X-15 flew 199 times, Spaceship One flew once. You have to divide the cost by the flight count.
Three times already, actually. And what kind of bizarre logic is that? As that article pointed out, just the research for the engine alone cost more than three times SS1's current complete development cost. If the X-15 had flown once, it wouldn't have cost just $1 million.
And even admitting that logic, you'd still have to back down after the next three flights, at which point the two vehicles would be at the same cost per flight in real dollars, and adjusting for 40 years of inflation is a lot of adjustment.
Yes, Spaceship One is not a Spaceship, it's a Spaceplane, true. NASA confirms this in their article on their own space plane, which bested spaceship one's mark forty years ago for roughly the same amount of money (adjusted for inflation) but without all the near-death control problems.
Uh... which X-15 program are you talking about? The one I'm thinking of - described here is described as costing $300 million - and adjusted for inflation that would be orders of magnitude larger than the $20 million that SS1 did it for!
This is most clearly evident with regard to the question of the program's original cost estimates and time frame. It is seldom acknowledged in the historical literature, but the X-15 program was a victim of what has become a fairly common occurrence in the U.S. space program, namely substantial delays and overruns. Three hundred million dollars does seem small in comparison to the cost of, say, Apollo or the shuttle, but it is still more than seven times the original estimate of $42 million. The final development costs of the engine alone were more than $68 million (plus a $6 million fee to Reaction Motors), a tenfold increase over what was expected when the project began. In addition, the complete vehicle, including the large engine, was ready for flight more than two years behind schedule. Despite all of this, development during the 1955-1957 period was never held up by a lack of funds, although in some years needed funding did not come through until the last minute.
Or am I missing something? No one has ever made a craft which has flown so high for so little, as far as I know.
((mass of Earth minus mass of Venus) plus mass of Mars) divided by mass of the moon = 23.7379076
(In a collision between Venus and Mars sufficient to impart enough angular momentum to Venus to make Earth's day, it's quite likely that only a Moon-sized object would form.)
It's a bit curious that the two nearest neighbors of Earth-Moon system could likely create another Earth-Moon system almost exactly. Bizarre.
..if the solar storms blew away water from mars upper atmosphere during a long period of time, which led to the drying of mars oceans, the same (but maybe in a lesser extent) should have happend to earth.
Mars has no magnetic field. Its core cooled off tremendously faster than Earth's, and so the magnetic field froze out.
Earth, being more massive, also has a much smaller scale height, so that liquid water vapor extends to much smaller heights. Combine those two and the water vapor loss rate for Earth is tremendously less.
Until the output of the Sun increases in a few hundred million years, the water cycle is stable. (Yes, a couple hundred million years - Earth will be out of the habitable zone *before* the Sun goes red giant).
(I think it is quite sad that we are surrounded by all these planets that once was easily terraformable but now they are all "dead"...and we are next):(
Well, yes - life requires energy, and the Sun is burning through it at an utterly incredible rate. Eventually it has to run out.
Of course, that's several hundred million years from now, and if we're still stuck on this planet by then, we deserve to go extinct.
If we really want a couple more, we could always move Venus out to Mars's orbit, and have Mars smash into it. Poof! Instant new Earth.
Fine, but would you volunteer to climb up and tie the ends together?
Spool it out from orbit (where it was spooled out from to begin with), not from the ground. You obviously want to have contingency cable anyway - probably need at least 25% extra cable length just to be safe. That's 25,000 miles, or a few thousand terrorist attacks.
You need to accelerate a 20-ton load from 463 m/s to 488 m/s in the total time it takes to climb the elevator. The reference design quotes that at one week.
First year-physics and the impulse equation says that to move a 18,143 kilogram mass from 463 to 488 m/s in 604800 seconds takes 0.74 newtons.
That's the force on the attachment point. Newton's third law.
there's not a lot of leeway in the design.
Yes there is - it's in the taper ratio, which the reference design gives as 4-6 (hence the 100% safety margin with a 100 GPa cable, when the taper ratio required calculated for a 150 GPa cable is 1.6). If you want more safety, you increase the taper ratio. But anyway, I think 0.74 newtons is within the safety budget.
How are they going to design it so that a bomb can't destroy the precious tether?
Make it 100,000 miles long.
Until the terrorists get ICBMs, any bomb they set off only affects the bottom 0.001% or so. They then spool out a few more km if something severs the bottom, and no one notices.
Yes it would be a very bad thing (tm) if someone crashed an airliner into a space elevator
Airplanes fly at 30K feet. That's 6 miles or so.
The space elevator would be 100,000 miles long, or so (of order).
A plane flying into the space elevator would do nothing except make the operators hold down a button for a few hours and move the anchor station to the elevator's new position.
Terrorists can't damage the space elevator without ICBMs or by putting a bomb on a payload. I don't know why people are so concerned about this. The elevator is primarily an orbital object. It has far more to worry about space debris than any Earth-based concerns.
Why did the scifi writers think that an asteroid would be used as the counterweight? There's no reason for that. Carbon is cheap - moving an asteroid is not. Just make the cable twice as long, and use the rocket remains as the counterweight. Easy enough. Can we propagate this idea? Here it is again: no asteroid counterweight!
It's quite reasonable to take terrorism into consideration when designing a structure.
Thankfully, the design of the structure takes it into account for you. How nice!
After all, consider this:
The structure is 100,000 miles long. What's the highest that someone could conceivably hit? Assume they have aircraft - so that's what, 30K feet, or 6 miles? So they've just severed the bottom 0.006% of the cable? This would do nothing to the cable itself. They would spool down 6 more miles, sigh, and no one would ever even know it happened. A terrorist organization might try it once, and then they'd realize they're wasting their money.
The only real threat comes when terrorists have orbiting satellites or ICBMs. I think we'll have other things to worry about then...
The other thing to consider is what precisely is the cost of losing the elevator. One might think that it's $10-15 billion, but it's not, unless you're stupid. The first thing you would do is send up another elevator, and leave it in orbit at GEO.
Then the cost of losing one elevator is high - probably a few million - but not that high. After all, once the first elevator has proven itself, putting up other elevators should be the highest priority. Eventually a meteorite swarm will destroy the elevator, and you don't want to be cast back to the dark ages of large exploding cylinders.
Are the upstream packages too broken to be good enough for debian?
Usually. Take a look at all the bugfixes that Debian applies to the packages (just look at bugreports from each package).
Has debian deviated so far from mainstream that the packages require extensive customization?
Not really. Upstream authors just don't really pay tons of attention to detail, and so while a release might work fine on their heavily customized system, it doesn't play well nice with the standards that Linux Standards Base and Debian Policy have defined.
Why can't the fixes be committed directly to upstream?
Sometimes they are - Debian package maintainers do a ton of backporting. Take, for instance, the Intel driver in XFree86 - for the longest time, it was improperly supported in X, and so the newest 855 (I think) couldn't run. You could fix it by going to a CVS update (4.3.99) but there were no releases of XFree86 that actually had this fix.
This is stupid, so Debian fixed it - the 855 driver change got backported to the version that Debian has, and Debian's version works.
A lot of people don't realize this, and so they think that "oh, I can't use Debian because certain packages are very old and contain many bugs" - that's not true. Lots of bugs are backported to packages. It's just that upstream authors many times change much, much more than just a simple bugfix, and to introduce all of those changes at once would, and does, break systems.
The other simple reason is that Debian supports more architectures, by far, than any other distribution, and it takes a large amount of time to verify all those architectures.
(Many people would say "who cares, I only use x86", and that's nice - but we need to have a distribution like Debian!)
It's also important to remember that Debian acts as a very solid "base" for operating systems. You can build on it very, very well, and many companies do! Knoppix is quite amazing (and is Debian, repackaged). Lindows/Linspire, Xandros, Libranet, etc. are all Debian-based operating systems, and the number really keeps growing. Those distributions don't stay out of date because they don't have the same concerns Debian does, and so if you're really a version-number whore, go with them.
The only measure of efficiency that matters much is peak Watts per dollar cost. ... in a really super-narrow view of the world, sure.
There are other things that matter besides cost... like size and weight. If a space-based satellite needs 1 KW of power, and you're comparing solutions that are 10% efficient and dirt cheap, and 100% efficient and 100X more expensive, you'd probably choose the 100% efficient version, as it's 10X smaller and 10X less weight.
Of course, you might say this does turn into cost for space-based applications (because weight turns into cost), but no one would want to send up something that's massively larger than it needs to be - the potential for damage is too high.
That being said, the article's misleading - commercially available solar cells can be found above 20% easily. It's just the really cheap ones that are 10-12% that these would be challenging.
The term copy-right means that they have a right to control who can get a copy of whatever has a copy-right attached to it.
No, it doesn't!
Copyright means that they have the right to control who makes copies, not whoever can get a copy! Otherwise I couldn't sell the copy that I have, nor could I even throw it away!
Title 17, thanks to the Copyright Act of 1976, means that the only thing they can control about the copy that they sell is the sale, and that's it. After first sale, they can't control anything about that copy. (They can of course prevent the sale of any copies of that copy, but you are absolutely allowed to make at least one archival copy of the copy that you purchase.)
e.g. it would be way cool if they distributed their entertainment products in digital files over the Internet that automagically provoked the recipient to pay a few cents to their coffers. That would exploit easy copyability, while hewing to copy-rights.
I am always allowed to make one archival copy of any copyrighted object that I purchase, regardless of what Nintendo tries to tell me, and they cannot charge me for making a copy. They have no standing to. Not being a lawyer, I'm not sure if I can make an archival copy of that copy if the first is destroyed, but I'd imagine so.
Are you talking about PCI Express cards (which is not PCI-X)? Do they make PCI-X graphics cards?
PCI-X is 4.3 GB/s (maximum!), and AGP 8X is 2.1 GB/s. Four graphics cards talking on a PCI-X bus would probably saturate it, especially given that it's shared-bus, so the number of bus arbitrations would be huge.
PCI-Express is point-to-point, so provided you could find a motherboard with 4 x16 links (good luck!) or at least had 4 x16 slots (again, good luck) you could do it.
But a 4 processor motherboard with 4 x16 slots doesn't exist right now (it'd be a *huge* motherboard!), so no, you can't do it.
I have looked at this... the problem is the way Linux handles the keyboard... To Linux, all keyboards are the same. If there was some way to either tell X to ignore all KBs except some, or have the kernel map them (say/dev/input/kb0/dev/input/kb1) similar to how mice are done, this would not be a problem at all.
That's not true! (And neither is the person who responded to you...)
The first exoplanets were discovered by Alexander Wolszczan in 1991, around PSR B1257+12.
They're pulsar planets, yes, but they're planets. Give the guy credit.:)
Story here. Curious that the first discovered planets were Earth-sized. Also the planetary system is very much like Earth's. Dead, yes, but still encouraging.
Intel CPU's do have this technology as well, although it's half the width (64-bits at a time, rather than 128-bit).
The MMX registers are 64 bit, although they're not the main limitation of the MMX implementation. For some inane reason, Intel decided to use the floating point registers for integer data (namely, MMX registers) and so MMX doesn't require additional registers to be added. However, since you're absconding with the floating-point hardware, you can't do floating point math at the same time, and you have to save the state of the floating point hardware before you switch to MMX. In other words, MMX was... "interesting", but in the end, not that useful. After all, for one thing, it eliminated your floating point capability unless you wanted to context-switch out. (AMD's 'improvement' to that was 3DNow! which was basically "MMX that you can use for floating point as well!" - okay, better, but... it still kinda sucked).
AltiVec didn't have those limitations - it was very, very improved over MMX.
SSE, however, *did* add 8 new registers, and 128-bit wide objects, for floating point. So an x86 processor with SSE extensions does have 128-bit vector abilities, albeit in floating point. Vectorized integer math is a little rare (hence why MMX isn't that useful anyway) so AltiVec and SSE are actually pretty comparable. AltiVec does have 32 registers (which makes sense, of course, given PPC's 32 register scheme), whereas SSE only has 8 registers. I'm sure some comp. eng. person can come along and tell me why it's efficient to have vector hardware that's the same depth as your register hardware (as x86 has 8 registers and 8 SSE registers, and x86-64 has 16 SSE registers, and 16 normal registers)
(SSE2 basically said "OK, MMX really blew - now you can just use the SSE registers for integer as well.")
When Apple posts benchmarks showing their machines to be faster than x86 machines, the benchmarks almost always make heavy use of these SIMD instructions... and rightly so.
When Apple used to post benchmarks. A modern G5 can keep pace with top end Athlons and P4s anyday, without any specialized benchmarks. Be nice to Apple - the days of the "G3 is 50% faster than a Pentium II using Photoshop's 'G3K1ckZA$$' filter on a mostly-red image of a cow... on Tuesdays!" are over, thank God. And if you had vectorized code on the x86 (using SSE), the comparison wouldn't be that unequal, unless it was heavily biased towards the PPC's obvious strengths (high register count). Then again, it's not like the x86 has any real strengths anyway...
But anyway, my point was that the SIMD implementation on x86 isn't really very different than on the PPC, once you count SSE. SSE is register-starved compared to PPC, sure, but so is x86 in general. x86-64 removes that last limitation (mostly, 16 registers is still starved compared to 32, I guess) but I doubt there would be a big performance jump going from 16 128-bit registers to 32 128-bit registers. There's not a ton of code that could efficiently utilize that. There is *some*, sure, but not a lot.
Wow, you know, there's an obvious solution to the atmospheric drag part that I didn't even realize. Just goes to show you that a human's normal experience sucks for trying to figure out orbital dynamics.
Incidentally, the full system design is here, and it's moderately more fleshed out than we're ever going to do in a Slashdot post. But anyway...
The solution to the atmospheric drag part is easy - only the perigee is at low altitude. The apogee is at high altitude. This, of course, solves the atmospheric drag part elegantly - the vast majority of the travel is well outside of the atmosphere. Thus you experience a small amount of drag once per orbit (which lowers your apogee), but you have a long time to recover that via solar power. Quite elegant - it basically utilizes the fact that the magnetosphere is immensely larger than the atmosphere.
The solution they're proposing is for assisting LEO to GTO, but the design is scalable and could eventually do suborbital to LEO. Oddly enough, they also used 2 gee acceleration, and listed mach 10-12 as the needed suborbital velocities (on a different page). Apparently I did my math somewhat right.
1000km cable, assuming 3cm in diameter, using a 2 GPa tensile strength steel weighing 7700kg/m^3... at best, this cable would carry 14,000kg (with little margin of error)
How'd you get this? The carrying capacity of the cable is completely dependent upon the rotational acceleration. 2GPa tensile strength with ~30 cm^2 area implies 6 MN maximum load capacity. At 1000 km, it's got a volume of 2827 m^3, so it's got a mass of 21,767,900 kg. The tension on the cable is just the centripetal acceleration times the mass of the cable. It'd never hold itself together at 1 gee... best you could do is say, 0.2 gee. Your load capacity would be essentially infinite (8,232,100 kg). Did I do something wrong here?
You'd be much better off with carbon fiber cables (the real kind, not the space elevator kind). You could pretty much assume at least 5X steel (10 GPa), and the *huge* benefit of lower density (1800 kg/m^3). This is unlike a space elevator, where only the tensile strength matters, because the acceleration is independent of mass. The lower weight (4X) and heigher strength (5X) means that the cable can support an acceleration 20X higher, or up to 4 gees. If you would run it at 2 gees, that'd give you a 100% safety margin. With a radius of 500 km, an acceleration of 2 gee, the tangential velocity is 3129 m/s, or mach 9.2. Orbital velocity at 500 km is 7500 m/s or so, Earth's rotation is 500 m/s or so, so the suborbital craft needs to be going about 3871 m/s, or mach 11.4. That's a fair mite faster than SS1 was going, and it's way faster than anything on the horizon as well, but it's still less than full orbital.
The air resistance doesn't worry me - it's not that much of an acceleration, and the exponential falloff of the atmosphere means that the drag really only matters for the tiny portion of time that it's at the bottom. And if it's really too bad at 100 km, just have the bottom at 200 km, and the drag is suddenly thousands of times smaller. The height of a suborbital hop isn't difficult, it's the delta-V.
The other problems, like oscillations, plasma forming in the cable, etc., I'd worry about more. But those are generic problems that space tethers need to solve anyway, and I think with the advent of carbon fiber cables and the massive benefits that tethers can provide that not trying to solve them is really silly.
I.e., snapped. It doesn't matter whether it snapped due to stress or due to an unforseen collision - both of them would be equally big problems!:)
Yah, but it would've been like criticizing the Wright brothers for a bumpy landing, or saying that the dust buildup on the Mars rovers makes the design a "failure". They weren't trying to have the tether hold together for the entire trip. It wasn't designed to, and someone probably calculated that it wouldn't, so it was probably expected. After all, as I said, the sensor only was supposed to transmit for 10 hours. The tether lasted for days.
Unless you've got a motor on the thing,.00003% atmosphere at Mach 3 means that, after a day or two, you are a) no longer orbiting and b) no longer going at Mach 3.
Sigh. It's encountering weak drag at the most extreme range of its spin. The vast majority of the tether would be located higher, in orbit. In order to make the acceleration tolerable, it'd probably have to be about 500 km long, so it'd be orbiting at 350 km up, where there's far, far less drag. 1000 km would probably be more feasible, but obviously more of a logistical nightmare.
C'mon, the concept is not hard to understand. It's like a pinwheel that laps at water at the bottom of its spin. And it would have a motor on the thing - an electric generator, powered by solar. Run current through the tether and it pushes against the magnetic field of the Earth, and moves its orbit. Brief calculations (not done by me!) showed that it's only half a newton or so drag for the portion at the low end.
And, re: tethers, I'd say an experiment where the tether essentially exploded would be a really good description of a "disaster". But hey, what the heck do I know?
Apparently not as much as the review panel that said that while the experiment showed that the insulation of the cable was more vulnerable than believed, it also showed that there were no fundamental problems in the concept of using orbital tethers for power generation, which is exactly what the mission was supposed to do.
Oh yeah. And where are all the successful tests of rendezvous-ing with a Mach 3 tether in the upper atmosphere? Oh yeah. They don't exist. Mostly because it's a targeting problem a little harder than ballistic missile defense, because you can't fudge a targeting error with a big ba-da-boom.
The relative velocity between the two would be zero - the suborbital craft is moving at Mach 3, the tether is orbiting at Mach 25 (ish), and its extreme ends are moving at mach 22, in retrograde. Net result is a rather slow moving rendezvous. The length of the cable determines how much of a margin of error you have in timing (and also the acceleration of the payload). Mach 3 is probably too slow - 2 to 3 times the velocity would be much more feasible.
The density of air D at a given altitude is roughly d0*e^(-alt*1.43e-4); d0=1.29kg/m^3. That means that we're looking at 5.769e-10 at 150km, 4.895e-13 kg/m^3 at 200km, and 1.076e-17 kg/m^3 at 275km. Our orbital velocity v will equal sqrt(G*Mearth/(alt+Rearth)) = 7782m/s. Drag = 1/2 * C * D * A * v^2. Substituting, we get drag=0.0296C at 200km, negligable at 275km... but at 150km, we get 3.88C. So, I suppose if you had a very low value for C, and a big stock of ion drives or a good solid electromagnetic propulsion system, it might be possible to overcome for atmospheric drag... but it'd be no easy task. But on top of that, you have to lift a spacecraft.
No, you don't need to "lift the spacecraft" - your momentum will do that. The ribbon will simply slow down and enter a lower orbit, depending on its relative mass compared to the spacecraft, at which point it will need to raise itself to a higher orbit again, which can take as long as you need. A few solar panels and probably in a day or so it's back at normal orbit. They're just orbital momentum storage devices, that's all.
And C_d for a thin ribbon would be of order or less than 0.1. At the low point it's less than a newton of force.
Well, more than that. The atmospheric drag equation doesn't work at high relative velocities and low densities like we're trying to use. Considering the shape of the cable, I doubt that it'd be proportional to v^2 at all.
Oscillations - both natural (such as atmospheric or magnetospheric drag) and induced (such as when the spacecraft is picked up). Charge buildup. Micrometeorites. Severe cable strain (I think you'll need a cable with a much bigger diameter than 3 cm!).
All the effort you spent trying to debunk atmospheric drag - very strange. You could've just worked out the math as to how fast a 200 km cable would've needed to be rotating to be at rest with a mach 3 aircraft. Unfortunately it'd need to have a centripetal acceleration of about 50 gees - it'd shred the craft to pieces.
Obviously the cable would have to be much longer than 200 km - probably 1000 km at least, and you'd still need to have SS1 be going about 4 times faster. But it's still practical.
Etc. It's a huge task.
Absolutely. But I don't believe that it's anything that we couldn't do right now, with enough engineering. Engineering that I'm certainly not going to be able to work out in a Slashdot post.
Oh, and with SEDS 2, the tether snapped unexpectedly 3.7 days after deployment.
Man, that's mean! On SEDS 2 the tether didn't 'snap' - it was severed due to an impact. Considering it was merely meant to be a proof-of-concept on the deployment, and not actually survive the length-of-mission, that's fine. I mean, c'mon. The satellite it was hosting only transmitted for 10 hours!
Lets assume a 3cm thickness cable, that extends from 100km to 300km, centered at 200km. That's a cross-sectional area of 6,000 m^3. Pretty hard to overcome that... that's the sort of cross sectional area of a blimp cutting through the atmosphere's fringes;)
6000 m^2, not m^3. Except for the fact that the atmosphere decays exponentially with a scale height of 10 km. So if you wanted to take that into account, the drag would be something like the equivalent of 300 m^2 at 100 km (integral of 6000*e(^-x/10000) from 0-200,000, roughly).
Plus it's rotating, so its profile is only 6000 m^2 when it's flat against the atmosphere. On average, it's half that. So its profile is 3000 m^2.
Plus you're not taking into account the differential velocities. At 100 km, it'd only be travelling at mach 3, or 1 km/s, right? Orbital velocity at 100 km is about 8 km/s. That means that, roughly, it's going to experience 64 times less drag than an orbital object at 100 km. I'm not going to take the time to work out all the math, but sufficient to say, I doubt it would be an insurmountable amount of drag. Even 300 m^2 is not much more than the ISS, and the ISS only needs reboosting every 90 days or so.
It's also important to realize you're talking about fluid flow here, and a tiny, thin piece of ribbon is not going to act the same as a very large flat object - the flow is going to probably be completely laminar, with very little resistance at all. They don't teach drag along with friction because drag is very, very hard.
I can't think of a single successful space tether experiment thus far.
Hmm, is this the same experiment? Were there two? This was a Columbia-based mission.
Although the board found that the tether's insulation was more vulnerable to damage than the experiment's designers had believed, they also found that the problem "is not indicative of any fundamental problem in using electrodynamic tethers." In fact, while the tether was operating it produced currents three times higher than theoretical models had predicted prior to the flight, the board reported.
As for successful space tethers, SEDS 2, SEDS 1. We'll never know about ProSEDS, since it was cancelled.
Furthermore, if they're dipping down to 62 km, they'll be having to counter a bit of wind resistance - more, I'd imagine, than an ion drive could make up for.
100 km, not 62 km. 62 miles. (What, you thought they chose it in imperial?:) ). At 100 km, you've got 0.00003% of the atmosphere left. While you will have air resistance, it's so small that I think it can be neglected. When the tether is unloaded, it probably also would have a negligibly small cross-section. In addition, it's only travelling at mach 3 with respect to the atmosphere at the worst density! It has very little to worry about when it comes to atmospheric drag.
Also, read the link I provided. The thrust is provided by electromagnetism, pushing against the Earth's magnetic field, powered probably by solar panels, most likely situated at the center of mass.
It should also be pointed out that our current experiments with space tethers have been phenomenally successful, not disastrous. The one experiment generated so much electricity that the tether essentially exploded. In this case, however, you'd prefer the tether not generate electricity. Again, not hard to do by design.
Not true. It is not trivially easy to build up that extra velocity, because you have to lug all of the extra propellant through the atmosphere.
Well, kindof true. If you want to have a single-vehicle-to-orbit solution, yah, you have to lug all of the extra propellant. But there are solutions in design which could definitely utilize a suborbital reusable spacecraft.
In fact, you can make docking and pickup very easy, if you make the rotation opposite the direction of orbit at the right rotation to match velocities. Someone would have to do the math on the acceleration to figure out how big it would be to allow for safe human transport on the craft, but it's definitely doable for small satellites.
Anyway, I know I'm glossing over a lot of details (like how do you guarantee the "human module" that an SS1-like craft would dump off has a safe abort procedure) but the point is that there can be very interesting uses for a suborbital craft. You're out of the atmosphere. That's a huge benefit right there.
Slashdot Mirror
X-15 flew 199 times, Spaceship One flew once. You have to divide the cost by the flight count.
Three times already, actually. And what kind of bizarre logic is that? As that article pointed out, just the research for the engine alone cost more than three times SS1's current complete development cost. If the X-15 had flown once, it wouldn't have cost just $1 million.
And even admitting that logic, you'd still have to back down after the next three flights, at which point the two vehicles would be at the same cost per flight in real dollars, and adjusting for 40 years of inflation is a lot of adjustment.
Uh... which X-15 program are you talking about? The one I'm thinking of - described here is described as costing $300 million - and adjusted for inflation that would be orders of magnitude larger than the $20 million that SS1 did it for!
Or am I missing something? No one has ever made a craft which has flown so high for so little, as far as I know.
Hit it with another planet of course.
What planet could possibly be perfect for this?
((mass of Earth minus mass of Venus) plus mass of Mars) divided by mass of the moon = 23.7379076
(In a collision between Venus and Mars sufficient to impart enough angular momentum to Venus to make Earth's day, it's quite likely that only a Moon-sized object would form.)
It's a bit curious that the two nearest neighbors of Earth-Moon system could likely create another Earth-Moon system almost exactly. Bizarre.
Mars has no magnetic field. Its core cooled off tremendously faster than Earth's, and so the magnetic field froze out.
Earth, being more massive, also has a much smaller scale height, so that liquid water vapor extends to much smaller heights. Combine those two and the water vapor loss rate for Earth is tremendously less.
Until the output of the Sun increases in a few hundred million years, the water cycle is stable. (Yes, a couple hundred million years - Earth will be out of the habitable zone *before* the Sun goes red giant).
(I think it is quite sad that we are surrounded by all these planets that once was easily terraformable but now they are all "dead".
Well, yes - life requires energy, and the Sun is burning through it at an utterly incredible rate. Eventually it has to run out.
Of course, that's several hundred million years from now, and if we're still stuck on this planet by then, we deserve to go extinct.
If we really want a couple more, we could always move Venus out to Mars's orbit, and have Mars smash into it. Poof! Instant new Earth.
Fine, but would you volunteer to climb up and tie the ends together?
Spool it out from orbit (where it was spooled out from to begin with), not from the ground. You obviously want to have contingency cable anyway - probably need at least 25% extra cable length just to be safe. That's 25,000 miles, or a few thousand terrorist attacks.
Speed at the ground (equator): 1670 km/hr
Altitude of low earth orbit: 350 km above sealevel
Speed at geostationary orbit: 27,400 km/h
Assume ascending speed of 100 km/hr:
Travel time to top: 12600 seconds
Difference in horizontal speed: 25730 km/h
Needed horizontal accelaration: 2.04 kmph/sec (0.567 mps/sec)
Speed at ground: 463 m/s
Speed at GEO: 488 m/s (altitude of GEO is ~22,300 km - add the radius of the Earth, and calculate circumference)
Delta-V: 25 m/s
Assume ascending *time* of 1 week, not 4 hours! (baseline design)
Force needed on a 20-ton payload: 0.67 N.
Thus the anchor needs to tug on the cable with a westward force of 0.67 N. This acceleration could be provided by a human being tugging.
Payloads at Earth orbit are moving at 463 m/s.
Payloads at GEO are moving at 488 m/s.
You need to accelerate a 20-ton load from 463 m/s to 488 m/s in the total time it takes to climb the elevator. The reference design quotes that at one week.
First year-physics and the impulse equation says that to move a 18,143 kilogram mass from 463 to 488 m/s in 604800 seconds takes 0.74 newtons.
That's the force on the attachment point. Newton's third law.
there's not a lot of leeway in the design.
Yes there is - it's in the taper ratio, which the reference design gives as 4-6 (hence the 100% safety margin with a 100 GPa cable, when the taper ratio required calculated for a 150 GPa cable is 1.6). If you want more safety, you increase the taper ratio. But anyway, I think 0.74 newtons is within the safety budget.
How are they going to design it so that a bomb can't destroy the precious tether?
Make it 100,000 miles long.
Until the terrorists get ICBMs, any bomb they set off only affects the bottom 0.001% or so. They then spool out a few more km if something severs the bottom, and no one notices.
Yes it would be a very bad thing (tm) if someone crashed an airliner into a space elevator
Airplanes fly at 30K feet. That's 6 miles or so.
The space elevator would be 100,000 miles long, or so (of order).
A plane flying into the space elevator would do nothing except make the operators hold down a button for a few hours and move the anchor station to the elevator's new position.
Terrorists can't damage the space elevator without ICBMs or by putting a bomb on a payload. I don't know why people are so concerned about this. The elevator is primarily an orbital object. It has far more to worry about space debris than any Earth-based concerns.
somehow dislodge the asteroid from orbit
Why did the scifi writers think that an asteroid would be used as the counterweight? There's no reason for that. Carbon is cheap - moving an asteroid is not. Just make the cable twice as long, and use the rocket remains as the counterweight. Easy enough. Can we propagate this idea? Here it is again: no asteroid counterweight!
It's quite reasonable to take terrorism into consideration when designing a structure.
Thankfully, the design of the structure takes it into account for you. How nice!
After all, consider this:
The structure is 100,000 miles long. What's the highest that someone could conceivably hit? Assume they have aircraft - so that's what, 30K feet, or 6 miles? So they've just severed the bottom 0.006% of the cable? This would do nothing to the cable itself. They would spool down 6 more miles, sigh, and no one would ever even know it happened. A terrorist organization might try it once, and then they'd realize they're wasting their money.
The only real threat comes when terrorists have orbiting satellites or ICBMs. I think we'll have other things to worry about then...
The other thing to consider is what precisely is the cost of losing the elevator. One might think that it's $10-15 billion, but it's not, unless you're stupid. The first thing you would do is send up another elevator, and leave it in orbit at GEO.
Then the cost of losing one elevator is high - probably a few million - but not that high. After all, once the first elevator has proven itself, putting up other elevators should be the highest priority. Eventually a meteorite swarm will destroy the elevator, and you don't want to be cast back to the dark ages of large exploding cylinders.
Are the upstream packages too broken to be good enough for debian?
Usually. Take a look at all the bugfixes that Debian applies to the packages (just look at bugreports from each package).
Has debian deviated so far from mainstream that the packages require extensive customization?
Not really. Upstream authors just don't really pay tons of attention to detail, and so while a release might work fine on their heavily customized system, it doesn't play well nice with the standards that Linux Standards Base and Debian Policy have defined.
Why can't the fixes be committed directly to upstream?
Sometimes they are - Debian package maintainers do a ton of backporting. Take, for instance, the Intel driver in XFree86 - for the longest time, it was improperly supported in X, and so the newest 855 (I think) couldn't run. You could fix it by going to a CVS update (4.3.99) but there were no releases of XFree86 that actually had this fix.
This is stupid, so Debian fixed it - the 855 driver change got backported to the version that Debian has, and Debian's version works.
A lot of people don't realize this, and so they think that "oh, I can't use Debian because certain packages are very old and contain many bugs" - that's not true. Lots of bugs are backported to packages. It's just that upstream authors many times change much, much more than just a simple bugfix, and to introduce all of those changes at once would, and does, break systems.
The other simple reason is that Debian supports more architectures, by far, than any other distribution, and it takes a large amount of time to verify all those architectures.
(Many people would say "who cares, I only use x86", and that's nice - but we need to have a distribution like Debian!)
It's also important to remember that Debian acts as a very solid "base" for operating systems. You can build on it very, very well, and many companies do! Knoppix is quite amazing (and is Debian, repackaged). Lindows/Linspire, Xandros, Libranet, etc. are all Debian-based operating systems, and the number really keeps growing. Those distributions don't stay out of date because they don't have the same concerns Debian does, and so if you're really a version-number whore, go with them.
The only measure of efficiency that matters much is peak Watts per dollar cost. ... in a really super-narrow view of the world, sure.
There are other things that matter besides cost... like size and weight. If a space-based satellite needs 1 KW of power, and you're comparing solutions that are 10% efficient and dirt cheap, and 100% efficient and 100X more expensive, you'd probably choose the 100% efficient version, as it's 10X smaller and 10X less weight.
Of course, you might say this does turn into cost for space-based applications (because weight turns into cost), but no one would want to send up something that's massively larger than it needs to be - the potential for damage is too high.
That being said, the article's misleading - commercially available solar cells can be found above 20% easily. It's just the really cheap ones that are 10-12% that these would be challenging.
The term copy-right means that they have a right to control who can get a copy of whatever has a copy-right attached to it.
No, it doesn't!
Copyright means that they have the right to control who makes copies, not whoever can get a copy! Otherwise I couldn't sell the copy that I have, nor could I even throw it away!
Title 17, thanks to the Copyright Act of 1976, means that the only thing they can control about the copy that they sell is the sale, and that's it. After first sale, they can't control anything about that copy. (They can of course prevent the sale of any copies of that copy, but you are absolutely allowed to make at least one archival copy of the copy that you purchase.)
e.g. it would be way cool if they distributed their entertainment products in digital files over the Internet that automagically provoked the recipient to pay a few cents to their coffers. That would exploit easy copyability, while hewing to copy-rights.
I am always allowed to make one archival copy of any copyrighted object that I purchase, regardless of what Nintendo tries to tell me, and they cannot charge me for making a copy. They have no standing to. Not being a lawyer, I'm not sure if I can make an archival copy of that copy if the first is destroyed, but I'd imagine so.
Are you talking about PCI Express cards (which is not PCI-X)? Do they make PCI-X graphics cards?
PCI-X is 4.3 GB/s (maximum!), and AGP 8X is 2.1 GB/s. Four graphics cards talking on a PCI-X bus would probably saturate it, especially given that it's shared-bus, so the number of bus arbitrations would be huge.
PCI-Express is point-to-point, so provided you could find a motherboard with 4 x16 links (good luck!) or at least had 4 x16 slots (again, good luck) you could do it.
But a 4 processor motherboard with 4 x16 slots doesn't exist right now (it'd be a *huge* motherboard!), so no, you can't do it.
Well, I don't, but this technology has been around a while for Windows.
Thinsoft sells BeTwin which does exactly that. (The first versions were "PC Buddy" back in '99. On an ISA card, even!)
Of course it's more expensive (you need to buy software) than the Linux solution, but what Windows solution isn't?
I have looked at this... the problem is the way Linux handles the keyboard... To Linux, all keyboards are the same. If there was some way to either tell X to ignore all KBs except some, or have the kernel map them (say /dev/input/kb0 /dev/input/kb1) similar to how mice are done, this would not be a problem at all.
Ah, young Jedi, but there is a way...
Ruby, or Backstreet Ruby.
Note that this was in the article, though the link for Backstreet Ruby here is down - probably Slashdotted.
Beauty of open source. Where there's a need, someone has found a way.
That's not true! (And neither is the person who responded to you...)
:)
The first exoplanets were discovered by Alexander Wolszczan in 1991, around PSR B1257+12.
They're pulsar planets, yes, but they're planets. Give the guy credit.
Story here. Curious that the first discovered planets were Earth-sized. Also the planetary system is very much like Earth's. Dead, yes, but still encouraging.
Intel CPU's do have this technology as well, although it's half the width (64-bits at a time, rather than 128-bit).
... it still kinda sucked).
The MMX registers are 64 bit, although they're not the main limitation of the MMX implementation. For some inane reason, Intel decided to use the floating point registers for integer data (namely, MMX registers) and so MMX doesn't require additional registers to be added. However, since you're absconding with the floating-point hardware, you can't do floating point math at the same time, and you have to save the state of the floating point hardware before you switch to MMX. In other words, MMX was... "interesting", but in the end, not that useful. After all, for one thing, it eliminated your floating point capability unless you wanted to context-switch out. (AMD's 'improvement' to that was 3DNow! which was basically "MMX that you can use for floating point as well!" - okay, better, but
AltiVec didn't have those limitations - it was very, very improved over MMX.
SSE, however, *did* add 8 new registers, and 128-bit wide objects, for floating point. So an x86 processor with SSE extensions does have 128-bit vector abilities, albeit in floating point. Vectorized integer math is a little rare (hence why MMX isn't that useful anyway) so AltiVec and SSE are actually pretty comparable. AltiVec does have 32 registers (which makes sense, of course, given PPC's 32 register scheme), whereas SSE only has 8 registers. I'm sure some comp. eng. person can come along and tell me why it's efficient to have vector hardware that's the same depth as your register hardware (as x86 has 8 registers and 8 SSE registers, and x86-64 has 16 SSE registers, and 16 normal registers)
(SSE2 basically said "OK, MMX really blew - now you can just use the SSE registers for integer as well.")
When Apple posts benchmarks showing their machines to be faster than x86 machines, the benchmarks almost always make heavy use of these SIMD instructions... and rightly so.
When Apple used to post benchmarks. A modern G5 can keep pace with top end Athlons and P4s anyday, without any specialized benchmarks. Be nice to Apple - the days of the "G3 is 50% faster than a Pentium II using Photoshop's 'G3K1ckZA$$' filter on a mostly-red image of a cow... on Tuesdays!" are over, thank God. And if you had vectorized code on the x86 (using SSE), the comparison wouldn't be that unequal, unless it was heavily biased towards the PPC's obvious strengths (high register count). Then again, it's not like the x86 has any real strengths anyway...
But anyway, my point was that the SIMD implementation on x86 isn't really very different than on the PPC, once you count SSE. SSE is register-starved compared to PPC, sure, but so is x86 in general. x86-64 removes that last limitation (mostly, 16 registers is still starved compared to 32, I guess) but I doubt there would be a big performance jump going from 16 128-bit registers to 32 128-bit registers. There's not a ton of code that could efficiently utilize that. There is *some*, sure, but not a lot.
Wow, you know, there's an obvious solution to the atmospheric drag part that I didn't even realize. Just goes to show you that a human's normal experience sucks for trying to figure out orbital dynamics.
Incidentally, the full system design is here, and it's moderately more fleshed out than we're ever going to do in a Slashdot post. But anyway...
The solution to the atmospheric drag part is easy - only the perigee is at low altitude. The apogee is at high altitude. This, of course, solves the atmospheric drag part elegantly - the vast majority of the travel is well outside of the atmosphere. Thus you experience a small amount of drag once per orbit (which lowers your apogee), but you have a long time to recover that via solar power. Quite elegant - it basically utilizes the fact that the magnetosphere is immensely larger than the atmosphere.
The solution they're proposing is for assisting LEO to GTO, but the design is scalable and could eventually do suborbital to LEO. Oddly enough, they also used 2 gee acceleration, and listed mach 10-12 as the needed suborbital velocities (on a different page). Apparently I did my math somewhat right.
1000km cable, assuming 3cm in diameter, using a 2 GPa tensile strength steel weighing 7700kg/m^3... at best, this cable would carry 14,000kg (with little margin of error)
:)
How'd you get this? The carrying capacity of the cable is completely dependent upon the rotational acceleration. 2GPa tensile strength with ~30 cm^2 area implies 6 MN maximum load capacity. At 1000 km, it's got a volume of 2827 m^3, so it's got a mass of 21,767,900 kg. The tension on the cable is just the centripetal acceleration times the mass of the cable. It'd never hold itself together at 1 gee... best you could do is say, 0.2 gee. Your load capacity would be essentially infinite (8,232,100 kg). Did I do something wrong here?
You'd be much better off with carbon fiber cables (the real kind, not the space elevator kind). You could pretty much assume at least 5X steel (10 GPa), and the *huge* benefit of lower density (1800 kg/m^3). This is unlike a space elevator, where only the tensile strength matters, because the acceleration is independent of mass. The lower weight (4X) and heigher strength (5X) means that the cable can support an acceleration 20X higher, or up to 4 gees. If you would run it at 2 gees, that'd give you a 100% safety margin. With a radius of 500 km, an acceleration of 2 gee, the tangential velocity is 3129 m/s, or mach 9.2. Orbital velocity at 500 km is 7500 m/s or so, Earth's rotation is 500 m/s or so, so the suborbital craft needs to be going about 3871 m/s, or mach 11.4. That's a fair mite faster than SS1 was going, and it's way faster than anything on the horizon as well, but it's still less than full orbital.
The air resistance doesn't worry me - it's not that much of an acceleration, and the exponential falloff of the atmosphere means that the drag really only matters for the tiny portion of time that it's at the bottom. And if it's really too bad at 100 km, just have the bottom at 200 km, and the drag is suddenly thousands of times smaller. The height of a suborbital hop isn't difficult, it's the delta-V.
The other problems, like oscillations, plasma forming in the cable, etc., I'd worry about more. But those are generic problems that space tethers need to solve anyway, and I think with the advent of carbon fiber cables and the massive benefits that tethers can provide that not trying to solve them is really silly.
I.e., snapped. It doesn't matter whether it snapped due to stress or due to an unforseen collision - both of them would be equally big problems!
Yah, but it would've been like criticizing the Wright brothers for a bumpy landing, or saying that the dust buildup on the Mars rovers makes the design a "failure". They weren't trying to have the tether hold together for the entire trip. It wasn't designed to, and someone probably calculated that it wouldn't, so it was probably expected. After all, as I said, the sensor only was supposed to transmit for 10 hours. The tether lasted for days.
Unless you've got a motor on the thing, .00003% atmosphere at Mach 3 means that, after a day or two, you are a) no longer orbiting and b) no longer going at Mach 3.
Sigh. It's encountering weak drag at the most extreme range of its spin. The vast majority of the tether would be located higher, in orbit. In order to make the acceleration tolerable, it'd probably have to be about 500 km long, so it'd be orbiting at 350 km up, where there's far, far less drag. 1000 km would probably be more feasible, but obviously more of a logistical nightmare.
C'mon, the concept is not hard to understand. It's like a pinwheel that laps at water at the bottom of its spin. And it would have a motor on the thing - an electric generator, powered by solar. Run current through the tether and it pushes against the magnetic field of the Earth, and moves its orbit. Brief calculations (not done by me!) showed that it's only half a newton or so drag for the portion at the low end.
And, re: tethers, I'd say an experiment where the tether essentially exploded would be a really good description of a "disaster". But hey, what the heck do I know?
Apparently not as much as the review panel that said that while the experiment showed that the insulation of the cable was more vulnerable than believed, it also showed that there were no fundamental problems in the concept of using orbital tethers for power generation, which is exactly what the mission was supposed to do.
Oh yeah. And where are all the successful tests of rendezvous-ing with a Mach 3 tether in the upper atmosphere? Oh yeah. They don't exist. Mostly because it's a targeting problem a little harder than ballistic missile defense, because you can't fudge a targeting error with a big ba-da-boom.
The relative velocity between the two would be zero - the suborbital craft is moving at Mach 3, the tether is orbiting at Mach 25 (ish), and its extreme ends are moving at mach 22, in retrograde. Net result is a rather slow moving rendezvous. The length of the cable determines how much of a margin of error you have in timing (and also the acceleration of the payload). Mach 3 is probably too slow - 2 to 3 times the velocity would be much more feasible.
The density of air D at a given altitude is roughly d0*e^(-alt*1.43e-4); d0=1.29kg/m^3. That means that we're looking at 5.769e-10 at 150km, 4.895e-13 kg/m^3 at 200km, and 1.076e-17 kg/m^3 at 275km. Our orbital velocity v will equal sqrt(G*Mearth/(alt+Rearth)) = 7782m/s. Drag = 1/2 * C * D * A * v^2. Substituting, we get drag=0.0296C at 200km, negligable at 275km... but at 150km, we get 3.88C. So, I suppose if you had a very low value for C, and a big stock of ion drives or a good solid electromagnetic propulsion system, it might be possible to overcome for atmospheric drag... but it'd be no easy task. But on top of that, you have to lift a spacecraft.
No, you don't need to "lift the spacecraft" - your momentum will do that. The ribbon will simply slow down and enter a lower orbit, depending on its relative mass compared to the spacecraft, at which point it will need to raise itself to a higher orbit again, which can take as long as you need. A few solar panels and probably in a day or so it's back at normal orbit. They're just orbital momentum storage devices, that's all.
And C_d for a thin ribbon would be of order or less than 0.1. At the low point it's less than a newton of force.
Well, more than that. The atmospheric drag equation doesn't work at high relative velocities and low densities like we're trying to use. Considering the shape of the cable, I doubt that it'd be proportional to v^2 at all.
Oscillations - both natural (such as atmospheric or magnetospheric drag) and induced (such as when the spacecraft is picked up). Charge buildup. Micrometeorites. Severe cable strain (I think you'll need a cable with a much bigger diameter than 3 cm!).
All the effort you spent trying to debunk atmospheric drag - very strange. You could've just worked out the math as to how fast a 200 km cable would've needed to be rotating to be at rest with a mach 3 aircraft. Unfortunately it'd need to have a centripetal acceleration of about 50 gees - it'd shred the craft to pieces.
Obviously the cable would have to be much longer than 200 km - probably 1000 km at least, and you'd still need to have SS1 be going about 4 times faster. But it's still practical.
Etc. It's a huge task.
Absolutely. But I don't believe that it's anything that we couldn't do right now, with enough engineering. Engineering that I'm certainly not going to be able to work out in a Slashdot post.
Oh, and with SEDS 2, the tether snapped unexpectedly 3.7 days after deployment.
Man, that's mean! On SEDS 2 the tether didn't 'snap' - it was severed due to an impact. Considering it was merely meant to be a proof-of-concept on the deployment, and not actually survive the length-of-mission, that's fine. I mean, c'mon. The satellite it was hosting only transmitted for 10 hours!
6000 m^2, not m^3. Except for the fact that the atmosphere decays exponentially with a scale height of 10 km. So if you wanted to take that into account, the drag would be something like the equivalent of 300 m^2 at 100 km (integral of 6000*e(^-x/10000) from 0-200,000, roughly).
Plus it's rotating, so its profile is only 6000 m^2 when it's flat against the atmosphere. On average, it's half that. So its profile is 3000 m^2.
Plus you're not taking into account the differential velocities. At 100 km, it'd only be travelling at mach 3, or 1 km/s, right? Orbital velocity at 100 km is about 8 km/s. That means that, roughly, it's going to experience 64 times less drag than an orbital object at 100 km. I'm not going to take the time to work out all the math, but sufficient to say, I doubt it would be an insurmountable amount of drag. Even 300 m^2 is not much more than the ISS, and the ISS only needs reboosting every 90 days or so.
It's also important to realize you're talking about fluid flow here, and a tiny, thin piece of ribbon is not going to act the same as a very large flat object - the flow is going to probably be completely laminar, with very little resistance at all. They don't teach drag along with friction because drag is very, very hard.
I can't think of a single successful space tether experiment thus far.
Hmm, is this the same experiment? Were there two? This was a Columbia-based mission.
As for successful space tethers, SEDS 2, SEDS 1. We'll never know about ProSEDS, since it was cancelled.
Furthermore, if they're dipping down to 62 km, they'll be having to counter a bit of wind resistance - more, I'd imagine, than an ion drive could make up for.
:) ). At 100 km, you've got 0.00003% of the atmosphere left. While you will have air resistance, it's so small that I think it can be neglected. When the tether is unloaded, it probably also would have a negligibly small cross-section. In addition, it's only travelling at mach 3 with respect to the atmosphere at the worst density! It has very little to worry about when it comes to atmospheric drag.
100 km, not 62 km. 62 miles. (What, you thought they chose it in imperial?
Also, read the link I provided. The thrust is provided by electromagnetism, pushing against the Earth's magnetic field, powered probably by solar panels, most likely situated at the center of mass.
It should also be pointed out that our current experiments with space tethers have been phenomenally successful, not disastrous. The one experiment generated so much electricity that the tether essentially exploded. In this case, however, you'd prefer the tether not generate electricity. Again, not hard to do by design.
Not true. It is not trivially easy to build up that extra velocity, because you have to lug all of the extra propellant through the atmosphere.
Well, kindof true. If you want to have a single-vehicle-to-orbit solution, yah, you have to lug all of the extra propellant. But there are solutions in design which could definitely utilize a suborbital reusable spacecraft.
In fact, you can make docking and pickup very easy, if you make the rotation opposite the direction of orbit at the right rotation to match velocities. Someone would have to do the math on the acceleration to figure out how big it would be to allow for safe human transport on the craft, but it's definitely doable for small satellites.
Anyway, I know I'm glossing over a lot of details (like how do you guarantee the "human module" that an SS1-like craft would dump off has a safe abort procedure) but the point is that there can be very interesting uses for a suborbital craft. You're out of the atmosphere. That's a huge benefit right there.