuels Resource – The use of residual cryogenics available in the ET. What value is a rocket engine that burns powdered aluminum ? Can you make the ET itself into reaction mass for OTVs ?
4. Structures – What can you do with a structure that carries all the loads imposed by two Solid Rocket Boosters plus an orbiter and carries over 1.5 million pounds of cryogenics at liftoff ?
5. Tethers – How can a tether be used in concert with an ET to expand capabilities in space ?
6. Miscellaneous – What science can you do with a very large bottle for biology ? What advantage can be taken of a potential payload diameter of 27.5 feet ?
The capabilities provided by the ET that are not available elsewhere include: large relatively inexpensive masses of aluminum (in excess of 53,000 pounds per tank (56)) in earth orbit; large factory tested pressure vessels; large diameter payload capabilities with small enhancements of the ET; and large enclosed volumes ready for use in orbital storage until needed.
The ramifications that tank utilization can have on the space program in the future are significant. At one stroke, the program changes from one based on limits imposed by high launch costs and relatively small launch and orbital volumes to a program which has started to make use of resources found in space. In this context, the ET is a space-based resource that is available for use at a very low cost. This resource can be adapted to a variety of specialized uses depending on the needs and imagination of the user. This resource is also something that has finely detailed engineering drawings available to the general public and is constructed out of Aerospace/MILSPEC grade materials (20). This is clearly a very attractive resource.
At the present time, there are several obstacles to the proposed use of the ET in orbit. The most significant problems are orbital lifetime (Skylab syndrome), and potential contamination (foam outgassing). The Skylab syndrome, which is tied to the orbital lifetime of the tank, is driven by the desire not to drop large massive objects on the earth in an uncontrolled manner. This is perceived as the primary problem in the use of the tank on orbit. As a result of the altitude of delivery and its size, the ET has a finite orbital lifetime due to aerodynamic drag and solar drag. Any program that will rely on the tank has to plan to expend reaction mass of some sort in order to keep the tank in orbit. Orbital lifetimes can be as long as years or as short as days depending on the altitude the ET is released, the attitude at which it is stored, and the activity of the sun (56). There are several tradeoffs that can be utilized to solve this problem. First, the residual cryogenics can be used to power low thrust gas burning hydrogen/oxygen thrusters which can boost the ET up to a very long lived orbit over the period of a few days (46). If this is done, little to no recoverable cryogenic residuals will remain in the ET, but the ET will be in a very long-lived orbit. Second, the orbiter can attach a tether, set up a libration with the tank, release at the proper point, and send the ET into a much higher orbit and deorbit the shuttle. There are two problems with this technique. First, it rotates a billion dollar spacecraft and a massive ET around one another on the end of a rope. Second, it has never been done on this scale before.
Contamination is the second major problem with the ET. The entire ET is coated with Spray-On Foam Insulation (SOFI) which will outgas on orbit in space. This pollution may not be tolerable in the environment inhabited by the space station. A potential fix is to strip the foam off the ET after release in orbit. Most of the foam can be stripped by the equivalent of a hot-wire cheese slicer. The remainder can be polished off by leaving the ET ‘hanging in the breeze’ over a period of a month or two exposed to the molecular oxygen present in the proposed orbits (14). There are other problems with the ET in orbit. Having more mass and volume available than is required for currently planned operations in orbit are factors. The partial disassembly of a tank has never been attempted in space and needs to be demonstrated. These problems are all solvable and in some cases may not be problems at all but advantages.
It is clearly in the national interest to do whatever is reasonable to better utilize equipment already purchased. With the External Tank, we have the potential to provide for an enormous expansion in space capabilities for minimal expenditures – extremely low cost and extremely high return. With this in mind, the following suggestions are made:
A. Initiate immediately a program to deliver the ET into orbit. This program should plan for on-orbit maintenance of the delivered tanks and the possible inclusion of Et based structures and modules in the US Space Station.
B. Arrive quickly at an ownership and sales policy. Suggest sales price be set to reflect ONLY the costs of operating the ET orbital storage facility, not an attempt to recover the costs of the entire STS. The rationale here is twofold. First, the ET is essentially a salvaged article. R&D funds expended were spent in making the STS operational, NOT in using the ET. Second, the ownership issue should be solved quickly and early. This will encourage private investors who want to purchase inexpensive space capability and not feed lawyers. The precedent for this issue comes from the experience with the KC-135. This aircraft was constructed to fill a defense need. The sale of aircraft of the same or a similar design was not constrained by any attempt to recover the cost of monies already spent. Boeing and other American aerospace companies sold no small number of these KC-135 class aircraft to airline companies worldwide. The net result was an unexpected creation of new jobs, new industries, and the generation of more tax revenues. The utilization of the ET will have a similar impact.
The External Tank is a very important item in the future of this nation in space. It provides the leverage necessary to make large scale space based operations of almost any kind affordable and therefore able to be done privately. The possible applications of the tank on-orbit are constrained only by the imagination. The ET should be stored in orbit for future use as soon as possible and the sales and ownership questions settled. This action will likely have an impact on this nation’s future in space comparable to Apollo.