Cutaway diagram of a fictional nuclear-powered airship, labelled in Cyrillic text. The title reads АТОМНЫЙ ВЛАСТНТЕЛЬ НЕБА, which translates to something like 'atomic ruler of the sky'.

This image, of a nuclear-powered airship presumably from some Soviet magazine, came across my dash. The title reads АТОМНЫЙ ВЛАСТИТЕЛЬ НЕБА, which seems to translate roughly to 'atomic ruler of the sky'. It inspired some questions...

While migrating this post to Jekyll, I found a different version of the picture, labelled in English:

The same airship image, this time labelled in English, with a dodgy photo backdrop printed behind it. The labels are rather vague, focussing more on how different facilities might be used by the crew than details of how the airship could fly.

I have so many questions!

For example, in this cutaway most of the zeppelin seems to be full of machinery and stuff? But like, most of the volume should be full of lifting gas! So I’m not quite sure how this works.

This is especially the case since… nuclear reactors are extremely heavy things for their size. Particularly because you need to shield it in all directions so your zeppelin doesn’t get irradiated, causing cancer in your crew and slowly accumulating structural weakness in the zeppelin.

And like… zeppelins need a huge amount of lifting gas! like have a look at this cutaway of the Hindenburg (or the technical drawings here):

Cutaway diagram of the Hindenburg, showing how much of the airship is lifting gas.

The actual cabins take up only a tiny space at the bottom of the airship (the gondola isn’t in this picture, but that was pretty small too).

According to airships.net

The actual lifting ability of each gas varies with temperature, pressure, and humidity, and to take account of varying atmospheric conditions and gas impurities airship designers often conservatively estimated helium’s lift at 60 lbs per 1,000 cubic feet and hydrogen’s lift at 68 lbs per 1,000 cubic feet.

OK, so, how heavy is a nuclear reactor? The main vehicles with nuclear reactors in them at the moment are ships, particularly the ones made by the US Navy. I’m finding it surprisingly difficult to find stats on how heavy they are, though.

To an extent, it depends how much power an airship needs. The Hindenburg’s four engines apparently each supplied 890kW. From the ones I can get stats on (reading this list), warship reactors tend to produce hundreds of megawatts, though maybe it’s less in submarines? None of the submarine reactor articles tell you how much power they produce.

That sounds like it’s pretty much definitely overkill, and thus the reactors on warships would be far too heavy.

The other kind of vehicle people have been researching nuclear reactors for are spacecraft, since nuclear power and solar power are the main ways to generate electricity in space. (I talked a little bit about using nuclear power in space in that spaceship design post!)

Here are the space nuclear reactors I’ve been able to find info on:

So those are basically way too low-power for an airship!

The USA has also thought aboout multi-megawatt space power. I found this report, which says the reactor designs they considered - designed to produce 15MW of electrical power - were capable of approaching the ‘3-5kg/kW’ required for the missions they were envisioning. Which suggests a 15MW reactor would have a mass of 45,000kg to generate those 15MW.

The two main reactor designs they came up with were…

a gas-cooled reactor based on the NERVADerivative design, operating a closed cycle Brayton power conversion system;and a molten lithium-cooled reactor based on SP-100 technology, driving a boiling potassium Rankine power conversion system

I don’t know if I’d want molten lithium and boiling potassium on my airship…

All in all, it sounds like a reactor capable of giving sufficient power would be required to have a mass of several tens of metric tonnes.

Anyway, my concern was the lifting gass wouldn’t be enough to carry that reactor. Cargo capacities of airships probably aren’t going to change much from when Germany was building big airships - I guess we can probably build lighter structural elements, but a cubic metre of helium will lift as much now as ever!

The Graf Zeppelin apparently had a usable payload of about 15 metric tonnes, and a total lifting capacity of 87 tonnes. The Hindenburg had a total lifting capacity of 232 tonnes, and the ship, fuel, crew, passengers, cargo etc. typically came in at about 215 tonnes.

With a nuclear-powered airship, the ship wouldn’t need to carry fuel, since everything is driven by the reactor. So that probably makes a bit more space for the reactor. But it still seems like it would be a very tight squeeze to fit a nuclear reactor in alongside everything else the airship would be carrying. Especially if you want it to carry helicopters as well!

I believe the hypothetical multi-megawatt space reactor would also be almost entirely unshielded, relying only on a shadow shield to protect the spaceship. This is why nuclear reactor spaceships are usually conical: the conical area consists of radiators and heat pipes for converting heat from the reactor to electricity, all carefully confined to the shadow of the shield.

If you’re going to put your nuclear reactor in the centre of an airship, it needs to be fully shielded in all directions, and I expect this would add significantly to the mass.

On the enlarged version of the image, you can see that the nuclear airship is 300m long. The Hindenburg was 245m long, so this is a fair bit bigger!

Assuming airships are more or less the same shape, the volume of an airship scales roughly as the cube of its length, so if we assuming total lifting capacity scales in the same way, the nuclear airship would have a total lifting capacity of about 430 tonnes. Which is a lot, and I could believe you if you said a nuclear reactor could be carried with room for other stuff!

So at least in terms of whether an airship could hypothetically have the lift to carry a nuclear reactor, I’m inclined to say it’s plausible.

Whether it’s a good idea, on the other hand…