Building Infrastructure: The Tether Option

by Paul Gilster on August 25, 2009

Conservation of energy means we never really get something for nothing. Nonetheless, the idea of propellantless propulsion is profoundly important for our future in space. A solar sail uses momentum from solar photons to get its boost, letting the Sun serve as the energy source so we don’t have to carry heavy fuel tanks and can maximize payload. So propellantless propulsion really means finding sources outside the spacecraft itself to do the work.

The Interplanetary Gambit

Recently I’ve finished Michel Van Pelt’s book Space Tethers and Space Elevators (Copernicus/Praxis, 2009), a treatment of a technology we seldom consider in these pages because it’s more practical in terms of near-Earth solutions. But Van Pelt surveys tethers — and the space elevator idea, which is built around what could be considered a giant tether — so comprehensively with regard to the implications of leaving the propellant behind that his book is a must read for those of us interested in deep space development. After all, building a space-based infrastructure will demand cheap access to the outer system, and it turns out tethers have interplanetary possibilities.

space_tethers

Consider the principles of momentum exchange tether systems of the kind known as ‘bolos.’ Here we’re talking about a rotating system that could be used to transfer spacecraft to higher orbits. A 100 kilometer cable in an elliptical orbit can be set to spinning vertically like a sling. Imagine it with a ballast mass on one end and a spacecraft catching and docking device on the other. The center of mass (and thus the center of rotation) will lie close to the ballast mass. Timing is all — the tether’s rotation can be timed so that when the bolo reaches its perigee, the tether is vertical and swinging backward, capable of matching the velocity of a slower moving satellite.

As the catching device swings past, the payload spacecraft can be hooked to its docking clamp. At the other side of the rotation, the docked spacecraft will now be at maximum altitude, and can be released at a higher velocity. We’ve given our spacecraft a cheap ride to a new orbit. Now imagine a larger system of this kind, as Van Pelt does:

A series of bolo tethers, each tether passing a spacecraft onto the next, could be used to achieve even larger orbit changes than a single system. For example, one tether system could catch a spacecraft from a very low orbit and swing it into a somewhat higher orbit. Another bolo picks it up from there and puts the satellite into a geosynchronous transfer orbit (GTO). A third tether catches the load again and imparts sufficient velocity to it so that it reaches escape velocity. A satellite initially orbiting just above the atmosphere could thus be slung all the way into an interplanetary orbit around the Sun, and all this without using any rocket propulsion and propellant…

Reverse the process and you can catch a spacecraft at the top of the tether rotation and release it into a lower orbit. We can imagine, then, a system that brings interplanetary missions returning to Earth into a series of orbits around the Earth that culminate in a reentry and landing that use no propellant at all. All those kilograms of fuel that would otherwise have had to make the interplanetary journey so as to be available for return are thus no longer needed, and once again we’ve flown with a much larger payload.

A ‘Rotavator’ in Lunar Orbit

This is absorbing stuff, and Van Pelt goes on to discuss Hans Moravec’s idea of a lunar ‘skyhook’ or ‘rotovator, a notion that, in turn, draws on John McCarthy’s work in the 1950s.’ Here we have two long tether arms in rotation and a massive central facility. The tethers would have the same length as the orbital altitude of the central unit, meaning that as the system rotates, their tips would periodically reach the surface of the Moon. Now this is important: The rotovator would rotate in the same direction as its orbit, and at a rate so that the velocity of each tether’s tip would equal the orbital velocity of the system’s center of mass. That means that when the tips reach the lunar surface, their velocity relative to the Moon would be zero.

We’ve built a system — call it a ‘lunavator’ — that can place an object on the surface or pick up something that needs to get into space. What a scenario. Here’s how Van Pelt describes it, noting that people on the surface would not see it as a rotating system at all:

To them it will look like a long cable reaching straight down vertically from the sky, then retreating back up exactly the same way. Depending on the tether length and the rotation speed (the combination always needs to be selected so that the tip speed at the lunar surface is zero), lunavators can be made to periodically touch down at the same single spot, at several fixed spots or at any number of varying locations every orbit. However, a rotovator that makes multiple rotations per orbit could service more than one lunar station. We may also use the tether as a kind of helicopter that picks up a payload at one site and then, after one or multiple rotations, drops it at another surface location without releasing it into space.

Tuning Up (and Reboosting) the Assembly

Now with a system like this, you have to assume that the amount of time available to pick up payloads or deposit them on the lunar surface would be short, but additional tether deployed from reels at the tether tips could lessen that problem. Van Pelt figures the unreeling tether mechanism could lengthen the time the end of the tether spends on the surface to several minutes, a more practical solution. It’s clear in this system that the central tether facility will have to be quite massive to prevent individual payloads from stealing too much momentum from the rotating tether system.

How to prevent orbital decay is one of many problems Van Pelt addresses in this volume — he goes on in the chapter I’m dealing with here to discuss the Momentum-eXchange/Electrodynamic-Reboost (MXER) tether system invented in the 1980s by Robert Hoyt of Tethers Unlimited. Here, a tether system in orbit around the Earth replenishes its transferred orbital energy by using solar arrays, running electrical energy through a metal wire in the tether. The Lorentz force caused by the interaction of the electric tether’s magnetic field with the Earth’s magnetic field then provides a steady push which can be used to propel the entire tether system back up to a higher orbit.

MXERTether_4

Image (click for a larger image at the Tethers Unlimited site): Momentum-Exchange/Electrodynamic-Reboost (MXER) tether systems can provide propellantless propulsion for a wide range of missions, including: orbital maneuvering and stationkeeping within Low Earth Orbit (LEO); orbital transfer of payloads from LEO to GEO, the Moon, and Mars; and eventually even Earth-to-Orbit (ETO) launch assist. By eliminating the need for propellant for in-space propulsion, MXER tethers can enable payloads to be launched on much smaller launch vehicles, resulting in order-of-magnitude reductions in launch costs. In order for MXER tethers to achieve their potential in real-world application, several key technologies must be developed and demonstrated, including space-survivable tethers incorporating both high-strength and conducting materials, technologies for rendezvous with and grappling of payloads, and techniques for predicting and controlling tether rotation and dynamics. Credit: Tethers Unlimited.

Making the Concept Credible

Who knew so many tether experiments had already been performed as are revealed in this book, going back to the days of the Gemini program? This is enlightening reading, and I’m glad to see a book focused on tethers (and in one chapter, on the space elevator concept) coming onto the popular science market. Van Pelt does a fine job acquainting us with the history and principles of tether systems and their possible uses in making near-Earth and interplanetary operations far cheaper than they are today. We may also find them useful for creating artificial gravity on long space missions and, interestingly, sweeping away dangerous charged particles around a spacecraft.

Credibility is a key to getting tether systems accepted as a viable technology, but it’s disheartening to see that they do not factor into the current planning of NASA, ESA or any other space agencies. “It will require considerable advocating, publicizing, convincing and lobbying,” writes Van Pelt, “to keep development going, and that may turn out to be even harder than meeting the technical challenges.” That’s a sentiment that scientists on numerous projects, from solar sails to nuclear propulsion, will understand.

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tacitus August 25, 2009 at 15:31

Interesting. I haven’t read much about rotating tethers but my imagination was certainly captured by Bradley Edward’s original paper on a carbon nanotube space elevator a few years back. I may have to pick up a copy of this book to read up on shorter tethers too.

Of course, Edwards’ paper also discusses the utility of a space elevator for launching interplanetary probes. Given a design that extends the elevator ribbon beyond geostationary orbit as a counterweight, the far end of the ribbon could have the capability of launching probes to destinations all the way from Mercury to Saturn.

Gregory Benford August 25, 2009 at 15:41

Has anyone studied how much power would it take to lift the space station enough to remain in orbit? An electrodynamic tether driven by onboard solar panels could then replace the Russian booster that we pay for every year to keep the station from falling into the atmosphere.

Administrator August 25, 2009 at 18:11

Gregory, I don’t know about the power numbers, but Van Pelt discusses this concept and believes it’s quite workable. The problem he cites is that the safety rules now in place after the Columbia disaster (not to mention tight budgeting) mean that this is unlikely to get implemented. Here’s a quote:

The risk of a return capsule accidentally colliding with the ISS or a tether wrapping itself around the station will probably be too high; in addition, no money is set aside for the development of the system, and there is no launch slot available in the current ISS flight planning.

george scaglione August 26, 2009 at 9:27

gregory,paul,after all the time and money that was used not to even mention the work by astronauts in orbit and the pursuant danger to them.it would be,to phrase it in a polite way.”not bright” to do anything but keep the ISS in orbit and productive (even added to) for a long time! my opinion. your friend george

Tulse August 26, 2009 at 14:56

“Has anyone studied how much power would it take to lift the space station enough to remain in orbit?”

NASA must have those figures for VASIMR, since they will be doing tests in 2011-12 of using that technology for station-keeping for the ISS. My understanding is that the 200kW version will be attached to the station, although because of ISS power constraints it will operate in “burst mode” of about 10 minutes of thrust at at time, with a trickle-charge of its batteries from the ISS solar panels when it is not firing.

Christopher L. Bennett August 26, 2009 at 21:43

It bugs me that people keep calling these things “bolos.” That’s the wrong word. A bolo is a large knife or machete. The correct word is “bola” or “bolas,” a weapon consisting of two or more heavy balls connected by cords. The mistake seems to have originated with the bola tie, a kind of string tie that started being called a “bolo tie” by mistake in the ’60s, or so the dictionary says.

forrest noble August 27, 2009 at 23:51

The tether idea sounds good to me. The question is how much will it cost, how well will it work, and how reliable will it be. It would seem that it could have many purposes in the long run.

george scaglione August 28, 2009 at 14:45

forrest yes sir.but,as always,i wonder,how long will it take to develop these great ideas?! obviously not comming down the road soon. there are just so many.maybe,lol,too many areas in which we just do not know what is next! as i have said maybe i was just spoiled as a young man.my god! mercury,gemini,apollo.they laid it out and then DID it!!! we have to get back to that kind of thinking.by the way in the newest issue of newsweek magazine for the end of august their cover story is “in search of aliens nasa is out there looking” soooo i spent $7.95 to read a one page article.had the impression that half the magazine would be about this!! however they are looking for “other earths” and feel that everybody will be interested.as i say,yes, if another credable earth is found then people will be in a hurry to get there! will be a huge shot in the arm for the space program! ohh – this other earth is 1000 light years off!? people will still want to go,lol,i can just hear it now…”why hasn’t nasa done more!? and some of them gull dang it will be the same folks who 10 minutes ago said,”nasa is a waste. shooting money into space!(?) too complicated to go into here and now without boring everybody to tears and having you all wish,”why don’t they shoot george off into space!? ” but as always i do hope to have begun a good conversation. respectfully to one and all your friend george

lars willen June 17, 2011 at 13:01

a combination of mxer tether and a spaceblimp is my idea. lift up a spaceblimp to 50 miles by a speed of 4.000 mph and then lift by a low flyby tether satellite in a orbit of 110 miles orbit to a distance of 40.000 miles and then the spaceblimp left the tether satellite and goes in a moon orbit.

nwgg.de and uctp.de also youtube videos i upload under my name lars willen

Jim Poynton May 6, 2012 at 10:12

I have wondered to myself about this form of transport and the apparent getting of something for nothing. It seemed to me that the transfer of cargo along the line should have some effect. My thought was a change in orbital velocity of the satellite, which would have serious effects on the vertical climb. Another is turbulence in the Earth’s atmosphere.

Anyway, my suspicion seems to be confirmed in the New Scientist article at the end of the link I have included.

Jim

Jim Poynton May 6, 2012 at 10:14

The Website doesn’t seem to show, so I’ll add it here!

http://www.newscientist.com/article/dn16223-space-elevator-trips-could-be-agonisingly-slow.html

Jim

Bert Murray February 17, 2013 at 16:31

Another concept mentioned in the book , that should be brought up here is the aerovator. This would require a constant power source, but could be build from the ground up, hence not requiring payloads required to launch in orbit

Article
http://en.wikipedia.org/wiki/User:Yakushima/AerovatorAW

Google Books
http://tinyurl.com/abhergs

Zo0tie March 4, 2013 at 13:46

So what would a rotovator look like? Here’s one on Mars.

http://youtu.be/Z81wpmqXQLo

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