Leaving The Solar System

[steve_bank]

I think Jimmy Higgins said it, we are adapted to Earth not space.

Lasers do not need on board power supplies? The energy required to accelate a mass by some ampount is required, there is no way around it. Doen't matter what the propulsion is. From the plots I posted even a 1g constant accertion for 1kg quickly becomes impossible. More so for a large spaceship.

 

[bilby]

I think Jimmy Higgins said it, we are adapted to Earth not space.

Lasers do not need on board power supplies? The energy required to accelate a mass by some ampount is required, there is no way around it. Doen't matter what the propulsion is. From the plots I posted even a 1g constant accertion for 1kg quickly becomes impossible. More so for a large spaceship.

A laser driven spacecraft needs an onboard power supply in exactly the same way that the solar panels on your roof need you to have the Sun in your house.

 

[steve_bank]

As you move away from the Sun energy density in W/m^2 goes down by 1/r^2. Inverse Square Law. Out from the solar system it is nothing.

Lasers were looked at to lift a space levator and there was a small scale demonstraion.

If you have a solar sail what would the force per unit area or pressure be where the laser beam hit the sail? How hot would it ge?

Imagine a platform in space. On one end is a sheet vertical. Fixed to the plate is a laser. The laser shines on sthe heet. Newton's 3rd Law wouldseem to say there will be an equal and opposite reaction through the mounts to the plate. Like standing on the plate and pushing against the sheet. Make a fist and start punching the sheet and the net change in momentum is zero. Or throw baseballs at the sheet.

 

[Jayjay]

I don't think we, as in biological humans, will ever leave the solar system. At least not in space ships. By the time technology is there, we'll have superhuman AIs, mind uploads, and far faster ships already colonizing and building infrastructure on other solar systems. Hauling a human meat bag, let alone a human society, all that distance is a bit like sending a horse to the moon with a catapult.

 

[Bomb#20]

Imagine a platform in space. On one end is a sheet vertical. Fixed to the plate is a laser. The laser shines on sthe heet. Newton's 3rd Law wouldseem to say there will be an equal and opposite reaction through the mounts to the plate. Like standing on the plate and pushing against the sheet. Make a fist and start punching the sheet and the net change in momentum is zero. Or throw baseballs at the sheet.

The sail you push on isn't fixed to the laser. The sail isn't fixed to anything but the payload. So it accelerates away from the laser.

Yes, that puts a reactive force on the laser, which will accelerate away from the sail with equal and opposite momentum change, unless there's some other force on the laser. So you'd engineer the thing to have another force on the laser. The obvious arrangement would be to add a solar sail fixed to the laser. So the sun pushes on the solar sail (and also powers the laser), the solar sail pushes on the laser, the laser pushes on the starship's sail, and the laser beam pushes the laser back away from the starship toward the solar sail, balancing the forces on the laser, so the laser stays still. The net overall effect is that the starship accelerates away from the home star and a bazillion photons bounce off the two sails, back in the general direction of the home star. Their collective momentum away from the destination star is what balances the starship's momentum toward the destination star.

 

[Jayjay]

Imagine a platform in space. On one end is a sheet vertical. Fixed to the plate is a laser. The laser shines on sthe heet. Newton's 3rd Law wouldseem to say there will be an equal and opposite reaction through the mounts to the plate. Like standing on the plate and pushing against the sheet. Make a fist and start punching the sheet and the net change in momentum is zero. Or throw baseballs at the sheet.

The sail you push on isn't fixed to the laser. The sail isn't fixed to anything but the payload. So it accelerates away from the laser.

Yes, that puts a reactive force on the laser, which will accelerate away from the sail with equal and opposite momentum change, unless there's some other force on the laser. So you'd engineer the thing to have another force on the laser. The obvious arrangement would be to add a solar sail fixed to the laser. So the sun pushes on the solar sail (and also powers the laser), the solar sail pushes on the laser, the laser pushes on the starship's sail, and the laser beam pushes the laser back away from the starship toward the solar sail, balancing the forces on the laser, so the laser stays still. The net overall effect is that the starship accelerates away from the home star and a bazillion photons bounce off the two sails, back in the general direction of the home star. Their collective momentum away from the destination star is what balances the starship's momentum toward the destination star.

The conversion efficiency of energy from sunlight to laser is probably so bad, that there are better ways to keep the local laser in place than a light sail. If the laser platform is close to the sun, you can resupply it with a propellant from time to time. I suppose there could be a chain of lasers along the way, using e.g. nuclear fusion for power, but even then, it's probably more efficient to use that energy source to shoot ions or something else in the opposite direction to counteract the force from the laser, than it would be to have another laser somewhere in the inner solar system.

Besides, assuming that the laser is powered by solar panels, having some of that real estate be taken by a solar sail that just does nothing except block out the sun seems very impractical.

 

[bilby]

Imagine a platform in space. On one end is a sheet vertical. Fixed to the plate is a laser. The laser shines on sthe heet. Newton's 3rd Law wouldseem to say there will be an equal and opposite reaction through the mounts to the plate. Like standing on the plate and pushing against the sheet. Make a fist and start punching the sheet and the net change in momentum is zero. Or throw baseballs at the sheet.

The sail you push on isn't fixed to the laser. The sail isn't fixed to anything but the payload. So it accelerates away from the laser.

Yes, that puts a reactive force on the laser, which will accelerate away from the sail with equal and opposite momentum change, unless there's some other force on the laser. So you'd engineer the thing to have another force on the laser. The obvious arrangement would be to add a solar sail fixed to the laser. So the sun pushes on the solar sail (and also powers the laser), the solar sail pushes on the laser, the laser pushes on the starship's sail, and the laser beam pushes the laser back away from the starship toward the solar sail, balancing the forces on the laser, so the laser stays still. The net overall effect is that the starship accelerates away from the home star and a bazillion photons bounce off the two sails, back in the general direction of the home star. Their collective momentum away from the destination star is what balances the starship's momentum toward the destination star.

The conversion efficiency of energy from sunlight to laser is probably so bad, that there are better ways to keep the local laser in place than a light sail. If the laser platform is close to the sun, you can resupply it with a propellant from time to time. I suppose there could be a chain of lasers along the way, using e.g. nuclear fusion for power, but even then, it's probably more efficient to use that energy source to shoot ions or something else in the opposite direction to counteract the force from the laser, than it would be to have another laser somewhere in the inner solar system.

Besides, assuming that the laser is powered by solar panels, having some of that real estate be taken by a solar sail that just does nothing except block out the sun seems very impractical.

The object of the exercise is to eliminate the need for the spacecraft to carry fuel.

Once that problem has been solved, then we can start worrying about efficiency and practicality; Though these latter issues don't need to be considered at all if our sole objective is to demonstrate that the inability to carry sufficient fuel need not be a show-stopper.

The physical possibility of powering a spacecraft from a source that need not be carried with the spacecraft eliminates the argument "it's impossible to carry sufficient fuel to travel interstellar distances under constant acceleration". It's not necessary to do this efficiently, as once we can use power from sources not traveling with the spacecraft, the power available becomes enormous - we can harness entire stars if necessary.

Nobody's arguing that it would be practical or easy - just that it's not physically impossible.

 

[steve_bank]

Imagine a platform in space. On one end is a sheet vertical. Fixed to the plate is a laser. The laser shines on sthe heet. Newton's 3rd Law wouldseem to say there will be an equal and opposite reaction through the mounts to the plate. Like standing on the plate and pushing against the sheet. Make a fist and start punching the sheet and the net change in momentum is zero. Or throw baseballs at the sheet.

The sail you push on isn't fixed to the laser. The sail isn't fixed to anything but the payload. So it accelerates away from the laser.

Yes, that puts a reactive force on the laser, which will accelerate away from the sail with equal and opposite momentum change, unless there's some other force on the laser. So you'd engineer the thing to have another force on the laser. The obvious arrangement would be to add a solar sail fixed to the laser. So the sun pushes on the solar sail (and also powers the laser), the solar sail pushes on the laser, the laser pushes on the starship's sail, and the laser beam pushes the laser back away from the starship toward the solar sail, balancing the forces on the laser, so the laser stays still. The net overall effect is that the starship accelerates away from the home star and a bazillion photons bounce off the two sails, back in the general direction of the home star. Their collective momentum away from the destination star is what balances the starship's momentum toward the destination star.

A recoiless laser?

 

[steve_bank]

In scifi movies they just turn on a light bulb at the rear of the spaceship.

 

[Loren Pechtel]

For that matter, we can already build artificial hearts, so how long can it be before we know how to build an artificial womb? Your robotic factory can be pre-stocked with a freezer full of in-vitro-fertilized embryos. This way we won't need humans to travel any faster than robots. If it takes 10,000 years to reach Alpha Centauri, no big woop, so it takes 10,000 years. If the human race wants interstellar extinction insurance, we must learn patience.

Vernor Vinge, Longshot.

 

[Loren Pechtel]

I think Jimmy Higgins said it, we are adapted to Earth not space.

Lasers do not need on board power supplies? The energy required to accelate a mass by some ampount is required, there is no way around it. Doen't matter what the propulsion is. From the plots I posted even a 1g constant accertion for 1kg quickly becomes impossible. More so for a large spaceship.

Laser-pumped lightsail has no lasers on the ship. The logical place for the laser is around Mercury--it's going to be huge, it needs gobs of power and you have to deal with the competing forces of the sail effect of the solar panels and the thrust from the beam--from a practical standpoint that means it must either be on the surface (night becomes a big issue) or in an "orbit" that can be shifted to a forced orbit to balance the forces involved. (Which will be constantly changing as Mercury goes around the sun.)

 

[Loren Pechtel]

As you move away from the Sun energy density in W/m^2 goes down by 1/r^2. Inverse Square Law. Out from the solar system it is nothing.

Which is why ordinary lightsail is limited to about .01c. By then you're too far from the star to get meaningful acceleration.

With laser-pumped lightsails you can use big enough lenses to compensate.

Lasers were looked at to lift a space levator and there was a small scale demonstraion.

If you have a solar sail what would the force per unit area or pressure be where the laser beam hit the sail? How hot would it ge?

Imagine a platform in space. On one end is a sheet vertical. Fixed to the plate is a laser. The laser shines on sthe heet. Newton's 3rd Law wouldseem to say there will be an equal and opposite reaction through the mounts to the plate. Like standing on the plate and pushing against the sheet. Make a fist and start punching the sheet and the net change in momentum is zero. Or throw baseballs at the sheet.

Actually, this produces thrust--after the light is reflected by the sheet it goes off into space. This effect has actually been measured in practice--there was a longstanding mystery about the Voyager probes being slightly off from where all the math said they should be. They finally figured out that it was radiation from the heat of the RTGs they carry.

 

[Loren Pechtel]

Yes, that puts a reactive force on the laser, which will accelerate away from the sail with equal and opposite momentum change,

Nope, the sail gets twice the effect--the photons hit it and bounce back, as opposed to the laser simply emitting them.

 

[Jayjay]

Imagine a platform in space. On one end is a sheet vertical. Fixed to the plate is a laser. The laser shines on sthe heet. Newton's 3rd Law wouldseem to say there will be an equal and opposite reaction through the mounts to the plate. Like standing on the plate and pushing against the sheet. Make a fist and start punching the sheet and the net change in momentum is zero. Or throw baseballs at the sheet.

The sail you push on isn't fixed to the laser. The sail isn't fixed to anything but the payload. So it accelerates away from the laser.

Yes, that puts a reactive force on the laser, which will accelerate away from the sail with equal and opposite momentum change, unless there's some other force on the laser. So you'd engineer the thing to have another force on the laser. The obvious arrangement would be to add a solar sail fixed to the laser. So the sun pushes on the solar sail (and also powers the laser), the solar sail pushes on the laser, the laser pushes on the starship's sail, and the laser beam pushes the laser back away from the starship toward the solar sail, balancing the forces on the laser, so the laser stays still. The net overall effect is that the starship accelerates away from the home star and a bazillion photons bounce off the two sails, back in the general direction of the home star. Their collective momentum away from the destination star is what balances the starship's momentum toward the destination star.

The conversion efficiency of energy from sunlight to laser is probably so bad, that there are better ways to keep the local laser in place than a light sail. If the laser platform is close to the sun, you can resupply it with a propellant from time to time. I suppose there could be a chain of lasers along the way, using e.g. nuclear fusion for power, but even then, it's probably more efficient to use that energy source to shoot ions or something else in the opposite direction to counteract the force from the laser, than it would be to have another laser somewhere in the inner solar system.

Besides, assuming that the laser is powered by solar panels, having some of that real estate be taken by a solar sail that just does nothing except block out the sun seems very impractical.

The object of the exercise is to eliminate the need for the spacecraft to carry fuel.

Once that problem has been solved, then we can start worrying about efficiency and practicality; Though these latter issues don't need to be considered at all if our sole objective is to demonstrate that the inability to carry sufficient fuel need not be a show-stopper.

The physical possibility of powering a spacecraft from a source that need not be carried with the spacecraft eliminates the argument "it's impossible to carry sufficient fuel to travel interstellar distances under constant acceleration". It's not necessary to do this efficiently, as once we can use power from sources not traveling with the spacecraft, the power available becomes enormous - we can harness entire stars if necessary.

Nobody's arguing that it would be practical or easy - just that it's not physically impossible.

I'm not arguing against an interstellar spacecraft with a light sail being pushed by a laser; that makes perfect sense. But there is no point in having a light sail on the laser. It stays in the solar system, and it can either carry fuel / propellant or be restocked if necessary. Or replaced: I would imagine multiple lasers would be better than one anyway, for redundancy purposes.

 

[steve_bank]

Yes, that puts a reactive force on the laser, which will accelerate away from the sail with equal and opposite momentum change,

Nope, the sail gets twice the effect--the photons hit it and bounce back, as opposed to the laser simply emitting them.

Stand in water and push on a boat and it moves. Throw a ball at the sail and the boat will nudge. Stand on the deck and push and it goes nowhere. Throw a ball at the sail and there is an opposite reaction transmitted to the boat by friction through your feet.

If the photons orginate in another inertial frame like the Sun then momtum will be trasfered to the sail. NASA tried an experiment with a solar sail and I believe it failed for some mechanical reason.

When machine guns were first put on helicopters in the VN War, long forward bursts actualy slowed down the helecopter.

Recoiless large caliber rifles mounted on vehicles use springs to absorb the reaction force keeping the vehicle from going backwards.

I'm guessing you would be better off pointing the lasers backwards for propulsion.

 

[Bomb#20]

The sail you push on isn't fixed to the laser. The sail isn't fixed to anything but the payload. So it accelerates away from the laser.

Yes, that puts a reactive force on the laser, which will accelerate away from the sail with equal and opposite momentum change, unless there's some other force on the laser. So you'd engineer the thing to have another force on the laser. The obvious arrangement would be to add a solar sail fixed to the laser. So the sun pushes on the solar sail (and also powers the laser), the solar sail pushes on the laser, the laser pushes on the starship's sail, and the laser beam pushes the laser back away from the starship toward the solar sail, balancing the forces on the laser, so the laser stays still. The net overall effect is that the starship accelerates away from the home star and a bazillion photons bounce off the two sails, back in the general direction of the home star. Their collective momentum away from the destination star is what balances the starship's momentum toward the destination star.

A recoiless laser?

Steve, I understand you have poor vision. But if you're going to argue with people you need to try to read what they write and try to understand it. I didn't propose a recoilless laser.

This would all be easier if I could just draw a picture, but I'm no artist. To simplify: the sun pushes the laser away, and the laser pushes its own beam away, and the recoil from the laser beam pushes the laser back toward the sun, and the two forces pushing on the laser cancel each other out. Nothing is recoilless. Momentum is conserved. The other end of the laser beam pushes on the starship. The laser stays here, next to the sun. The starship flies away to Alpha Centauri.

Stand in water and push on a boat and it moves. Throw a ball at the sail and the boat will nudge. Stand on the deck and push and it goes nowhere. Throw a ball at the sail and there is an opposite reaction transmitted to the boat by friction through your feet.
...
I'm guessing you would be better off pointing the lasers backwards for propulsion.

Yes, you already made that argument, and we already understood you. But you say it over and over, because you keep making the same incorrect assumption about what design other people are proposing. Nobody except you is talking about putting a propulsion laser on a starship. Take that picture out of your head.

 

[Bomb#20]

I'm not arguing against an interstellar spacecraft with a light sail being pushed by a laser; that makes perfect sense. But there is no point in having a light sail on the laser. It stays in the solar system, and it can either carry fuel / propellant or be restocked if necessary. Or replaced: I would imagine multiple lasers would be better than one anyway, for redundancy purposes.

Well sure there's a point in it -- it makes restocking the propellant unnecessary. Higher up-front costs, lower ongoing costs. Both solutions are feasible, and if you know enough that you can calculate that economic tradeoff hundreds of years in advance of the technology being ready, hey man, will you be my investment advisor?

 

[Jayjay]

I'm not arguing against an interstellar spacecraft with a light sail being pushed by a laser; that makes perfect sense. But there is no point in having a light sail on the laser. It stays in the solar system, and it can either carry fuel / propellant or be restocked if necessary. Or replaced: I would imagine multiple lasers would be better than one anyway, for redundancy purposes.

Well sure there's a point in it -- it makes restocking the propellant unnecessary. Higher up-front costs, lower ongoing costs. Both solutions are feasible, and if you know enough that you can calculate that economic tradeoff hundreds of years in advance of the technology being ready, hey man, will you be my investment advisor?

Where are you going to put the laser, that pushes the laser, that pushes the spacecraft? And what's going to keep that laser in one place?

It's not really an economic tradeoff, but physical one. It's less efficient to keep a laser in place with another laser, than it would be to... say, just have two lasers on the same platform pointing to opposite directions. Which itself would be kind of silly if you have access to better sources of propulsion.

Too many lasers! Unless there is a nuclear war in the future and cats develop the ability to shoot lasers out of their mouths.

 

[Loren Pechtel]

I'm not arguing against an interstellar spacecraft with a light sail being pushed by a laser; that makes perfect sense. But there is no point in having a light sail on the laser. It stays in the solar system, and it can either carry fuel / propellant or be restocked if necessary. Or replaced: I would imagine multiple lasers would be better than one anyway, for redundancy purposes.

Something has to be done to keep the laser in place. Rockets would be quite a problem to refuel--remember that the laser is going to be pushed half as hard as the probe, that's a lot of fuel! Thus you either need a gravitational tether (ie, Mercury) or a lightsail tether.

I think you'll also need to do something to handle the final focusing lens. Thinking about it I can't see how it was handled in Flight of the Dragonfly even though he was careful with the numbers otherwise.

 

[Loren Pechtel]

Yes, that puts a reactive force on the laser, which will accelerate away from the sail with equal and opposite momentum change,

Nope, the sail gets twice the effect--the photons hit it and bounce back, as opposed to the laser simply emitting them.

Stand in water and push on a boat and it moves. Throw a ball at the sail and the boat will nudge. Stand on the deck and push and it goes nowhere. Throw a ball at the sail and there is an opposite reaction transmitted to the boat by friction through your feet.

If the photons orginate in another inertial frame like the Sun then momtum will be trasfered to the sail. NASA tried an experiment with a solar sail and I believe it failed for some mechanical reason.

When machine guns were first put on helicopters in the VN War, long forward bursts actualy slowed down the helecopter.

Recoiless large caliber rifles mounted on vehicles use springs to absorb the reaction force keeping the vehicle from going backwards.

I'm guessing you would be better off pointing the lasers backwards for propulsion.

I don't see your point at all.

Put the laser on the ship and you have to power them somehow. Leave the lasers behind and they can use solar power and you're not faced with the tyrrany of the rocket equation. You also get the benefit of twice the momentum out of the same amount of power.