Firebombing doesn't poison everyone and everything in a huge radius.
Neither did nuclear attacks, as far as the people calling the shots knew, after the initial flash. Hard to claim malicious intent for long-term poisoning under those conditions.
...our DNA is equipped with raid-1 capabilities ? Maybe it requires some added Reed-Solomon codes, everyone knows raid-1 doesn't protect against data-corruption...
It does have error correction codes, in that only a subset of possible sequences validly code for things. If one strand contains valid sequences and the other doesn't, you know which to use when rebuilding.
A molecular biologist can give you a more detailed explanation of how this works than I can.
Since when does one call 40% efficiency efficient?
Since efficiencies of less than 1% are typical for most lasers that aren't diodes or CO2, that's when.
And since the _energy_ efficiency of chemical rockets is multiplied by the mass fraction (typically 5% or less), that's when.
If you can get a 40% efficient laser dumping most of its energy into either a carried-hydrogen stream with an Isp of 600-900, you get a system that is _vastly_ more efficient than chemical rockets.
It remains to be seen if this ends up being a system that's cheap enough in practice, though. Chemical rocket cost certainly isn't dominated by fuel (otherwise we'd be launching at $100/kg right now).
You can vary the wavelength across the array, and that avoids most of the problems with speckle patterns.
They already do this within each launcher node. This is described in detail in the description of how bar lasers would be combined into arrays with sufficiently high luminance to drive the craft.
I personally doubt the combined light from all of the lasers would be coherent enough to get much speckle, and even if it was, the craft is moving fast enough that it would see a constantly-changing pattern that would average out to something reasonable, but I certainly haven't run the numbers to prove this.
correction: liquid oxygen weight is responsible for most of the shuttle weight (8 tons of oxygen are needed to burn 1 ton of hydrogen) but the volume of liquid oxygen is actualy rather small, much smaller than the volume of liquid hydrogen There are 2 tanks within the external tank - a small one for LOX sits on top, the rest underneath of it is filled with LH.
The reason for this "paradox" is extremely low density of liquid hydrogen.
And this is why liquid hydrogen is bad as a fuel for a craft that has significant flight stresses - you need a huge tank, which weighs a lot and is large enough that the square/cube law starts to bite you for structural strength (increasing the weight even more). It turns out that the drawbacks of increased craft size per unit cargo weight outweigh the benefits of using alternate fuels. This is why most new craft proposals use hydrocarbon-based fuels. There have been several papers on this topic (some linked from Slashdot).
The reason why the proposed laser craft can get away with using hydrogen is that they claim an Isp of 600-900, vs. an Isp of around 400 for H2+O2, and around 300 for CH4+O2. This drastically reduces the propellant:cargo mass ratio, to the point where the hydrogen tanks aren't cripplingly large. Whether this Isp can be achieved in practice remains to be seen (though a fair bit of work has been done on the type of engine they cite, and their numbers for lasers are fairly accurate, suggesting similarly thorough engine research).
The only place it makes sense to use hydrogen in a chemical rocket is for a low-thrust drive in space, and chemical rockets are out-competed there by electric drives.
If projects like OpenCores catch on and FPGAs become cheaper then maybe open source can perform as well in that region as it does in software. Then I think people would not be happy with ARM taking down compatible products, just as people would not be happy if Microsoft went after WINE.
The big problem that opencores.org and other open hardware projects will run into, if they aren't already, is patents. For hardware, there is distressingly often only a handful of good ways of doing something, which will have been patented long ago. I'd love to design a 2D/3D graphics chip core (and I have the expertise to pull it off - IAACompEng with graphics experience), but if I tried I'd have everyone from Matrox on down knocking on my door with a big stick in hand.
Perhaps if the patent revolution finally comes for software, hardware patent lifetimes will be shortened as a side effect.
There's also ARM's lurking in games-consoles (GBA, Dreamcast), routers, PDA's, portable music players, mobile phones, infact just about every type of small device.
The Dreamcast uses a Super H 4 as its primary processor, as it needs the SH4's ability to manipulate floating-point vectors natively at reasonable speed.
There may be an ARM core tucked in there for other purposes (sound?), but SH4 is the heart of the machine.
Re:Nice technology
on
Broadband Blimps
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· Score: 3, Informative
Sounds like an attempt to overcome the runaway expenditures of Teledesic's failed LEO project. The problem with these high-altitude sender/receivers is that--while they offer a technology solution--there is a corresponding weakness in application.
For example, latency in these systems make it unattractive for many internet applications (who wants to play FPS's over a spread-slotted Aloha CDMA system?).
As long as you have a relatively nearby ground station to relay to, latency isn't a horrible problem. Right underneath one of these things, round-trip latency is about 0.13 milliseconds. At the edge of a blimp's broadcast range (around 100 km if I'm reading things correctly), it's 1.3 milliseconds round-trip.
Think of these as a much cheaper way of building a very tall relay tower, for something closer to reality than the "satellite" analogy.
We can't change its name to be a 'space bridge'. If we did, we couldn't have the same hilarious jokes in every Slashdot article about elevator music.
Don't worry; we'll have plenty of Transformers jokes to replace them with.
("No, Prime, _you_ will activate it for me - for the secret cargo is: Cybertron!")
Re:Three times redder than they human eye can see?
on
Titan's Surface Revealed
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· Score: 2, Informative
Visible 'red' light is around.65 to maybe.75 micrometers. So are they are saying 2.1um or so?
I do wish these articles would just say what they mean and not try to make it seem more 'amazing' with fuzzy statements like that. It's like "WOW! THREE TIMES REDDER!" - when in fact, near IR is nothing special - most cheap camcorders can take pretty good pictures in that frequency range.
Silicon photodetectors, like the silicon CCD chips in camcorders, have a cutoff at about 1.1 micron. They won't see 2.1 micron infrared.
Furthermore, John Q. Public reading that press release will have no idea what a "micron" is, but probably _will_ get the general idea from a phrase like "three times redder". If you want an accurate description of what's going on, why on earth are you reading a press blurb?
As for interstellar exploration, we need a financial incentive, much like the X-Prize. Only, in this case, first company sponsoring a colonization mission to an Earth-like planet, claims it.
The problem is that unless we discover new physics, sending even a flyby probe across interstellar distances within anything approaching a reasonable timeframe is an absurdly expensive project. It's within the capacity of Earth's richest nations to do it if they bent all of their resources to the task, but that's about it. This means that any financial incentive an X-prize like contest could offer would be insignificant compared to the project's cost. Similarly, not even Bill Gates could afford the ticket price for interstellar tourism, so using the prize to leverage advertising for another market doesn't work either.
As manufacturing and especially automated manufacturing technology improve, the cost of an interstellar craft may drop to the point where it's feasible for only a large organization to produce a probe, or even send colonists. However, for the time being, an Alpha Centauri shot is at about the same stage as a Moon shot was 50 years ago - something that could be done as a pissing contest (or military turf battle) between the richest governments, but by nobody else.
I hope to see a probe launched in my lifetime, but I don't expect it to be any time soon, or by profit-driven private organizations.
Probably not, in the whole scheme of things, very little gets spent finding new, better energy sources.
Perhaps that is due to the controlling interests not wanting to give up that control.
Or just because we don't need them at present, and there's greater return on investment in other branches of development.
Think about it. If we knew we _had_ to switch to, say, hydrogen power storage and nuclear power generation within 50 years, wouldn't the big oil companies invest scads of money to make sure _they'd_ be the big hydrogen/nuclear companies?
It won't be for this launch, or even the next one, but hopefully Canada will soon have companies making launch vehicles:
I doubt this. We're in a horrible launch location (you want to be close to the equator), and the market is already pretty saturated (Arienne-5 and other solutions on the expensive side and Russian boosters on the less expensive side).
I'll be the first to cheer if we do get Canadian launch facilities (way too much red tape getting things launched by the US), but I'm not holding my breath.
No, you can scan your spam folder in seconds, because you will recognise the subject lines. The duration is not comparable.
I do this. It takes a large amount of time. Comparable to the time required to hit "delete" on seeing a flash of spam-text.
Time savings: zero.
The only reason I spam-filter at all is that the spam filter serves adequately as a _priority_ filter, letting me get to _most_ of the real stuff first. Doesn't save me from slogging through the rest.
I swear I can go weeks on end without a single spam getting through, no false positives -- and between 20 and 100 SPAM in my "spam" box per day!
This is what I don't get - in order to be sure you have no false positives, you have to comb through all of the spam by hand, which for the most part defeats the purpose of a spam filter. If you don't do so, then you can't claim zero false positives - you can only claim that you haven't _noticed_ any false positives.
I have a whitelist at work, and it works quite well, but combing through and emptying the spam bucket is still an annoying part of each day.
However, without doing so, I'll never know if I missed that one message in (about) a thousand that's from a vendor that's not in my whitelist.
QOTD: "I don't have a solution, but I do admire the problem.".
I'm afraid I don't know offhand, but it seems you've already found that information:). There are likely at least half a dozen project names that the US military has cycled through in its research programs on nuclear propulsion, too. I think they did actually get a nuclear jet flying (passing air through the core instead of stored reaction mass).
Why is hydrogen the best, here?
It has a very low atomic weight. "Temperature" is the amount of kinetic energy per particle (molecule, usually). So for a constant exhaust temperature, using hydrogen gives you the greatest energy per unit mass, and so the greatest exhaust velocity. I'd have to grind through formulae to see how this meshes with gas law, though, as this seems inconsistent with what I learned about gas law in first-year. So, I could be misunderstanding the reason hydrogen is used.
What's the deal with engine components having to be at the exhaust temperature? Wouldn't this almost always be true, since the fuel is going through and combusting inside the engine, or is there more to this?
It turns out that this doesn't have to be true in a chemical rocket. In a nuclear rocket, the exhaust is heated by the reactor, so the reactor has to be at exhaust temperature. In a chemical rocket, the exhaust is self-heating, so you can spray reactants into the middle of the combustion chamber and have a blanket of cooler gas at the chamber walls to protect them. Building the combustion chamber and nozzle out of ablative materials gives you this in a straightforward manner, though your nozzle lifetime is finite with this scheme. By this method you can get exhaust temperatures far in excess of what just about any reasonable structural material will stand (even carbon composites).
There has been talk of building nuclear drives that use a cloud of uranium gas to provide energy, allowing similar schemes, but my understanding was that this is only actually feasible in space (and is low thrust, as well).
I see. This would be the main reason that an upper atmospheric launch would be of some benefit, is that right?
It's one of the reasons. The other reason is that it allows smaller spacecraft to be used. In order to be able to mostly ignore atmospheric losses, your craft's mass must be substantially greater than that of the atmosphere it plows through on the way up. That's about 10 tonnes per square metre of cross-sectional area - but at 10km up, it's more like 3 tonnes, so you can use a craft a third as long (and presumably narrower, making the smallest practical craft size 10x-30x smaller by mass).
A smaller craft means easier to handle structural stresses (thanks to the square/cube law), which means less mass spent on structure and more on cargo, or cheaper materials used for construction, or both.
So, launching small craft from the upper atmosphere is very attractive. The Pegasus launch system already does this.
So, if you had a big enough aircraft that could lift enough to haul all of the fuel needed to add on the last Mach 20-25 of delta-v and the spacecraft into the upper atmosphere, you could launch from there with a nearly-horizontal trajectory and save the lost acceleration during the lift phase of a ground launch. I would imagine that a problem with this is that there are no such aircraft available at this time.
There are (e.g. Pegasus). The craft are just very small. Space Ship One also used a scheme like this, though they weren't orbit-capable (just sub-orbital boost to 100 km altitude).
If Scaled Composites goes into orbit-capable craft desgn, they'd quite possibly use a scheme like this. I recall hearing that they were mainly positioning themselves as a supplier for space tourism businesses wanting sub-orbital craft, though. I don't have hard confirmation either way.
So, are there any good hydrocarbon fuels that have comparable Isp to hydrogen?
I'm afraid not. Typical best Isp is on the order of 250 or so for hydrocarbon/oxygen rockets.
What makes a good hydrocarbon rocket fuel?
Hydrogen-rich, easy to handle at room temperature, and ideally not very nasty to humans. For best Isp, use liquid methane, though that's a bit pickier to handle.
What are the downsides of using hydrocarbon fuels?
Low Isp, mostly.
Why does liquid hydrogen seem to win in the end?
It doesn't, most of the time. At low altitudes, rocket nozzle concerns reduce the Isp you can actually squeeze out of fuels, so the shuttle only uses it for high-altitude burn. The size and mass of the LH2 tank makes it questionable whether or not this is a better solution than LOX/hydrocarbon upper stages. There are a few papers around that discuss this (don't have a link handy, sorry).
It might be a good solution in space, especially in the outer solar system where solar heating is less of a problem (so you have less overhead for the insulation and cooling systems).
The NASA Space Shuttle uses solid rocket boosters, and I had gathered that these use some kind of hydrocarbon fuel or other. What's in them and what's good/bad about them?
The shuttle SRBs use ammonium perchlorate and aluminum as the oxidizer and fuel, respectively. The composition breakdown also lists iron oxide (which probably means they use thermite to light the thing), and a rubberizing compound to bind it all in place.
At low altitudes, gas expansion concerns limit the Isp you can usefully get from a fuel without a variable-geometry nozzle, so using a lower-Isp fuel in the SRBs is still competitive with using the main (LOX/H2) engines for boost (actually, these are still turned on). A core-burning solid fuel rocket can also give you greater thrust, especially if the boosters are disposable (i.e. you don't mind materials degradation as long as they last one flight). So the SRBs are used to help with high-thrust boosting when the shuttle is low in the atmosphere and near its fully-fueled weight. Higher up, the boosters are dumped and the LOX/H2 engines (optimized for high altitude/vacuum) take over.
In other words, the delta-v is a fixed quantity for a given orbit, and the exhaust velocity of the fuel is also fixed (in practice 10 times the Isp), and that becomes the exponent on 1/e to yield the dry:wet ratio for the spaceship, right?
Yes, this is correct.
I take it that given some "wonder fuel" that provided an Isp of 800 (an exhaust velocity of 8000 km/sec), the launch weight of the fuel would only need to be 63% of the total mass of the craft, is that right?
This is also correct. This is part of why NERVA-style nuclear rockets are so attractive. Passing hydrogen through a fission core gives you pretty much the best exhaust velocity you can get with an engine where engine components have to be at the exhaust temperature.
In principle, a nuclear-electric drive could get better Isp, but in practice, power to weight ratios and heating problems limit you to far less than one gravity of thrust, which means they don't help with the ground-to-orbit problem.
This 1/e business sounds like it comes from a solution to some sort of definite integral with respect to time and mass of the rocket equation. Does that mean that this 1/e to the power of the delta-v over exhaust velocity only applies directly to single-stage rockets, and that the situation would be improved for multi-stage rockets?
Parts of the rocket that you throw away count as fuel weight, to the first order, so you can look at them as degrading the Isp of the fuel instead of adding to craft mass. It still makes life easier to use multi-stage rockets to reach orbit. In fact, this is the only way we've been able to reach orbit at all (Single Stage To Orbit projects keep running into materials problems; we'll get there eventually).
The effect of a higher exhaust velocity is not only a higher impulse, but also the fact that you're ejecting your fuel at a higher rate, thereby lightening your launch vehicle as it flies at a faster rate, which basically translates into a higher overall effeciency. Is that a fair statement?
I'm afraid not. It's just impulse delivered per unit fuel weight that ends up mattering.
So, hydrogen is a light molecule, but it burns pretty well. What about something like a liquid sodium and flourine or chrlorine engine?
The highest-Isp chemical fuel mixture is hydrogen as the fuel and fluorine as the oxidizer. This gives the most energy per unit fuel weight, which means that exhaust temperature is as high as you can get, which gives you the best exhaust velocity achievable with chemical fuels.
In practice, exhaust temperature and materials handling problems make H+F impractical as a fuel. H+O is almost as good, and liquid oxygen is much easier to handle than liquid fluorine. Hydrogen still takes enough fuel tank mass overhead that it's questionable whether it's worth it as a fuel, though (hydrocarbons are stored at much higher density, meaning a smaller tank and less craft weight).
It turns out that metals like sodium are heavy enough to offset their reactivity benefit. It's hard to beat hydrogen for combustion energy per unit weight. Ditto heavier halogens like chlorine (oxygen gives the same or better reactivity at lower atomic mass).
One final thought: I assume that there is a limit to how great of a "wonder fuel" you could use for launches of human cargoes, since the ideal seems to be getting a lot of acceleration right at the start right away, and that brings you back to the problem of launching humans with rail guns, i.e., the acceleration would be fatal, and so you're stuck to a certain extent hauling a large part of the fuel for a large part of the flight just to provide a reasonably gradual acceleration as opposed to a big jerk at the beginning.
This turns out not to be a problem. It's only the Isp of the fuel that determines the craft's mass fraction, so there's no requirement to boost at 100 gravities when boosting at 3-4 will do.
The actual problem is the vertical
Re:Further reading...
on
Amorphous Steel
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· Score: 2, Informative
The article was a little thin, so I mosied on down to Wikipedia.
You can also find an abstract and a PDF of the whole article on the physical review letters site (a few links in from the article Slashdot linked).
These are letters, so they haven't been through rigorous peer review, but the authors take great pains to cite related work and describe their experiment in excruciating detail, so their results are almost certainly perfectly valid.
Capsule summary: Adding about 1.5% ytterbium to steel alloys makes it *far* easier to get amorphous phases of them, which is normally a royal PITA for metals (you tend to get very fine grains instead). This has been shown before, but they map out a range of alloy compositions and show where alloys with good properties lie within that range, and do a large number of tests to a) prove that they really have produced amorphous steel and b) measure the materials properties of the steel they've produced.
Let me just make sure I'm clear on some terms and concepts. Isp means specific impulse or something?
Yes. The original definition was "pound-seconds of thrust per pound of fuel". The modern definition is "newton-seconds of thrust per kilogram of fuel", divided by 9.8 (Earth's surface gravity) to get back to the old units. Easy to handle chemical fuels have an Isp of around 200-250. Difficult to handle chemical fuels have Isps of around 300. Very nasty and/or extremely difficult to handle fuels have Isps around 350-400. NERVA-type nuclear drives, which pass reaction mass through a fission reactor core, have Isps of about 550 or so, but are not politically feasible (if one crashes, you get radioactive crud spread around, and your engine sprays out radioactive crud as the core gets etched away, and you can't afford to carry much shielding, so the reactor harms crew and electronics while in operation - geometry requirements for use as an engine prevent safer reactor designs from being used).
In practice, Isp times 10 is about equal to your exhaust velocity, and every multiple of your exhaust velocity adds a ratio of 1/e to your dry:wet weight ratio. So, a rocket with a fuel with an Isp of 250 that wanted to go at 8 km/sec would need a delta-v of 3.2x its exhaust velocity (8000 / 2500), for a mass fraction of about 4% (e^-3.2) - i.e., 96% fuel, 4% structure plus cargo plus crew. High Isp helps a _lot_. It just turns out that most really high Isp fuels require enough mass overhead for the ship or cause enough other problems to not be worthwhile.
Is "delta-v" the same as acceleration, or just that you have to get from sitting still on the earth to 8km/sec in a short period of time to get to orbit?
Delta-v is the change in velocity needed to perform any specific maneuver in space. It doesn't matter whether this is accomplished all at once or over a long period of time. To get from being at rest on the surface of the earth to being in orbit, you need to change your velocity by 8 km/sec. In practice, we have to do this very quickly for a surface-to-orbit launch, because a sub-orbital trajectory will bring us crashing back to earth fairly quickly, and because atmospheric drag slows us down if we hang around in the atmosphere, and because if we're thrusting upwards, earth's gravity is pulling us downwards, and we end up losing velocity if we just coast (most of the thrusting to orbit is sideways, but the initial burn to get above most of the atmosphere is upwards).
Is 8km/sec escape velocity, by the way?
8 km/sec is the delta-v required to get to low Earth orbit. An object in a circular orbit near the surface of the earth moves at 8 km/sec relative to the surface. Escape velocity is the delta-v needed to get out of Earth's gravity well altogether. This is about 11 km/sec. An object on an escape trajectory fires straight up from the surface of the earth at 11 km/sec. As it climbs out of the gravity well, it slows down. For an object launched at precisely escape velocity, it'll keep drifting away, but by the time it's far from earth it's moving very slowly (almost at rest). A probe intended to tour the solar system would have to be launched at more than 11 km/sec.
In practice, an efficient space launch would use the smallest possible delta-v (8 km/sec) to get off of Earth's surface, as this is the delta-v that must be provided with inefficient chemical rockets. To climb further out of the gravity well, and move around the solar system, a craft would use a more efficient type of drive (like an ion drive, plasma drive, or other electric drive that produces a higher Isp). So far, only the Deep Space 1 probe has done this.
Once you're out of Earth's gravity well (delta-v: 11 km/sec from launch), you can use "gravitational slingshot" trajectories to steal momentum from other planets or from the moon, as an alternative to carrying vast amounts of fuel or a high-Isp electric drive, but this takes quite a while (typically years between slingshot passes,
You're only using a 10,000 T elevator because if it's ever cut, it impacts with a yield of about 7 times its weight in TNT. While you can design it so that naturally occurring failure modes won't cause the whole thing to come down (or even much at all of it), sabotage is a real enough concern that you don't want to drop a _million_ tonnes of elevator on the planet. Incompetence is also a potential failure source (there's a raised highway around here that they only started repairing _after_ chunks of concrete fell down on to cars running beneath it - it's always politically better to delay repairs just another year).
There are other styles of ground-to-orbit device besides elevators. Skyhooks are one of the more popular. The problem is that the stresses on a rotating skyhook that has a tip moving at ground speed at ground level are comparable to those on a space elevator, so the materials problems end up being the same. For travelling between orbits in space, they're easier to build, but you could just as easily use high-Isp electric drives and avoid the investment in the first place. So, I'm not sure skyhooks are a good idea to build (an elevator has less administrative overhead). Might be possible to have higher throughput, though.
Why rockets? Are space elevators the only alternative?
There are other alternatives, but they are either very hard to build or very expensive to build, to the point where it's questionable if they would make economic sense compared to chemical rockets. Chemical rockets, if commoditized to the point where fuel was the dominant cost, are "cheap enough" for most purposes (the only thing they're not cheap enough for lifting is huge amounts of bulk structural material for city-sized space stations).
Anything that carries its energy source with it _and_ has enough thrust to get from ground to orbit will have the problems of chemical rockets (be it chemically powered or NERVA-style fission powered, your exhaust has to be cool enough to not destroy your engine too quickly, which limits Isp). Nuclear-electric and even solar-electric work fine once you're already in orbit, because you can afford to build up delta-v over months, but ground to orbit requires delivering at least 8 km/sec delta-v in a few minutes at most.
Schemes that supply energy from elsewhere are things like laser launchers and mass drivers. Mass drivers _might_ be practical for bulk cargo that can take thousands of Gs acceleration. Delicate cargo like humans that's limited to a few Gs requires a mass driver a thousand kilometres long, which is not economical to build (and has the problem of firing tangentially, with a very long path through the atmosphere between the cannon muzzle and space). Building a cargo-capable mass driver is still a very difficult proposition (thousands of Gs is a tall order for field coils to produce on a multi-tonne cargo capsule).
Laser launchers have to plow through lots of atmosphere, and only give you an advantage when operated in "jet" mode (taking reaction mass from the atmosphere). Mass carried with the craft gives you the same problem as chemical rockets (exhaust temperature limits Isp). Boosting in jet mode means flight must be at a fairly shallow angle to have a long enough launch path, which probably means multiple laser stations (range through the atmosphere isn't that great). This is very pricey to build, to the point where you have a hard time amortizing construction costs to a competitive level (mass drivers are a bit better here because they can have higher throughput).
A space elevator is nice because it lifts material with pretty much the bare minimum possible energy investment. The problem is that it not only requires near-magical composites, it requires _lots_ of them. That means not only waiting for reliable nanotube composite production, but waiting for it to be cheap enough to let you build and maintain a structure massing tens of thousands of tonnes (and you'll _need_ to maintain it, in an environment with micrometeors and enough radiation to degrade highly ordered materials). Space elevators also have throughput problems - they can't carry more than around 10%-30% their own mass in cargo at any given time (depending on how much extra strength you spec for it when you build it - and extra strength costs a _lot_ for this type of structure). You have to haul this mass at least 40,000 km (amd more like 100,000 if you're going to the counterweight tip for maximum launch velocity). That takes a while. So for a 10,000 T elevator you can only haul 1,000 T at a time, and have to schlepp it at _minimum_ a distance equivalent to circumnavigating the world at the equator. This will take a while. So throughput ends up being lower than something like a mass driver, which is very nearly as energy efficient.
So, I think chemical rockets are what we'll be using for quite a while. Commoditized chemical rockets are actually pretty nice.
It seems to me that there is not much attention being given to the idea of large, flying disk/wing aircraft that could fly up to high altitudes and then switch to rocket propulsion as a means for lifting large payloads on a reliable, safe, and regular schedule. The commercial aerospace industry is pretty close to the "fuel as the pr
First, the disclaimer. Most of what follows is educated guesswork. However, the "educated" part means that most of the guesses are made for a reason, and should be at least partly correct. Enjoy:).
How long do you think it will be until we start to see:
Inflatable space stations that are rotating so as to provide artificial gravity
If you have a large station rotating for pseudogravity, most of the stresses are tensile stresses caused by the rotation. Whether the hull is rigid or not doesn't make a huge structural difference at that point.
A small rotating station would just be one or more bods and/or counterweights on the ends of cables. Making the habitat modules inflatable would save a bit of mass for the structure, so there's a reasonable chance it would be done if inflatable technology is sufficiently trusted.
Space stations that are environmentally self-contained, i.e., with plants and animals and goodly amounts of open water and topsoil-like material on board, using the sun's energy to maintain a self-supporting ecosystem
It turns out that the cost of going from a 95% efficient recycling scheme to a 100% efficient recycling scheme is high enough that it isn't likely to be done any time soon (not until we have space habitats with populations of thousands, and maybe not even then).
Air recycling would be done electrochemically, not boilogically (burn CO2 in hydrogen to get H2O and methane, electrolyze H2O for oxygen, strip hydrogen from the methane for use as H2 feedstock and sequester the carbon as waste). Water recycling can done adequately by dehydration of wet waste and fractional distillation of wastewater. As long as you have abundant energy, recycling air and water at high efficiency is very practical.
You'd still need to ship up dry foodstuffs and take down waste, as well as top up the water supply, but the volume is low enough that this isn't a big load.
I don't think biological food production and waste reclaimation will be practical in small stations for the forseeable future. It'll take large colonies, as mentioned above, and you have to be willing to fork over a _huge_ mass allowance for the biosphere you're building (an equivalent mass of air, water, and food would last years before running out, so this is a _big_ investment in mass for only a long-term payoff).
Microcosms of endangered plant and animal ecosystems on earth transplanted or reproduced into large biospheres in space
Unlikely to ever happen, as it's cheaper to build contained environments for them on Earth. Space environments of this type would arise as a side effect when self-contained space station biospheres were built for other purposes, but building space stations just to preserve the biospheres is a horribly inefficient way of achieving the goal.
Biosphere "prisons in space".
Not happening. Jails on earth are far cheaper. If you want to get rid of someone so that they're no longer a drain on your system, there are plenty of places on Earth you can dump them, or you can just kill them outright (not a debate I'm getting into in this thread, thank you).
Jails in space would only exist as part of larger self-contained colonies, because it's cheaper to keep prisoners on-site. Prisoners would not be shipped up from Earth.
Universities (and cities) in space
50 years if we're lucky, 200 if we're not. We'd need a way of cheaply moving a vast amount of structural material to the building side, and a very large amount of finished goods that are difficult to manufacture on-site. In practice, this means we need a mining colony on the moon manufacturing aluminum, steel, and glass fiber as structural materials before anything else of significant size is built in space. A space elevator would allow material to be brough up from earth (though still more expensively than from the moon), but there are very formidable materials challenges to be overcome before we c
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Firebombing doesn't poison everyone and everything in a huge radius.
Neither did nuclear attacks, as far as the people calling the shots knew, after the initial flash. Hard to claim malicious intent for long-term poisoning under those conditions.
...our DNA is equipped with raid-1 capabilities ? Maybe it requires some added Reed-Solomon codes, everyone knows raid-1 doesn't protect against data-corruption...
It does have error correction codes, in that only a subset of possible sequences validly code for things. If one strand contains valid sequences and the other doesn't, you know which to use when rebuilding.
A molecular biologist can give you a more detailed explanation of how this works than I can.
Since when does one call 40% efficiency efficient?
Since efficiencies of less than 1% are typical for most lasers that aren't diodes or CO2, that's when.
And since the _energy_ efficiency of chemical rockets is multiplied by the mass fraction (typically 5% or less), that's when.
If you can get a 40% efficient laser dumping most of its energy into either a carried-hydrogen stream with an Isp of 600-900, you get a system that is _vastly_ more efficient than chemical rockets.
It remains to be seen if this ends up being a system that's cheap enough in practice, though. Chemical rocket cost certainly isn't dominated by fuel (otherwise we'd be launching at $100/kg right now).
You can vary the wavelength across the array, and that avoids most of the problems with speckle patterns.
They already do this within each launcher node. This is described in detail in the description of how bar lasers would be combined into arrays with sufficiently high luminance to drive the craft.
I personally doubt the combined light from all of the lasers would be coherent enough to get much speckle, and even if it was, the craft is moving fast enough that it would see a constantly-changing pattern that would average out to something reasonable, but I certainly haven't run the numbers to prove this.
correction: liquid oxygen weight is responsible for most of the shuttle weight (8 tons of oxygen are needed to burn 1 ton of hydrogen) but the volume of liquid oxygen is actualy rather small, much smaller than the volume of liquid hydrogen There are 2 tanks within the external tank - a small one for LOX sits on top, the rest underneath of it is filled with LH.
The reason for this "paradox" is extremely low density of liquid hydrogen.
And this is why liquid hydrogen is bad as a fuel for a craft that has significant flight stresses - you need a huge tank, which weighs a lot and is large enough that the square/cube law starts to bite you for structural strength (increasing the weight even more). It turns out that the drawbacks of increased craft size per unit cargo weight outweigh the benefits of using alternate fuels. This is why most new craft proposals use hydrocarbon-based fuels. There have been several papers on this topic (some linked from Slashdot).
The reason why the proposed laser craft can get away with using hydrogen is that they claim an Isp of 600-900, vs. an Isp of around 400 for H2+O2, and around 300 for CH4+O2. This drastically reduces the propellant:cargo mass ratio, to the point where the hydrogen tanks aren't cripplingly large. Whether this Isp can be achieved in practice remains to be seen (though a fair bit of work has been done on the type of engine they cite, and their numbers for lasers are fairly accurate, suggesting similarly thorough engine research).
The only place it makes sense to use hydrogen in a chemical rocket is for a low-thrust drive in space, and chemical rockets are out-competed there by electric drives.
If projects like OpenCores catch on and FPGAs become cheaper then maybe open source can perform as well in that region as it does in software. Then I think people would not be happy with ARM taking down compatible products, just as people would not be happy if Microsoft went after WINE.
The big problem that opencores.org and other open hardware projects will run into, if they aren't already, is patents. For hardware, there is distressingly often only a handful of good ways of doing something, which will have been patented long ago. I'd love to design a 2D/3D graphics chip core (and I have the expertise to pull it off - IAACompEng with graphics experience), but if I tried I'd have everyone from Matrox on down knocking on my door with a big stick in hand.
Perhaps if the patent revolution finally comes for software, hardware patent lifetimes will be shortened as a side effect.
There's also ARM's lurking in games-consoles (GBA, Dreamcast), routers, PDA's, portable music players, mobile phones, infact just about every type of small device.
The Dreamcast uses a Super H 4 as its primary processor, as it needs the SH4's ability to manipulate floating-point vectors natively at reasonable speed.
There may be an ARM core tucked in there for other purposes (sound?), but SH4 is the heart of the machine.
Sounds like an attempt to overcome the runaway expenditures of Teledesic's failed LEO project. The problem with these high-altitude sender/receivers is that--while they offer a technology solution--there is a corresponding weakness in application.
For example, latency in these systems make it unattractive for many internet applications (who wants to play FPS's over a spread-slotted Aloha CDMA system?).
As long as you have a relatively nearby ground station to relay to, latency isn't a horrible problem. Right underneath one of these things, round-trip latency is about 0.13 milliseconds. At the edge of a blimp's broadcast range (around 100 km if I'm reading things correctly), it's 1.3 milliseconds round-trip.
Think of these as a much cheaper way of building a very tall relay tower, for something closer to reality than the "satellite" analogy.
We can't change its name to be a 'space bridge'. If we did, we couldn't have the same hilarious jokes in every Slashdot article about elevator music.
Don't worry; we'll have plenty of Transformers jokes to replace them with.
("No, Prime, _you_ will activate it for me - for the secret cargo is: Cybertron!")
Visible 'red' light is around .65 to maybe .75 micrometers. So are they are saying 2.1um or so?
I do wish these articles would just say what they mean and not try to make it seem more 'amazing' with fuzzy statements like that. It's like "WOW! THREE TIMES REDDER!" - when in fact, near IR is nothing special - most cheap camcorders can take pretty good pictures in that frequency range.
Silicon photodetectors, like the silicon CCD chips in camcorders, have a cutoff at about 1.1 micron. They won't see 2.1 micron infrared.
Furthermore, John Q. Public reading that press release will have no idea what a "micron" is, but probably _will_ get the general idea from a phrase like "three times redder". If you want an accurate description of what's going on, why on earth are you reading a press blurb?
As for interstellar exploration, we need a financial incentive, much like the X-Prize. Only, in this case, first company sponsoring a colonization mission to an Earth-like planet, claims it.
The problem is that unless we discover new physics, sending even a flyby probe across interstellar distances within anything approaching a reasonable timeframe is an absurdly expensive project. It's within the capacity of Earth's richest nations to do it if they bent all of their resources to the task, but that's about it. This means that any financial incentive an X-prize like contest could offer would be insignificant compared to the project's cost. Similarly, not even Bill Gates could afford the ticket price for interstellar tourism, so using the prize to leverage advertising for another market doesn't work either.
As manufacturing and especially automated manufacturing technology improve, the cost of an interstellar craft may drop to the point where it's feasible for only a large organization to produce a probe, or even send colonists. However, for the time being, an Alpha Centauri shot is at about the same stage as a Moon shot was 50 years ago - something that could be done as a pissing contest (or military turf battle) between the richest governments, but by nobody else.
I hope to see a probe launched in my lifetime, but I don't expect it to be any time soon, or by profit-driven private organizations.
Sorry about the beef ban, but if you would stop making cannibals out of your cattle and spreading mad cow, perhaps it could be lifted.
You do realize that the US does exactly the same thing, and that there was a reasonable chance the Canadian cow came from a US herd, right?
Probably not, in the whole scheme of things, very little gets spent finding new, better energy sources.
Perhaps that is due to the controlling interests not wanting to give up that control.
Or just because we don't need them at present, and there's greater return on investment in other branches of development.
Think about it. If we knew we _had_ to switch to, say, hydrogen power storage and nuclear power generation within 50 years, wouldn't the big oil companies invest scads of money to make sure _they'd_ be the big hydrogen/nuclear companies?
It won't be for this launch, or even the next one, but hopefully Canada will soon have companies making launch vehicles:
I doubt this. We're in a horrible launch location (you want to be close to the equator), and the market is already pretty saturated (Arienne-5 and other solutions on the expensive side and Russian boosters on the less expensive side).
I'll be the first to cheer if we do get Canadian launch facilities (way too much red tape getting things launched by the US), but I'm not holding my breath.
No, you can scan your spam folder in seconds, because you will recognise the subject lines. The duration is not comparable.
I do this. It takes a large amount of time. Comparable to the time required to hit "delete" on seeing a flash of spam-text.
Time savings: zero.
The only reason I spam-filter at all is that the spam filter serves adequately as a _priority_ filter, letting me get to _most_ of the real stuff first. Doesn't save me from slogging through the rest.
I swear I can go weeks on end without a single spam getting through, no false positives -- and between 20 and 100 SPAM in my "spam" box per day!
This is what I don't get - in order to be sure you have no false positives, you have to comb through all of the spam by hand, which for the most part defeats the purpose of a spam filter. If you don't do so, then you can't claim zero false positives - you can only claim that you haven't _noticed_ any false positives.
I have a whitelist at work, and it works quite well, but combing through and emptying the spam bucket is still an annoying part of each day.
However, without doing so, I'll never know if I missed that one message in (about) a thousand that's from a vendor that's not in my whitelist.
QOTD: "I don't have a solution, but I do admire the problem.".
What does NERVA stand for, by the way?
:). There are likely at least half a dozen project names that the US military has cycled through in its research programs on nuclear propulsion, too. I think they did actually get a nuclear jet flying (passing air through the core instead of stored reaction mass).
I'm afraid I don't know offhand, but it seems you've already found that information
Why is hydrogen the best, here?
It has a very low atomic weight. "Temperature" is the amount of kinetic energy per particle (molecule, usually). So for a constant exhaust temperature, using hydrogen gives you the greatest energy per unit mass, and so the greatest exhaust velocity. I'd have to grind through formulae to see how this meshes with gas law, though, as this seems inconsistent with what I learned about gas law in first-year. So, I could be misunderstanding the reason hydrogen is used.
What's the deal with engine components having to be at the exhaust temperature? Wouldn't this almost always be true, since the fuel is going through and combusting inside the engine, or is there more to this?
It turns out that this doesn't have to be true in a chemical rocket. In a nuclear rocket, the exhaust is heated by the reactor, so the reactor has to be at exhaust temperature. In a chemical rocket, the exhaust is self-heating, so you can spray reactants into the middle of the combustion chamber and have a blanket of cooler gas at the chamber walls to protect them. Building the combustion chamber and nozzle out of ablative materials gives you this in a straightforward manner, though your nozzle lifetime is finite with this scheme. By this method you can get exhaust temperatures far in excess of what just about any reasonable structural material will stand (even carbon composites).
There has been talk of building nuclear drives that use a cloud of uranium gas to provide energy, allowing similar schemes, but my understanding was that this is only actually feasible in space (and is low thrust, as well).
I see. This would be the main reason that an upper atmospheric launch would be of some benefit, is that right?
It's one of the reasons. The other reason is that it allows smaller spacecraft to be used. In order to be able to mostly ignore atmospheric losses, your craft's mass must be substantially greater than that of the atmosphere it plows through on the way up. That's about 10 tonnes per square metre of cross-sectional area - but at 10km up, it's more like 3 tonnes, so you can use a craft a third as long (and presumably narrower, making the smallest practical craft size 10x-30x smaller by mass).
A smaller craft means easier to handle structural stresses (thanks to the square/cube law), which means less mass spent on structure and more on cargo, or cheaper materials used for construction, or both.
So, launching small craft from the upper atmosphere is very attractive. The Pegasus launch system already does this.
So, if you had a big enough aircraft that could lift enough to haul all of the fuel needed to add on the last Mach 20-25 of delta-v and the spacecraft into the upper atmosphere, you could launch from there with a nearly-horizontal trajectory and save the lost acceleration during the lift phase of a ground launch. I would imagine that a problem with this is that there are no such aircraft available at this time.
There are (e.g. Pegasus). The craft are just very small. Space Ship One also used a scheme like this, though they weren't orbit-capable (just sub-orbital boost to 100 km altitude).
If Scaled Composites goes into orbit-capable craft desgn, they'd quite possibly use a scheme like this. I recall hearing that they were mainly positioning themselves as a supplier for space tourism businesses wanting sub-orbital craft, though. I don't have hard confirmation either way.
So, are there any good hydrocarbon fuels that have comparable Isp to hydrogen?
I'm afraid not. Typical best Isp is on the order of 250 or so for hydrocarbon/oxygen rockets.
What makes a good hydrocarbon rocket fuel?
Hydrogen-rich, easy to handle at room temperature, and ideally not very nasty to humans. For best Isp, use liquid methane, though that's a bit pickier to handle.
What are the downsides of using hydrocarbon fuels?
Low Isp, mostly.
Why does liquid hydrogen seem to win in the end?
It doesn't, most of the time. At low altitudes, rocket nozzle concerns reduce the Isp you can actually squeeze out of fuels, so the shuttle only uses it for high-altitude burn. The size and mass of the LH2 tank makes it questionable whether or not this is a better solution than LOX/hydrocarbon upper stages. There are a few papers around that discuss this (don't have a link handy, sorry).
It might be a good solution in space, especially in the outer solar system where solar heating is less of a problem (so you have less overhead for the insulation and cooling systems).
The NASA Space Shuttle uses solid rocket boosters, and I had gathered that these use some kind of hydrocarbon fuel or other. What's in them and what's good/bad about them?
The shuttle SRBs use ammonium perchlorate and aluminum as the oxidizer and fuel, respectively. The composition breakdown also lists iron oxide (which probably means they use thermite to light the thing), and a rubberizing compound to bind it all in place.
At low altitudes, gas expansion concerns limit the Isp you can usefully get from a fuel without a variable-geometry nozzle, so using a lower-Isp fuel in the SRBs is still competitive with using the main (LOX/H2) engines for boost (actually, these are still turned on). A core-burning solid fuel rocket can also give you greater thrust, especially if the boosters are disposable (i.e. you don't mind materials degradation as long as they last one flight). So the SRBs are used to help with high-thrust boosting when the shuttle is low in the atmosphere and near its fully-fueled weight. Higher up, the boosters are dumped and the LOX/H2 engines (optimized for high altitude/vacuum) take over.
In other words, the delta-v is a fixed quantity for a given orbit, and the exhaust velocity of the fuel is also fixed (in practice 10 times the Isp), and that becomes the exponent on 1/e to yield the dry:wet ratio for the spaceship, right?
Yes, this is correct.
I take it that given some "wonder fuel" that provided an Isp of 800 (an exhaust velocity of 8000 km/sec), the launch weight of the fuel would only need to be 63% of the total mass of the craft, is that right?
This is also correct. This is part of why NERVA-style nuclear rockets are so attractive. Passing hydrogen through a fission core gives you pretty much the best exhaust velocity you can get with an engine where engine components have to be at the exhaust temperature.
In principle, a nuclear-electric drive could get better Isp, but in practice, power to weight ratios and heating problems limit you to far less than one gravity of thrust, which means they don't help with the ground-to-orbit problem.
This 1/e business sounds like it comes from a solution to some sort of definite integral with respect to time and mass of the rocket equation. Does that mean that this 1/e to the power of the delta-v over exhaust velocity only applies directly to single-stage rockets, and that the situation would be improved for multi-stage rockets?
Parts of the rocket that you throw away count as fuel weight, to the first order, so you can look at them as degrading the Isp of the fuel instead of adding to craft mass. It still makes life easier to use multi-stage rockets to reach orbit. In fact, this is the only way we've been able to reach orbit at all (Single Stage To Orbit projects keep running into materials problems; we'll get there eventually).
The effect of a higher exhaust velocity is not only a higher impulse, but also the fact that you're ejecting your fuel at a higher rate, thereby lightening your launch vehicle as it flies at a faster rate, which basically translates into a higher overall effeciency. Is that a fair statement?
I'm afraid not. It's just impulse delivered per unit fuel weight that ends up mattering.
So, hydrogen is a light molecule, but it burns pretty well. What about something like a liquid sodium and flourine or chrlorine engine?
The highest-Isp chemical fuel mixture is hydrogen as the fuel and fluorine as the oxidizer. This gives the most energy per unit fuel weight, which means that exhaust temperature is as high as you can get, which gives you the best exhaust velocity achievable with chemical fuels.
In practice, exhaust temperature and materials handling problems make H+F impractical as a fuel. H+O is almost as good, and liquid oxygen is much easier to handle than liquid fluorine. Hydrogen still takes enough fuel tank mass overhead that it's questionable whether it's worth it as a fuel, though (hydrocarbons are stored at much higher density, meaning a smaller tank and less craft weight).
It turns out that metals like sodium are heavy enough to offset their reactivity benefit. It's hard to beat hydrogen for combustion energy per unit weight. Ditto heavier halogens like chlorine (oxygen gives the same or better reactivity at lower atomic mass).
One final thought: I assume that there is a limit to how great of a "wonder fuel" you could use for launches of human cargoes, since the ideal seems to be getting a lot of acceleration right at the start right away, and that brings you back to the problem of launching humans with rail guns, i.e., the acceleration would be fatal, and so you're stuck to a certain extent hauling a large part of the fuel for a large part of the flight just to provide a reasonably gradual acceleration as opposed to a big jerk at the beginning.
This turns out not to be a problem. It's only the Isp of the fuel that determines the craft's mass fraction, so there's no requirement to boost at 100 gravities when boosting at 3-4 will do.
The actual problem is the vertical
The article was a little thin, so I mosied on down to Wikipedia.
You can also find an abstract and a PDF of the whole article on the physical review letters site (a few links in from the article Slashdot linked).
These are letters, so they haven't been through rigorous peer review, but the authors take great pains to cite related work and describe their experiment in excruciating detail, so their results are almost certainly perfectly valid.
Capsule summary: Adding about 1.5% ytterbium to steel alloys makes it *far* easier to get amorphous phases of them, which is normally a royal PITA for metals (you tend to get very fine grains instead). This has been shown before, but they map out a range of alloy compositions and show where alloys with good properties lie within that range, and do a large number of tests to a) prove that they really have produced amorphous steel and b) measure the materials properties of the steel they've produced.
Let me just make sure I'm clear on some terms and concepts. Isp means specific impulse or something?
Yes. The original definition was "pound-seconds of thrust per pound of fuel". The modern definition is "newton-seconds of thrust per kilogram of fuel", divided by 9.8 (Earth's surface gravity) to get back to the old units. Easy to handle chemical fuels have an Isp of around 200-250. Difficult to handle chemical fuels have Isps of around 300. Very nasty and/or extremely difficult to handle fuels have Isps around 350-400. NERVA-type nuclear drives, which pass reaction mass through a fission reactor core, have Isps of about 550 or so, but are not politically feasible (if one crashes, you get radioactive crud spread around, and your engine sprays out radioactive crud as the core gets etched away, and you can't afford to carry much shielding, so the reactor harms crew and electronics while in operation - geometry requirements for use as an engine prevent safer reactor designs from being used).
In practice, Isp times 10 is about equal to your exhaust velocity, and every multiple of your exhaust velocity adds a ratio of 1/e to your dry:wet weight ratio. So, a rocket with a fuel with an Isp of 250 that wanted to go at 8 km/sec would need a delta-v of 3.2x its exhaust velocity (8000 / 2500), for a mass fraction of about 4% (e^-3.2) - i.e., 96% fuel, 4% structure plus cargo plus crew. High Isp helps a _lot_. It just turns out that most really high Isp fuels require enough mass overhead for the ship or cause enough other problems to not be worthwhile.
Is "delta-v" the same as acceleration, or just that you have to get from sitting still on the earth to 8km/sec in a short period of time to get to orbit?
Delta-v is the change in velocity needed to perform any specific maneuver in space. It doesn't matter whether this is accomplished all at once or over a long period of time. To get from being at rest on the surface of the earth to being in orbit, you need to change your velocity by 8 km/sec. In practice, we have to do this very quickly for a surface-to-orbit launch, because a sub-orbital trajectory will bring us crashing back to earth fairly quickly, and because atmospheric drag slows us down if we hang around in the atmosphere, and because if we're thrusting upwards, earth's gravity is pulling us downwards, and we end up losing velocity if we just coast (most of the thrusting to orbit is sideways, but the initial burn to get above most of the atmosphere is upwards).
Is 8km/sec escape velocity, by the way?
8 km/sec is the delta-v required to get to low Earth orbit. An object in a circular orbit near the surface of the earth moves at 8 km/sec relative to the surface. Escape velocity is the delta-v needed to get out of Earth's gravity well altogether. This is about 11 km/sec. An object on an escape trajectory fires straight up from the surface of the earth at 11 km/sec. As it climbs out of the gravity well, it slows down. For an object launched at precisely escape velocity, it'll keep drifting away, but by the time it's far from earth it's moving very slowly (almost at rest). A probe intended to tour the solar system would have to be launched at more than 11 km/sec.
In practice, an efficient space launch would use the smallest possible delta-v (8 km/sec) to get off of Earth's surface, as this is the delta-v that must be provided with inefficient chemical rockets. To climb further out of the gravity well, and move around the solar system, a craft would use a more efficient type of drive (like an ion drive, plasma drive, or other electric drive that produces a higher Isp). So far, only the Deep Space 1 probe has done this.
Once you're out of Earth's gravity well (delta-v: 11 km/sec from launch), you can use "gravitational slingshot" trajectories to steal momentum from other planets or from the moon, as an alternative to carrying vast amounts of fuel or a high-Isp electric drive, but this takes quite a while (typically years between slingshot passes,
A couple of other notes on elevators:
You're only using a 10,000 T elevator because if it's ever cut, it impacts with a yield of about 7 times its weight in TNT. While you can design it so that naturally occurring failure modes won't cause the whole thing to come down (or even much at all of it), sabotage is a real enough concern that you don't want to drop a _million_ tonnes of elevator on the planet. Incompetence is also a potential failure source (there's a raised highway around here that they only started repairing _after_ chunks of concrete fell down on to cars running beneath it - it's always politically better to delay repairs just another year).
There are other styles of ground-to-orbit device besides elevators. Skyhooks are one of the more popular. The problem is that the stresses on a rotating skyhook that has a tip moving at ground speed at ground level are comparable to those on a space elevator, so the materials problems end up being the same. For travelling between orbits in space, they're easier to build, but you could just as easily use high-Isp electric drives and avoid the investment in the first place. So, I'm not sure skyhooks are a good idea to build (an elevator has less administrative overhead). Might be possible to have higher throughput, though.
Why rockets? Are space elevators the only alternative?
There are other alternatives, but they are either very hard to build or very expensive to build, to the point where it's questionable if they would make economic sense compared to chemical rockets. Chemical rockets, if commoditized to the point where fuel was the dominant cost, are "cheap enough" for most purposes (the only thing they're not cheap enough for lifting is huge amounts of bulk structural material for city-sized space stations).
Anything that carries its energy source with it _and_ has enough thrust to get from ground to orbit will have the problems of chemical rockets (be it chemically powered or NERVA-style fission powered, your exhaust has to be cool enough to not destroy your engine too quickly, which limits Isp). Nuclear-electric and even solar-electric work fine once you're already in orbit, because you can afford to build up delta-v over months, but ground to orbit requires delivering at least 8 km/sec delta-v in a few minutes at most.
Schemes that supply energy from elsewhere are things like laser launchers and mass drivers. Mass drivers _might_ be practical for bulk cargo that can take thousands of Gs acceleration. Delicate cargo like humans that's limited to a few Gs requires a mass driver a thousand kilometres long, which is not economical to build (and has the problem of firing tangentially, with a very long path through the atmosphere between the cannon muzzle and space). Building a cargo-capable mass driver is still a very difficult proposition (thousands of Gs is a tall order for field coils to produce on a multi-tonne cargo capsule).
Laser launchers have to plow through lots of atmosphere, and only give you an advantage when operated in "jet" mode (taking reaction mass from the atmosphere). Mass carried with the craft gives you the same problem as chemical rockets (exhaust temperature limits Isp). Boosting in jet mode means flight must be at a fairly shallow angle to have a long enough launch path, which probably means multiple laser stations (range through the atmosphere isn't that great). This is very pricey to build, to the point where you have a hard time amortizing construction costs to a competitive level (mass drivers are a bit better here because they can have higher throughput).
A space elevator is nice because it lifts material with pretty much the bare minimum possible energy investment. The problem is that it not only requires near-magical composites, it requires _lots_ of them. That means not only waiting for reliable nanotube composite production, but waiting for it to be cheap enough to let you build and maintain a structure massing tens of thousands of tonnes (and you'll _need_ to maintain it, in an environment with micrometeors and enough radiation to degrade highly ordered materials). Space elevators also have throughput problems - they can't carry more than around 10%-30% their own mass in cargo at any given time (depending on how much extra strength you spec for it when you build it - and extra strength costs a _lot_ for this type of structure). You have to haul this mass at least 40,000 km (amd more like 100,000 if you're going to the counterweight tip for maximum launch velocity). That takes a while. So for a 10,000 T elevator you can only haul 1,000 T at a time, and have to schlepp it at _minimum_ a distance equivalent to circumnavigating the world at the equator. This will take a while. So throughput ends up being lower than something like a mass driver, which is very nearly as energy efficient.
So, I think chemical rockets are what we'll be using for quite a while. Commoditized chemical rockets are actually pretty nice.
It seems to me that there is not much attention being given to the idea of large, flying disk/wing aircraft that could fly up to high altitudes and then switch to rocket propulsion as a means for lifting large payloads on a reliable, safe, and regular schedule. The commercial aerospace industry is pretty close to the "fuel as the pr
First, the disclaimer. Most of what follows is educated guesswork. However, the "educated" part means that most of the guesses are made for a reason, and should be at least partly correct. Enjoy :).
How long do you think it will be until we start to see:
Inflatable space stations that are rotating so as to provide artificial gravity
If you have a large station rotating for pseudogravity, most of the stresses are tensile stresses caused by the rotation. Whether the hull is rigid or not doesn't make a huge structural difference at that point.
A small rotating station would just be one or more bods and/or counterweights on the ends of cables. Making the habitat modules inflatable would save a bit of mass for the structure, so there's a reasonable chance it would be done if inflatable technology is sufficiently trusted.
Space stations that are environmentally self-contained, i.e., with plants and animals and goodly amounts of open water and topsoil-like material on board, using the sun's energy to maintain a self-supporting ecosystem
It turns out that the cost of going from a 95% efficient recycling scheme to a 100% efficient recycling scheme is high enough that it isn't likely to be done any time soon (not until we have space habitats with populations of thousands, and maybe not even then).
Air recycling would be done electrochemically, not boilogically (burn CO2 in hydrogen to get H2O and methane, electrolyze H2O for oxygen, strip hydrogen from the methane for use as H2 feedstock and sequester the carbon as waste). Water recycling can done adequately by dehydration of wet waste and fractional distillation of wastewater. As long as you have abundant energy, recycling air and water at high efficiency is very practical.
You'd still need to ship up dry foodstuffs and take down waste, as well as top up the water supply, but the volume is low enough that this isn't a big load.
I don't think biological food production and waste reclaimation will be practical in small stations for the forseeable future. It'll take large colonies, as mentioned above, and you have to be willing to fork over a _huge_ mass allowance for the biosphere you're building (an equivalent mass of air, water, and food would last years before running out, so this is a _big_ investment in mass for only a long-term payoff).
Microcosms of endangered plant and animal ecosystems on earth transplanted or reproduced into large biospheres in space
Unlikely to ever happen, as it's cheaper to build contained environments for them on Earth. Space environments of this type would arise as a side effect when self-contained space station biospheres were built for other purposes, but building space stations just to preserve the biospheres is a horribly inefficient way of achieving the goal.
Biosphere "prisons in space".
Not happening. Jails on earth are far cheaper. If you want to get rid of someone so that they're no longer a drain on your system, there are plenty of places on Earth you can dump them, or you can just kill them outright (not a debate I'm getting into in this thread, thank you).
Jails in space would only exist as part of larger self-contained colonies, because it's cheaper to keep prisoners on-site. Prisoners would not be shipped up from Earth.
Universities (and cities) in space
50 years if we're lucky, 200 if we're not. We'd need a way of cheaply moving a vast amount of structural material to the building side, and a very large amount of finished goods that are difficult to manufacture on-site. In practice, this means we need a mining colony on the moon manufacturing aluminum, steel, and glass fiber as structural materials before anything else of significant size is built in space. A space elevator would allow material to be brough up from earth (though still more expensively than from the moon), but there are very formidable materials challenges to be overcome before we c