On the subject of wildly optimistic deadlines
New York Times 1989 : Dr. Emmett Brown has demonstrated his Mr. Fusion domestic reactor which he claims will be commercially available by 2015 (worst case).
A British startup has proposed combining the "maybe one day" technology of fusion power with the "slowly, slowly" tech of ion propulsion to create an engine capable of sending humanity to the stars. The Register sat down with Richard Dinan, CEO of Oxford-based Applied Fusion Systems (AFS), to learn a little more about the …
yeah without the urgency of winning a war [i.e. Manhattan Project to get fission to work] fusion power is likely to be "researched" until the cows come home [with no results, "always more research"].
Of course, being "the guy" to make it work for realz is a true motivator for SOME scientists. Good luck getting that past the upper management bureaucracy that is perpetually funded by "research" grants.
In any case I worked the maths out on this quite some time ago.
In order to get propulsion to be efficient, you have to accelerate a large mass by a small amount, but if that amount is too small, the engine isn't efficient.
There's a point at which the amount of fuel you need, accelerated by the engines you can build, will be minimized for maximum acceleration for whatever mission you're running. I'm pretty sure NASA's "rocket surgeons" run these numbers a LOT.
Now, the best fusion reactor design wouldn't (primarily) produce electricity. Fuel to produce elecricity along with thermal losses would be essentially _wasted_ unless you're accelerating fusion products, but they're too lightweight to be effective fuel.
To make up for that a hydrogenous material [like water or methane] would be used. You'd have to separate out the hydrogen fusion fuel from that, but no big deal. Then the engines get to use that fuel to product propulsion with the bulk of the hydrogenous material. And "some electricity" can be produced along the way. So yeah you'd have to run the engines occasionally to recharge the batteries [or use fuel cells, or both].
In any case, using current tech [that does not involve warp fields and wormholes] you'd need to produce a reaction engine (one that produces a given amount of energy from the fusion of hydrogen into helium, then transfers that as kinetic energy of the propellant), such that the energy produced becomes velocity of the propellant, and gives you an impulse for acceleration. All of that coupled against the total weight on takeoff [with 100% fuel load] etc. etc. etc. even if that 'take off' is in earth or lunar orbit.
Anyway, thought I'd mention it. Sorry about bursting any bubbles. The equations really aren't that hard.
post-edit - looks like my fan club is already thumbing be down. thank you!!!
Fusion has always been a decade or two away since I wad a small child, so it's easy to think any deadline to be optimistic. This time it "might" be real. Big improvements have happened to superconducting materials that make ITER somewhat obsolete, and it now seems that you can get the magnetic field density needed to make small fusion reactors viable. A thin film of superconducting material deposited on steel tape revolutionizes the way these things can be built. The steel not only gives mechanical strength, it's a also the only insulator required as it's conductivity is so poor compared to the superconducting film.
The idea that any scientist would try and hide a working reactor just to keep getting research cheques is just so much bollocks.
For starters, researchers get paid fuck all, and secondly (and most obviously), if you could build a working fusion reactor you could pretty much name your price.
you heard that story about a turtle moving half way to it's destination with each step......?
I just meant that it's been pushed back continuously, if it is demonstrably 'working' in 2025 I will be surprised. If I am buying electricity that was generated using a Tokamak in the next 50 years I will be elated
@Yet Another Anonymous coward
Ah, a stellarator. It is interesting because it solves one of the biggest problems with plasma instability in a tokomak. But tokomak lets you "research" indefinitely...
Good luck getting "research" scientists and the bureaucracy that funds them to actually USE a stellarator... it might actually produce stable power, and we can't have THAT, now can we?
This has "Investment scam" written all over it. I don't believe the chap expects his audience to understand the power requirements of Ion thrusters, or the engineering challenges of building a fusion reactor (on earth or in space.)
Hell, if you can build a fusion reactor in space, you probably have the capability to build small ones for powering cargo ships. There would be a *lot* of money in that, which could finance your mission to mars if that's what you wanted to do...
You only need roughly one kilowatt to power a single Ion thruster suitable for an unmanned interplanetary mission. You'll need two pairs of two thrusters, so that you can have one fail and still use a pair to keep the thrust acting along the main axis of the spacecraft. That means you have a power budget for thrust of 2 kW, which is easily within the ability of a sensibly sized solar panel, leaving plenty of power for communications and sciencey stuff. This power input gives you maybe 400 miliNewtons of thrust, so the craft, weighing maybe 6 or 7 Tonnes, accelerates quite slowly. (i.e. the same force as exerted on your hand by a couple of 2p coins being pulled downwards by gravity....)
If you want to move a bigger spacecraft, just add more solar cells and Ion thrusters. (There are some difficulties in making bigger Ion thrusters, although the efficiency could be improved if the engineering problems with scaling can be overcome..)
As long as you are in the interplanetary space, that is you have escaped the practical extent of the gravitational field of Earth, even a solar sail will do the trick. Or you can use radioactive carbon as mentioned in here earlier, with a half life of about 5000 years.
To escape Earth's field you need a bit more oomph to get out in a reasonable amount of time. And that is where nuclear power might help.
"To escape Earth's field you need a bit more oomph to get out in a reasonable amount of time"
If you want to accelerate quickly, it would be a lot easier to use a big chemical rocket than a fusion reactor, and would require getting less mass into orbit given our current understanding of how to engineer fusion reactors.
Would you invest in a fusion powered ocean going cruise liner, construction beginning next year? If not, then don't even think about investing in fusion reactors in spaaaaaaaaace.
I agree, this sounds like an investment scam on much the same lines as the Moller Skycar or whatever it was Steorn were wibbling about.
The first step towards doing anything interplanetary is improving on rockets for getting stuff into orbit. The techno-beanstalk technology is about the only thing that will work here, and it is just about within our current technology. What you do is use normal rockets to get a hub station up to geostationary orbit, then you start lowering a line of super-strong cable down towards the ground, using some sort of counter-weight to stop it dragging the station down.
When you have a line from ground to geostationary orbit, probably terminating at an artificial island in the Pacific, you start reinforcing this until you can send loads of a few tonnes from ground to geostationary orbit. At this point your costs of getting stuff from ground to orbit drop by a couple of orders of magnitude, and the safety of doing so increases tremendously. At this point, space tourism becomes possible (you have to be thinking of the money-making aspects of all of this), lunar colonisation becomes much easier, and once you have a moon base a mission to Mars becomes a going proposition as well.
The whole problem here is that space, even low earth orbit, is hostile to humans. We need complex life support to live up there and even then, solar storms emit huge amounts of radiation. Humans off the surface of the earth need somewhere to hide, and a few tens of metres of lunar regolith are one good place to be. The moon is a good jumping-off place for interplanetary missions because there's almost no atmosphere and much lower gravity; you can build complicated stuff on the surface there which can get to lunar orbit quite easily, and thence to interplanetary space, but on the moon is a much more forgiving place for humans than is free fall.
Once you have your beanstalk, lunar base etc then you can start looking to get to Mars. At this point I'm not really seeing a reason for not using the Orion drive; it works, it is simple and away from Earth radioactive contamination is going to get pushed away on the solar wind.
>If you want to accelerate quickly, it would be a lot easier to use a big chemical rocket than a fusion reactor, and would require getting less mass into orbit given our current understanding of how to engineer fusion reactors.
Correct, given the constraint you equally correctly identify. My point was rather more forward looking, to the time when fusion power was available, in which case a fusion equivalent of NERVA would be feasible.
"Once you have your beanstalk"
*If* someone had a credible material for a beanstalk, it would be being built right now. The trouble is we're not just talking about huge tensile and shear strength, we need a safety factor on those strength values of about 6 to allow for manufacturing variability, wear and tear and general cock-ups in predicting weather patterns.
We don't even have a plan to keep the orbiting end stationary:- thermal expansion and contraction of the beanstalk will cause variations in the orbital altitude of the top end, meaning that it will have to be constantly firing thrusters to stay above the base station, *assuming there is no wind*. The beanstalk would be situated at the equator, and would have to withstand hurricane force winds sometimes... You *could* use gas turbines distributed along the beanstalk to counteract the wind load, but then you just made it a lot heavier, requiring more tensile strength and creating a rather unique fuel distribution problem....
@AC - slight correction: solar power isn't much use beyond Mars because the energy collectable for a given area of solar collector decreases as the square of distance from the sun.
It took solar-powered Dawn, with its 36 sq.metre solar panels, dry mass of 747kg and 425kg of xenon propellant at launch, 15 months to get from Earth orbit to Mars and a further 29 months (plus a gravity slingshot from Mars) to get to Vesta. While its enormous solar array provided 10 kW in Earth orbit (1AU radius), this had dropped to 3kW at 3AU. Vesta, its first target, orbits at 2.15 AU from the sun. It then took another 30 months to get from there to Ceres,which orbits between 2.56 and 3 AU from the sun.
I think this shows that Ceres at, a bit under 3AU from the sun, is about the practical limit for solar powered spacecraft. The next planet out, Jupiter, is at 5.2AU, so if Dawn was orbiting Jupiter, its solar cells would only be providing 1kW.
"if you can build a fusion reactor in space, you probably have the capability to build small ones for powering cargo ships"
ACK, and a virtually unlimited supply of potential fuel - aka "the ocean".
It'd be _perfect_. And every Navy on the planet would want YOUR design for their warships.
Not *relevantly* true with current technology because:
Solar radiation to power the solar panels falls off as 1/r^2, which means your ion engine has produced most of the delta-v it is ever going to, by the time it has gone more than 0.5AU outwards from the Sun. Your standard 400mN thruster achieves 0.6mm s-2 on a 6 tonne spacecraft. Unfortunately, that only gives you a delta-v of about 5-8 km/s. That’s *good*, but it certainly isn’t a game-changer compared to conventional chemical.
To be a game-changer for delta-v, it would have to accelerate *much* faster over the “runway” of the first 50 million km outwards from the Sun. 3-5 mm/s2 at least. That means much higher power requirements, so very much larger solar panels (mass starts to exceed what we know how to fold away in the launcher), and plain beefier ion thrusters.
But god knows why fusion......
Actually a standard nuclear RTG is just the ticket to generate electricity independent of distance from Sun and therefore a much longer “runway”. Just, we aren’t allowed to launch RTGs any more for political and environmental reasons.
The man dislikes the word 'nuclear' as he says people think 'bomb'. He has named his mini-tokamak STAR where A stands for Atomic. I've just tried the word association game with a few friends and 'Nuclear' gives 'Reactor' as the usual response, but 'Atomic' was 'Bomb' in every single case. Either Dinan is much mistaken or it says something about my circle of friends.
>I've just tried the word association game with a few friends and 'Nuclear' gives 'Reactor' as the usual response, but 'Atomic' was 'Bomb' in every single case. Either Dinan is much mistaken or it says something about my circle of friends.
That could be something about your circle of friends. My experience is that
- if you are a physicist you automatically become responsible for nuclear weapons, in all countries, and you are about to end the world in a nuclear Armageddon.
- if you are a chemistry graduate you automatically become responsible for pollution, in all countries, and you are about to cause the end of the world by ozone hole depletion, global warming and drowning in waste
- if you are a biologist you automatically become responsible for GMO, in all countries, and you are about to cause the end of the world by hideous deranged mutant monsters.
One of my friends therefore always said he was a upholsterer when people asked him what he did for a living.
When I hear "Atomic" my usual response is Debbie Harry / Blondie
(One of my degrees is a (proper) science degree & I had to undergo a course in radiation protection before they let me play* with some radio-nucleotides)
Pop culture references beat my scientific knowledge in word association most of the time!
* tedious research stuff no actual playing
"If one wanted a realistic improvement in thrusters, why not work on nuclear thermal rockets. These were all but flight-ready already in the 1960's (NERVA)"
Even today, that is still probably the most promising and practical nuclear propulsion technology. It's either that or VASIMR.
Would be a good idea to get ion thrusters to use methane as a fuel... Capture the farts of the spacefarers, and use it for propulsion!
A number of propulsion systems utilizing wastes from the ISS's life support have been seriously considered. It isn't unusual for waste systems optimized to recover oxygen (from CO2 or water) to end up with waste methane, hydrogen, and/or carbon that could be adopted to ion- or arcjet propulsion.
"While Orion, which could have lofted immense amounts of mass at the cost of, er, quite a bit of nuclear fallout, was axed in the 1960s"
Actually, no. The quantity and quality of fallout from a modern-day Orion using the cleanest warheads we know how to build would be minimal, vastly less even than the single small device used on Hiroshima. Launched from a remote location, most of the pulse-bombs detonated at altitude, with careful pauses for different layers of the atmosphere, you could get a massive Orion into orbit with negligible environmental impact. I think the most recent analyses suggest that a big Orion (say, something big enough to put an entire self-sustainable colony on Mars in a single voyage, supplies and all, maybe a 25,000-tonne ship) could be launched for the statistical "price" of one or two extra cancer cases worldwide.
Put another way, the solid science behind pollution analysis tell us that we could colonise the entire solar system with dozens of Orions and thousands of people, for about one-fortieth of the environmental impact and increased disease/death rate caused by Volkswagen cheating its emissions tests. And you could build the fleet of ships and pulse-bombs for about four times what VW has lost because of that same fraud. Any of the top twenty armed nations' miltary budgets could handle sedveral Orion missions if turned to exploration. The UK could swap Trident for Orion and have money to spare; along with bragging rights for the first serious spaceship ever flown. Oh, and immediate global dominance.
Indeed, if the benefit of an Orion fleet is decreased pollution and greater wealth on Earth—through building orbital and Moon-based solar power arrays; pulling metal-rich asteroids into the Earth-Moon system for mining; moving the worst polluting industries up into space—then we are insane not to do it. The likely benefits of Orion to our species so vastly outweigh the possible downsides that, from an economic and scientific point of view, we are verging on suicidal stupidity by ignoring this opportunity.
We don't even need to invent any radical new tech. Orions are surprisingly basic and easy to build. Even the pusher plate survives nuclear hammering much better than you'd imagine (mild ablation can be tolerated), and for sure we know how to build tidy, tiny warheads with precision launch and detonation ...
Orion sits there as a stunningly obvious, practical solution to spaceflight and arguably our best chance of survival as a species. We ignore it because of the N-word: in almost complete, oblivious, witless ignorance and foolishness.
a big Orion (say, something big enough to put an entire self-sustainable colony on Mars in a single voyage, supplies and all, maybe a 25,000-tonne ship) could be launched for the statistical "price" of one or two extra cancer cases worldwide.
But before you launch your colony ship, you certainly have to make numerous test launches, some of which will fail (quite spectacularly!)
For probaly the only situation where Orion might be feasible to push through (for desperate reasons), see "Footfall" by Niven and Pournelle.
"I think the most recent analyses suggest that a big Orion (say, something big enough to put an entire self-sustainable colony on Mars in a single voyage, supplies and all, maybe a 25,000-tonne ship) could be launched for the statistical "price" of one or two extra cancer cases worldwide."
That's really interesting. Do you have a source for that? I'm fervently pro-nuke myself and I'd love to have that in my, ahem, arsenal.
Yes. Not much fallout at all. Tiny selectable-yield H-bombs engineered to be clean.
Freeman Dyson & co generated estimates of how many fatal cancers would be caused globablly for each launch, and the number was 'less than 10', but they knew that was politically unacceptable.
I wonder how many folks die of lung cancer caused by exposure to nuclear radiation from trace elements in coal smoke? Gotta be way more than 10
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