Is it possible to make a "safe" electromagnetic "jet engine" using extremely long wavelengths?

Question

Here's the idea: Building a "jet fighter" that can operate in extended combat conditions with no atmosphere without losing air superiority in atmosphere.

It's not going to have both traditional jet engines and rocket engines. The jet engines would be deadweight in purely space combat, and the rocket engines (oxygen tanks included) add extra weight in atmosphere. Worse, an engine that needs to expel matter to produce momentum risks running out of fuel in an extended fight.

In this setting, practical "impulse engines" that push against the fabric of space-time haven't been invented yet and probably won't reach the market for another 100-200 years.

This calls for an electromagnetic engine that emits a massive volume of photons with combined momentum per second comparable to a late 2010s-era jet fighter engine.

Assume a suitable power source is already available and the combined thermal profile (generator + engine) is comparable to a modern jet fighter.

The main challenge: figure out an appropriate wavelength for the engine output that doesn't burn, cook, blind, or irradiate the ground crew. The hazard zone behind the engine should be comparable to a traditional jet engine - or smaller. Additionally, it must not produce EMP-type effects that damage or interfere with electronics further than 100 meters away in a cone shape behind the engine.

The volume of photons would be massive considering that each photon has only a tiny amount of momentum.

Obviously, any sort of ionizing radiation is out of the question due to environmental damage and health concerns. Visible-spectrum light is a poor choice because it would instantly blind anyone who happens to look at the engine the wrong way.

Infrared can't be used due to the known health hazards of intense infrared radiation (https://en.wikipedia.org/wiki/Infrared#Health_hazards): "Strong infrared radiation in certain industry high-heat settings may be hazardous to the eyes, resulting in damage or blindness to the user. Since the radiation is invisible, special IR-proof goggles must be worn in such places."

A microwave engine is a bad idea because the engine output might knock out electronics from miles away in a manner comparable to EMP weapons. NOTE: the engine in question is open-ended, not a closed reflective "EM-drive" chamber.

This leaves only extremely low frequency/long wavelength radio waves as the remaining option.

The question: Is it possible to pick a wavelength long enough to avoid causing thermal or optical damage to the ground crew and nearby population while also preventing EMP-type damage to electronics?

Answer 1

No

What you're describing is a photon rocket. For a perfectly collimated beam, thrust vs. energy output is 300MW/N.

An F-16 has a maximum, full-afterburner thrust of 128.9kN. To achieve that same thrust, you would need to output a total photon energy of 38.7TW. For comparison, total electrical generative capacity on the planet is 10TW.

There is no radiation type that would produce thrust and not do horribly destructive things to the atmosphere behind it, and by extension everything looking at it, near it, under it, or probably for many kilometres in every direction.

You would also need a magic energy generator, but the text of the question indicates that that is not a problem.

Generally speaking, it's wise to remember the Kzinti Lesson:

a reaction drive's efficiency as a weapon is in direct proportion to its efficiency as a drive.

Your proposed spacefighter's primary drive is the mother of all laser cannons. Once in atmosphere, it would carve a blinding, blazing line of dissociated atmospheric gases across the sky. (All assuming that the thrust is never directed downwards.)

Answer 2

Why not a hybrid engine.

Problem: Whatever you do, whichever method you pick, you need to expel matter at specific speed or equivalent amount of energy to obtain trust. Since matter energy equation (e = mc², c² is a huge number) favors matter so much, expelling pure energy for propulsion is not a good idea. You will be expelling too much energy for it to be safe.

This hybrid engine intakes a gaseous matter and heats it up to plasma, and pushes itself using this. In atmosphere, it will use intake to get the matter, heat it up as much as it can and it will provide trust. In space, or less dense atmosphere, the highly compressed gas stored in the tanks will be used, it will be heated more to obtain better push with less material expended. You will need to fill up often if you want a lighter fighter. Or if you store large amounts of matter, you will be able to last longer.

Best of them all, if you can handle life support, out of combat, these fighters could cover as much distance as you want in space. For instance, you can dispatch them from Jupiter to fight at Mars. When they reach to their destination, their gas tanks will still be almost full. When they are lower, they can hit the atmo to fill up the tanks. As long as there is fuel to burn they will be in the fight.

We are very close to (or we already have) this technology. Only problem is a very strong power source. A nuclear/antimatter powered fighter would be an excellent idea.

Answer 3

Abuse the power source

The answer by jdunlop indicates the main problem. However, let's assume that this power source can produce arbitrarily energetic photons in exact ways.

We can now borrow from supernovae, specifically pair-instability supernovae. These occur when photons are produced with enough energy that a collision can convert energy into mass. We'll be scaling up slightly, and converting our output into neutron-antineutron pairs. These will be moving "rather fast" and have no electric charge. This means that, if we direct them appropriately, we can fire them into empty space.

But wait ... if we can create particles with our photon drive, why are we using uncharged particles? Instead, let's make some electrons and positrons, and give them an electromagnetic kick via the photoelectric effect. We suddenly have a decent reaction mass to work with, and handwavium can scale this up to whatever size we need.

Firing near-negligible masses very fast is far more effective than shining a beam alone.

Answer 4

The problem of running out of fuel is going to exist whether you're expelling the "fuel" directly or converting it to radiation and expelling that. Obviously if you can convert matter directly into energy then very little fuel is required; but if you have access to that kind of energy there are a variety of ways to use it for propulsion. As @jdunlop points out in their answer, radiation of any form is going to be dangerous in that quantity. Indeed, anything with high enough momentum/energy coming out of the back of your jet is going to be dangerous, which presents an unfortunate trade-off between efficiency of the engine and danger to bystanders.

Answer 5

I think you might be looking for a vasimr engine.

It uses radio waves to ionize and heat an inert propellant, forming a plasma, then a magnetic field to confine and accelerate the expanding plasma, generating thrust.

It is an engine, uses long wavelength EM(radio). Test engines have been constructed and tested. Problem: they operate in vacuum.

VASIMR engines are inefficient, and to get energy density nuclear power plant would be needed. if those are being used then there are other more efficient options.

A fusion power plant could allow a rocket to be run both atmosphere and space with some reaction mass. But that is not the engine type you are asking about.

I thought I read many years ago about combining VASIMR with RAM/SCRAM but that would still leave the problem of how to get from ground to mach.

A single atmosphere and space engine that works using EM to add energy to a reaction mass? Currently no, near future no. Far future, probably not. Personally I believe fusion engines would fill the role, if the role is even reasonable in the future.