SSI External Tank Report
V – Tethers
V. ET Project – Tethers
Tethers are essentially long flexible ‘ropes’ which connect two bodies. They have been suggested for a number of future space applications that are meaningful when discussing External Tank use in space. The following section will detail possible tether enhancements of space shuttle, space station, and ET based missions.
Tethers are important to future operations in space for several reasons. These include momentum transfer and storage, energy storage, stabilization and control enhancements, controllable microgravity environments, structural strength of a tether based rotating station, and electrical power generation and / or propulsion. Any operation that can take advantage of tethers for these applications will expand capabilities significantly. Enhancements of the energy and momentum storage, structural strength, and attitude control translate directly to increased capabilities without the increased launch costs (13, 56, 84).
The basic tether makes use of the dynamics of long flexible bodies in orbit. Typically, two bodies are connected by a tether of some length (on the order of miles in some proposals) and either swung around one another or left to dangle in a gravity gradient stabilized mode. Tether based stabilization can be accomplished in a gravity gradient mode, a swinging or rotating (librating) mode, or in a drag stabilized mode. The more massive that the body is on either end of the tether, the more energy it takes to move it and disturb the structure. The ET is ideally suited for tethers due to its large inexpensive mass on-orbit. This allows it to be used as a counterweight or momentum storage device for either static or librating tethers. It also is suited to tethers because it is the strong structural member of the STS stack. In an ET based tether application, tethers can be attached to the SRB attach points and the Orbiter attach points and the ET can be spun up (50, 56). Tensile loads on the ET are far lower than those experienced during launch. This allows a substantial safety margin without tank modifications.
The tether has been flown on manned missions in the past (13, 50). There were tether experiments carried out on the last two Gemini missions. Gemini XI used a 100 foot long tether between it and the Agena to conduct artificial gravity experiments at two different rotation rates. Gemini XII used a similar tether to conduct gravity gradient stabilization experiments. Both sets of experiments were extremely successful and the results warrant further study.
The next planned tether experiment is a proposed Italian subsatellite designed to investigate the upper atmosphere from LEO. The concept is to deploy the satellite from the orbiter on the end of a 50 km (over 30 nautical mile) long tether and sample the upper atmosphere. This would accomplish several objectives. The primary science objective would be to study the upper atmosphere. Engineering objectives include the behavior of a very long tether in space, dynamics of the mass-tether combination, and the testing and operation of a winch in space. This is an important proof of concept experiment that should lead to increased emphasis on tethers in the future (56).
A long object will tend to stabilize with its long axis pointing at the center of the earth. The combination of forces will lead to small artificial gravitational forces on either end of a stable platform. This artificial gravity is equal to 1.5 times the change of gravity gradient in LEO at about .4 miligee per km. The extra third is due to centrifugal force gradients induced by the end masses attempting to pull into different orbits away from the center of mass. The presence of these forces also causes a tension on the tether. This force doubles as the length of the tether doubles (13, 56).
In addition to a stationary tether, artificial gravity can also be induced by rotating the mass-tether combination. This has been proposed in several space station applications including Dr. Gerard K. O’Neill’s original High Frontier proposal (13, 37, 56, 84, 89). This proposed using rotating hydrogen tanks in a tethered facility 200 meters in diameter at a rotation rate of 3 rpm to induce internal gravity for habitation from .7 – 1.0 G.
There are several items of interest with a rotating station. The first is that there are indications that the body cannot tolerate rotation rates much greater than 1 – 3 rpm (89). Thus the station should be kept large enough to keep the rotation rates small. The second concern is to minimize corriolis forces (13). This is also accomplished by keeping the station large. The third concern is operational. There are docking problems associated with arriving at and leaving a rotating station. The typical proposal is to make the center of rotation the docking port. This is feasible and should work. The last concern is with the very real problem associated with adaptation to zero ‘G’ by visiting crews. If a station is designed for habitation which includes living quarters with artificial gravity, there may be problems when the crew transitions to zero ‘G’ for the ‘workday’. This may or may not be a problem. However, the unhealthy effects of long-term weightlessness on the human body are serious enough that this solution should seriously be studied.
Momentum storage and exchange are related in this context to the raising and lowering of orbits. Due to the forces discussed above, if a tethered mass is released from a stationary orbiting station, it and what it was released from will assume stable orbits with the closest approach seven times the length of the tether. If the release wro from rotating tether, the closest approach of the final orbits would be as much as 14 times the length of the tether (13, 56). The equation for mass and orbital radius is also on the diagram below.
The orbit of any facility or structure can be raised or lowered by momentum exchange procedures alone. If the operation is conducted with enough planning, the station may not require thrusters to keep it orbit. For example, if an orbiter arrives at a space station, it uses a certain amount of OMS fuel to get there. If the orbiter meets a tether tens of miles below that same station and is winched up into the station, that same OMS fuel becomes excess and can be transferred to the station for OTV refueling (16, 37). When the orbiter leaves, it can be winched down to the capture altitude or even lower and released. If it is released far enough away from the station, no OMS burn will be necessary for reentry. The net gain in this operation is the OMS fuel excess delivered to the station. The net loss is the time that the station spends in the lower net orbit due to the winching up of the orbiter.
The use of momentum transfer by the space station, orbiter, or the orbiter/ET combination would result in overall savings in onboard fuels, possible opening up of higher orbits to the orbiter, and insertion of the ET into a high orbit without expenditure of any residual cryogenics. If the ET and orbiter are tethered together, spun up to an appropriate rate, and released at the correct time and attitude, the ET orbit could be raised 40 x 560 km and the orbiter could have its orbit lowered 10 x 130 km, or low enough to deorbit. Loads on a tether in this case are less than 4,000 lbs tension. This is well within existing tether material capabilities today (50, 56).
The final possible application discussed here involves the use of a tether as an electrical device. In addition to moving through space, bodies in LEO are also moving through a plasma in a magnetic field. As a result of these effects, there is a net potential difference between the bodies on either end on the order of 200 volts/km. If the tether is made of a conducting material and properly connected, a substantial current could be drawn for onboard power. A proposed design for power has been made that will deliver between 5 – 65 kilowatts continuously (13, 56, 84).
As with any other application, there is a tradeoff with this form of electrical power generation. It induces additional drag on the station by interaction with the magnetic field during power generation. This means that the momentum of a tethered station becomes an electrical energy storage device. As electrical power is generated, the station’s orbit decays. Any method of supplying additional mass to be released below the station thereby becomes a method of supplying future electrical requirements. Calculations of a station that will lower and then release a visiting orbiter 150 km below the station will generate over 9,000 kilowatt hours (kwh) of electricity without orbital decay (56).
There is a reverse application to this power generation scheme. The Alfven Engine has been proposed by Drell et al. as a method of orbit maintenance, plane changing, orbit raising, maneuvering, and excess energy conversion (13, 56, 84). If excess electrical power exists, a current flow could be forced against the potential to work against the magnetic field. Theoretical efficiencies of 50% have been suggested. If this proves feasible, there exists an orbital engine with a better power to thrust efficiency than present ion engines. Another technique would be to store excess photovoltaic energy by raising altitude through the Alfven Engine during daylight periods of the orbit and use the excess to power the station during periods of darkness in the orbit.
There are significant tether related enhancements of the basic shuttle mission. As has been mentioned in earlier sections, the momentum exchange technique is clearly the most attractive when combined with the ET. There are several variations to this theme that are of interest when applied to shuttle missions.
As mentioned previously, the timed release of the ET into the proper orbit can be used to transfer the orbiter and / or its payloads into orbits unreachable with current techniques (50, 56). There are several choices in this regard. The first is the reentry of the orbiter from altitude while lofting the ET into an orbit which will not decay in a short period of time. The second choice is to loft the orbiter into a higher orbit by the controlled release of the ET from a rotating ET / Orbiter system and deorbit the ET after release. This is a possible use of the ET, but may not be the best choice for actual applications. A third choice would be to release the ET in either direction from a non-rotating system. A fourth possible use would be to release a payload from the orbiter itself while the orbiter is still connected to the ET. The advantage of this technique is in two areas. The first is that a swinging release coupled with a burn by the payload at release is even more effective than a swinging release. The second advantage is that the combined mass of the Orbiter/ET is more than that of the Orbiter itself and therefore allows more energy to be put in the loft of the payload.
There are significant advantages in placing the tether in a space station and only launching the orbiter partway to the altitude of the station. This is referred to as Tether Mediated Rendezvous (16). The basic idea is to launch the STS to a highly elliptical orbit that intersects that of the end of a space station mounted tether 41 – 47 minutes after launch. The orbiter will rendezvous with the tip of the tether using the ET nose itself as a docking probe for the Orbiter/ET. After the docking is complete, the stack is winched the remaining 40 – 55 km up to the station. The advantages include: the delivery of several tons of OMS fuel to the space station holding tanks fur future use in OTV operations; the delivery of the ET and Orbiter to the station; the settling of the residual cryogenics and possible scavenging of them during the winching procedure into tether mounted holding tanks. All of these advantages translate directly into fuels delivered to the station.
There is also an additional safety factor in the use of the ET as the docking probe. The ET nose will protect the nose and windows of the orbiter from possible impact with the tether tip by being 50 feet closer to the tip than the orbiter itself.
This appears at first glance to be a procedure that requires precise timing. This is partially true in that it does inflict another constraint on the launch timing itself and continue to narrow the launch window. The arrival at the tether is timed based on midcourse corrections flown enroute and the venting of residuals as they slowly boil off for thrust through cold-gas engines. The orbiter arrives at the tip of the tether with sufficient OMS fuel to conduct several abort scenarios. The mission rules should always allow the orbiter to carry enough OMS fuel to complete the rendezvous in case the tether technique does not work for whatever reason. The strategy also allows the orbiter to ‘brute force’ the departure and reentry by the use of the OMS engines. It is important to point out that there are three levels of success in this plan. The most successful mission would be one that allows the orbiter to dock with the tether tip with a minimum of hovering, delay, or the requirement to ‘wave off’ the docking for another orbit.
This delivers the maximum OMS fuel to the station in the minimum time. An intermediate success would be an OMS burn for rendezvous with the station and a tethered release for departure. This allows the scavenging of the OMS fuel necessary for an unaided release for the station reboost. The most unsuccessful mission would be precisely what is planned for current shuttle to station missions – unaided arrival and departures (16).
There are several tether and ET based applications that are possible in the design and operation of a space station. These are all combinations of previously mentioned applications that will enhance the operation of the space station.
The microgravity applications of bodies on tethers can be utilized to store liquids so they will be available for use. The problem with weightlessness is that normal liquid flows will not take place. It will likely be very important to store the cryogenics, scavenged OMS fuels, water, or any other liquid a certain distance above or below the station for use. This takes advantage of the combined effects of the gravity gradient and centrifugal forces to settle the liquids on one end of the storage facility where they can be normally pumped (13, 56).
Tank storage on orbit can also be enhanced by the use of tethers to minimize the cross sectional area ‘into the wind’ (56). The intention is to hang a mass below the ET or the ET farm on the end of the longest possible tether. This serves several needs by putting the ETs in the best possible attitude relative to the ‘wind’ on orbit. It also puts them at the best possible altitude relative to the ‘wind’ by placing the ETs at the highest portion of the orbit above the atmosphere. It places a massive object in space for possible use by visiting orbiters. The orbiter can rendezvous and dock with the end of a tether carried on the tank farm and run the same momentum exchange as was discussed earlier for a space station. It can deposit the ET it carries at the ‘tank farm’ and winch itself to the lowest possible altitude for release into a reentry trajectory.
In a paper by Dr. Giuseppe Colombo et al., the use of multiple tethers and platforms made of connected tanks is suggested as a way to fly a space station (56). The suggestion is to construct massive platforms so as to minimize the cross sectional area into the ‘wind’. The tasks performed on the different platforms can be designed to take advantage of the separation of the two units. For example, the top platform can launch and service OTVs, launch satellites to higher orbits, conduct scientific observations higher above the atmosphere than the lower platform. The lower platform can conduct operations that tend to contaminate the environment around the station such as materials processing, tank stripping, disassembly, cutting, and melting.
Because it is lower in altitude and inside more of the upper atmosphere, emissions at this level will not pollute the upper level and will reenter the atmosphere sooner (15). A lower level would also be the location of a orbiter retrieval and payload reentry operation. The space equivalent of an elevator would operate between the two levels. There would be low gravity at both levels, so any applications that require zero gravity would need to be conducted at a separate station or at the center of mass of the station. A typical schematic is detailed below.
An additional space station architectural application would be to use a rotating station to induce artificial gravity for the station crew (14, 89). This would be advantageous in the preparing the crew for the gravities on the moon, Mars, or elsewhere. Additionally, the spinning ET based station is easy to construct, structurally sound, and capable of stopping the deterioration of the human body under weightlessness if the gravity level induced is high enough.
There are additional tether related uses that are farther from realization. These include the use of the ET as a landing module deposited on the surface of the moon or Mars from the tip of a rotating tether (14). The concept is to use the momentum transfer to kill most or all of the velocity differential between the ET and the surface and load the ET with whatever is required for the lunar or Martian base. An additional tether use would be the ‘Rotating Skyhook’ (84). This uses a long tether for launch and landing of small (or not so small) payloads on the lunar or Martian surface. The ET application here would be as a counterweight for momentum storage.
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