A Partial Shopping List of ET-Related Feasibility Issues

Tasks for Near-Term Applications:

Tasks for Materials-Processing Applications:

DISCUSSION OF ET-RELATED TASKS

Generic Enabling Tasks

G1. Final Disposal of ETs from Orbit After Use. ETs may be taken into orbit for immediate use, for storage and later use, or merely in order to delay ET disposal. (Delayed disposal allows retrieval of residual propellants and ET-mounted payloads, and permits missions that have no safe disposal area in the first orbit.) If the ET is delivered directly to a space station, the station can use the ET or cause controlled reentry using tether techniques. If the ET is to be stored for later use, then provision for on-board drag-makeup and/or controlled reentry must be made, to prevent a repetition of Skylab's fate. One possibility is a solar-powered electrodynamics tether to gradually drive the ET into a long-life, low-debris orbit above 1200 km (from which it can later be retrieved by using the tether as a brake). An alternative -- or backup -- is solid rockets (plus attitude control), either to boost the ET into a higher orbit, or to precipitate reentry if an ET reaches the end of its orbital life before recovery. For cases where delayed disposal is desired, the mission itself must include the disposal operation. Tether and rocket options for the ET-storage case also require analysis.

G2. Evaluation and Control of Outgassing from ETs. The ET TPS foam has been identified as a possible cause of surface contamination (due to outgassing, deposition of the outgassed species on an optical surface, and UV-fixation). Possible cures for this problem include depositing a vacuum-process coating on orbit or putting a non-porous cover on the ET. Such coatings or covers might also enhance thermal control and micrometeorite shielding. Some alternatives include changes in the TPS formulation (perhaps using NCFI instead of CPR), or removing the TPS on orbit (by heating it from within to cause bond failure, as proposed in M1). The major task here is to evaluate the problem and identify what responses may be effective; a detailed study of their practicality might best await completion of some of the other tasks described here. (IRAD work on this problem is underway at Martin Marietta in Michoud.)

G3. Deactivation of the ET Range Safety System. For short-term use of the ET in space, "safing" the RSS arming hardware may provide adequate protection against accidental detonation of the two linear shaped charges mounted in the LO2 and LH2 tanks. However, for longer-term uses, the hazard of detonation due to chemical degradation or micrometeorite impact may be unacceptable. One possible solution is to redesign the ET RSS to facilitate removal of the fuses and/or charges in orbit. Another is to entirely eliminate the ET RSS (but not the SRB RSS) in those missions where on-orbit ET use is desired. (This change is apparently already being considered for some launch azimuths from ETR.) Study of the hazards and options for reducing them should be a high priority.

G4. Orbiting Debris Hazards to (and from) ETs. For use of ETs as part of a space station at altitudes near 400 km, the probability of impact with man-made space debris appears to be acceptably low, and the fragments generated by any such impact will have a rather short orbital life. However, if ETs are stored at higher altitudes (600-1100 km) for later use, the probabilities of impact and the lifetime of the resulting fragments each grow by an order of magnitude, and the cycle of collision and fragment generation could become regenerative. The general space debris effort on the collision hazards to -- and from -- orbiting ETs is also needed, for various ET storage and use scenarios.

Tasks For Near-Term Applications

N1. Scavenging Residual Propellants for Storage and On-Orbit Use. The most obviously valuable ET resources are the residual cryogenic propellants. In addition to over 1.5 tons of pressurized gas, there are typically about three tons of flight performance reserve plus trapped fuel. In addition, the payload margin on each mission (typically about 10 tons, although much less if an ACC or ET-intertank is used to provide extra payload volume), can be used to transport additional propellants (either cryogenics or storables) into orbit. Hardware and procedures for transferring and storing SSME and OMS propellants are needed, for a variety of applications. Direct rendezvous with and refueling of autonomous reusable OTVs or space tugs are one possibility; off-loading of propellants to an orbiting fuel depot is another. Residual pressurization gases might best be stored in ETs until they are used (in low-pressure fuel cells, or in low-pressure gas-fed unpumped rocket engines). Launch and rendezvous strategies that sacrifice the ET residuals to reduce OMS usage should also be studied, because such strategies allow the liberation of the more easily storable OMS propellants for on-orbit storage and use (perhaps in OMS-type thruster assemblies).

N2. Transferring Propellants from the ET into a Dry-Launched Centaur. One near-term application of residual fuel recovery in in loading a dry-launched Centaur. This does not utilize any otherwise-unusable fuel (since the flight performance reserve margins may be used up in reaching orbit), but it greatly simplifies launch abort operations. Using slightly subcooled LO2/LH2 in the ET should provide enough pressure difference between the ET and Centaur to allow unpumped transfer in a reasonable time. Transfer might even begin before MECO (as studied by Convair), to use the 3-gee dynamic head in LO2 transfer.

N3. Unbolting Operations: Manhole Covers, Intertank Access Door, & Intertank. A variety of proposed uses require access to the intertank, or to the interior of one of the propellant tanks. To do this requires removing dozens of bolts. Exerting torque requires some sort of reaction point or restraint attachment at the work station. In addition, a powered socket-driver (either electric or turbine) would be useful to save time and reduce astronaut workload. (For some more ambitious ET applications, the propellant tanks must be unbolted from the intertank structure. This would require removing hundreds of heavy bolts.) Hardware and procedures for such operations need to be developed.

N4. Use of the Intertank as a Supplemental Payload Carrier or Supply Cache. STS payload manifests to low-inclination orbits are generally volume-limited rather than mass-limited. In addition, structural limits limit the payload bay to 65K lbs, even on low-altitude, low-inclination missions where the STS might lift more than this. Getting around the payload volume constraints and orbiter structural constraints is clearly valuable. If the ET is taken into orbit, additional payloads and supplies might be carried in the ET intertank region, which has about half the volume of the payload bay. The intertank cannot compete with the proposed Aft Cargo Carrier in providing orbiter-like payload accommodations, a convenient shape, and easy access. However, for delivering supplies to a space station and similar uses, cheap "second-class" accommodations in the intertank may often be adequate. For initial evaluation of the intertank as a payload carrier, concepts for attaching objects to the intertank structure are needed, along with ways to load and unload payloads. Structural load calculations are also needed to establish payload limits.

N5. Adapting Airlocks to Fit the Manholes. Use of the propellant tanks as pressure vessels is attractive. To permit this, airlocks must be mounted on one or more of the five manholes (2-LO2, 3-LH2) in each ET. Adapter fittings could allow attachment of existing airlocks to the manhole fittings, or new airlocks (based mainly on existing hardware) could be developed. A means of inserting very long objects through airlocks would also be useful; this might be accomplished with an extender tube that fits over one airlock door for such operations. Eventually, an airlock scavenger pump would be valuable, to save most of the 5 kg of air ($15k lift cost) used each time an airlock is cycled. Development and operating costs should be estimated both for existing airlocks with adapters, and also for new airlocks designed for use with ETs.

N6. Determination of Maximum On-Orbit Pressure Vessel Ratings. The propellant tanks of the ET are design to withstand internal pressure loads of several atmospheres (the LH2 tank is tested at 40 psig at room temperature). However, this is for short-term room-temperature loads only; no allowance is made for creep or the possibility of higher metal temperatures, and safety factors are much lower than traditional for man-rated pressure vessels. A study is needed of the maximum internal pressure ratings that should be assigned for several on-orbit conditions, such as: unmanned but attached to or near the orbiter; manned with restrictions; and manned without restriction. Identifying any low-cost changes that could raise the allowable ratings would also be useful.

N7. Interconnection of LH2 and LO2 Tanks On-Orbit. Some scenarios for manned use of the LH2 tank involve venting the LH2 tank, removing one or more manhole covers, installing airlock(s), and repressurizing the tank with O2 or O2/N2. If the O2 pressurization gas in the LO2 tank could be used for this purpose, roughly a ton of payload could be saved. Attaching the necessary plumbing and redundant valving before launch may be possible, but it may create difficult safety problems. The alternative is to make the connection in space, after the LH2 tank has been evacuated. One possibility is to connect the two tank pressurization lines to each other after the ET is separated from the orbiter. Hardware and procedures for pre-launch or post-launch installation need to be developed and compared, to determine which alternative is preferable.

N8. Low-Leakage Valve Seals for Long-Term Gas Storage in ETs. The valves used in the ET, particularly the 17" feedline poppet valves, may or may not have gas leakage rates low enough to allow long-term on-orbit gas storage. Leakage rates should be checked. If the valves are not tight enough, improved valve designs should be identified. An alternative is to design piping caps that could bolt on to the aft ET-orbiter attachment flanges to reduce leakage. The need for such measures and their approximate costs should be evaluated.

N9. Assessment of the Value of Micrometeorite Protection Provided by ET TPS. For use of the ET as a man-rated pressure vessel, a micrometeorite shield is desirable (but, as Skylab showed, not absolutely necessary). Traditional designs use a discrete surface layer; this stops small bodies, and fragments larger ones so their momentum is absorbed by the pressure vessel over a large area. It would be useful to assess the degree of inherent protection provided by that portion of the TPS that remains on the tank after MECO. If additional protection is desired, it might be provided by a surface coating or cover sheet installed in orbit. This cover might also minimize outgassing and temperature excursions. If the TPS provides inadequate micrometeorite shielding, it would be useful to determine the added mass necessary for such a cover layer, both for covers in contact with the TPS, and for covers spaced away from it.

Tasks For Materials-Processing Applications:

M1. Methods of Removing ET TPS in Orbit. Many applications for intact ETs and for ET materials require removal of the Thermal Protection System (TPS) foam that covers most of the ET. Doing this in orbit may not be as difficult as it seems, since the silicone adhesive that bonds the foam to the ET debonds at 350-375F. However, heating the ET to debond the TPS foam will create large amounts of outgassing and loose debris. This problem might be best handled by deploying the ET at the bottom of a 20 km tether before heating it. Then the outgassing will not affect surfaces at the top of the tether, and loose debris will fall into a very short-lived low orbit. The major tasks here are to investigate methods of heating the ET from within (such as aiming low-grade solar concentrators through a manhole, or inserting radiant heating coils), and methods of pulling or knocking loose pieces of foam that remain attached to the ET (such as attaching a vibrating device to the ET). Ways of catching the foam to save it for later use (such as a net below the ET) should also be considered. Effects of the heating process on aluminum alloy properties also must be checked. Moderate heating will not remove ablators, so investigation of ways of removing ablator materials would also be useful.

M2. Concepts for Mobile On-Orbit Metal-Cutting Robots. Many proposed ET uses such as hangars, shielding, and wire formation require cutting the ET into pieces. Speed and reliability do not seem to be as important as safety, cost, maintainability, and debris control. The robot might position itself in any of several ways: it might move along cables attached to ET structures, or move around a cable looped around the ET, or simply hold onto the ET through the cut just made. Cutting debris might be collected or, if the ET is put at the bottom of a long tether, caused to drop into a short-lived orbit. Power might be provided by tether or by on-board solar arrays. If tethered below a space station, it could be controlled from there; otherwise it might be commanded when it is in view of a suitable ground station. The position of the robot on the ET might be obtained by an on-board TV camera, perhaps using patterns of black TPS plugs installed on the ET. (Such plugs have already been used on the ET to evaluate TPS ablation.)

M3. Concepts for a Furnace to Melt Down Scrap Aluminum. Many ET materials applications require that the ET be reduced to molten form (from which it can be easily converted to powder, flake, wire, or ingot). Conceptual designs for such furnaces are needed. Safety may be best obtained by putting the furnace at the end of a tether away from a space station; the resulting gravity field may also simplify operations considerably. The cheapest way of providing heat may be a tracking paraboloid solar concentrator (using a deployable umbrella design), but use of surplus electrical power may also be considered if a solar concentrator creates difficult safety problems. The furnace capacity should be at least one ET (about 10 m3 of aluminum). Electromagnetic pumping of the molten material can be used. Vapor deposition on nearby surfaces may require that operations be viewed from the other end of the tether, by telescope. The primary emphasis in this task is to reduce fairly large pieces of scrap to molten form, but designs that can as an integral feature also make ingots, powder, flakes, or wire would be useful.

M4. Thrusters (Thermal, Chemical, Electric) Using ET Materials. One of the largest mass requirements for a space station is propellants for use on board and for fueling OTVs; this may account for more than half the mass that must be delivered to a space station over its life. Using the ET as reaction mass could double the effective mass throughput of the Space Transportation System, if it could offset most propellant uses. One way is to use solar concentrators to decompose TPS or evaporate aluminum, and duct the vapor to generate thrust; Isp values of over 100 should be possible. Contamination can be isolated by putting the thruster at the end of a tether. Chemical engines might be use fuel-rich mixtures of molten aluminum and residual LO2; Isp values over 200 should be possible. Another option is a tri-propellant engine that uses an Al/H2/O2 mixture; an Isp typical of H2/O2 engines should be obtainable, with twice the propellant mass for a given H2/O2 mass. Finally, molten aluminum can be used in an electromagnetic repulsion gun (as currently under study by Peter Mongeau at EML in Cambridge, Mass). This engine concept sprays molten aluminum onto an insulated face coil, and repels the aluminum by discharging a capacitor through the coil. Eddy current heating can vaporize the exhaust to eliminate solid debris, or the gun can simply be vectored so that exhausted particles reenter rather than remaining in orbit. Isp values over 600 seem possible, with specific power requirements on the order of 6 kW/Newton. Such engines might raise satellites to GEO in a few weeks, and could be used for station-keeping once GEO is attained.

M5. Vapor Deposition Processes for ETs (or Using ET Materials). Coatings on an intact ET (including TPS) may be a simple way to adjust its optical/thermal surface properties. A vapor-rocket such as described in M4 could deposit a 1 micron coating of aluminum in less than an hour, even if deposition is limited by the rate at which the low-emittance deposited surface can reject the heat of condensation without overheating. By continuing to bathe an ET in such an exhaust for a month of so, 1 mm thick layer could be applied. This is enough to contain outgassing and protect against micrometeorites. A top coat of some other material (evaporated or sputtered) could provide the desired surface properties for the final use. The major task here is to design an assembly that suspends and rotates an ET in the view of a vapor generator so as to allow a controlled coating process. The whole assembly can be deployed at the end of a tether during use to isolate contamination. A separate task is to design an evaporator for evenly depositing aluminum on the inside surfaces of inflatable balloons and tubes. By keeping a high-emittance surface outside for heat rejection, deposition rates as high as 1mm/day should be possible. This concept would allow ET metals to be recycled into antennas and structural members of arbitrary size. For use as structural members, alloy uniformity and state (annealed or precipitation hardened?) should be investigated.

M6. Distillation or Other Processes for Separating ET Alloy Constituents. The aluminum alloys used in the ET contain about 91-93% aluminum, with copper, zinc, and magnesium making up most of the remainder. Ways of removing these alloy constituents, either to recover them or to purify the aluminum, might be useful for some applications.