Dynamic Structures

A dynamic structure is a seemingly fixed structure with compression support provided by the deflection of high speed streams of mass. There are many variants for space launch:

• vertical, like the space fountain

• circular, like Paul Birch's orbital rings

• orbiting segment, like Roger Arnold's spaceport

• ground anchored loop composed of circular segments, such as the launch loop

These ideas were all developed independently, most "negatively inspired" (that won't work but this might) by Arthur Clarke's science fiction novel "Fountains of Paradise", which was inspired by Yuri Arsutanov's 1960 "electric train to the cosmos" , which was inspired by Konstantin Tsiolkovski's 1895 "space tower". See Jerome Pearson's informative history of space elevator concepts.

We did not learn about each other until we published and presented at conferences; in my case four years after I started working on a series of ideas that led to the launch loop. Another inspiration was Shirley Smith (mother of a long-ago girlfriend) who mentioned the old folk idea of a sky-hook, an imaginary device that hooks onto nothing in midair.

All dynamic structures are intrinsically unstable. They require an absolute reference, precision electronic control, and some way to create lateral (sideways) forces to correct for inevitable (and typically exponentially growing) variance. Structures that rise through the atmosphere will be subjected to wind loading (fastest in the lower stratosphere), ice accumulation, and perturbations from the vehicles they support. While it may be possible to correct for all these problems merely by local spacing control, and deflecting the upbound mass stream, structure movements must "thread the needle" through every lumped mass attachment (vehicles, high altitude stations, and back down to the bottom terminus) to submillimeter accuracy.

It is an open question whether this can be done without forces launched through diagonal guy wires, which can also be deflected by wind and ice, and add failure risks. Guy wires must be tapered, heavier at the top, and the taper will be far too great if they are taller than 100 kilometers or so.

• OrbitalRings - why these are difficult.

• Don Dolan's 2017 "Spaceport" ... orbital rings, decades after Paul Birch and Ken Brakke's orbital rings

• DomesticStructures - why you don't want to build a bridge or a building with launch loop technology.

More on origins

From a 2005 Wikipedia talk page comment for the Space Fountain

Keith Lofstrom here, inventor of the launch loop. Seeing this article reminded me that I have a few file boxes of papers from the 70's and 80's, when all this stuff was new and a lot of enthusiasts were working on it. I can provide lots more information.

I have a bad photocopy of Rod Hyde et al's papers, the original 30 Jan 1981 paper and the 2 April 1982 addendum. I have quite a few other papers from the era, too. I am heading out of town for a week, but when I get back I can scan them and put the images up on the launch loop server. . . . and I never did, way too many things to do. KHL 2018

The first published paper in the general area of momentum storage launchers (my term) or dynamic structures (Bob Forward's term) would be the Roger Arnold and Don Kingsbury articles about The Spaceport in the November and December 1979 issue of the Analog Science Fiction Magazine. This is an orbiting mass driver that captures payloads. Immediately upon seeing the November Analog, I went up to visit Roger Arnold in Kent, Washington - he was working at Boeing at the time. I had been working on a long "flying cable" (tether) system (I presented it at the Orycon 1 science fiction convention in 1979), and Roger's rather elaborate (and high gee) capture system intrigued me. We had a bit of an argument about "frozen spinach launchers", but parted the best of friends. I wonder where he is now?

The launch loop was born out of the flying cable - the problem with all these systems is stability and control, and I decided I needed a rigid fixed surface to work against for lateral stability. For example - if you are aiming flying rings at a cm-accuracy target 40,000 Km away, and transit time is an 5000 seconds, you need to measure lateral and rotational velocities within 2 micrometers/second (about one part in 2E-10 compared to breech velocity). If you are measuring between two points a kilometer apart, you need to measure displacements of 200 nanometers, and know with exteme precision where the datums are on every individual ring. Of course, with rings you need to control 6 velocities and 6 positions to this positional accuracy. And the rings that miss ... Yikes!

So, assuming a planet as a source of basic stability and reaction mass, and a grid of laser interferometers to make up for the fact that planets are actually rather mushy things (tides, earthquakes, and all that), and realizing that there were dozens of other problems with vertical launchers, I went horizontal.

The first thing I came up with was something like Ken Brakke's Skyrail (L5 News, July 1982) or Paul Birch's Orbital Ring (JBIS, Nov 1982) but I abandoned that in mid-1980 as having most of the same problems, as well as being impossible to build from the surface. Since I only needed a 2000 km launch path for 3-gee-to-escape, I chopped off the useless extra 38,000 km and made a closed loop. Hence, the launch loop. It evolved from there, and resulted in a submission to the AIAA Advanced Space Propulsion competition in April 1981. That was followed by a short paper in the November 1981 American Astronautical Society Reader's Forum, another short non-technical paper in the August 1992 L5 News (Ken and I learned about each other from this, and this let Rod Hyde know about both of us), and a presentation at the April 1993 L5 Conference in Houston. I presented just after Eric Drexler presented his first public paper on nanotechnology. We were both mobbed by enthusiastic fans in the hallway for hours after the presentations. That night, Eric and I watched the Soviet Mir space station pass overhead from the roof of the hotel, and discussed Ricardo's Law of Comparative Advantage in a world of big, slow humans and quick, cheap nanointelligences. I also met Bob Forward there, which began a long friendship. Robert Heinlein came to my room party ...

In May 1983, I met with Ken and Paul at the 6th Princeton Space Manufacturing Conference. None of our papers were accepted, but we took over a classroom for a couple of days afterwards to formalize some analysis conventions and do some calculus together. There we explored the "two stream instability", with Paul becoming convinced that we could never make it work. I argued that with damned good measurement ("many wavelengths of light"), and smart active control ("only 14 millimeters per microsecond"), and frequent (100 Km apart) cables to the ground to shed correction forces, taking out the instabilities was quite doable (what Bob Forward called "A mere engineering detail").

I had been working with Stan Schmidt at Analog since September 1982 on yet another short non-technical paper, which appeared in the December 1983 Analog magazine (my first paid sale of writing). This connected me to science fiction author Dean Ing, which connected me to Leik Myrabo, which finally got me a slot at the July 1985 AIAA Advanced Propulsion Conference, where I could finally publish some damned equations!

This was all like pushing rope - work was getting busy (I was managing a design team), and I was getting approximately zero outside help. I came to the depressing conclusion that this wasn't going to happen in my lifetime unless I earned enough money to build the damned thing myself. Note, as of July 2005, I haven't made the money thing happen yet. Sigh.

DynamicStructures (last edited 2018-05-07 17:53:23 by KeithLofstrom)