Note: This paper was originally written as a much longer thesis in 1985 and was repeatedly shortened to the version here, of February 1989, cleaned up just a little in 1996. I had the pleasure of presenting the paper as an 18-viewgraph pitch to several groups in the Denver area and even represented the company by presenting it at the PISSTA Conference in Beijing, in June, 1987, a year before the Tiananmen Square massacre. A decade later, by September 1996, orbiting ETs is still possible, but several problems have made themselves known, as discussed at the end.

ABSTRACT

The exploration and development of the solar system may be furthered after the Space Station era by use of Space Shuttle External Tanks. Over the next decades 250 external tanks can be flown to earth orbit. Modification of tanks before flight would enhance utility on orbit without degrading the external tank's primary use as strongback and fuel tank. This paper describes the tanks, proposes modifications, and reviews on-orbit setup and assembly configurations. Included are two pages of illustrations.

SHUTTLE TRANSPORTATION SYSTEM

The American Shuttle Transportation System consists of three major elements: the Orbiter, which re-enters the atmosphere at the end of a mission and glides to a landing on conventional aircraft runways; two Solid Rocket Boosters, jettisoned early in the flight ascent; and the External Tank, which provides fuel for the orbiter main engines. The tank is carried to a near orbital altitude of 150 kilometers and to 98% of orbital velocity before being jettisoned and allowed to tumble and burn up in the atmosphere.

The external tank consists of two pressure vessels, the smaller forward tank for liquid oxygen, and the aft tank for liquid hydrogen. These are connected by a structure called the intertank. The external tank is 28 feet in diameter, 152 feet long, and weighs 67,000 pounds empty. It is of stressed skin construction in aluminum alloy, welded and bolted together. The exterior surface is covered with a foam insulation.

The 5 story tall oxygen tank has a volume of 19,500 cubic feet, that of a 2400 square foot house. Its nose contains equipment designed to use gaseous oxygen remaining in the tank after separation from the orbiter to impart a tumbling motion on atmospheric re-entry to minimize the wreckage footprint. Pressure is limited to 1.6 atmospheres or 24 psi. The 10 story high hydrogen tank has a volume of 54,000 cubic feet, over twice that of the oxygen tank. Tank pressure is limited to 2.4 atmospheres or 36 psi (1).

Solid rocket booster thrust loads feed into a beam which crosses the unpressurized intertank between the tank end domes and distributes those loads into the intertank ring structure.

Using a direct insertion launch trajectory, continuing into orbit with the external tank attached, would allow the shuttle and tank combination to attain the standard circular 160 nautical mile orbit and allow a small payload increase. Residual fuel would remain in the tanks, fuel which is now discarded with the tanks. Currently an Orbital Maneuvering System engine burn is required for orbit insertion. This would not be required if the tank remained attached to the orbiter, but a longer than normal OMS burn would be used at orbit apogee to circularize the orbit (2).

Maximization of orbit altitude may be accomplished by continuing the main engine burn until fuel exhaustion. Sensors currently built into the orbiter hydrogen feedline cut off oxygen flow in the event of hydrogen depletion, preventing engine damage from an oxygen rich shutdown. The sensors can be used to shut down the engines, maximizing engine burn time. Remaining hydrogen fuel may be vented through the main engines, preventing the torque reaction resulting from venting through the fueling ports on the underside of the orbiter (3). Residual oxygen may be retained for use as a breatheable air component, as will be shown later.

PREFLIGHT MODIFICATIONS

Any spacebound external tank can be modified before flight to enhance on-orbit utility without degrading its primary mission of structural strongback and shuttle main engines fuel tank.

Adding three rows of fittings to the tank exterior, with twelve equally spaced fittings per row, allows:

The rows of fittings should be placed at both ends of the intertank where it splices to the oxygen and hydrogen tanks, and at the aft end of the constant section of the hydrogen tank. These areas are already reinforced to accept externally applied loads.

The tank exterior can also be fitted with circumferential lines of handholds around the tank adjacent to the fittings, and at the forward and aft hatches. Two nose-to-tail lines of handholds on opposite sides of the tank would connect these, spaced for easy movement along the tank surface. Handholds would lie flush with the insulation and be deployed to attach Skylab-type shoe cleats and clip-on safety lines.

The oxygen tank nose cone should be redesigned for easy removal in space, and the oxygen tumbling subsystem eliminated. The latches connecting the tank and orbiter would be redesigned to release with zero velocity. To retain the option of emergency early tank release, the latches can be supplemented by pyrotechnic thrusters similar to those used on the solid rocket boosters to ensure separation from the tank. Thrusters can induce tumbling by differential burn time and remain unarmed for normal on-orbit release.

Existing foam tank insulation is approximately 1 inch thick and is not a meteoroid or debris shield. Kapton or aluminum shielding, integrated with the tank insulation to optimize efficiency, thickness and weight, could be designed for preflight installation.

Each tank is now fitted with forward and aft access hatches with 36 inch diameter openings. Existing hatch access covers bolt to the outside of the tank end skins and carry hoop tension loads (4). Changing to a hinged internal mounting would keep hatches closed under pressure and permit easy interior access, but would require strengthening the area around the hatch openings. To allow for docking and grouping tanks, the aft hydrogen and forward oxygen tank hatch mounting plates should include docking flanges similar to those planned for Space Station.

A tunnel built into the intertank could allow access between the oxygen and hydrogen tanks. The distance between the tank domes is 4 feet at the center. The aft oxygen and forward hydrogen tank hatches would be realigned with their centers offset 30 inches from the tank axis. The small tank-to-tank distance would prevent standing in the tunnel with both hatches closed. Enlarging the tunnel outward to accomodate two or three people would provide a limited safe haven in the event of pressure loss in either large tank. Making the tunnel into an airlock could be accomplished by adding a hatch at the intertank skin.

Opposite the intertank tunnel is space to mount vacuum pumps and valves to permit evacuation, charging and pressure equalization of the tanks and tunnel. In the areas between the pumps, beam and tunnel, space is available for small storage tanks similar to those used on the orbiter for storage of propellants and breathables. These tanks would be for storage of air when either of the large tanks or the tunnel were evacuated, and could be charged with liquid nitrogen before launch and mixed with the residual oxygen after the tank is serviced on orbit.

The pumps, storage tanks and tunnel would be mounted to the intertank skin. The tunnel would include metal bellows at the tank hatches to minimize unwanted load coupling between the three pressure vessels when at various levels of pressurization or during launch and ascent. All intertank equipment will fit within the external tank outer envelope and would not reduce tank volumes.

The shuttle orbiter uses an earth sea level atmosphere mixture and pressure. Sea level equivalence within the external tank would pressurize the oxygen and hydrogen tanks to 61% and 41% their rated values. Reducing internal pressure to 0.8 atmosphere, 12 psi, the equivalent of 5000 feet altitude as found in the Colorado Front Range cities of Denver and Colorado Springs, would not degrade crew capability. Standardizing pressures aboard Shuttle, Space Station, and within external tanks inhabited in space at the lower pressure offers several advantages:

Estimated weight of the modifications, excluding interior decking and equipment carried on a space-and-weight-available basis in the orbiter cargo bay, but including meteoroid shielding, is approximately 11,000 pounds.

ON-ORBIT TANK SERVICING

Once on orbit, the external tank would require servicing. Servicing extra-vehicular activities would take approximately one day for each end of the tank, carried out by two or three astronauts, one possibly using a Manned Maneuvering Unit for external inspection and transporting equipment between the orbiter cargo bay and tank hatches, and one or two astronauts stationed inside the tanks to remove and install equipment.

The orbiter would first orient the tank aft end toward the sun to maximize boil-off of any hydrogen after opening the hatch, and to minimize loss of oxygen. The hydrogen feedline suction bell and interior feedline would be removed at a flange installed where the feedline passes through the aft tank wall. The oxygen tank has an X-shaped vortex baffle at the feedline opening which would be disassembled and removed. Both tank fuel openings would be fitted with covers carried aboard the orbiter.

Tank interiors can be divided into decks and the decks into cabins. External tanks at the rim of rotating stations should use horizontally oriented Decks, as will Space Station modules. With a 28 foot external tank diameter, a two-story configuration is practical.

The large diameter also allows an efficient vertical layout. Mounting decks to existing structure on 10 foot centers would divide the tanks into a total of 15 stories. In the oxygen tank two decks would attach to the top and bottom slosh baffles. Within the hydrogen tank, four decks would attach to existing internal ring stiffeners with three other decks utilizing existing ring frames as corbels.

Interior wall partition frame and deck supports could be mounted at 60 degree increments. Central in each deck would be an access opening for moving between decks. A ladder running full tank length, installed along one side of the opening, would aid astronaut movement in any gravity gradient. Decking would be a lightweight structural grid similar to that used on Skylab, with triangular openings to provide receptacles for boot sole cleats and for cargo tiedowns. Clips on the underside of the deck grid could be added to accept ceiling panels. Brackets added to the tank wall before flight would attach equipment modules.

Ladders, deck supports, deck segments and interior wall and ceiling panels in 32 inch wide sections would be carried in bundles within the orbiter cargo bay, then guided through the hatch openings for installation. Non-standard decking due to ring frame size differences could be installed before flight without diminishing fuel capacity or disturbing flow.

Small segments of decking could surround all four hatches to allow astronauts to be anchored via their cleated boots while working the hatches or guiding cargo through the openings.

Forced air circulation may be added to those tanks used for living accomodation and in factory modules that generate heat. Dissipation would be via radiators mounted to the outside of the tank similar to those on the orbiter cargo bay doors or in arrays as on the early Space Station configurations. The circulation ducting, manifold, fan, filters and cooling coils could be modularized with one unit used within the oxygen tank and two in the hydrogen tank. Carried aloft collapsed to 10 foot lengths in the orbiter cargo bay, then fitted through the tank hatches, they would be extended to 50 feet long and mounted to the deck section edges opposite the ladder in the central opening.

Once the tank is serviced, it can be oriented into the solar wind to maximize time before reboost, or assembled into a group of tanks. Parking tanks in a 300 nautical mile orbit allows them to remain stored for up to 15 years before reboost (5).

TANK ASSEMBLY CONFIGURATIONS

External tanks may be arranged into groups in several ways. A hexagonal close pack cluster of 7 tanks would be well suited for interplanetary ships. Tank clusters could be one or two deep depending on mission and engine limitations, with connecting tunnels between each cluster of tanks.

A rigid structure is achieved with a tetrahedral pentagon array, which would not pack the tanks closely together as in the cluster arrangements. Peripheral links need to be 1.8% longer, 3 feet, than the other links, which would be made up in the connecting airlocks.

The planar hexagonal array is ideal for a rotating station, which would have 5% gravity at the rim at 1 revolution per minute, which is below the coriolis sickness threshold.

Interconnecting hallways are not required in either array layout but airlocks at the intersections need to be large to ensure tank clearance. Airlocks would also carry structural loads. Hallway and airlock segments would be shipped in the orbiter cargo bay as space and weight are available and would be installed as part of the tank parking orbit servicing.

Tanks could be assembled into groups with the tank attached to the orbiter, which contains maneuvering thrusters for guiding the tank into position in the assembly. The center of gravity of the orbiter with the external tank attached falls within the controllable region of the maneuvering thrusters (6). Jacking points and alignment pins would move the tank to final position before mating attachment flanges.

MODIFICATION COSTS

Assuming 250 tanks were modified, the $500 million tank recertification cost for the preflight modifications could be amortized at $2 million per tank. Each tank would cost an additional $7-10 million to modify before flight and service for storage on orbit. The total additional cost would be $9 to $12 million per tank (7). This is the total additional cost to place a large, pressurized volume in storage in earth orbit. The basic tank as currently configured costs $24 million and is destroyed on each flight.

CONCLUSION

The tank modifications outlined here are entirely within existing manufacturing capabilities. If NASA were to sell modified tanks for their 40% additional cost, they would be extremely inexpensive pressurized on-orbit real estate. The 250 plus tanks available could speed development of space industry after Space Station, and promote planetary exploration and settlement. It is conceivable that modified Space Shuttle External Tanks will find their way beyond the solar system on the first missions to the near stars.

Figures and Diagrams:

References

UPDATE -- A DECADE LATER

September 1996: Orbiting ETs is still possible, but several problems have made themselves known:

1: A means of reboosting tanks would need to be added to prevent uncontrolled re-entry. The U.S. was lucky (or very skillful) Skylab debris did not impact a populated area.

2: The insulation tends to flake off on orbit, leaving a cloud surrounding the tank, and wrapping the tank to contain debris after orbit insertion would be virtually impossible.

3: The tanks are so large that it would be difficult to build enough boosters to loft sufficient hardware to utilize the volume! This problem could go away with construction of an inexpensively reusable orbiter, but then the tanks will no longer be used.

4: The installation of hardware in the tanks on orbit would be extremely labor intensive and dangerous if done with space suits.

5: To install hardware from the Shuttle orbiter cargo bay direct to the tanks would best be accomplished by a nose hatch in the orbiter with a straight-line tunnel from the cargo bay along the flight deck lower level. This would allow work to be carried out without space suits, assuming a) the tanks were pressurized and b) the hardware were contained within a pressurized tank in the cargo bay. The nose hatch was proposed by NASA to aid Space Station docking but rejected as being too costly a modification.

6: Orbiter life and total number of missions have been severely reduced, to some 6 per year for the next 15 years, though there is doubt about NASAs ability to meet both goals.