Explain why my rocket won't work.

[TheFaithfulStone]

Okay, so I've been thinking about "future propulsion" of late, and I had this dumb idea. You guys tell me why it won't work.

Given - Nuclear Photonic Rockets have a light producing reaction, and the radiation pressure drives the ship. (Like standing behind the sail on your yacht with a blowdrier, only this actualy works.) Theoreticaly this is possible, right?

So - take a cylinder of arbitrary dimensions (although probably thin and very large) and fill it with water. Heat it with a laser or with an equally giant microwave so that it is full of suspended bubbles. Cover one side of it with a reflective surface, like a solar sail. Now, using batteries, nuclear power, whatever, use ultra-sonic speakers to induce sonoluminesence in the water. Half of the photons produced impact the reflector, the other half stream out the back - then the half that impacted the reflect go out the back too - voila! A very small amount of thrust with a high specific impulse. Propellant isn't consumed, just fuel for the powerplant.

Now - why is that all a bunch of bullcrap? I'm sure it is, but I don't know enough about physics to know why. I suspect the reaction is ridiculously inefficient - that is, it's better to spend the energy you used producing that much ultra-sound on accellerating the spaceship.

TFS

[alxian]

unless you are trying to explain an orion type propulsion system in your own words....


and not some kind of boat with a sail on a windless day so you blow on your sail growing ever more perplexed with every breath to the point of exasperation as to why you aren't moving...

(Like standing behind the sail on your yacht with a blowdrier, only this actualy works.)

oh look you said as much yourself... *giggles

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it is scientific fact that with enough surface area light from a source such as a star will push you. if there is no resistance theoretically you could keep right on accelerating, the rate of which is so languid that you'd be dust before you finally achieved speeds required to cross interstellar distances.. how you plan to stop is failry simple too.... IF you are pointed at your destination star (assume you can even steer) simply deploy your sail in the opposite direction... once you get close enough and slow enough conventional chemical or ion propulsion will do to get you to the solid ground of other planets... which you'd wanna scope out from orbit to avoid being eaten by any native life.. like a hippobronccolus or sharkoceros...

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So - take a cylinder of arbitrary dimensions (although probably thin and very large) and fill it with water

um, carrying any amount of water into space from the surface of our planet is a feat of engineering and if its not meant for life support it would be an extravagance.

water is very heavy. fresh water is a relatively rare and precious resource even to us here on earth. entire industries are built on providing people with "mountain fresh" spring water. filling a soyuz full of it and hoping to reach orbit might not be possible.

if you could capture a comet for your water source then that could work. if that in itself was easy...

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Cover one side of it with a reflective surface, like a solar sail. Now, using batteries, nuclear power, whatever, use ultra-sonic speakers to induce sonoluminesence in the water. Half of the photons produced impact the reflector, the other half stream out the back - then the half that impacted the reflect go out the back too

individually those two things would create more thrust.

the water jet, with would be put to better use as ionized gas, and the light sail.

but together as you propose the photons leaving the water chamber would create as much drag (versus the intended direction of travel) as they would pushing against the sail, furthermore their vector would not be perpendicular to the sail thus the design would be inefficient given the investment.

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voila! A very small amount of thrust with a high specific impulse. Propellant isn't consumed, just fuel for the powerplant.

ah ok, you wish to turn the free electrons in the water into photons.. and hope to get as much of them (like a laser) pointing out of the back of your craft.

it would indeed go, mainly because the sail would be absorbing light from the sun. and possibly would be more powerful than the solar sail idea, if only marginally so. the light from the water could be use to steer the sail like a rudder... if only light was as powerful a propellant as super heated water might be. it would be like expecting a flashlight to behave like a rocket in space. the potential of the light beamed out the one end really won't overcome the mass of the flashlight, not that anything is holding it still the light will fade and die (baterries going out) before the flashlight moved very far at all.

just like the flashlight the problem remains that eventually you will run out of electrons to convert into photons in your water supply..

what happens to that dionized water if there is an atmosphere in that tank? does its chemical properties change? would it become like some form of acid? corroding the metal tube?

looks like i was paying attention is chemlab afterall

**Deionized water easily changes pH while storing it. This is because carbon dioxide from the air disolves in the water and causes a drop in pH by forming carbonic acid H2CO3**

thank you wikipedia.

so if you ever managed to get to another star and planet and least you would have one hell of a weapon to protect yourself. or more likely the creature won't be as wussy as pathetic humans and thus may consider your tank of acid a mighty gift, and quite a rare vintage. (having taken billenia to cross the void).

[TheFaithfulStone]

Is there a limit to how long the sonoluminescent effect will last? That would certaintly be a deal killer.

TFS

[alxian]

the transmission rate versus reabsorbption of the water would determine the efficiency.

powering the sonic amplifyers would be a primary conscern.

how much water? how much energy are you using to make it luminesce? at what rate is light escaping the chamber? how much insulation is there to stop ambient energy loss into the void of space.

i couldn't give you any numbers without much more information than you've provided.

[Qfwfq]

Why use such a complicated manner of producing light?

[TheFaithfulStone]

Originally Posted by Qfwfq

Why use such a complicated manner of producing light?

The idea is that it's "cheaper" to use the natural effect of sonoluminesence to "compress" the sound energy into the light pulses than to make that much light energy outright.

Basically, rather than having to have a nuclear reaction strong enough to produce huge amounts of light, you could have a much lower yield reaction which would only make sound.

Okay - so, basically it takes energy X to produce B amount of photons. It takes energy Y to vibrate C amount of water at ultra-sonic frequencies. Theoretically, in a well designed acoustic container, the efficiency of this reaction should be very high - (That is, every sound wave that hits the bubble should be "transformed" to heat and light pretty pretty completely.) Therefore if Y < X and the amount of photons produced by C in sonoluminesnce is close to B then it's a good system.

Now, if sonoluminesnce is a vacuum energy effect then you could concievably get "extra energy" out of it. If not, then you should still get good efficiency, and you'll only have the heat buildup for the "sound system" and not for the whole "light system."

Of course, the water comes in handy for other things too - like radiation shielding for the crew, and an emergency supply once you've accelerated to your velocity.

I suspect the main problem is that there is no way to generate enough thrust to move that much mass just with radiation pressure - but it might work for steering a solar or magnetic sail or something.

Besides, the whole thing is fictional, and it doesn't really have to work, it just has to sound somewhat plausible.

TFS

[Qfwfq]

[TheFaithfulStone]

Originally Posted by Qfwfq

I know it's all bull crap - is it marginally reasonable bull crap or such complete horse puckey that no one who passed high school physics will buy it?

Besides, I think I found another problem - your water will absorb most of your sound waves before you can create enough light to actually do anything. If you could get sonoluminesence in a solid, or something where the sound travels a lot farther before being eaten by the medium, that would work, of course, then how would you get light out of it?

Whooo, this is a dumb idea. Nevermind.

TFS

[UncleAl]

The Starflight Handbook Eugene Mallove and Gregory Matloff, 1989.

The math is all in there. A 100% mass conversion drive will get you nowhere. To hit 90% of lightspeed vs. your starting inertial frame and stop on arrival, annihalating matter and antimatter, payload is 5% of initial mass - and that includes the vessel. If you want to return, the numbers are [i]vastly[i] worse for you must carry the return antimatter as dead weight.

Whether you use oars or dust from Tinkerbelle's butt, the math cannot be avoided. A crappy 400 tonne vesel (displacement of the smallest research nuclear submarine with a crew of 12) would then need 4000 tonnes of matter and 4000 tonnes of antimatter as fuel to arrive at the Centauri system some 4.5 years after launch as viewed from Earth. Do you have a clever source for 4000 tonnes of antimatter?

We're talking a 172 million megatonne explosion if antimatter containment is breached in the above one-way example. The glow from bubbling water won't outperform a kid on a scooter. How much thrust does a flashlight give?

[EWright]

Sounds to me like travel via a shaken Coke can