Nuclear Airships

[Panhomo]

Lighter than air vehicles (LTAs), blimps, hot air balloons, dirigibles, airships, have a number of advantages over heavier than air vehicles (HTAs), airplanes, helicopters:

HTAs generate lift over their surface, but weight over their volume, and so are only efficient at relatively small sizes, and can only lift relatively small loads. LTAs, like most other machines, become more efficient at larger sizes.

Lifting surfaces only work while in particular motion with respect to the air, which means HTAs continuously produce drag, and must work to counteract it. HTAs are either unable to remain in one position, or their lifting surfaces must move relative to the vehicle, which adds complications and inefficiencies. LTAs may remain still with respect to the air, or move in any direction.

It seems to me that LTAs chief disadvantage is its lifting gas. LTAs function by having a volume of low density gas which offsets the density of the rest of the vehicle. An ambient temperature gas can be used, but the options are limited. Helium is expensive, and leaks, hydrogen is dangerously reactive, and slowly leaks, the other possible gasses aren't much lighter than air. The other possibility is to use hot air, but hot air cools down, and so must be continuously reheated. Heating the air by burning a fuel requires taking a store of fuel with you, which adds weight. The buoyancy of hot air is proportional to its temperature, but the rate it looses heat is proportional to the temperature to the fourth power, which limits conventional hot LTAs to either have a very high proportion of their volume dedicated to the lifting gas, or to operate for a short time.

I say conventional, because all hot LTAs have heated themselves by burning a chemical fuel. Nuclear reactors are by far the best heaters humanity possesses, a gram of natural uranium contains as much stored energy as two million grams, two tons, of jet fuel. Even given that a more massive engine is needed to burn a gram of uranium than a gram of kerosene, a nuclear powered hot air balloon could operate with the same size balloon as a kerosene burner, with temperatures 30 times higher, and lift 30 times as much mass for the same period, or lift the same mass for a million times longer.

It seems to me then, that the invention of nuclear reactors could have, and perhaps should have, kept LTAs alive. Imagine cargo-airships, traveling at 200 kms per hour 10 kms above the earth, unconstrained to canals or sea ports. Imagine an aircraft carrier floating at 20 kms, high enough to not need a takeoff ramp, the jets can just be dropped. Imagine when launching rockets to space we start above 90% of the earth's slowing atmosphere.

Do we have any airship experts on this forum? How plausible is a nuclear airship, for peace or war? Surely some other people must have considered this besides the half dozen mentions I can find online?

 

[King Augeas]

Weeelllllll... the Hindenburg had a lifting capability of about 215 t and a deadweight of about 118 t, apparently.
The nuclear B-36 thing had an 11 t cockpit and a 16 t 1 MW reactor. So put them in the Hindenburg and you'd have about 75 t left over of lift.

But... that's for hydrogen, and we're using hot air, meaning that we'd need four times the volume of hot air to get the same lift. Er, so we only have lift of 55 t or so, against a deadweight of 100 t or so. So it just sits there.

 

[James Ricker]

Lighter reactor technology is needed, but cutting down on shielding is a bad idea.
A small reactor used primarily for heating the air might work.
A composite frame and plastic fittings would drastically reduce weight. Possibly making a Hindenburg size airship feasible.
But not everyone will be happy about having a nuclear reactor flying over their heads.

 

[Mad Bad Rabbit]

http://www.historynet.com/atomic-airships.htm

 

[Panhomo]

Weeelllllll... the Hindenburg had a lifting capability of about 215 t and a deadweight of about 118 t, apparently.
The nuclear B-36 thing had an 11 t cockpit and a 16 t 1 MW reactor. So put them in the Hindenburg and you'd have about 75 t left over of lift.

Why did you put that particular reactor in the Hindenburg? Did you do some calculation as to what power the nuclear Hindenburg needs?

But... that's for hydrogen, and we're using hot air, meaning that we'd need four times the volume of hot air to get the same lift. Er, so we only have lift of 55 t or so, against a deadweight of 100 t or so. So it just sits there.

Air is 13.6 times denser than hydrogen, so you only need four times the volume if your air is 3.4 times as hot as the ambient temperature. if you heat the air to 14 times ambient temperature then you lift better than hydrogen, at least until your vehicle melts.

These vehicles get dramatically better as they get larger.

The volume scales as the cube of the radius of the balloon.
The mass you can lift scales as the cube, which means the installed power, stored fuel, etc. also can also scale as the cube.

The cost of your balloon scales closer to the square of the radius.
The mass of the skin of the balloon goes as the square.
The drag is also proportional to the square of the radius.
The heat loss also goes as the square.

The cost and mass of some other parts scales somewhere between the square and the cube, ie no matter how large your vehicle the steering wheel will be similar in size, weight, and cost.

A vehicle that is twice as wide in any dimension will be about 4 times as expensive, but can lift about twice as much.

Similarly the mass of a nuclear reactor doesn't increase as fast as its power output; a 100 MWt reactor isn't 10 times heavier than a 10MWt reactor. (MWt is Mega-Watt-thermal, great because we get about 60% of that as electrical power, and we get nearly 100% to heat our air)

So I was thinking quite a bit bigger.

I've figured out some numbers for a vehicle limited by heat loss and the power generation.

A roughly spherical "balloon", with the bulk of the mass situated in a structure fore and aft, with smaller structures around the "equator" and on each pole. Insulated to lose heat at roughly 1 tenth an ideal blackbody emitter, powered by twin 500MWt reactors, weighing 5000 tons each (including shielding and fuel). Air inside is heated to twice ambient temperature, giving a density half ambient. The ideal size for this balloon is about 500 metres radius. It has a volume of half a cubic kilometre. The ballon will have 3 square kilometers of skin, which made of kevlar and an insulating foil coating might weigh 10000 tons. At 10 kms altitude it can lift 90 000 tons. At 1 km altitude it can lift half a million tons. If the balloon skin and reactors make up half the vehicles total mass then it will have a flight ceiling around 15 kms.

At 10 kms it has a maximum speed of about 70 kph. At 15 kms it can go 90 kph, near sea level 45 kph.

It might need to refuel once every decade or two.

 

[SsgtC]

Why did you put that particular reactor in the Hindenburg? Did you do some calculation as to what power the nuclear Hindenburg needs?

I believe it's because that was one of the only air cooled reactors in history, if not the only one. Every other design requires either pressurised water or liquid metal

 

[King Augeas]

I used it because it was light. I've no idea what power you need!
Lift of various gases I took from here - http://www.heidas.de/docs/HeiDAS_AIAA2003.pdf
More heat equals more lift, of course, but those were the figures available.

 

[asnys]

The NB-36 reactor is not really comparable. It was used exclusively as a radiation source, to test shielding technology and radiation propagation modeling. It was not intended for producing power, so its power density was correspondingly pretty sucky.

What you want is one of the planned propulsion reactors from the Aircraft Nuclear Propulsion Program, which the NB-36 was built for. The two main options are the Direct-Cycle Engines developed by GE - three of which were actually static-tested, though none flew - and the Indirect-Cycle Engines developed by Pratt & Whitney. The Direct-Cycle Engines used uranium oxide fuel elements and beryllia moderator and heated air by passing it directly through the reactor core. The Indirect-Cycle Engines were cooled by liquid sodium, and the heat transferred to a heat exchanger in the engine. Unfortunately, there's not much information about the indirect-cycle out there, but in the long run they were considered the superior technology because they could achieve a higher volumetric power density, which translated to lighter shielding and therefore higher power/weight.

The designs of these engines were in flux throughout the lifespan of the program, so there's no one number for the power and weight. But the XNJ140E is representative of the later Direct-Cycle designs, producing 120 MWth at a weight of 30 tons, with an exhaust temperature of 2500 F - though note that I'm not sure how much of the shielding that includes.

 

[riggerrob]

Good idea about using the nuclear reactor to heat the air. A nuclear reactor would be able to rapidly heat the envelope. To rapidly cool the envelope: vent hit air and pump in some cool air. Rapidly cooling the envelope - after landing - drastically reduces the risk of damage during wind storms. Cooling and collapsing the envelope also greatly reduces hangar fees. Collapsing would probably require winches tugging on internal support wires.
Structurally I am envisioning a semi-rigid airship with a stiff keep.

Since radiation risk reduces with the square of the distance from the source, the key to reducing crew casualties is installing the reactor as far as possible from the crew. Perhaps install the reactor near the tail and keep the crew near the bow. Space cargo amidships to balance.

 

[Panhomo]

Good idea about using the nuclear reactor to heat the air. A nuclear reactor would be able to rapidly heat the envelope. To rapidly cool the envelope: vent hit air and pump in some cool air. Rapidly cooling the envelope - after landing - drastically reduces the risk of damage during wind storms. Cooling and collapsing the envelope also greatly reduces hangar fees. Collapsing would probably require winches tugging on internal support wires.
Structurally I am envisioning a semi-rigid airship with a stiff keep.

I'm not sure winches and wires would be necessary. The envelope could have creases or divots in it to provide some rigidity and direct the expansion and contraction, gravity would also help. Sticking the thing in a hanger wouldn't be necessary very often, one of the chief advantages of nuclear power is the ability to operate continuously for long periods. Floating above the weather might be more cost effective than harbouring in a hanger.

Since radiation risk reduces with the square of the distance from the source, the key to reducing crew casualties is installing the reactor as far as possible from the crew. Perhaps install the reactor near the tail and keep the crew near the bow. Space cargo amidships to balance.

Their's another advantage to size. Airships as among the largest manmade structures, a kilometer long, would be incredibly cool and seems almost plausible.

 

[Puzzle]

One thing about doubling in temperature to remember is that you’re doubling in kelvin, so you’re going to need really hot gas pretty quickly. Most elastomers won’t take the heat needed.

 

[e of pi]

One thing about doubling in temperature to remember is that you’re doubling in kelvin, so you’re going to need really hot gas pretty quickly. Most elastomers won’t take the heat needed.

Yeah, that's what I was thinking too. "Room temperature" ambient air at 70F/21C is about 294 K, so doubled temperature would be ~590 K, which is 316 C or just shy of 600 F. That's pretty hot for most materials, especially for only getting you a 50% decrease in density over ambient.

 

[riggerrob]

Like most hot air balloons, a nuclear dirigible would be temperature limited. Just hang thermometers inside the envelope and attach them to a thermostat near the reactor. Sure, hot air exiting the reactor might be 316 degrees Celcious, but by the time it has mixed with air already inside the envelope, it would cool to a temperature that would not melt the envelope.

 

[e of pi]

Like most hot air balloons, a nuclear dirigible would be temperature limited. Just hang thermometers inside the envelope and attach them to a thermostat near the reactor. Sure, hot air exiting the reactor might be 316 degrees Celcious, but by the time it has mixed with air already inside the envelope, it would cool to a temperature that would not melt the envelope.

Exactly, but it's the mixed temperature inside the envelope that sets the balloon's buoyancy and lift capacity. Being limited to more reasonable envelope temperatures will limit the balloon's lift much faster than thermal energy source.

 

[Puzzle]

it would cool to a temperature that would not melt the envelope.

But if it cools then it doesn't have the lifting force needed.

That's pretty hot for most materials, especially for only getting you a 50% decrease in density over ambient.

Yeah, Mylar melts at 260°C, Kevlar melts at 500°C, so you can only drop the density by a factor of less than three and I'm sure they weaken as they get close to their melting point. Maybe there's some other exotic compound that's lightweight, strong, and also capable of taking a thousand degrees or so, but I sort of doubt it. I think a better approach might be to try to keep some sort of cool air layer entrapped near the walls, but that would be a pretty fun CFD problem.

I wonder if Nitrogen would be a better base, it's a bit lighter than normal air, and it's not too hard to get.

 

[e of pi]

I wonder if Nitrogen would be a better base, it's a bit lighter than normal air, and it's not too hard to get.

Not all that much lighter--molecular weight of 28 for N2 vs. 28.97 for dry air--only about a 3.6% savings in density.

 

[Puzzle]

Not all that much lighter--molecular weight of 28 for N2 vs. 28.97 for dry air--only about a 3.6% savings in density.

Yeah, but unlike hydrogen it's not flammable and there's tons of it everywhere. Also with the temperatures needed to get the envelope that hot getting rid of oxygen in close proximity to the reactor might be worth the hassle.

 

[Panhomo]

Yeah, Mylar melts at 260°C, Kevlar melts at 500°C, so you can only drop the density by a factor of less than three and I'm sure they weaken as they get close to their melting point. Maybe there's some other exotic compound that's lightweight, strong, and also capable of taking a thousand degrees or so, but I sort of doubt it.

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.

 

[urielventis]

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.

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.

 

[Archibald]

Welcome back @e of pi

Nuclear zeppelins are just like nuclear battleships, or nuclear bombers. Such a very high coolness factor.