Post by Glom on Jul 8, 2005 6:54:08 GMT -4
To build a nuclear powered high bypass turbofan. I've been discussing this elsewhere, but I thought I summarise my thoughts here because there are smart people here.
The most obvious way to do it is the replace the gas turbine core with a fission core that drives the LP fan directly. The fan pushes some air through the core for cooling purposes and the bulk through the bypass generating the thrust.
Shielding and containment
This is obviously a critical issue. Most people react to the idea of a nuclear airliner by saying that a crash would be a repeat of Chernobyl. But if any nuclear powered airliner were to take to the air, it would obviously be sufficiently protected.
One critical need is a material capable of shielding against neutrons and gamma rays without being too bulky. I don't know of any material capable of doing the job at this time so we might have a showstopper. The key is probably in maritime technology, with which I'm not as familiar.
Naturally, the fission must be encased sufficiently to withstand a worst case scenario without being too heavy or bulky. I don't know where we're at with such technology, but I don't think it would be this would be the most troublesome problem. Basically, we need to put it in a black box. The fact that this fission core sits in the middle of the engine, is an advantage because any impact event will impact the surrounding structure first serving as a kind of protection like the crumple zone in a car.
Deadman's switch and throttling
Any fission core would have to be an accelerator driven system in my opinion. Control rods are failsafe in generating stations because of gravity. But while it is reasonable to assume a generating station would never find itself upside down, the same assumption can't be made with an aircraft even if it's not a normal mode of flight. Control rods are therefore not a failsafe way of controlling the reaction. An ADS depends on a proton beam producing spallation neutrons to maintain fission in a reactor that is subcritical. If anything goes wrong, the accelerator fails and the reactor can no longer sustain the reaction. This is the deadman's switch.
This, combined with the necessary containment measures would eliminate any terrorist threat and make aircraft a darn sight safer than now without that chemical fuel they must carry.
The ADS would also allow for more rapid throttling assuming the accelerator is so readily controllable. It's an improvement of the old control rod system, but I admit that I'm not sure if it would be enough.
Fuel
Obviously it is highly important that fuel and waste remain contained. I have no doubt that HTR technology would be applied to ensure that the fuel and waste would be safely locked up in the impermeable blocks or spheres not to mention quelling any proliferation concerns (although if you are in charge of large airliners, I think you already have all the weapons of mass destruction you will ever need). This will keep the coolant clean and make the fuel easily changeable. The other advantage is negative temperature coefficient.
The flaw is that I believe HTR technology requires more space than conventional fuel rods and as such it might be too bulky.
Cooling
One word: helium. It's the obvious choice. It doesn't activate, so combined with HTR fuel technology, it will remain as clean as the day it loaded into the engine. It's also lightweight and uses high temperatures for good thermodynamic efficiency (probably greater than that of current gas turbines), which is not applicable thanks to advances in metallurgy. I believe the engine would operate through the Brayton cycle.
The flaw is that I'm not sure the helium could be allowed to return to the original state in space available. If it couldn't, it would have to be ejected and fresh helium introduced, which would turn it into a consumable propellent, which would be no good. In order for this to work, the helium must be able to operate through the complete Brayton cycle. That's fine in a large generating station, but I'm not sure about the confines of an engine.
Of course, cooling will be made possible by the airflow while a small fraction of the air passing through the core picking up the waste heat and exitting out the back, generating a bit of thrust thanks to it high speed. In this way, we have a similar operation to conventional high bypass turbofans where a small amount of thrust is generating from the hot gas exitting from the core while the bulk is derived from the cooler gas that went through the bypass.
If at any point increased cooling is needed, for example during takeoff when the engine is at high power and low speed, additional cooling measures can be made. Water cooling is obvious but perhaps a better idea might be something along the lines of what I mentioned earlier and dumping hot helium overboard. This obviously is out of the question for normal operation, but if it was only necessary for brief periods, it could work provided air cooling is sufficient during cruise. I'm thinking something analogous to the operation of the afterburners on Concorde. Using them throughout the entire flight would mean they wouldn't be able to get much further Ireland, but using it just for critical stage like takeoff and acceleration through the sound barrier worked fine.
Advantages and limitations
Obviously the concept of aircraft range will disappear since an aircraft will have all the range they would ever need. We wouldn't have a problem with air quality and fuel costs wouldn't be such an issue to airlines. Aircraft would be safer because they would no longer be carrying thousands of pounds of highly flammable fuel and instead a properly contained reactor.
There are some problems to overcome though:
The most obvious way to do it is the replace the gas turbine core with a fission core that drives the LP fan directly. The fan pushes some air through the core for cooling purposes and the bulk through the bypass generating the thrust.
Shielding and containment
This is obviously a critical issue. Most people react to the idea of a nuclear airliner by saying that a crash would be a repeat of Chernobyl. But if any nuclear powered airliner were to take to the air, it would obviously be sufficiently protected.
One critical need is a material capable of shielding against neutrons and gamma rays without being too bulky. I don't know of any material capable of doing the job at this time so we might have a showstopper. The key is probably in maritime technology, with which I'm not as familiar.
Naturally, the fission must be encased sufficiently to withstand a worst case scenario without being too heavy or bulky. I don't know where we're at with such technology, but I don't think it would be this would be the most troublesome problem. Basically, we need to put it in a black box. The fact that this fission core sits in the middle of the engine, is an advantage because any impact event will impact the surrounding structure first serving as a kind of protection like the crumple zone in a car.
Deadman's switch and throttling
Any fission core would have to be an accelerator driven system in my opinion. Control rods are failsafe in generating stations because of gravity. But while it is reasonable to assume a generating station would never find itself upside down, the same assumption can't be made with an aircraft even if it's not a normal mode of flight. Control rods are therefore not a failsafe way of controlling the reaction. An ADS depends on a proton beam producing spallation neutrons to maintain fission in a reactor that is subcritical. If anything goes wrong, the accelerator fails and the reactor can no longer sustain the reaction. This is the deadman's switch.
This, combined with the necessary containment measures would eliminate any terrorist threat and make aircraft a darn sight safer than now without that chemical fuel they must carry.
The ADS would also allow for more rapid throttling assuming the accelerator is so readily controllable. It's an improvement of the old control rod system, but I admit that I'm not sure if it would be enough.
Fuel
Obviously it is highly important that fuel and waste remain contained. I have no doubt that HTR technology would be applied to ensure that the fuel and waste would be safely locked up in the impermeable blocks or spheres not to mention quelling any proliferation concerns (although if you are in charge of large airliners, I think you already have all the weapons of mass destruction you will ever need). This will keep the coolant clean and make the fuel easily changeable. The other advantage is negative temperature coefficient.
The flaw is that I believe HTR technology requires more space than conventional fuel rods and as such it might be too bulky.
Cooling
One word: helium. It's the obvious choice. It doesn't activate, so combined with HTR fuel technology, it will remain as clean as the day it loaded into the engine. It's also lightweight and uses high temperatures for good thermodynamic efficiency (probably greater than that of current gas turbines), which is not applicable thanks to advances in metallurgy. I believe the engine would operate through the Brayton cycle.
The flaw is that I'm not sure the helium could be allowed to return to the original state in space available. If it couldn't, it would have to be ejected and fresh helium introduced, which would turn it into a consumable propellent, which would be no good. In order for this to work, the helium must be able to operate through the complete Brayton cycle. That's fine in a large generating station, but I'm not sure about the confines of an engine.
Of course, cooling will be made possible by the airflow while a small fraction of the air passing through the core picking up the waste heat and exitting out the back, generating a bit of thrust thanks to it high speed. In this way, we have a similar operation to conventional high bypass turbofans where a small amount of thrust is generating from the hot gas exitting from the core while the bulk is derived from the cooler gas that went through the bypass.
If at any point increased cooling is needed, for example during takeoff when the engine is at high power and low speed, additional cooling measures can be made. Water cooling is obvious but perhaps a better idea might be something along the lines of what I mentioned earlier and dumping hot helium overboard. This obviously is out of the question for normal operation, but if it was only necessary for brief periods, it could work provided air cooling is sufficient during cruise. I'm thinking something analogous to the operation of the afterburners on Concorde. Using them throughout the entire flight would mean they wouldn't be able to get much further Ireland, but using it just for critical stage like takeoff and acceleration through the sound barrier worked fine.
Advantages and limitations
Obviously the concept of aircraft range will disappear since an aircraft will have all the range they would ever need. We wouldn't have a problem with air quality and fuel costs wouldn't be such an issue to airlines. Aircraft would be safer because they would no longer be carrying thousands of pounds of highly flammable fuel and instead a properly contained reactor.
There are some problems to overcome though:
- Can a suitable method of shielding be developed?
- Can the fission core be properly contained?
- Can a reactor work on this small scale?
- Can enough useable power be generated from such a small reactor?
- Can the Brayton cycle be closed?
- Can the ADS provide sufficiently rapid throttling?
- Can this be prevented from turning into a maintenance nightmare, particularly given that no doubt 4 engines 4 long haul would return if not for power limitations on a single engine but for the fact that ETOPS was only possible due to confidence in proven technology?
