Polywell + Space Elevator = Hyper Cheap Access To Space

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BSPhysics
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Post by BSPhysics »

Keegan, I'm aware of the awesome tensile strength necessary to construct the ribbon and the current limitations on CNT length. But, I'm not sure we need 100,000 km CNT molecules to make an adequate ribbon. One millimeter tubes woven into a matrix would make a ribbon capable of supporting a modest elevator car. It's curious to me that as futuristic as a SE might seem, fusion powered SSTO's may leapfrog right past them in affordability. Or, maybe the combined technologies will create an earth to orbit system better than fusion or elevators alone could provide.

From Wikipedia...


Cable

The cable must be made of a material with a large tensile strength/density ratio. A space elevator can be made relatively economically feasible if a cable with a density similar to graphite and a tensile strength of ~65–120 GPa can be mass-produced at a reasonable price.


Carbon nanotubes would be a highly useful material for creating a space elevatorBy comparison, most steel has a tensile strength of under 2 GPa, and the strongest steel resists no more than 5.5 GPa, but steel is dense. The much lighter material Kevlar has a tensile strength of 2.6–4.1 GPa, while quartz fiber[citation needed] and carbon nanotubes[28] can reach upwards of 20 GPa; the tensile strength of diamond filaments would theoretically be minimally higher.

Carbon nanotubes' theoretical tensile strength has been estimated between 140 and 177 GPa (depending on plane shape),[28] and its observed tensile strength has been variously measured from 63 to 150 GPa, close to the requirements for space elevator structures.[28][29] Even the strongest fiber made of nanotubes is likely to have notably less strength than its components.

Improving tensile strength depends on further research on purity and different types of nanotubes.

Designs call for single-walled carbon nanotubes. While multi-walled nanotubes are easier to produce and have similar tensile strengths, there is a concern that the interior tubes would not be sufficiently coupled to the outer tubes to help hold the tension. However, if the nanotubes are long enough, even weak Van der Waals forces will be sufficient to keep them from slipping, and the full strength of individual nanotubes (single or multiwalled) could be realized macroscopically by spinning them into a yarn. It has also been proposed to chemically interlink the nanotubes in some way, but it is likely that this would greatly compromise their strength. One such proposal is to take advantage of the high pressure interlinking properties of carbon nanotubes of a single variety.[30] While this would cause the tubes to lose some tensile strength by the trading of sp² bond (graphite, nanotubes) for sp³ (diamond), it will enable them to be held together in a single fiber by more than the usual, weak Van der Waals force (VdW), and allow manufacturing of a fiber of any length.


A seagoing anchor station would incidentally act as a deep-water seaport.The technology to spin regular VdW-bonded yarn from carbon nanotubes is just in its infancy: the first success in spinning a long yarn, as opposed to pieces of only a few centimeters, was reported in March 2004; but the strength/weight ratio was not as good as Kevlar due to the inconsistent quality and short length of the tubes being held together by VdW.

As of 2006, carbon nanotubes cost $25/gram, and even a space elevator that did not reach GEO would have a mass of 20,000 kg. However, this price is declining, and large-scale production could result in strong economies of scale.[31]

Carbon nanotube fiber is an area of energetic worldwide research because the applications go much further than space elevators. Other suggested application areas include suspension bridges, new composite materials, lighter aircraft and rockets, armor technologies, and computer processor interconnects. This is good news for space elevator proponents because it is likely to push down the price of the cable material further.

http://en.wikipedia.org/wiki/Space_elevator


BS

cuddihy
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Post by cuddihy »

I wonder what the effects of atomic oxygen would be on a carbon nanotube yarn?
Tom.Cuddihy

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Tom Ligon
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Post by Tom Ligon »

Cuddihy,

Interesting concept you have there!

There are a host of environmental problems which have been brought up which would challenge a space elevator. Atomic oxygen is included, but one might be able to keep a layer of "paint" on the cable, frequently refurbished.

Orbital debris is a major hazard. Belts might better resist this scourge.

The long haul thru radiation is worse for passengers, but probably detectably rough on a molecule as specific as a carbon nanotube.

Does anyone else recall a tether experiment run from a shuttle some years ago? They paid out a weight on a tether, using tidal force to pull it outward. The tether had a conductive loop of some sort, and the idea they were testing was an electromagnetic braking system. It was a sort of "Diet Smith Magnetic Space Coupe" application, intended used for deorbit braking against Earth's magnetic field instead of for propulsion.

Anyway, they spooled the thing out, and fireworks ensued from the spool. A huge current discharge occurred down the length of the thing, and they were baffled by it. SOMETHING up there created a large voltage gradient down the tether. This kind of nasty surprise could set a space elevator project back, or kill it. Or, it could be a source of power to exploit.

A recent proposal to make a solar-panel-powered "magnetic sail" was on the internet a few days ago. The idea is to use electrical power to deflect the solar wind for thrust. A Polywell might work even better. Tweak the design a little and you get ... a Bussard ramjet! A very low-performance one, but I think Doc would be tickled if anybody built one.

Nanos
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Post by Nanos »

Perhaps we should look at Maurice Ward's Starlite polymer, in case his Sidney Stratton type discovery is what we are after..

Keegan
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Post by Keegan »

BSPhysics wrote:Keegan, I'm aware of the awesome tensile strength necessary to construct the ribbon and the current limitations on CNT length. But, I'm not sure we need 100,000 km CNT molecules to make an adequate ribbon. One millimeter tubes woven into a matrix would make a ribbon capable of supporting a modest elevator car. It's curious to me that as futuristic as a SE might seem, fusion powered SSTO's may leapfrog right past them in affordability. Or, maybe the combined technologies will create an earth to orbit system better than fusion or elevators alone could provide.


Current state of the art nanotube length = 1mm = 10^-3

Tether length = 37,500km = 3.75x10^7

Consider current Nanotubes are 10's of gigapascals short of required strength

That leaves us with 11 orders of magnitude from where we are and where we need to be. I realise that nanotubes can be woven but then the whole is rarely as strong as parts.

Consider Computer Technology. Utilizing the finest efforts of the human race transistors have shrunk 5 orders of magnitude in 50 years. Note that this is making things smaller and not really building complete structures piece by piece, merely etching silicon.

Just by considering the magnitudes involved, I dont think its going to happen in my lifetime, even if we live longer and technology is accelerating at an exponential rate. So it would be a waste of my energy to think too much more about it. I thought about it alot once. (Diamond had my hopes up)

On the subject of Polywell SSTO vehicles, i share much the same pessimism. The SSTO wiki has an excellent breakdown of the challenges involved

Have a look at the Reaction Engines A2, by far the most viable hypersonic/ssto transport design today. Engines are heavy. It is widely known that you must utilize varying engine technology at varying air speeds. The weight penalty involved for extra engines kills you everytime. Add a fusion reactor ? you gotta be kidding me. The A2 sidesteps this with its ingenious SABRE Engine. It is interesting to note that the heat exchanger proposed in this engine is definitely worth studying for polywell applications. SABRE dramatically cools the air from 1000 C down to -140 C, approaching 1 GW/m³ !! :shock:

Polywells are going to be bigger than first thought thanks to the complex electric fields and peripherals. Unless the transport is going to incredibly massive this will also reak havoc with the Area Rule compromising hyper sonics greatly.

Also redundancy. If i had a shot at being first to Mars i would take a risk and ride a single reactor for sure. Commercial on the other hand ? your gonna want sort of back up. Drifting off into oblivion with cabin fever would seem a horrible way to go. So eventually your gonna want a second reactor thats going to complicate things even further.

No need to get down though, there is a solution. I think the launch loop is much more viable. It is often criticized for its chances of catastrophic failure. Much the same can be said about the space elevator. What very few people realize is that if a few launch loops were built in parallel, it would only take a few more loops to support a compromised loop = Safety. Building extra launch loops would still be far far far cheaper than a single space elevator.

So power multiple launch loops through cheap early generation fusion power. Next, form the main reactor space from triangular modules so that the final product may resemble something like a Geodesic Dome. Launch fusion modules and other space craft modules from your multiple launch loops, 5 tonnes at a time @ up to 80 per day, per loop @ xx cents per kilo.

Assemble the fusion spacecraft modules in orbit at a space station. Make sure the space craft can handle 2x the crew numbers. Send 2 spacecraft off to Mars and with Polywell QED/ION Drives you should get to Mars in a few weeks with redundacy.

I call shotgun ....... :D
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MSimon
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Post by MSimon »

I did some BOE calculations.

If you use a maglev track (Inductosyn) to get you up to Mach .8 then a LH2 reaction mass SSTO is very feasible. The only cooling required would be from the reaction mass. The whole deal would be done without decelerator grids. Thermal all the way to limit size.

We will know a lot more once we get BFRs operational.
Engineering is the art of making what you want from what you can get at a profit.

Keegan
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Post by Keegan »

^ When i read up on SSTO, the thing that got me was the launch profile. Wildly different to a conventional rocket. Much more horizontal for much more longer. You will need a large reaction mass. It incurs a huge weight and volume penalty.

Still though, Clean, Nuclear Thermal Single Stage To Orbits envouge in 20XX ? i like the sound of that. While we are at how about a pure diamond SP3 space frame ? That would definitely solve all our thermal, strength, weight and pimping needs 8)
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MSimon
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Post by MSimon »

We are probably on the verge of carbon nanotube being good for space frames. A lot of the design effort in space frames goes towards dealing with shear stress i.e. the outer edge is where the peak forces are. Low density high strength materials are a big help in that respect.

You can increase the distance between framing elements.

Also good for compression elements as the L/D ratio is better (smaller) for a given strength which gives better buckling resistance.
Engineering is the art of making what you want from what you can get at a profit.

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