Depends what you're doing. Something like google or SETI or frame rendering scales very well to a cluster, because the amount of internode communication required is very low.
Something like CFD or FEM is about in the middle, which is to say that clusters and SCs do about as well as each other. This is because, although there is a requirement that nodes communicate, the amount of communication is relatively low compared to the amount of internal computation. ie each cell is mostly affected by the cells directly neighbouring it and the previous status of the cell.
Something like plasma or MHD modelling just doesn't work on a cluster because at each iteration, every node needs to hear from every other node, so the speed of the computation is basically dependant only on the rate of communication between nodes.
What that means is that one google PC can calculate at a significant fraction of the speed of the whole cluster. Which is why you need a supercomputer.
This isn't the 1890s, guys. Our soldiers aren't over there raping the women and stealing everything they can find as war trophies.
Some slashdotters have a funny sense of the pace of history. The 1890s were a time when your modern way of thinking wouldn't have been too out of place. As far as I remember, the crying of havoc was banned some time in the late 1300s (Richard II?) and hasn't been officially supported by a government for about the same time.
But powergaming takes advantage of the niceness of the DM. For instance, it is pretty common in a number of RPGs for a two handed weapon to deal more damage than a one handed weapon. Many powergamers take advantage of this to increase their damage. But this (generally) has the downside that you can't use a shield. So if you play in a group with a powergamer, better get used to them wining about how the monsters are getting "lucky" and hitting more often.
Because you're playing for fun, the DM is reluctant to kill a character for making bad character building decisions. So, even though the penalty in the rules is clear, the actual penalty is nothing, because the DM wants to be nice.
For instance: Your group falls into a lake. All of the power gamers are wearing heavy armour and can't swim. They should all die. But we can't have that, because it wouldn't be fun; so everybody lives, regardless of the fact that the power gamers didn't "pay" to live.
I think that's the main problem with computer RPGs, (and with RPGs generally when not played with a smart DM). There's not much checking of characters to see if they make sense. If you want to powergame, that's OK, but you'd better be prepared to accept the negative sides of having a character with only a few specialities.
In other news, my experience is that often powergamers tend to have the kind of personalities that feel persecuted in the real world because nobody recognises thair genius. Is it connected? I don't know.
You can look at all the includes, headers, and modular source code files for a software package like OpenOffice all at the same time and make sense out of them?!?
Yes, modular normal code is a problem as well, but you don't get the problem that every 10 (or so) lines is split off into its own module. With small programs (which is pretty much all I know about), labview is in the 10-100 blocks range, where c/fortran might be in the 1-5 blocks range. So take the module problem of a large code and multiply by 10-100 and you'll see what I'm getting at.
Maybe he's talking about something like labview, where new programs are made mostly by linking together little boxes on the screen. Each box contains some components which are either prespecified, or can be filled in.
I've used labview just for writing programs to link to IEEE hardware, and it certainly is much easier to deak with a large number of modules when they're visually represented, and very fast to kludge together a fast fix.
The only thing is that the debugging and maintainence is a nightmare because unlike a normal C/Fortran, not all of the program is visible at once (it's in a thousand tiny blocks), and so looking at several related bits of code is very time consuming. So much so that we recently rewrote some labview code in c, just to improve the clarity and maintainability.
OK, Clearly you think that there's nothing more efficient than a rocket. Depends, of course on how you define efficiency. Modern rockets achieve very close to their theoretical efficiency, I think that there's no arguing with that. However other types of engine have a higher "Efficiency" than a rocket.
Consider the rocket equation. If we compare an airbreathing engine burning hydrogen with a rocket engine breathing hydrogen/Lox, then for the same delta V, and the same launch weight, (because we have the same exhaust velocity),
then if we assume the same combustion efficiency, the airbreathing engine will have launched four times the mass. So, for the same "Efficiency", we achieve four times the launch. Now, of course, you're right that there are a lot of factors which can reduce this margin, but the fact remains that a scramjet operating at 25% efficiency will have the same effect as a rocket launched at 100% efficiency.
I agree that scramjets are not currently competition for rockets. They may never be, but that doesn't mean that they shouldn't be investigated.
I would note that from 1980-1999 that the reliability for liquid rocket launches was 97.5%. Most of the failures are inherently due to constraints caused by the low superstructure weight requirements of rockets. If you think that it's acceptable that 2.5% of launches fail, forever, then that's your perogative. No other form of transportation finds this acceptable however, and I don't see that we should be content with it in the space industry.
"Shielding and structure have to be sufficient to withstand reentry anyway, so they should withstand the load peak during railgun acceleration. Or are we talking about a fundamentally different load pattern here?"
You're thinking re-usable SSTO. Nothing says scramjets are only for SSTO or reusable. Think of replacing the second stage on your favourite rocket with a scramjet, and you'll see one possibel option.
"Two, the structural weight needed to withstand pushing through atmosphere at extreme Mach (stand up to both drag and temperatures) outweighs what you need for tanks and LOX to do it the rocket way."
Sez you. Not a scientist, and not, quite frankly someone who has any idea. It's about the numbers. You don't know them. Those who do know (Including me, I might add), know that it's a close thing which is unclear, even within the scientific community.
"Think, there's a reason that we've used rockets for 50 years instead of scramjets -- it's easier."
Since when is easier a reason for stopping development?
"To make a practical, re-usable, passenger carrying scramjet we have to solve problems of designing a lightweight, temperature resistant vehicle that doesn't require a major overhaul between flights (that's the practical part), plus the problems of designing an effective, large-scale (compared to current test models) scram-jet engine. To make a practical, re-usable, passenger carrying rocket we have to solve problems of designing a lightweight, temperature resistant vehicle that doesn't require a major overhaul between flights (that's the practical part) -- but we already know how to make efficient, large-scale rocket engines.
Which seems easier?"
It'd be easier to stay at home, but that's not the point.
I'd point out that there's nothing saying that Scramjets: 1. must be in a lifting body 2. must be SSTO or 3. must be reusable.
In fact a lot of proposals have them being none of the above.
I think the question is: Would you like more efficient rockets? If yes, then airbreathing engines are one of the available options.
"We all look forward to that day -- but air-breathing jets are no more advanced than rockets, and in fact they're rather Rube Goldbergish. Using jets and wings to get to space is equivalent to the "horseless carriage" era of automobiles -- or worse. It's like trying to come up with a 100MPH, 200 HP vehicle by inventing a harness that will let you hook up 200 actual horses, because they can "forage for themselves" rather than just building in a gas tank and engine."
No, it's the equivalent of the car in the age of horses. Anybody looking at a horse, even today can see that its a superior engine to a car. better design, reliability, efficiency, and just plain better looking. But if you want to go faster that 30Mph, there's no way to train your horse to take you there.
Rockets have reached their limit. Just because their possible successors are not doing better than rockets now, doesn't mean that they won't eventually do better.
The point that your parent was making about scramjet was that there's no inherent reason why a scramjet must have higher drag than a rocket of the same frontal area. Scramjets don't push through the air any more than any other vehicle.
A railgun launch would be possible, but also has a lot of associated problems. You want a trajectory to orbit where the speed is increasing as the altitude is increasing (basically a constant Pitot pressure ascent). This means that the heat and pressure loads are basically constant over the flight. If you have a heat and pressure load peak, as will probably be caused by a high-speed rail launch, then you need extra structure and shielding, which will increase the flying weight of the vehicle.
Obviously there are a lot of people studying whether the trade off comes off positive or negative for the cost to orbit.
For the other readers, I'll add to this that the parent touches on several other points which have some bearing on scramjet research, particularly magnetohydrodynamic engines, and MHD control of combusting flow, both of which have been extensively researched, particularly in the Soviet Union. This is also some pretty great stuff.
I would note that there a a lot of methods of fuel mixing, ignition and flameholding other than thos mentioned in the linked article. This is a rich field, in which what the public sees is just the tip of the iceberg.
And in addition, all of this is without active cooling. No blowing into the boundary layer, circulation under the skin or other tricks. All in all, the heat question is the least of the problems of a hypersonic lifting body.
Yes, fuel is cheap, but a rocket is a mountain of fuel, and reliable, lightweight superstructure for a mountain is neither cheap nor easy.
The thing is that rockets are great, unless you want to improve reliability or efficiency from current levels, then you need something else.
The fact that they accelerate Hyperx with a rocket is rather beside the point. A lot of the early flights of carrier aircraft were made from ordinary landing fields. That doesn't in any way mean that they don't have the potential to be used in other ways.
I'm also not a rocket scientist, I've been working in scramjet research for 7 years.
Rockets work well, but that doesn't mean that there's not something better out there. There are two main problems with rockets: 1. They've pretty much reached their peak efficiency. 2. They've pretty much reached their peak reliability.
As scientists, we're trying to improve the efficiency of our engines. With rockets, we're at the point where you won't get another 30% out. So you need something new. The comment about drag is correct, but the promise of airbreathing technology is that you lose an extra 30% in drag and gain 60% in efficiency (insert your own numbers where necessary).
In addition, the problem with rockets is that about 95% of the takeoff weight is fuel. That means not much superstructure, and so lower reliability than an aeroplane, for instance. Obviously, is the structure is such a low fraction of the takeoff weight, the gains to be made from composite materials are minimal, thus, again, the wish for a new engine.
Right sort of numbers, Hypersonic is different from supersonic in that in unseparated flow there are no subsonic regions, whereas supersonic flow is generally a mix of supersonic, transsonic and subsonic flow. In addition, there's a "hypersonic similarity law", which basically states that when you get above Mach 8, the effect of Mach number differences on the flow structure is minimal.
Interestingly, Mach 8 is also the boundary where if you decelerate the flow in your engine subsnic, that it is not possible to re-accelerate the combustion products enough to recover the losses from deceleration of the incoming air. Thus supersonic combustion is needed.
The current thinking is so: Use turbojet stage for takeoff. Bring in Ramjet stage at transsonic speeds, transitioning to full ramjet about Mach 1.5 to Mach 2. Bring in Scramjet stage from Mach 3-4, transitioning to full scramjet at Mach 5-7. Bring in Rocket stage at mach 10-12, transitioning to full rocket at Mach 14-16.
You see, that it's rare that any single stage is purely one thing or the other. Scramjets are not the solution to space travel. They're one piece of the puzzle. Reducing the cost of flight to space by 5% is something which would still be worthwhile, and airbreathing flight certainly has great promise to do far more than that.
The problem is that at the moment it's only that: promise. These tests are to see if we can turn promise into reality.
Not entirely true. Jet fuel b is a hydrocarbon mix of 4-16 carbon atom molecules. For a hydrocarbon with N carbon and 2N+2 Hydrogen, the oxygen required for full combustion is between N and 2N for the carbon and N+1 for the Hydrogen. The ratio of fuel to oxygen mass is between 1:2 and 1:3. Therefore by switching to airbreathing you'll save 60% to 75% on your takeoff weight (on the scramjet stage only of course).
My grandmother has osteoperosis. Would you buy her a segway? I think that the group of people who can't walk, but would not be severely injured by a fall from a segway is vanishingly small.
Actually, since if you're writing in code, even the "plaintext shouldn't be plaintext, that's not quite right. Figure you could also write T qik brn fx jump ovr Thu lzzy dg, and many such similar (Lets say about 5 opteions per word, for a possible 1.953 million options even for such a simple message. If its not required that every group makes a word, then the requirement is loosened even further. I would say that for a given group of letters and numbers, probably on the order of 10% of the arrangements could be deciphered to be some sort of message. Guess also that you may not know the language that the message is written in, or that there may be three or more languages in the same message, and that the message may also contained pre-arranged codegroups, and pretty much no group of letters can be free of suspicion.
Slashdot Mirror
Depends what you're doing. Something like google or SETI or frame rendering scales very well to a cluster, because the amount of internode communication required is very low.
Something like CFD or FEM is about in the middle, which is to say that clusters and SCs do about as well as each other. This is because, although there is a requirement that nodes communicate, the amount of communication is relatively low compared to the amount of internal computation. ie each cell is mostly affected by the cells directly neighbouring it and the previous status of the cell.
Something like plasma or MHD modelling just doesn't work on a cluster because at each iteration, every node needs to hear from every other node, so the speed of the computation is basically dependant only on the rate of communication between nodes.
What that means is that one google PC can calculate at a significant fraction of the speed of the whole cluster. Which is why you need a supercomputer.
This isn't the 1890s, guys. Our soldiers aren't over there raping the women and stealing everything they can find as war trophies.
Some slashdotters have a funny sense of the pace of history. The 1890s were a time when your modern way of thinking wouldn't have been too out of place. As far as I remember, the crying of havoc was banned some time in the late 1300s (Richard II?) and hasn't been officially supported by a government for about the same time.
But powergaming takes advantage of the niceness of the DM. For instance, it is pretty common in a number of RPGs for a two handed weapon to deal more damage than a one handed weapon. Many powergamers take advantage of this to increase their damage. But this (generally) has the downside that you can't use a shield. So if you play in a group with a powergamer, better get used to them wining about how the monsters are getting "lucky" and hitting more often.
Because you're playing for fun, the DM is reluctant to kill a character for making bad character building decisions. So, even though the penalty in the rules is clear, the actual penalty is nothing, because the DM wants to be nice.
For instance: Your group falls into a lake. All of the power gamers are wearing heavy armour and can't swim. They should all die. But we can't have that, because it wouldn't be fun; so everybody lives, regardless of the fact that the power gamers didn't "pay" to live.
I think that's the main problem with computer RPGs, (and with RPGs generally when not played with a smart DM). There's not much checking of characters to see if they make sense. If you want to powergame, that's OK, but you'd better be prepared to accept the negative sides of having a character with only a few specialities.
In other news, my experience is that often powergamers tend to have the kind of personalities that feel persecuted in the real world because nobody recognises thair genius. Is it connected? I don't know.
You can look at all the includes, headers, and modular source code files for a software package like OpenOffice all at the same time and make sense out of them?!?
Yes, modular normal code is a problem as well, but you don't get the problem that every 10 (or so) lines is split off into its own module. With small programs (which is pretty much all I know about), labview is in the 10-100 blocks range, where c/fortran might be in the 1-5 blocks range. So take the module problem of a large code and multiply by 10-100 and you'll see what I'm getting at.
Perhaps CANT SPM -Have to add the "A" or we arrive at a new form of enlargement spam I really don't want to think about.
Maybe he's talking about something like labview, where new programs are made mostly by linking together little boxes on the screen. Each box contains some components which are either prespecified, or can be filled in.
I've used labview just for writing programs to link to IEEE hardware, and it certainly is much easier to
deak with a large number of modules when they're visually represented, and very fast to kludge together a fast fix.
The only thing is that the debugging and maintainence is a nightmare because unlike a normal C/Fortran, not all of the program is visible at once (it's in a thousand tiny blocks), and so looking at several related bits of code is very time consuming. So much so that we recently rewrote some labview code in c, just to improve the clarity and maintainability.
You mean like the for-profit corporation registered at slashdot.org?
What, you mean like the for-profit company registered at slashdot.org?
And Pluto was discovered by Percival Lowell, thus the "PL" symbol for the planet Pluto.
I seem to remember that Lowell used a standard refracting telescope which was something like 6m long. anyone got a link to a picture?
OK, Clearly you think that there's nothing more efficient than a rocket. Depends, of course on how you define efficiency. Modern rockets achieve very close to their theoretical efficiency, I think that there's no arguing with that. However other types of engine have a higher "Efficiency" than a rocket.
Consider the rocket equation. If we compare an airbreathing engine burning hydrogen with a rocket engine breathing hydrogen/Lox, then for the same delta V, and the same launch weight, (because we have the same exhaust velocity),
then if we assume the same combustion efficiency, the airbreathing engine will have launched four times the mass. So, for the same "Efficiency", we achieve four times the launch. Now, of course, you're right that there are a lot of factors which can reduce this margin, but the fact remains that a scramjet operating at 25% efficiency will have the same effect as a rocket launched at 100% efficiency.
I agree that scramjets are not currently competition for rockets. They may never be, but that doesn't mean that they shouldn't be investigated.
I would note that from 1980-1999 that the reliability for liquid rocket launches was 97.5%. Most of the failures are inherently due to constraints caused by the low superstructure weight requirements of rockets. If you think that it's acceptable that 2.5% of launches fail, forever, then that's your perogative. No other form of transportation finds this acceptable however, and I don't see that we should be content with it in the space industry.
"Shielding and structure have to be sufficient to withstand reentry anyway, so they should withstand the load peak during railgun acceleration. Or are we talking about a fundamentally different load pattern here?"
You're thinking re-usable SSTO. Nothing says scramjets are only for SSTO or reusable. Think of replacing the second stage on your favourite rocket with a scramjet, and you'll see one possibel option.
Clearly the rocket equiation is too difficult for you. Try this, and I say this in the honest hope that you will, not in the interests of a flame war.
Try a rocket with mass M and accelerate to Mach 25.
Look at the remaining mass.
Try a rocket with mass M/2, but thrust-for-carried-mass efficiency of 3 times the first rocket.
Look at the remaining mass.
Note that Mass2>Mass1.
This is the point of scramjets.
"Two, the structural weight needed to withstand pushing through atmosphere at extreme Mach (stand up to both drag and temperatures) outweighs what you need for tanks and LOX to do it the rocket way."
Sez you. Not a scientist, and not, quite frankly someone who has any idea. It's about the numbers. You don't know them. Those who do know (Including me, I might add), know that it's a close thing which is unclear, even within the scientific community.
"Think, there's a reason that we've used rockets for 50 years instead of scramjets -- it's easier."
Since when is easier a reason for stopping development?
"To make a practical, re-usable, passenger carrying scramjet we have to solve problems of designing a lightweight, temperature resistant vehicle that doesn't require a major overhaul between flights (that's the practical part), plus the problems of designing an effective, large-scale (compared to current test models) scram-jet engine. To make a practical, re-usable, passenger carrying rocket we have to solve problems of designing a lightweight, temperature resistant vehicle that doesn't require a major overhaul between flights (that's the practical part) -- but we already know how to make efficient, large-scale rocket engines.
Which seems easier?"
It'd be easier to stay at home, but that's not the point.
I'd point out that there's nothing saying that Scramjets:
1. must be in a lifting body
2. must be SSTO or
3. must be reusable.
In fact a lot of proposals have them being none of the above.
I think the question is: Would you like more efficient rockets? If yes, then airbreathing engines are one of the available options.
"We all look forward to that day -- but air-breathing jets are no more advanced than rockets, and in fact they're rather Rube Goldbergish. Using jets and wings to get to space is equivalent to the "horseless carriage" era of automobiles -- or worse. It's like trying to come up with a 100MPH, 200 HP vehicle by inventing a harness that will let you hook up 200 actual horses, because they can "forage for themselves" rather than just building in a gas tank and engine."
No, it's the equivalent of the car in the age of horses. Anybody looking at a horse, even today can see that its a superior engine to a car. better design, reliability, efficiency, and just plain better looking. But if you want to go faster that 30Mph, there's no way to train your horse to take you there.
Rockets have reached their limit. Just because their possible successors are not doing better than rockets now, doesn't mean that they won't eventually do better.
The point that your parent was making about scramjet was that there's no inherent reason why a scramjet must have higher drag than a rocket of the same frontal area. Scramjets don't push through the air any more than any other vehicle.
A railgun launch would be possible, but also has a lot of associated problems. You want a trajectory to orbit where the speed is increasing as the altitude is increasing (basically a constant Pitot pressure ascent). This means that the heat and pressure loads are basically constant over the flight. If you have a heat and pressure load peak, as will probably be caused by a high-speed rail launch, then you need extra structure and shielding, which will increase the flying weight of the vehicle.
Obviously there are a lot of people studying whether the trade off comes off positive or negative for the cost to orbit.
For the other readers, I'll add to this that the parent touches on several other points which have some bearing on scramjet research, particularly magnetohydrodynamic engines, and MHD control of combusting flow, both of which have been extensively researched, particularly in the Soviet Union. This is also some pretty great stuff.
I would note that there a a lot of methods of fuel mixing, ignition and flameholding other than thos mentioned in the linked article. This is a rich field, in which what the public sees is just the tip of the iceberg.
And in addition, all of this is without active cooling. No blowing into the boundary layer, circulation under the skin or other tricks. All in all, the heat question is the least of the problems of a hypersonic lifting body.
Yes, fuel is cheap, but a rocket is a mountain of fuel, and reliable, lightweight superstructure for a mountain is neither cheap nor easy.
The thing is that rockets are great, unless you want to improve reliability or efficiency from current levels, then you need something else.
The fact that they accelerate Hyperx with a rocket is rather beside the point. A lot of the early flights of carrier aircraft were made from ordinary landing fields. That doesn't in any way mean that they don't have the potential to be used in other ways.
This comment is almost completely wrong.
I'm also not a rocket scientist, I've been working in scramjet research for 7 years.
Rockets work well, but that doesn't mean that there's not something better out there. There are two main problems with rockets:
1. They've pretty much reached their peak efficiency.
2. They've pretty much reached their peak reliability.
As scientists, we're trying to improve the efficiency of our engines. With rockets, we're at the point where you won't get another 30% out. So you need something new. The comment about drag is correct, but the promise of airbreathing technology is that you lose an extra 30% in drag and gain 60% in efficiency (insert your own numbers where necessary).
In addition, the problem with rockets is that about 95% of the takeoff weight is fuel. That means not much superstructure, and so lower reliability than an aeroplane, for instance. Obviously, is the structure is such a low fraction of the takeoff weight, the gains to be made from composite materials are minimal, thus, again, the wish for a new engine.
Right sort of numbers, Hypersonic is different from supersonic in that in unseparated flow there are no subsonic regions, whereas supersonic flow is generally a mix of supersonic, transsonic and subsonic flow. In addition, there's a "hypersonic similarity law", which basically states that when you get above Mach 8, the effect of Mach number differences on the flow structure is minimal.
Interestingly, Mach 8 is also the boundary where if you decelerate the flow in your engine subsnic, that it is not possible to re-accelerate the combustion products enough to recover the losses from deceleration of the incoming air. Thus supersonic combustion is needed.
The current thinking is so:
Use turbojet stage for takeoff.
Bring in Ramjet stage at transsonic speeds, transitioning to full ramjet about Mach 1.5 to Mach 2.
Bring in Scramjet stage from Mach 3-4, transitioning to full scramjet at Mach 5-7.
Bring in Rocket stage at mach 10-12, transitioning to full rocket at Mach 14-16.
You see, that it's rare that any single stage is purely one thing or the other. Scramjets are not the solution to space travel. They're one piece of the puzzle. Reducing the cost of flight to space by 5% is something which would still be worthwhile, and airbreathing flight certainly has great promise to do far more than that.
The problem is that at the moment it's only that: promise. These tests are to see if we can turn promise into reality.
Not entirely true. Jet fuel b is a hydrocarbon mix of 4-16 carbon atom molecules. For a hydrocarbon with N carbon and 2N+2 Hydrogen, the oxygen required for full combustion is between N and 2N for the carbon and N+1 for the Hydrogen. The ratio of fuel to oxygen mass is between 1:2 and 1:3. Therefore by switching to airbreathing you'll save 60% to 75% on your takeoff weight (on the scramjet stage only of course).
My grandmother has osteoperosis. Would you buy her a segway? I think that the group of people who can't walk, but would not be severely injured by a fall from a segway is vanishingly small.
Actually, since if you're writing in code, even the "plaintext shouldn't be plaintext, that's not quite right. Figure you could also write T qik brn fx jump ovr Thu lzzy dg, and many such similar (Lets say about 5 opteions per word, for a possible 1.953 million options even for such a simple message. If its not required that every group makes a word, then the requirement is loosened even further. I would say that for a given group of letters and numbers, probably on the order of 10% of the arrangements could be deciphered to be some sort of message. Guess also that you may not know the language that the message is written in, or that there may be three or more languages in the same message, and that the message may also contained pre-arranged codegroups, and pretty much no group of letters can be free of suspicion.