Laser Propulsion in space -- please see the attached diagram....

[drjamesroe]

Please help me understand why this is a concept that has not been studied? The laser sail is really the only info I can find in this area using a laser from a fixed source. Using liquid nitrogen to cool the mirrors and reflecting lasers off of them for propulsion. I'm a Doctor of Pharmaceutical Science, and certainly not an engineer by any stretch of the imagination, I'm just trying to wrap my brain around why this wouldn't work. Probably a stupid question, just looking for some insight. Thank you!

 

[PeroK]

The so-called photon rocket is one idea for propulsion through empty space. The idea is that emitting photons, rather than massive particles, is more efficient. See, for example:

https://en.wikipedia.org/wiki/Photon_rocket

 

[drjamesroe]

Thank you!

 

[Ibix]

Your diagram is over complicated. You just point a laser backwards and ride the recoil - no need for your mirror setup, which deflect the beams from going straight backwards and therefore reduce efficiency. You want the beam to be going out the back in as near a perfectly colimated parallel beam as you can arrange.

 

[anorlunda]

Please do not ask me to download and open a .DOCX file coming from a stranger. That is a huge malware risk.

@drjamesroe , if it is a diagram, please take a screenshot of it and post the JPG of the screenshot.

 

[drjamesroe]

Your diagram is over complicated. You just point a laser backwards and ride the recoil - no need for your mirror setup, which deflect the beams from going straight backwards and therefore reduce efficiency. You want the beam to be going out the back in as near a perfectly colimated parallel beam as you can arrange.

Hello, by using 2 lasers you have bi-directional control, combined with tiltable mirrors providing you have computer controlled movable mirrors to deflect the laser/photons and control your vector by 360 degrees alternating each laser as needed. Single laser would not give you that capability.

 

[drjamesroe]

Please do not ask me to download and open a .DOCX file coming from a stranger. That is a huge malware risk.

@drjamesroe , if it is a diagram, please take a screenshot of it and post the JPG of the screenshot.

Hello, you are right, I should have known better then to attach it as word. . I have a jpeg pic below. Having 2 separate lasers with moving/tilting mirrors gives you 360 vector/thrust and bi-directional steering capabilities. A one laser design doesn't provide that. Using laser nuclear fusion powered 'uranium nuclear fusion' reactor for interstellar transport. Perhaps a hybyrd engine in case one method breaks down or has a problem, there will always be a back up.

 

[Ibix]

You can do that with one laser - you mount it on gimbals so that you can tilt it out of line with the center of mass (many traditional rockets do this - the Apollo landers, for example). Then you lose no thrust from non-parallel beams when operating tilted. Or you can use vernier systems with little rockets/lasers mounted on the sides of the ship to orient it and a fixed main drive. With a laser drive it might make sense to use mirrors to tap some light off from the main drive for that, but I suspect that would depend on your mirror efficiency.

 

[drjamesroe]

Please help me understand why this is a concept that has not been studied? The laser sail is really the only info I can find in this area using a laser from a fixed source. Using liquid nitrogen to cool the mirrors and reflecting lasers off of them for propulsion. I'm a Doctor of Pharmaceutical Science, and certainly not an engineer by any stretch of the imagination, I'm just trying to wrap my brain around why this wouldn't work. Probably a stupid question, just looking for some insight. Thank you!

 

[Ibix]

Using laser nuclear fusion powered 'uranium nuclear fusion' reactor for interstellar transport. Perhaps a hybyrd engine in case one method breaks down or has a problem, there will always be a back up.

There are a number of theoretical hitches here. For a start, fission is hopelessly mass inefficient. You really need a straight up matter/antimatter reactor (the Hiroshima bomb had more than 60kg of uranium, and you get the same energy release from less than half a gram of antimatter). And even with such a reactor the numbers are extremely depressing. Assuming a 100% efficient fuel-to-drive-photon conversion (unlikely to say the least), then to accelerate to 0.87c, which gets you a time dilation factor of 2, something like 75% of your craft needs to be fuel. If you intend to stop again 75% of the remaining mass needs to be fuel. That means that for every kilogram of ship or payload you need 15kg of fuel, half matter, half antimatter.

Bear in mind that this vessel still needs nearly five years (2.5 ship time) to get to the nearest star. How big a ship do you need for a 2.5 year trip with a crew? Now multiply that by sixteen to make room for the fuel and remember that antimatter storage is extremely difficult and dangerous (see my earlier point about the Hiroshima weapon). Even a 1kg robot probe needs enough fuel to be a hundred megaton range bomb if anything goes wrong.

This is the main reason people look at remotely powered spacecraft . The problem with a self-contained craft is that the fuel is heavy, so you need more fuel to accelerate the fuel, and more fuel to accelerate that extra fuel, and more fuel... That's why we built something the size of a Saturn V to carry three men. But if you use a static laser to power a moving sail then you don't need to accelerate the fuel, so it is enormously more efficient.

 

[anorlunda]

IMO, the best strategy for a successful interstellar spacecraft is to reduce the mass of the entire vessel and fuel.

In the 1970s, there was a proposal for a solar sail spacecraft . The sail would be made of aluminum foil only 2 atoms thick and a diameter comparable to the Moon. Light energy would come from multiple lasers in near solar orbits. The payload would be a chip. The entire spacecraft and payload would mass only 3 grams. The author said it could be accelerated to 0.25 c before passing the orbit of Mars.

I'm sure that proposal was just fantasy, but lightweight is the direction we need to look.

 

[mfb]

Antimatter to electricity conversion is quite inefficient. Lasers are inefficient, too. Something like 30% of the electric energy goes to light, maybe 50% if we are optimistic, the rest goes to heat. Combine it with the antimatter conversion and we are looking at maybe 10-20% overall system efficiency? You can use part of the heat for propulsion (thermal radiation is radiation, too) - but only with very low thrust values. Generally waste heat will limit your thrust. A photon rocket has the highest propellant efficiency but it's a really slow propulsion method.

Thrust vectoring is not needed, or only in a really tiny amount. You can rotate the spacecraft to change the direction of acceleration and move its center of mass to avoid torque.