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comments by Keith Lofstrom, 2008 July 10

It will be a long time before something like a launch loop gets built. In 2008, there is no significant market. I hope that the people building that market using expendable rockets will proceed with confidence - as launch volume increases, there will be alternatives that can bring prices down as much as needed.

Although in mass and cross section and length it is smaller than a trans-regional power line, the launch loop is still too damned big. I sell products that are 40 by 70 microns (see http://www.siidtech.com ). I also work with signals that move at large fractions of the speed of light, so 14 micrometer per nanosecond rotors seem rather slow :-). I hope the launch loop inspires some clever person to invent something with the same essential behavior ( 3g, 11km/sec, 80km altitude, ground powered ) that gets around the need for a long structure.

The tolerable g forces and high orbit capability are critical. A lot of people are thinking about stuff I call "frozen spinach launchers", as the highest value commodity they can move. I want to ride the thing, and to interesting places, too. I want to bring along assembled industrial equipment, much of which won't tolerate the g forces people can tolerate.

I could be wrong - John Hunter, builder of prodigious experimental hydrogen hypervelocity cannons for the Air Force, showed me his cell phone. He added some structural plastic at the right spots, and accelerated it inside a container at thousands of g's to about half orbital velocity. I rather doubt that a 3 ton NC milling machine will tolerate that, though.

I did not patent the launch loop on purpose - I want it to get built, I want to make it easier for others to do so, and I have other ways to earn my living. It will be a long time before the first one, and longer before the second; the first operator will have a sustainable advantage without resorting to government granted monopolies.

The rotor is segmented and jointed or in some way "stretchy"; as it drops from 80km altitude to the turnarounds it picks up speed, and thus has to stretch longer to maintain the same mass flow rate without high axial tension. The rotor compresses a little after losing momentum to a payload and slowing down, but less than it does in the 80km drop. There is probably a better way to get to the same stretchiness. The rotor, segmented or not, shouldn't be too bendy; that makes the segments harder to control. At the shortest wavelengths, the transversa rigidity of the rotor segments and the joints keeps the rotor lined up and permits the rotor-to-track spacing to be small, especially in the turnarounds.

The cross section and mass per meter (about 3 to 7kg per meter) is an issue. It could be smaller and launch smaller payloads - single passenger vehicles are possible. However, the biggest scaling effect is keeping it from getting kicked around in gusty high altitude winds. I talked with some folks that did weather research at what is now NOAA in Boulder CO, and learned something about wind variations along the equator. Between the hurricane belts, the wind is boring. I computed the needed "rigidity", added a safety factor, and came up with the mass per length. The cross section can probably be optimized down, especially with doppler laser providing some warning about oncoming air packets.

The smallest, simplest vehicle might resemble a single seat with a plastic fairing in front and a magnet rail running between the legs. I expect there will be daredevil versions of single passenger launch "vehicles" - Hell's Angels in space. Unmanned payloads may be as simple as wooden shipping pallets balanced on either side of a magnet rack, with plastic fairings stapled the front.

At the larger end, a launch loop can certainly be made a lot larger in diameter. Using a large hollow pipe for a rotor, with cross stringers and other goodies to constrain vibrations, a launch loop could be meters in diameter and launch stuff as big as 747s. You have to make it wider; the payload rate to surface area ratio has to stay roughly constant or the rotor will cool too slowly. A large diameter rotor makes possible the interesting notion of long, skinny robots that ride around inside and do rotor repairs. Sorry, no human workers, too much acceleration at the turnarounds for people to work in there, and besides, going home on weekends is a bit of a problem.

Large diameter loops will present a larger cross section to occasional bits of space debris, though. Launch loops will exacerbate the problem by increasing the mass in orbit. Anything below geosynchronous eventually falls down. Hopefully, we will be able to send up enough robots and lidar imaging systems to eventually track down every little bit of near-earth space junk, and a larger loop can support elaborate lateral meteor bumper systems.

The 80km altitude is a compromise between launch drag and safety from orbiting space junk. The original launch loop design was at 120km altitude, and the drag on launched payloads is negligable. However, junk at 120km altitude stays aloft for a long time. Orbits decay towards circularity, but by the time most objects reach 80km altitude they only have an orbit or two left before they plummet into deeper atmosphere and fall. It could go higher with some minimal meteor bumpers on the side, and an aggressive robotic cleanup program. I don't know how to do that.

However, unlike many objects like orbital rings, you can build lots of launch loops. Around the equator, between the hurricane belts between 5 degrees north and south latitude, you can build thousands of launch loops, and with some care you can even limit the interaction if some of them fail catastrophically, by disposing of the rotor into the ocean or into solar orbit. This is not probably possible with space elevators - the orbiting debris from the upper portions of the first one to fail will eventually take out all the rest.

Superconductors and supermaterials. Feh. A good concept doesn't need them starting out. If "magic technologies" are used later on used to improve things, that is OK, but the extra complication is just another failure mode in the beginning. As my former boss Winthrop Gross said, "too many things that oughta work is a not-oughta work". The launch loop itself is risky enough already.

Tether propulsion makes a good complement to the launch loop. Tethers can be used for apogee insertion, and they don't need to be that big to provide 1000m/s scale velocity changes. At some point, you can be trading momentum with the moon to drive the system up there. I really don't like the idea of sending up rocket engines with every payload - it adds cost, cuts deliverables, and if one cooks off during launch it can take down the loop.

Since I like tethers, need hanging cables for long-wave stabilization, and need to winch payloads up to West station somehow, I encourage the space elevator folk to keep dreaming their dreams. If they fail to achieve their goals by a factor of 10 (or they cannot be dynamically stabilized, there are issues), they have still created a lot of technology that will be useful for the launch loop. So recruit them if you can, but please don't discourage them. A person needs dreams in order to get out of bed in the morning.

About the "Lofstrom" loop and other appearances in print. Frederick Pohl used the name "Lofstrom loop" in his SF novel Heechee Rendezvous. He landed stuff on it to save energy. And broke it, which is dramatic. Hey, folks, the energy saved is cheap compared to the risk. Energy is free. It just takes expensive attention and capital equipment to move it and deliver it. You don't trade off expensive equipment to save a little energy. But if I don't like it, I can write my own novels. I asked Fred not to call it a "Lofstrom loop" I prefer launch loop. The idea of fame upsets me a little. I would rather it be named after the team that actually builds one. I just want a ride.

Bob Forward was a friend, and he used the launch loop in his SF novel Dragon's Egg. He had the neutron star inhabitants build one a meter or two long, to launch themselves to neutron star escape velocity (a significant fraction of the speed of light). He broke his launch loop in a starquake. More dramatic, I guess.

If you want a less technical description of the Launch Loop, you can find it in the December 1983 Analog magazine. Or in the L5 News around that time.

I would love it if someone good at drawing or computer animation would make pictures and movies of the Launch Loop. The problem is, from a distance far enough to see the whole thing, it is thin enough to be quite invisible. Much dramatic license will be needed, a way to make it visible without implying that it is massive.

* end of comments by Keith Lofstrom, 2008 July 10

WikiPedia (last edited 2013-06-24 02:25:36 by KeithLofstrom)