III. An Incremental/Flexible ET-based Station Development Process

A. Preface

The approach to space station design presented in this section makes a number of significant departures from most proposals made to date. It is based on the assumptions presented in section II and takes advantage of the ET resources as described elsewhere in the report.

No claim is made that this process is the only way to develop an ET-based space station. Indeed, the variety of options offered by external tanks seems to be much larger than can be encompassed by a report of this size.

The following major features are significant:

- The design offers an opportunity to create a full-scale space station, providing major in-orbit services, large volumes, and economic and scientific returns, at a very early date (perhaps as early as 1990-1991). The path to this station requires few technological break-throughs, and less of the complex system integration that would be called for in an architecture based strictly upon orbiter-carried modules.

- Construction of the incrementally developed structure in orbit may begin several years before the first habitation modules are introduced. This means (a) that economic returns may be seen while those modules are still undergoing ground testing, and (b) that utilities and services will already be in place when the habitation modules are ready for attachment.

- The scheme emphasizes recovery, storage in orbit, and re-use of STS system propellants. The outline includes a large number of possible liquid, gas, and energy exchange systems -- under the assumption that only the best of these will finally be chosen for implementation. It is clear that tons of residual propellants (now dumped in the Indian Ocean) and residual OMS propellants (now wastefully carried home in the shuttle) can open the way to great new capabilities. They form a central part of the "service station" aspect of this design.

The plan has a remarkable range of features which may offer cost savings, such as:

- Architectural independence for subsystems -- allowing users to arrange equipment within a wide variety of large pressure vessels, including ETs -- can release designers from tight space restraints, and the need to carefully nest unrelated devices together.

- Construction of major structural support beams in orbit will not be necessary under this plan. The external tanks can serve as the structural backbone of the space station.

- Once the "service station" is operational, habitation modules may be plugged into already available supplies of oxygen, water, make-up air (needs N2), and power. This means that already developed hardware such as Spacelab or even the mothballed Orbiter 101 (Enterprise) might be used as elements of the station.

Some possibilities presented by this design such as tethered rendezvous and momentum transfer techniques, offer intriguing future opportunities. However, we emphasize that the station process presented here is not fundementally paced or critically dependent upon development of tether technology, but does benefit from its successful development.

An outline of the incremental approach is shown in Figure 2. There are seven development tracks, which should progress in parallel. Seven stages are also shown, leading (perhaps by the early 1990's) to a fully operational space station.

B. Tracks of Parallel Development

To arrive at a final implemented structure in orbit, several parallel lines of technological development should be pursued. Some of these tracks rely upon parallel development in other tracks. In most cases, however, development is not critically paced by other programs. This is particularly true of tether techniques, which add to the value of a heavy space station, but which are not required.

1. Conventional Dynamics: In order to put together a large permanent structure out of ET, techniques will have to be developed for controlling such large objects for long periods. Tethers seem to offer promising ways to achieve orbital stability, turning mass into an asset rather than a liability - (see #7 below), but conventional reaction-control systems must be developed in the meantime. This track presents no unusual development problems.

2. "Front-End" Station Development: This track traces a few of the necessary milestones leading toward the installation (at Stage VII) of the first man-rated modules.

3. EVA - Extra-Vehicular Activity: A number of stages in the design will require manned activity in open space, to inspect ETs, install airlocks, etc. The EVA events are shown in track three.

4. Materials, Science, Biology: There will be many opportunities to pursue science and low-gee technology while the incremental station is growing. Key features are pointed out as milestones.

5. "Service Station": The most important advantages of the design can be found in the services and utilities which are available once propellant reclamation and solar cell / fuel cell power systems are in place. Many options are presented in this track, any one of which might return great economic and scientific benefits.

6. Miscellaneous: A few miscellaneous milestones are mentioned here.

7. Tethers: This important technology promises large payoffs in the long run. However, progress towards full tether capability is not linked to either the timing or success of the basic design.

C. Stages of development - Details and Increments

The ordinate of the process schematic (Figure 2) displays seven stages of development (from top to bottom), leading to the establishment of a major space station by (nominally) the year 1991. Here we go into some detail regarding each of the milestones shown. The year of start for this program is denoted by t'.

In some boxes are small keys representing the uniqueness offered by this process.

Legend U = A capability universal to all space stations. E = A capability that is significantly enhanced by this process. T = A capability offered to any significant degree only by using external tanks. R = A technology or procedure that must be developed in order to use tanks in space.

Stage I. Preliminary Studies. (t' + 2 years est.)

Tasks

Track 1. To satisfy planners that the ET is a structure which can support space construction, instrument packages can be added to one or more ETs in the near future to return data on the tanks' physical integrity after the stresses of lift-off. Behavior of residual cryogenic propellants in the tanks is monitored, as well as rate of evaporation before re-entry. No ET is orbit-inserted at this stage.

Track 2. Engineering studies are begun, leading to the design and start of construction of habitation modules to be orbited at stage VII.

Track 3. EVA techniques are developed looking towards the type of assembly activity called for at later stages. An airlock compatible with the manhole flange on the hydrogen tank is developed, as well as the Aft Cargo Carrier (ACC) and tools for work in low gee.

Track 6. Recovery of debris from the Indian Ocean re-entry of an ET will determine to what extent fears of the "Skylab Syndrome" are relevant to the ET.

Stage II. Dispose of an ET from Orbit. (t' + 3 years est.)

At this stage we envision a task-oriented mission in which an ET is for the first time carried into orbit along with the shuttle. Techniques for controlling the ET safely are tested, including both reaction motors and tether-mediated disposal. (These tasks are not expected to interfere with an otherwise completely normal Orbiter mission, delivering standard cargoes, etc.) This time, the orbit-inserted ET will not remain in orbit.

Tasks

Track1. Test an RCS (Reaction Control System) stabilization package. This might be integrated, pre-launch, into the intertank region between the O2 and H2 tanks, or strapped on during EVA.

Track 2. Continue manned module development on the ground.

Track 3. EVA activity will involve inspecting the tank to determine rate of evaporation and leaking, if any, and the amount of ablative material left over after orbit-insertion, with an eye toward reducing ET weight at liftoff.

Track 5. The ET is instrumented to observe performance of the residual propellants within the tanks. The RCS motors demonstrate settling of liquids at one end under modest acceleration.

Track 7. The first tether experiment is the simplest and easiest. From a small cannister, a hollow spool deployer lets out a Kevlar tether that is attached to the ET during EVA. By very simple Orbiter maneuvers, orbital momentum held by the ET is stolen by the Orbiter, causing the orbiter to rise and the ET to fall. The tether is then cut at the right moment to cause ET re-entry to a proper disposal zone. (The RCS system acts as a backup.) The shuttle benefits from the momentum exchange, and tether techniques are tested.

Stage III. ET to Semi-Permanent Storage. (t' + 3 to 4 years est)

The first ET to be left in orbit will test various new techniques, ideally including actual collection of residual propellants. (We have conservatively left that milestone for stage IV, however.)

The External Tank left in orbit will be orbit-stabilized by a tether-dipole, assisted by an RCS package.

It is expected that by this time the Aft Cargo Carrier (ACC) will be a reality. This could easily be the first test of the ACC. If so, it would travel light, carrying primarily the prototype airlock to be attached to the hydrogen tank.

Tasks

Track 1. Full RCS/booster system in intertank or ACC.

Track 2. ACC proof and test.

Track 3. EVA activity involves deploying and entering the ACC, removing the manhole cover from the H2 tank, inspecting the interior with a camera, and attaching the airlock. The tank is then repressurized to one atmosphere and resealed.

Track 4. Now that a semi-permanent ET + tether is in place, experiments may begin to study the effects of the Lorenz force on conducting cables cutting the Earth's magnetic field. Some of the interesting possibilities this might lead to are tested.

Track 5. During EVA, techniques for on-orbit interconnection of H2 and O2 tanks and lines are tested. (Perhaps the evacuated hydrogen tank will be repressurized from the oxygen tank, rather than from special bottles.) [N2 required by doctrine]

Track 6. New types of remote control devices, teleoperators, "climbing monkey" robots can be tested, using the ET as a test bed.

Track 7. The "kite" dipole stabilization technique is now tested. It should result in the tank assuming a low-drag orientation, extending its orbit lifetime by a factor of up to seven. The study will eventually lead to Lorentz-force technologies, plus new rendezvous techniques.

This might be the time to test "negative momentum transfer," in which, at the end of mission, the Orbiterdoes part of its de-boost by lowering itself via tether, giving up momentum to the ET.

Stage IV. Full Scale, ACC/ET-based Test Bed. (t' + 4 to 5 years)

This stage returns to the ET that had been left in orbit in Stage III. True cryogenic propellants storage and fuel-cell tests are begun. This mission's ET is pre-modified with the necessary lines so that reiduals can be easily collected into an ACC-carried resevoir. These residuals can run a test fuel-cell, whose water by-products can be stored (perhaps in the O2 tank.)

Tasks

Track 1. Link two tanks together, using Orbiter attachment points. Test stabilization techniques.

Track 2. Continue manned systems development on the ground.

Track 3. EVA will involve inspecting the older tank through the airlock left in place. If leakage was slight, small biological packages might be left this time. The systems in the new ACC are tested. Newly developed cutting, welding tools, plus new teleoperators, are tested.

Track 5. Full test begins of residual propellants recovery and storage, use in fuel cells, and stowage of water. Demonstrate pumps, coolers, etc.

Track 7. The kite dipole from the last time is inspected, and possibly replaced by a more sophisticated hook-and-reel system, capable of active compensation features. Full-scale de-orbit test of the shuttle takes place, and savings of OMS propellants are evaluated.

Stage V. Liquids, Gases, Power and Utilities Test. (t' + 5 to 6 years)

Techniques for standard RCS dynamics control are now fully developed. Track 1 is completed.

This is the first mission whose lift budget is seriously affected by the space station program. (The greatest investment in prior missions was time rather than mass or volume.) For instance, it would be desirable at this point to carry up and test the "escape capsule" described elsewhere.

The third tank-plus-ACC added to the growing complex has been pre-modified so that sophisticate liquid and gas transfer techniques may be tried. For the first time these involve transfers between the ET/ACC and the Orbiter itself.

The list of possibilities given in the box shown is long. By Stage V the best of these options will have been chosen on the basis of engineering studies.

Tasks

Track 2. The habitation structures now undergo ground testing. The "escape capsule" (perhaps a stripped down mini-Apollo) is tested. (Unmanned re-entry.) Methods for linking three or more tanks are tested, as well as attaching solar cell modules and experiment packages to ETs.

Track 3. EVA activity at this stage includes inspecting the prior cryo-propellant store and setting in motion the more sophisticated system in ET/ACC #3. The airlock attached to tank#1 is used to introduce packages, lighting, and supplies to convert Tank #1 to a simple Emergency Orbital Habitat (see August report). The Oxygen tank on #1 may be used as a garbage dump (500 cubic meters).

Track 5. Multiple residual propellant transfer and re-use techniques can be tested during this mission. These include stirage of hydrogen and oxygen for use in either fuel-cells or in filling Centaur-type upper stages thereby simplifying the STS-rating of that high-performance stage, by allowing it to be sent into orbit unfueled.

Residual OMS propellants may be stored for use in smaller teleoperators and free-flyers. Hydrazine might be used as a source of N2 for advanced life support systems. The box in Schematic I shows other possibilities worth considering.

Track 7. The hook-and-reel dipole system left attached to ET/ACC#2 may be used to test tether-mediated rendezvous between orbiter and cluster, using the growing mass of the cluster to raise the orbiter's altitude after rendezvous.

Stage VI. Full Scale Utilities Platform Operational. (t' + 6 to 7 years)

With this stage we see inauguration of a complete, unmanned "service station" in orbit. Cryogenic propellants are recovered and transfered at the start of the mission. Excess residual OMS propellants may be similarly transferred and stored at the end. (Tether techniques are developing into valuable supplements, allowing recovery of even more residuals.) (It should be emphsized that this pace of development is considered rather conservative. It is very liekly that Stages V and VI can be combined in a single mission.)

The cluster is now capable of providing low-rate solar power, high-rate fuel-cell power, H2, O2, OMS propellants, and vast, rigid, air-tight volumes. ET/ACC#4 will be pre-modified to provide power, liquid and gas conduits so that these resevoirs will be accessible to the man-rated modules now almost ready for launch.

Tasks

Track 2. The power/gas/fluid interconnects for the manned station are prepared.

Track 3. The "emergency habitat" is upgraded to become a semi-man-rated laboratory. Experimental animals might be left in orbit until the next mission to test the soft habitat. During this mission, time and motion studies in large volumes and materials processing techniques will be possible in a very large shirtsleeve environment.

Track 4. The initial, large-scale Controlled Ecological Life Support experiment is put into place, using water produced in fuel cells out of residual propellants. The water may also be used for prototype chemical processing.

(Recent studies indicate that lifting water to orbit drives the cost of conventional systems. Already, larger amounts of water are available in this design than via any conceivable scenario of which ETs are not a part, and this reservoir will only keep growing.)

Track 5. The "service station" is inaugurated, making available some of the utilities shown in the box in Schematic I. Some, such as the reconversion of water into cryo-propellants, may come later. Each of these techniques has great value by itself. The prime function of the service cluster will be to provide large, redundant reservoirs of basic power and consumables to the monned modules, soon to arrive.

Stage VII. Establishment of Full-Scale, Manned Station (t' + 7 to 9 years)

We see, at this stage, the establishment of a large, safe, manned station. For one thing, the habitation modules need not be built with the depth of safety oriented redundancy that has dominated spacecraft design to date. The availability of several very large shirtsleeve environments (sealed hydrogen tanks each filled with enough air to keep a man alive for months) means that the active hardware themselves may be simpler. The special, man-rated modules need not include large water and oxygen reservoirs, for instance, since these are already available in the ET/ACC-based utilities center.

Another advantage is that ACC-carried modules are not limited to the 15 foot width of the cargo bay. The module added at Stage VII will allow more elbow room even without considering the hydrogen-tank workshops.

Science and technology experimental panels and pallets may be installed within tanks, allowing architecture-independent design. The cost of adding to the station, once the initial stages are completed, should be modest and incremental.

Tasks

Track 2. Establishment of the manned station. One launch at this stage is completely dedicated to space-station establishment, and carries no other cargo. The man-rated modules are now attached to the service station cluster.

There are several conceivable approaches. The modules may be based on the ACC. Or a specially refurbished Orbiter 101 (Enterprise) might be taken up, with no intention of ever returning it to Earth. The Orbiter will automatically come equipped with all the systems to maintain humans, to maneuver if necessary, and to support EVA activity. A spacelab carried in the cargo bay, plus ACC-based structures, could together comprise a sophisticated manned core to station-cluster as a whole, when combined with the servie reservoirs already in place.

Now the utilities and services offered expand with the presence of working men and women. From the day of first habitation, economic returns are clearly seen.

Track 3. Inflatable partitions within modified hydrogen tanks might provide quite safe "soft" man-rated areas for working and living.

Track 4. Now the cluster will have one or two expendable ETs. One of these might be taken apart in materials processing experiments, including the production of aluminum wire for a variety of uses. Shingle-shielding may be used to create a radiation shelter for astronauts to use during solar proton storms.

Major CELS (Controlled Ecological Life Support) and ALS (Advanced Life Support) experiments will have ample room and water without interfering with other station activities. Near-term goals will be ecological studies, animal exposure to long-term weightlessness, and biological or wet chemical air-replenishment. Long-term goals will include "space farming," to still further reduce the mass of consumables that must be carried up in Orbiters.

Track 5. The hydrogen/oxygen economy will be perfected as tons of additional residuals are added at regular intervals. The water resource (which can be accumulated economically in no other way) may attract customers who will bid for the opportunity to use it.

Track 7. The cluster will by now be a storehouse of orbital momentum. Tether rendezvous techniques will save even more propellants. Promising applications, such as Lorentz-force methods, could help secure the station from long-term orbital decay, without use of propellants. Other tether techniques might open a door to still more dramatic opportunities.

Beyond Stage VII.

We contemplate an operational manned station in orbit by Stage VII. As it continues to grow, this station will become the first stop for transfer of cargoes to geosynchronous orbit, since it is here that upper stages may be conveniently fueled.

It is here that studies of humans in the space environment will be made. Many free-flying satellites will be on station, within range of retrieval (possibly taking advantage of residual OMS propellants). Materials and biological science studies will be pursued. External tanks will be gathered and accumulated for multiple future uses.

The growth of this major facility in orbit still leaves open options of developing smaller stations, in other orbits and inclinations. This design, which leads conveniently to a massive space station, afterwards allows the deployment of many small stations based on the lessons learned. This is because, once the development is finished, all one will really need for a primitive but usefule manned station will be an external tank plus a single attached module packed into an ACC. Solar cells can be attached to ETs manually without sophisticated deployment techniques. Five tons of residual propellants can provide reservoirs of oxygen and water. The ACC module will be larger than any spacelab. Such min-stations can be assigned to special functions or organizations, left for later use, or kept in orbit as emergency orbital refuges.

This study's concept does not rely upon intrinsically expensive, tightly integrated modules, but rather on a flexible approach that becomes simpler to develop and modify as time passes. This seems to us an especially attractive feature.

A number of long range ideas suggest themselves. The large shirtsleeve enviroments offered by the main "Tank Farm" station may make it possible to offer visits to space to a far wider range of persons than previously imagined, without impinging seriously on the work taking place in the "core" areas. (This might even extend to offering physically qualified winners of a national lottery a chance for a wavation in space.) On a station as large as the version envisioned here, extra personnel might be housed without seriously interfering with important activities.

The availability of large, rigid tanks also offers, as well, the possibility of creating rotating structures in which significant artifical gravities can be created.

All of these advanced concepts follow as possibilities from the inherent flexibility of the basic, utilitarian design achieved by Stage VII. However, none of these advanced concepts are required in order to reap the substantial benefits of Stage VII itself.