SSI External Tank Report

III – External Tanks as Propellant Resources

III. ET Project – ET as Propellent Resource

This section details external tank applications that are related to the use of the tank and what it carries as a fuels resource. The possibilities include scavenging the residual cryogenics remaining after launch, powdering the ET and using it as fuel in a special rocket engine, and the use of ETs as reaction mass in electric space engines. NASA has a requirement for 2.5 million pounds of cryogenic fuels in low earth orbit (LEO) based on potential orbital transfer vehicle (OTV) traffic models. The scavenging of residual cryogenics from the ET can fill over 92% of this requirement at a very low cost (69). The potential cost savings of these type operations is great when compared to the basic launch cost of $2000 per pound to LEO (29, 32, 69).

I. Scavenging of residual cryogenic fuels.

As mentioned previously, The direct insertion trajectory will also deliver an average of 15,000 pounds of residual cryogenics into orbit. These residuals are available for scavenging, storage, or use immediately after MECO or longer if steps are taken to retard boiloff. In a propellant scavenging study, a plan utilizing residual cryogenics in the ET will meet the requirements of 2.5 million pounds of OTV fuels over a ten year period. The scavenging of residual cryogenic is cheaper than launching the required fuels into orbit in an orbiter based tanker by almost an order of magnitude. The numbers on the figure below are costs per pound of fuel delivered to orbit. Note how the ET/ACC based scavenging is by far the most economical fuels supply choice (69).

The scavenging operation can be conducted in a variety of ways. If there is a requirement to dispose of the tank, the scavenging operation needs to be done in the 20 minutes of time available after MECO. This can be accomplished using equipment carried in the payload bay of the shuttle or in an ACC. If the ET is carried into orbit, the scavenging operation can be done later. This could use scavenging equipment carried in an ACC or a separate facility located at the space station. It could also use a free flying scavenging flyer carried in the ACC, the payload bay, or already on orbit. It could also use the payload bay based scavenging equipment. The payload bay based method is the most expensive way to scavenge residuals because it takes up payload bay space, payload mass, and requires orbiter modification. The ACC based tanker does not impact the orbiter payload bay or the orbiter and can be used any time inflight after launch. An ACC based tanker and cryogenic scavenging operation can supply 2.3 million pounds of fuel (92% of the requirement) at a cost of $350/lb over a ten-year period (69). If the required cryogenics are brought into orbit by STS tanker, the projected cost runs about $2000/lb (69). Over ten years the program savings due to this ET application alone will be on the order of $3.5 billion primarily due to launch costs. Cryogenic scavenging also enhances the space station program. Every shuttle visit to the vicinity of the station can deliver an average of 15,000 pounds of cryogenics at almost no cost per mission. This is clearly an important advantage to an operation limited by congressional funding.

II. Reaction Mass.

Support requirements have been extensively studied for future space based operations. One of the results from these studies has been the identification of a requirement for large quantities of reaction mass for orbital operations (19, 56). This reaction mass is used for such things as orbital maintenance of large space platforms (prevention of orbital decay due to drag), fuel for an OTV operation, fuel for satellite launch and recovery, fuel for future large manned space expeditions, and fuel for emergency requirements (56). Typically, the studies focus on some sort of liquid fuel either supplied by an orbiter based tanker, scavenged from the ET itself, or the orbiter OMS system in orbit. The reaction mass applications to follow suggest reaction mass applications based on using the mass of the tank itself as the reaction mass. The mass of the tank on-orbit of about 69,000 pounds, can by itself fill all refueling requirements proposed with no additional shuttle visits. Once again, the program savings are primarily in launch costs for the 69,000 pounds already in orbit.

A. Aluminum fuelled rocket.

The first proposal is to powder the aluminum of the tank structure and use it as fuel in an Hydrogen / Oxygen / Aluminum based rocket engine (18, 19). As mentioned before, each ET delivered to LEO will also deliver an average of 15,000 lbs of residual cryogenics and over 53,000 lbs of aluminum. Performance studies of advanced propulsion engines have indicated that an H2/O2/Al based engine that burns fuel on a H:O:Al = 1:3:4 mix will give a specific impulse (Isp) over 400 seconds (18). This Isp is somewhat less than proposed advanced H2/O2 engines (480 – 490 sec) and less than state of the art H2/O2 engines (460 sec) proposed for OTV applications. However, a comparison of the economics of using powdered aluminum as the component of the fuel burned in a rocket versus the economics of burning only liquid fuels suggests that this is a capability well worth the time and effort to study. The energy advantage of the Aluminum fueled rocket is that the Aluminum – Oxygen reaction delivers 22% more energy per unit mass than the Hydrogen – Oxygen reaction does (18, 19). The economic advantage is that the aluminum burning engine utilizes already orbiting ETs as a primary source of fuel.

In two studies of advanced propulsion for OTV applications by Dr. A.H. Cutler of the California Space Institute, the Aluminum fueled engine is compared with an advanced technology engine and the Centaur RL-10 (a proposed OTV candidate) (18, 19). The comparison was made through models of future OTV traffic levels and mass requirements. The assumption was made to use scavenged ET cryogenic residuals as the primary OTV fuel for all three rockets. Analysis of costs showed that if the fuel demand for OTV traffic models remains below the available scavenged cryogenics, it is more economical to use the RL-10 for primary OTV propulsion. However, if there are not enough scavenged cryogenics available, then it becomes far more advantageous to develop and use the Aluminum fuelled rocket. This engine is a better choice for two reasons. First, it reduces the mass required in LEO for both fuel and tankage by about 30% over the RL-10 and about 15% over the advanced engine. Second, it is about 40-50% cheaper to fuel because ET aluminum is used for reaction mass and does not need to be launched.

The problem with the Aluminum engine is a very expensive development cost – on the order of $1 – 2 billion (18). This cost, the processing facility cost, and the engine production cost all were added into the analysis. Even with these high costs, the Aluminum engine is far more economical to develop and operate in a scenario which includes a shortfall of scavenged cryogenics. The cost savings is entirely due to the availability of Aluminum at a low cost in LEO.

Support equipment for this application includes SOFI stripping equipment and a method of melting the aluminum cut from the ET (15, 18, 20, 92). The cut aluminum is fed into an induction furnace for melting. The asymmetric winding of this furnace pools the molten aluminum at one end where it can be drawn off and powdered. These are examples of space operations that use tools and techniques which are familiar to industry on the earth and can be adapted to on-orbit operations.

B. Mass Driver Fuel.

In the middle 1970’s, Dr. O’Neill proposed the Mass Driver as a tool necessary in the construction of very large structures in space (52). It was proposed as a means of recovering asteroids, launching large quantities of lunar soils off the moon, and even as a very large OTV for use in Cislunar space. The beauty of the Mass Driver is that it is not sensitive to what is used as reaction mass. In this context, Dr. O’Neill proposed powdering the tanks and using them as reaction mass for a mass driver. A moderately efficient mass driver was proposed that could move 850 tons from LEO to Lunar orbit expending 1050 tons (about 30 powdered tanks) of aluminum (62). This is a possible use of the ET which may be significant if there are no competing uses of the ET in an unpowdered form.

C. Reaction Engines.

There are two other possible reaction mass applications. The Railgun, currently being discussed in SDI research as a kinetic kill weapon, was proposed as a space engine (11). A railgun reaction engine could use powdered ET materials as reaction mass. It is not as efficient as the mass driver or the aluminum rocket, but may serve as an emergency maneuvering system for military battle stations. Problems with the railgun include a fairly serious pollution problem due to large masses of debris being thrown away behind the engine. A similar device with the same capabilities and the same problems is the coilgun also being researched in the SDI effort.

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