Developing a near-term small space tug for LEO and station use is one option - look where it could get us
US space workers are currently faced with both the loss of the Shuttle program (correctly set in motion by the Bush administration years ago), and also by the temporary gap in space jobs caused by the probable cancelation of the Ares Program. Understandably they are all very concerned about their personal future, and also the seeming end of the manned space program. There is a way to at least partly alleviate both of these problems: (one financial and the other perceptual).
When the Shuttle design using solid rocket boosters was picked in 1971 (almost 40 years ago), it received sharp opposition from many quarters including von Braun himself, according to some sources. The solid boosters are safe to use for unmanned launches, but since they cannot be turned off in case of an accident, I have always (starting in 1971) regarded them as far too risky to use on manned launches. In addition, the emphasis of the current NASA leadership is rightly on launch cost reduction, where it should have been for the last 6 years. There is no practical way to significantly reduce the cost of using solid boosters, since the propellant itself has to be manufactured and cast, and the cost of refurbishing and flying a solid booster is about 80% of the cost of a brand new booster. (Compare the cost of (chemically and structurally) complex solid propellant (as cast in the booster) per pound to that of LOX and RP1 or equivalent liquid propellants per pound.)
Therefore, if we are looking for a 5 or 10 fold launch cost reduction, which is what we need to support any kind of economically sustainable operations beyond Low Earth Orbit (LEO), (or even the space station itself over the long term), liquid fueled boosters are clearly the way to go. This is apparently why there is currently a lot of interest in re-usable Heavy Lift Vehicles, (HLVs), based on liquid fueled engines. Unless a private company takes the project as its own initiative, it will probably be many years before any program to manufacture large HLV's begins. Although many operations in and beyond LEO can be accomplished without use of HLV's, there are some objects which absolutely need to be launched pre-assembled, and others for which the cost of in-space assembly would be extremely high.
However, the problems we are facing right now requires a close examination of our options. What can we do that will both pave the way to on-orbit re-fueling operations and station expansion in 5-10 years, and also soak up a good measure of the out-of-work NASA employees and contractors? Obviously, no program will provide an exact match for the job skill proportions existing, but we want one that will help, without wasting money on wasteful make-work programs such as extending Ares testing.
What is the capability that we most need right now?. There are currently constant cries of "keep the shuttle flying - we cannot live without its capability". What are those critical capabilities? The shuttle can deliver crew, cargo, and entire modules directly to the space station and other LEO locations, with payloads of about 20 tons, and also return modules to the surface. The capacity of Atlas and Delta Heavy launchers can deliver about the same payloads to LEO, but they cannot directly deliver them to the station for either docking or unloading. There have been very few times when a really large module needed to be returned to the surface, so replacing large module return capacity is not economically practical for now.
One of the features of the original space station and shuttle concept was a space tug. The concept was called the S.T.S: (Space Transportation System), which would have allowed access to other locations in LEO (in the same or similar orbital plane) from the station. Such a tug would be able to capture large payloads, either modules or large cargo containers, and deliver them safely back to the station. The tug was originally in the post-Apollo plans, but by the mid-1970's, like many critical components of the current Station, such as the Large Centrifuge Facility, it got chopped by budget cuts and budget overruns of other parts of the shuttle program, leaving just the Shuttle. Thus this component was lost long before the 1984 Reagan Space Station proposal. We still only have two components of what was intended to be a three-component system.
What is clearly needed is a fast-paced program to replace the shuttle's capability with the existing heavy launcher (Atlas and Delta) capabilities and a new LEO space tug (LEOT). There are a number of required components to such a program.
- Make sure that any payloads launched to LEO can be located, mechanically acquired and safely captured by the tug.
- Build a set of 3 LEO tugs that can retrieve a payload from anywhere in the space station's orbital plane (within a degree or two of the plane. We assume that the tug might weigh up to 20 tons with some stored propellant on board. It could thus be launched itself by one of the Atlas or Delta Heavies.
- Build a small propellant depot that could be docked to the space station on a long boom. The depot would have venting safety systems to protect the station from being "spun up" by any uncontrolled leakage and would store only non-hypergolic propellants due to safety. The depot could alternately have a small engine and be placed near the station; it could follow the station during any re-boosts using its own propellant. The small depot should have enough capacity to re-fuel the LEO space tug.
- Design the mini-depot to be able to re-liquefy its propellants as fast as they boil off; with dewar-style tank designs and sunshades to reduce the boil-off rate. The depot would be powered by its own solar panels. Adequate electrical power is critical to being able to re-liquefy the propellants.
- The propellants stored could be LOX and RP1 or Methane, to reduce the higher power levels needed to re-liquefy liquid hydrogen.
- Design the depot so it could be launched by one of the current (Atlas or Delta) heavies.
- Create a COTS-like competition to build a tanker system using any of the existing non-heavy booster configurations to replenish the depot. As part of the deal, the government would agree to launch service guarantee contracts with the winning company (a specified number of launches during a specified period of time at a specified price per launch).
- Manage the program all-up so that all components become available for use at about the same time.
Such a space tug development program has lots of advantages.
- It would have a near-term effect on reducing the space job problem.
- It would soon replace the capacity of the shuttle of delivering heavy cargos and large modules to the station. This would allow resumption of near-term planning and construction of new space station modules such as the critical centrifuge facility, and additional crew hab space.
- Initiation of such a concrete program would quickly demonstrate the current administration's intent to pursue human space operations.
- It would demonstrate and prove out the in-space re-fueling system required for beyond-LEO exploration, and quickly gain us invaluable experience needed for building larger depots and tugs.
- It could be modified for crew rescue, or, with a docked hab module, create a crew rescue vehicle that could rescue any crew stranded in or near the space station's orbital plane.
- It would stimulate into existence the system of small tanker vehicles that would be require to re-supply the larger fuel depots that would be built after 2020.
- It would provide greater flexibility and safety in re-boosting the space station if the VASIMR engine system is not implemented by then.
- Much larger versions of it (High Earth Orbit Tugs (HEOTs) would be able to reach Geosynchronous Earth Orbit and lunar orbit and repair or move problem communications satellites like the currently drifting one, which is now menacing other satellites. It seems crazy that we have this enormous global multi-billion dollar investment in comsats, with still no way to repair, refuel or control them other than with remote radio commands.
- The same larger tug could be used as the basis for an LEO - to High Earth orbit and lunar orbit in-space ferry system, with return of the tug to LEO facilitated by the use of aero-braking and subsequent orbital adjustment (circularization). This large re-usable tug would replace the expendable earth departure stage of the Constellation. It would open the way for a similar depot-based infrastructure to be built up in the Geostationary and Lunar orbits, which would then allow relatively inexpensive access to the lunar surface. The HEOT tug would be recovered by slow, multi-pass aero-braking, returning to a LEO station and depot.
- Progress is being made at Ames and other centers which will allow the design of narrower aero-capture (single-pass) crew return vehicle designs, able to return to LEO from GEO and beyond, without the very wide and hard-to-launch shape of a cone-shaped Apollo or Orion capsule. This will allow the "service module" portion of the crew vehicle to remain as an integral part it during aero-capture into LEO, making the whole vehicle re-usable. The HEOT would return separately from the crew vehicle via the slow aero-braking method; both would rendezvous with a station and depot.
- The combination of a large tug, Orion-derived crew vehicle designed for aero-capture and compatible habitat module would thus lead to the ability to take crews far beyond LEO and GEO, such as to asteroids or Mars orbit. Much of the expertise that went in to the design of the Orion capsule could be used in expanding the tug/crew vehicle/habitat design. An Orion capsule all by itself is obviously of little use in long-distance missions. It would need a propulsion module and a hab module for such trips, where a larger tug could serve as the propulsion system. Such a whole in-space transportation system based on separately launchable crew vehicle, propulsion and hab modules would be much easier to design and launch than an integral system. These vehicles would not be used for Earth to orbit and back transport. They just need to be placed into LEO before they can be used!
All of this sequential progress, of course, depends on the success of the current privatization effort. The political stresses caused by the loss of space jobs and the unfair perception that the administration is trying to end the manned program, are damaging the cause of privatization and threatening to extinguish the promising new companies in their infancy. A rapid study (90 days or less) could determine the feasibility of the LEO space tug /mini-depot and tanker system and how soon design and construction could begin. The resulting reduction in political pressure would then create a greater assurance that the move toward privatization would continue. Launch cost reductions resulting from privatization will also make the Station much more valuable and useful. Eventually the private companies will have enough muscle to build their own re-usable HLV boosters, whether the government builds them or not. That will eventually assure the ability to launch large integral payloads when in-space assembly is impractical, such as for Mars orbit to surface and return ferries, which need to have an integral aero-shell and where all the cargo must be fully enclosed within the vehicle.
Much of the space policy discussion over the last 25 years has been about destinations. We will never be able to afford to repeatably get to those destinations and sustain crews or scientists at them without getting the transport costs down, so worrying about destinations without getting cheap launchers and cheap inter-orbital transport first, is like counting your chickens before they hatch. If we start getting the costs down, the extra money freed from operations will enable continuing efforts to reduce launch costs. Then, at some point, we will be able to go to all the valid destinations and do development, science, exploration and eventually create bases and settlements. To do that, we must protect the privatization process, and initiate at least some movement toward modern in-space operations and infrastructure. The LEO space tug and min-depot represent the start of at least one valid and practical path toward those goals.
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