The Mega-Module Path to Space Exploration (Or: How to Use an HLV)

by John K. Strickland, Jr., National Space Society

Date: 07 October 2005 Time: 06:26 AM ET

Ever since the abrupt demise of the Saturn V rocket system at the end of the Apollo era, engineers and space advocates have dreamed of what they could do with a booster of similar capacity. In my opinion the recent correct decision by NASA Chief Mike Griffin and his team to go for the largest available booster which can be created at a reasonable cost, will now allow us to make big plans for the first time in 35 years. This article focuses on how to exploit the wide variety of large payloads, which a truly large Heavy Lift Vehicle (HLV) makes possible.

Replacing the shuttle orbiter and external tank (ET) with a second stage based on the ET itself will provide greatly increased flexibility and capability. The only capability lost is that of returning large payloads, and this capability has been used only a few times to advantage. The extreme annual cost of maintaining the shuttle system could have paid for duplicating these payloads many times over. The ability to launch payloads of 100 or more tons with a payload shroud diameter of over 27 feet far outweighs that loss. Using a "hammerhead" type shroud could allow payloads of at least 30 feet across.

There are several obvious reasons for wanting a large booster, (beyond just the ability to launch a bigger payload), such as avoiding multiple launches and the massive complications and delays that would accompany them. However, one kind of multiple launch system deserves a second look. Most of the potential problems with supporting a single mission with multiple launches result from launch failures or delays in launching one or more payloads needed for a given mission. But what if one of the two payloads is already in space. A large portion of every spacecraft is propellant, and there is no need to launch fully-fueled vehicles if the fuel could be already waiting for them in orbit. Admittedly, this does call for the exploration vehicle to go into orbit instead of on a direct trajectory to the Moon or Mars and it does result in some disadvantages.

However, in this case, the advantages far outweigh the disadvantages. Launching payloads with empty tanks would in many cases more than double the available launch mass of any integral exploration spacecraft which could be launched on a given HLV, allowing the spacecraft being launched to be larger. Every ton of propellant that does not have to be on the spacecraft can be replaced by an equivalent ton of useful spacecraft structure. It would allow post-launch checkout of all spacecraft systems before departure from vicinity of Earth. It would require temporary docking to load fuel, but no assembly of modules in space. The key to this tactic is the orbital fuel (or propellant) depot.

Having an HLV capable of putting 100 or 120 tons in orbit enables us to launch a complete (but empty) fuel Depot into orbit with a single launch. If the Depot is capable of storing propellant for extended periods, the propellant itself can be delivered long enough in advance of mission dates to prevent any delays. In order to do this, the depot needs to be able to continuously re-liquefy propellants as they boil off using solar energy from its own solar panels. The Depot would have several redundant fuel and oxidizer tanks. It should be able to handle several types of fuel and oxidizer. It would be human-tended (operated by astronauts to put propellants in from a propellant-carrying launch or to take propellant out for a mission spacecraft.) There would be no permanent human crew needed.

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The Depot would need to be well shielded from space debris and thermal cycling in orbit, as well as extra insulation to reduce boil-off. The external surface of the Depot could also serve as part of the shroud. The depot would have a primary 3-axis attitude control system assisted by its shape, allowing gravity gradient forces to reduce the use of attitude control fuel. The large weight allowances would permit extra shielding, extra insulation and more redundant systems than a minimal version might allow. The total amount of propellant a depot could handle would depend primarily on the propellant's density, since there is no weight penalty in orbit.

Propellants to fill the depot could be delivered using a second launch of the HLV. Timing for these launches would not be crucial. Propellants could be delivered in a lightweight tank with minimal insulation (similar to the existing ET), and then immediately transferred to the depot. The lightweight tank could then be discarded and set for re-entry. Fueling operations in orbit are in some ways safer than on the ground, since any vapors escaping from propellant transfer operations would almost instantly dissipate in the vacuum and would present virtually no explosive hazard. One problem that orbital propellant transfer operations needs to deal with is, of course, the behavior of liquids in micro-gravity. Transfer pumps need to be delivering liquids, not pockets of gas mixed in the liquids. This problem has been solved in the past and there are multiple ways to handle it.

If an automatic propellant transfer technology could be developed, no on-site crew would even be required. Any such automatic system would depend on a docking system, which (in addition to primary docking), would also have to connect fuel lines, transfer the propellant, and then detach the connections. These operations could be done by using a miniature version of the primary docking system, except that the respective positions of pipe connections would already be known to within about a millimeter. Control of such an operation could be handled remotely by a ground crew using video cameras, full data readouts and manual controls for connections, valves and pumps. Alternately, a crew in a Crew Exploration Vehicle (CEV) could be launched with a lighter version of the tanker. Depending on the orbit used by the Depot, the CEV could separate from the tank, and then rendezvous with a space station, or could even be used to deliver a crew to an exploration vehicle. Fuel transfer technology experiments should therefor be given high priority for space on remaining shuttle missions to the space station.

Such a large propellant depot could be described as one type of a "mega-module". The standard shuttle payload limit for space station modules is about 20 tons, so these payloads would be 5 or 6 times larger than that. Once one type of mega-module, such as a depot, has been defined and designed, it will quickly become apparent that it could be used in multiple locations. It then becomes obvious that you need to build several copies of some kinds of modules, such as the fuel depot. A depot could also be used at a transfer location, such as Lunar Orbit, or the Earth-Moon Lagrange L1 or L2 points. A smaller version would be very useful on the Lunar surface if production of Lunar oxygen begins. A depot would even be needed in Mars orbit once extensive human exploration operations begin. Once you have several copies under construction, there is less of a problem and/or program delay if one copy is lost during a launch failure.

Once you decide to design and launch one kind, the possibilities of creating other types of mega-modules are immediately obvious. (If we have a big booster, we should use it to full advantage.) Some of these could be components of exploration vehicles, while others could be part of unrelated space development (which includes scientific infrastructure). For example, a refuge module, such as would provide a safe retreat from the space station, or at L1, L2, or Lunar orbit, would have much in common with the kind of habitation module used to transit between earth and Mars orbit. It would also be much easier to design a solar storm shelter area in a 100-ton module than in a 20 ton module. The large available diameter would provide room to place the "storm cellar" in the middle of a variety of food and water stores. It also might be possible to create refuges at Depot sites, which would already have solar power and attitude control available.

There are many kinds of integral structures for which it is difficult to design a modular version if they have to be assembled in space. Imagine having to design a large motor home so it can be re-assembled in several pieces with a small crew in a short time. Segmentation of structures and space vehicles causes a lowering of structural integrity, and requires additional horrendously expensive crew time to re-assemble them in space. It is also better for vehicles that will be subjected to thrust to be orbited as one piece. This also reduces the amount of potential air leakage at permanent seal joints of a composite structure.

Any kind of integral re-usable vehicle tends to be larger than the equivalent set of expendable components. Re-entry vehicles are a good example of something with a minimum functional size. For example, a re-usable Mars orbit to surface ferry needs to enter the Martian atmosphere, and be large enough to reach Mars orbit after re-fueling on the surface. Such a ferry would need to have a very large integral aero-shell which itself could not be launched on a small vehicle due to its bulk. The same is true of a re-usable lunar ferry, even though it does not need an aero-shell. The expendable ferry can discard the descent stage when returning to lunar orbit or a nearby L-point. The re-usable ferry must carry enough fuel to lift the equivalent of the descent stage back into orbit. On the other hand, less structure is needed, since only a single module (for ascent and descent) is needed. Such a ferry could carry either a CEV with passengers or bulk cargo. For all these reasons, having a very large booster makes the design of fully re-usable deep-space vehicles much easier.

In a similar fashion, a host of other types of mega-modules would practically demand to be built. For exploration purposes, lunar transfer vehicles are needed for people and cargo, possibly using a CEV as the primary crew cabin and emergency capsule. The 60-day report indicates that an expendable trans-lunar stage would be used for lunar expeditions. There is no reason why this stage could not be re-designed into a re-usable Cis-lunar "tug", which could return to LEO using aero-braking. A large space tug with crew cabin which could also retrieve space station modules or even do repair missions to Geosynchronous orbit would be very useful. This tug should be able to re-fuel from the depot. For Mars expeditions, a standard Earth orbit to Mars Orbit propulsion module would be needed. Such a module could also use fuel brought in tanks from the surface of Mars to send crews back towards Earth orbit.

The use of multiple types of Mega-modules would make it easier for international space expeditions to cooperate, since each country could build one or more types of modules. Since each module type would be integrated and have standardized interfaces, the complexity of having several countries work on the same module would be minimized.

Astronomers would love to be able to design a space telescope with a 25-foot or larger diameter mirror. Some of the incipient flagship space telescope missions currently delayed by cost over-runs might be able to save money and greatly increase their light-gathering capacity by being re-designed as a 100 ton instrument. With 100 tons, you could also place a large outer-solar-system probe on a very fast trajectory by using additional boost stages. You could also build an oversize space station module complete with human-sized centrifuge.

Last but not least, the HLV makes it possible to build and test a full-scale collector module for a Solar Power Satellite. It would be uneconomical to launch enough modules for a functional Powersat on the proposed NASA HLV, but such a test could prove out the ability of the module to fully deploy its huge array of solar film. Such a single module, if fitted with a microwave transmitter, could provide power for a solar-powered tug or other heavy power demand. Based on advanced designs done in the late 1990's, a 100 ton collector module could theoretically deploy solar (photo-voltaic) film with a total area up to 1 square kilometer, intercepting 1.3 Gigawatts of sunlight, and providing about 100-150 Megawatts of power if the film is about 12% efficient. If this test was successful, it could stimulate enough business interest to create a really cheap, fully re-usable and privately owned large HLV. With that, we could build a full system of PowerSats to supply the Earth with pollution-free power and permanently end both the energy and global warming problems within a single generation.

John K. Strickland, Jr. is a director of the National Space Society and holds memberships in several other space groups.

NOTE: The views of this article are the author's and do not reflect the policies of the National Space Society.

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