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Airships!

Many people love them, many see them as a flying safety hazard, especially the hydrogen-filled kind. And recently, I've thought about the other alternative:

Propeller-Airships!

enter image description here enter image description here

I'm currently in the process of writing a lil steampunk/ retro-sci-fi comic series, taking place around the late 1880's-1960's, in a world where many things such as the first practical helicopters, and small nuclear reactors have been invented during said time. And even tho it'd be perfectly reasonable to have blimps or semi-dirigibles during this time, i wanted to find an alternative. The thing that mainly motivated me was the idea of using an SMR as a power source for these ships, since an equivalent to them exists within this world.

enter image description here

The airship's weight generally hangs around that of pre-dreadnought and/or dreadnought type battle ships of our world, at about 15,000-30,000 tons, including the reactor's mass. Unlike the rotor blades of helicopters, the lifting-propellers on these ships are not articulated and aren't meant for steering, meaning any complicated mechanisms to do so aren't needed.

The exact purpose for the existence of these airships would extend beyond the frame of this question, so it isn't a part of it for now. So for the answer's sake, you can assume that their purpose in this world is considered important enough for there to still be a need for their construction.

The broad "technological level" of the time in which these ships fly should be around the early-to-mid 70's of our world, so any materials, manufacturing processes or techniques found during this period would be available to build them.

So to sum it up, the main question here would rather be:

Is there a way to make it possible for an airship, using only propellers and being powered by one or more of said reactors, to lift itself off the ground? And how heavy could it possibly be before being unable to do so?

Note: The answer doesn't need to be 100% science-based, but it would be amazing if it's as plausible as possible. So no fancy hand-wavium elements!

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    $\begingroup$ You need dense atmopshere to lift an airship this big. Otherwise, compare with helicopters of current, they have a lot larger rotors and a lot smaller mass? and remember, the lifting power is proportionate to rotors' square while mass is to volume, so scaled up ship won't lift while scaled down will. $\endgroup$
    – Vesper
    May 15 at 8:44
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    $\begingroup$ The word airship has a very well defined meaning, and it is does not mean helicopter. $\endgroup$
    – AlexP
    May 15 at 8:56
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    $\begingroup$ @NimRad: Why wouldn't it be the case? Practical large helicopters heave to use the characteristic complex linkage in their rotors because they have only one or at most two of them. But those fantastic helicopters have many rotors, so that they can achieve the same effect by using differential thrust between groups of rotors, dispensing with the need of cyclic control. Just like quadcopters (or hexacopters and so on) do. $\endgroup$
    – AlexP
    May 15 at 9:21
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    $\begingroup$ +1 for "hydrogen-filled gas bags are too dangerous; let's put nuclear reactors in the sky instead!" Loving the aesthetic. I doubt there's any way to make it practical, but it sounds fun. $\endgroup$ May 15 at 17:20
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    $\begingroup$ @AlexP they're ships, they're in the air .. yes airship has a specific established meaning in 'this' world, but I don't think it's unreasonable to use it otherwise for a piece of fiction, it can count as 'locale flavour' for the world in question .. maybe acknowledging that it's non-standard (for this world) use of the word may be helpful to avoid confusion for questions here but he can use it in his world as he likes. $\endgroup$
    – Pelinore
    May 15 at 18:57

6 Answers 6

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There aren't many Small Modular Reactor designs for which a mass has been given. One for which there is a mass figure is the Hyperion Power Module, which weighs 'less than 50 tons' and produces 25MW. I'll use the figures of 50t and 25MW, for 0.5 MW/t power to mass ratio.

Hovering vehicles use disk loading to describe hover efficiency. Effectively, this means that the larger the rotor and the lower the mass it is lifting, the lower the downwash speed and the more efficient the aircraft is.

So, with a 50t aircraft and a 25MW powerplant, ignoring the mass of the aircraft, we have a hover efficiency of 2 kg/kW, which is equivalent to the hover efficiency of a Lift-Fan aircraft. This is the least efficient that such an aircraft could be and still fly.

Note that this discounts everything other than the weight and output of the reactor. The rotors and airframe are going to add more weight, and this will reduce the hover efficiency. It may be possible to build lighter reactors, since current SMRs aren't designed for absolute minimum mass, just reasonable portability, but it will still be necessary to add an airframe and rotors. Carbon fiber composites began to be introduced in the 1970s. With a lightweight reactor made from a material such as Titanium and a carbon-fiber composite airframe and rotors, such an airship may approach the hover efficiency of a lift-fan aircraft.

The downwash from such an airship is likely going to exceed 500kph, and will probably be very noisy. That's going to affect where it can be used and what it can be used for. They will probably have to fly well above the ground in order to diffuse their downwash.

The NuScale reactor weighs around 500t with an output of 77MW. This gives a minimum hover efficiency of around 6.5 kg/kw. It would have to perform more like a helicopter than like a more maneuverable, more compact aircraft.

Why couldn't these airships have larger propellers? They can, but because they would be made from real-world materials, not super-strong, super-light unobtainium, and the bigger they are, the heavier they become in order to support the weight of the aircraft. The bigger the rotors, the heavier the aircraft becomes, but the lower the disk loading and the higher the hover lift efficiency. While the raw weight/power is 2 kg/kW, this doesn't mean that this can't be increased.

Larger rotors make the aircraft less stable, slower, require more landing space, and are harder to make. This is the tradeoff that would have to be made... small, fast, noisy and stable or large, slow and less stable... and more expensive, but with slower downwash. It would be up to the OP what sort of aircraft to build.

Hovering aircraft with extremely low power and high hover efficiency is possible. A human-powered helicopter, the AeroVelo Atlas, had a very high hover efficiency, but was the size of a sports field, with a hover efficiency of over 116 kg/kW. However, this aircraft was impractical for any purpose other than winning a prize in a human-powered helicopter competition.

So, in conclusion, the issue is not "Can it get off the ground?", but "Can it get off the ground and do anything useful?" The lower the hover efficiency figures in kg/kW, the more capable the aircraft will be of being compact and maneuverable.

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    $\begingroup$ By a curious coincidence, the real-life Mil Mi-26 has a max take-off weight of 50 tonnes using two ZMKB Progress D-136 turboshaft engines providing a total of 17 MW. $\endgroup$
    – AlexP
    May 15 at 9:54
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    $\begingroup$ Love the detailed answer; will just point out that SMR systems have some additional weight which might not be covered in the usual specs: cooling, turbines (to turn the heat into electricity) and radiation shielding, for example. The HPM in the diagram at en.wikipedia.org/wiki/Gen4_Energy is only a small portion of the overall system. Maybe some of that can be reused between reactors, or dropped from OP's design, or otherwise minimized via clever engineering... $\endgroup$ May 15 at 17:38
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    $\begingroup$ @SarahMesser and some turns into lots when you don't have an ocean or a river to use for heat rejection. $\endgroup$
    – g s
    May 15 at 18:15
  • $\begingroup$ Did you discount the weight of the eels? (Different Monty, I know.) $\endgroup$
    – Theodore
    May 15 at 21:07
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    $\begingroup$ The AeroVelo Atlas is a ridiculous machine. The thing is 1,282 m2 (13,768 sq ft) in size... And weights only 55 kg empty. $\endgroup$
    – Nelson
    May 16 at 6:14
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Have a look at the Convair NB-36H. This is 1950's technology. It had a nuclear reactor and propellors. One was not actually connected to the other but it could have been, and would have been if a later version had been built.

The reactor was lighter than ordinary reactors, as it had little or no shielding on the side. When it was in the hanger it had to be coned off so people walking past it did not get irradiated.

How about using the heat of the reactor to generate buoyancy? This would mean you could have a hot-air blimp providing most of the lifting, and the propellors giving drive and any up/down motion.

The reactor could hang underneath the body of the craft to further reduce the exposure of those on board to the radiation, or reduce the weight of cladding. As you land, the reactor could be shut down. It could then be wound on board, or you could land it first and the rest of the ship would land somewhere else using it as a teacher. This would not look like that marvellous galleon with propellors model, but it might work.

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  • $\begingroup$ The Convair NB-36H was a conventional aeroplane. The OP is after a VTOL/hovering aircraft $\endgroup$
    – Monty Wild
    May 15 at 10:04
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    $\begingroup$ Yes, I know. But it was what people were thinking about in the 50's, and it gave an idea of how a reactor might be re-designed to minimise weight. Project Pluto might have given you something more like the City of the Birdmen from Flash Gordon. $\endgroup$ May 15 at 10:26
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Your propeller-airship is just a very large helicopter. So yes, it's possible, but just like with helicopters, it uses absurd amounts of energy to stay aloft.

A side note: the suction on the top of your propellers contributes about twice as much to lift as the downwash. with your picture, most of the downwash hits the deck of the ship, which is less than ideal.

Realistically, due to the prohibitively large amount of energy needed, helicopters are only used where dirigibles or fixed-wing aircraft cannot operate. And for a monstrosity of that size, you'd really have to come up with a really good justification. Other than the rule of cool, that is.

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"Can you launch an ICBM horizontally?"

"Sure, why would you want to?" (The Hunt for Red October)

From a science-based perspective, yes, you can create the airship you're talking about using the statistics of existing SMRs. But that same expectation for science begs the question, why would you want to? The technology for a vertical propeller necessitates the technology for a horizontal propeller and the "buoyancy" provided by the lift due to horizontal flight compares to the buoyancy of nuclear-powered ships traveling the oceans, offsetting the weight of the SMRs and allowing for more cargo.

Science is a harsh taskmaster as it demands efficiency. So does economics. Thus, why would you want a nuclear-powered vertical-propeller airship when a nuclear-powered horizontal-propeller airplane would do a better job? It should be noted that historically a number of projects tested the feasibility of nuclear powered aircraft. The reason we don't have them today isn't technical infeasibility, but impracticability.

The issue of impracticality is overcome with having an in-story reason for using an atomic helicopter (which is what you have). Monty did a better job than I explaining the basic physics behind atomic helicopters, so we know that you have a fundamentally sound idea.

So we’ve established my proposal is sound in principle now we’re just haggling over price. (Pirates of the Caribbean: Curse of the Black Pearl)

And Monty's not wrong about the prop wash problem. A multi-propeller helicopter-style airship doesn't make much sense. Scientifically, it would be much less efficient as much less of the air moved by the propellers would find its way beneath the aircraft. So we really do need a reason to justify not using the more efficient, more practical solution that's technologically simpler than an atomic helicopter. So, what is it you might really have?

enter image description here

The Marvel Helicarrier, which debuted in 1965 and would likely need to use more then one SMR. (Image courtesy "15 Things You Didn't Know About S.H.I.E.L.D.'s Helicarrier," ScreenRant.)

A rose by any other name would smell as sweet. (Shakespeare, Romeo & Juliet)

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Frame-ish Challenge

So - in your question - you never specified how high up said vehicle was to travel. Which gives me a chance to talk about one of my favourite bits of Aviation lore:

WIGs or as the Russians call them: Ekranoplans

As you get closer to the ground, the area of high pressure under the wing cannot disperse in the same manner as when you are high up, which creates a Cushion of air underneath the vehicle, increasing the amount of lift available.

More detail available Here

This means if you want to build a large airborne vehicle and you don't need any high-altitude performance (the certification has 3 classes - with the middle one being certified up to 150m) - then using this phenomenon allows you to build much bigger for a certain amount of lift than you would otherwise need to do....

Which sounds like exactly the sort of scenario you want.

TL;DR - so long as you want to stay within 150m of the ground, using Ground Effect is a realistic method of having a large aerial vehicle

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    $\begingroup$ Not exactly what i'm going for with this vehicle, but +1 for mentioning Ekranoplans! Love these vehicles as well, and they do find applications in the story, tho at a smaller scale and with different roles. $\endgroup$
    – NimRad
    May 15 at 20:04
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    $\begingroup$ Of course, this makes the downwash under the vehicle even more annoying to anything on the ground, I guess? $\endgroup$ May 16 at 20:13
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Yes, it is 100% possible. However, there would be some questions about why not just use (non-flammable) helium bags to provide the lift, instead of expending massive amounts of energy on propellers to lift the ship. Of course though, rule of cool trumps all, so if you don't like helium bags, go for it!

Although, it may be better to use fewer larger propellers than lots of small ones, as it would be easier to build and more efficent.

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