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Speculative - cislunar transport infrastructure, 2054

Posted by: Centrillium - Thu Jan 19, 2006 3:49 pm
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Speculative - cislunar transport infrastructure, 2054 
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Post Speculative - cislunar transport infrastructure, 2054   Posted on: Thu Jan 19, 2006 3:49 pm
Okay, a hypothetical question, a bit wierd probably but it relates to some science-fictional musings on my part.

Suppose: its year 2054, and there is routine travel to the Moon. I imagine this happening along something like the following lines - suborbital tourism running profitably through the 2010s, orbital tourism beginning in the 2020s, lunar tourism driving the establishment of lunar colonies through the 2030s - space tourism a major industry by 2054 (turnover some hundreds of $billions). Satellite and lunar solar power and asteroid/lunar resource return (platinum group metals etc) are smaller but growing industries by the present time. This may be complete nonsense - but the projections are based on the work of Patrick Collins and co.

Fusion power developed and growing on Earth (still only 1-2% of world electricity supply), mainly DT fusion, DD reactors still experimental and Lawson criterion for D-He3 fusion not yet met. A few large fusion-powered spacecraft may exist but the technology isn't widespread (fusion reactors/drives still extremely large and expensive - that's just realistic).

What I wonder is - how is travel between Earth orbit and the moon, or Earth orbit and the Lagrange points, carried out? In other words, what is the most economical approach at this tech level? Chemical rocketry could certainly do the job but there are other options - solar electric propulsion, solar thermal, nuclear electric, nuclear thermal, momentum exchange/tethers, solar sails...

I'm guessing that SEP and solar sails will probably entail very long travel times - maybe used for cargo? Could a solar thermal rocket leverage the propellant savings of an NTR without the daunting 'N' word? I understand that STRs have significant limitations...

Given that thermal rockets require propellants with low molecular weight to leverage their advantages against chemical rockets, would they be used at all? Chemical rocket propellant can be manufactured at need from easily storable water (using abundant solar power) - dumping the O2 and using just H2 in a thermal rocket probably makes no sense though. I assume that creating an infrastructure of H2 propellant dumps for thermal rockets is much more difficult and expensive than an equivalent infrastructure for water (fuel depots will surely be needed for a fully reusable transportation infrastructure...?)

The other thing I mentioned was momentum exchange... I really have no clue here. Assuming regular Earth/Moon/Ln (n = 1,2,...,5) traffic, could a system of tethers enable entirely propellantless transfer? If so, given the expense of shipping fuel about, it seems to me that this is probably what would emerge... but its a technology I know next to nothing about.

Okay, wierd post, but I'd love to pick some of you guys' brains about this. :)


Last edited by Centrillium on Thu Jan 19, 2006 11:28 pm, edited 1 time in total.



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Post Re: Speculative - cislunar transport infrastructure, 2054   Posted on: Thu Jan 19, 2006 5:15 pm
Centrillium wrote:
A few large fusion-powered spacecraft may exist but the technology isn't widespread (fusion reactors/drives still extremely large and expensive - that's just realistic).
Assuming the above, I would think a 2 or 3 step transport system would exist. Chemical launch to LEO. Transfer to a fusion craft for the trip from LEO to LLO. If the fusion ships exist at all, that would be the logical use for them. Then possible transfer to chemical lunar landing vehicles which use propellant produced on the Moon, or possibly in orbit from Asteroid resources. Depending the level of fusion technology, the fusion craft may land on and take off from the Moon, making it a 2 step trip. But I don't see fusion launch to LEO due to environmental concerns.


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Post    Posted on: Thu Jan 19, 2006 5:25 pm
id think you should add another 20 years onto the date but maybe not. I think it would be a huge jump if the private sector could decide to move away from rockets as we know then today and perhaps go for ion engines after entering orbit, not sure how they would get hold of the technology but if the private sector could get into orbit happily in 30 years then use some sort of ion propulsion, then we would have something special!

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Post    Posted on: Thu Jan 19, 2006 6:35 pm
The private sector has been using ion engine technology for decades in the form of plasma etching of semiconductors. When ion engines were first developed, they were tested but not used in space. The technology spun off for plasma etching and has been in use for that purpose ever since. Only recently has the technology actually made it into space for it's originally intended purpose.

And note that Centrillium did not limit his hypothetical future to private enterprise.


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Post    Posted on: Thu Jan 19, 2006 11:23 pm
campbelp2002 wrote:
Transfer to a fusion craft for the trip from LEO to LLO. If the fusion ships exist at all, that would be the logical use for them.


The kind of ships I had in mind aren't going to be used for this. Essentially my prediction is that fusion technology develops along the 'mainstream' line projected by the actual fusion community (with whom I am somewhat familiar, I worked at JET for a time) - that is, large magnetic confinement devices such as the tokamak, stellarator, RFP et al. People often talk about inertial-electrostatic fusion or dense plasma focus or any number of supposed 'short cuts', but I'm skeptical - the people who really know, are betting on the tokamak, and they have good reasons for doing so.

Given the above, a fusion engine is going to be big, and would only be justified on a big ship. In particular, I imagine by 2054 there might be a few government-built fastliners taking large numbers of colonists to Mars on high-energy trajectories, with travel times of a few weeks. The kind of delta-vee you can leverage with a fusion engine is surely overkill in cislunar space, where minimum-energy transfers are fast enough.

campbelp2002 wrote:
But I don't see fusion launch to LEO due to environmental concerns.


Well, tritium isn't healthy, but a fusion drive with enough thrust to be useful for a launch vehicle - the 'fusion torch' of quasi-hard SF - is unrealistic. They don't outright break any laws of physics, but in engineering terms they're pretty fantastical. I guess its conceivable that a really compact fusion reactor could be used in a thermal rocket (achieving the same sort of Isp as a familiar NTR), although the physics of toroidal magnetic confinement systems guarantees that they will never be that small.

Rob Goldsmith wrote:
id think you should add another 20 years onto the date but maybe not.


I guess the premise rests on the assumption that Cheap Access To Space is the hard part. To suppose that large-scale orbital tourism starts in the 2020s may be optimistic, but it may also be conservative (given that orbital tourism is a reality today). Basically, the hope is that once the market for tourism is clear and undeniable, capital will become available to finance the development of robust SSTO RLVs and bring the cost of space access down by 100-1000 times. At that point, who knows what's possible? :)

Rob Goldsmith wrote:
if the private sector could get into orbit happily in 30 years then use some sort of ion propulsion, then we would have something special!


Well - ion drives have to use very slow trajectories due to their low thrust. SMART-1 took more than a year to travel to the Moon, for instance. Also, the propellants (xenon, argon) are relatively expensive - that's unimportant now, but if you project to the point where propellant costs are actually a significant portion of total cost (a necessary condition if lunar tourism is a large scale enterprise), then it might matter a great deal. So I'm not sure ion drives are really the technology of choice given the kind of scenario I'm considering.

Speaking of which - campbelp, I didn't know that about ion drives, thanks. :) Interesting factoid...


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Post    Posted on: Thu Jan 19, 2006 11:57 pm
Centrillium wrote:
The kind of ships I had in mind aren't going to be used for this. ... Given the above, a fusion engine is going to be big, and would only be justified on a big ship. In particular, I imagine by 2054 there might be a few government-built fastliners taking large numbers of colonists to Mars on high-energy trajectories, with travel times of a few weeks.
I don't agree, but it all depends on the assumptions. For example, in your initial post you said noting about Mars settlements but did say there were Lunar settlements. A big expensive fusion ship that has less than 1G acceleration (so it can't lift itself off the Earth even if it wanted to) would be valuable in cislunar space, IMO. If there is a thriving lunar tourism business it would be a good source of income. And it would not need large amounts of propellant to be brought up from the ground at great expense.

On the other hand, if there were thriving Mars settlements, and a tourism industry, of course they would be perfect for that too. Assuming they could really carry enough propellant to thrust all the way there for the kind of short trip times that tourists would demand. If the fusion ships were less developed, so that they could not thrust for the entire trip but could only make low energy transit with lower propellant needs, there would be less demand for them in that capacity.

So it all depends on the assumptions. I guess I was assuming slightly less capable ships and less Mars activity that you were.


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Post    Posted on: Fri Jan 20, 2006 1:33 am
campbelp2002 wrote:
A big expensive fusion ship that has less than 1G acceleration (so it can't lift itself off the Earth even if it wanted to) would be valuable in cislunar space, IMO.


Well, sitting here in 2006, of course, its impossible to predict the performance characteristics of a fusion drive exactly but most of the sources I trust suggest that fusion engines will be big, heavy machines producing very high impulse and low thrust. Perhaps more thrust than electric propulsion - the physical limiting factors are different, a fusion drive is essentially governed by thermodynamics so the main issue is cooling (drive power going with the square of exhaust velocity and all), whereas ion drives have to be low thrust due to the diminishing returns you get when you try to ramp up the charge density beyond a certain point. At least I think that's how it works.

Anyway, minimum-energy transfers in cislunar space only take days. I'm not sure whether the big fusion ships are even going to accelerate fast enough to do better. Assuming they can... each one is going to be a huge machine, basically the equivalent of a nuclear-powered aircraft carrier in terms of its value and significance. I doubt that carrying people to the Moon and back with such vehicles makes much economic sense.

campbelp2002 wrote:
For example, in your initial post you said noting about Mars settlements but did say there were Lunar settlements.


Well, I didn't want to go on too long. :) I'm kind of imagining that maybe by this time, there's an international 'Mars Agency' whose job it is to organise the settlement of Mars, and they operate a few of the aforementioned fastliners. NASA, ESA and/or their successors/contemporaries may be operating large fusion-powered exploration vessels in the outer system as well. There won't yet be a commercial basis for Martian settlements, but as main belt mining begins to get underway, the Martian colony will be in a great position to supply the miners with food, low-tech manufactured goods, etc. They may also be exporting inventions and ideas as Zubrin suggests in Case for Mars, although they have to compete with the Moon, orbitals, asteroid mining colonies... :)

campbelp2002 wrote:
Assuming they could really carry enough propellant to thrust all the way there for the kind of short trip times that tourists would demand. If the fusion ships were less developed...


I was thinking of highly eccentric hyperbolic transfer orbits with extended accel/decel periods of maybe a couple of days at each end - but crunching the numbers, and looking at how much ITER weighs (20,000 tonnes and that's just the reactor), even that probably isn't realistic. I'll have to do a more careful analysis and get back to you - as I say though, at this stage its all highly speculative.

In any case, travel to Mars will still take weeks at minimum, so the prospects for tourism will be limited. The very rich may be customers for 'interplanetary cruises', but I think the main market will be for one-way trips. Things may even still be in the 'selective' phase where the government(s) choose colonists for their expertise.


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Post    Posted on: Fri Jan 20, 2006 1:58 am
Well, as I said, it all comes down to the assumptions. I am assuming that the fusion drive will be very heavy compared to its thrust. I am also assuming that it would not allow much higher speeds than could be gotten with chemical or NTR, but it would use less propellant, because of a very high ISP, to get that speed. In other words the main advantage is not speed; it is economy, in terms of propellant used per trip.

By the way, I cannot imagine any space drive based on a tokomak. Those things are just waaaay to massive. I assume one of the other fusion reactors you mentioned as not being front runners would have to be made workable. Some of them, if they work as promised, could be quite small.


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Post    Posted on: Fri Jan 20, 2006 5:44 am
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Those things are just waaaay to massive.
And I can't see any way around this..

I would expect the _deeply_ speculative magnetized target fusion
http://trs.nis.nasa.gov/archive/0000066 ... 212691.pdf
to at least offer better theoretical (thrust) performance.

[EDIT] 5MB file + link may be dead :(


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Post    Posted on: Fri Jan 20, 2006 8:21 am
Hello, Centrillium,

the german astronaut Ulrich Walter explains in his german book "Außerirdische und Astronauten" ("Aliens and Astronauts") that the only usable fusion drive would be the Pulsed Fusion Drive which wouldn't have nothing to do qith a Fusion reactor but with micro-fusion-bombs which are too small for military use and propell a space ship by thier fragments impacting a magnetic field.

What do you think about that?



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Post    Posted on: Fri Jan 20, 2006 10:30 am
campbelp2002 wrote:
I assume one of the other fusion reactors you mentioned as not being front runners would have to be made workable.


Hmm - I didn't really mean to imply that the ship would have an ITER-like tokamak strapped to the back end, although reading my posts again that definitely is the impression I give. What you're looking for, obviously, is a high specific power - there are 'mainstream' fusion concepts that have a much higher specific power than the basic tokamak. For instance the spherical tokamak geometry leverages a much higher beta, coupled with high fields you could get 1-10 kW/kg which is attractive for propulsion. (Regular tokamaks are at about 300-500W/kg.)

I wouldn't rule out a cylindrical rather than toroidal geometry, such as the tandem mirror or gasdynamic mirror, but the problem is that these have not been seriously persued in the real fusion effort since the early 80s so they're a long way behind the front runners. But as far as IEC or focus fusion go - I don't really believe in those ideas. The people pushing them (focus fusion particularly) are saying 'oh look, I've just invented a wonderful new geometry for a fusion reactor and I can make a power plant in a couple of years for a million dollars'. Heh.

Ekkehard Augustin wrote:
the only usable fusion drive would be the Pulsed Fusion Drive


Right - external pulse propulsion has the great advantage that it makes cooling a lot easier! I think that's why fusion pulse drives are considered superior to magnetic confinement fusion drives in some literature. Unfortunately, as I see it building these things isn't going to be easy. What you're looking at essentially is an inertial confinement fusion reactor configured for propulsion. Now, ICF is at least a generation behind MCF in the real world (yes, NIF will probably achieve 'ignition' before ITER demonstrates Q=10, but what they don't tell you is that the definition for ignition in laser fusion is different from that in magnetic fusion, and achieving ignition in an ICF machine doesn't mean you've basically got a power plant as it would for MCF). For my purposes, I'm assuming that ICF is still at the research stage in 2054.

I'm aware of novel pulsed fusion concepts - such as the magnetised target fusion you mentioned - but my thinking is, 'every concept we've tried to push towards breakeven has been hard work, why would MTF be any easier?' Basically the tokamak and co. have a huge headstart. I disagree with Mr. Walter's assertion, if indeed it is his assertion, that MCF can't make an attractive propulsion system.


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Post    Posted on: Fri Jan 20, 2006 12:06 pm
Ulrich Walter's main reasons are

1. the density of protons and/or hydrogen in space is that low that the amount which can be collected by Bussard collectors never would be sufficient to get a fusion reaction. I have to add that he explicitly is speaking about interstellar space then.

2. reasons which don't have to do with doubting the MCF - I will have to reread the book regarding that and post the result here.

3. weight

Ulrich Walter is talking about it while explaining which way the required high velocities for interstellar travel could be achieved.



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Post    Posted on: Fri Jan 20, 2006 2:17 pm
In the December 16 space show, about 28 minutes into the program, Jim Benson of SpaceDev. says he met Dr. Robert Bussard a few years ago and likes the idea he has been working on, IEF or Inertial Electrostatic Fusion.
It uses Hydrogen and Boron for fuel and has the same benefits as Helium 3 (no neutrons).
Benson says that Bussard has achieve fusion in his lab a number of times but he is getting old and lacks funding.
Bussard has a lab just 10 minutes away from SpaceDev and SpaceDev has acquired Bussard's equipment and is trying help him find funding.

I got this link by googling "Robert Bussard ".
http://www.ibiblio.org/lunar/school/Int ... stems.HTML

By the way, according to Wikipedia, this is the same Bussard who is famous for proposing the interstellar ramjet fusion engine.


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Post    Posted on: Fri Jan 20, 2006 5:36 pm
Ekkehard Augustin wrote:
Ulrich Walter is talking about it while explaining which way the required high velocities for interstellar travel could be achieved.


Ah, well, that's something else I guess. :) My requirements are much less demanding, I'm only talking about trips to Mars. As for points 1, 2, 3 - I'm definitely not considering a ramjet, that's a very science-fictional concept. Weight isn't the problem so much as specific power, provided that you're okay with big ships... if you've got a 300 tonne reactor and it delivers 3GW of thermal power, then you're set, as long as you don't mind building a ship that weighs thousands of tonnes... considering that Mars missions with total to-orbit masses on that order have been contemplated before, I don't think its that much of a stretch.

campbelp2002 wrote:
Jim Benson of SpaceDev. says he met Dr. Robert Bussard a few years ago and likes the idea he has been working on, IEF or Inertial Electrostatic Fusion.


I mentioned IEC (inertial electrostatic confinement) before... all the abbreviations that crop up when discussing the many fusion reactor concepts can be quite confusing sometimes. :) Anyway, IEC=IEF. I (with help) actually built a Farnsworth-Hirsch fusor last year, although it was a fairly feeble effort (we never got any neutrons). Prof. Kulcinski is another IEC fan, so is Harrison Schmitt. I've asked plasma physicists about it and they say its a real concept, but the proponents talk it up out of proportion - there are problems which rarely get mentioned. For instance, the plasma is strongly non-Maxwellian which greatly increases the bremmstrahlung radiation losses. No IEC project has ever demonstrated Q>10^-6 or so, let alone Q=0.7 (JET) or Q=10 (ITER). You might say that's just because they've never had as much funding, but I think that the fusion community know what they're doing and they're pursuing the tokamak, stellarator, RFP etc because these really are the best bets. I know some of these people, they want to build fusion reactors more than they want to protect their jobs, they're not pursuing lame duck concepts to keep their cosy R&D grants or anything (which is what some IEC fans might suggest).

campbelp2002 wrote:
Benson says that Bussard has achieve fusion in his lab a number of times but he is getting old and lacks funding.


Achieving fusion is easy. :) Achieving Q=1 (breakeven), that's difficult.

I should maybe comment a bit on my background seeing as I'm getting a bit pompous and know-it-all about this fusion stuff. :) I'm an undergraduate physicist, I say 'I worked at JET' impressively but it was just a placement writing software, so what I know on the subject is mostly second-hand from the scientists I worked with. It is an interest of mine though, obviously.


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Post    Posted on: Fri Jan 20, 2006 5:48 pm
As I promised I am rereading Ulrich Walter's book. He says that

1. fusion like going on in the sun is of an crossection of efficiency which is by 10^20 orders of magnitude smaller than fusion of Deuterium or Helium-3 (next he explains a fusion drive by the example of a magnetic bottle),

2. the crossection-efficiency of a ramjet is too small and would require a giant reactor (he is quoting calculations which resulted in the size of 7,000 km),

3. the magnetic funnel at a technological limit of 1000 Tesla woul catch only 10^-12 of the interstellar protons (but he also says that this could improved).

These are the reasons he is listing in his book dating from 2001.

Regarding the Pulsed Fusion Drive he quotes Freeman Dyson from 1968 as follows (retranslation from German): "We that time believed that there was a real chance that the United States transit to the technology of nuclear propulsion directly and could avoid the construction of chemical rockets like the Saturn V. Our plans were to send space vehicles to Ma<rs and to Venus until the year 1968 at costs which would have been a fraction only of what is spent in the Apollo program today."

Next he tells that political reasons and causes prevented the nuclear drives After that he explains what could be achieved by the Pulsed Fusion Drive if it only wouldn't be barred by treaties and politics.



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