Hydrogen-Powered Aircraft == Anti-Terrorist Device? 701
maladroit asks: "Today on NPR's Talk of the Nation/Science Friday , Harry Braun of the Phoenix Project said that a hydrogen-powered airplane would not have produced the fire and intense heat that brought down the World Trade Center towers. Is this true ? What are the other advantages and disadvantages of hydrogen fuel ? Details on the Phoenix Project's website are a bit sketchy, but I'm sure the Slashdot crowd has some answers (and Richard Dean Anderson jokes)." Sounds like a good theory, it doesn't account for the hostage aspect, but it would prevent the use of aircraft as cheap bombs. Would there be any drawbacks? How much would such a refit cost for your average commercial aircraft?
actually, you're right. (Score:5, Informative)
Please read up on these things before spouting retardedness.
- A.P.
It does have good points. (Score:5, Informative)
Hydrogen: Pros and Cons (Score:5, Informative)
to burn "up", i.e. to rise. The plane would be
fueled with liquid hydrogen at 20 degrees K
(only Helium liquifies at a lower temperature) and would evaporate quickly into a gas. Unlike the current JPx fuels, the hydrogen disipates rapidly and would stick to stuff and burn. The hydrogen would burn and disipate rapidly and
leave behind only those pre-existing materials which have been ignited.
One problem is that even liquid hydrogen is very light (very low density) and so requires very large tankage. The Shuttle's external fuel tank is mostly a hydrogen tank (something like 80% of the volume?) with a surprisingly small liquid
oxygen tank at the top. I have seen a liquid hydrogen bubble chamber being filled and marveled at the droplets of liquid hydrogen entering the chamber and just floating down (drifting really, not falling like water droplets do).
Re:Hindenburg (Score:3, Informative)
Re:Less Boom, Yes, but Safer? (Score:3, Informative)
each "cube" of the building was designed to withstand a certain amount of pressure, when the ones that were superheated colapsed, it increased the pressure on the lower cubes that they could not handle it, thus they collapsed, thats why the building fell straight down and not fall over when the plane hit
Well, ONE problem (Score:5, Informative)
It wasn't the jet fuel that rammed the plane all the way THROUGH tower two on live TV. It might not have burned hot enough to cause the tower collapses, but having hydrogen fuel wouldn't have made the planes bounce off the towers, either...
Challenger (Score:5, Informative)
Now, as to continued flame, that's a different matter. It is unlikely that the hydrogen would act as an effective fuel to continue the fire for much after the initial impact.
The fundamental energetics of hydrogen combustion suck compared to fossil fuel combustion.
Hydrogen comes into its own more in the context of things like fuel cells. I don't think that the high demands of take-off powering would be well met by fuel cells. Cars can take longer to accelerate on a highway for instance with less loss of functionality. Either the airplane gets off the ground by the end of the runway, or it doesn't. The ability to abort a landing and lift off again is an important safety consideration.
The reason the site is short on details is that anyone who can make hydrogen work better than fossil fuels will make billions in the first year. It's a fantasy for anything but fringe applications. (Compare the Motorola fuel cell story today. Even that is methane-based, not hydrogen.)
Looks like our journalist at NPR had to fill a slot by deadline and went with what he could get to fill it.
Dispelling a few misconceptions (Score:5, Informative)
1. Hydrogen is extremely explosive - Hydrogen is not *extremely* explosive. It can be explosive, but it needs a certain amount of oxygen in order to explode.
2. The Hindenburg explosion was caused by the hydrogen. - It is widely believe that the explosion was caused by the flammable fabric covering of the ill-fated airship.
3. Myth#1 is why we don't have Hydrogen-powered cars - Actually, the biggest problem is that hydrogen is, for lack of a better term, sparse. (Opposite of dense). It's difficult to package a sufficient amount of it in a reasonable volume. There is ongoing work to change this by combining it / embedding it in other materials or packages, i.e. Carbon nanotubes.
4. Hydrogen is hazardous flammable substance - Because of its being the lightest (least dense) gas, a hydrogen fire will bascially burn in an upward direction. In addition, the gas will dissipate quite rapidly - imagine what would happen if you 'spilled' some Helium - it would just float straight up, even if it was on fire. Hydrogen does the same.
5. The fire was not a significant part of the tower collapse - While the kinetic energy of a fully loaded 757 / 767 cannot be ignored, if that was *all* there was, the towers would be standing today, and probably repairable as well. The collapse was caused by the extremely hot (1500+ degree) fires burning long enough to weaken the steel structure. The beams were rated for 1 hour of fire resistance. They held for at least that long, and then gave way, causing the 6 million lb. floor to fall and begin the domino effect.
Re:Jet Fuel (Score:3, Informative)
Why do I know? I'm a general aviation pilot. As such, I'm pretty close to jets frequently, and I've asked the fueler monkeys.
On to the topic. I'm not sure why hydrogen isn't used for jets, other than the fact that it's a bitch to store and transport. I'd think it'd be an ideal fuel for just about everything, since in it's pure form the only burn by-product is water.
BTW, the Hindenberg got smoked mainly because of the aluminum-oxide paint on it's exterior surfaces. Think solid fuel rocket.
Re:Hydrogen burns (Score:2, Informative)
Re:low energy density (Score:1, Informative)
Hydrogen Exonerated in Hindenburg Disaster (Score:1, Informative)
The memory of the spectacular destruction of the Hindenburg airship affects people's perception of hydrogen and their acceptance of the gas as an energy source. The lighter-than-air craft burst into flame--in full view of a crowd of reporters and newsreel cameras--while landing in Lakehurst, New Jersey, U.S.A., on 6 May 1937. Hydrogen has long taken the blame for the disaster, which effectively ended travel by zeppelin.
But retired NASA [National Aeronautics and Space Administration] engineer and long-time hydrogen advocate Addison Bain, who has been conducting extensive research on the incident, concludes that hydrogen played no part in starting the Hindenburg fire. To learn what really happened 60 years ago, Bain used NASA's latest investigative techniques to analyze original wreckage from the Hindenburg; conducted interviews with the few remaining survivors and those who have detailed knowledge of the Hindenburg's construction; examined original film footage and other documentary evidence; and visited the airship's former mooring sites in Lakehurst and Akron, Ohio, U.S.A. The dramatic findings of his research were reported at the National Hydrogen Association's 8th Annual U.S. Hydrogen Meeting and are the subject of the cover story of the May 1997 issue of the Smithsonian Institution's Air and Space magazine, published in observance of the incident's 60th anniversary. (Bain also plans to publish a complete manuscript with all data as well as two books for the general public and young adults.)
Observations of the incident show evidence inconsistent with a hydrogen fire: (1) the Hindenburg did not explode, but burned very rapidly in omnidirectional patterns, (2) the 240-ton airship remained aloft and upright many seconds after the fire began, (3) falling pieces of fabric were aflame and not self-extinguishing, and (4) the very bright color of the flames was characteristic of a forest fire, not a hydrogen fire (hydrogen makes no visible flame). Also, no one smelled garlic, the scent of which had been added to the hydrogen to help detect a leak.
This colorized photograph of the Hindenburg airship as it burned gives several proofs to the theory that it was the extreme flammability of the fabric cover, not the hydrogen inside, which caused the disaster.
Bain's study uncovered two contributing factors: the prevailing atmospheric conditions and the unorthodox method of landing at Lakehurst. First, thunderstorms had come through the Lakehurst area that day; lightning could still be seen at the time of the Hindenburg's landing. Secondly, the airship made a "high" landing: the zeppelin was moored at a high altitude and winched down to the ground via landing lines dropped from the airship. This, in effect, created a ready-made ground-to-cloud electrical path in the highly charged atmosphere. This combination of factors could prompt severe corona activity on any airship. In fact, an eyewitness reported seeing a blue glow of electrical activity atop the ill-fated Hindenburg before the fire started, which is indicative of the extremely high temperatures typical of a corona discharge.
Bain's suspicions of the zeppelin's fabric covering were raised when he learned that a cellulose nitrate dope with powdered aluminum might have been used on the Hindenburg. Bain was able to obtain two 60-year-old fabric samples representative of those used on the airship. At the NASA Materials Science Laboratories at Kennedy Space Center, testing included chemical and physical analysis using the scanning electron microscope, X-ray energy dispersive spectroscopy, optical microscopy, infrared spectroscopy, and tests of flammability, electrostatics, conductivity, surface and volume resistivity, thermogravimetric analysis, and corona discharge exposure.
At the NASA lab, one of the fabric samples subjected to a flame propagation test burnt up in seconds, still volatile after six decades. The remaining sample was subjected to high-voltage electrical fields, replicating the atmospheric conditions surrounding the Hindenburg that fateful night. The electric arc burned a hole in the fabric; however, when the sample was mounted so it remained parallel to the arc (as the airship was), the fabric ignited and disappeared in seconds.
The Hindenburg fabric was found to be made of a cotton substrate with an aluminized cellulose acetate butyrate dopant. The observations of the fire listed above, in fact, are consistent with a huge aluminum fire. (The brightness of the space shuttle's rocket boosters are an example of aluminum-based combustion.) So, it was the extreme flammability of the Hindenburg's fabric envelope which caused the disaster and not the lifting gas inside.
Files examined at the Zeppelin Archive in Friedrichshafen, Germany, yielded final confirmation of Bain's theory. Several handwritten letters, when finally translated from German, corroborate what Bain uncovered. Wrote electrical engineer Otto Beyersdorff on 28 June 1937, "The actual cause of the fire was the extreme easy flammability of the covering material brought about by discharges of an electrostatic nature."
Re:Remember the Hindenburg? (Score:3, Informative)
It caught fire because of the way the airship was designed, and how it dissipated the electrostatic charge that built up on the outer skin.
The ship's skin was a series of panels that were stitched (not really, more like tied) together. When the landing lines are lowered, the charge in these panels are supposed to flow through the entire skin of the airship and down the landing lines.
Because of poor design/construction, some of these panels retained their electric charge because of poor contact with their surrounding panels. The voltage between the charged panels and the non-charged panels was great enough to produce a spark.
BUT... the spark did NOT ignite the hydrogen.
The spark ignited the SKIN.
Since the skin had to be reflective to reflect heat, the germans coated the skin with a mixture that contained aluminum oxide powder.
Sound familiar? Aluminum oxide powder is used as solid rocket booster propellant in the space shuttle.
It was the fire on the skin that ignited the hydrogen cells. Hydrogen burns clear, and is barely visible in daylight. The initial fire on the airship was orange-red.
- Ben
Re:low energy density (Score:3, Informative)
what is the weight difference between the two.
Interesting idea, but not practical. If you're still worried about planes flying into buildings (it's been used once, if they're smart they'll now switch tactics) I see your point, but this is liking saying they'll never use a truck bomb again because they used them before. If we leave them the opportunity, they or someone else will do it again.
actually, just putting in solid cockpit doors(and using them) would stop this.
the auto pilot would need to engage at a least a mile to have any real use. and thats only if the pilot is doing something that the uto pilot can recover from.
Re:Less Boom, Yes, but Safer? (Score:4, Informative)
This got an "insightful" moderation???
It takes more energy to split hydrogen from water, than you get back by running the fuel cell on the hydrogen. This makes sense in some situations, for example a solar-power application where you can build up a supply of hydrogen when the sun's shining and then convert it back to electricity at night. It's like a storage battery, and as far as commercial aviation is concerned it'd be about as useful as a cargo hold full of lead-acid batteries.
As for splitting the hydrogen out from a hydrocarbon as needed, well, wasn't the whole point of the exercise to get rid of the hydrocarbons so that they wouldn't cause high-temperature fires in a crash?
Maybe there's a compromise, like using methane to power the jet engines (probably don't even need a fuel cell). Methane is a lot more volatile than traditional jet fuel, so it might disperse more quickly in a crash situation [and no jokes about the smell please; pure methane is odorless]. However this probably isn't economically viable.
Hydrogen myths and facts (Score:3, Informative)
I don't know how feasible powering a plane with hydrogen is - I sort of follow hydrogen energy news and don't recall ever coming across any prototype jets or prop planes. I don't know that hydrogen could power a jet sufficiently. Storage methods (tanks etc.) are heavy, possibly too heavy for economical flight. I question whether this is a realistic scenario or just wild speculation.
The big problems with hydrogen are cost, lack of a production infrastructure, lack of a distribution infrastructure, difficulty of storage, and the unlikllihood of a widespread manufacture of any kind of hydrogen vehicle lacking resolution of all these other issues. Making a plane fly on hydrogen would certainly not be a simple "retrofit". This would be a transition from a liquid to a gasseous fuel with totally different combustion characteristics.
Hydrogen is clean to burn either chemically (fuel cell) or through combustion and simple (if not easy or necessarily efficient) to generate, and therefore may become a valid way to transform renewable forms of energy into a storable fuel, and to make energy from conventional fuels more efficeintly and cleanly. But I doubt very much it will be the fuel of choice in planes any time soon
Abestos would have helped too... (Score:3, Informative)
What really gets me is the possibility that abestos could have delayed the colapse for up to 4 hours longer. They stopped spraying asbestos in the buildings above the 64th floor becase NYC banned it. They were wetspraying, which was a technique used to eliminate asbestos from getting in the air. While we'll never know how long if any those building would have stayed up, the belief at the time was that asbestos would provide 4 hours longer before the girders melted, hopefully giving time to evacuate the building.
Asbestos, much like hydrogen has been demonized, somewhat unfairly. While there is no argument that it is not dangerous, there can be safe ways to utilize dangerous materials. Unfortunately people jump on these bandwagons too quickly to make informed decisions.
I remember when they removed asbestos from my elementary school, the teacher told us that dust from the ceiling tiles was asbestos, probably exactly what she thought. In actuality, it was normal dust, and the asbestos was covered by fiberglass and foil insulation, and was harmless, until they started scraping it all off that is...
Re:It's not only the fuel (Score:4, Informative)
The use of asbestos was banned (or cut back severely) after they had already insulated floors -10 through 70. That's why it is more dangerous on 70+.
Re:Hydrogen airplanes (Score:3, Informative)
Buildings eventually come down. Hopefully it's a controlled process after a long and useful life, to make way for a new building; or it may be due to natural or man-made disaster. When the inevitable end comes, you do not want a large building falling over sideways (IIRC, this was the objective of the WTC truck bomb several years ago); you want it to collapse in on itself.
So it's not a question of designing ot to collapse - it's designing how it will collapse when the time comes.
Re:It's not only the fuel (Score:3, Informative)
The upper floors are lighter than the lower floors because they do not need to support the whole building. They make them lighter by using fewer materials. Fewer materials means that the upper floors are weaker. Granted they are under a lighter load, but they are still weaker.
If you want to make them stronger, then you need to increase the weight. If you increase the weight, then you need to beef up the lower floors. This adds expense to the system.
The other issue is that fuel fires cannot be adequatly handled by a water based sprinkler system. WTC's fire suppression (and most other buildings) was never designed to suppress a liquid fuel fire. They are principally designed to suppress solid fuel fires like a filing cabinet or an electrical fire.
The building was not poorly designed. It survived long enough to allow 15000+ people to escape. Sure we learned some lessons, but I doubt that liquid fuel fire suppression is going to make it into the building codes.
Would asbestos have saved the day? I do not know. It may have given the occupants an extra 15 minutes to escape. But the main issue was not the fire itself, but the unanticipated fuel type.
Re:It's not only the fuel (Score:2, Informative)
Asbestos junk science [cryptome.org]
Basically, the fire was way too hot for asbestos to handle, even if it had been used. Nice try.
Here we go... (Score:4, Informative)
First, the planes would be using liquid hydrogen, not gaseous, so all the comments about needing to compress the gas, or contain the gas, or the gas not having as much energy as jet fuel need to read the damn article!
Secondly, there are about a zillion different opinions based on guesses and I didn't see a single person who was qualified (at least no one say why they were qualified) to say what would happen when the plane hit a building. It's all just a bunch of speculation.
Third, we know what caused the Hindenburg to explode. About half the messages are saying that it wasn't the hydrogen, it was the coating. It should also be noted that the Hindenburg was not filled with liquid H2 so the comparison again is not really valid.
What I'd like to see is someone who works with H2 in a liquid form to post their thoughts on what would happen based on their own observations and experiments with liquid H2. The closest I saw was one individual who talked about seeing the drops of H2 liquid just kind of floating downwards instead of just falling like water.
The other point is that I saw many posts saying how H2 contains a lot of energy, and others saying it contains very little. Logic would say that one of those is wrong. Once again, I would like to see someone who knows what they are talking about post something and include either a reference or a credential, or something to compare H2 and jet fuel in regards to energy density.
I guess this is slashdot, I'd just like to see more people that do know what they are talking about post and less people that think they know what they are talking about claiming that they do... But again, this is slashdot, so I cannot expect much different.
<-- end rant...
Re:Intermediate energy source (Score:4, Informative)
Not a chance (Score:3, Informative)
The simple answer is way the Hell too much. The airline industry (and the leasing industries associated with it) operate on very tight profit margins. Just look at how disasterous the last month has been for the airlines. They are NOT going to be paying to design, much less paying to install, an entirely new type of engine. The bill would be trememdous!
My father runs a small aircraft leasing business, and basically the industry, which already had a glut of aircraft, is looking at total and utter devestation. This is an industry where every time the government imposes new noise level limits for aircraft, firms go out of business by the dozens. His firm is small enough to find a way through it, and well diversified besides, but the firms that own most of the aircraft that the airlines fly, firms like GE Capital, are going to take huge hits. There simply will not be any money available for refits, and barely enough to meet government mandated maintainence checks, much less a project of this magnitude.
Re:Remember the Hindenburg? (Score:2, Informative)
Advantages of H2:
--being lighter than air, will rise upwards before it ignites,
--the combustion product of hydrogen is H2O, better known as water. None of the nasty smoke that kerosene, plastics and the like put out.
The reason you don't see it in cars is twofold. Fuel Cells are very expensive, and storage of a significant amount of H2 is difficult. It is either highly pressurized or stored in a heavy metallic matrix.
I suppose you can add to that that Hydrogen filling stations aren't on every street corner yet.
Michael
The Physics of Hydrogen (Score:5, Informative)
Two points: the Hindenburg burned because of the envelope, not the hydrogen (see the many posts explaining this), and the Hindenburg used diesel fuel for power, and hydrogen for lift.
> No, it wouldn't burn for a sustained time, like jet fuel did,
> but it would burn even more violently, hence causing more initial
> injuries. In fact, a more violent explosion mith have collapsed
> the towers right away, and those 10,000 or so folk wouldn't have
> had the chance to escape like they did.
Not likely a problem. Most of the experts consulted believe that the sustained fire is what caused the collapse of the buildings. As the videos show, the force of the initial explosions was mostly external anyway (remember that huge fireball?) so even if the force of the hydrogen was significantly more powerful (which it wouldn't have been, for reasons below), the extra force would simply have thrown debris farther, not done much more damage. It's very likely that if the planes used to hit the WTC towers had been hydrogen powered, they'd still be standing. In addition to not having burning jet fuel all over the building (which actually trapped people in the floors directly below the impact, by running down the stairwells), the force of the explosion would tend directly out along the impact vector, then upward. The rapid expansion of the hydrogen as it escapes the tanks tends to inhibit explosive force (increase in volume means decrease in temperature and pressure), so the extra force gained by the fact that hydrogen burns very efficiently is offset.
> Then there's the issue of storage... wouldn't high-pressure
> crtyogenic fuel tanks be prohibitively heavy for an aircraft?
Yes, they are, and that's why hydrogen fuel cells aren't more popular. Once that problem can be solved (materials scientists have been working on this for a long time, mostly for space vessels) hydrogen has a chance against fossil fuels, but not until then.
Virg
Hydrogen goes boom. (Score:1, Informative)
Re:Less Boom, Yes, but Safer? (Score:3, Informative)
When people talk about Hydrogen powered aircraft, they are talking about using rocket engines, not fuel cells. Using hydrogen as a fuel pretty much dictates that you need to use an oxidizer as well, as there is not enough free oxygen in the atmosphere to get the energy levels required for flight. Liquid Oxygen is nasty stuff; you don't want to get it anywhere near a flame.
Also, fuel cells don't have a high enough power-to-weight ratio to get an airplane off the ground. To move an airplane (or a car, for that matter) you need KINETIC energy. Fuel cells produce ELECTRICAL energy. In order to convert the electrical energy into kinetic energy, you need an electric moter. Electric motors are nowhere close to having the kind of efficiency needed for an aircraft engine.
Re:just floating down (Score:2, Informative)
Although I have never seen it or heard of it previously (it sounds awesome to watch), I believe it. Here are the two dominant reasons according to Physics:
1) The buoyant force of air. Air has a density of ~1kg/m^3. Displacing 1m^3 of air produces 9.8 Newtons of upward lifting force (about 2.2 lbs of force). If your 1m^3 balloon has a mass of less than 1kg, it floats. This is reason why hydrogen gas balloons/blimps float.
2) When falling, an object reaches it's terminal (maximum) velocity when the gravitation force pulling it down is equal to the viscous air resistance (and buoyant force) opposing it balance out.
Here's the math with "up" being positive:
@ terminal velocity:
Fnet = ma = -mg + Fbuoyant +Fviscous = 0
Fbuoyant = (1kg/m^2)Vg (on earth - V=volume of object)
Fviscous ~ bv (b= constant , v = speed of object (simple model)
bv = mg - (1kg/m^2)Vg = DVg - (1kg/m^2)Vg
= (D - (1kg/m^2))Vg
velocity = (D - (1kg/m^2))Vg/b
where D is the density of the object. So, assuming the same object shape (which results in the same "b"), the closer an object's Density gets to the atmospheric density (1kg/m^2 on earth), the slower it falls. If it drops below the atmospheric density, it floats.
Sorry for the lecture folks, I used to be a physics teacher.
Liquid Hydrogen is not Hydrazine (Score:3, Informative)
To make things more interesting, it's also highly toxic (hydrogen - h2 - is not).
Osha [osha-slc.gov] comments on it as:
1.1.2. Toxic effects (This Section is for information only and should not be taken as the basis of OSHA Policy.)
Hydrazine is a severe skin and mucous membrane irritant in humans; in animals, it is also a convulsant and a carcinogen. In humans, the vapor is immediately irritating to the nose and throat and causes dizziness and nausea; itching, burning, and swelling of the eyes develop over a period of several hours. Severe exposures of the eyes to the vapors causes temporary blindness lasting for about 24 hours. Recurrent exposure to hydrazine hydrate has been reported to cause contact dermatitis of the hands without systemic intoxication.
In humans, hydrazine is absorbed through the skin, by inhalation, and orally; systemic effects include weight loss, weakness, vomiting, excited behavior, and convulsions; the chief histologic findings are fatty degeneration of the liver and nephritis. (Ref. 5.6.)
Of course hydrogen still reacts well in the presence of oxygen, but unlike hydrazine, requires a spark or other catalyst to start cumbustion.
- The Ravnos
FreeSpiritMind.com [freespiritmind.com]
Re:Remember the Hindenburg? (Score:3, Informative)
Hydrogen Fuel Safety (Score:3, Informative)
One method of storing this is just to store it as a compressed gas. This is not a very efficient way to store it, and it will have considerable negative impact on the range and or cargo capacity of the aircraft. In other words, air travel would get a lot more expensive and more rare. Not likely to happen. This method would also be dangerous because a rupture of a compressed hydrogen storage tank would release a lot of energy just from the bursting of the pressurized tank. The hydrogen would then be mixed with the surrounding air in a potentially VERY explosive combination, much like an FAE bomb. It is unlikely that a large portion of the hydrogen cloud would have the right fuel/air mixture to explode, but even a small percentage would be a big explosion. In an open area a large portion of the hydrogen might escape without burning, but in an enclosed area like the WTC, a large portion of it would probably still have burned. No benefit to using this method, and a lot of negatives.
The Hydrogen could be stored in a metal hydride. Basically the hydrogen is "soaked up" into metal like water soaking up into a sponge. Amazingly you can get quite high hydrogen storage densities with this method, even higher than storing it as a compressed gas. It will be much less of a fire hazard than conventional jet fuel. The hydrogen will not come out of the metal-hydride "sponge" all at once; so even if there is a fire it will be a small but long fire instead of a big, quick one. This method will be even safer because of the fact that the planes will never leave the ground. Metal-hydride may give good storage densities for automobiles, but the fuel tanks would be way too heavy to use on an aircraft.
The third method is cryogenic storage, as either liquid or slush hydrogen. This method gets the best storage densities as hydrogen storage goes; but it is still a lot less dense than normal jet fuel. That means you still need much bigger fuel tanks to get the same range. This might not be more expensive (and might even be cheaper) because of possible engine improvements. But you have the problem of handling a cryogenic fuel, which adds to costs (and the possibility of a ground crew injury). Then you have the problem of where to put the fuel. There isn't enough room in the wings to put all the fuel there, like is done with normal jet fuel. One possibility is to put it in the fuselage, but that is VERY dangerous because you now have the double threat in a crash of killing the passengers with cryogenic hydrogen before they have a chance to be killed in the resulting fire. Putting the extra tanks out on the wings makes sense from a structures point of view because you have shorter load paths, and would get the cryogenic fuel somewhat further away from the passengers; but it still would not eliminate the fuel as a risk. Yes a puddle of liquid hydrogen WILL burn. As the liquid boils it mixes with the air, creating a flammable mixture. As the mixture over the puddle burns the heat increases the rate of boiling of the puddle. This is actually not too different from what happens when a puddle of non-cryogenic fuel burns. Will it be less of a fire hazard? Maybe. A hydrogen fire will not emit as much thermal radiation, which seems safer; but for the same reason it is invisible and therefore harder to fight (a problem that might be solved with trace impurities). In an open area a hydrogen fire will dissipate more quickly and cover less area, but that doesn't apply to the WTC case because it wasn't in an open area and hydrogen might have actually been worse because of the possibility of explosion instead of just fire. I also wonder what that high a volume of cryogenic hydrogen would have done to the steel structure upon impact; the huge temperature swings from ambient temperature to cryogenic to a hydrogen flame might have caused the collapse to happen sooner. In a normal crash that happens in an open area hydrogen is theoretically safer, but modern jet fuel is not as explosive as most people believe, thanks to evolutionary refinements in its composition and I have not seen any full up aircraft tests (such as have been performed with modern jet fuel) that assesses the added hazard of storing large volumes of cryogenic fuel in a passenger aircraft.
On the plus side, hydrogen powered aircraft could have smaller (possibly cheaper and more quiet) engines. They would not pollute as much (though they still generate NOx). Despite the extra tankage, the aircraft might even be lighter and cheaper. It is possible, therefore, that a fleet of hydrogen-powered airliners might be cheaper to buy and operate than a fleet of normal ones. Or at least it would be if you didn't have to factor in the capital cost of rebuilding the entire fuel production, fuel transport, and refueling infrastructure. But of course you do. If something (a huge terrorist campaign or a sudden shortage of oil) were to wipe out our current fuel infrastructure and we had to rebuild it from scratch, then we might want to look at hydrogen again. Until then it will take a revolution in fuel storage density, hydrogen production and transport technology, or some new super hydrogen-only super engine to justify junking a fuel infrastructure we have already paid for.
This is not a new idea. Hydrogen has been considered as an alternate fuel in airliners since at least the 1970s. There are good reasons why it has not been adopted.
Bottom line, for now liquid or slush hydrogen is the only practical storage method for large aircraft. Even then, the storage densities of Hydrogen suck. Fire hazards are safer, but it almost certainly would not have prevented the WTC collapse (it might even have hastened it). The added hazards of cryogenic fuel (especially if stored in the fuselage) may more than make up for the reduced fire hazard. Like so many other technologies, it offers the tantalizing potential for reduced costs; if only we didn't have an already-paid-for infrastructure that supports the current technology of choice... but we do. If you are building a scramjet then it is probably worth the effort to put up with the extra tankage and the cryogenics and the custom fuel infrastructure. If you are building a passenger jet, then you are just asking for more cost with only incremental benefits that have yet to be demonstrated in full up testing. If the gov't wants to help this along, they could have an X-plane program to demonstrate full up development of a hydrogen cargo plane or bomber (the engineering would be similar enough to a civilian airliner for lessons to carry over), then slam one of the planes into the ground in a simulated crash when the program is over to get data on the actual safety of large hydrogen powered aircraft. Until then, the technology will (rightly) lie dormant until something makes it more economically attractive (i.e. a more efficient use of resources).
References: Hawkins, W.M. and Brewer, G.D., "Alternate Fuels Make Better Airplanes: Let's Demonstrate Now," _Astronautics_and_Aeronautics_, Sept. 1979
Raymer, D.P., _Aircraft_Design:_A_Conceptual_Approach_, AIAA, 1992
Re:hydrogen (Score:2, Informative)
It is probably about 3,500 degrees, not 60,000. A 60,000 degree temperature is 6 TIMES HOTTER than the Sun, which is only at 10,000 on the surface, and where the corna gas is at 100,000 degrees it would take more than a cubic mile of that very thin plasma to warm a cup of coffee.
Hydrogen would be a VERY SAFE fuel, either as a liquid or a gas. Here is a url of a paper demonstrating the difference. A Hyrdrogen powered car is MUCH safer than a gasoline powered car. First, gasoline is much heavier than air, and a leak would result in flashback ignition, even if the ignition source were many many feet away. Secondly, Hydogen burns UP, not down. Even the liquid form vaporized into gas much lighter than air, so it won't accumulate or get trapped. The Challanger disaster was caused by the leak in the solid booster burning a hole in the liquid Hydrogen tank. Seventy seconds into the flight, just after maximum powerup, the tank ruptured, spewing liquid Hydrogen into a Mach 5 slipstream, instantly vaporizing and igniting the Hydrogen. The rupturing liquid Oxygen tank added to the malstrom, a factor that wouldn't be present in most Hydrogen fires related to cars or planes.
http://www.eren.doe.gov/hydrogen/pdfs/Swain_Fue
The USA and the rest of the Free World needs to began a "Manhatten Project" designed to covert our countries to Hydrogen. The best way is with Solwer Power Tower II installations. Excess power can be used to convert non potable water to Hydrogen , releasing the Oxygen to the atmosphere.
http://rhlx01.rz.fht-esslingen.de/projects/alt_
This technology is 'low tech' and could be built and maintained by craftsmen in any community.
Re:Liquid Hydrogen is not Hydrazine (Score:3, Informative)
The Space Shuttle Main Engines do use, as you put it, "pure liquid Hydrogen" and that is the fuel in the External Tank (along with liquid oxygen as an oxidizer).
Re:Liquid Hydrogen is not Hydrazine (Score:3, Informative)
Re:Biological warfare (Score:3, Informative)
Due to a combination of dilution and the effect of clorine, attempting biowarfare attacks against a water supply is not very effective.
But you are right, there are better methods. They tend to involve generating aeresols, usually with a moving or area source. That is the reason for the worry over cropdusters.
Notes on Hydrogen Planes (Score:3, Informative)
Virg