I'm not so sure about that. Gas turbines have a higher power density (kW/kg and kW/m^3), but I believe they have lower efficiency than diesels for automotive use.
Aircraft operating at high altitudes, where the incoming air is very cold, can be more efficient. That's because the work to compress the air to get it into the combustion chamber is proportional to volume, not mass, and cold air has a smaller volume. Also, I believe that small turbines are less efficient than large ones due to gap clearance (related to inefficiency) does not scale exactly.
Gas turbines used for electrical generation are more efficient, but I believe that's because they use the waste heat in a "combined cycle" to run a steam turbine or some other secondary generator.
Can someone who knows for sure chime in?
The article says "Transportation of the solar panels into space is too expensive at the moment to be commercially viable, so Japan has to figure out a way to lower costs," so the transportation costs cannot be included in the stated $21B figure, making it seem of little value. At first I was really impressed since $21 a watt is within striking distance of being economically competitive. (Fossil fuel powered plants cost in the vicinity of $5 per watt to build PLUS fuel costs. And any new technology tends to come down in price with experience.) Another possible problem: The article says the satellite "produces" one gigawatt, which may not be the same as receiving one gigawatt on the ground. Anyone know the answer to that question?
Making multiple backup copies protects against losing the secret (the root key in this case), but clearly increases the risk of theft.
Secret sharing is the way to backup and still be secure. In a "k out of n" secret sharing system, the secret is divided into n pieces, any k of which allow perfect reconstruction of the secret. What's amazing is that any k-1 tell absolutely nothing about the secret!
The easiest to understand is a k-1 out of k system. For example, taking k=5 and assuming the secret is 1000 bits long, the first four pieces of the secret are totally random bit strings, each 1000 bits long. The fifth piece is the XOR of the secret and these four strings. It's not hard to see that any four pieces tell nothing, but all five produce the secret when XORed together.
More complex k out of n systems are not too much harder to understand. For example, a 3 out of 5 system can be based on the coefficients (A,B,C) of a quadratic function y = Ax^2 + Bx + C. The coefficients can be determined by any three points (x,y) which lie on the graph. If C is the secret, and the 5 pieces of the secret are five points (x1,y1), (x2,y2), (x3,y3), (x4,y4) and (x5,y5) on the graph, then any 3 of them determine (A,B,C) and hence the secret C. But any 2 or less of them tell us absolutely nothing about C. Arithmetic is done in a finite field so that C is a bit string or similar.
Martin Hellman
http://www-ee.stanford.edu/~hellman/http://nuclearrisk.org/
FAA Advisory Circulars are just that - advisory and not mandatory. But, if deviance from that advice leads to accidents, especially fatalities, then they have a tendency to become mandatory regulations. Depending on where you guys are flying, the risk may be small or large, hopefully something your safety committee is taking into account.
As a pilot who has occasionally had to dodge birds and stray helium balloons, what worries me is the small, but non-zero chance that such an autonomous glider could collide with a manned (or womanned) aircraft. FAA Advisory Circular 91-57 recommends that model aircraft fly no higher than 400' and take other precautions not to interfere with full-scale aircraft. Given the length of his flights, I strongly suspect he is flying well above that.
"how pilots experienced in the area and are still alive know that these downdrafts can rip the wings off an airplane?"
In the incident I know best, the pilot parachuted and survived, though I believe he suffered significant injuries. See page 36 in
http://www.quovadis-aero.com/pdf_ext/2004_winning_on_the_wave.pdf
for a description. But that describes a very different day in terms of wind conditions compared to the day Fossett was killed.
The article referenced above is describing strong mountain wave when the winds were probably averaging 50-75 kts, with local gusts significantly higher. In contrast, the NTSB report indicates that the winds on Fossett's accident day were averaging less than 20 kts, with gusts (at the mountain top level) to almost 50 kts. That's a big difference in wind conditions and, as the NTSB reports, the accident was probably caused by a downdraft (possibly induced by a strong gust interacting with the terrain), not an in-flight breakup.
There's a page "How destructive would a failure of deterrence be?" at the web site which briefly treats both a partial failure like nuclear terrorism and complete failure:
http://nuclearrisk.org/2destructive.php
That page has links to references with more details, plus my paper accessible at
http://nuclearrisk.org/paper.pdf
has a longer section on the issue you raise. Mild warning: The paper is about 2 MB.
While I'm posting, I'll mention that a number of the posts here seem to miss a key point of my effort. It is not to get my preliminary analysis used as the basis for decision making, but rather as the basis for calling for more detailed studies to either confirm or correct my preliminary conclusion.
Hope this helps.
Martin Hellman
Slashdot Mirror
I'm not so sure about that. Gas turbines have a higher power density (kW/kg and kW/m^3), but I believe they have lower efficiency than diesels for automotive use. Aircraft operating at high altitudes, where the incoming air is very cold, can be more efficient. That's because the work to compress the air to get it into the combustion chamber is proportional to volume, not mass, and cold air has a smaller volume. Also, I believe that small turbines are less efficient than large ones due to gap clearance (related to inefficiency) does not scale exactly. Gas turbines used for electrical generation are more efficient, but I believe that's because they use the waste heat in a "combined cycle" to run a steam turbine or some other secondary generator. Can someone who knows for sure chime in?
The article says "Transportation of the solar panels into space is too expensive at the moment to be commercially viable, so Japan has to figure out a way to lower costs," so the transportation costs cannot be included in the stated $21B figure, making it seem of little value. At first I was really impressed since $21 a watt is within striking distance of being economically competitive. (Fossil fuel powered plants cost in the vicinity of $5 per watt to build PLUS fuel costs. And any new technology tends to come down in price with experience.) Another possible problem: The article says the satellite "produces" one gigawatt, which may not be the same as receiving one gigawatt on the ground. Anyone know the answer to that question?
Making multiple backup copies protects against losing the secret (the root key in this case), but clearly increases the risk of theft. Secret sharing is the way to backup and still be secure. In a "k out of n" secret sharing system, the secret is divided into n pieces, any k of which allow perfect reconstruction of the secret. What's amazing is that any k-1 tell absolutely nothing about the secret! The easiest to understand is a k-1 out of k system. For example, taking k=5 and assuming the secret is 1000 bits long, the first four pieces of the secret are totally random bit strings, each 1000 bits long. The fifth piece is the XOR of the secret and these four strings. It's not hard to see that any four pieces tell nothing, but all five produce the secret when XORed together. More complex k out of n systems are not too much harder to understand. For example, a 3 out of 5 system can be based on the coefficients (A,B,C) of a quadratic function y = Ax^2 + Bx + C. The coefficients can be determined by any three points (x,y) which lie on the graph. If C is the secret, and the 5 pieces of the secret are five points (x1,y1), (x2,y2), (x3,y3), (x4,y4) and (x5,y5) on the graph, then any 3 of them determine (A,B,C) and hence the secret C. But any 2 or less of them tell us absolutely nothing about C. Arithmetic is done in a finite field so that C is a bit string or similar. Martin Hellman http://www-ee.stanford.edu/~hellman/ http://nuclearrisk.org/
FAA Advisory Circulars are just that - advisory and not mandatory. But, if deviance from that advice leads to accidents, especially fatalities, then they have a tendency to become mandatory regulations. Depending on where you guys are flying, the risk may be small or large, hopefully something your safety committee is taking into account.
As a pilot who has occasionally had to dodge birds and stray helium balloons, what worries me is the small, but non-zero chance that such an autonomous glider could collide with a manned (or womanned) aircraft. FAA Advisory Circular 91-57 recommends that model aircraft fly no higher than 400' and take other precautions not to interfere with full-scale aircraft. Given the length of his flights, I strongly suspect he is flying well above that.
"how pilots experienced in the area and are still alive know that these downdrafts can rip the wings off an airplane?" In the incident I know best, the pilot parachuted and survived, though I believe he suffered significant injuries. See page 36 in http://www.quovadis-aero.com/pdf_ext/2004_winning_on_the_wave.pdf for a description. But that describes a very different day in terms of wind conditions compared to the day Fossett was killed. The article referenced above is describing strong mountain wave when the winds were probably averaging 50-75 kts, with local gusts significantly higher. In contrast, the NTSB report indicates that the winds on Fossett's accident day were averaging less than 20 kts, with gusts (at the mountain top level) to almost 50 kts. That's a big difference in wind conditions and, as the NTSB reports, the accident was probably caused by a downdraft (possibly induced by a strong gust interacting with the terrain), not an in-flight breakup.
There's a page "How destructive would a failure of deterrence be?" at the web site which briefly treats both a partial failure like nuclear terrorism and complete failure: http://nuclearrisk.org/2destructive.php That page has links to references with more details, plus my paper accessible at http://nuclearrisk.org/paper.pdf has a longer section on the issue you raise. Mild warning: The paper is about 2 MB. While I'm posting, I'll mention that a number of the posts here seem to miss a key point of my effort. It is not to get my preliminary analysis used as the basis for decision making, but rather as the basis for calling for more detailed studies to either confirm or correct my preliminary conclusion. Hope this helps. Martin Hellman