Cislunar transportation: the space trucking system
by John K. Strickland
|All along our interstate and other major roads, we have truck stops designed to service and refuel large trucks. The space equivalent we really need is the waystation.|
You do not use an expensive truck to carry a load just a single time, and then immediately send the truck to the junkyard to be scrapped. Trucking businesses could not operate this way. Some truck cab and trailer combinations today are probably worth close to a quarter million dollars new. Some cabs alone are close to $100,000 used. Most of the current rockets used today cost over $100 million, so large rockets can be up to 1,000 times more valuable than a tractor-trailer, yet all of them smash into the ocean or desert and become scrap metal after just one flight.
For rockets that take off from the ground, one obvious way to allow re-use is for them to land on the ground intact. SpaceX and some other companies are trying to do just that. Quite a few rockets have now accomplished short flights and landed again safely. Without wings, the landings must be vertical. Re-use with a vertical landing was first done by the DC-X at White Sands on September 11, 1993.
For rocket vehicles in space, the problem is different. We do not want to bring the vehicle back to the ground to refuel, since it is extremely costly to get it up into space in the first place. Once it is in orbit, we want to be able to re-use that vehicle in space over and over again.
However, there is currently no fuel supply or “space gas station” in space where such a vehicle could refuel. After all, what good is a truck without fuel? What good would the Interstate routes be without gas stations? When we refer generically to space “fuel”, we usually mean two propellants: a real combustible fuel such as hydrogen or kerosene, and an oxidizer like oxygen.
All along our interstate and other major roads, we have truck stops designed to service and refuel large trucks. The space equivalent we really need is the waystation, and its most important feature is the propellant (fuel and oxidizer) supply—the depot.
At truck stops, the gasoline or diesel fuel is safely kept in underground tanks. The kind of high energy rocket propellants used in space and in the shuttle’s External Tank are liquid oxygen and liquid hydrogen, which are so cold that they want to boil away constantly. Non-cryogenic propellants like hydrazine do not boil away but have much less energy.
Cryogenic propellant has to be kept in a cryogenic propellant depot, which consists of heavily insulated storage tanks, shaded so the sun never shines on them, and equipped with a cryo-cooler, a refrigeration unit which keeps the fuel below its boiling point, hundreds of degrees below zero, so it never boils away. The cooler (and the rest of the waystation) is powered with arrays of solar panels like those used on the space station. Some people at NASA refer to waystations as “Gateways,” but those proposed officially so far by NASA have no depots and are not built or serviced by reusable vehicles (see “A glimpse at a gateway”, The Space Review, November 12, 2012).
|A waystation/logistics base would act like a space version of a freight terminal yard and warehouse.|
Just like a truck stop, a waystation needs equipment to transfer or pump the fuel, which would be in zero gravity, from tankers bringing fuel to the depots and, later, from the depots into the vehicles that need to be re-fueled. In this case, the tankers delivering the fuel are other reusable rocket-powered vehicles with tanks of fuel as their cargo, or extra fuel in their tanks. The fuel (propellant and oxidizer) would be transferred into and from the depots at special fuel transfer docking ports attached to the depots. The technology to prove that the fuel transfer systems would work in space is in development, but the funding for it is currently very limited.
The fuel could be produced on and launched from the Earth, Moon, or Mars, and be sent into orbit. The fuel produced on Earth would be off-loaded at a small waystation in low Earth orbit (LEO), at an altitude similar to the Space Station. However, due to the artificial space debris problem there, we need a better location, if we want to safely accumulate and store large numbers of space vehicles and large amounts of propellant.
So, what does cislunar mean? It simply refers to the space around the Earth that includes the Moon, and its orbit, and all of the five Earth-Moon Lagrangian points or L-points. These L-points are locations or zones in empty space near the Moon and in its orbit around the Earth, that provide ideal locations for space transport hubs like the waystations. The “points” are actually areas of “gravitational balance” between the Earth and Moon, that constantly move around the Earth in synchrony with the Moon about once a month. Some of these points are also very good locations for waystations.
L5 has been considered as a good location for a space colony for about 40 years. L1 and L2 are both close to the Moon: L1 is an area about 58,000 kilometers (36,000 miles) in front of the Moon, while L2 is a similar distance behind it, and almost as wide as the Moon.
Besides their convenient location near the Moon, one reason that both L1 and L2 are ideal locations for waystations is that dangerous artificial space debris cannot accumulate there. Objects have to be kept there deliberately by small amounts of stationkeeping thrust. These safe zones would allow us to accumulate large numbers of space vehicles and large amounts of propellant with a much lower risk of damage or puncture, since only natural objects such as meteoroids and tiny asteroid fragments could hit them.
Another reason that L1 and L2 are good locations is that vehicles can leave from them and go to many different locations, such as the Moon, Mars, asteroids, and also back to LEO, using only a minimal amount of fuel. This is like having your trucking company’s terminal right next to the intersection of two main interstate routes, with an on-ramp right outside the company’s gate. The Moon is only 12 hours flight time away from L1 and L2.
The existing space station is not a waystation since it lacks the ability to refuel even one space tug. Another thing the station lacks is cargo-handling ability. While it can dock or attach large pressurized modules, it has very limited ability to transfer large cargo items between vehicles outside the pressurized volume where the astronauts live. A real waystation would be able to do this and serve as a logistics base. If we want to mount expeditions to the Moon or Mars, we will want to be able to move large objects from one vehicle to another without needing spacewalks. In any case, the objects would weigh many tons, far too large for astronauts to move by themselves.
Companies that have large fleets of trucks need to have freight terminals: large yards next to factories and distribution warehouses where the big trucks can be loaded and unloaded. In space, just like on the ground, you cannot do very much without equipment and supplies. When equipment is delivered, you need some place to deliver it to, or you may need to trans-ship it to another location.
A waystation/logistics base would act like a space version of a freight terminal yard and warehouse. At the LEO waystation, cargo and/or fuel would be transferred from the reusable tug and tanker “capsules” launched into orbit (which would then return to Earth and land), to in-space tugs and tankers meant to deliver it to other locations such as L1 or L2. Each vehicle’s cargo hold would need movable restraints to hold the cargo item rigidly once it was placed.
|The whole point of having a cislunar transportation system, with reusable vehicles as well as waystations equipped with propellant depots to refuel them and with cargo handling capability, is to make access to the Moon, Mars, GEO, and asteroids much less expensive and much more reliable.|
To move and store cargo, a waystation needs docking positions to keep the vehicles firmly in one spot, and a large, mobile robot arm that can reach out in a vacuum, grab cargo from one vehicle, and place it into another one. The robot arm takes the place of forklifts and similar cargo-moving equipment used on the ground. The arm does not have to lift the cargo, but it must contend with the mass of cargo without breaking itself. It needs to be able to move the cargo precisely out of one vehicle and into (or attach them to) another vehicle. This work could be done under the direct supervision of crew members or by remote teleoperation. Most waystations would probably have the robot arm mounted on rails running along one side of an open truss beside one or more rows of docking stations (mounted on the adjacent sides of the same truss), where cargo vehicles are parked. This “docking truss” takes the place of the freight yard.
The waystation’s docking positions are not like the pressurized docking ports that the International Space station uses. Most of the cargo items, like a lunar habitat module, would be too large to be moved into the pressurized area. Cargo items would be moved in vacuum, directly from one unpressurized cargo hold to another. The waystation would probably also have some thermally shielded cargo storage areas to temporarily store cargo items. Cargo could be inside pressurized cargo containers if needed. If crew access is not needed, these pressurized containers would not need to be docked to a pressurized port.
Why is in-vacuum large cargo handling needed? In previous expedition concepts, an item would be loaded into a lander, which would go to a destination, land, be unloaded and then be abandoned—the “Land and Abandon” model. If we are able to reuse the vehicles, we will need to be able to refuel them and also load them with new cargo items when they return to the waystation.
A waystation would have a crew habitat and reserve food and life support stocks for any crewmembers that were transiting through to use, just as some truck stops have restaurants and accommodations for truckers. The habitat could double as a refuge in case of emergencies, to protect crew members from solar radiation storms or problems with vehicles. In deep space, there is twice as much galactic radiation (cosmic rays) than in LEO, and also the danger of a solar radiation storm. The crew module would need to have a “solar storm shelter” surrounded by lots of mass (food, water, and equipment) to protect the crew from the radiation.
If a waystation had very heavy traffic, it might eventually have a permanent crew stationed there. The crew module area would also have one or more multiple docking adapters attached to it, just like the ones on the space station, to allow multiple vehicles with crews on board to dock and let the crews enter and maintain the waystation. It would also have large solar panel arrays and heat radiators similar to the space station’s set to power the crew habitats, the cryo-coolers for the depots, the small station-keeping electric thrusters, and the cargo-handling robot arm.
A variety of vehicles would work with the waystations to gain access to various locations in space. Reusable boosters would have reusable second stages and reusable cargo and fuel capsules (tugs and tankers) that would deliver only to the LEO waystation. Then liquid-fueled or electrically (plasma or ion rocket) powered tugs would move the cargo and fuel to other locations, such as L1, L2 or geosynchronous orbit (GEO). These vehicles then would return back down to LEO via aerocapture (using the Earths atmosphere as a free speed brake) to get another load.
At L1 or L2, lunar ferries would carry cargo, equipment, and fuel down to the lunar surface and then return to L1 or L2 for another load. Such a round trip is only possible if the high-powered liquid oxygen and hydrogen is always available for the vehicles to use, stored in the depots at the waystation. To keep the cost of developing and building the various vehicles low, they would use many of the same components, such as rocket engines, tanks, and crew cabins.
The first thing that we would probably do with a waystation located at L1 or L2 is to set up a base at one of the lunar poles, to gain access to the lunar surface for science and to mine water ice there for rocket fuel. Some of the cargo items would be crew habitats, lunar rovers, power generating equipment, and fuel production and storage equipment.
Once produced, the lunar propellants could then be sent up to the waystation on the lunar ferries, as it would take far less energy to move fuel from the Moon to L1 than to move fuel from Earth to L1. To move the lunar fuel to L1 as cargo, the lunar ferries would also use more lunar fuel as propellant. The cost savings from the fuel production would allow continued scientific exploration of the lunar pole. Other areas of the Moon would also be accessible from L1, and the presence of multiple ferries at the waystation would allow rescue missions. Additional lunar water could be used as radiation shielding around the waystation’s crew habitat.
If we wanted to send a manned expedition to Mars, about 20 reusable vehicles with the required propellants would be accumulated at the L1 or L2 waystation. Transit windows for Mars expeditions open about every 26 months and only stay open for about a month.
With the very large stores of cryogenic fuel stored at the waystation’s depots, one vehicle a day could easily depart from the waystation on its way to Mars, creating a true Mars fleet. Having multiple vehicles creates a much safer expedition for the crew. A similar mission launched from the waystation could investigate individual asteroids, but would need only a few vehicles.
The whole point of having a cislunar transportation system, with reusable vehicles as well as waystations equipped with propellant depots to refuel them and with cargo handling capability, is to make access to the Moon, Mars, GEO, and asteroids much less expensive and much more reliable. As long as our space transportation system depends on expendable rockets (large or small) and has no reusable in-space vehicles, fuel depots, or logistics capability, it is not truly a system at all, and doing anything in space will remain either exorbitantly expensive or impossible.