Nuclear Airships

Sure their is, it's called carbon fiber. It's considerably more expensive than kevlar, and was developed later, which is why I didn't suggest it.
Carbon composites tend to be limited more by the matrix than by the fibers. The fabric has to be sealed somehow to prevent leakage, and that temperature sets the envelope limit even if carbon fibers itself may be good to much higher temperatures.

Also, I was using 10 kms altitude as my main reference point, where ambient temperatre is about 220 Kelvin. 440 K is only 167 C.
It's all well to fly at 10 km, but to be able to take off you have to have lift greater than weight at sea level, in sea level ambient air--unless you build the balloon at 10 km and never descend below that height.
 

Puzzle

Donor
Sure their is, it's called carbon fiber. It's considerably more expensive than kevlar, and was developed later, which is why I didn't suggest it.

The balloon that went around the world years ago was made of kevlar and carbon fiber.

Also, I was using 10 kms altitude as my main reference point, where ambient temperatre is about 220 Kelvin. 440 K is only 167 C.
I've been having a hard time finding thermal limits for carbon fiber fabrics, but several places have noted that while the carbon fiber itself is very resilient the resins used to bond it are much less so. Past that, the ship still has to rise through the lower atmosphere.
 
Air is 3 times denser at sea level than at 10 kms, so a vehicle that is neutrally buoyant at double ambient temperature at 10 kms would only need to be 4/3 ambient temperature to be neutrally buoyant at sea level. 4/3 x 300K = 400K = 130C.

I've noticed now that I screwed up some math in my example earlier, the numbers are only correct at 10 kilometres.
 

Puzzle

Donor
Air is 3 times denser at sea level than at 10 kms, so a vehicle that is neutrally buoyant at double ambient temperature at 10 kms would only need to be 4/3 ambient temperature to be neutrally buoyant at sea level. 4/3 x 300K = 400K = 130C.
You need the internal and external pressures to be equivalent though, or your balloon will collapse or explode. If you take a big bag of 10,000 m air down to sea level you've got 70 kPa acting against the envelope. To get that thinner air up to the correct pressure you need to triple its temperature.
 
You need the internal and external pressures to be equivalent though, or your balloon will collapse or explode. If you take a big bag of 10,000 m air down to sea level you've got 70 kPa acting against the envelope. To get that thinner air up to the correct pressure you need to triple its temperature.

That's not necessary at all, and if we did that we would be much to light and go rocketing up above 10 kms. The vehicle has to take in and expel air from the atmosphere.
 
What about material weakening with radiation? You may protect the crew by placing them far of the reactor but the part of the airship holding the reactor to the airship will suffer.

It's probably a very bad idea to not use unit shielding. Which will drive up the weight of the reactor - the XNJ140E didn't use it, it used a shadow shield - but will keep you from accidentally frying anyone who comes too close, along with eliminating most concerns about radiation degradation of materials outside of the core itself.

About the reactor itself, should air pass directly in contact of the hot fuel? or with a primary coolant system? It will completely change the type of reactor you need, the neutronic moderator, the type of fuel, his size.
If you want to go small size, submarine type reactor (Pressurised Water Reactor and Fast Neutron Reactor) are good, but you will have weight problem (primary coolant system are heavy, several tons of water, lead, pumps...). With gas cooled reactor you will be bigger, but a bit less heavy.
In the case of air going directly on the nuclear fuel... forget it, no sane country will allow a system like this flying above them, it will spread contamination all around.

"Primary coolant" systems - what ANP called an indirect cycle - actually have a better power/weight ratio if you use a decent coolant and factor in the shielding. Air isn't a very good coolant, so if your reactor is air-cooled, your fuel elements need a lot of surface area to transfer heat to the air. That in turn means your reactor needs to be relatively large, volumetrically. The weight of the system is dominated by the shielding - especially for unit shields, but this is true of shadow shields as well. Since the weight of the shielding is proportional to the surface area of the reactor, that in turn means an air-cooled reactor is relatively heavier compared to a system that uses a coolant like molten sodium or molten salt to transfer heat to a heat exchanger.

The downside of an indirect cycle is that it's technologically a lot more complicated.
 
"Helium is expensive, and leaks"

Not only that -- it's also non-renewable, and much more valuable in other applications such as MRI machines.
 
Here's the best previous discussion on the topic that I've found.

Interesting points include ballast being very important, and steam being considerably better than air at the same temperature.

Also, I think I underestimated the potential aerodynamic improvement possible over a sphere, while maintaining low surface area to volume ratio. These vehicles could potential go quite fast, although their acceleration would be horrible.
 
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With all that hot air the question of not cooking the crew becomes a concern.
Control cars, passenger as well as cargo holds would have to be hung below the envelope or heavily insulated but every ounce of insulation is one once less payload. So it looks like everything will be hung below the envelope.
 
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