EM Drive Paper Published

[scaryjim]

... they haven't tried to explain where the energy comes from. 'Bouncing stuff round a cone' doesn't really do much for me in the way of explaining how that comes to impart energy to a thing.

They pump electrical energy into a microwave generator that feeds the whole system. This isn't energy from nothing. It's about generating thrust without any apparent ejection of matter - that's important for travelling interstellar distances because those interstellar services stations are a long way apart, and charge ridiculous prices for refuelling

More to the point, there are ways of generating electricity that don't require large amounts of consumable fuel, so if you can generate thrust from a purely electrical system you don't have to carry as much physical fuel around. Reading up on the stuff linked in this article it looks like the current theory is that matter is ejected from the "engine" in the form of paired photons - my guess is the "bouncing around" actually sets up standing waves that are amplified through feedback from the incoming microwaves to the point where they generated this paired-photon exhaust. If that mode of action is supported by further experimentation it should be possible to increase the efficiency of the system; really it's a question of whether we'll ever get a fuel source with high enough power:weight for it to realistically drive a self-powered EM Drive vehicle. But for that it's worth remembering that, if you can avoid the gravity wells of the planets and other bodies you pass, you don't need to apply thrust continuously - once you get up to speed you can kill the drive and just coast...

EDIT:
Indeed, as TheAnimus said (much quicker & more concisely), it's a fancy magnetron. Just like your microwave oven at home.

[TheAnimus]

I thought it was also that it has a potential to be quite efficient, so that with fission or even a PV array you could power it almost indefinitely, constantly.

[EndlessWaves]

This makes me wonder, just how much electricity can you generate in space to crank one of these things up? No-one want a 100W space ship

You could lift a small nuclear power plant into space easily enough. The ones in warships and submarines tend to be a few dozen megawatts and a few hundred tons. That would be too heavy for current infrastructure (the ISS is just over 400 tons). Something in the megawatt range might be conceivable though.

[DanceswithUnix]

You could lift a small nuclear power plant into space easily enough. The ones in warships and submarines tend to be a few dozen megawatts and a few hundred tons. That would be too heavy for current infrastructure (the ISS is just over 400 tons). Something in the megawatt range might be conceivable though.

I remember reading some scifi stories where spaceships were powered by a sort of nuclear tug on a long cable, so the tug didn't require shielding to keep the weight down.

[scaryjim]

I thought it was also that it has a potential to be quite efficient, so that with fission or even a PV array you could power it almost indefinitely, constantly.

Key word

At the minute they're in the region of mN/kW. That's not efficient. That's horrifically inefficient. To scale that up to rocket engine levels of thrust (generally thousands of kN) you'd need a power source generating terawatts of power. For comparison, in 2004 (according to this ofgem report) the combined generating capacity of the UK's major producers was less than 100 GW.

So yeah, they really need to work on the efficiency

EDIT:
Of course, that's a comparison to the sort of rocket engines used to get vehicles into space from Earth. Once you've escaped Earth's gravity you don't need anywhere near as much thrust; you just accelerate slowly and steadily until you hit the required speed. So you could build ships in orbit, or you could use conventional rocket engines as part of a multi stage setup where the final stage is an EM drive. The bottom line is that there are very few fuels with the energy density of hydrocarbons, and very few ways of employing them to generate thrust that are as efficient as a rocket engine. I'm pretty sure that if we ever use EM Drives in actual vehicles, we'll still see them getting off Earth by using conventional rockets, one way or another...

[DanceswithUnix]

Guess these might be handy for keeping things in orbit. I gather the ISS gets regular propellant shipments to keep it whizzing around and out of the way of whatever wanders close. That might still be necessary for avoidance etc, but perhaps could be minimised.

[EndlessWaves]

I remember reading some scifi stories where spaceships were powered by a sort of nuclear tug on a long cable, so the tug didn't require shielding to keep the weight down.

Sounds like a disaster waiting to happen. You'd have to time the burns perfectly to keep both in line when flipping the vessel over to decelerate.

Robotic ships might have it at one end of the vessel though, maybe even on a pole, so only a hemisphere or less has to be shielded.

For human ships it may be more economical to make use of the heat output directly and put it at the heart of the ship.

Still, reactionless drives are real.

Wow.

[Xlucine]

That was my first thought, tbh. On current thrust/power ratios you'd need gigawatts of power to get something off the ground at all. To make a craft that was solely EM Drive powered would take efficiency enhancements of many orders of magnitude, or the development of a much higher energy density power source than we currently have (fusion, anyone? ).

OTOH, using conventional rocketry to get components into orbit then building EM Drive powered craft in space so you don't have to overcome Earth's gravity well might have mileage - the thrust can be generated for as long as you have power, and we can already do long-lasting lower-power sources just fine, so over an extended period of continuous thrust you could accelerate a ship, free from the interference of gravity and drag, to significant speeds. That said, making course adjustments and stopping at the other end might be an issue....

Stopping is easy, you just turn around halfway through the journey and thrust in the opposite direction. Get a rocket that'll accelerate at 9.81m/s^2 and you'll even get gravity out of it!

[Xlucine]

Everyone read this site, no discussion of spacecraft propulsion is complete without it:
http://www.projectrho.com/public_htm...enginelist.php

You could lift a small nuclear power plant into space easily enough. The ones in warships and submarines tend to be a few dozen megawatts and a few hundred tons. That would be too heavy for current infrastructure (the ISS is just over 400 tons). Something in the megawatt range might be conceivable though.

It's already been done - the voyager probes ran off a nuclear power plant, as does the curiosity rover

Key word

At the minute they're in the region of mN/kW. That's not efficient. That's horrifically inefficient. To scale that up to rocket engine levels of thrust (generally thousands of kN) you'd need a power source generating terawatts of power. For comparison, in 2004 (according to this ofgem report) the combined generating capacity of the UK's major producers was less than 100 GW.

So yeah, they really need to work on the efficiency

EDIT:
Of course, that's a comparison to the sort of rocket engines used to get vehicles into space from Earth. Once you've escaped Earth's gravity you don't need anywhere near as much thrust; you just accelerate slowly and steadily until you hit the required speed. So you could build ships in orbit, or you could use conventional rocket engines as part of a multi stage setup where the final stage is an EM drive. The bottom line is that there are very few fuels with the energy density of hydrocarbons, and very few ways of employing them to generate thrust that are as efficient as a rocket engine. I'm pretty sure that if we ever use EM Drives in actual vehicles, we'll still see them getting off Earth by using conventional rockets, one way or another...

Engines to get into orbit are an entirely different beast to engines for moving around in space. To get into orbit you need lots of thrust per unit weight, to move around in orbit efficiently you want the highest exhaust velocity possible (the hardest part of moving around in space is bringing the fuel you need with you, so you want to make the best use of the stuff you do have). Chemical rockets offer great thrust to weight but low exhaust velocity; while ion engines (using an electric field to accelerate mass, kinda like a particle accelerator) have terrible thrust to weight and good exhaust velocities.

[DanceswithUnix]

It's already been done - the voyager probes ran off a nuclear power plant, as does the curiosity rover

I was aware of those, but they don't seem to scale to high output though, do they?

[EndlessWaves]

It's already been done - the voyager probes ran off a nuclear power plant, as does the curiosity rover

It has been done, but the examples you give are radioisotope thermal generators rather than nuclear reactors.

There are few full nuclear reactors up there too, in the US experimental SNAP satellite and some of the Soviet reconnaissance satellites.

They're tiny examples producing a few kilowatts though, nothing on the scale of what would be possible if you built the biggest one we could get up there in a single launch.

[scaryjim]

Everyone read this site, no discussion of spacecraft propulsion is complete without it:
http://www.projectrho.com/public_htm...enginelist.php ...

That's a lot of information to dump all at once

OTOH it's nice to know that my point - based on assumptions and application of first principles - is basically correct: that EM Drives would be no use for reaching orbit, but might be practical thereafter. I found it particularly interesting that existing ion thrusters don't have that much greater thrust: power ratios - if wikipedia is to be believe they generate up to 250mN off up to 7kW input energy. That's within 2 orders of magnitude, which sounds like a much more realistic gap to close.

[badass]

That's a lot of information to dump all at once

OTOH it's nice to know that my point - based on assumptions and application of first principles - is basically correct: that EM Drives would be no use for reaching orbit, but might be practical thereafter. I found it particularly interesting that existing ion thrusters don't have that much greater thrust: power ratios - if wikipedia is to be believe they generate up to 250mN off up to 7kW input energy. That's within 2 orders of magnitude, which sounds like a much more realistic gap to close.

Play Kerbal Space program. If you would have spent some time on that you would have immediately come to the same conclusion

What it really makes sink in is just how much fuel mass you spend getting something into orbit and how quickly you get there!

Once there, it's all about specific impulse and delta V