SSI External Tank Report
I – Tank Information
The External Tank of the STS is designed to fill two primary needs. These are to safely carry sufficient cryogenics to deliver the orbiter into Low Earth Orbit (LEO) and to serve as the structural heart – the strongback – of the STS during the launch phase. The basic empty weights, volumes and dimensions are detailed in the first figure.
The tank is constructed in three primary segments, which are the hydrogen tank, the intertank and the oxygen tank. The Oxygen tank rests on top of the stack. It is constructed of aluminum in four sections and includes slosh baffles. It is bolted to the intertank section, which carries its loads in compression during flight. The intertank is the primary strength member of the tank. It is unpressurized and constructed using a skin stringer and ringframe arrangement. It also carries a beam which carries the thrust loads of both solid rocket boosters (SRBs) during flight. The intertank is bolted to the top of the hydrogen tank, the largest section of the external tank. The hydrogen tank is designed to carry the required liquid hydrogen during launch. It also carries the loads imposed on the stack by the orbiter engines (SSME) during launch. The entire tank is covered with Spray-On Foam Insulation (SOFI) which serves primarily to prevent ice buildup on the tank before launch and to prevent excessive boiloff of the cryogenics. Typical depth of the insulation is between one and one and a half inches (1-1 1/2″). There are also additional items attached including range safety hardware, feedlines, sensors, electrical lines, tumble valves and venting systems (63).
After construction and testing, the ET, with low pressure in both the hydrogen and oxygen tanks, is shipped by barge to the launch site. The hydrogen tank is pressure tested to over 36 psi. The oxygen tank is pressure tested by filling it with water at the factory. Neither can tolerate an outside pressure differential greater than 0.2 psi (48). Shipping it pressurized also keeps the interior clean. The tank is then checked out, attached to the appropriate stack and launched in normal sequence.
A typical launch inserts the orbiter and ET into a very low orbit where the tank is jettisoned to reenter somewhere over the Indian or Pacific Oceans (56). The orbiter then conducts an OMS burn to loft itself into the desired orbit. The tank is jettisoned with over 98% of the energy required to keep it in orbit. It is important to control the reentry of the tank to insure that it comes down in an unpopulated area. There is an alternate trajectory for launch called a direct insertion. This trajectory has been flown successfully with the ET splashing down near the Hawaiian Islands. If the tank is retained even longer and taken into orbit, the orbiter can deliver the tank, orbiter payload and additional orbiter payload into all possible STS orbits. It attains this capability by flying a more efficient trajectory and using the more efficient SSMEs to put it all the way into orbit. The following figure is a comparison of Shuttle performance to orbit without the ET, with the ET, and with an ET enhancement called the Aft Cargo Carrier (ACC) which will be discussed later. It is important to note that if the ET is taken to orbit, this action alone improves the STS payload capability by as much as 2,000 pounds to orbit (56). At an average launch cost of $2,000 per pound (69), this is a substantial enhancement of the STS capability.
Taking the ET into orbit presents several opportunities and capabilities that we do not yet have. It also produces some additional problems. The opportunities available are related to the mass and size of the tank itself. The capabilities are related to things that can be done with the tank on orbit. The primary problem is related to safety considerations. All will be discussed in the following section.
The size and mass of the external tank on orbit compare favorably with past and proposed future orbital facilities. The 53,000 cubic foot volume of the hydrogen tank alone is far larger than any facility flown or planned. It is more than twice as large as the Skylab at 18,300 cubic feet (70). It is more than five times the volume of the proposed Space Operations Center (SOC) module of 7,100 cubic feet (75). The volume available in the oxygen tank at 19,500 cubic feet is also larger than all the above mentioned facilities. These volumes can be accessed through three foot diameter inspection hatches in the respective tank domes. The domes can also be completely removed by pulling the attachment bolts if the desire is to open the entire end of the tank.
The mass of the tank is also an advantage. At an average mass of 69,000 pounds apiece, the tank can provide the inertia and strength required for large scale operations in space. Once released, the tank will assume a gravity gradient stabilized attitude with the long axis pointing at the center of the earth (56). A telescope mounted on this platform could rely partially on the mass and inertial of the tank itself rather than an active three axis stabilization system. The tank can be used as a workbench or a strongback in orbit. This concept utilizes the actual structural strength of the tank itself as a construction platform. The construction of large space structures need not be based on specially designed lightweight members that could not support their own weight on the surface.
Due to reserve, pressurization and ullage requirements, the tank should contain an average of 15,000 pounds (5,000 to 40,000 pounds) of residual cryogenics when it arrives in orbit (16). This liquid hydrogen and liquid oxygen can be scavenged from the tank shortly after launch for a variety of uses in orbit (69). These residuals also pose a safety problem, which must be addressed. The current use of the liquid oxygen tank pressurization gas is to tumble the tank after it is jettisoned for reentry. The on-orbit problem is that there is concern about the boiloff rate and possible overpressurization and rupture of the tank. There are several ways to deal with this problem. The first is to scavenge the residuals relatively early with a set of catch tanks located on the aft end of the tank. This can be also be done with the required equipment located in the payload bay of the orbiter but is less desirable because it uses payload bay space that can be better sold to customers. A second method would be to install a heat reflector to keep the sunlight off the tank. The first Skylab crew did this to Skylab to make it habitable. The ET could be made a better storage container for cryogenics by wrapping it with mylar blankets to retard boiloff (46). The next figures detail boiloff rates for residual cryogenic propellants.
A third method involves trading the cryogenics as they boil off for orbital altitude. There have been several proposals for low thrust, gas burning hydrogen – oxygen rocket engines for station keeping and orbit raising. The engines proposed by Martin Marietta are 1,500 lbf thrust with a specific impulse (Isp) of 375 seconds (46). A proposed arrangement of four of these engines mounted on the intertank as thrusters using the residuals left. The tradeoff here is between orbital lifetimes desired and the amount of scavenged cryogenics desired.
As was demonstrated by the reentry of Skylab, the orbital lifetime of large space objects is often far less than predicted. One of the primary considerations with taking the tank into orbit is keeping it there. The orbit of the tank will decay over time due to aerodynamic drag and effects of the solar activity. The desire is to put the tank into the highest possible orbit. Cross section ‘into the wind’ is also a factor. The gravity gradient stabilized tank with a ‘nose down’ attitude is in the worst possible attitude for long orbital lifetimes because it presents the largest area to the wind. The best attitude is either end of the tank pointed into the direction of motion. The following graphs detail calculated orbital lifetimes for a single external tank in three different storage modes (56).
It is particularly important to control the orbital altitude of the tank if brought into orbit. The reentry point and impact footprint are extremely difficult to predict for an uncontrolled reentry (56). The leads to a requirement to reenter the tank after launch, reenter the tank after it has served its purpose on orbit, or to keep it in orbit using active measures. Thus, any plan, which proposes external tank applications on orbit, must address this issue. Some form of orbital maintenance such as momentum transfer involving tethers or the low-pressure cryogenic boiloff thrusters needs to be used. This is not a particularly difficult problem to solve. It is however, a fact of life for every structure in earth orbit. There are current studies of appropriate propulsion for keeping the NASA space station in orbit.
The last problem with the External Tank in orbit concerns outgassing from the SOFI on orbit. This amounts to an unwanted fouling of the environment and the equipment in the vicinity of the tank on orbit (14). Like most problems in the past that have been referred to as fatal flaws, this is not insoluble. Preliminary calculations suggest that the SOFI will deteriorate at a rate of about 0.8 grams per second for the entire ET (3). This rate is purely an order of magnitude prediction subject to change with further investigation. Outgassing may be a problem to vehicles and structures in the vicinity of the ET. Possible solutions include: bagging the tank in a mylar blanket which will retard the rate; applying metals by vapor or liquid deposition to the outside of the tank in orbit; removing the SOFI from the tank; or keeping the tank based structures lower in orbital altitude than the rest of the operation by the use of tethers (14). The ramifications and magnitude of this problem warrant further investigation as tanks are proposed for orbital applications.
The first enhancement of the STS is likely to be the Aft Cargo Carrier (ACC) (4). The ACC is detailed in the figure below. It is basically a cargo volume constructed similar to the ET components using the same tooling and bolted onto the bottom flange of the hydrogen tank. There are three basic advantages to the ACC. The first is that it provides a volume nearly equivalent to the cargo bay of the shuttle that is not constrained by a diameter of 15 feet. Its volume of 9,100 cubic feet compares well with the orbiter cargo bay volume of 10,600 cubic feet. The ACC dimensions of 27.5 feet in diameter and 20 feet in height allow large diameter relatively low mass payloads to be flown without the constraints imposed by the 15 feet diameter orbiter cargo bay. A typical ACC weighs near 14,000 pounds (4). It consists of a skirt, which connects to the ET, a payload support structure and a shroud and is insulated for protection from the SRB plumes and blast effects. Payload penalty to orbit for the ACC is from 9,800 – 11,400 pounds (68). This is below the actual ACC weight as a result of shroud jettison after SRB separation. Center of gravity problems during flight have been addressed and do not appear to be a problem.
The beauty of the ACC is that it provides another cargo volume at a minimal cost. This important to the STS program because it helps remove a payload constraint. The orbiter imposes two constraints on payloads. These are volume and mass limitations. The mass limitation cannot be changed without changing the actual orbiter performance. The volume constraint can be changed by the addition of the ACC. If the STS is launched carrying less than the maximum mass, this leaves excess mass to orbit capability, which may be exploited by mission planners using an ACC. This could be payload such as communications satellites that will generate additional revenue for that particular sortie. This would allow non revenue generating sorties such as planetary missions, science missions, DoD missions, Spacelab missions or Space Station visits to generate revenue that they would not be able to do otherwise. It also gives the mission planner and scheduler an option that can be used to recover from the impact of a mission cancellation. If a flight is canceled, rather than impact the entire flow for years to come by bumping payloads from flight to flight, the planner could fly a few loaded Acts on appropriate future missions to catch up.
The ACC can be configured to carry almost everything that can be currently carried in the cargo bay within the limits of a length of 20 feet (26). If a payload is designated for an ACC mission, which makes the ACC the primary cargo carrier, the volume remaining in the cargo bay can carry additional payload. The figure that follows shows a cross section of possible ACC payloads including space station modules, storm shelters, satellite modules, Centaur G booster, wide OTVs, cryogenics scavenging, large diameter mirrors and antennas, service modules, lifeboats, entry ports for ET based structures, and shuttle mission enhancements. A proposed ACC payload includes the servicing structure for the space telescope. A typical mission scenario would be to launch the orbiter, deploy satellites from the payload bay, retrieve the service structure from the ACC with the RMS while the rendezvous with the space telescope is conducted, and conduct the necessary servicing of the telescope (45, 48). The ACC can also carry thrusters in the skirt, which can be used to either boost the tank into a higher orbit or deorbit it to a controlled splashdown in the ocean.
The ACC is designed for minimal impact in the current STS operations. It can be constructed using available tooling within the Michoud facility. It has minimal impact on the tank itself and the interfaces between the ACC and ET are relatively simple. The largest impact on the orbiter will be adapting the flight software to an ACC mission. Total cost of the ACC is around $150 – 250 million (4, 48) and it can be flown within three years of the decision to buy. Like the rest of the ET based applications studied, this is a relatively simple, low cost enhancement, which has a very high return and significant positive impact on the STS.
There are several additional ET enhancements which have been proposed (25). These include a Forward Cargo Carrier (FCC), an exterior track system, an interior track array, and a variety of flanges for opening the end of the tank for orbital applications. With the exception of the FCC, all the enhancements are relatively inexpensive and easy to add to the tank.
The FCC is the same size as the ACC and can carry a similar payload. It does not require the protective shroud that the ACC does. However, it poses a larger problem than the ACC because it will require additional mass and structure in the intertank and possibly the hydrogen tank to support the loads imposed by it on the tank. This makes the impact of the FCC greater on the STS program and thus increases the potential costs of the FCC. This cost increase due to program impact and increased tank costs may not be as attractive as the ACC.
The next enhancements are the interior and exterior rails or tracks. These tracks allow things like mobile robots, mobile cranes, interior hangar operations, the attachment of station modules to the exterior of the tank, and the possible use of the tank itself as a linear accelerator. The track/rail enhancement will allow a variety of tank enhancements which utilize the structural strength and inertia of the tank (25).
The last class of proposed enhancements are lumped together as things that will make the tanks easier to attach together end to end, easier to enter, and easier to attach external payloads for launch. These include flanges, attach hardware, large seals, hangar hinges, a variety of entry hatches, and additional hardware. These all are relatively simple enhancements that are inexpensive, lightweight and easy to add to a tank (25).