# Is it possible to make a “safe” electromagnetic “jet engine” using extremely long wavelengths?

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?

• The advantage of jets in earth's atmosphere is that they do not have to carry their own reaction mass. They take the existing reaction mass of the atmosphere and just add energy to it. You are proposing that they lose this advantage? – Justin Thyme the Second May 3 at 23:47
• @JustinThyme problem is that those jet engines are several tons of deadweight outside of atmosphere, compromising space superiority versus a pure space-fighter. – user1258361 May 3 at 23:51
• Given the reaction mass required to achieve high delta-V in space, "several tonnes of deadweight" isn't really that big a deal, and presumably would not substantially affect the performance of the fighter in vacuum. – jdunlop May 4 at 0:29
• I think it'll be perfectly safe, in that it'll never be able to leave the ground. – user_1818839 May 4 at 16:34
• Moving about in space is a different proposition to in atmosphere. You may be able to get thrust in one direction, which is great, because you can probably turn the engine off after a bit of acceleration and just let gravitational forces get you to where you want to go. Once you arrive, you need to slow down (a lot) - for that you either need a forward facing engine, or to re-orientate and burn in the opposite direction. How are you going to turn around? Thrusters of some sort? Gas eject type things as we have now or something different? How are these fuelled? – Ralph Bolton May 5 at 9:07

# 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.)

• +1 for the statement, "horribly destructive," which I predict that history will prove to be the single greatest understatement of the entire third millennium of the common era. – JBH May 4 at 2:49
• As an additional point of reference, 38.7 TW of continuous energy output is the energy equivalent of roughly 9.25 kilotons of TNT every second. – GrumpyYoungMan May 4 at 3:08
• Maybe you have identified the source of Gamma Ray Bursts. – DrMcCleod May 4 at 7:11
• @DrMcCleod - no, it would be short of that by thirty orders of magnitude. It would be a godawful laser, but only by terrestrial standards, not interstellar ones. (Not really relevant to the question, but never miss an opportunity to point out exactly how powerful astronomical events are. :D ) – jdunlop May 4 at 7:27
• +1 for the Kzinti lesson – Qami May 4 at 18:55

Why not a hybrid engine.

Problem: Whatever you to, 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.

• Antimatter maybe. Nuclear, no. – jdunlop May 4 at 18:08
• In the computer game Elite, you used to be able to fuel up by "skimming the sun". That is, by scooping up coronal matter from around a star, and somehow filling the fuel tanks with it. Could a similar idea additionally be applied to this fighter for extended missions, perhaps using planetary upper atmosphere? – Ralph Bolton May 5 at 9:01
• @jdunlop I am not so quick to dismiss nuclear. The link you provided was excellent read. At end, they dropped as there would be no defense against it and pentagon was sure Soviets would make the same missile if US made it. Also, a missile in air for months is not very beneficial, a fighter is another matter. – Cem Kalyoncu May 5 at 11:03

## 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.

• nice idea... but I'm afraid it's no less destructive than a photon beam. Even if you spread out the propellant so that it doesn't immediately recombine, the antimatter will recombine with the atmosphere... and both sides will hold a ton of excess charge which will produce enough current to - by itself - wreak havoc in anything organic or electronic in its general area... and if you don't spread it out, like you suggest, all of the reaction mass will turn into gamma radiation flying out in every direction. – John Dvorak May 4 at 13:04
• @JohnDvorak With that much power you could shunt the particles out at basically arbitrary speeds - you don't need to generate very much antimatter if you can sling it out the back of your craft at 99% C. It's basically an ion engine drive, but it makes it's own propellant. Still probably lethal as hell, but I'm going to guess less lethal than a 38 TW microwave beam. – Iron Gremlin May 4 at 23:33
• "Firing near-negligible masses very fast is far more effective than shining a beam alone." Not true if you need to produce those masses from pure energy. en.wikipedia.org/wiki/Energy%E2%80%93momentum_relation The goal of a rocket is to produce momentum. E^2 = (pc)^2 + mc^2, where p is momentum, E is energy, c is speed of light, and m is rest mass. From the equation it is clear that, if you need to make the mass too, you're better off using photons rather than massive particles. (Normal rockets have the mass on hand, skipping most of the energy cost.) – BBeast May 5 at 10:36
• The equation also shows that firing near-negligible masses at ultrarelativistic speeds ((pc)^2 >> mc^2) is only slightly better than using a photon drive (excepting the possibility of using particles which might not interact strongly with your environment), in terms of energy output per unit momentum. – BBeast May 5 at 10:39
• Ooh, how about we use dark matter as propulsion material? Would "barely interacts with baryonic matter" make the drive impossible, or just ridiculously heavy? – John Dvorak May 5 at 19:21

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.

• The OP does say "assume a suitable power source is available", so I'm treating the energy question as "magic, and not requiring fuel". – jdunlop May 4 at 1:42
• This constitutes a frame challenge, however would benefit from much much more explanation and perhaps reading this meta post regarding frame-challenges. – A Rogue Ant. May 4 at 2:26

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.

• The OP is explicitly looking for an engine that expels no reaction mass. Though VASIMR is fascinating. – jdunlop May 4 at 18:22
• VASIMR's got a similar problem to a photon rocket: it requires incredible amounts of energy to produce fighter-level thrust. It's about three orders of magnitude more energy-efficient, but still requires 5 GW to match an F-16's engines. That's comparable to the peak output of one of the world's largest power plants. – Mark May 4 at 21:42
• @jdunlop i made the assumption of reaction mass because pure energy thrust is not viable full stop. Think bigger then death-star beam weapon as propulsion for a fighter jet. – Gault Drakkor May 4 at 22:22
• @GaultDrakkor - that's not true - see my answer. A multiterawatt laser is a bad thing, don't get me wrong, but the Death Star (from the destruction it wreaks) is estimated to have a $2.4 \times 10^{32}$-watt laser. Twenty orders of magnitude larger. Both the power source and the emitter would have to be magic, but there's nothing intrinsically incorrect about a photon rocket. – jdunlop May 5 at 0:02