I've got news for you, guy: dairy farmers have been freezing bull sperm for a lot longer than they've been freezing embryos, and a bull is darn lucky if he ever gets to hump a real live cow. Why do you think you can get liquid nitrogen delivered to your doorstep in the boondocks?
Granted, the foam would have slowed due to friction with the air, but why hundreds of miles per hour? Can someone explain?
Because the slipstream in the neighborhood of the bipod ramp was transsonic or even supersonic, and the foam had a huge amount of surface area for its mass. The combination of high velocity air and high area made the force quite high, while the low mass increased the acceleration (a = F/m, by Newton). If I understand correctly, the size of the foam piece and its speed were measured more or less directly from the film; the mass of the foam was computed based on the calculated area, which determined the drag force on it.
You know why we lost the Columbia? Because NASA regulations didn't allow anyone to go out and look at the damned wing in orbit without specific orders.
And if they had looked and did find a hole in the wing, then what?
Same outcome, one way or another. Either they try to de-orbit regardless and crash, or they don't de-orbit, run out of power and oxygen and then lose the Orbiter when it comes down due to air drag. There was no place to park it and no way to rescue the crew on orbit. Screwed.
All the orbiters should have been in museums by now. The whole Shuttle system should have been replaced long since. Our failure to do so is testament to the triumph of inertia and pork over concrete goals.
I would like for someone to simply give them a fixed budget for 3-5 years, saying "Here's all the money you are getting for a while. In 5 years, I want a fully detailed report on how you spent it. If you don't operate at X level of efficiency, you're getting budget cuts."
You'll get that just as soon as you can keep the congresscritters from insisting that the money flows back to their districts. Efficiency is the enemy of pork, and pork is how the votes for things are assembled (both money and safety were sacrificed to put the SRB business in Jake Garn's district in Utah). Efficiency is also the enemy of bureaucratic empires; managers will fight to have (and spend money on) more personnel as long as it means that their pay grade and visibility goes up, and hang the organization's alleged objectives.
The cure for this may be competition. If NASA was put out of the transport biz and the money went to whoever could deliver X tons of consumables to the ISS on schedule and provide taxi and emergency rescue service, we'd probably have tons of money left over... as long as we could keep the pols from skewing the contract awards to their campaign contributors. Unfortunately, I don't see this happening either.
Let me clarify - you shave 4 km/sec off the delta-v you need to produce using chemical rockets.
It was clearly wrong (either erroneous or grossly mis-stated) the first time.
Getting to LEO from the ground requires roughly 5 miles/second of delta-V, about 8 km/sec. Getting to escape velocity requires about 7 miles/second, or 11.2 km/sec. If you chop 4 km/sec off the delta-V for escape, you're down to 7.2 km/sec and you will not even achieve orbit.
You're right about the mass-ratio, so I suspect that you're just fumbling the expression of the concepts. A forum like Slashdot isn't a good place to do that; there are too many people out there who accept any authoritative-looking thing as gospel. While they may well deserve what they get, I think it's incumbent upon those of us who actually know something to try to help the public understand things rather than obfuscate them.
You shave at least 4 km/sec off of your required delta-v if you can use ion drives and have a longer trip.
The delta-V is a function of the path; the required delta-V for a typical ion-drive trajectory is actually a bit higher than a two-impulse elliptical transfer. What's reduced by ion drives is mass ratio, which you would expect from the rocket equation.
(that word in the subject means "study of the moon", in case you were wondering.)
Quoth the poster:
First off, the moon could be set up as a base to launch more investigative, futher expeditions into our solar system. (prolly for cheaper, cuz it wouldn't cost as much to blast off from the moon, due to lower gravity)
Wrong. Getting to the moon is about as expensive as getting to Mars, more or less, largely because Mars has an atmosphere that you can use to brake against for free. Only a fool would go to the Moon, stop there, then launch off again to go to Mars; for one thing, you're much more efficient doing your boosting near the bottom of a gravity well rather than at the top of one (em vee squared, dude).
Second of all, perhaps there will be a different set of minerals up there that we could start mining and build stronger, yet lighter materials.
The Moon is largely made up out of minerals we are quite familiar with here on ol' Terra, and nature has done us a favor by differentiating them using water-based sorting processes which don't exist on Luna. You should do some studying of the subject; not only might you learn something, you might put yourself in a position to actually contribute something useful.
...that pesky second law of thermodynamics prevents burning stuff from being very efficient. I think I heard somewhere around 30% efficiency for the best internal combustion engines.
Try over 40% for medium-speed diesels (based on the higher heating value of the fuel), something like 50% for low-speed diesels such as the ones which power ships, and I recall seeing 60% claimed for a combined-cycle turbine powerplant in Britain (gas-fired). This is not to say that fuel cells can't meet or beat this, but the competition is tough at the top.
I'd rather be around a leaky hydrogen tank than a leaky propane tank.
Maybe your neighbors would prefer the reverse. Hydrogen is a fairly stable molecule, and would drift upwards until it reached the upper stratosphere where high-energy UV could crack it. There it would form water, much higher in the atmosphere than water normally forms. The resulting high-altitude ice crystals would form great surfaces for the catalytic breakdown of ozone, which your neighbors would probably not appreciate very much.
A world which uses H2 heavily might not be quite as much of the eco-paradise as some paint it.
nd why hydrogen over, say, gasoline or propane? Because you can't make gasoline out of water and sunlight.
I'm not so sure about that. If I had a bunch of bottomland (nice and soggy), a source of fertilizer like the effluent of a sewage-treatment plant, and a device to convert organic matter to hydrocarbons such as described here, I think I could do it and clean up the sewage problem in the bargain. I'd just grow a lot of nitrogen- and phosphorous-hungry plants like water hyacinths in the sewage and keep scooping them up and feeding them to the fuel processor.
Are we cursed forever to avoid using the single most commmon element in the universe, one that will burn clean, simply because someone burned a balloon with it once decades ago?
If only it were so simple. Safety is not the issue in public consciousness (how many million dead in automobile crashes, yet people barely give safety a second thought most days?) Instead, the use of hydrogen presents a ton of problems that are far less tractable:
Current production is almost entirely non-renewable. Signatories to the Kyoto treaty will not be able to make their targets by "switching" to hydrogen if they make it from natural gas (or, heaven forbid, coal).
Production is highly inefficient. Whether it's made from hydrocarbons, carbohydrates (polysaccharides such as wood) or electricity, the hydrogen only embodies a relatively small fraction of the energy which goes into the process. This further increases the cost, as well as CO2 production if the raw material is any kind of carbon-based fuel.
Production is costly, relatively speaking. Storing energy as hydrogen appears to cost several times as much as gasoline.
For these reasons, it looks like not such a good idea to plan an economy around this. AAMOF, it looks like a diversion by enemies of change; they can point to hydrogen as the panacea, but use all the very real difficulties as excuses for the glacial pace of achievement.
Now compare that to our current state of affairs: the vast majority of our electricity coming from coal or gas, much of it imported; our cars running on gasoline, almost all of it imported.
Now try and tell me it doesn't make sense to switch.
Oh, it does.... but not to hydrogen. Batteries (such as lithium-ion) are far more efficient and have much lower costs already. If you want to power a transportation system, using a Calcars-style system of grid-feeding hybrid vehicles would do a much better job, for less, using today's technology. Such vehicles would have no problem stabilizing the grid.
Maybe he's talking about the little rad-hard chip that used to be favored for space applications, and really meant "COSMAC proportions". Of course, that would be a pretty small disaster even for those big old dice.
These craft typically use heat to make electricity, and then use the electricity to power a thruster of some kind; the thruster accelerates propellant. Ion thrusters have been made to use a great many different things as reaction mass (propellant), ranging from argon to bismuth to Buckyballs to xenon. The Deep Space One probe which was so phenomenally successful used xenon in its ion engine.
You are correct that the Carnot efficiency cannot be calculated for this case, as it is not returning to it's original position. However, in the postulated case of a perfect mirror, there is no heat sink, as the photons will be reflected back towards the sun, and not radiated from the back of the sail.
Actually, you can treat it as a Carnot-cycle engine, and in at least two different ways. If you assume that a volume of photons is trapped in a perfectly reflecting volume with the photon sail as one of the boundaries, the motion of the sail amounts to an expansion or contraction of the volume and the work done against the sail shows up in the photons being red- or blue-shifted.
The other way is to consider the sail as a Steady State, Steady Flow machine. Photons come in on one vector in the reference frame, bounce off the (moving) sail, and leave on another vector. For a sail thrusting forward in its orbit or moving away from the Sun, the photons will be red-shifted by some tiny amount; this loss in energy is equivalent to the delta-temperature between the inlet and outlet of the "machine". As with any Carnot-cycle engine running on a very small temperature difference, the efficiency is lousy.
The real problem is that the efficiency you measure depends on the reference frame, but you have this same issue when trying to compute the energy change from gravity-assist maneuvers so anyone doing astronautics should have some idea of what to use.
Personally, I'm with the 'bulkhead without a door school of thought (The pilots have a seperate external door. That makes it impossible to physically coerce pilots, because you can't get to them. Problem solved.
Which creates another set of problems (which you ignore; I gather you are of the L. Neil Smith school of thought):
Now the pilots require their own toilet facilities. (Size and weight must be subtracted from payload.)
The cockpit has to be expanded to accomodate yet another exit door. (Size and weight must be subtracted from payload.)
(the real killer) The routes flyable with the aircraft are strictly limited to those which do not exceed the flight crew's allowable flying time. Missions such as over-the-pole flights between Europe and Chicago, which require a second crew to take over mid-flight, would not be possible.
I'm of the opinion that "soft walls" are probably less desirable than a "hijack, autoland me" system which can be activated by the pilots before any hijacker has enough time to breach the cockpit door.
Do you think this could boost payloads delivered from small non-NASA suborbitals like Rutan's[?]
In a word, yes. That is exactly the kind of thing it's intended for, and if I could find the web page where I stashed my Hohmann orbit transfer formulae I'd be able to give you numbers on what you could achieve. (My calculations would be simplified by treating the two-body system as a point mass for gravitational energy considerations, but I doubt that would affect the answer significantly.)
Scratching numbers on my calculator watch, it looks like a 60-ton tether sat coming in at 9 km/sec could grab a 5-ton suborbital craft moving at 1 km/sec and be slowed to 8.38 km/sec (still above circular-orbit speed). If we assume that the tether itself is massless, the center of mass of the combined craft would be 12/13 of the way toward the counterweight. If the tether length was 100 kilometers, the angular speed would be (8km/sec/100km)=0.08/sec, and the acceleration on the smaller craft would be (omega^2*r)=0.0064*(1e5*12/13)=591m/sec^2, or about 60 G. That's too much for people, but it shouldn't be any difficulty to design unmanned spacecraft to handle it.
Now imagine something which could boost a 5-ton payload from suborbital-hop speed to LEO, GTO or even further out... once a week.
As I see it, the other end of the cable is hooked to a bigass weight - at least, that's what the massive, spacecraft-size block looks like in the schematics. Where does that block come from, and how do you boost it even to LEO?
One obvious possibility is to buy a spent Shuttle external tank (ET) or five (pay NASA to take it all the way to orbit for you), and launch something like a giant robotic tin snip with the first one. You slice the tank into pieces and stuff them into a Kevlar bag (or just cram pellets of the hydrogen tank into the much-smaller oxygen tank). One ET is about 60,000 pounds of mass, so not a bad start.
Solar sails might work too, but I suspect you'd need some honkers to get adequate results.
Below a certain altitude (which depends on the solar cycle) solar sails are worse than useless. The atmosphere is tenuous at those altitudes, but until you get high enough that the drag against your sail is smaller than the thrust you get from light pressure you'll just drag yourself down faster.
This is why electrodynamic propulsion is so attractive for this purpose. The same soup of ions which drags against a sail forms a current-return path for a conductor. Pump some current through your tether, and you can push against Earth's magnetic field; the lower you are the denser the ions, the stronger the field, and the better it works.
The Japanese have failed recently with using the slingshot for space purposes, although in a different application.
... in other words, you have no idea what you are talking about.
The Japanese probe was using a "gravity assist". If you would just RTFA, you'd know that the scheme being talked about here involves an actual sling (a tension member with masses at both ends) not dissimilar from the one in the legend of David vs. Goliath. The only similarities between the two are that:
Energy and momentum are exchanged between two masses, and
Ignorant people see the word "sling" in popular descriptions and think they're the same.
Why can't I watch my local high school or local college's sports teams on TV?
Broadcast range and economies of scale, mostly. For a HS team game, you'd probably have no interest outside the school district, but any TV station of economical size would have a coverage area going over dozens of high schools (and perhaps dozens of districts). Either you'd be picking and choosing what school's game to broadcast (discriminatory) or you'd have to devote a channel to each school (overwhelming the bandwidth and hardware with things that very few people would want to see, proportionate to those able to see them).
Cable at least has the option of finer granularity, and real high-speed Internet streaming could put individuals in complete control. Broadcast stuff has to be of interest to the masses, or it doesn't work.
Chicken maturity matters?
on
Chicken Run
·
· Score: 2, Interesting
That line about the chickens being basically babies in adult bodies gave me pause, though -- I am a confirmed carnivore, but some of the stuff we do in the name of taste and profit is hard to digest.
Eggs are essentially the most-immature form of chickens. Pot pies, chicken franks and such are, IIRC, made from old laying hens: the most mature chickens in commercial farming. Broilers are in the middle. I suspect that you've eaten all of the above.
Is there anything about broilers (the 8-week wonders) being so young that makes them more pitiful than the other ends of the spectrum? All they are is a population which has been bred (selected) for certain traits; I doubt very much that they feel any more discomfort in their lives than laying hens, and probably less.
If the plastic is reduced to monomers, it's hard to see how degradation of a few molecules could affect the usability of the result; they'd be filtered out during the separation process. I'm sure the same thing happens when monomers are made in the first place.
If you're just trying to re-melt and mold (or blow, or extrude) the same polymer again, I'm sure you're right.
Because the United States becomes more and more dystopian every day, let's look at another possibility. At your pre-employment screening, you have a mandatory breathalyzer test to see if you've been drinking. Unbeknownst to you, your breath is also analyzed for the presence of indications of cancer. If you have any, the company decides that you might very well be too expensive and drive up their insurance costs, so you don't get hired.
Pre-employment "drug" tests have been used to screen women for pregnancy, so I have no doubt that a cancer-detecting breathalyzer will be used to screen for other expensive conditions (or at least certify them as "pre-existing" and thus not covered by the company).
But ratifying Kyoto might at least have shown the USA's intention to do something about its mass consumption. It might have shown they feel responsible for burning over 25% of worldwide resources, while constitutin less than 10% of its population/surface.
You might have missed this point, but the US also pays for the resources it gets from other countries. It returns value in the forms of goods and services.
There are a lot of good arguments against the USA using so much oil, and producing such a large fraction of humanity's excess CO2. However, the "over-consuming society" argument is logical junk. If the USA consumed 50% of the world's human-handled energy but produced it all from solar and wind, the "mass consumption" claim would still be true! That just goes to show how little sense it makes. Find another argument.
Slashdot Mirror
I've got news for you, guy: dairy farmers have been freezing bull sperm for a lot longer than they've been freezing embryos, and a bull is darn lucky if he ever gets to hump a real live cow. Why do you think you can get liquid nitrogen delivered to your doorstep in the boondocks?
Now recompute them as fatalities per hour, or per departure.
Same outcome, one way or another. Either they try to de-orbit regardless and crash, or they don't de-orbit, run out of power and oxygen and then lose the Orbiter when it comes down due to air drag. There was no place to park it and no way to rescue the crew on orbit. Screwed.
All the orbiters should have been in museums by now. The whole Shuttle system should have been replaced long since. Our failure to do so is testament to the triumph of inertia and pork over concrete goals.
The cure for this may be competition. If NASA was put out of the transport biz and the money went to whoever could deliver X tons of consumables to the ISS on schedule and provide taxi and emergency rescue service, we'd probably have tons of money left over... as long as we could keep the pols from skewing the contract awards to their campaign contributors. Unfortunately, I don't see this happening either.
Getting to LEO from the ground requires roughly 5 miles/second of delta-V, about 8 km/sec. Getting to escape velocity requires about 7 miles/second, or 11.2 km/sec. If you chop 4 km/sec off the delta-V for escape, you're down to 7.2 km/sec and you will not even achieve orbit.
You're right about the mass-ratio, so I suspect that you're just fumbling the expression of the concepts. A forum like Slashdot isn't a good place to do that; there are too many people out there who accept any authoritative-looking thing as gospel. While they may well deserve what they get, I think it's incumbent upon those of us who actually know something to try to help the public understand things rather than obfuscate them.
Quoth the poster:
Wrong. Getting to the moon is about as expensive as getting to Mars, more or less, largely because Mars has an atmosphere that you can use to brake against for free. Only a fool would go to the Moon, stop there, then launch off again to go to Mars; for one thing, you're much more efficient doing your boosting near the bottom of a gravity well rather than at the top of one (em vee squared, dude).The Moon is largely made up out of minerals we are quite familiar with here on ol' Terra, and nature has done us a favor by differentiating them using water-based sorting processes which don't exist on Luna. You should do some studying of the subject; not only might you learn something, you might put yourself in a position to actually contribute something useful.A world which uses H2 heavily might not be quite as much of the eco-paradise as some paint it.
- Current production is almost entirely non-renewable. Signatories to the Kyoto treaty will not be able to make their targets by "switching" to hydrogen if they make it from natural gas (or, heaven forbid, coal).
- Production is highly inefficient. Whether it's made from hydrocarbons, carbohydrates (polysaccharides such as wood) or electricity, the hydrogen only embodies a relatively small fraction of the energy which goes into the process. This further increases the cost, as well as CO2 production if the raw material is any kind of carbon-based fuel.
- Production is costly, relatively speaking. Storing energy as hydrogen appears to cost several times as much as gasoline.
For these reasons, it looks like not such a good idea to plan an economy around this. AAMOF, it looks like a diversion by enemies of change; they can point to hydrogen as the panacea, but use all the very real difficulties as excuses for the glacial pace of achievement. Oh, it does.... but not to hydrogen. Batteries (such as lithium-ion) are far more efficient and have much lower costs already. If you want to power a transportation system, using a Calcars-style system of grid-feeding hybrid vehicles would do a much better job, for less, using today's technology. Such vehicles would have no problem stabilizing the grid.Maybe he's talking about the little rad-hard chip that used to be favored for space applications, and really meant "COSMAC proportions". Of course, that would be a pretty small disaster even for those big old dice.
These craft typically use heat to make electricity, and then use the electricity to power a thruster of some kind; the thruster accelerates propellant. Ion thrusters have been made to use a great many different things as reaction mass (propellant), ranging from argon to bismuth to Buckyballs to xenon. The Deep Space One probe which was so phenomenally successful used xenon in its ion engine.
The other way is to consider the sail as a Steady State, Steady Flow machine. Photons come in on one vector in the reference frame, bounce off the (moving) sail, and leave on another vector. For a sail thrusting forward in its orbit or moving away from the Sun, the photons will be red-shifted by some tiny amount; this loss in energy is equivalent to the delta-temperature between the inlet and outlet of the "machine". As with any Carnot-cycle engine running on a very small temperature difference, the efficiency is lousy.
The real problem is that the efficiency you measure depends on the reference frame, but you have this same issue when trying to compute the energy change from gravity-assist maneuvers so anyone doing astronautics should have some idea of what to use.
- Now the pilots require their own toilet facilities. (Size and weight must be subtracted from payload.)
- The cockpit has to be expanded to accomodate yet another exit door. (Size and weight must be subtracted from payload.)
- (the real killer) The routes flyable with the aircraft are strictly limited to those which do not exceed the flight crew's allowable flying time. Missions such as over-the-pole flights between Europe and Chicago, which require a second crew to take over mid-flight, would not be possible.
I'm of the opinion that "soft walls" are probably less desirable than a "hijack, autoland me" system which can be activated by the pilots before any hijacker has enough time to breach the cockpit door.Scratching numbers on my calculator watch, it looks like a 60-ton tether sat coming in at 9 km/sec could grab a 5-ton suborbital craft moving at 1 km/sec and be slowed to 8.38 km/sec (still above circular-orbit speed). If we assume that the tether itself is massless, the center of mass of the combined craft would be 12/13 of the way toward the counterweight. If the tether length was 100 kilometers, the angular speed would be (8km/sec/100km)=0.08/sec, and the acceleration on the smaller craft would be (omega^2*r)=0.0064*(1e5*12/13)=591m/sec^2, or about 60 G. That's too much for people, but it shouldn't be any difficulty to design unmanned spacecraft to handle it.
Now imagine something which could boost a 5-ton payload from suborbital-hop speed to LEO, GTO or even further out... once a week.
This is why electrodynamic propulsion is so attractive for this purpose. The same soup of ions which drags against a sail forms a current-return path for a conductor. Pump some current through your tether, and you can push against Earth's magnetic field; the lower you are the denser the ions, the stronger the field, and the better it works.
The Japanese probe was using a "gravity assist". If you would just RTFA, you'd know that the scheme being talked about here involves an actual sling (a tension member with masses at both ends) not dissimilar from the one in the legend of David vs. Goliath. The only similarities between the two are that:
- Energy and momentum are exchanged between two masses, and
- Ignorant people see the word "sling" in popular descriptions and think they're the same.
Try to learn something before posting.Cable at least has the option of finer granularity, and real high-speed Internet streaming could put individuals in complete control. Broadcast stuff has to be of interest to the masses, or it doesn't work.
Is there anything about broilers (the 8-week wonders) being so young that makes them more pitiful than the other ends of the spectrum? All they are is a population which has been bred (selected) for certain traits; I doubt very much that they feel any more discomfort in their lives than laying hens, and probably less.
If you're just trying to re-melt and mold (or blow, or extrude) the same polymer again, I'm sure you're right.
Pre-employment "drug" tests have been used to screen women for pregnancy, so I have no doubt that a cancer-detecting breathalyzer will be used to screen for other expensive conditions (or at least certify them as "pre-existing" and thus not covered by the company).
There are a lot of good arguments against the USA using so much oil, and producing such a large fraction of humanity's excess CO2. However, the "over-consuming society" argument is logical junk. If the USA consumed 50% of the world's human-handled energy but produced it all from solar and wind, the "mass consumption" claim would still be true! That just goes to show how little sense it makes. Find another argument.