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Laser propulsion...

  1. It is said that aiming a laser from earth, the moon, etc, could be used to propel a spacecraft receiving such a laser beam. It is also said that a solar sail could use the reflection of photons to move through space.

    Now my question is, while impractical what stops an on board laser being used and aimed at a slight angle to a surface that reflects the light in the opposite direction of intended traveling direction?
     
  2. jcsd
  3. If you're using an on board laser, you don't need an extra surface. The backside of the laser cavity will do. Just like a rocket. Unbelievably impractical and inefficient though, as you said.

    By aiming the laser on a second surface, we're not going anywhere.

    www.gentec-eo.com
     
  4. My question was regarding the consideration of using lasers even for interplanetary travel. And the suggestion that stationary lasers could be used, to get even to near lightspeed.

    Given that a large ship could carry enough fuel to perform fusion or fission to generate large quantities of energy for millenia, it seems that generating photons could be done long term.

    If stationary lasers are considered viable, why not on board lasers(a side from the current impracticalities of putting a fusion or fission reactor). Some modern advances have been suggested to even enable x-ray laser generation seemingly cheaply.
     
  5. The current impracticality of putting a fusion or fission reactor on board a spacecraft is a very serious hurdle!

    Photonic propulsion for high precision maneuvering may be feasible, but I don't expect it as a main propulsion system in the near future. Either way, it is still a very new and exotic technology.
     
  6. Yes, but I imagine while currently financially impractical, a large Colonization ship could well have an on board reactor and lots of fuel(generating vast quantities of energy for multiple millenia), though it could run out of physical propellant on very long voyages, photons could be easily generated throughout the voyage.
     
  7. Well whichever propellant is used, you'll need a lot of it, because you'll need to produce quite a bit of high energy photons to push the weight of such a large vessel. It's very speculative. The fact that you have a nuclear reactor means you need radiation shielding which are almost by definition dense, heavy metals. Heavy means more photon propulsion, which means a stronger reactor, which means more shielding, which means more weight...

    These are amongst the most powerful diodes (diodes are compact and efficient) available:
    http://www.as.northropgrumman.com/businessventures/ceolaser/products/laserdiodes/index.html

    As for the most powerful CW research laser (neither compact or efficient):
    http://www.engadget.com/2010/12/12/northrop-grummans-100-kilowatt-laser-fired-for-six-hours-straig/
     
    Last edited: Dec 19, 2010
  8. mheslep

    mheslep 3,450
    Gold Member

    My first thought when I saw the OP's part about a fixed planetary laser propelling a spacecraft was not momentum transfer from photons but a photoelectric energy capture, which then is used to produce thrust via some high velocity acceleration of on board matter, plasma, etc. Seems like that's more likely to be successful, at least superficially.
     
  9. D H

    Staff: Mentor

    To illustrate the silliness of this idea, let's up the ante a bit. Well, not just a bit. A whole lot. How about a laser that consumes, without any losses, the equivalent of the entire world's electric energy production? That was about 6.25×1019 joules for 2005, or about 2 terawatts. The thrust produced by a 2 terawatt laser: 6,700 newtons.

    A spacecraft that produces 6,700 newtons of thrust and contains a 2+ terawatt power plant and a 2 terawatt CW laser will not go anywhere.
     
  10. The 6 kN of thrust is significant, but continuous TW is for science-fiction books.
     
  11. D H

    Staff: Mentor

    For a light vehicle, yes. For a vehicle that contains a 2 terawatt power plant and a 2 terawatt laser, no. Even in science fiction land, the combined mass of those two would be beyond immense. The resultant acceleration would be next to nothing.
     
  12. I am not arguing with you D H!

    But I'm not about to say the OP's ideas are "silly" neither!:smile:

    And I'm certainly not aware of rules within science-fiction land!:wink:
    (now which one is that where they go "0.5 past lightspeed"... "in less than 12 parsecs"...)

    Good night guys.:zzz:
     
  13. but it is said laser and solar sails can propel themselves from EM pressure


    Ion thrusters accelerate slowly but they do gain in speed through time. In space one may assume the object will tend to remain in motion, and even small acceleration should add up over long periods of time. Also fission reactors need not be extraordinarily large, and shielding requirements depend upon the construction of the ship.

    nuclear waste from which no more energy could be extracted could also be expelled to supplement efforts by reducing mass and providing additional propulsion
     
    Last edited: Dec 20, 2010
  14. D H

    Staff: Mentor

    I'm not disputing solar sails. IKAROS (see post #8) is on route to Venus right now. Note however that chemical rockets were used to launch the vehicle and to give the vehicle Earth escape velocity. It would take a long time to escape from high earth orbit to solar orbit via solar sails. They have hardly any oomph.

    Solar sails work because solar radiation pressure at 1 AU is 1.412 kW/m2. IKAROS is a 315 kg vehicle with a 200 square meter solar sail. Tilting the sail at the optimal 35.3 degree angle between the sail normal and the sun axis yields a thrust is 1.25×10-3 newton at 1 AU, or an acceleration of about 4×10-6 m/s2. Want more thrust? Go closer to the Sun or make a bigger sail. Using a solar sail to get to the outer planets is going to run into the same 1/r2 problems that preclude use of solar arrays beyond Mars.

    Ion thrusters, puny as they are, give about 50 times the thrust that IKAROS will achieve. The only advantage of solar sails is that no fuel is required. They eliminate the nastiness of the rocket equation.

    Now you're talking about a laser powered vehicle with the laser onboard the vehicle. All of the nastiness of the rocket equation comes into play. This is a non-starter of a concept. Photons are quite simply the absolute worst choice for exhaust.

    The kinetic energy of a non-relativistic exhaust particle relative to the vehicle is 1/2mv2. The momentum imparted to the vehicle by ejecting that particle is mv. The momentum to energy ratio is 2/v. Perversely, a slow-moving exhaust stream generates more thrust per unit of energy than does a high velocity stream. The downside to a slow-moving exhaust stream is that you have to eject a lot more mass to get the same delta V, something you don't want to do because of the rocket equation. Nonetheless, there still remain some advantages to a slower-moving exhaust stream. This is the basic idea behind the VASIMR engine.

    That factor of 2 in the momentum to energy ratio drops as exhaust takes on relativistic speeds, eventually becoming 1/c for photons. This makes photons doubly worse as a choice for exhaust. About the only time photons make sense is when the photons comes from matter / antimatter annihilation, and that is pure science fiction.
     
  15. Nuclear proponents claim even current global energy consumption could be maintained for over 100+B years with resources in the earth's crust. I imagine the nuclear fuel is but a very small fraction of the crust which is a small fraction of the earth's mass.

    It would seem that a 100+B years of such vast energy generation, should one be able to concentrate similar quantities of fuel, is virtually inexhaustible. One could use say 50% of the energy for acceleration for 100+B years, acceleration would have to be very very little to not have an impact if we allow for long intervals of time.

    If scaled down to several millenia(for acceleration times) with similarly reduced fuel mass requirements, would it truly be completely impractical?

    As for antimatter, it seems artificial generation of such is prohibitively energetically expensive, concentration of nuclear fuels is infinitely more practical.
     
    Last edited: Dec 20, 2010
  16. edited

    Rough calculations, indicate that while one will eventually reach one's target, the acceleration speed for reasonable masses using simple photon emission are too low leading to unacceptable duration of interstellar trips.

    edited 2

    This link seems to shed some light on the original bae comments about overcoming inefficiencies of photon emissions with multiple reflections.

    Hypothetically it seems that a ship could drop thin mirrors behind it as it moved and thus gain 1000xmore momentum per photon emitted, via multiple reflection,vastly improving the situation. As the laser elevator paper implied a stationary mirror at one end, laying mirrors closer along the way may allow for even greater than 1000x gain, depending on what factors cause this limit.

    edit 3

    It seems the photonic laser thruster system shows 3000xamplification of radiation pressure

    For deceleration mirror mass should be negligible as the mirror could be thrown in front of the ship(such throws should provide backwards propulsion), and can be recaptured as the ship moves forward, the reflections would provide more deceleration, the mirror or a few mirrors could be used and reused until the ship is fully decelerated.
     
    Last edited: Dec 20, 2010
  17. D H

    Staff: Mentor

    First off, a space mission that spans millennia has zero chances of coming to pass anywhere in the near future. It will not happen in my lifetime, and probably not in yours. The problem of how to get to the stars belongs to some future generation. If such a mission does come to pass it almost certainly will not be using any kind of propulsion technology that we know of now.

    That said, let's play with this. The best space-based, fission-powered generator is Russia's TOPAZ generator. It's specific power (power per unit mass) is about 10 watts/kilogram. NASA's cancelled Project Prometheus hoped to improve this to 15 watts/kilogram. Let's assume an order of magnitude improvement to 100 watts/kilogram. Ignore that to run for millennia the vehicle will need to carry a lot of extra nuclear fuel onboard. Ignore that lasers are rather imperfect devices; they consume a lot more power than they produce as coherent light. Ignore that this laser will have significant mass. In short, we have a vehicle that magically produces photons for propulsion at the cost of a 100 watts/kilogram power plant, and does so forever. The mass of the power plant cancels out in computing the acceleration. The result is 100 watts/kg/c, or 0.334 micrometers/second2. Expressed another way, that acceleration amounts 10.5 meters per second per year.

    A vehicle is not going to get to the stars anytime soon with 10.5 meters per second delta V per year.
     
  18. Ahhh, but if you did not take mirror reflections into effect, I think you'd have to multiply acceleration by 3000 as the vehicle receives 3000 times the radiation pressure from each photon emitted.
     
    Last edited: Dec 20, 2010
  19. D H

    Staff: Mentor

    Are you talking about the mirrors inside the laser? There is no net force from the light bouncing back and forth inside the laser. All that matters is the number of photons exiting the laser per unit time and the frequency of those photons. "What happens in the laser stays in the laser" (not quite right of course; the tiny fraction of light that does gets out is the laser's reason for being).
     
  20. mheslep

    mheslep 3,450
    Gold Member

    Townes? WC Fields? :tongue:
     
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