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#76 2008-01-07 08:15:27

Terraformer
Member
From: Ceres
Registered: 2007-08-27
Posts: 3,607
Website

Re: Un- conventional ways to LEO

Fusion bombs that don't use a fission primer? Interesting. How do they work?

(I'd like to build one.)


"I'm gonna die surrounded by the biggest idiots in the galaxy." - If this forum was a Mars Colony

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#77 2008-01-07 09:26:51

Antius
Member
From: Cumbria, UK
Registered: 2007-05-22
Posts: 1,003

Re: Un- conventional ways to LEO

Fusion bombs that don't use a fission primer? Interesting. How do they work?

(I'd like to build one.)

No.  Fusion bombs with fission primers, but used in such a way that there are negligable radiological consequences to any member of the public.

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#78 2008-01-18 13:56:23

John_Frazer
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From: Boulder, Co. USA
Registered: 2002-05-29
Posts: 75
Website

Re: Un- conventional ways to LEO

"jumpboy11j" on Dec 13, 2007 wrote about the hypersonic skyhook, and "terraformer" wrote about it being slowed down, and needing to be re-boosted.
My favorite way around this, is the realization that the cable extends up to orbit, and cuts the Earth's magnetic field. Perfect for a ED tether reboost.
As well, it gets back a little of what it lost while lifting something, when it lets anything go to drop back down. Empty returning passenger spaceplanes or cargo lighters, captured space trash, etc. Of course this won't amount to anything until we're bringing lots of payload back down, like asteroidal platinum group and precious & strategic metals.
In the mean time, it's long past time for ED tethers to be used.

This version of a skyhook is an upper stage, while laser thermal is a lower stage "zero stage". Airbreathing would seem to make the most sense.
It doesn't seem to make sense to investigate laser thermal for anything but close-in to the spaceport where it took off from. Beyond that, an airbreathing jet or maybe rocket is better, and I'm not even convinced that it needs to be very high speed. Just get it moving high and fast, and a spaceplane to the landing/capturing platform on the lower end of the skyhook is entirely within reason.

I have a more difficult time seeing this as a solution for larger payloads. Things that can be lifted in small payloads make sense for it, because it doesn't strain the system (or our credulity to consider it). For a people mover, it makes sense, because it's redundant and forgiving, compared to just riding a rocket up.
It never gets fast enough to need advanced TPS (not much more than the "SpaceShipOne"needed), and it doesn't carry much load of fuel/propellant.

For larger payloads, like station segments or interplanetary upper stages, I can't see anything near term that makes as much sense as BDBs.
We definitely need a modern day look at Aerojet's and Truax's figures for the Sea Dragon. I don't mind that it's basically a big roman candle with only one flight, If it lifts big cargos cheaply.

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#79 2008-05-06 10:55:13

JoshNH4H
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From: Pullman, WA
Registered: 2007-07-15
Posts: 2,526
Website

Re: Un- conventional ways to LEO

I have come up with another adaptation to a shorter space elevator.  It starts with a tower that is 6000 km high. 

I'm assuming that young's modulous is the right thing to use for this.  (www.en.wikipedia.org/wiki/young_modulus).  For silicon, this is 150 GPa.  This means that each meter squared of silicon can have 150,000,000,000 newtons of pressure.  if the whole thing is in 1 G, then that is 6,385 kilometers tall, which is about the radius of the earth.  But wait! that means that the end will be in 1/4 G, doesn't it?  so it can hold much more than that!  however, I will stick with 6000 km because I'm not sure I'm using the right modulus.

The distance from there to GEO is 30,000 km.  At GEO, there will be 0 G, while at 6 KKm, there will be 1/4 G.  Due to tapering the cable, and the slowing of the decreace of gravity, I will assume an average G of 1/25, or .4 Newtons per kilogram.


Kelvar: Necessary tensile strength: 18 GPa  Tensile strength: 3.6 GPa
UHMWPE fibers: Necessary TS: 12 GPa TS: 3.5 GPa
Monocrystalline Si: N- TS: 28 GPa TS:7 GPa
Carbon fibre: N- TS: 21 GPa TS: 5.7 GPa

and the TS / N-TS:

Kelvar:      .2
UHMWPE:  .29
Mc- Si:      .25
C F:          .27

Higher is better. 

I would suggest work into Monocrystalline Silicon because it hasn't really been investigated as a tensile material, and weaving it into threads can likely increase its tensile strength.    A higher tower can compensate for losses.


-Josh

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#80 2008-05-08 06:43:20

Gregori
Member
From: Baile Atha Cliath, Eireann
Registered: 2008-01-13
Posts: 297

Re: Un- conventional ways to LEO

This is just my ridiculous over the top idea, but how bout we use a type of catapult?

Drop a very very heavy weight a short distance and use hydraulics to transfer its energy upwards into a smaller craft.

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#81 2008-05-22 19:11:48

Rune
Banned
From: Madrid, Spain
Registered: 2008-05-22
Posts: 191

Re: Un- conventional ways to LEO

If we are serious about achieving large-scale access to space with present day technology, I would suggets that we reopen discussion on ground-launched nuclear powered spacecraft (Orion), using small and ultra-clean nuclear fusion bombs for propulsion.  The radioactivity released by each launch would be small, resulting in less than one death globally.  The spacecraft would mass anything from ten thousand to millions of tonnes and could carry many thousands or even millions of people into low earth orbit and beyond.

The only thing preventing the construction and use of Orion ships are political difficulties, the technical issues are more or less solved.  If you want largescale access to space, this is the way to achieve it with technology available today.

I signed up two pages ago (great forum, by the way) in the hope of being the first to point out this certainly "unconventional" method. Bad luck, I guess, someone out there is always faster... Anyhow, international treaties against nuclear testing aside, this is certainly feasible, using 70's technology at most. Of course, "clean nukes" (designed for a more fusion-like and less radioactive blast) would be desirable, and I´m not sure when those were developed, or how little fallout they produce.

It also certainly overpowered, since even a single trip to mars surface and return was proposed in the original study. And with the theorical capaticy to lift a city (thousands or millions of tons in the largest studied proposals) to orbit in one launch, it could be tho only feasible way of building large-scale projects (how to get all the thousands of kms of cable for a space elevator into orbit?). Me, I´m just happy with getting something the size of a colony, for example, out in a single Big-Dumb launch. Let's not forget that with that much power to spare, you could greatly increase safety sactors, reduce engeneering requirements, and use heavier materials for craft and cargo alike.

What can I say, if somebody asked me the fastest, cheapest way to go to space, I'd have to say nukes and agree with Antius. There's plenty of them around anyway...

P.S: Non-english speaker, so hope there aren't too many mistakes. roll

Rune. I liked the old Orion better.


In the beginning the universe was created. This has made a lot of people very angry and been widely regarded as a "bad move"

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#82 2008-05-22 21:12:02

GCNRevenger
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From: Earth
Registered: 2003-10-14
Posts: 6,056

Re: Un- conventional ways to LEO

Four catches though:

  • First, it will be very hard if not impossible to build a pure-fusion atomic bomb. They also will probably not be all that efficient, requiring lots of heavy chemical explosives and a thick radiation case. And even then, they will be too expensive to make enough for common use.

    Second, the "pusher plate" becomes more efficient geometrically as it gets wider, so bigger and bigger ships can carry more and more massive loads... however the reverse is also true, that small Orion's are awful and don't get that great of an efficiency at all.

    Third, how does one land such a contraption? Getting all that mass up there is nice and all, but what then? How do you get up and down to it efficiently? ...And if you can do that efficiently, what do you need Orion for?

    Fourth, you can't fly the thing often from a launch pad that keeps getting hit by atomic bomb blasts, now can you?


"The power of accurate observation is often called cynicism by those that do not have it." - George Bernard Shaw

The glass is at 50% of capacity

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#83 2008-05-23 08:19:58

Rune
Banned
From: Madrid, Spain
Registered: 2008-05-22
Posts: 191

Re: Un- conventional ways to LEO

Yeah, you're right, it probably wouldn't be a good system for rutinary access to space. This launch system would probaly only work for limited launches and extremely large cargoes. However, as a one-time-only first launch of materials for orbital construction, its unbeatable. Such as in, say, the first launch for construcion materials for a new colony, in mars or in the moon, or extremely large proyects like the space elevator.

Think of it as a method for using once a decade, at most. Besides, fuel may be expensive, but considering the payload capacity, it may be economical. As for landing, I'm not sure, but I'd guess the same way you lifted off: in a big bang and with style. Depending on the frecuency of blasts and the kind of shock absorbers the pusher plate is connected to, the ship may survive undamaged a powered touchdown.

However, I agree completely about the launch/landing site. You would want it remote, and you wouldn't want to hang around after a take-off or touchdown.


Rune. What can be more macho than to ride a nuke? big_smile


In the beginning the universe was created. This has made a lot of people very angry and been widely regarded as a "bad move"

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#84 2015-01-29 17:04:31

JCO
Member
Registered: 2015-01-22
Posts: 35

Re: Un- conventional ways to LEO

I am sure someone has posted this some time in the past. In the long term I think this will be the best solution for getting to Mars. I have a theory on why it has attracted so few backers. It is too good for its own good. What I mean by that is it would work so well it would put itself out of business. A small design could lift 5 tons a day. In the first year of operation it would be able to lift almost 1,800 tons to GEO. That is more than ALL the payloads currently scheduled globally. Most of those payloads will not be ready to lift off for years.

Most think that we just do not have the technology for it. The fact is the only piece missing is the material for the cable. It will probably surprise people to find out that solving this may be closer then you may suspect. KONE is currently producing an elevator cable called UltraRope containing CNT that is one seventh the weight of a steel cable and twice the strength. Doing the math that should mean the their design should have 14 times the breaking length of steel. That would make it as good or better than any other material available. UltraRope is likely making use of somewhat dated CNT manufacturing techniques as it has been under development for the past decade. Rice University 2 years ago reported a new method of making CNT thread that should be scalable to mass production. The thread is reportedly has 10 times stronger than the next best CNT thread. The individual CNTs making up the thread are all less than a centimeter in length. Peking University has reported making individual CNT fiber of nearly 20 cm. As this was reported in 2009 and I have not heard much more about it they are not progressing quickly toward mass production of them. That said I think that the only thing in the way is the funding to mature the technology.

So what will an SE GEO do for transportation to Mars? Other that cutting the cost just to get too GEO the elevator cable can be its own counter weight. A payload released from the end of the cable would have enough velocity to reach the orbit of Mars. Once released the trip to Mars could take as little as 2 months. One hitch to that is that the ride up in the SE could take over 2 weeks.

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#85 2015-01-29 19:25:26

SpaceNut
Administrator
From: New Hampshire
Registered: 2004-07-22
Posts: 27,177

Re: Un- conventional ways to LEO

I would like everyone to try and find a topic title that is close to the one you wish to create such as to keep the forum from having duplicates and growing in excess. It also would be nice to try and stay on topic as well.

To do a search of newmars for titles.... http://www.google.com/advanced_search
enter to the form the word  or combinations such as to ferret out the topic of interest. The enter the site or domain box http://www.newmars.com/forums

and submit to get a list just on this forum...

Sample Space Elevator

https://www.google.com/search?as_q=spac … gws_rd=ssl

As you can see the generation of the same topic is explosive in duplications...

Sorry for the rant...

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#86 2015-01-29 21:10:29

JoshNH4H
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From: Pullman, WA
Registered: 2007-07-15
Posts: 2,526
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Re: Un- conventional ways to LEO

This topic has been merged from JCO's space elevator topic.


-Josh

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#87 2015-01-30 08:00:22

Tom Kalbfus
Banned
Registered: 2006-08-16
Posts: 4,401

Re: Un- conventional ways to LEO

I think an inflatable space tower is a good idea. You see materials have greater tensile strength than compressive strength, it is easier to dangle a cable down than build a tower with bricks or steel girders. But if you inflate a structure you convert compression to tension, the strength that holds the tower up is the strength of the inflatable's walls, it has to contain the compressed gasses within. Let me give you an example. Suppose we were to contain a section of Venus's atmosphere within a balloon. It would stack higher than Earth's atmosphere. So lets suppose we have a 100 bar atmosphere composed of nitrogen inside of a balloon, how high would it reach into space?

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#88 2015-01-30 14:51:13

JoshNH4H
Member
From: Pullman, WA
Registered: 2007-07-15
Posts: 2,526
Website

Re: Un- conventional ways to LEO

Edit: I was looking over this post after reading it, so before I launch into my very long-winded and technical refutation of JCO's post, I'd like to say a couple things:

Firstly, welcome to Newmars!

Secondly, I consider space elevators to be an extremely interesting technology which is probably why my post is so long. 

Thirdly, I insist that you don't take this personally because it's not personal.  All for the fun of argument and all that.

I wrote up a reply to this, and it was promptly lost when I hit submit.  Ack!  So here we go again:

I am sure someone has posted this some time in the past. In the long term I think this will be the best solution for getting to Mars. I have a theory on why it has attracted so few backers. It is too good for its own good. What I mean by that is it would work so well it would put itself out of business. A small design could lift 5 tons a day. In the first year of operation it would be able to lift almost 1,800 tons to GEO. That is more than ALL the payloads currently scheduled globally. Most of those payloads will not be ready to lift off for years.

There are fundamentally two sources of investment capital:  Private and Public.  I'll look at private first, because it's the default source of investment capital.

My basic contention is that your statement that "it is too good for its own good" is another way to say that the current demand for launch services doesn't justify the construction of a space elevator.

A space elevator is a huge construction project, on a scale never even contemplated before in the history of mankind.  If the tether is fully extended out on the other side of GEO, its length will be twice as long as the total length of the entire US Interstate System.  And if you think that's not that much, according to wiki the US Interstates are "the largest highway system in the world and the largest public works project in history" having cost us (give or take) 500 billion dollars, which also makes it arguably the most expensive thing ever.

Let's say the space elevator would cost $50 billion.  For comparison, it will cost $1.5 Billion plus inevitable cost overruns to build a new bridge from New Jersey to Staten Island, a distance of just 200 m.  $50 billion is probably a low-end estimate, but I'll run with it.

As of 2014, there were 92 attempted orbital launches, only some of which were successful.  Let's say the mean payload for each launch was 10 tonnes, and that the average cost to launch was $10,000/kg.  These are both probably high end estimates, but I'll run with them too.  That would make the total value of the launch market* $9.2 Billion/yr.  However, launch costs are falling.  The Falcon 9 costs roughly $5000/kg, which will in the short term halve the value of the launch market.  The Falcon Heavy costs $1600/kg, which would reduce its value down to $1.5 billion/yr.  This would likely increase over time, but frankly nobody knows what the rate would be.  Realistically, financing on that $50 billion investment would cost 15% per year or more (The US stock market grew by 11.4% in 2014; Why would you invest your money on a riskier investment with a lower rate of return?).  Let's say the space elevator captures 100% of the global launch market by charging half of what SpaceX does, or $750/kg.  The annual value of the launch market will be $750 million/year.  Given the value of the investment and the interest on it, it's literally impossible to turn a return on investment here, even if you have zero operating expenses. [Coincidentally, at $750 million/yr and 15% interest, the most the elevator could cost and still be feasible would be $5 billion, again if your operating expenses are zero.

How fast will the launch market grow as prices fall?

It's impossible to say.  But it might not be as much as you think.  In economics, elasticity is used to measure the percent change in demand compared to the percent change in price.  A perfectly elastic demand corresponds to an elasticity of -1.  It can be proven (I've satisfied myself with the math and would be glad to show you if you're interested) that in a perfectly elastic market, revenue remains constant even as prices fall.  While we have no reason to believe that the elasticity of the market is perfect (most aren't) I would be surprised if it were very far from this value.

Let's put it another way:  Have you tried to invest in LiftPort Group**?  Would you?  Would you put your retirement fund into it?  I might invest a few dollars in them (If I thought they were a serious organization) but not because I expected to make any money off of it.  The reason why is that it's a bad investment from a profit perspective.

What about the government?  Well, there are by my count 6 public-sector space programs on Earth worth mentioning: NASA (USA), ESA (Europe), CNSA (China), RFSA (Russia), ISRO (India), and JAXA (Japan).  Not one of them has the budget, vision, or capability to do such a thing. NASA actually has about as much money as the rest of these space agencies put together.  But really we're very far away from a space elevator, and I'll address why in my response to your next paragraph.

Most think that we just do not have the technology for it. The fact is the only piece missing is the material for the cable. It will probably surprise people to find out that solving this may be closer then you may suspect. KONE is currently producing an elevator cable called UltraRope containing CNT that is one seventh the weight of a steel cable and twice the strength. Doing the math that should mean the their design should have 14 times the breaking length of steel. That would make it as good or better than any other material available. UltraRope is likely making use of somewhat dated CNT manufacturing techniques as it has been under development for the past decade. Rice University 2 years ago reported a new method of making CNT thread that should be scalable to mass production. The thread is reportedly has 10 times stronger than the next best CNT thread. The individual CNTs making up the thread are all less than a centimeter in length. Peking University has reported making individual CNT fiber of nearly 20 cm. As this was reported in 2009 and I have not heard much more about it they are not progressing quickly toward mass production of them. That said I think that the only thing in the way is the funding to mature the technology.

There's this pervasive myth among space elevator advocates that the only remaining impediment to building an elevator is finding a cable strong enough.  I would like to make it exceedingly clear that this is very wrong.

Here are some other major technological hurdles to building a space elevator:

  • Building a tether that can avoid thousands of LEO, MEO, GEO, and HEO satellites.  Arthur C. Clarke proposed that slight oscillations could be used to avoid pieces of space junk whose location and trajectory are known precisely.  He was right, but the dynamics of such a thing are highly non-trivial.  This is actually a matter I have some experience in: I did research a couple summers ago with a professor working on Earthquake engineering.  My research, in particular, involved computer models of using an applied force instead of a shake-table to simulate the effects of an earthquake on single story structures.  And you know what?  Even in the relatively trivial case of a single story structure it's really hard to design a system that can get it right.  I can only imagine the incredible difficulty involved in a 150,000 km long tether (That's how long it has to be without a counterweight) with both continuous and discrete mass distributions, subject to Earth's non-constant gravitational field and several other forces (atmospheric drag, electrostatics, magnetic fields, and the gravitational force of the Moon all come to mind) that are trivial on small scales but will have significant effects on the dynamics of such a gossamer structure.

  • Building a tether that will not fail, or will fail gracefully, when struck with a piece of space junk.  There's space junk out there, and we can't see all of it.  Space elevators are necessarily equatorial, and a space elevator will probably remain in service for a long time (10 years?  20 years?) so this is a real threat.  The space elevator will have a whole lot of elastic (tensile) potential energy stored up due to the extreme forces applied to it.  The value is about 10-100 GJ/m^3 at GEO and less elsewhere, but not really all that much less because taking into account the 1/r^2 law of gravity and the even more rapid decrease due to the rotation of the cable, most of the force will come from the first few thousand kilometers.  That means that when the cable is damaged it won't snap-- it will explode.  When you were a kid, did anyone ever snap a rubberband against your skin?  We called it a spiderbite.  Again, the dynamics are complicated because the elevator has the energy to physically move faster than the release of energy within it, but imagine that happening at 5 km/s.  It has the potential to wreak havoc, and because the cable would be going straight down it might remain mostly intact as it travelled through the atmosphere.  The cable needs to be designed so that this is impossible.

  • Deploying the tether without destroying the tether.  While the tether will be very strong in tension, it will also be very flexible, and Coriolis forces will tend to cause it to spin as it's extended in both directions from GEO.  Some degree of control over this will be necessary-- The cable can't be allowed to bunch up, to spin too quickly, or to build up harmonic vibrations.  I will repeat that the dynamics here are extraordinarily complex.

  • Powering the climbers.  The difference in potential energy between an object in GEO and an object on the surface of the Earth is about 60 MJ/kg.  All of this energy will need to be provided by the motors of the climbers, which will be less than 100% efficient.  This is higher than the energy of any battery or any chemical fuel.  If your power source is fully 10% the mass of your climber, you need about 600 MJ/kg.  Given system inefficiencies call it 1 GJ/kg.  Where does that come from?  Are you going to run a wire all the way up from the ground to GEO?  How heavy will that be (Answer:  Realistically, heavier than the tether).

  • End of life operations for the tether.  What do you do with it once its serviceable life is over?  How can you guarantee that it will be strong enough to be disposed of without causing damage to other in-space infrastructure or to Earth?

I'm not saying that any of these are insolvable because they're not.  But they're not easy either, and the solution requires time, effort, and testing to develop technologies far ahead of what we have now.

At this time, a space elevator has not been conclusively proven to be possible.  The development of a cable material is the first step, but by no means the last.

By the way, speaking of tether material:

In a fantastic essay on the matter published in 1981, Arthur Clarke noted that a tether hanging down from geostationary would be subject to approximately the same force as a tether hanging 5,000 km in a 1-g gravitational field.  Space Elevator advocates have seized on the implied tensile strength resulting therefrom as their golden standard for something that is "strong enough".  Let me be very clear on this matter:  It's not. 

The space elevator is going to be a huge structure whose failure would certainly result in the deaths of most of the people on the cable at any given time, but also possible a danger to many people on Earth.  On Earth, where the failure of a structure leads to danger for human beings, safety factors are as high as 5 and sometimes even higher.  That means that the figure of merit for breaking length is not 5,000 km in a 1 g field, but 25,000 km.  For a material with a density similar to water or CNT, that means that the figure of merit is not 65 GPa, as is often claimed, but 250 GPa.  Just to be very clear: The nature of a space elevator means that it needs a higher safety factor than normal structures.  This is because if any particular meter of the 150,000,000 m structure fails, the whole structure fails.  The chances of there being significant defects in any particular part of the structure are pretty high when one is that large.  Even if 999,999 out of every 1,000,000 meters of tether is good enough, you're still screwed because you have hundreds of defects, each of which means a lack of structural integrity.

Furthermore, the tether will probably have a lot of ancillary structural mass such as tracks or magnetic rails, perhaps wires (?) maybe coatings to protect it from chemical/radiation damage, and of course there will be a certain amount of payload on it at all times.  Let's go with a nice round number and say the required strength is about 500 GPa (For a material with a density of 1,000 kg/m^3; It will be proportionally higher for higher densities.  Also, this corresponds to a breaking length in 1 g of about 50,000 km). 

How does ultrarope compare?  Not very well.  Its breaking length is only twice that of steel, when used as an elevator cable.  By the way, here's an interesting tidbit:  Although using Steel's density of 7,800 kg/m^3 and its strength of up to ~500 MPa (Actually it can go even higher, but that doesn't matter right now), the ideal breaking length should be around 6.5 km.  KONE seems to believe that its actual, useful breaking length is just 500 m.  This implies a factor of safety around 13.  If you were skeptical of my numbers before, perhaps you're willing to give them more credibility now?

The best fiber I've seen is T1000 fiber.  Its tensile strength is 6.37 GPa and its density is 1,800 kg/m^3.  This implies an ideal breaking length of about 350 km.  Putting this another way, the best material we have is still about 150 times too weak to be used in a space elevator.  The space elevator material will be better than T1000 fiber by about the same amount that the steel typically used in contruction is better than... Well, frankly, it's hard to find materials that weak.  After some research, I've discovered that Mozzarella Cheese*** is about right.

It's an awesome idea, but it's not close.

*It's not really right to talk about market value when the market is not a functioning one, but I'll do it anyway because these numbers aren't exact and I need a figure of merit to use.

**LiftPort actually created kickstarter which got about 3500 backers and $110,000.  However, they weren't selling shares in a prospective space elevator but instead little nick-nacks and memorabilia.  This shows that there is a good deal of interest in such a structure but not a whole lot of investment capital.

***I would like to thank M. MEHMET AK and SUNDARAM GUNASEKARAN for expending the time and effort to test the mechanical properties of cheese; The Wisconsin Center for Dairy Research for funding this study; and the Journal of Food Science for publishing it.

Last edited by JoshNH4H (2015-01-30 15:12:19)


-Josh

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#89 2015-01-30 18:13:02

Tom Kalbfus
Banned
Registered: 2006-08-16
Posts: 4,401

Re: Un- conventional ways to LEO

An inflatable tower made of the same stuff as this would be space elevator cable might be a good alternative. Lets say we had a shaped balloon made out of carbon nanotube fiber. There are atmosphere that have supported greater weight than Earth's atmosphere, say the atmosphere of Venus for instance, if we put that atmosphere within a nanotube fiber balloon and inflated it at up to 100 atmospheres at sea level, the top of the structure would rise to well above Earth's atmosphere. A piece of space junk would punch a small hole in it, it would take time for all the atmosphere to leak out, giving plenty of time to patch the hole caused by the orbiting space junk. The tower would probably work best as an inflatable arch with enough length to accelerate a payload to orbital velocity at around 300 kilometers above Sea Level. I think this structure would be more robust that a tether supported by centrifugal force.

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#90 2015-01-31 12:40:25

Terraformer
Member
From: Ceres
Registered: 2007-08-27
Posts: 3,607
Website

Re: Un- conventional ways to LEO

Any idea what happened to the skyhook idea though, Josh?


"I'm gonna die surrounded by the biggest idiots in the galaxy." - If this forum was a Mars Colony

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#91 2015-01-31 12:58:41

Tom Kalbfus
Banned
Registered: 2006-08-16
Posts: 4,401

Re: Un- conventional ways to LEO

A skyhook is just a smaller version of a space elevator that rotates to synchronize its ends with the ground, an inflatable tower or arch is built from the ground up.

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#92 2015-01-31 14:14:17

JoshNH4H
Member
From: Pullman, WA
Registered: 2007-07-15
Posts: 2,526
Website

Re: Un- conventional ways to LEO

Terraformer, what do you mean?  It is subject to some of the same issues as a space elevator, although the tensile strength requirements are significantly lower. On the other hand it's very possible that there will be differential gravitational forces on the skyhook as it spins which can cause other oscillation problems.


-Josh

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#93 2015-01-31 14:20:21

JoshNH4H
Member
From: Pullman, WA
Registered: 2007-07-15
Posts: 2,526
Website

Re: Un- conventional ways to LEO

Tom Kalbfus-

I'm a bit pressed for time at the moment, but a few issues come to mind.

Firstly, you've described three (really four) different systems:  A tensile inflatable space tower, a compressive inflatable space tower, and a launch loop supported by one of these two.  Each of these has its own issues.  In general, accelerators are closer to current technology and thus require less development work, but will probably result in higher operating cost per kilo.  I would expect them to initially be cost-competitive with reusable chemical rockets.


-Josh

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#94 2015-01-31 16:05:51

GW Johnson
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From: McGregor, Texas USA
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Re: Un- conventional ways to LEO

So what's wrong with a light gas gun for payloads hard enough to withstand 1000's of gees? 

That's something that could be built "right now",  and with all existing technologies and materials.

GW


GW Johnson
McGregor,  Texas

"There is nothing as expensive as a dead crew,  especially one dead from a bad management decision"

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#95 2015-01-31 17:48:29

Terraformer
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From: Ceres
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Re: Un- conventional ways to LEO

I wasn't thinking of the rotating skyhook, but the one where the bottom tip is moving at 6-7km/s and vehicles hover to rendezvous...


"I'm gonna die surrounded by the biggest idiots in the galaxy." - If this forum was a Mars Colony

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#96 2015-01-31 20:12:55

JCO
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Registered: 2015-01-22
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Re: Un- conventional ways to LEO

JoshNH4H wrote:

There are fundamentally two sources of investment capital:  Private and Public.  I'll look at private first, because it's the default source of investment capital.

My basic contention is that your statement that "it is too good for its own good" is another way to say that the current demand for launch services doesn't justify the construction of a space elevator.

I do not take this personally but this did seem to provide a large number of examples of the assumption that it is impossible. Your statement that it was another way there was not enough demand for it seems to ignore the fact that I went on to say just that.

Comparing it to the highway system I think is a huge distortion of the scale. The main thing it ignore is the fact that the entire mass of the ribbon (cable) would be less than any of the numerous record breaking high rises recently built. The scale of the project would be epic but well within our current engineering capacity.

Your discussion of cost is for the most part choosing numbers. From what I have heard SpaceX hope to reduce launch cost by an order of magnitude. SE advocate suggest that it could reduce launch cost by 2 orders of magnitude. Also the cost to build payloads would be reduced by the fact that they would not need to be constructed to deal with the stresses of a rocket launch.

JoshNH4H wrote:

There's this pervasive myth among space elevator advocates that the only remaining impediment to building an elevator is finding a cable strong enough.  I would like to make it exceedingly clear that this is very wrong.

Here are some other major technological hurdles to building a space elevator:

The fact is that all your technological hurdles are only engineering challenges. The ribbon is the only thing that is impossible today, we have yet to develop a material that is strong enough. Also you have a major factual error, the 350 km break length should actually stated as a sea level break length. It is stated that way because it is a way to ignore the change in gravity with height. A material would actually only need a 5,000 km sea level break length to reach GEO. So the T1000 fiber would only need to be 15 times stronger. Still a long way to go but a lot more doable.

Most of your "Technical hurdles" relate to ribbon failure. You discuss it as if you are unaware that the issues might have already been considered. For example cable designs that are more robust so micro meteor do not cause failure are being developed for use with currently available fibers because NASA intends to make use of tethers.

Your argument that it will have to be stronger to carry passengers assumes it will carry passengers. This is a myth of many space elevator advocate. It is very likely that a payload on the SE will take one week to reach GEO. It will take an hour just to reach the edge of the atmosphere. Unless it is as a tour bus to the edge of space I do not think many people will be riding the SE. The SE will be a cargo train, rocket will be the passenger planes.

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#97 2015-02-02 12:35:06

JoshNH4H
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From: Pullman, WA
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Re: Un- conventional ways to LEO

I do not take this personally but this did seem to provide a large number of examples of the assumption that it is impossible. Your statement that it was another way there was not enough demand for it seems to ignore the fact that I went on to say just that.

I suppose I wasn't disagreeing with you on the matter, but phrasing is important.  Saying that a space elevator is too good for its own good is an inherent value judgment.  It's one that I agree with, but it's neither neutral nor objective.  I was trying to show how things look from the viewpoint of the dual central planning apparatuses of the American economy, Wall Street and Washington, to show why it hasn't been done yet but also to establish a paradigm for what would need to happen for the elevator to get built.

The implicit thrust of the argument was that for a space elevator to happen it would need to be conclusively shown to be possible.  It would need to be something that could actually be begun.

Comparing it to the highway system I think is a huge distortion of the scale. The main thing it ignore is the fact that the entire mass of the ribbon (cable) would be less than any of the numerous record breaking high rises recently built. The scale of the project would be epic but well within our current engineering capacity.

I agree and I disagree.  A space elevator is a very different type of project from the Interstate highways, but I do still think it's instructive to consider its length in comparison to them. 

I think the statement that it's well within our current engineering capacity is categorically false.  That would imply we could start building tomorrow if we wanted, which we simply can't, for any number of reasons.

Your discussion of cost is for the most part choosing numbers. From what I have heard SpaceX hope to reduce launch cost by an order of magnitude. SE advocate suggest that it could reduce launch cost by 2 orders of magnitude. Also the cost to build payloads would be reduced by the fact that they would not need to be constructed to deal with the stresses of a rocket launch.

This is true, but I think you're missing the point:  The value of the launch market is small; The cost of the space elevator is large.  Therefore a space elevator would be unlikely to pay for itself over its useful lifetime.  Here's a thought experiment:  Let's say you're willing to take a loss of $40 billion dollars on constructing a space elevator.  Now let's say you instead spend that money on subsidizing launch costs; Let's say 80% of the cost of the launch (Provided costs don't rise, maybe?), and dial back the subsidy by some predetermined amount each year.  This will increase the value of the launch market, and would also create a known demand for low-cost launch services.  As the subsidy is scaled back there will be a clear incentive to develop cheaper space launch systems in order to prevent your potential customers from being priced out of the market.  This may ultimately result in a space elevator becoming economically viable.

Which has done more good after ten years?  After 20?

The fact is that all your technological hurdles are only engineering challenges.

This is in my opinion the fundamental cause of my disagreement with you on the matter.  To quote myself:

I wrote:

I'm not saying that any of these are insolvable because they're not.  But they're not easy either, and the solution requires time, effort, and testing to develop technologies far ahead of what we have now.

A technological hurdle is an engineering challenge.  They are one and the same.  I'm not arguing about whether a space elevator is physically possible or not because I know it is.  I'm arguing that it doesn't make economic sense now and it's not going to for quite a while.  The reason people like the space elevator is that they see it as a way to bring down costs.  But if it costs a trillion dollars (This number is an example to illustrate my point, not an estimate) to do the basic research before you can get to the point where you spend $50 billion to build it (It's not just launch costs!) then you're probably not going to get a return on your investment.  The following is an example of what I mean:

Let's take the following possibilities [Again, these are examples that I'm using to illustrate my point]:

1) Space elevator

Research and Development costs: $450 billion
Construction costs: $50 billion
Operating profit: $1 billion per year
Lifetime: 25 years

Total costs: $500 billion
Total Profits: $25 billion
Net return on investment: -$475 billion
Annualized return on investment: -16.2%

2) Much Cheaper Chemical Rocket Technology (e.g. reusability, or SSTO, or whatever)

Research and Development costs: $4.9 billion
Up-front Construction costs: $0.1 billion
Net increase in annual profit: $0.5 billion
Useful lifetime for this technology: 15 years

Total Costs: $5 billion
Total (incremental) Profits: $7.5 billion
Net return on investment: $2.5 billion
Annualized return on investment: +5.5%

It is my claim that these numbers are representative of the costs associated with each of these two options.  It's all about the money here.  Realistically, because the space elevator would need to charge more money to make up the costs associated with building it, it wouldn't be able to create a reduction in the cost of sending payload to orbit.

The ribbon is the only thing that is impossible today, we have yet to develop a material that is strong enough. Also you have a major factual error, the 350 km break length should actually stated as a sea level break length. It is stated that way because it is a way to ignore the change in gravity with height. A material would actually only need a 5,000 km sea level break length to reach GEO. So the T1000 fiber would only need to be 15 times stronger. Still a long way to go but a lot more doable.

In fact I do not have a factual error, you misunderstood what I was saying.  You're also misusing the word impossible; It is 100% possible to develop a material strong enough for use in a space elevator.  Just as it is 100% possible to solve every technical issue with the space elevator (both those that I raised and those that I didn't).  The only things standing between us and a space elevator is time and money.  That is to say, lots of time and lots of money. 

The relevant part of my post:

I wrote:

In a fantastic essay on the matter published in 1981, Arthur Clarke noted that a tether hanging down from geostationary would be subject to approximately the same force as a tether hanging 5,000 km in a 1-g gravitational field.  Space Elevator advocates have seized on the implied tensile strength resulting therefrom as their golden standard for something that is "strong enough".  Let me be very clear on this matter:  It's not.

The space elevator is going to be a huge structure whose failure would certainly result in the deaths of most of the people on the cable at any given time, but also possible a danger to many people on Earth.  On Earth, where the failure of a structure leads to danger for human beings, safety factors are as high as 5 and sometimes even higher.  That means that the figure of merit for breaking length is not 5,000 km in a 1 g field, but 25,000 km.

In engineering, you do not build things to withstand exactly the theoretical force that will be applied to them.  A tether material with a breaking length of 5000 km would snap at the drop of a pin.  You need to build things to be stronger than the theoretical forces to be applied to them if you want them to work in the real world.

The magnitude of this overbuilding is called a "Safety Factor".  You take the theoretical value of the force applied (here, represented as a breaking length of 5000 km at sea level gravity) and multiply it by some constant.  In construction, that constant is typically around 5.  In other applications it's much higher.  Please note that this is not an unrealistic thing to do; Have you not heard of bridge failures despite the large safety factors used in their construction?  High safety factors are a logical response to the unpredictability of the universe in applications where failure would result in losses of human life or large amounts of money. 

Even if there aren't humans physically on the cable (Which I would argue with, but that's beside the point) there will be people at the base and at the GEO location who could be harmed by a cable failure, and also along the equator who could be harmed if it were to fall.  Maybe you're willing to risk their lives for the space elevator, but I assure you that they aren't.

If a material were used in the construction of a space elevator which had a breaking length of 5000 km, it would constantly be on a knife's edge of failure.  That is to say, it would certainly fail either very shortly after or during construction.

Again, elevator cable is instructive on this matter.  Despite the fact that Steel has a notional breaking length of 6.5 km, it is only considered safe to use elevators cables up to 500 m.  That implies an even higher safety factor of 13 (I used 10 relative to the strength of the cable material because there will certainly be other mass affixed to it).  So effectively if you're disagreeing with this you're saying that actual lived experience with elevator cables (Which used to fail fairly often about a century ago) is subordinate to your notions of what ought to be.

To visualize this:  The breaking length of Mozzarella Cheese is 15 m.  Let's say a 12 m rope of cheese were suspended 100 m in the air.  Would you sit in a chair suspended from the bottom of it? 

Most of your "Technical hurdles" relate to ribbon failure. You discuss it as if you are unaware that the issues might have already been considered. For example cable designs that are more robust so micro meteor do not cause failure are being developed for use with currently available fibers because NASA intends to make use of tethers.

I hold to my claims on this matter.  Short tethers in scientific missions are not subject to nearly the same safety standards as the commercial transportation industry. 

The case of a Space Elevator cable is particularly extreme.  The elastic energy stored in the cable is about 12 times denser than the energy stored in TNT.  We should be thinking of cable failure as an explosion, a detonation, because that's what it will be.  No solid structure could easily withstand that.


-Josh

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#98 2015-02-02 15:42:03

GW Johnson
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From: McGregor, Texas USA
Registered: 2011-12-04
Posts: 4,993
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Re: Un- conventional ways to LEO

The Red Mars/Blue Mars/Green Mars trilogy does indeed point out the consequences of elevator failure.  That tale describes one falling on Mars.  Wherever/whenever such a thing is first attempted,  it shouldn't an inhabited body,  precisely because of the side effects of a failure-and-fallback event. 

GW


GW Johnson
McGregor,  Texas

"There is nothing as expensive as a dead crew,  especially one dead from a bad management decision"

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#99 2015-02-02 19:49:19

JoshNH4H
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From: Pullman, WA
Registered: 2007-07-15
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Re: Un- conventional ways to LEO

I think Robinson may have dramatized that a bit, since the cable will not actually be all that big.  Even at just* 65 GPa, an elevator which could support 100 tonnes under Earth's gravity would have a cross-section of about 1.5e-5 m^2, or put another way it would be a string with a diameter of 5 mm.  He described it as being something closer to 10 m. 

Having said that, the small theoretical size does beg the question of how exactly one is supposed to affix large masses to such a small tether.

*Given the discussion we're having here, 65 GPa is indeed a low failure strength in context


-Josh

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#100 2015-02-03 12:08:48

Tom Kalbfus
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Registered: 2006-08-16
Posts: 4,401

Re: Un- conventional ways to LEO

JoshNH4H wrote:

I think Robinson may have dramatized that a bit, since the cable will not actually be all that big.  Even at just* 65 GPa, an elevator which could support 100 tonnes under Earth's gravity would have a cross-section of about 1.5e-5 m^2, or put another way it would be a string with a diameter of 5 mm.  He described it as being something closer to 10 m. 

Having said that, the small theoretical size does beg the question of how exactly one is supposed to affix large masses to such a small tether.

*Given the discussion we're having here, 65 GPa is indeed a low failure strength in context

The moment it hits the atmosphere, it burns up and becomes carbon dioxide! A Space elevator will have an enormous surface area relative to mass. I really don't think most of it will get through the atmosphere to make a linear crater around the equator.

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