Author Topic: Propellantless Field Propulsion and application  (Read 688633 times)

Offline QuantumG

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Re: Propellantless Field Propulsion and application
« Reply #1760 on: 12/03/2012 10:53 PM »
isp is just a peculiar way of measuring thrust per unit of energy. Saying isp is infinite, even for an externally powered thruster, gives the wrong impression.
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Offline 93143

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Re: Propellantless Field Propulsion and application
« Reply #1761 on: 12/03/2012 10:56 PM »
It also has to do (primarily, even, it could be argued) with how much delta-V you can get out of your thruster before you run out of propellant.  That's why it's in units of impulse per unit mass (ie: "specific impulse").  If all of your propellant is externally supplied, you can keep thrusting until you either break down or exit the region where the propellant supply is available.
« Last Edit: 12/03/2012 10:58 PM by 93143 »

Offline MP99

Re: Propellantless Field Propulsion and application
« Reply #1762 on: 12/03/2012 10:59 PM »
Quote
Maybe I do not understand your statement, but isn't that obvious?

The EMDrive will only have thrust when there is external power being supplied to it from solar panels or a nuclear reactor or some other source of electricity.


No, I meant a powersource that is not in the same reference frame as the EM drive (e.g. attached to it). From what I understand all current experiments were run with the power supply not beig moved with the drive itself.

IIUC, the EM Drive is claimed to reduce in efficiency as it speeds up. Don't understand how that is compatible with Relativity.

cheers, Martin

Offline Afrocle

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Re: Propellantless Field Propulsion and application
« Reply #1763 on: 12/03/2012 11:09 PM »
How did you calculate your 8.3 x 10-6 Newtons of thrust from a 2.5-kw power source? It is already established that a solar sail achieves a larger 9.15 x 10-6 Newtons of thrust from a smaller 1.37-kw power source (i.e. the sun at 1 AU).

photons reflect off surfaces at twice the radiation pressure that they are absorbed by surfaces

This is because the incoming and outgoing momenta are equal and opposite.  With absorption - or radiation - you only have one or the other.

(9.14/2)*2.5/1.37 = 8.34

Quote
What or where is this "equal and opposite force somewhere else in the power system" that balances the other EMDrive forces and prevents the EMDrive from working?

It's not a question of "preventing the EMDrive from working".  Conservation of momentum must be satisfied.  If the source of the thrust is ordinary electromagnetism (which I'm not claiming), then the reaction is most likely electromagnetic, and while it could be interacting with Earth's magnetic field or an unrelated piece of lab equipment someone forgot to turn off, another possible explanation is that the circuit is producing some sort of inductive effect, or perhaps a charge separation - some sort of electromagnetic effect internal to the integrated test setup, which would naturally sum to zero over the whole apparatus.

I don't know what the source of the thrust is.  We don't have enough information yet.

What are you comparing this "too small" thrust to? An ion drive engine at 5,000-sec Isp would have 10 times less thrust for the same power input as the EMDrive, so what ion drive are you basing your comment on?

Yes, but we don't know the EM-Drive works yet.  He's comparing the ion drive to a conventional photon drive, which has a power-to-thrust ratio of 299792458 W/N.

In your equation of (9.14/2)*2.5/1.37 = 8.34 you are combining the 1.37-kw propulsive source of the solar sail being propulsed by radiation pressure from the sun at 1 AU and the radiation pressure from a 2.5-kw power source located 6 centimeters away from the intended surface. That does not make sense.

Radiation pressure on a surface area is related to 1/3 of the radiant energy by unit volume within a space. The unit volume of a 6-cm microwave cavity is orders of magnitude smaller than the unit volume of energy coming from the sun.

Maybe I should re-ask my question in a different way.....What is the radiation pressure in Newtons/m2 on a surface within a microwave oven at a 2.5-kw power setting?


Offline 93143

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Re: Propellantless Field Propulsion and application
« Reply #1764 on: 12/03/2012 11:11 PM »
Radiation pressure is a surface effect, not a volume effect.

That calculation is not how I got my number originally; it just shows that it's consistent with yours.
« Last Edit: 12/03/2012 11:18 PM by 93143 »

Offline Afrocle

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Re: Propellantless Field Propulsion and application
« Reply #1765 on: 12/03/2012 11:13 PM »
Quote
Maybe I do not understand your statement, but isn't that obvious?

The EMDrive will only have thrust when there is external power being supplied to it from solar panels or a nuclear reactor or some other source of electricity.


No, I meant a powersource that is not in the same reference frame as the EM drive (e.g. attached to it). From what I understand all current experiments were run with the power supply not beig moved with the drive itself.

IIUC, the EM Drive is claimed to reduce in efficiency as it speeds up. Don't understand how that is compatible with Relativity.

cheers, Martin

I think that the Chinese paper steered away from that assertion. The Chinese paper seemed to have a tone that suggested that the original founder of the EMDrive did not understand what he was doing when he started this in 2001.

I am not being negative on the founder of the EMDrive, but it appears that we should start with the recent Chinese paper to analyze what makes sense and what does not.

Offline Afrocle

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Re: Propellantless Field Propulsion and application
« Reply #1766 on: 12/03/2012 11:15 PM »
Power-to-thrust is 299792458 W/N, as I mentioned above.  This means that if you generate the power internally, you need to convert (assuming 100% efficiency) 3.33564095e-9 kg into energy and expel it for every NĚs of impulse you get from your drive.

In other words, the specific impulse is 299792458 NĚs/kg, or 30570323 seconds.

If you use solar power, on the other hand, the Isp really is infinite...

Thank you.

Offline Afrocle

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Re: Propellantless Field Propulsion and application
« Reply #1767 on: 12/03/2012 11:16 PM »
isp is just a peculiar way of measuring thrust per unit of energy. Saying isp is infinite, even for an externally powered thruster, gives the wrong impression.


I agree. Thanks.

Offline Afrocle

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Re: Propellantless Field Propulsion and application
« Reply #1768 on: 12/03/2012 11:24 PM »
Radiation pressure is a surface effect, not a volume effect.

And did you notice that the calculation completely answers your question?  It's not how I got my number originally; it just shows that it's consistent with yours.

The Wikipedia page on radiation pressure (http://en.wikipedia.org/wiki/Radiation_pressure) states that:

"It may be shown by electromagnetic theory, by quantum theory, or by thermodynamics, making no assumptions as to the nature of the radiation, that the pressure against a surface exposed in a space traversed by radiation uniformly in all directions is equal to one third of the total radiant energy per unit volume within that space."

Radiation pressure is a surface effect that is driven by the radiant energy per unit volume within that space. The microwave cavity is a high Q (i.e. low losses) space with a very small unit volume that does not let the radio waves escape as you add more waves/energy.

I do not think that your calculation answered my question.

Offline 93143

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Re: Propellantless Field Propulsion and application
« Reply #1769 on: 12/03/2012 11:49 PM »
Your question had nothing to do with a resonant cavity.  You asked how my value for a flashlight drive was consistent with your value for a solar sail.  I pointed out that the 2x difference between reflection and generation explained the discrepancy.

This assumes that the radiation is unidirectional (which is pretty realistic in both cases but inconsistent with the scenario described by the Wiki page), and that the solar sail is facing the sun straight on, which gives maximum thrust.

Besides, your talk of 1 AU vs. 6 cm seems to imply that you missed the significance of "per unit volume".  All that matters is the radiative intensity, and since we already know that, the volume doesn't matter.

Kilowatts per square metre are not a volumetric quantity; how far away the radiation came from is entirely irrelevant.

Offline sfuerst

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Re: Propellantless Field Propulsion and application
« Reply #1770 on: 12/03/2012 11:52 PM »

The Chinese paper gives a direction for how the EMDrive works (i.e. it is like a solar sail working under the understood principles of radiation pressure), but it does not give specific math of how they come to the 720 mN thrust with a 2.5-kw 2.45-Ghz input.

If you want thrust from radiation pressure, there are much easier ways of doing it.  Just use a light bulb and a parabolic mirror.  The isp from such a photon-rocket "thruster" is enormous.  However, obviously no one uses such a thing, the reason being that the thrust is too small.  It's better to add a small amount of reaction mass and use something like an ion drive.


What is the performance in terms of thrust in Newtons versus power input in kw for your easier "light bulb and parabolic mirror" thruster? The Isp is theoretically infinite for these propellantless thrusters so Isp is not the goal for your improvement on the photon-rocket.

What are you comparing this "too small" thrust to? An ion drive engine at 5,000-sec Isp would have 10 times less thrust for the same power input as the EMDrive, so what ion drive are you basing your comment on?

Assuming perfect collimation a photon rocket spends 300MW per Newton of force.  To derive this, you just need dimensional analysis.  Work = Force * velocity, and the relevant velocity here is the speed of light.

The isp is not infinite since E=mc^2.  You are spending energy (and hence rest mass) to power your rocket.  For a photon rocket, the isp in "sensible" units is equal to the speed of light.  Add in a factor of Earth's gravitational acceleration 'g' to convert to seconds.

The EMDrive is getting larger thrust than allowed by physics.  So... either they are lying, or miss-measuring something.  My guess is that they aren't correctly measuring the far field, and thus getting the thrust wrong.

Antimatter-powered photon rockets are the ideal thing for interstellar travel.  However, basically anywhere else they are highly inefficient and you are better off using some reaction mass.

Offline 93143

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Re: Propellantless Field Propulsion and application
« Reply #1771 on: 12/03/2012 11:56 PM »
The EMDrive is getting larger thrust than allowed by physics.  So... either they are lying, or miss-measuring something.

Or the operating principle is something in between the cracks in our current understanding of physics.

I do not comment on the likelihood that this is true.

Offline sfuerst

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Re: Propellantless Field Propulsion and application
« Reply #1772 on: 12/04/2012 12:30 AM »
The EMDrive is getting larger thrust than allowed by physics.  So... either they are lying, or miss-measuring something.

Or the operating principle is something in between the cracks in our current understanding of physics.

I do not comment on the likelihood that this is true.

Typically new physics appears at the extremes of parameter space.  So much so, that it usually is seen to be useless for applications.  (This is because everything inside the extremes has been well explored beforehand, pretty much by definition.)

Hoping that something as well understood as basic electromagnetism has applications that break the laws of conservation of momentum and energy is just silly.  There has been so many experiments testing that that the probability of someone finding anything new that doesn't conflict with those earlier results is close to zero.  If there is something, then why hasn't someone else noticed it beforehand?  Then you add in Noether's theorem that links momentum and energy conservation to symmetries of the universe, and the probability shrinks even further.  (Time and spacial translation invariance is tested by many more experiments than just electromagnetism.)

On the other hand, if you were investigating something like the properties of the Higgs boson, or gravity in the strong field regime, outlandish or unusual properties wouldn't be so much of a surprise.

Offline QuantumG

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Re: Propellantless Field Propulsion and application
« Reply #1773 on: 12/04/2012 12:36 AM »
I don't rule out the possibility that these researchers have constructed a device that defies current understanding of momentum, etc, and that it might actually be practical for some purposes. I just don't have enough information to reproduce it, and there's no good reason for them to have failed to publish that information.

What is that information? The exact dimensions of the cavity, the materials used, and the exact frequency of the microwaves. In short: what's the recipe? Spell it out. Neither group has done that and yet both groups are claiming success. How are we supposed to know they're even seeing the same phenomena?
Jeff Bezos has billions to spend on rockets and can go at whatever pace he likes! Wow! What pace is he going at? Well... have you heard of Zeno's paradox?

Offline Afrocle

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Re: Propellantless Field Propulsion and application
« Reply #1774 on: 12/04/2012 12:40 AM »
Your question had nothing to do with a resonant cavity.  You asked how my value for a flashlight drive was consistent with your value for a solar sail.  I pointed out that the 2x difference between reflection and generation explained the discrepancy.

This assumes that the radiation is unidirectional (which is pretty realistic in both cases but inconsistent with the scenario described by the Wiki page), and that the solar sail is facing the sun straight on, which gives maximum thrust.

Besides, your talk of 1 AU vs. 6 cm seems to imply that you missed the significance of "per unit volume".  All that matters is the radiative intensity, and since we already know that, the volume doesn't matter.

Kilowatts per square metre are not a volumetric quantity; how far away the radiation came from is entirely irrelevant.

I do not remember asking for the value of a flashlight drive.

If you have 1.37-kw applied a meter away from a surface versus 2.5-kw applied 6-cm away from a surface then there should be a significant difference in pressure on that surface. I think that I understand your point, however.

Let me ask a different question. Is there an advantage to restricting the energy flux within the tiny volume of a microwave cavity versus the energy flux of the sun on a solar sail not being restricted (except for the restriction provided by the solar sail)?


Offline 93143

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Re: Propellantless Field Propulsion and application
« Reply #1775 on: 12/04/2012 12:45 AM »
Hoping that something as well understood as basic electromagnetism has applications that break the laws of conservation of momentum and energy

Who said anything about breaking conservation?  M-E doesn't.  If the EM-Drive works (which I am not claiming), whatever makes it work can be assumed to also not break conservation unless very good evidence shows up that it does.
« Last Edit: 12/04/2012 12:48 AM by 93143 »

Offline sfuerst

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Re: Propellantless Field Propulsion and application
« Reply #1776 on: 12/04/2012 12:58 AM »
Hoping that something as well understood as basic electromagnetism has applications that break the laws of conservation of momentum and energy

Who said anything about breaking conservation?  M-E doesn't.  If the EM-Drive works (which I am not claiming), whatever makes it work can be assumed to also not break conservation unless very good evidence shows up that it does.

M-E does.  Its math depends on a vector theory of gravity.  The reason everyone else uses the more complex tensor theory known as GR is because vector theories break energy-momentum conservation.

If EM-Drive works at the efficiency claimed, then it also needs to break momentum conservation.  A photon rocket is the best you can do isp-wise with a thruster-like setup.

Solar sails, plasma bubbles pushed by the solar wind, tethers etc. are propellentless propulsion techniques that don't break the laws of physics...

Offline 93143

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Re: Propellantless Field Propulsion and application
« Reply #1777 on: 12/04/2012 01:01 AM »
I do not remember asking for the value of a flashlight drive.

Right here (sort of):

I do know that the maximum thrust achievable by radiation pressure at 2.5 kW is 8.3 micronewtons.
How did you calculate your 8.3 x 10-6 Newtons of thrust from a 2.5-kw power source? It is already established that a solar sail achieves a larger 9.15 x 10-6 Newtons of thrust from a smaller 1.37-kw power source (i.e. the sun at 1 AU).

Now, it seems I wasn't entirely clear what I meant by "radiation pressure".  So let me rectify that:  I was referring to the use of a directed radiation source as a thruster.  If you don't carry the radiation source with you, the available thrust doubles - if you can maintain the total power impinging on your vehicle, which is possible in principle if the source is unidirectional.  The sun's light is roughly unidirectional over planetary distances, but over interplanetary or interstellar distances obviously it is not.

Quote
If you have 1.37-kw applied a meter away from a surface versus 2.5-kw applied 6-cm away from a surface then there should be a significant difference in pressure on that surface.

The sun is not 1.37 kW.

Solar radiation is 1.36 kW/m▓ at 1 AU.  So if you have a solar sail of area 0.919 m▓, your impinging radiation power is 1.25 kW, which due to the reflection doubling is equivalent in thrust to a 2.5 kW unidirectional light source on the vehicle.
« Last Edit: 12/04/2012 01:06 AM by 93143 »

Offline Afrocle

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Re: Propellantless Field Propulsion and application
« Reply #1778 on: 12/04/2012 01:12 AM »
I don't rule out the possibility that these researchers have constructed a device that defies current understanding of momentum, etc, and that it might actually be practical for some purposes. I just don't have enough information to reproduce it, and there's no good reason for them to have failed to publish that information.

What is that information? The exact dimensions of the cavity, the materials used, and the exact frequency of the microwaves. In short: what's the recipe? Spell it out. Neither group has done that and yet both groups are claiming success. How are we supposed to know they're even seeing the same phenomena?


I am reading a NASA funded paper right now that might have some of the recipe for this type of photon thruster. I am trying to see if it is flaky before posting it.

Offline QuantumG

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Re: Propellantless Field Propulsion and application
« Reply #1779 on: 12/04/2012 01:24 AM »
I don't rule out the possibility that these researchers have constructed a device that defies current understanding of momentum, etc, and that it might actually be practical for some purposes. I just don't have enough information to reproduce it, and there's no good reason for them to have failed to publish that information.

What is that information? The exact dimensions of the cavity, the materials used, and the exact frequency of the microwaves. In short: what's the recipe? Spell it out. Neither group has done that and yet both groups are claiming success. How are we supposed to know they're even seeing the same phenomena?


I am reading a NASA funded paper right now that might have some of the recipe for this type of photon thruster. I am trying to see if it is flaky before posting it.

I was talking about EM-Drive, but okay.
Jeff Bezos has billions to spend on rockets and can go at whatever pace he likes! Wow! What pace is he going at? Well... have you heard of Zeno's paradox?

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