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Solar rocket

Solar powered rocket - what are the limits?
 
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Solar powered rockets (mirror to concentrate the sunlight, heat a material to extreme temperatures and use that as rocket exhaust) have been considered.

And I've been told that this is limited, in that an optical system can at most concentrate reflected light to the same brightness as the source - so the limit would be the temperature of the surface of the sun. You can't focus the sun's image down to a single point using a parabolic mirror - the image of the sun can only be so small.

In rockets, the key is getting your exhaust velocity/energy higher. For a given exhaust velocity, you can only get a certain amount of delta-V. So it would be good if one COULD concentrate solar energy to brighter than the sun's surface, to put more energy into your exhaust mass.

So what if I put a mirrored light pipe right at the focus of a mirror concentrating sunlight, and collect all that light energy and direct it down the pipe. The pipe narrows by a factor of 10, to 1/100th the area of the focused solar image (say from a centimeter to a millimeter across), and directs that concentrated light energy - 1 hundred times as hot as the sun - through that one tiny point onto a tiny amount of exhaust material, heating it to extremely high temperatures and giving extremely high exhaust velocities.

This would have to be done in very short pulses, so that the mirrored tube would not have time to absorb enough energy to disintegrate. There would be energy losses to absorption - but as long the losses aren't 99%, the light coming out the small end should be more intense than the source of that light, allowing a more efficient solar rocket.

So - can anyone give a physics reason why this won't work?

TomRC, Sep 05 2002

Solar Rocketry http://www.permanent.com/t-steam.htm
Example of the solar rocket idea [TomRC, Sep 05 2002, last modified Oct 17 2004]

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       I don't know that it definitely wouldn't work, but I can spot at least one problem.   

       You say that you will limit the reflected light to very short pulses so the pipe won't heat up. a) How? b) The pipe will still probably heat up and melt down - why not just have more mirrors? c) Why not make the end of the light pipe itself the ablating material?   

       Btw, you can easily experiment with this structure on Earth - the sunlight is filtered, but we got plenty of it. I have seen a televised demo of a sun-powered furnace, and it didn't look to me like you needed to concentrate the light further after passing it through the parabolic mirrors.
DrCurry, Sep 05 2002
  

       Why is exceeding the temperature of the sun important?   

       Are you looking here for pure light power, or are you intending using the concentrated beam as a heating device for a lump of fuel, and using the reaction from that to power the rocket?
drew, Sep 05 2002
  

       (Cloud passes overhead, rocket turns ass over teakettle) "Oh the humanity!"
Mr Burns, Sep 05 2002
  

       [DrCurry] To shorten the light pulses, you could have the focused beam move very quickly across the end of the light pipe and then away, for example. But no point worrying about it, if the light pipe idea doesn't work.   

       [drew] The point of exceeding the sun's surface temp is that has been claimed to be the limit to how high a temp you can create optically, hence with a solar rocket, which in turn limits how hot you can get your reaction mass, and in turn the exhaust velocity.   

       The sun's surface temp really isn't all THAT hot - 6000K. Lightning can hit over 25000K. I want millions of degrees - up in the fusion/sun's core range.
TomRC, Sep 06 2002
  

       Now you're staring to sound like a mad scientist.
madradish, Sep 06 2002
  

       [TomRC] I think relying on such high performance paraboloic mirros for human piloted space craft would be unreliable after a short time in use due to micro asteroid sandblasting and pithing effects. [thcgenius] made his point well.   

       "Just after lift off unexpected bird flock causes rocket man to eject after his thrusters peter out...
hollajam, Sep 06 2002
  

       Hmmm... There is a material limit to how hot you can make your rocket engine, while still retaining a useful rocket to sit in. Not much point in making it so hot that it frazzles away. This kind of engine would be more suited to long steady burns, gradually accelerating to very high velocities.   

       Ground based solar furnaces typically generate concentrations of several thousand suns, but this still doesn't make the temperature higher...
drew, Sep 06 2002
  

       The question is not whether this is a practical a solar rocket, but rather "can sunlight be concentrated by any means, such as this example". If it cannot, solar rockets are inherently limited, regardless of the practicality of any particular design.
TomRC, Sep 07 2002
  

       "And I've been told that this is limited, in that an optical system can at most concentrate reflected light to the same brightness as the source"   

       The way it works is to take the light that would be shining on an area this O big and concentrate it to an area this . big. You concentrate the light into as small an area as possible to collect all of its energy where you want it.   

       I don't know if this would have enough oomph to lift off from Earth, but it would work in space, until you run out of powerful sunlight. Your link mentions nuclear reactors, which would work as well, but both have the problem that water is heavy, and you have to get it up there first. He says that "Water is probably the most abundant volatile in most volatile rich asteroids near Earth." which may be true, but is a bit much to base this on...
StarChaser, Sep 07 2002
  

       Why not use your propellant to cool the lightpipe? That way it would be pre-heated when it reaches the focal point.
BunsenHoneydew, Sep 07 2002
  
      
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