Is a jet engine with no moving parts possible?

I imagine something that would heat the air as it enters causing it to expand and exit rapidly thusly propeling the engine. Is this possible?

Isn’t that what a ramjet is?

I was thinking of a machine that had no combustion, but heated the air through, although it would be terribly impractical, an electric heater.

You mean, somehow heat the air outside the engine? The combustion of the fuel must take place inside the engine in order to cause the gases to expand and drive the jet forward.

Oh, I see what you mean. sorry.

I recall something of a jet or jet-like engine where the air was heated by a ground-based laser pointed at it.

Would an air based version of a magnetohydrodynamic drive count ( the drive featured in The Hunt for Red October, and yes, it’s real ) ? If you gave the air a charge, you could use electrical or magnetic fields to accelerate it for thrust. For a somewhat speculative metntion of this kind of technology, look here . It’s mainly about “air spike” technology, but mentions magnetohydrodynamic engines.

How plausible all this really is, I don’t know; I’m a layman, not an engineer.

Actually, I believe it was a rocket, not a jet; it was featured in the science fiction novel Footfall. It’s also mentioned in the link above ( pulse detonation engines ).

Jet engines can use nearly any high temperature source for the required heat…burning of hydrocarbons just happens to be the lightest one.

Jet engines have been built using a nuclear reactor as the heat source. Pretty sure they were just feasability demos, and too heavy to fly.

And yes, ramjets have no moving parts at all. They have been termed "flying stovepipes. They rely on the shockwave of supersonic flight to cause the exhaust to exit the rear…thus will not operate at subsonic speeds. They must therefore have some suplimental propulsion system to get them up to the speed where the ramjet becomes effective. This can be a “mother ship” rocket boosters, or more conventional turbo machinery.

I saw some little boats at the fairground powered by ‘pop-pop’ engines - essentially a tiny ramjet, using water/steam as the propulsion medium; in this particular case, the heat source was a candle, but any heat source would work - a candle just happens to be compact and light and so ideal for this application.

This site deals with valveless pulse jet engines, and says that they have no moving parts.

I’ve seen RC models fly with smaller versions of pulse jet engines. They appear to do a fine job of providing thrust, but make the most hellish noise it would be possible to imagine.

What’s the distinction between a rocket and a jet in this case? A rocket generally carries all of its reaction mass with it, along with its oxidizer. It burns it, and expels the hot gas out the back.

A jet takes in air, mixes it with fuel, and shoots it out the back. The difference is that a jet breathes air, while a rocket carries its own oxidizer.

This means the ground-based laser craft are more like jets than rockets. The way they work is that they have a bell-shaped reaction chamber that fills with air. The laser heats the air, which is expelled out the bottom as reaction mass. Laser shuts off, bell re-fills with air, repeat. It’s basically a laser-pumped pulse jet.

Given a suitably dense power source, there’s no reason you couldn’t have your laser in the airplane with you, creating a ‘laser jet’. And if you need to operate out of the atmosphere, you could carry reaction mass and heat it with the laser, or even burn up the structure of the spaceship (with an ablative material) and use that.

Yeah, but it’s fun to see the noise make old folks from London duck for cover. :smiley:

A surpising amount of work was done towards a similar idea, but powered by a nuclear reactor. They even test fired the engine.

Project Pluto - the “Flying Crowbar”, a nuclear ramjet powered cruise missile.

I assume you are talking about an engine has thrust at zero velocity for the engine as a unit. Somehow you have to get a lot of air into the engine. In addition there has to be something that keeps the heated air from going out the front end. How would you do both of these things with no moving parts and the engine standing still? A turbojet does it because the air is being brought in and compressed before being admitted to the combustion chamber. The exhaust goes out the back and not the front because the region outside the rear end is at a lower pressure than the air entering from the compressor. A ramjet does it because the high velocity of the engine rams the air into the intake where it is slowed to subsonic speed raising the pressure and preheating it. From there back it acts like a turbojet except there is no turbine which is there in a turbojet to power the compressor.

If you get your electric jet up to sufficient speed to get ram air then you are better off to use a ramjet. Burning a hydrocarbon fuel gets much more energy for the same weight than somehow generating electricity, with no moving parts mind you, and then heating the air.

Notice that even the nuclear powered Pluto was a ramjet.