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Here's the set up. In my slightly-futuristic mildly-sci-fi novel, I have a need for an aerial transport vehicle (I have been informed what I need is closer to a VTOl than a hovercraft). These crafts need to be able to go from the ground to a building hundreds of stories high with reasonable speed, while still maintaining the comfort of their passengers. They do not need to be large, but should have sufficient space to carry around 40-50 passengers. They are public transportation vehicles, similar to an aerial bus. Most of their movement would be vertical, though they would need to travel horizontally every now and then. They would not need to go far.

I would like these vehicles to be nuclear powered. As I understand it (not being a nuclear scientist, I am likely wrong), one could use a nuclear reactor to turn water to steam, which one could then use to create thrust, thus moving the vehicle. The obvious drawback is the chance of radiation leakage, or worse, explosion.

How can I make my VTOL so safe that the radiation risk is minimal (or non-existent)?

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  • $\begingroup$ You said "able to go from the ground to a building hundreds of stories high". That's not a hovercraft, that's an aircraft. Maybe you're specifically looking for a VTOL aircraft. $\endgroup$
    – Samuel
    Commented Jul 21, 2015 at 23:09
  • $\begingroup$ Wiki have solution for you here but your hovercraft should operate in desert or open areas. $\endgroup$
    – user6760
    Commented Jul 21, 2015 at 23:27
  • $\begingroup$ @user6760 - a SMR still has the crappy power-to-weight-ratio that makes reactors impractical in aircraft. $\endgroup$
    – jdunlop
    Commented Jul 27, 2021 at 17:23
  • $\begingroup$ Using steam directly to drive the vehicle is kind of a terrible idea for a couple of reasons - one, nuclear contamination, but two, it means you're exhausting your heat transfer medium. You have to carry not only the reactor, but sufficient mass of water to propel your vehicle. Water's heavier even than jet fuel. $\endgroup$
    – jdunlop
    Commented Jul 27, 2021 at 17:30

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What you are describing is a VTOL aircraft, not a hovercraft. I suspect you want the rotors to be enclosed, rather than open like a helicopter, which would make operating in urban environments a bit easier, since there is little risk of the rotor blades coming into contact with the buildings.

Why nuclear power isn't used in aircraft is the crappy power to weight ratio. Nuclear energy is extremely energy dense, but the ionizing radiation and neutron release of a fission reactor will quickly kill the passengers and crew, not to mention everyone along the flight path, unless the radiation is shielded by dense materials like lead. The mass of the shielding and the weight of the power conversion machinery of a fission reactor essentially negate the energy density argument for aircraft (ships and submarines are a different matter, since they can use displacement to carry the mass of the reactor). We can see a bit of this in regular aviation history. Helicopters were delicate, expensive and difficult to fly because the piston engines were heavy and offered very little power to overcome the weight and drag of the helicopter. Only when gas turbines became common did helicopters really take off (heh), since the turbines offered a compact, lightweight power source for the helicopter. Helicopters were actually invented in the late 1920's early 1930's, and a few were actually in service in WWII (for example the Focke-Achgelis Fa 223 "Drache"), but there was no German "Air Cavalry" because these machines were underpowered. The turbine powered UH-1 "Huey" was a much different proposition...

But your coleopter can still use nuclear energy, just indirectly. The vehicle needs to have a microwave receiver (rectenna) built into the fuselage, and the city have powerful microwave emitters beaming energy to receptive coleopters as they fly. Safe corridors for flight include keeping people away from the beam path, and ensuring buildings and structures are not placed in such a way that they randomly scatter microwaves. The pilot may have to engage battery power to take off and climb into the beam path (and the batteries better be charged when it is time to land!), but electric aircraft powered by microwave beams have been demonstrated experimentally (for example, the Stationary High-Altitude Relay Platform), so this isn't theoretical.

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  • $\begingroup$ There was to be a nuclear fixed-wing bomber; the concept was tested by installing a submarine reactor in a B-36, flying the aircraft, and even actually operating the reactor in flight (though the original 6 piston engines did all the work of flying the airplane). The concept was abandoned more for safety concerns than being "impractical" under a military definition. Also worth reading an Analog magazine serial, "Steam Bird" from the 1984 (Hilbert Schenck, also available on Amazon) for why this doesn't work well even in fixed wing form... $\endgroup$
    – Zeiss Ikon
    Commented Jul 27, 2021 at 16:59
  • $\begingroup$ @ZeissIkon - the reactor that was installed in the B-36 only produced a fifth of the power of the aircraft's engines. It was a proof of concept, but insufficient. $\endgroup$
    – jdunlop
    Commented Jul 27, 2021 at 17:24
  • $\begingroup$ @jdunlop No argument. The end product was to have been able to stay aloft for months or years at a time, provisioned and crew rotated by serving (in part) as a flying aircraft carrier. Avengers and S.H.I.E.L.D., anyone? $\endgroup$
    – Zeiss Ikon
    Commented Jul 27, 2021 at 17:30
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Go fusion

With a compact fusion device, you could make the VTOL aircraft as big as you wanted since power output is no longer a limitation. The heavy penalties associated with shielding fission reactions don't appear in fusion reactions. Two benefits of fusion is that first, the output is a stream of alpha particles and second, fusion reactions yield more energy than fission reactions. These are easily turned into electric current and can be shielded with a few inches of air or human skin. Paper is also cited as an effective shield for alpha particles.

Fusion is hard

We don't have a way to make fusion power plants yet, compact or otherwise, because it's really difficult. Humanity has been working on the problem for 60 years and still haven't cracked it. I have a question over on Physics SE to see if someone can point me towards an interesting approach that I haven't heard about in long time that was supposed to be clean, powerful and light on radiation.

The VTOL would have to be built around the fusion plant but on a larger craft more options are available in where to place the power plant. Perhaps a craft reminiscent of the V-22 Osprey with tilt rotors for VTOL operations, might do the trick. Just trade out the turboprops for electric motors feed by the fusion power plant.

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  • $\begingroup$ The approach I was looking for was called Focus Fusion. $\endgroup$
    – Green
    Commented Aug 23, 2015 at 19:04
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    $\begingroup$ On the fusion side of things can you imagine the look on the pilot's face when he asks the engineer "What about the radiation?" and the engineer says, "We've taken extra precautions, it uses two layers of cardboard" $\endgroup$
    – Scott Downey
    Commented Jan 22, 2016 at 13:20
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1. Use induced gamma emission.

Nuclear isomers store a great deal of energy. With the right stimulation (we may not know what that is...) they can be persuaded to release that energy abruptly, serving as an abundant source of gamma radiation of known frequency (and energy). Blocking (and absorbing/converting/using) this gamma radiation should be easier than usual because you can design a crystal structure that absorbs that one frequency especially well. In case of accident, the debris may not even be radioactive.

2. Make the citizens radiation proof.

Copy the elephant or Deinococcus, or both. Oh, it's just a small leak in the reactor .. no cause of alarm!

3. Put the nuclear reactor somewhere else

The Wonkavator operates in a wide but preset grid in space - up, down, sideways, back and forth, and there's even a button for special excursions. The power source is a nuclear reactor squirreled safely away over Packaging and Product Sterilization. The power is beamed out along whichever one of the tubes in space the Wonkavator travels on, and it uses that to create the magnetic field oscillations, ion drive, steam propulsion or other form of motile force is appropriate for whichever environment it happens to need to cross.

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The easiest way of dealing with the problem is basically to magic the issue away. A wizard made it safe by technobabble magic. But, I don't think you're asking for an answer like that. :P

The easiest realistic way of dealing with the issue is to make your ships powered by thorium, which requires a Uranium breeder to work, and hence, can be shut down at any time.

Making the radiation risk minimal will never happen. Unless you're going to place reactor at the other end of the ship and make it very very long. The weight necessary to keep radiation from a reactor away is also likely unrealistic for a hovercraft in the slightly-futuristic world.

Lead shielding

You could use magnets to deflect the particles away and heavy lead blocks to keep the gamma and neutron radiation contained, but for volume (not weight) bulk and transport purposes, you may want a superheavy element of some sort.

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  • $\begingroup$ Superheavy elements and lead don't pair well with "aircraft". $\endgroup$
    – jdunlop
    Commented Jul 27, 2021 at 17:22

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