RF Resonant Cavity thruster-based spacecraft

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RF Resonant Cavity thruster-based spacecraft

Postby buggy » Tue Sep 30, 2014 6:17 am UTC

This is a idea that I've had for a while, ever since seeing the news article about the NASA research group that tested the dubious sounding RF Resonant Cavity thruster, a.k.a. EmDrive/Cannae Drive, and found that it appears to actually work. Theres a lot of debate as to whether or not it actually does work, but for this thought experiment we're going to assume that it does, infact, work.


So say you want to get to space. Conventionally, you would use a chemical rocket. Chemical rockets are inefficient, dangerous, and loud, but this is the only practical method to get to space that we can currently use. Generally, you would have to build a massive, staged rocket to get any kind of reasonable mass to orbit. This means that getting mass to orbit is very expensive.

Now lets look at the EmDrive. Current experiments have shown a input energy-thrust efficiency comparable to Ion drives, which is excellent if you are already in space but nowhere near enough to get you off of the earth. Calculations have suggested that a properly designed resonance cavity could obtain efficiencies as high as .1N/KW, which is still not enough to get to space. Thus, while extremely useful outside of a atmosphere since thrust is linearly proportional to input energy and it requires no propellant, it just doesn't produce enough thrust to get you off the ground on Earth.

This would be the end of it, but the thrust of the EmDrive is based on the Q of the resonance cavity. A higher Q value means more energy is converted to useful thrust, and less is converted into useless heat. The Q of the cavity is determined by the conductivity of the materials it is constructed out of. A cavity constructed out of superconducting materials would have a efficiency approaching 100%, and this is backed up by the creator of the Cannae Drive, who has claimed to have constructed a superconducting version of his drive and found it to be in the range of 90% efficient.

So, theoretically, utilizing a supercooled EmDrive, you could be getting as high as a 90% or above conversion ratio of electrical energy-thrust. This is where things get interesting.

So, here is the design I've been toying with. We want to get to space, and we want to do it with a reasonable budget. To get to space we need a object, i.e. spacecraft, that has:

1: A sufficiently strong structure that can hold everything together
2: A means to produce thrust sufficient to counteract gravity and accelerate the object, a engine
3: Whatever is required to keep the engine functional, i.e. fuel
4: A guidance system and means to control the spacecraft
5: A means to keep its occupants alive, this means you need the following
5A: A hermetically sealed internal compartment so passengers do not suffer decompression
5B: A source of fresh air (moderately long trips), and source of food and water (very long trips)

So lets start with the hull, which is largely responsible for requirement 5A. A cost effective solution I've come up with is a Intermodal Shipping Container, generally just known as a cargo container or shipping container. These are the large metal containers that can be seen on shipping vessels. A standard 40' shipping container is approximately 12M by 2.5M by 2.5M, and weighs 3,800 kg. These are not air tight, but are close enough that it would be reasonably possible to make them so. They are also quite durable, capable of holding more than 10x their weight, but may still not be durable enough to survive 1 atmosphere of outward pressure. I've assumed that the modifications and reinforcements required to make them durable enough for space and airtight would raise the weight to 5,000kg.

So we have our hermetically sealed internal compartment and sufficiently strong structure. Lets talk about number 2 and 3.

RF Cavity thrusters produce thrust from electricity, no propellant or reaction mass required. This means that no exhaust is produced, so you can safely put them inside the hermetically sealed portion of the spacecraft. This removes a lot of issues with complexity, maintenance, and even guidance since you can easily put them infront of the center of mass. As they produce thrust from electricity, you do not have to deal with the high temperatures of chemical propellants and reactants, or the issues of storing and piping them. You simply have to generate enough electrical energy.

Assuming that we have a 100% efficient EmDrive, we would require approximately 9 watts of power per kg to counteract gravity. A reasonable figure would be to have 20 watts of power per kg, to provide a little over 2 gs of acceleration. This means that we would need 20kW per 1,000kg, and thus about 100kW for our 5000kg structure. Lets add a little leeway for the mass of occupants, the propulsion system, and the electrical systems, say about another 5000kg. So we need 200kw.

A widely used option for power in space is solar power. In direct sunlight, about 1kW of power hits a square meter. Most modern commercial solar panels are between 15-20% efficient. So lets assume 150W per m^3. The top of our container, or any side of it really, is 12m by 2.5m, so 30m^3. This means that if you covered the top of the container with solar panels, you could produce approximately 30*0.150=4.5kW, or about one fiftieth of what we need. So how about we build some scaffolding 2m out from each side of the container, and put solar panels on it. We now have 14m by 4.5m, or 63 m^3. This gives us 63*0.150=9.45kw, still not enough. Top of the line, space-grade solar panels have upwards of 29% efficiency, so lets say we use 25% efficient solar panels. This gets us 63*0.25=15.75kw, even still not nearly enough. But we need to power the lamps inside the spacecraft, so lets do it for now anyway.

For the portion directly ontop of it, the container provides all the structure the solar panels need. The mass per unit area of solar panels is not a easy figure to locate, but what sources I've found suggest between 0.9kg/m^2 for space-grade solar cells, and 1kg per 10 watts for commercial units with structural reinforcement, so between 0.9kg/m^2 and 15 kg/m^2. Lets just assume that its 2kg/m^2. This means our 63m^2 of solar panels has a mass of 126kg. Lets add 174kg for the scaffolding so we have a nice round 300kg.

So, we have a spacecraft that now weighs 5300kg but can only produce 15.75Kw out of 200. At this rate we could probably get enough if we just kept adding solar panels, but it would be quite complicated and expensive to add over 500m^2 of panels to our little cargo container. So we need another power source. I've found that high density rechargable lithium ion batteries would be a good source.

Lithium ion batteries have energy capacities of around 100-260 watt hours per kg. They can output between 250-340 watts, so they can usually discharge themselves completely in less than a hour. Of course, batteries work differently than solar power, they are a limited source. Instead of going through a lot of math to figure out how many batteries we need for enough energy to get to orbit, we can just use watt hours. If we have 200 kwh of batteries, we can power the engines for 1 hour. If we can get to orbit in a hour, then we have enough.

Lets give 2 hours of time to get to orbit, just to be safe. So we need 400kwh, at 100-260 kw per 1000 kg of batteries, that gives us between 1500-4000kg of batteries, depending on the kind we use. Lets assume we use 3000kg of batteries. We now have a spacecraft that weighs 8,300kg and has more than enough power to get to orbit. Once in orbit, you can use those 300kg of panels we decided to leave on the craft to recharge the batteries, and within a day you should have enough power to go back down again.

So the final issue is guidance. EmDrives produce thrust from microwaves, these microwaves are produced by a microwave source such as a magnetron. Depending on what kind of microwave source is used, you may or may not be able to increase or decrease input power on-the-fly to increase or decrease the amount of microwaves, and therefore thrust, produced. However sources suggest that EmDrives pratically have no warm-up/cool-down period, thus even if you cannot lower the thrust dynamically you can rapidly switch the drive on and off to simulate reduced thrust. This is exactly how microwave ovens run at lower power levels, the magnetron is just periodically switched on and off instead of running at a lower power level.

Assuming that EmDrives use magnetrons, that means we have a upper limit of approximately 3kw of thrust per drive, as magnetrons have a upper limit of about 3kw of sustained output. This means we would need a lot of them. Luckily they are fairly small, so it isn't unreasonable to fit nearly a hundred of them on the inside of the cargo container. By having power controls for all of them hooked up to a computer, we could control the thrust at points around the entirety of the craft. This makes steering fairly easy, assuming we have gyroscopes or some other method to detect rotation, it simply becomes a matter of programming.

We now have everything we need for a viable spacecraft. With 1,700kg of mass free, we should have no problem carrying people and a few oxygen tanks, if the air left in the quite roomy cargo container isn't enough already. Lets see how we did as far as making it cheap.

Cargo container: 3,000$~
Solar panels: 1$/w~, 16,000$~ total
Li-ion batteries: varies massively, anywhere between 80,000$ and 1,000,000$
EmDrives: No clue, they aren't terribly complex but I can't find any prices for their components
Guidance: Under 10,000$ depending on control method
Wiring: Under 10,000$
Structural additions: Probably under 10,000$
This comes to around 150,000$-300,000$ for the lower end of estimates, 1,200,000$ or so at most.

There are a LOT of 'if's and 'ish's here, and I've assumed a lot of things. For one, we assume that RF Resonant Cavity Thrusters work, but a research team at NASA, a chinese research team, and the european space agency, all have found that it appears to work, even if they aren't sure how. They all have similar and consistant findings as well that fit with the theory of how exactly these engines work, so I believe that they work despite the relatively little testing that has been reported so far.

I designed the craft with a LOT of leeway to account for inaccuracies, we have nearly double the amount of batteries we need and we still have nearly two tons of 'free' mass. A more accurate design could plausibly halve the cost of the craft, meaning you could potentally build this thing for under 100,000$. This is a BIG reduction compared to the billions that current spacetravel costs, and moreover this is both completely reusable and solar powered, so it essentially costs nothing to launch repeatedly ignoring maintenance.

As well, the craft is still partially solar powered. Once in space, you could maintain about 0.1gs of constant acceleration, depending on how far from the sun you are. This means you could get to a LOT of interesting places once your in orbit, and pretty quickly too. The moon would take a day or two, mars a week or two. My original plan for this set mars as a goal, but, for the sole reason of being unable to figure out how to get or make a cheap space suit, I set orbit as my goal.

There are still a few issues I haven`t accounted for, some of which are more significant than others:
Big:
1: Cooling. To be so efficient, the EmDrives have to be superconducting. We don't have viable room-temperature superconductors yet, so it has to be made from low-temperature superconductors. I don't know how much waste heat a properly designed EmDrive will produce in the important parts that need to be cold. Theoretically, it won't produce any. Practically, it could be as high as 90%. I can't see any way to really test this without actually building one.

I considered using a stirling heat pump, which could get it cold enough, but those are extremely inefficient, 5% or so, so if the Drives produce a lot of heat then the pumps produce a lot more heat and things quickly get very hot and inefficient. Another method would be to just use a tank of liquid nitrogen that cools by evaporative cooling, and vent the gaseous nitrogen into space. This would be fairly cheap, liquid nitrogen is very cheap in bulk but it probably wouldn't last long enough for long trip. It would also take big tanks, and add a lot of complicated piping and insulation, pretty much exactly the things we were happy to get rid of when we got rid of chemical propellants.

Medium:
1: Radiation. Not too big of a issue near Earth, and really if I had a chance to take a trip to mars I'd happily accept a bit of radiation on the way there. This issue is a little complicated but not deal breaking, and there's a bunch of ways you could potentally go about solving it.

Small:
1:Engine orientation: This is a bit of a silly one. You can steer by turning the whole craft, not a issue. And theoretically on the way up your 95% battery power anyway, and your launching upwards towards the sun for at least part of it. But if you want to go to, say, mars, you'd be thrusting away from the sun, so the solar panels would be facing away from the sun too, since you put them on the top of the craft. So you can't power the engines continously.

If you put them on the bottom of the craft, you can't sit on the ground and charge via solar power, you can't use them for extra power during takeoff, you have the added complexity of having to flip the four ton container during construction, and you risk landing on them and breaking them. Really not sure how to solve this without adding a whole extra set of solar panels on the bottom that you still might land on and break, but you can work around it anyway.


Thank you for taking the time to read this, I know there is probably a bunch of issues and mistakes and misconceptions I've gleamed over, as well as grammer issues. I've also not cited ANY of my sources, but it's already taken over four hours to type this and I'd prefer not to spend more time noting sources. I hope that despite the fact that I haven't posted here very much, and despite the mistakes and errors, you still take the time to read this massive post. For those that don't want to read through it all:

Tl;dr: I thought of a theoretical spacecraft that could potentally get to space for a thousandth of the cost of current spacecraft by utilizing a brand new propulsion technology.


Obviously NASA, or anyone else really, would have jumped on this if this had been possible for a while, but the technology behind it is brand new and still a little dubious.

So tell me, why can't we get a brand-new spaceship for the cost of a small house?

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Re: RF Resonant Cavity thruster-based spacecraft

Postby Zamfir » Tue Sep 30, 2014 10:28 am UTC

Assuming that we have a 100% efficient EmDrive, we would require approximately 9 watts of power per kg to counteract gravity.

This is the part that might need some elaboration. Keep in mind that watts are not newtons...

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Re: RF Resonant Cavity thruster-based spacecraft

Postby johnny_7713 » Tue Sep 30, 2014 12:04 pm UTC

To get to a 400 km altitude in 1 hour you need an average speed of 111 m/s. If you are purely counteracting gravity (at sea level) on 8,300 kg, you need 81.4 kN. This gives an absolute minimum required power of just over 9 MW (111*81.4e3), at least initially. Note that is the actual thrust power required, so input power into your propulsion system will need to be more. All of the above is neglecting atmospheric drag and needing to accelerate to orbital velocity.

Regarding your mass estimates you seem to have left out the mass of the actual EM-drives as wel as wiring, flight computers, all manner of life support and thermal control systems and even the seats for the passengers.

Regarding your cost estimates a bunch of components do not make a spacecraft. You've left out all the personnel costs for production, as well as engineering costs for producing a detailed design, and the costs of testing everything. Actually developing a 90% efficient EM-drive will also be massively expensive.
Incidentally as your design gets more accurate you generally identify more stuff you need, so the price of your design goes up, not down.

A shipping container is built to resist vertical compressive force and to a lesser extent vertical tensile force, not internal overpressure. Also shipping containers are made of steel (afaik) which is not the best choice if you're going for a light-weight structure.

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Re: RF Resonant Cavity thruster-based spacecraft

Postby Zamfir » Tue Sep 30, 2014 6:00 pm UTC

This gives an absolute minimum required power of just over 9 MW (111*81.4e3), at least initially.

The EMDrive already violates conservation of momentum as we know it. It might work itself around conservation of energy as well. Alternatively, you could fly up at a millimeter per second, to reduce the power requirements.

I wonder, if you hover with a reaction less drive of 90% ( or 1%) efficiency, would you consume any energy? It's 90% of zero, after all.

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Re: RF Resonant Cavity thruster-based spacecraft

Postby Copper Bezel » Tue Sep 30, 2014 7:32 pm UTC

Huh? Aren't you still fighting gravity the whole way? Or is it like sticking a Command hook to the ether and pulling yourself up on it?
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Re: RF Resonant Cavity thruster-based spacecraft

Postby Izawwlgood » Tue Sep 30, 2014 7:47 pm UTC

buggy wrote:Assuming that we have a 100% efficient EmDrive
Waitwaitwait, when did this happen?
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Re: RF Resonant Cavity thruster-based spacecraft

Postby Zamfir » Tue Sep 30, 2014 8:04 pm UTC

Huh? Aren't you still fighting gravity the whole way? Or is it like sticking a Command hook to the ether and pulling yourself up on it?


Yes, a reactionless drive is pretty much a skyhook. The EMDrive supposed to work through quantum or through relativity, because the ether is unfashionable. Its proponents mightily disagree on how it works. NASA did some test which measured a little residual force, hence the attention. But in the same test, they also removed the feature that was supposedly creating the effect and the magic was still there. So either the effect is some unaccounted error in the measurements, or the effect is not actually understood by its builders.

For the sake of the discussion, we're basically assuming that second option, plus that we can increase the strength of the machine a hundred thousandfold or so. In which case you could indeed airlift containers to space with a bunch of microwave ovens. Of course, we could also assume that the strength increases 100 million times, then we could lift skyscrapers with a light bulb.

At that point, it becomes relevant whether the EMdrive can also break (apparent) conservation of energy. By drawing energy from the quantum foam or so. If not, the skyscraper will move upwards very slowly. But who knows.

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Re: RF Resonant Cavity thruster-based spacecraft

Postby buggy » Tue Sep 30, 2014 8:52 pm UTC

Izawwlgood wrote:
buggy wrote:Assuming that we have a 100% efficient EmDrive
Waitwaitwait, when did this happen?
This is, as far as I know, the theoretical max. I assumed we were using a perfect EmDrive to simplify calculations.

johnny_7713 wrote:To get to a 400 km altitude in 1 hour you need an average speed of 111 m/s. If you are purely counteracting gravity (at sea level) on 8,300 kg, you need 81.4 kN. This gives an absolute minimum required power of just over 9 MW (111*81.4e3), at least initially. Note that is the actual thrust power required, so input power into your propulsion system will need to be more. All of the above is neglecting atmospheric drag and needing to accelerate to orbital velocity.

Regarding your mass estimates you seem to have left out the mass of the actual EM-drives as wel as wiring, flight computers, all manner of life support and thermal control systems and even the seats for the passengers.

Regarding your cost estimates a bunch of components do not make a spacecraft. You've left out all the personnel costs for production, as well as engineering costs for producing a detailed design, and the costs of testing everything. Actually developing a 90% efficient EM-drive will also be massively expensive.
Incidentally as your design gets more accurate you generally identify more stuff you need, so the price of your design goes up, not down.

A shipping container is built to resist vertical compressive force and to a lesser extent vertical tensile force, not internal overpressure. Also shipping containers are made of steel (afaik) which is not the best choice if you're going for a light-weight structure.

As for the unaccounted mass of wiring and other things, I assumed that would be part of the 1,700kg that is 'free'. As per engineering costs, from my understanding, the design would be 'simple' enough that a single person could do this. The solar panels would directly power the batteries, and power from the batteries would be converted to A.C. and power the magnetron. Relays on each drive's circuit would control the drive. The structure itself is subpar but I, probably wrongly, assumed that EmDrives were so efficient that a lightweight, carefully constructed spacecraft wouldn't be required. Thus, a heavy shipping container with added internal supports would suffice.

As for actually designing a 90% efficient drive, the creator of the Cannae drive has reported that he already created one. I still can't find enough info about it's actual construction to determine how hard it would be to produce or how heavy the cavity itself is.
Zamfir wrote:
Assuming that we have a 100% efficient EmDrive, we would require approximately 9 watts of power per kg to counteract gravity.

This is the part that might need some elaboration. Keep in mind that watts are not newtons...

Then I may have fudged one of the most important unit conversions. "When an object's velocity is held constant at one meter per second against constant opposing force of one newton the rate at which work is done is 1 watt." So a newton is 1 meter per second squared, gravity is about 9m/s^2 or 9 newtons, so opposing gravity would require 9 watts?

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Re: RF Resonant Cavity thruster-based spacecraft

Postby Izawwlgood » Tue Sep 30, 2014 9:11 pm UTC

buggy wrote:This is, as far as I know, the theoretical max. I assumed we were using a perfect EmDrive to simplify calculations.
Well, this is posted to 'Science', not 'Fictional Science', which is what was confusing me. The EmDrive hasn't been shown to do much of anything assuredly.
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Re: RF Resonant Cavity thruster-based spacecraft

Postby buggy » Tue Sep 30, 2014 9:20 pm UTC

Izawwlgood wrote:
buggy wrote:This is, as far as I know, the theoretical max. I assumed we were using a perfect EmDrive to simplify calculations.
Well, this is posted to 'Science', not 'Fictional Science', which is what was confusing me. The EmDrive hasn't been shown to do much of anything assuredly.

Yes, but like I said in my original post I'm assuming it does to avoid a largely off-topic debate.

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Re: RF Resonant Cavity thruster-based spacecraft

Postby gmalivuk » Wed Oct 01, 2014 4:06 am UTC

buggy wrote:
Zamfir wrote:
Assuming that we have a 100% efficient EmDrive, we would require approximately 9 watts of power per kg to counteract gravity.

This is the part that might need some elaboration. Keep in mind that watts are not newtons...

Then I may have fudged one of the most important unit conversions. "When an object's velocity is held constant at one meter per second against constant opposing force of one newton the rate at which work is done is 1 watt." So a newton is 1 meter per second squared, gravity is about 9m/s^2 or 9 newtons, so opposing gravity would require 9 watts?

It would require about 10 watts/kg minimum to rise at a constant 1m/s near Earth's surface. If it's rising at 10m/s it would require 100W/kg. If it's rising at 100m/s it would require 1kW/kg. The relationship you have quoted is one between power and speed*force.

To get a relationship between power and force directly, you need to know how that force is being generated. My desk is exerting approximately 0 watts of power to hold my phone stationary against the pull of gravity, but if I attached a (magically weightless) photon drive to make the phone hover, it would require a few hundred megawatts of power even at perfect efficiency.
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Re: RF Resonant Cavity thruster-based spacecraft

Postby gmalivuk » Wed Oct 01, 2014 4:07 am UTC

Izawwlgood wrote:
buggy wrote:This is, as far as I know, the theoretical max. I assumed we were using a perfect EmDrive to simplify calculations.
Well, this is posted to 'Science', not 'Fictional Science', which is what was confusing me. The EmDrive hasn't been shown to do much of anything assuredly.

Yeah, I moved it to Fictional Science, because if we're taking for granted that a reactionless drive exists, we're doing science fiction.
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Re: RF Resonant Cavity thruster-based spacecraft

Postby johnny_7713 » Wed Oct 01, 2014 7:37 am UTC

buggy wrote:
johnny_7713 wrote:To get to a 400 km altitude in 1 hour you need an average speed of 111 m/s. If you are purely counteracting gravity (at sea level) on 8,300 kg, you need 81.4 kN. This gives an absolute minimum required power of just over 9 MW (111*81.4e3), at least initially. Note that is the actual thrust power required, so input power into your propulsion system will need to be more. All of the above is neglecting atmospheric drag and needing to accelerate to orbital velocity.

Regarding your mass estimates you seem to have left out the mass of the actual EM-drives as wel as wiring, flight computers, all manner of life support and thermal control systems and even the seats for the passengers.

Regarding your cost estimates a bunch of components do not make a spacecraft. You've left out all the personnel costs for production, as well as engineering costs for producing a detailed design, and the costs of testing everything. Actually developing a 90% efficient EM-drive will also be massively expensive.
Incidentally as your design gets more accurate you generally identify more stuff you need, so the price of your design goes up, not down.

A shipping container is built to resist vertical compressive force and to a lesser extent vertical tensile force, not internal overpressure. Also shipping containers are made of steel (afaik) which is not the best choice if you're going for a light-weight structure.

As for the unaccounted mass of wiring and other things, I assumed that would be part of the 1,700kg that is 'free'. As per engineering costs, from my understanding, the design would be 'simple' enough that a single person could do this. The solar panels would directly power the batteries, and power from the batteries would be converted to A.C. and power the magnetron. Relays on each drive's circuit would control the drive. The structure itself is subpar but I, probably wrongly, assumed that EmDrives were so efficient that a lightweight, carefully constructed spacecraft wouldn't be required. Thus, a heavy shipping container with added internal supports would suffice.

As for actually designing a 90% efficient drive, the creator of the Cannae drive has reported that he already created one. I still can't find enough info about it's actual construction to determine how hard it would be to produce or how heavy the cavity itself is.


Even if your propulsion system is super simple and can be designed by one person, you still need to figure out thermal control (don't want your astronauts to boil alive or freeze to death), life support (air, water, waste product processing), GNC (how are you going to point your space craft? Also, software to run the flight computer is not off the shelf), communications, orbital mechanics, etc. All of that has to be done down to the very last screw on the drawing, not just to back-of-the-envelope size estimates. It's not a one-man job.

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Re: RF Resonant Cavity thruster-based spacecraft

Postby cjameshuff » Fri Oct 03, 2014 12:22 pm UTC

Zamfir wrote:
This gives an absolute minimum required power of just over 9 MW (111*81.4e3), at least initially.

The EMDrive already violates conservation of momentum as we know it. It might work itself around conservation of energy as well.


It does. Check page 6 of http://www.emdrive.com/2Gupdate.pdf, where Shawyer claims it conserves energy. You'll find that, according to him, a drive accelerating in the direction opposite to its thrust experiences an increase in energy. So just point one down and sit it on the ground (which will accelerate it upward at a constant 9.8 m/s^2 at no cost), and it'll pour out energy. (Shawyer is a bit fuzzy on things like the equivalence principle, the lack of absolute velocity in relativity, etc.)


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