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Your typical aircraft propels itself by expelling something, may it be air, combusted fuel or even ions. Would it be possible for an aircraft to do none of those things and simply heat the air around it in order to propel itself? Like a plane shaped microwave that was turned inside out. The high-tech vehicle would emit powerful microwaves or lasers or something to heat the surrounding air, thus generating lift and thrust.

I am not so much concerned with the technology behind it as I am with the physics themselves. Like, imagine a plane that heats the air under it with a death star style laser beam. Bald eagles already use rising air currents to soar high into the air. It's the same principle but the heat source can be wherever you want.

There's also something called plasma actuators that alters the airflow for stability by heating the air above the wings.

The aircraft could get off the ground like a hot air balloon, focusing its beam inside a mantle. Once it has gained altitude it would change shape, tuning the mantle into wings and gliding forward. The beam emitter would heat the air in front of it for lift and control.

Also, if we heat the air enough it will explode into a plasma, which can serve for propulsion. Probably.

I am not sure how this would work so please, enlighten me.

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    $\begingroup$ "...like an hot air balloon". What else haven't you already answered? $\endgroup$
    – L.Dutch
    Jun 12, 2022 at 20:51
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    $\begingroup$ Depends on how much the use of "mantle" is allowed. If we can use it as a sack to create a jet of air - yes. If we are allowed to heat only unenclosed air around the aircraft - that would be possible only in theory, because it requires much higher energy use, such aircraft would be balancing on a cloud of plasma. $\endgroup$
    – Alexander
    Jun 12, 2022 at 20:54
  • $\begingroup$ Or, say, inside a turbine? $\endgroup$ Jun 12, 2022 at 20:54
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    $\begingroup$ Behold: The Flying Solar Chimney! Halfbaking at its best. Good times, good times. halfbakery.com/idea/Flying_20solar_20chimney#1393940855 $\endgroup$
    – Willk
    Jun 12, 2022 at 22:19
  • $\begingroup$ en.m.wikipedia.org/wiki/Lightcraft uses a laser to turn air into expanding plasma, and uses that to provide propulsion $\endgroup$
    – sdfgeoff
    Jun 13, 2022 at 4:38

1 Answer 1

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This is a Frame Challenge

And it's amazing how often I've used the following quote on this site....

Can you launch an ICBM horizontally?

Sure. Why would you want to? (The Hunt for Red October)

Let's assume you have the ability to both heat and cool air.

  • You heat the air below your "ship" and cool the air above it. This creates lift.
  • You heat the air behind your "ship" and cool the air before it. This creates propulsion.

The problem? It's whomping inefficient. Your example of birds rising on thermals is, well... a bad example. The birds are already gliding! They used their wings to provide lift and propulsion and are simply taking advantage of the little extra boost the thermals give them. And it's not the heat of the air that's causing the extra boost - is the updraft the heat caused (which is more complex than simply heating the air). But my point is they were already in motion and the updraft enhanced that motion.

No bird can lift itself off the ground (that I know of) simply because the air around them is hot.

Which means your problem, which is the main problem with all flight, is weight

Birds are honking light and still can't do what you're suggesting without some other power source (flapping their wings). But we can ignore that because a hot air balloon does some of what you're asking by containing the hot air, which has a lower density than the surrounding cool air, and thus rises.

Having proven the fundamental principle is possible what remains is making a heavy airplane do it... without hot air containment....

Which it kinda already does. Airplanes, like birds, can take advantage of thermal updrafts to enhance flight. So if we ignore the idea of primary lift and propulsion and only focus on enhancing flight, could the airplane cause the thermal updraft?

If the equipment and its supporting energy source was light enough (so as to not overwhelm the benefit with the negative consequence of weight) — sure.

But why would you want to?

The problem with finding a justification for a wonderful but wholly impractical idea like this is that it is impractical. There are many other ways of achieving the effect that are cheaper and simpler. They'll always be used and it's pretty much impossible to rationalize an impractical solution by inventing a way to prohibit the more practical solutions.

enter image description here
(Image courtesy This City Knows)

So why did I include that cool antique French postcard? Because the answer is "Who cares?!" People have been dreaming about impractical flight for a very long time. The card (and the collection it came from) where an invitation to illustrators in 1899 to dream about flight in the year 2000. The ideas were wonderfully mocked by Avery Brooks when he said, "It's the year 2000, but where are the flying cars? I was promised flying cars!"

Why aren't there flying cars? Weight vs. propulsion....

Should that stop you?

No

My recommendation is that you don't worry about the physics behind making an idea like this work. Reality is overrated! Simply invoke a world rule. You have flying cars airplanes that create lift through the insert technobabble method, which heats the air in a narrow column, lifting the plane. It uses the same tech to push it along.

Cool!

It's very Flash Gordonesque. I like it!

So I'll leave you with one more quote, and I'd like you to note the emphasis, which is mine.

A measure of how seriously people take the science of the Ringworld – and how daft it has driven them – comes in a story from the 1971 World Science Fiction Convention, when excited students from MIT apparently crowded out the venue chanting: "The Ringworld is unstable!" Apparently, the Ringworld would need giant thrusters to maintain orbit around its sun (a problem that Niven addressed in a follow-up 10 years later). The significant thing isn't that Niven was wrong but that people took so much of the rest of his science seriously enough to worry about such matters. His ideas have traction. The Ringworld is splendidly improbable but perhaps not impossible. (Source)

And Larry didn't explain the physics.

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