Question Regarding Solar Sails

[GreylockDS2]

I am unsure if this has been addressed before , and if it has i apologize.

I have been reading a bit about solar sails and there use in propulsion in space , one of the things that seems to come up as speculation when traveling interstellar distances is the use of lasers . A laser on planet A in solar system A shines upon the sails and helps push the ship towards planet B in solar system B.

Now it seems to be that this idea seems to be a one way trip unless the crew places a new laser on planet B to help them travel the void between systems back to planet A.

Now i was wondering what would the flaws of instead of the lasers being independent of the ship , having them be mounted on the ship . So that when the ship is out of the effective range of a sun it can use internal power to assist in propulsion.

[GrandMasterTerwynn]

I am unsure if this has been addressed before , and if it has i apologize.

I have been reading a bit about solar sails and there use in propulsion in space , one of the things that seems to come up as speculation when traveling interstellar distances is the use of lasers . A laser on planet A in solar system A shines upon the sails and helps push the ship towards planet B in solar system B.

Now it seems to be that this idea seems to be a one way trip unless the crew places a new laser on planet B to help them travel the void between systems back to planet A.

Now i was wondering what would the flaws of instead of the lasers being independent of the ship , having them be mounted on the ship . So that when the ship is out of the effective range of a sun it can use internal power to assist in propulsion.

What you're asking is about the same as asking "What happens if I built a small sailboat, stuck a great big fan on it, and then tried to use that to blow wind onto the sail and make me go?" The answer is the same in both cases: "You end up not going anywhere, and manage to look foolish while doing it."

Why? Simple physics.

The wind in the sailboat case, or the laser mounted on Planet A are an external force. The air/laser beam impacts the sail and causes the ship to go off in the opposite direction on account of Newton's Third Law of motion; which states that for every action, there must be an equal and opposite reaction.

In your idea, and the sailboat with the fan mounted on it; the force being generated is now tied to the ship. So, while the air/laser beam hitting the sail will try to push the ship in one direction, the thrust generated by the laser/fan will push the ship in the opposite direction. The two forces will cancel out, and you won't go anywhere.

So how does the crew of the photon sail ship get from Planet B back to Planet A? They build a laser on Planet B. That, or their sailing ship is a rocket with just enough fuel to stop the ship at Planet B, and then accelerate it back up to its cruising velocity (while relying on the laser mounted on Planet A to stop it at the end of the homeward journey. The advantage of this scheme is that the ship only has to carry half the fuel it would've otherwise had to carry.)

[Starglider]

In your idea, and the sailboat with the fan mounted on it; the force being generated is now tied to the ship. So, while the air/laser beam hitting the sail will try to push the ship in one direction, the thrust generated by the laser/fan will push the ship in the opposite direction. The two forces will cancel out, and you won't go anywhere.

This is incorrect. A perfectly black solar sail would produce equal force to the laser, but all practical solar sail designs are mirrors, which produce slightly less than twice the recoil force of the laser. So the ship would move, but somewhat slower than if you got rid of the sail and just fired the laser on its own.

[Patrick Degan]

Dr. Robert L. Forward addressed this issue in a paper on advanced space propulsion. In a course of his discussion of a hypothetical laser-sail mission to Epsilon Eridani, Dr. Forward suggests that the main payload sail of the spacecraft would be detached and used to reflect the light of the laser beam from Earth onto a secondary return sail to drive the ship on the return flight.

We will want a starship design that can carry out round trip missions to stars as distant as Tau Ceti and Epsilon Eridani within a human lifetime. The lightsail would be built in three sections [see Figure 2]. There is an inner payload sail that is 100 kilometers in diameter. Surrounding that is an inner ring-shaped sail that is 320 kilometers in diameter with a 100 kilometer hole. Surrounding that is an outer ring-shaped sail that is 1000 kilometers in diameter with a 20 kilometer diameter hole. The total structure would mass 80,000 tons, including 3,000 tons of payload consisting of the crew and their habitat, supplies, and exploration vehicles.

The entire lightsail structure would be accelerated at 30% of Earth gravity by 43,000 terrawatts of laser power. (Since the Earth only produces about 1 terrawatt of electrical power, we would certainly want to use the free solar power in space instead of trying to get our power from Earth.) At this acceleration, the lightsail would reach a velocity of half the speed of light in 1.6 years. The expedition would reach Epsilon Eridani in 20 years Earth time and 17 years crewtime, and it will be time to stop. At 0.4 lightyears from the target star, the outer ring sail would be separated from the two inner portions. The inner portions would be allowed to lag behind while they are being turned to face the large outer ring sail. The laser light coming from the solar system would reflect from the outer ring sail acting as a retro-directive mirror. The reflected light decelerates the two inner portions and brings them to a halt at Epsilon Eridani.



After the crew explores the system for a few years (using their lightsail as a solar sail), it will be time to bring them back. To do this, the smaller ring sail is detached from the payload sail and they turn to face each other. Provided someone back in the solar system remembered to turn on the laser beam 12 years earlier, the laser beam from the solar system hits the ring-shaped sail and is reflected back on the payload sail. The laser light then accelerates the payload sail back toward the solar system. As the payload sail approaches the solar system 20 Earth-years later, it is brought to a halt by a final burst of laser power. The members of the crew have been away 51 years (including 5 years of exploring), have aged 46 years, and are ready to retire and write their memoirs.

[aieeegrunt]

So how does the crew of the photon sail ship get from Planet B back to Planet A? They build a laser on Planet B. That, or their sailing ship is a rocket with just enough fuel to stop the ship at Planet B, and then accelerate it back up to its cruising velocity (while relying on the laser mounted on Planet A to stop it at the end of the homeward journey. The advantage of this scheme is that the ship only has to carry half the fuel it would've otherwise had to carry.)

The scary thing about this scenario, since it's STL is it requires the crew of the ship (or their descendants, depending on whether this is a freezer ship or a generation ship) to trust that the people on Planet A are going to willing and able to fire the laser up again.

[Junghalli]

I think maybe the OP is thinking of a photon drive.

This works in principle, in practice the problem is that propulsion by light is pretty energy-inefficient. Put it this way. Take a flashlight, point it away from yourself, and turn it on. You now have your own personal photon drive. Do you feel a perceptible force pushing you in the direction opposite the flashlight's beam, as if you were holding onto a small rocket that was firing with the exhaust pointed away from you? No? That's because this kind of propulsion is pretty weak.

To be precise, it takes a 300 megawatt photon drive to generate 1 newton of thrust, and 1 newton is enough to make a 1 kilogram mass accelerate at 1 m/s^2. For perspective, enough power for a small city wouldn't be able to lift a couple of Hungry Man dinners against the force of Earth's gravity with this kind of propulsion. I trust you can see the issue with trying to incorporate a photon drive capable of getting a serious mass up to relativistic speed in anything that might vaguely be described as human-convenient timescale into a vehicle.

This is less of a problem with a stationary laser pushing a seperate spacecraft, because that way you can make the laser as big and heavy as you want without it effecting how much energy you need to push the spacecraft.

[LadyTevar]

Int he original Tron, the vessels between cities were basically Solar Sails. The beam hit the ship and was split into four to guide the sail.

[Molyneux]

What you're asking is about the same as asking "What happens if I built a small sailboat, stuck a great big fan on it, and then tried to use that to blow wind onto the sail and make me go?" The answer is the same in both cases: "You end up not going anywhere, and manage to look foolish while doing it."

Actually, the Mythbusters took this on. It's extremely inefficient and requires precision, but if you set this up, you do wind up propelling yourself with the sail - albeit slowly.

[GreylockDS2]

thank you for all of the responses , it makes sense what your all saying about how it wouldn't work in a way I had envisioned it. I guess thats what happens when you get a visual idea for some concept sketches and start to wonder if it makes sense in the real world.

What i had sketched and was wondering about was a space ship with sails , and lasers pointed towards the sails. Though i know bad science number one was having the lasers visible, but none the less the idea was that the lasers began to look like the rigging of a age of sail vessel. It was just a silly idea in truth.

[GrandMasterTerwynn]

What do I get for posting off-the-cuff answers first thing in the morning? Schooled, that's what.
I'd forgotten the MythBusters covered this one, I'll have to go dig up the appropriate DVD and revisit it.

The scary thing about this scenario, since it's STL is it requires the crew of the ship (or their descendants, depending on whether this is a freezer ship or a generation ship) to trust that the people on Planet A are going to willing and able to fire the laser up again.

Not really. The people on planet A are going to have to leave the laser on for at least a few decades, and possibly a couple of centuries just to get the ship out to planet B. Remembering to turn it back on for the return trip should be easy . . . i.e. you have to be comfortable with thinking and acting in timescales of centuries just to undertake the mission in the first place.

[Guardsman Bass]

What would you build a thousand-kilometer-wide light sail out of?

[Junghalli]

What would you build a thousand-kilometer-wide light sail out of?

http://www.transorbital.net/Library/D001_AxA.html:

The lightsail is made of thin aluminum film stretched over asupporting structure that in turn is attached to the payload.