I'm busy writing a proposal or two, but here's an innovative way to get to the Moon. Much lower marginal cost, and exactly the kind of things that NASA should be considering.

[via Mark Whittington]

If NASA does put significant effort into innovative approaches like tethers (which are great for delivering material down to the lunar surface, as alluded to in that link, not just for getting away from LEO), then VSE will end up being more useful than I have feared. Lunar nuclear power would also be very useful.

Having said that, ISS included all sorts of attractive technologies that were dropped along the way (remember that helium-xenon solar dynamic power system?), so I will not be surprised if the same thing happens to these items.

One "problem" with NASA actually deploying a MXER this is that equatorial orbits appear to be the most logical place for MXER tethers to be located. Getting to zero degrees of inclination from a NASA launch facility isn't the easiest thing to do.

Now, if an alt-space outfit were to deploy a MXER via SeaLaunch or SpaceX at Kwajelein (Kouru and Alcantara, Brazil for non-US pads) that MXER could be used to fling supplies to NASA crews or tourist operations doing lunar exploration.

= = =

Using an equatorial inclination for MXERs will greatly reduce "launch window" issues when it comes time to match payloads with the tether tip.

NO!! Building a gigantic booster is the one true way!!

How does the math look on using the charged tether as a form of propulsion?

That, together with the catch mechanism, is my biggest concern wrt tethers.

Long term, they also can get in the way of elevators, but I'm sure things can be shuffled around to accomodate whatever is needed.

The above comment is meant as sarcasm. The comment engine stripped off my sarcasm tags.

If this idea would work around Earth, couldn't a similar system be installed in orbit of the Moon, for the return journey?

Big D

The electrodynamic tether works great at low altitudes, up to about 1500 km. The TSS1-R mission gathered twice the current that was expected by the initial conservative calculations. They were generating several kilowatts when the tether broke at 19.5 km in deployed length.

Dennis

Duh, forget my inane comment above. Moon's got no magnetic field...

Dennis,

But isn't this the other way around? Instead of generating power by dipping through the upper atmosphere, we're talking about boosting orbit by electrifying the tether with power generated by other means. IANAE, but I'd like to see someone who is give me some warm fuzzies on the issue before I start getting my hopes up.

D

The system that generates power by cutting magnetic field lines is equally efficient in pumping the orbit by putting power into the tether that cuts the magnetic field lines. The reason that the e-tether wants to be at the equator is that this is the most efficient lattitude for cutting the field lines.

Do a search on the Nasa technical reports server for the scientific results of the TSS 1-R mission. It was a pretty good mission.

Dennis

Like space transports tethers like high flight rates. Unlike space transports direct cost reductions with increased scale are not as strong. This infers that the first tethers should be small, generally necessitating a lot of orbital assembly. I do not see this fitting into the NASA/VSE way of doing things. If they do still try it, it may result in a space shuttle of tethers putting everyone else off of them for years to come.

I also do not much like the idea of directly including the electrodynamic tether. This is a very low flight rate solution, with all that infers.

Perhaps the first viable application of tethers might be for the orbiting/deorbiting of space transports. This only needs a few 100m/s of delta v and so can be a very small and simple system yet still work with large payloads.

I also do not much like the idea of directly including the electrodynamic tether. This is a very low flight rate solution, with all that infers.

OTOH, why just have one? Given the size, you couldn't have thousands, but there should be room for dozens of them.

My take is that this would be interesting if merely to develope a working tether application in space. I certainly see something like this being created well before a space elevator.

Mark,

Rotating tethers are ideal for the moon. You cannot use electrodynamic reboost, but you can balance the incoming/outgoing traffic, using bags of regolith as ballast. You can also use magnetic launchers from the lunar surface to reboost the tether. See www.tethers.com for more details.

A "Lunavator" tether on the moon becomes a "momentum bank" - Instead of using valuable propellant to reduce your incoming velocity enough to land on the lunar surface, you put your momentum "in the bank" and withdraw it when it comes time to go back to earth.

A "Lunavator" tether on the moon becomes a "momentum bank"...

A good point, not often enough recognized (although McCarthy and Moravec saw it long ago). Once there's frequent two-way traffic, rotating tethers with "catchers" at each end allow you to keep using the same momentum over and over -- vs. rockets, for which half the momentum is lost in dissipating gases every time you use them. It's an even purer win than Aldrin's cycler.

Two way traffic?

Supplies from Earth to Luna & PGM bearing asteroid fragments from Luna to Earth.

There you go.

Yes one tends to forget that in continually trading momentum and energy around the solar system total energy and momentum must be conserved. This infers that a solar system wide transport system is possible that requires no propellant or net energy to operate. One might even use it to extract orbital energy at the outer reaches where solar power might be prohibitive.

Coping with the inaccuracy of tether throws is a problem, typically this limits one to larger payloads with on board guidance. One way to help mitigate this might be to use an active system on the tether tip. I am thinking of a harpoon here, but with the pointy bit replaced by a large flying net. This would seem a more capable form of catcher and should enable far more aggressive tether designs, hopefully including tether mass launchers from the Lunar surface. Mass drivers being so heavy, awkward and payload limited.

Tethers are a cool concept, but I'd hate to crew a system using one. Having to make your target windows with fraction-of-a-second accuracy is asking for a lot of aborted flights.

A spinning loop might be easier to catch than a tether. A big bicyle wheel in space would only be an extra pi length of tether. No more syncopation.

Another easy target is the center. Doing a zero-zero intercept with the center, then sliding out the tether may be easier than catch and toss (albeit half the benefit and no "banking").

Can you do some kind of counterrotating pair of tethers where there is no magnetic field?

I happen to agree with Pete that the best use for tethers is for orbital boost/reboost in LEO. We are a very long way from knowing how to operate a >100 km tether for any application.

There was a proposal in the late 80's early 90's for an e-tether for ISS reboost but JSC killed it. It is still a good idea for that application. The SEDS-2 mission proved the ability to accurately target a re-entry corridor with a downward deployed tether.

Dennis

I think we just need Chuck Norris to round house kick space ships to the Moon.

Chuck Norris once bet NASA he could survive re-entry without a spacesuit. On July 19th, 1999, a naked Chuck Norris re-entered the earth's atmosphere, streaking over 14 states and reaching a temperature of 3000 degrees. An embarrassed NASA publically claimed it was a meteor, and still owes him a beer.

.....Okay I've gone stupid since reading this site:

http://chucknorrisfacts.com/

Y'know, I've heard of the slingshot effect, but never in the context of actually using one.

We start by building this really huge Dennis the Menace...

A circumferential tether prevents the use of taper, so is very limited in tip speed. It might also be difficult to grab on to as mass prevents it from being covered in catchers. One could add tapered spokes which would help. Still, it might work well for low tip speeds.

One of the difficulties in such systems is physical contact speeds. Above maybe a 100m/s or so one probably has to go to a non contact magnetic approach, this is almost equivalent to a linear accelerator.

Counter rotating tethers can be used to trade rotational momentum back and forward and even cancel it out. However great care needs to be taken that their paths never cross - gear them together?

One additional point: a LEO rotating tether could catch suborbital launch vehicles. Maybe SS1 and its immediate descendants won't be as far from being orbital launchers as some have complained.

Pete: "Coping with the inaccuracy of tether throws is a problem, typically this limits one to larger payloads with on board guidance."

A valid point, but I'm optimistic about drecreasing mass and increasing sophistication of on-board guidance, so that only trivial thrust would be needed for corrections. The smarter you are, and the sooner you apply it, the smaller the tweaks required.

One other point about rotavator jai-alai even before we have two-way cargos to trade momentum: while electrodynamic tethers are slick, a magnetic field is not essential. Solar-electric, beamed power, or other low-thrust sources could spin up a tether gradually between launches/catches. Not free, but still a whole lot cheaper than the high power from >1G rockets.

Paul: "a LEO rotating tether could catch suborbital launch vehicles."

The deorbit burn effectively comes off the total delta v requirement for orbit. So one sort of gets the benefit of a ~300m/s tether twice. Doing just this alone probably doubles the space transport payload and so this might be the most economically viable starting point.

At this the tether mass is a very small fraction of the space transport mass ~10%, and probably within the space transport payload mass. Due to very high current launch costs I find it works out cheaper to develop your space transport with orbital capacity, (though the payload might be minimal), and use it to delivery the first tether. The tether will I expect want to be incrementally grown from small beginnings in conjunction with the tether tender. Even in the beginning, tether tender launch costs will be far less than current larger launchers and the money should be better spent there.

So, if these tethers are possible, and if the orbital-airship ideas at http://www.jpaerospace.com/ are possible, and if a lunar space elevator is possible (described at http://www.universetoday.com/am/publish/lunar_space_elevator.html?18112004
), then we could go from Earth-surface to Moon-surface and back again, without ever using a rocket ...