How to calculate laser thrust?

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    Laser Thrust
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Discussion Overview

The discussion revolves around calculating the thrust generated by a laser beam acting on a mirrored object, particularly in the context of propelling an interstellar probe. Participants explore various theoretical and practical aspects of laser propulsion, including conservation of momentum, the efficiency of using mirrors, and the implications of redshift and relativistic speeds.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Some participants suggest using conservation of momentum to calculate thrust, noting that the momentum change of the probe equals the momentum change of the photons when they reflect off the mirror.
  • There is a proposal to use two mirrors, one on the ground and one on the spacecraft, to create a continuous thrust system, although some participants express skepticism about its feasibility due to factors like redshift.
  • One participant argues for placing the laser on the probe itself to avoid complications associated with remote laser systems, while others counter that this would require carrying fuel, complicating the rocket equation.
  • Participants discuss the formula for thrust, with some asserting that thrust is calculated as power divided by the speed of light, while others argue that using a mirror doubles the thrust due to the reflection of photons.
  • There are mentions of historical concepts and papers related to laser propulsion, indicating that the idea is not new and has been explored in various forms over the years.
  • One participant expresses a desire to understand the amount of laser power required to achieve significant acceleration for a kilogram mass over extended periods, highlighting a lack of mathematical knowledge.

Areas of Agreement / Disagreement

Participants express multiple competing views on the efficiency and practicality of different laser propulsion methods. There is no consensus on the best approach, and several technical details remain debated.

Contextual Notes

Participants note limitations such as the need to consider redshift, the ideal rocket equation, and the practical challenges of building high-power lasers. The discussion also reflects uncertainty regarding the exact calculations for thrust and acceleration.

Who May Find This Useful

This discussion may be of interest to those exploring advanced propulsion concepts, particularly in the context of interstellar travel, as well as individuals curious about the technical challenges associated with laser-based propulsion systems.

CosmicVoyager
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Greetings,

I want build an interstellar probe.

Given the power of the laser beam and the mass of a mirrored object a laser is shining on, how does one calculate the thrust or acceleration?

Thanks
 
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Hi CosmicVoyager! :smile:

Use conservation of momentum …

the increase in momentum of the probe equals the total change in momentum of the photons in the laser when they bounce off the mirror :wink:
 
Why are you using a mirror for laser thrust in an interstellar probe??
 
The laser isn't on the spacecraft . The mirror is.
 
For laser, Thrust = Power / Speed of Light.
 
While at first this may seem like the least efficient way possible to propel a spacecraft he may be on to something.

Suppose you have 2 mirrors, one on the ground and one on the space craft. Each can be precisely angled so that when light bounces off the ground mirror it will be directed to where the spacecraft will be and when light bounces off the spacecraft it will be directed to where the ground station will be. Each mirror can also be distorted to compensate for beam divergence. On top of the ground based mirror you have a quantity of lasing material which is kept in an excited state. The two mirrors and the lasing material now form a single laser with no outlet. If the engineering works out what you would have is continuous, albeit low level thrust. You would also have the additional benefit of the spacecraft not having to cary any reaction mass for its propulsion system.
 
mrspeedybob, it won't work because of the Red Shift and the fact that laser cavity has to be the right length to resonate.
 
"The laser isn't on the spacecraft . The mirror is."

Ah ha! Thanks DH...good grief...
so what happens to net laser power delivered wrsp to distance? and with interstellar gas and planets and things intruding??

Why not put the laser on the probe? And just accelerate photons out the back?? Sure, you have to provide local power and accelerate the additional weight of the laser apparatus, but it sure seems to eliminate a lot of issues...either doesn't seem such a probe will never be able to land and return.

Has a space probe ever been powered by a remote laser?
 
Naty1 said:
Why not put the laser on the probe?
Because the ideal rocket equation is one mean SOB.

One way to get around the nasty implications of the rocket equation with regard to interstellar travel is to simply not take the fuel with you. Otherwise, getting to even the closest of stars is a bit like trying to get to Millinocket: You cahn't get theyah from heah" (Aside: this must be http://www.bangordailynews.com/external/bertni/bunker2.mp3" .)

The idea is not new; it goes back to at least 1984 with Robert Forward's paper "Roundtrip interstellar travel using laser-pushed lightsails" (http://adsabs.harvard.edu/abs/1984JSpRo..21..187F). Greg Landis refined the concept in 1999 in this white paper: http://www.niac.usra.edu/files/studies/final_report/4Landis.pdf.
 
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  • #10
@Naty1:

No. There have been a few projects, but they involved a propellant. Laser on a planet/station delivers heat to the probe where some cheap propellant (like water) is heated to high temperatures and used for propulsion.

If you look at the formula I provided, and consider the fact that building a 1MW laser is a huge challenge, it should be clear why this isn't done.
 
  • #11
K^2 said:
For laser, Thrust = Power / Speed of Light.
It has been a while since I derived this myself, but I believe thrust is twice as much as you said if you are using a mirror. For each photon, the change in momentum is Δp = 2p, where p is the photon's momentum.
 
  • #12
Correct, assuming a perfect mirror and ignoring redshift. A factor of two, however, doesn't help much. That a 1 megawatt laser will generate 1/150 Newtons of thrust instead of 1/300 Newtons doesn't help all that much.
 
  • #13
Redbelly98 said:
It has been a while since I derived this myself, but I believe thrust is twice as much as you said if you are using a mirror. For each photon, the change in momentum is Δp = 2p, where p is the photon's momentum.
With a mirror, you also have to figure in the red shift, so it's a bit more complicated. I gave the value for thrust of laser by itself.
 
  • #14
K^2, your factor of one ignores that the photons are reflected. Saying that thrust is power / speed of light, F=P/c, assumes that the photons are absorbed rather than reflected. Since the photons are reflected, the thrust is (ignoring redshift) F=2P/c\,\cos\theta where \theta is the angle of incidence.
 
  • #15
I didn't realize we were considering relativistic speeds. Otherwise we wouldn't be concerned with Doppler effects, which are of order v/c. But I guess for interstellar travel we would want to.
 
  • #16
DH... Thanks for the reference in your post #9...I skimmed the 1999 paper and the practical problems enumerated there are formidable...

In practical terms, the force produced by reflecting a light beam is 6.7 Newtons per gigawatt of light reflected. This force comes with no expenditure of fuel
whatsoever.

The thing that's astonishing to me is that's the received power/force conversion not the emitted power thousands or millions of miles or more distant...

Maybe that overall difficulty is just as well as I'd hate for such a probe to be really, really easy...we'd be overrun with alien snoops and maybe even weapons systems...
 
  • #17
"Why not put the laser on the probe? And just accelerate photons out the back??"

One could not reach relativistic speeds carrying the fuel. The point of the laser is the engine isn't on the craft. An indefinitely powerful laser or lasers could be used.

Imagane of countless square miles of solar panels in space powering gigawatts of lasers.
 
  • #18
Greetings,

Thanks for the replies :-)

I want to get some idea of how many gigawatts or terawatts or whatever of laser power would be needed to propel a kilogram for example to somewhere between half the speed of light to the speed of light over a period of years.

Once I know how much laser power I need I can figure out how many millions of square miles of solar panels in space I need for power.

Someone posted that a megawatt laser generates 1/150 Newtons of thrust. How much would that accelerate a kilogram?

I'm sorry I'm lacking the mathematical knowledge.

Thanks
 
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