Is Microwave-Based Jet Propulsion Viable for Marine Vehicles?

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Discussion Overview

The discussion revolves around the viability of microwave-based jet propulsion for marine vehicles, exploring theoretical and practical aspects of using microwaves to heat water for propulsion. Participants examine the feasibility of this concept compared to existing propulsion methods like pumpjet and magnetohydrodynamic drives.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant suggests that a microwave-based propulsion system could be efficient and easy to maintain, proposing that it would not require boiling water to function.
  • Another participant questions the feasibility of propulsion without boiling the water, arguing that water does not expand like air when heated, and proposes a pulse-jet mechanism instead.
  • Some participants note that water's thermal expansion characteristics limit its effectiveness as a propulsion medium, with one referencing a graph indicating water's greatest volume at 4ºC.
  • Historical context is provided regarding steam engines and their efficiency, with references to Carnot's work on maximum efficiency and comparisons to diesel engines.
  • Concerns are raised about the efficiency of RF antennas and power conversion processes in relation to microwave technology.

Areas of Agreement / Disagreement

Participants express differing views on the feasibility of microwave-based propulsion, with some supporting the initial idea while others challenge its practicality based on the physical properties of water. The discussion remains unresolved regarding the viability of the proposed concept.

Contextual Notes

Participants highlight limitations related to water's thermal properties and the efficiency of energy conversion in microwave systems, but do not resolve these issues.

No Otters
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Hi,

I'm new to the forums and I'd like to bounce an idea off of someone :biggrin:

Although I've got a physics degree, I have successfully managed to forget a lot of it as people tend to do when not practicing, and I was wondering if someone would kindly pick some holes in an idea I've had.

Firstly, I should imagine at least someone has heard of pumpjet propulsion for marine vehicles, as well as its nerdier cousin the magnetohydrodynamic drive. What I can't understand is why no one has mentioned a simple jet based on heating water using microwaves. It would not be necessary to boil the water, and in fact this would be unwise as it would cause cavitation, and the direction of flow could be controlled by simply shaping the tube so that it expands in the desirable direction.

I think I'm right saying that microwave absorption would be very efficient if done correctly, and although it would not make for particularly responsive propulsion, it would be quiet and easy to maintain. The technology required is much more readily available than for MHD, and it would have the capacity for greater velocity.

Can anyone see something I'm missing? I've got my suspicions you see :-p

Thanks!
 
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Welcome to PF, No Otters.
I have no education, so I can't really address your issue other than to say that it sounds very reasonable to me on the face of it, but I can't see how you would get propulsion without actually boiling the water. It won't expand like air does just from being heated. The only way that I can think of this operating would be like a pulse-jet, where you flash-boil a small amount of water to force out a liquid slug, then intake new water and repeat. I'm not the proper one to answer this, though. Hang tight, and some experts will be along.
 
Yes, air expands a lot when heated, water does not. That's why it works with air, but wouldn't work with water.
 
Danger said:
Welcome to PF, No Otters.
I have no education, so I can't really address your issue other than to say that it sounds very reasonable to me on the face of it, but I can't see how you would get propulsion without actually boiling the water. It won't expand like air does just from being heated. The only way that I can think of this operating would be like a pulse-jet, where you flash-boil a small amount of water to force out a liquid slug, then intake new water and repeat. I'm not the proper one to answer this, though. Hang tight, and some experts will be along.

I have one of those pulse-jet boats, Danger. Circa 1960, or so. It was made of 'tin', powered by a candle and cost a whole 10 cents at the time.
 
Ah yes... Just found a nice graph showing thermal expansion. Water is at its greatest volume at 4ºC, and is officially useless as a medium for propulsion!

Thanks anyway guys
 
You may recall in the past steam engines were used...Didn't CARNOT figure out the maximum efficiency of such an engine...around 15% or 20% or something at that level...and that was using steam expansion...diesel is MUCH more efficient...
 
No Otters said:
Ah yes... Just found a nice graph showing thermal expansion. Water is at its greatest volume at 4ºC, and is officially useless as a medium for propulsion!
You have that backwrds, but still...
 
Naty1 said:
You may recall in the past steam engines were used...Didn't CARNOT figure out the maximum efficiency of such an engine...around 15% or 20% or something at that level...and that was using steam expansion...diesel is MUCH more efficient...
Steam engines are roughly the same efficiency as diesel engines (30-40%) and partly for that reason, most of our electricity comes via steam.
 
No Otters said:
Ah yes... Just found a nice graph showing thermal expansion. Water is at its greatest volume at 4ºC, and is officially useless as a medium for propulsion!

Thanks anyway guys

At its greatest density at 4º C so a given mass is at its least volume.
 
  • #10
Where does the electricity come from.. A Diesel generator? Losses all they way.

Makes more sense to use the mechanical power from the Diesel directly.
 
  • #11
Phrak said:
I have one of those pulse-jet boats, Danger. Circa 1960, or so. It was made of 'tin', powered by a candle and cost a whole 10 cents at the time.

Neat. I hadn't heard of one of those. There was something that I used to read when I was a kid (can't remember what) that had all sorts of plans to make stuff like that. The only one that I can sort of remember was a toy boat that had 3 (or 4 or 5) sewing thread spools wound up with rubber bands.

Pumblechook said:
Where does the electricity come from.. A Diesel generator? Losses all they way.

Makes more sense to use the mechanical power from the Diesel directly.

There are some exceptions to that, though, particularly in submarines.
 
  • #12
Also if I remember correctly RF antenna's are very inefficient and transform about 5% of the energy into RF wave.
 
  • #13
Antennas are virtually 100% efficient as long as they are well (impedance matched) to the source of RF. Whether the RF is going in a direction you want to to is another matter.

Power conversions can be inefficient.. AC to DC to supply a magnetron and DC to RF output...overall is typically 60% efficient. Our 850 Watt microwave oven takes 1400 Watts from the mains.
 

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