Slowing down a light sail by bouncing the laser off a second lightsail.

[3blake7]

I was wondering how difficult it would be to use lasers to propel a light sail (let's say 500m x 500m) to 12% of the speed of light. The payload would be 5 megagrams. Without a way to slow it down, it would fly through the destination star system too quickly. I thought about crashing the payload into something but with that much kinetic energy, it's like a nuke going off. My payload probably wouldn't survive.

I was thinking, what if you have two light sails and when you near your destination, you release the second light sail and the laser array back home bounces the lasers off the front light sail. The front light sail would be speeding up with the back light sail (plus the payload), slowing down. 

Is it even possible to hit a 500m x 500m target 4 lightyears away? The front light sail would probably need several adjustable light sail panels that can bounce the lasers at different angles as the distance increases. I'm also aware that lasers can fan out but not sure how much in my case. I think you might have one array of lasers accelerating and another set with a different focal point for slowing it down.

Edited September 9, 2017 by 3blake7

[Strange]

1 hour ago, 3blake7 said:

I'm also aware that lasers can fan out but not sure how much in my case.

You could extrapolate from this: "At the Moon's surface, the beam is about 6.5 kilometers (4.0 mi) wide^[9]"

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

I am guessing that at that distance, the laser would be imparting minimal momentum to the light sail and so trying to use it to slow down would have negligible effect.

[swansont]

The force light exerts with perfect reflection is 2P/c, where P is the laser power.

1 GW gives you 6.67 N of thrust

6.67N/5000 kg = 0.00133 m/s^2

Doing the calculation purely using Newtonian physics, it will take more than 700 years to reach 10% of light speed. Assuming Doc Brown can get you a 1 GW laser.

A complication: as you speed up, the laser power decreases in the frame of the solar sail, owing to the Doppler shift, so it will take longer. Plus the laser divergence as noted above.

[3blake7]

Assuming we have fusion power with an autonomous self-replicating industry to mass produce power plants and lasers, what's the biggest issue? Lasers are like 20% efficient so we would need like 150 terawatts for 44 days of acceleration?

[dimreepr]

2 hours ago, 3blake7 said:

Assuming we have fusion power with an autonomous self-replicating industry to mass produce power plants and lasers, what's the biggest issue? 

 

Distance... as explained above; I doesn't matter how much power you pump in, it's the distance it has to travel that makes the idea redundant, even within the solar system the percentage of energy IN, that's useful at the end of its journey is nearly zero.

[swansont]

7 hours ago, 3blake7 said:

Assuming we have fusion power with an autonomous self-replicating industry to mass produce power plants and lasers, what's the biggest issue? Lasers are like 20% efficient so we would need like 150 terawatts for 44 days of acceleration?

Mirrors aren't 100% efficient. You would melt them unless your sail was fantastically large.

which lasers are 20% efficient? How are you going to get to 30 TW?

[3blake7]

I found this: http://www.georgedishman.f2s.com/solar/Calculator.html

Just realized I had a typo on my spreadsheet. The site has diamond sail at 11.8 million W/m^2 and I had 118 million.

I already looked at beamed core anti-matter rockets, inertial confinement fusion rockets, and magnetic confinement fusion rockets. They all had huge problems.

I guess best bet might be nuclear pulse propulsion or maybe a hybrid setup, laser propelled light sails with nuclear pulse propulsion to slow it down.

[Moontanman]

20 minutes ago, 3blake7 said:

I found this: http://www.georgedishman.f2s.com/solar/Calculator.html

Just realized I had a typo on my spreadsheet. The site has diamond sail at 11.8 million W/m^2 and I had 118 million.

I already looked at beamed core anti-matter rockets, inertial confinement fusion rockets, and magnetic confinement fusion rockets. They all had huge problems.

I guess best bet might be nuclear pulse propulsion or maybe a hybrid setup, laser propelled light sails with nuclear pulse propulsion to slow it down.

In fiction Physicist Robert Forward has used this idea before. Possibly you can see how he did it.

 

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

 

Quote

Forward's light sail propulsion system[edit]

The light sail system consists of three functional parts: a powerful laser, a large focusing lens, and a giant space-sail. The idea behind the solar sail is that the laser provides a small force on the sail when the sail reflects the light. This small force provides the acceleration of the spaceship. With the ship's primary source of energy coming from the outside, it would not be limited to traveling distances that it had enough fuel for.

The light used in the system was an array of a thousand laser generators, which were focused through lenses and aimed at the sail. The lasers provided up to 1,500 terawatts of power. Two different lenses were used to magnify the laser beams. The acceleration lens was 100 km in diameter and was able to accelerate the ship at 0.01g; the deceleration lens was 300 km in diameter and was able to decelerate the ship at 0.1g. Although these accelerations are relatively small, over time they result in enormous speeds.

To catch the energy, Forward used a 1,000-km-diameter, circular aluminum sail. The sail resembled a flattened disk with a 300-km diameter removable center portion. When traveling to Rocheworld, the entire sail was used. When the ship needed to decelerate, the smaller sail was separated from the larger outer sail. The large sail was used as a reflecting lens, focusing light onto the smaller sail, slowing the craft.

Using the light sail system, the spaceship Prometheus continued to accelerate for 20 years, traveling 2 light years' distance toward Barnard's Star before going into coast mode and traveling an additional 20 years' time at a constant speed of 0.2 c, covering the remaining 4 light years (ca. 23 trillion miles; 38 trillion km).

 

[3blake7]

Cool! It's funny, I was just reading his variation of Project Daedalus, with self-replicating probes.