SSI External Tank Report – Executive Summary
This report will review possible applications of the External Tank (ET) of the Space Transportation System (STS) in orbit. Enhancements of the space program through ET utilization in orbit will be covered in depth. Problems will be reviewed. Recommendations will be made. This report is intended to make a coherent case for the use of the External Tank in the American space program.
I. Tank introduction
The ET is carried almost to orbit with the orbiter and jettisoned with approximately 98% of the energy necessary to insert it in orbit. When jettisoned, each ET carries internally an average of 15,000 pounds of residual cryogenic fuels. These residuals are available for scavenging from the tank in a variety of scenarios. The availability of cryogenics already in orbit can potentially fuel planned OTV operations at a cost far lower than if the cryogenics are carried aloft in a tanker version of the orbiter.
The ET mass is over 69,000 pounds. Of this mass, there are approximately 53,000 pounds of aerospace grade aluminum. This aluminum can be cut, melted, powdered, welded and manipulated to suit any number of present and future structural needs. If the tanks are partially disassembled in orbit, the pieces can also be reassembled in the construction of large structures.
The oxygen and hydrogen tanks making up the ET provide two factory tested pressure vessels that are two to five times larger in volume than any space station yet flown or planned for the future. These large volumes are clean and able to be entered through inspection manholes in the respective tank domes. The on-orbit adaptation of the respective tank interiors for habitation, storage or maintenance facilities will require minimal time and effort.
There are two major problems with the use of the ET in orbit. the first and most critical is orbital maintenance. This is a result of the desire not to randomly drop large bodies on the surface of the earth from space. At typical STS orbits (160 – 220 nautical miles), the orbital lifetime of a tank inserted into a parking orbit can be measured in days to months. A plan to use the ET in orbit must address this problem. A quick solution would be to install small thrusters that use the boiloff of residual cryogenics to insert the ET into a very long lived (200 – 500 nm higher) orbit. Other solutions to this problem are possible and vary depending on the planned on-orbit use of the ET. They are also not particularly expensive. The second problem is possible contamination due to outgassing of the Spray-On Foam Insulation (SOFI). This may prove to be a pollution problem for a small number of proposed space base operations. However, it will require further study.
There are several relatively inexpensive enhancements to the ET that can be purchased that will enhance STS operations. the most important of these is the Aft Cargo carrier (ACC). The ACC is constructed using ET tooling and attaches to the aft end of the hydrogen tank. Cost of the ACC is between $150 – 250 million and it can fly three years after the go-ahead is given. The ACC is designed for minimal impact to the ET, orbiter and operations. It provides an additional cargo volume measuring 27.5 feet by 20 feet for payloads. This is valuable to operations because it deals with the volume restriction imposed by the orbiter payload bay. In other words, there is normally additional mass to orbit capability available in each STS launch. A typical example would be a Spacelab flight. The cCC can carry additional payload or primary payload to orbit. This gives the orbiter additional payload capability that can be sold to paying customers for minimal cost.
II. Tin Can Uses
The ET can be used as a ‘tin can’ in orbit for a number of applications. These include storage facilities for liquids, gasses and prepositioned vehicles. The ET/ACC combination can be launched with the ACC as a fully functioning manned space station. This type vehicle has an enormous expansion space inside and outside the ET itself for a variety of orbital applications. The ET/ACC as a space station can also be flown with a single shuttle launch. This capability will allow any number of potential customers to purchase independent space stations for the cost of two or three generic communications satellites. Potential customers for this capability include DoD, corporations, foreign nations and private consortiums.
The ET can also be used as part of any space station. It can be partially disassembled to make a hangar or easily turned into a space station habitation module of a far larger volume than any past, present or future space station module. The oxygen tank can be turned into a liquid or gas storage reservoir.
The cost savings by ET utilization in these operations are unspecified at this time. However, any specially designed space station module which will fill the needs addressed above must be compared against the ET in tow ways. the first comparison is launch cost. With the ET, you get a large rigid body already in orbit. You must lift anything else at $ 2,000 per pound. The second comparison concerns possible future expansion of the structure. If a future expansion is being planned, then the costs of R&D and on-orbit construction from the STS payload bay during an EVA must be compared to the cost of on-orbit modification of a body that is already in space. This analysis should show in most cases, that the adaptation of an orbiting ET will provide enormous cost savings to the program.
In addition, the use of the ET as part of a manned space effort will give the program a new perspective. As soon as the ET is inserted into orbit, the program has made large, massive structurally strong bodies available to prospective users at a very low cost. The volume restrictions for manned habitations are removed. The storage limitations for liquids and gasses are removed. This means that a specially designed structure does not need to be planned, sold to uninterested congressmen, launched, and constructed over a period of several flights. The planning turns to an emphasis on the adaptation of structures already in orbit. This adaptation will take a bit more EVA time (at over $40,000 per hour), but the savings in launch cost alone will more than cover the difference.
III. ET as a Propellant Resource
Analysis of future requirements for space based operations show the largest mass requirement is for fuels. These include OTV operations, satellite launch and recovery, and space station orbital maintenance. NASA has a requirement for 2.5 million pounds of propellants over the next ten years. Analysis based on this requirement show the scavenging of residual cryogenics from the ET can fill up to 92% of the requirement at a total cost saving of $3.5 billion over the period. The saving s comes primarily from the launch cost savings. The scavenging operation can be performed in a variety of ways. These include scavenging into the orbiter after MECO, scavenging into the ACC, scavenging into the space station after rendezvous, and scavenging into a free flyer. Each tank will provide an average of $30 million worth of residual cryogenics available for scavenging.
Another use of the ET as propellant is to powder the aluminum and use it as reaction mass in an Aluminum – Oxygen rocket for OTV applications. Analysis of OTV traffic models show that current technology engines (RL-10) are sufficient if the OTV fuel requirements do not exceed the availability of scavenged cryogenics. However, an Aluminum/hydrogen/oxygen engine rather than an advanced oxygen/hydrogen engine appears to be the most cost effective choice. This is once again due to the savings in launch costs because 40-5fˆ of the reaction mass is already in orbit as tanks. The analysis of the Aluminum rocket engine includes the $1-2 billion R&D costs, the high production costs, and the cost of flying a processing plant to grind the tanks into powder. Even with the inclusion of all these additional costs, the aluminum engine is potentially far cheaper than an advanced cryogenic engine because the mass requirements to orbit are far lower. Each tank used in this application is worth about $107 million in powdered Aluminum (computed at $2000 per pound to LEO).
The problems with these type engines are known. It is not clear at this time why the choice not to develop these engines in the 1960s was made. There were fairly serious problems with Propellant transport that may be solved by zero ‘G’ in orbit. There was also a problem with the time constraints of the Apollo program. The scientists investigating the Aluminum engines are well aware of the past history of the engine. They feel that the problems are solvable and that the Aluminum engine has great potential for OTV applications.
The ET can be used in any number of structural applications. These range from partial disassembly to complete melting and refining operations in orbit. The ET can be partially disassembled and reassembled into a variety of rigid structures. The tank domes can be removed and the hydrogen tank barrel can be reattached end-to-end to construct long rigid tubes. The tank can be cut into 5 feet by 60-80 feet long strips using known technologies and welded into any desired shape. Once again, welding is a known technology that has been tested in space.
The ET can be completely melted using electrical or solar methods. The melt can then be used to extrude structural members such as channels, I beams or rods. It can be used to make thin metal films by vapor deposition processes. It can also be used to make thin metal shells by an inflation technique. The shells and other manufactured structural members can be used for construction. It can also be powdered and used �ar casting and forming operations.
Another use of the ET is as a strongback or a testbed for the construction or anchoring of large structures. The advantage in doing this is that the ET is far more massive and structurally sound than planned space structures. This is because it is the structural heart of the STS during launch. A typical strongback use would be the construction large antennas on the ET. The mass and stability of the tank is also an advantage. Due to gravity gradient effects, the ET will tend to stabilize with the long axis pointing to the center of the earth. Any structure that can use the ET as a base or an anchor will require less active attitude control systems and thus be cheaper to build, fly and operate.
The use of tethers with the Et also provide significant advantages to the future space program. These advantages include artificial gravity, momentum exchange, electrical power generation, electric propulsion, and significant enhancements in shuttle and space station missions.
The physics of tethers allows artificial gravity to be generated in two ways. First, two tanks can be attached to each other by a tether and stored in a gravity gradient mode. This is useful in liquid storage applications. Second, the system can be made to rotate. Artificial gravity levels of 1 ‘G’ can be induced by a system 200 meters in diameter rotating at 2-3 rpm. This artificial gravity will negate he undesirable effects of long term weightlessness on the body and lengthen crew stays on station.
Momentum exchange is also useful. This can be done with either a static or rotating system. Release of an object from the end of a long tether will insert it into a much higher or lower orbit. This could lead to the use of a tethered release from a space station for a shuttle deorbit without an OMS burn. A singing tethered release could also be used to insert the ET into a high orbit that is long lived and drop an orbiter to a reentry. This exchanges momentum only and uses minimal fuel. A swinging release can also enhance the payload carrying capability of vehicles to higher orb(is (including escape orbits). A swinging release ‘steals’ momentum from the system and can be used to launch a far more massive payload away from LEO than could otherwise be launched with the same onboard propellants. Savings is in fuels and the advantage is that a more massive and more capable payload is possible.
Electrical uses of a conducting tether include the generation of station electrical power and the orbital raising and lowering of the station. A properly designed tether can generate electricity by interacting with the magnetic field. This induces drag and will lower the station. A current can be forced through the tether with excess electricity and the tether can generate a net thrust. this Alfven Engine can be used for orbital maintenance, orbit changing, and energy storage through momentum transfer. Efficiencies are higher than ion engines and no propellants are required.
Shuttle and station mission enhancements with a tether include additional payload capability to orbit, the saving of OMS fuel by a tether mediated rendezvous with a station, and storage of tanks and liquids. The tether mediated rendezvous can enhance the payload to station capability of the orbiter. It can also allow the scavenging of excess orbiter OMS fuel to the station for station requirements. Storage of liquid in a tethered tank takes advantage of the gravity gradient to store liquids where they can be pumped using conventional methods.
The Et in orbit can also be used in a variety of scientific and military applications. The scientific uses include the use of the ET/ACC combination as a way to launch large mirror arrays for telescopes. The hydrogen tank can be used as an affordable high energy observatory of a far larger size than planned. The exterior of the tank can be used as the structural base for large antenna arrays. The tank itself can also be used to study the interaction of plasmas with bodies in orbit.
The two tanks of the ET can be used to perform biological and life science experiments in space. The large volume of each tank can be used to perform experiments in farming, genetics, and waste management. The large size is advantageous because it provides significant biological inertia. this means that in the event of a problem, the biological system can be changes before the entire system dies. Farming becomes possible in the large volume available. Cost savings here are based on the launch cost of food and consumables produced in space as compared against the cost of launching these consumables. An orbiting ‘truck farm’ becomes possible.
Additional life support advantages are the use of the ET as a passive lifeboat. If ETs are inserted to long lived orbits while pressurized with oxygen or an air mixture, they can be entered and used by a crew in an emergency. The large volume of air can be used for weeks to months for life support without active equipment.
Military uses are related to the use of the ET as a cryogenic storage facility, space base, or ‘Coast Guard’ type operation in space. Ets can also serve as decoys, battle stations, ‘junk’ ASAT, and military space stations. The ET/ACC based space station for military purposes is something that would be affordable and attractive to those interested in manned military operations.
The use of the external tank in the American space program is potentially an enhancement that will have an impact greater than the decision to go to the moon. The reason is that the decision to insert the ET into orbit will make resources available for purchase at a cost far below that of the basic launch cost of $2000 per pound. The volume available internally is great. The metals available for use are aerospace grade aluminum. The technical drawings are all in the public domain.
Actual overall return based on the decision to regularly insert the ET are unknown. There are two numbers that may hint at the overall value. They are presented on the graphs to follow. he first number is the value of scavenged residual cryogenics at a rate of 12 and 24 launches per year for ten years starting in 1986. This number is based on the $2000 per pound launch cost. The second number is the value of raw aluminum at 53,000 pounds per ET at the same two launch rates and cost per pound to orbit. Note that each flight which inserts an ET into LEO is worth $30 million in residual cryogenics available for scavenging and $106 million in Aluminum.
Additional returns from the decision to use the ET in space depend on the actual application. Intangibles such as increased commercial interest and business expansion into space due to relatively inexpensive facilities are very difficult to measure. The increased capabilities of a program that extensively uses the ET are also difficult to measure monetarily, but they certainly are extremely valuable in the long run. There is also no way to measure the positive value to the space program that suddenly becomes rich in terms of mass in orbit, structures in orbit, and reaction mass.
The two analysis of actual tank applications involving hardware – the ACC and cryogenic scavenging – have not presented any unpleasant surprises. Both applications appear to not only possible but are potentially very valuable to those interested in affordable space based operations. At this time there appear to be no unknowns. Other than the materials processing facilities and the Aluminum engine, there are no applications that require any great investment of R&D funds to accomplish. With few exceptions, every suggested tank application appears to be possible. These proposed applications also appear to be significant improvements in the capabilities of this nation in space.
The figure that follows lists hardware R&D, and questions to be answered for the future of the ET in orbit. Most of the major questions have been answered. Most of the R&D is already being done as part of the space station program. Most of the necessary hardware either already exists or is in development for other space-based operations.
Total Value of scavenged residual cryogenics (12-24 Shuttle flights/yr)
Total Value of ET aluminum (12-24 Shuttle flights/yr)
The use of the ET in space is limited only by the imagination. Making large massive objects available to customers will open space base operations to every interested party at a reasonable cost. The overall possibilities presented by flying the ET are diverse and extremely valuable to all concerned.
This report concludes that it is in the best interest of the United States to take the ET into orbit and store it there permanently as soon as possible. This will make large scale space operations affordable. Three general recommendations follow:
1. Quickly insert the ET into a permanent storage facility in a relatively high earth orbit.
2. Arrive at a pricing policy that covers the cost of orbital storage and maintenance. Sales cost should cover only the cost of storage. The purchase agreement should simply define ownership issues. the government should not attempt to recover all costs of the last 25 years of spending on the space program through sales of the ET. The intent is to expand space capability. This is best done commercially. It can not be done if the sales costs are set arbitrarily high.
3. Design and fly the ACC as an enhancement of the STS. This will provide additional cargo space at a very low cost.
The external tank is an extremely flexible and potentially valuable enhancement of the space program. There is no other action that can be take today to expand space capability that will cost so little and provide so much potential. The choice to use the ET will be a welcome addition to the American space program that will pay for itself many times over the in the years to come. The tank should be flown to permanent orbital storage for future use early and often.