Tuesday, October 18, 2016

The Rocket Engine Nobody Understands Getting Verified

It's called an EM Drive, where EM is short for electromagnetic.  It's a spacecraft thruster developed by a single inventor, Roger Shawyer, and it's looking poised to revolutionize some aspects of space travel.  Think of 10 weeks travel time to Mars, instead of around 8 months.  Originally, scientists refused to acknowledge it could work because of two glaring, incredible, differences between the EM Engine and conventional engines.

It requires no chemical or nuclear fuel; no propellant at all.  It doesn't use exhaust in an action/reaction system like conventional engines. 

Let me point out it does require fuel in the sense of an electric supply.  It is not a "something for nothing" situation that would immediately be dismissed as a "perpetual motion" machine, violating the laws of thermodynamics.  But electrical power isn't something that's terribly hard to design into spacecraft; designers can use solar cell arrays, or even radioisotope thermal generators (RTGs).   The electrical power is used to run a microwave power source - the same frequency as microwave ovens, 2450 MHz - and the microwave power is dumped into a sealed cavity that's wider at one end than the other.  This generates thrust outside the cavity by some seldom-utilized properties of physics.
The EM drive engine, photo from the UK company, SPR Limited, that appears to be at the center of it all.  It looks like a conventional engine except for two things: there are no obvious fuel pipes on the left, and what would be the exhaust bell on the right is sealed. 

The fact that engine produces thrust is relatively easy to verify, and has been verified by labs that have studied it.  Among the groups studying it, the Chinese Academy of Sciences built a model and verified that at an input power of 2.5kW, their EmDrive thruster provides 720mN of thrust.  (720 milliNewtons isn't much thrust, but in the vacuum of space and running continuously, it can be a small, steady acceleration that can have a big effect).  NASA's paper is set to be released soon.  A paper in the journal Advances in Physics concludes it produces thrust in the same way as any other engine, using the electromagnetic energy in the cavity (referring to the energy as particles, photons, and not waves) . 
We consider the possibility that the exhaust is in a form that has so far escaped both experimental detection and theoretical attention. In the thruster’s cavity microwaves interfere with each other and invariably some photons will also end up co-propagating with opposite phases. At the destructive interference electromagnetic fields cancel. However, the photons themselves do not vanish for nothing but continue in propagation. These photon pairs without net electromagnetic field do not reflect back from the metal walls but escape from the resonator. By this action momentum is lost from the cavity which, according to the conservation of momentum, gives rise to an equal and opposite reaction.
The narrator here is AIP author Arto Annila.  Notice in this summary (from the abstract) there's no mention of how the thrust becomes directional.  Later in the paper, they say the asymmetry of the cavity directs more photons in one direction than other, causing the thrust.  In the original UK Mail article, they include an explanation that invokes a seldom-referred-to effect of general relativity.
Mike McCulloch of Plymouth University came up with a possible explanation based on a new theory of inertia.

McCulloch's suggests inertia arises from an effect predicted by Einstein's theory of general relativity called 'Unruh radiation'.

The Unruh radiation effect states that if you're accelerating in a vacuum, empty space will contain a gas of particles at a temperature proportional to the acceleration.

According to McCulloch, inertia is the pressure that the Unruh radiation exerts on an accelerating body.

When the accelerations involved are smaller, such as is the case with the EmDrive, the wavelength of Unruh radiation gets larger.

At extremely small accelerations, the wavelengths become too large to fit in the observable universe.

As a result, inertia may only take on whole-wavelength units over time, causing it to become 'quantized.' This means it can only in some multiple of a unit of measure, causing sudden jumps in acceleration.

But because of the EmDrive's truncated cone, the Unruh radiation in tiny.

The cone allows Unruh radiation of a certain size at the large end but only a smaller wavelength at the other end, according to an in-depth report by MIT.

This means the inertia of photons inside the cavity change as they bounce back and forth. To conserve momentum, they are forced to generate thrust.

This is a really interesting development.  I came across the EMDrive a while back but frankly didn't know what to make of it.  Now that it has been verified to produce thrust by several independent labs and the theoreticians are starting to develop models that explain how it works, things look up for it.  The inventor filed a patent (pdf warning) on an improvement to it, using a superconducting plane in the engine to improve efficiency.  The EMDrive has the potential to completely revolutionize the satellite industry.  Currently, satellites use small thrusters rated around one Newton - about the same size as the thrust EMDrive thrusters can generate.  (SPR says that a thruster like the one the Chinese Academy built, 750 mN, if deployed on the ISS, would easily provide the necessary acceleration to compensate for orbital decay, thus eliminating the need for the reboost/refueling missions.)  Satellites carry fuel which can be the ultimate life determinant for a mission.  The EMDrive can convert that to larger solar panels and microwave transmitter, giving potentially much longer life.  Right now, satellites in the Geostationary Orbit are out of reach of any known way to repair.  What if a satellite could be slowed down by constant light thrust from an EMDrive until it's in low Earth orbit and reachable by technicians on orbit?  Of course, the idea of using that constant low level thrust to speed trips to Mars is rather interesting, too.


  1. If we can only combine low energy nuclear reactions (cold fusion) with this tech before we fall back into another dark age we may yet get off this rock. We're so close to achieving these advances yet right on the cusp of civilizational collapse.
    Engineers....work faster!
    Politicians....quit working!
    Parasites.....start working!

    1. Cold fusion may or may not ever work. There is not reason a simple fission reactor cannot be used, however, over the objections of brainless primitivists.

      Heck, for robotic missions, a simple nuclear battery (the most reliable bit of kit ever made) could push 50 kilowatts into one of these for 20 years. That's 14.4N or ~3.25 pounds of thrust.

      Just for kicks, let's pencil this out: if your vehicle weighs 5000 kg, and you use a 14.4N thruster, this gives you 0.00288 m/sec^2 acceleration. If we run this for 10 years, we will achieve a velocity of 908,236.8 m/s, or 2,031,667 miles per hour.

      Since the acceleration is constant, the average velocity is just half this, or 454118.4 m/s, resulting in a traveled distance of 143,210,778,624,000 meters. This is the halfway point, so let's slow down to reach the destination. We will have traveled 2.86 x 10^14 meters, or 0.026 light years.

      Hmmm. We're going to have to decrease the weight of our vehicle or increase the power and efficiency of the thrust to get to the nearest star, but this in the realm of possibility which has never been the case before.

      Let's get cracking!

      (I've left out relativistic effects for simplicity)

  2. Why not strap a bunch of them together to increase thrust? It works for every other kind of engine, but due to the weird physics here, I'm not totally sure. Don't see why not though; at worst it would be some sort of proximity thing. Keep them X wavelengths apart and it will work. Probably easy since a wavelength is about 4 1/2 inches.

    1. Since everything is inside the conical waveguide I see no reason not to strap them together. I would like to see the watts-per-Newton increase, though. Now that we may be starting to understand why the damn things seem to work, we should be able to seriously improve the thrust efficiency.

      I want to see an on-orbit demonstration of this effect. I am still skeptical. It seems to violate the conservation of momentum. Also, are you certain that the Chinese number wasn't in micro Newtons rather than milliNewtons? 720mN is 0.16 pounds of thrust – and this certainly isn't difficult to measure.

    2. I meant to say "decrease", not "increase". More thrust per watt.

    3. The result was from the SPR Ltd. page, EMDrive.com. They link to a "professional translation" of the CAS paper. It's a pdf, and the abstract says,

      "when the microwave source output is 2.45GHz, with a microwave power of 80-2500W, the propulsion produced by the thruster is located in the range of 70-720mN, and the total measurement error is less than 12%."

      I read that as the RF output power being varied from 80 to 2500W - a ratio of just about 31 times (15 dB), and the thrust went from 70 to 720 mN, much closer to 10:1 or 10 dB (10.2 for the terminally anal retentive). My point is that for a 15 dB increase of the input to result in a 10 dB increase in the output is very similar to the RF amplifier characteristic of compression. At some point, you can drive the snot out of it and not get more power (you sometimes will see an amplifier deliver less power when you're driving the snot out of it). I can imagine there might be ways around that. I'm way out of my experience base here when it comes to thrusters, but I know RF amplifiers. The paper doesn't seem to address this.

      As for whether mN is for milli-Newton or micro-Newton, I don't know. I found it interesting that EMDrive.com also stated "Note that the Chinese thruster, if deployed on the ISS, would easily provide the necessary delta V to compensate for orbital decay, thus eliminating the need for the reboost/refueling missions." That certainly sounds more like milli than micro.


    4. Malatrope - head over to EMDRive.com to read other results. They're talking over 10 Newtons/kW RF.

      Teaser: "Clearly this simple form of compensation cannot completely compensate for the Doppler shift throughout a full pulse cycle, but fig 4 shows that the specific thrust at 20m/s/s can be improved to 92 N/kW". Closing in on 100N/kW (this is a cryogenically cooled, superconducting cavity).

      Complete paper:

    5. Jesus. Ok, here we go, folks. It's a new era.

    6. These are still computer models, though. I want to push some electric into an actual thruster and watch it skitter across the desk before I'll believe this thing. We've been burned before.

    7. For sure. The papers appear to refer to a 2nd Generation thruster with superconducting parts inside. Like you say, I want to see it working, but if they're really getting "whole numbers" of Newtons, that's a game changer. One of the slides was talking about accelerations of 20m/sec^2 - 2 Gs - and another talked about 100 m/sec^2 - 10 Gs. That's serious.

  3. Wouldn't you expect to detect the (tunneled?) photon exhaust in the form of more microwave leakage on the exhaust end?

    Wouldn't it be spiffy if electrostatic confinement fusion could be arranged so the reactor is a resonant cavity which leaks preferentially at one end? A multi gigawatt nuclear powered laser spacecraft drive? Photons have the highest exhaust velocity. Cue the Kzinti wars.

    1. I guess I'm not sure the photons that constitute the momentum transfer are at the same frequency as the transmitter. I don't know if the process robs them of that information.

      An analogy they use is the cancellation of waves that are 180 out of phase. The wave is gone, but the medium is still there. Do the photons shift to VLF? I don't understand the math well enough to answer this.

    2. I think we are finally seeing some questions forced to be asked at the very fundamental levels of physics, that should have been investigated decades ago.

      I also like the idea of a combined nuclear/resonance engine design using electrostatic confinement fusion – another area I had great hope for but hasn't attracted enough funding to answer the question of feasibility resoundingly. I still want to build a big fusor in my garage!

    3. (I wish we could edit comments, but...to add:)

      One of the major issues with electrostatic confinement fusion is extracting the energy, which comes out as generalized electromagnetic radiation and raw particulate heat. Can we not imagine a design where that energy comes out into a tuned cavity so that we can output pure microwave radiation? And can that not be channeled directly into one of these incredibly simple boxes? Can we not imagine what we could do with a simple, few-to-no-moving-part energy generator and EM drive converting many gigawatts of power directly to linear thrust?

      If these two technologies pan out, we could become an interplanetary species long before Elon Musk can get there on methane and liquid oxygen!

  4. Referring to the new NASA paper "leaked":

    Here is the thing, let's say it is true, and it's 1.2mN/kw.
    Solar panels top out at about 200W/kg, and even when I worked on design of ultra-low-mass liquid metal REMHD nuclear reactors for the interstellar precursor mission, they were about 300 W/kg (which was 3-10x better than existing designs)

    So that means the specific thrust of the power + thruster in this case (assuming massless thruster) would be 0.24-0.36 mN/kg.

    A hall thruster with the same assumptions is about 12-18 mN/kg, but uses roughly 50mg / mN-sec of thrust.

    So a 10kg spacecraft with 1mN of thrust would require:
    3-4 kg of EM thruster+power
    0.08-0.055 kg of hall thruster + power.

    The difference in mass (effectively 3-4 kg) is sufficient to run the hall thruster for 60-80 million seconds
    (Roughly 80,000 N-s of impulse). During this time, both spacecraft would accelerate at about 1/100,000th of a G, or a total delta-V of 6-8 km/s (roughly 10 km/s for the hall thruster), however, this is nearly 3 -years- of thrust.

    If one wanted to increase the effective acceleration by a factor of 10, the hall thruster example would still close...it would be less than 1kg of thruster+power, basically the same delta-V -and- payload, and an actual usable acceleration (i.e., about 3 months). Meanwhile, the EM thruster simply can't do any better than a 1-2x improvement, and only at a HUGE expense in payload.

    This means that to be competitive AT ALL with existing (and flight proven) systems for ANY mission, either the power density of the power source needs to increase by 5-10x, or the thrust efficiency of the EM thruster needs to be 5-10x better. (Well, and be proven to work at all)

    Now, this seems like a simple solution...but 2-3kW/kg is roughly the power to weight ratio of an F1 engine...and it would have to do that w/o using any substantial fuel over 6-8 million seconds (meaning, an energy density including "fuel" of 6.4 GJ/kg...or roughly the THEORETICAL limit for ultra-thin-film gossamer solar panels (at earth orbit) or actually close to the theoretical burnup limit of some nuclear reactors, and nearly 10x better in power density than even the insane types I've designed)

    Or the thruster efficiency could be improved by 10x...which, no one claims is possible (even those that claim this actually works).

  5. AND the above assumes a massless thruster. This is a huge problem, as even the best power electronics are on the order of a 5-10 kW per kg for low frequency, and that isn't including radiators and such needed for use in space, and RF ones are a factor of a few below that. That means including electronics barely effects the hall thruster example, while substantially changing the math (either by completely eliminating the payload if you use current state of the art, or increasing the mass of the propulsion system by 20-30% if you assume what is theoretically possible for both power system and electronics.

    Note that EXISTING and flown hall thrusters and PSU's are at the required level to make the above work--so EVEN IF the EM drive worked as advertised, either the power system needs to be either beyond what is physically possible, OR the thruster needs to be 10-20x better than what is even predicted. And given they been tweaking the design for literally years, if this were possible, (or even a 2-3x improvement) they would have done so as it would have made the results almost entirely unambiguous.

    Applying the same insane power density assumptions to the hall thruster (or better yet, an MPD thruster at 5x the ISP and 25x more power and means it is that much better--it would have an additional 2-3x advantage of the EMdrive, meaning for the same assumptions in power, the EM drive would have to be 20-30x the efficiency it is now...which literally no one thinks is even theoretically possible with even he most favorable assumptions. When including thruster and power system mass, the numbers are even worse for the EMdrive.
    (Note, if you include tankage and other crap with the hall thruster, it gets 1.1-2x worse depending on overall fuel mass, but that's actually negligible in the mission space that could be accessed with either system)

    So there you have it, EVEN IF IT WORKS AS ADVERTISED, it's still more than an order of magnitude worse than what would make it competitive with current state of the art systems (that are proven and flown) let alone competitive with what is theoretically possible with future systems.

    This means that EVEN IF there is some misunderstood physics that makes the EMdrive possible, EVEN THAT physics would have to be wrong by 20-30x for this to be useful IN ANY WAY.

    This is like saying "this is awesome, because we think this totally non-understood system could actually be workable, but only if it's actually 20-30x better than we hope (even though, if we could do that, we would as it would settle any questions) AND we make fundamental, effectively impossible improvements to power systems.