No Catcher's Mit
Earlier writers and illustrators envisioned a mass catcher at L2 actually catching payloads like a catcher's mit catching a baseball. The loads come rushing in at 200 meters per second or 720 kilometers per hour. That's 445 mph. Although Kevlar can stop a .44 magnum bullet, I don't buy it. There's no way in the world that the mass catcher will stay on station with thousands of tons of material rushing into it at that speed. Lagrange points are not that stable! Even if it uses one third of the material for reaction mass in nuclear electric rotary pellet launchers to stay on station it will be like a ship fighting a loosing battle with a storm and 1/3 of the material will be wasted! Even the NASA document SP-413 that comes on the CD with the 3rd edition of "The High Frontier" page 73 says that passive mass catchers were rejected. They cited the high precision needed to hit the 100 meter wide mouth of the passive catcher and recommended active catchers. Active catchers must maneuver around to intercept incoming payloads. Even those will have trouble with kilotons of material hitting them at 200 mps.
Another Way
I have suggested launching ten ton payloads into a 100 km. high LLO and firing cold gas thrusters for a mere 15 fps burn to reach a catcher or collector a little higher at 120 km. altitude. View Taxi Port. A scuba tank has enough energy stored in it to do this. Don't ask me to locate that source because it is lost in tons of files in my computer. The modules would do another 15 fps burn upon rendezvous with the collectors and gently enter their 100 meter wide glass fiber bags. This will ease navigational precision demands. The modules will have miniature guidance systems. Honeywell makes one that weighs only eight pounds. Radar tracking and supercomputers will control the flight of thousands of modules. Although this could be done, it seems rather byzantine. If we are going to put thrusters on the modules and make them 10 to 100 tons mass instead of just 10 kg. packages blasted by a mass driver running like a machine gun, why not just put monopropellant retros on them using a suspension of LUNOX and aluminum, silicon and magnesium? Using the rocket equation, a 100 ton module mass and a ISP of 250 seconds we find that eight tons of monopropellant could brake the modules by 200 m/s. The modules will amass a few tons but they will just be aluminum drums that are cannibalized in space. The compressed oxygen cold gas thrusters that assist navigation and the monopropellant motors could be removed and sent back to the lunar surface by rocket for reuse. So what if the catchers, or collectors or receivers if you prefer, when loaded up use NEP to reach a factory in L1 or L2 halo orbit or at L5 and robot and human workers have to disassemble the modules to get the motors and thrusters and load them up for return? Robotic and human workers on the lunar surface will have to load the modules up with regolith, silica, alumina, metals, etc. anyway. They can also assemble the modules and motors and fuel them. Factory type operations could do this fast. To launch 6,000,000 tons a year we need to launch one 100 ton module every 8.76 minutes. They could work that fast; have no doubt. Ever see a factory going full tilt? The system will be largely automated for high productivity.
This system does away with the LLO collectors and their NEP systems for dragging everything to a Lagrange point station for use. It is more direct, thus it should be speedier and more efficient timewise. The mass collectors at L1 or L2 could be made of steel nets to resist the hot exhaust from module retros, but these will fire at some distance from the net so the gases will be dissipated and cooled off before hitting the net. Mass drivers at different locations on the lunar surface could launch at slightly different velocities to send payloads on what Heppenheimer called "chromatic trajectories" that all hit the same spot.
Massive Mass Drivers
Since mag-lev trains can carry a string of cars amassing hundreds of tons I see know reason why we can't build mass drivers large enough to handle 100 ton modules after ten or twenty years of development on the Moon. Originally, O'Neill and others envisioned a mass driver that was stowed in sections in a Space Shuttle. This would shoot ten kg. packages rapid fire. I think it will be better to build the mass driver on the Moon and make it huge. We will still need radar tracking and computer control, but that's no biggie. Since mag-lev trains cost $50-$60 million per mile, a ten Gee mass driver that was 2.81 kilometers long that launches at 2.347 kps would cost $87 million to $104 million at these rates. That price tag is one reason most of the railways on the Moon will be conventional monorails running at 150 mph+ with no air friction and mag-lev run will only exist when industry is highly developed and only between major population centers.
Payload Destinations
Not only will loads go to L2 and then L5, they will go to L2 factories in halo orbit where fuel for taxis is derived. They can also go to L1 and the material transferred to an aerobraking module and shot down to LEO with a small 100 mps burn. Heat shields and other stuff will be cannibalized in LEO as there will probably be a large space hotel industry there someday. More people will be able to afford an orbital vacation than a lunar one to be sure. Aerobraking modules might just use big parafoils made of ceramic fibers instead of massive heat shields similar to the inflatable heat shields tested by the Russians. These will be easier to ship back to L1 for reuse. We won't have to chase down loads in LEEO; we will launch them when the timing is right for them to rendezvous with LEEO stations.
Also of interest, we find that a load launched onto a minimum energy trajectory to L2 takes 90 hours. At apolune it is traveling at 57 meters per second but L2 is moving at 106 mps faster than the Moon. If shot "from behind" it would have to fire its motors to catch up with the mass receiver. This would only require about 50 mps so we might use even less monopropellant. Of course, the gravity of Earth and the Sun complicates the astrodynamics, but this can all be worked out in the future.
