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Comment Re:Minecraft version (Score 1) 24

Oh geez, if any group ever wants to hack a website "for the lolz", they should totally hack NASA's server for this service and insert some ancient ruins or a monolith or something. ;) The prank would hit twice - first by the people thinking it was proof of aliens, and then when NASA corrected it, people thinking it's a coverup ;)

Comment Re: Batteries just don't store enough energy... (Score 1) 232

Also, there's a lot of diversity in terms of aircraft electrification that one can take, it's not an all-or-nothing thing. There's lots of different proposals for varying degrees - for example, high bypass with electric turbofans, using onboard electricity to spin the compressor so that you don't have to have a turbine, and so forth.

Comment Re: Batteries just don't store enough energy... (Score 5, Insightful) 232

Yes. Also, you can't ignore comparative efficiencies of engines. Or engine mass to weight ratios. Or the length of time to market, and the expected level of battery change during that time period. Or side benefits (for example, the ability to have small, very light engines was made use of in one NASA experiment that placed numerous small engines along a wing, causing an effect that created drastically more lift at low speeds and allowing for a much shorter takeoff distance).

And beyond that, you can't ignore economics. Having reduced range but getting your fuel at a fraction of a cost may ultimately prove to be more desirable. It's a very complex issue that one can't just make all-encompassing statements based on a single figure like "energy density of batteries vs. energy density of fuel".

Anyway, this is hardly Elon's first time to mention it. Years ago he mentioned that he wants to be the first person to have an electric plane break the sound barrier. If there's anything one can say about Elon, it's that he sure doesn't set the bar low...

Comment Re:Icebergs float on glaciers (Score 5, Informative) 37

No, it does not form "one huge crystal". Nitrogen ices at these temperatures have little structural integrity. It was well known before we got to Pluto that if we saw any sort of relevant topography, we'd know immediately that it was from water ice, as nitrogen ices are so weak that they'd just flow slack over time.

Comment Re:Icebergs float on glaciers (Score 5, Informative) 37

Nitrogen ices at these temperatures, while crystalline, have rather low viscosity. If you put weight on them, they slowly diffuse around it until the object either sinks or is buoyantly balanced out. The latter happens in the case of water ice.

Also, it's worth noting that it's not pure nitrogen ices, it's a nitrogen-carbon monoxide-methane eutectic. Nitrogen is the most common component, however. Also, there are multiple crystal phases that can be taken, depending on the conditions. Nitrogen ices are most famous for having some rather "explosive" phase transitions between different states.

Comment Re:sunfire / in my stellerator / makes me... happy (Score 1) 98

Fast neutron cross scattering sections in the couple MeV range barely vary over more than the range of 1-10 barns

1-10 barns is, of course, by definition, an order of magnitude. There is a massive difference between 10 barns and 1 barn. Tenfold, to be precise. ;)

More to the point, you can't just combine all cross sections like that. The energy imparted from an elastic collision isn't the same as from an inelastic collsiion, which isn't the same as an (n, gamma), and so forth. Elastic collisions are particularly low energy, particularly the higher Z the target. Taking them out of the equation yields much greater differences between materials in the range of a couple MeV. The upper end of the neutron energies are "somewhat" similar (up to about one order of magnitude), but down below 6 or 7 MeV or so there's quite a few orders of magnitude difference.

Likewise, total cross sections have no bearing on the accumulation of impurities in the material. The particular cross sections are relevant not only in terms of reaction rate, but also what sort of impurities you tend to accumulate and what effect they have on the properties of the material. Which of course varies greatly depending on what exactly they are.

Integration of annealing cycles into blanket design is not brought up enough in some design studies, but is a consideration to help

It's not a side issue, it's a fundamental issue to the design of a material designed for high temperature operation under a high neutron flux.

Blanket design is extremely constrained by tritium breeder ratio to ensure more tritium is produced than used, which squeezes volume allowed to be used by coolant, ... but they have much lower neutron flux to worry about. Gen 4 reactor designs are in the 500-1000 C temperature range, exceeding in some cases what is thought reasonable for fusion blanket design. ... Blanket replacement is considerably more complex than fuel replacement in a fission reactor

Perhaps they've been heading in a different direction since I was last reading on the topic, but I was under the impression that a prime blanket material under consideration was FLiBe. Which operates in a temperature range of 459-1430C, and is its own coolant. That doesn't change what the first wall has to tolerate, but as for the blanket itself, you have no "structural properties" to maintain, and cooling is only limited by the speed that you can cycle it.

The last paper I read on the subject also suggested that for breeding purposes one needs not only beryllium (they were reporting really poor results with high-Z multipliers), but the optimum ratio (to my surprise) worked out to be significantly more beryllium than lithium. So building structural elements out of beryllium serves double purpose, you don't have the excuse of "I need to use steel because it's cheaper" - you need the beryllium either way. It's strong, low density, similar melting point to steel, but retains strength better with heat, and highly thermally conductive. Beryllium swelling from helium accumulation stops at 750C+ as helium release occurs. So pairing a beryllium first wall with a FLiBe-based blanket seems like a very appropriate option.

Please don't get me wrong, I'm not at all disputing the great amount of engineering work left to do. I'm just more optimistic that appropriate solutions will be found. Perhaps I'm just naive in that regard ;)

Comment Re:sunfire / in my stellerator / makes me... happy (Score 1) 98

So on average the fission reactor material only has about 10% of its atoms displaced over the lifetime, while the fusion reactor would have, on average, every atom displaced hundreds of times over the lifetime.

How can you make generalized statements like that? Cross sections vary by many orders of magnitude Fission reactors are generally made of steel, which is hardly setting any records in terms of low cross sections. The smaller the reactor, the less material you have to replace, and the more expensive the material you can use. And being "displaced" is not a fundamental universal material property effect, it depends on how the material responds to radiation damage, which varies greatly. Generally materials respond better at high temperatures (annealing), and fusion reactors operate of course at far higher temperatures than fission reactors.

I have trouble seeing how one would consider neutrons per square meter to matter more than neutrons per MeV. Because neutrons determine what you're going to have to replace, and energy determines how much money you get from selling the power to pay for said maintenance. You can spread it over a broad area and do infrequent replacements, or have it confined to a tight area and do frequent replacements, the same amount of material is effected. Some degree of downtime for maintenance is normal in power plants - even "high availablility" fission plans still only get ~85% uptime.

Comment Re:sunfire / in my stellerator / makes me... happy (Score 1) 98

Hmm, thought... and honestly, I haven't kept up on fusion designs as much as I should have... but has there been any look into ionic liquids as a liquid diverter concept? In particular I'm thinking lithium or beryllium salts. They're vacuum-compatible, they should resist sputtering, they're basically part of your breeding blanket that you need already... just large amounts, flowing, and exposed. Do you know if there's been any work on this?

Comment Re:sunfire / in my stellerator / makes me... happy (Score 2) 98

The plasma facing material faces a flux of 1 neutron per 17,6Mev. By contrast, nuclear fuel cladding faces a flux of ~2,5 neutrons per 202,5 Mev, or 1 per 81 MeV. It's certainly higher, but it's not a whole different ballpark. And yes, you're dealing with higher energy neutrons but in a way that can help you - you've often got lower cross sections (for example), and in most cases you want the first wall to just let neutrons past.

There's a number of materials with acceptable properties. Graphite is fine (no wigner energy problems at those temperatures). Beryllium is great, and you need it anyway. In areas where the blanket isn't, boron carbide is great. Etc. These materials aren't perfect, but they're not things that get rapidly "converted into dust" by neutrons. Really, it's not the first wall in general anyway that I'd have concerns about, it's the divertor. The issue isn't so much that it takes a high neutron and alpha flux and "erodes" fast - that doesn't change the reactor's overall neutrons per unit power output ratio, and if you have a singular component that needs regular replacement, said replacement can be optimized. The issue is that you have to bear such an incredible thermal flux on one component. Generally you want to spread out thermal loads, it makes things a lot easier.

Comment Re:Fusion energy is impractical (Score 1) 98

When a fast neutron hits an atom it knocks it out of its position and frequently changes it to a different element/isotope.

The same applies to slow neutrons, so....? Your average 14,1 MeV neutron is most likely to inelastic scatter down to the point where more exotic reactions than (n, gamma) are basically impossible (excepting a few specific cases, like 6Li(n,t)4He - again, not dangerous). Only a small percentage of your 14,1MeV neutrons (depending on the material they're passing through) have a chance of undergoing anything more than a standard (n, gamma) transmutation. Unless the system is specifically designed to cause that (for example, a beryllium multiplication in the lithium blanket). The standard case is inelastic scatter once or twice -> elastic scatter a bunch -> become partially or completely thermalized -> capture.

This turns a solid structural material into a radioactive powder

What happens depends entirely on what's being bombarded. Many materials are perfectly fine after long periods of exposure - slow or fast neutrons. Light ions in particular are usually either A) relatively unaffected (sometimes requiring sufficient heat for proper annealing, sometimes not), or B) incredibly good absorbers, leaving nothing dangerous behind. See a more detailed breakdown above.

Comment Re:Venus (Score 1) 310

Plant cultivation is far, far harder on Mars, for many reasons.

1) Natural light: the solar constant is 1/5th as much on Mars as on Venus, and you're guaranteed to have dust clinging to your greenhouse glazing. More on this later.

2) Electricity: Same for solar power. And fission power systems (as opposed to radiothermal, which is far too weak) are 1) a rather expensive line-item to your development costs, 2) heavy to transport, and 3) complex (complexity is not good when it comes to operation in space). Beyond this, most people vastly underestimate how much power it takes to grow plants under lights - you need 1-2 orders of magnitude more area of solar panels than the area of plants you can grow. And the size of the LED lighting systems you'd need is very significant in its own right. Plants consume way more light to grow than most people give them credit for. The real world isn't The Martian where one can grow potatoes on normal room lights ;)

3) Room: Abundant, practically unlimited space comes free with a Venus colony. Space is extremely expensive on a Mars colony - it's a pressure vessel. Another downside to limited space: plants don't like it. It leads to humidity and temperature instabilities and buildups of gases like ethylene that are far more poisonous to plants than carbon monoxide is to humans. These gases break down, particularly in sunlght, so in big areas they're not a huge problem - but in confined spaces, they can deform and kill your plants readily. Pests and diseases also thrive much more in confined spaces.

(My comments on plants come from experience: I grow a small "jungle" in an indoor environment, entirely on artificial light)

So, while it is of course possible to grow plants on Mars, it's far, far easier on Venus.

As for opressiveness, once a wall is opaque, you can't really perceive how thick it is.

Indeed, I wasn't talking about wall thickness :) Just the issue of being enclosed in small spaces. Most designs call for integrating as many windows as they can, but that's always going to be limited - windows are a lot heavier for a given amount of surface area and can't be shielded for radiation exposure.

And I'm not sure how attractive Venus would be in comparison

So, you don't get a landscape, that's true - the surface isn't visible there. But at the desirable altitudes, there is still a "view", the clouds are dynamic there. A few kilometers further up and it's just a continuous haze (which may lead to rainbow effects below, there are some papers debating this ;) ), but in the "earthlike" layers clouds will come and go. Like living among the clouds on Earth.

But no, you don't get a landscape outside. Your landscape is the Garden of Eden you make inside, surrounded by clouds. :)

There's also those ever-present lightning storms all around you - that's going to be noisy, and a serious maintenance issue

The current state of research isn't "ever-present lightning". Again, unfortunately our knowledge of Venus is so poor compared to Mars, so it's hard to make definitive statements. But lightning appears to be "about" as common on Venus as it is on Earth.

Another thing that we need to learn more about is atmosphere variation. We've seen what appears to be significant variations in sulfur levels on Venus over time - it seems that the sulfur may be the result of frequent or continuous volcanic activity. So how the atmosphere will vary over time is an important question to be able to answer before we can send humans.

And how do you plan to prevent lightning strikes through your habitat?

Again, we don't know the distribution of lightning between a) different altitude layers, b) different latitudes, and c) over time. We actually don't know at this point if it's ever a risk at all - and if it ever is, whether it's avoidable or not. If it's not avoidable, then yes, one would need lightning protection (I presume faraday cage-style rather than any sort of ion shield), which would add mass and require a more difficult testing regime. If it is avoidable, or is never a problem: then there's no issue.

Definitely need more data on this one before we can send humans! It's time to stop neglecting Venus.

but since you're in the middle of the cloud layer they won't actually be getting anywhere near as much sunlight as they would in orbit, maybe not even as much as they would on Earth or Mars

Actually no :) The light levels at acceptable flight altitudes (~51-55km) are comparable to Earth on a clear day (except that you also have almost as much light also reflecting up at you as coming down at you). Depending on the frequency, it blocks about half of the light from the sun - but twice as much light hits Venus. Mars, however, gets 40% as much light as on Earth - when the dust isn't blowing. Sometimes you get dust storms which can last for months, easily enough to kill plants from lack of sunlight.

Note that solar panels don't have to be outside the envelope, if the envelope is transparent (which I've been assuming thusfar). They can even be built into structural elements (for example, solar roofs on shelters or walkways). It'd cost under 10% of the power, and in turn they'd be shielded from winds, lightning (if a risk), icing (if a risk), corrosion, etc, and your wiring needs would be greatly reduced. I really don't see a point to having anything outside the envelope except for the return rocket (even that's not 100% necessary, but probably a lot easier than a rocket-sized drop-bay ;).

If the ambient pressure is ~1atm, then you have roughly as much air above you as you would on Earth, but without a magnetosphere you're going to be counting on that air to block a lot more radiation.

I read a paper about this before but can't be bothered to dig it up again ;) Okay, okay, just a second.... hmm, this may have been the one. They simulated the Carrington Event and one previous one that was even stronger, and found that even they wouldn't be problematic at 62km (let alone a more realistic 53-54km, which has orders of magnitude more atmosphere over it). That is to say, they calculate 0,09Gy. Radiation therapy in humans is 45-80Gy. A CT scan is 0,008 Gy. So it's like getting a dozen CT scans, but nothing like undergoing radiation therapy. And that's at a much higher altitude than people would actually live at. Long-term GCR at actual colony height, according to their graphs, would be about 1e-8Gy/20h, or 4,4e-6Gy/year - not at all "dangerous". Levels are indeed higher than on Earth, but they're not problematic like they are on Mars. You don't need added shielding, you're sitting under a mass of shielding equivalent to a ~5 meter tall column of water. And the atmosphere above you creates a small induced magnetic field to boot.

Comment Re:Better transistors? (Score 2) 306

And if they're having a significant reduction in power consumption, then adding more cores gets all the easier.

Its always seemed to me that the best approach to processing is to offer a variety of cores and let the scheduler handle what to put where. You can have one or two extremely fast cores, half a dozen moderate speed cores, and dozens or more low speed cores - why insist that all cores be the same in "general purpose" computing?

Comment Re:Caller ID Blocker (Score 2) 242

Back when I lived in the states (I've never gotten a single telemarking call here in Iceland) I've often been tempted to respond with, "Why should I buy your product when I'm going to kill myself as soon as I get off the phone?" Suddenly making their job waaaay more stressful than they expected when they picked up the phone.

Never did it, but... ;) Honestly, I just couldn't get myself to be that mean to them, they're just normal people on the other end working menial, low paying jobs.

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