Laser space propulsion

[Jacques]

If I turn on a laser in space, will it accelerate ?

Usually rocket engine send mass to the back to accelerate in the opposite direction. Light has no mass, but has momentum and by conservation of momentum law the laser should accelarate...

I am not sure and that is why I am asking.

[pmb]

If I turn on a laser in space, will it accelerate ?

Usually rocket engine send mass to the back to accelerate in the opposite direction. Light has no mass, but has momentum and by conservation of momentum law the laser should accelarate...

I am not sure and that is why I am asking.

Yes, it will accelerate.

[Spyman]

If I turn on a laser in space, will it accelerate ?

Usually rocket engine send mass to the back to accelerate in the opposite direction. Light has no mass, but has momentum and by conservation of momentum law the laser should accelarate...

I am not sure and that is why I am asking.

Solar sails use a phenomenon that has a proven, measured effect on spacecraft. Solar pressure affects all spacecraft, whether in interplanetary space or in orbit around a planet or small body. A typical spacecraft going to Mars, for example, will be displaced by more than 1,000 km by solar pressure, so the effects must be accounted for in trajectory planning, which has been done since the time of the earliest interplanetary spacecraft of the 1960s. Solar pressure also affects the attitude of a craft, a factor that must be included in spacecraft design.

http://en.wikipedia.org/wiki/Solar_sail

[pmb]

Solar sails use a phenomenon that has a proven, measured effect on spacecraft. Solar pressure affects all spacecraft, whether in interplanetary space or in orbit around a planet or small body. A typical spacecraft going to Mars, for example, will be displaced by more than 1,000 km by solar pressure, so the effects must be accounted for in trajectory planning, which has been done since the time of the earliest interplanetary spacecraft of the 1960s. Solar pressure also affects the attitude of a craft, a factor that must be included in spacecraft design.

http://en.wikipedia.org/wiki/Solar_sail

I never quite grasped the solar sail thing. The acceleration is so small that it would take forever to get even to Mars. The amount of acceleration is independant of the area of the sail so 1 cm² will accelerate at the same rate as a sale of area 10²⁰ cm². The size of the sail is only to allow for the acceleration of a ship. The larger the sail the greater the force it can exert. But the acceration won't increase past the acceleration of a no load 1 cm² sail.

Edited June 29, 2012 by pmb

[swansont]

The amount of acceleration is independant of the area of the sail

 

Why?

 

For reflection the force is given by 2P/c, and the total power (P) scales with the area. If there is a payload, the mass does not scale the same way; doubling the area does not double the mass.

[Jacques]

I am not speaking of solar sail although it is very interesting.

pmb

Yes, it will accelerate.

That what I thaught.

It is the ultimate specific impulse. Why is it not used ?

[pmb]

Why?

If you increase the area of the sail by a factor R then the mass will increase by the same factor. This means that the acceleration remains unaffected.

 

Think of placing a solar panel of area 1 cm^2 in space with its surface normal being parallel to the light hitting it. The panel will have a force F exerted on it and will accelerate at the rate g. Now place another identical solar panel right next to it. They willl accelerate at the same rate. That won't change if they're fused together to make one solar panel but now you have a solar panel twice the area of the original sail.

Edited June 29, 2012 by pmb

[Enthalpy]

I never quite grasped the solar sail thing. The acceleration is so small that it would take forever to get even to Mars...

You should put some figures on it. As far from our Sun as Earth is, the incident pressure is 4.5µPa, plus some reflected pressure depending on the design. Polyester and polyimide weigh around 1300kg/m3, and film 25µm thin is a shelf product.

 

Then, putting a spacecraft on a Solar polar orbit for instance takes much shorter with a sail than going first to a Jupiter flyby to achieve the huge delta-V needed, and your craft is on a more interesting orbit. Same if going to Mercury. That's why a Japanese craft tests solar sails presently.

 

By the way, if you desire a thinner film (25µm is still easy to handle) I describe at Saposjoint.net a process for the controlled thinning of a standard film. This should improve Solar sails.

 

--------------------------------

 

A spacecraft emitting light (by laser or any means) is an old idea. It's impractical for many reasons, essentially because it needs a huge power to produce any thrust.

• chemical energy would be better used in a chemical rocket;
• nuclear energy (radioactivity or fission) would be better used to expell the reaction fragments or to heat hydrogen;
• Solar energy is better used in a Solar sail of the same area.

Some people consider a laser somewhere (Earth orbit, Moon surface...) whose light pushes the sail of a spacecraft. I suppose we don't achieve light more concentrated than Solar light at any significant distance.

Edited June 29, 2012 by Enthalpy

[D H]

We've got two very distinct threads of discussion going on here. The opening post was about using a laser as a rocket. The more recent posts were about solar sails. Very distinct concepts, unless you meld the two by using a laser on the Moon to illuminate a solar sail.

 

Using a laser as a rocket

This is a very silly idea. That photons are massless means you need to eject boatloads of photons to get even a tiny bit of thrust. A continuously operating 1.21 gigawatt laser generates about [imath]1.21\times10^9 watts/c\approx 4.04\,\text{N}[/imath] of thrust. There is no such thing as a continuously operating 1.21 gigawatt laser, and 4 newtons is tiny. The best the US has done is a 1 megawatt continuous wave laser, and it fired continuously for all of 70 seconds. Then it had to cool off for a long time. So problem #1 is no oomph. Problem #2: Powerful lasers are big beasts. No oomph divided by large mass = minuscule acceleration. Problem #3: Big lasers are incredibly inefficient. They consume a lot more power than they produce in the form of a laser beam. All of that other consumed power is just waste heat. Some small lasers have exceeded 60% electrical-to-optical efficiency, but these are small 13.5 watt laser diodes. There's ongoing work to bring a 25 kilowatt CW laser up to 30% efficiency. A 25 kilowatt laser would generate all of 83 micronewtons of thrust.

 

Where's that power come from? You have a rocket (a Rube Goldberg style rocket) if you use some internal form of power plant such as a fusion plant. The rocket equation with all it's nastiness comes into play. How about solar cells? You would be far better off forgoing the laser and just having a solar sail. The solar sail is going to beat the solar-powered laser in terms of thrust thanks to the second law of thermodynamics, and eliminating the laser drastically increases acceleration thanks to the big mass of the laser.

 

 

Solar sails

Thrust is very, very small. It's about 5.4 micronewtons per square meter of sail area at 1 AU. You'd get slightly more than 9 μN/m² if the sail was a perfect specular reflector (it's not) and if the sail's outward normal was aimed right at the Sun. Aiming the sail at the Sun has minimal effect on orbital energy. When firing a rocket, you get the best bang for the buck with respect to changing orbital energy if the thrust is along or against the velocity vector. You can't do that with a solar sail because that would mean aiming the sail edgewise to the Sun. The optimal angle is an angle of about 53 degrees between the outward normal and the vector toward the Sun, leading to about a 40% reduction in the thrust from aiming directly toward the sun.

 

You need a big sail. A very big sail. Mass does not scale with area. It's worse, closer to r³ than r². The sail is light. The structure needed to keep the sail rigid and aimed properly, not so light. That structure is subject to the cube-square law.

 

There are two advantages to solar sails. One is that it's always on. A small amount of thrust applied continuously over a long period of time can eventually add up to a big Δv. Ion thrusters are similarly appealing because of this. The other advantage is that unlike ion thrusters, solar sails are not subject to the nastiness rocket equation.

 

 

Powering a solar sail with a remote laser

The Sun doesn't provide all that much energy, 1.361 kilowatts/m² at 1 AU. What if we beef this up with a laser cannon on the Moon? Now we can use solar power to operate the laser. All it takes is a huge array of solar cells to power that 1.21 gigawatt laser. Simple! (Not really.) It's going to take a huge amount of infrastructure on the Moon, we still don't know how to make that big honkin' laser, and now we have some brand new problems.

 

Problem #1: You'll shoot your eye out!

The military is interested in big lasers because they do shoot the eyes out of enemy vehicles. Aim a 1.21 gigawatt laser at a solar sail that isn't a perfect specular reflector and what you have is a former solar sail.

 

Problem #2: You won't shoot your eye out. You'll miss.

This laser is going to take very precise aiming capability. The craft is going to be speeding away from the laser base (that's the whole point). It won't take long before microradian accuracy is required. For a solar sail with an area of 1 km, this is about 500,000 km, a bit more than the 384,400 km distance between the Earth and the Moon.

 

Problem #3: You'll just spread the energy over a wide area.

Getting the center of the beam focused on the solar sail is only half the problem. The other problem is beam divergence. A laser cannon with a milliradian beam divergence is going to be effective for a very short distance. The beam divergence needs to be about the same as the accuracy: About a microradian just to give a bit of a helpful shove at the start of the mission. After that short distance, the solar sail is just a solar sail.

[swansont]

If you increase the area of the sail by a factor R then the mass will increase by the same factor. This means that the acceleration remains unaffected.

 

Think of placing a solar panel of area 1 cm^2 in space with its surface normal being parallel to the light hitting it. The panel will have a force F exerted on it and will accelerate at the rate g. Now place another identical solar panel right next to it. They willl accelerate at the same rate. That won't change if they're fused together to make one solar panel but now you have a solar panel twice the area of the original sail.

 

Now rework the problem with a payload of mass M, and a sail of mass m, where M >> m.

 

I mean, there's no point of discussing a practical propulsion system with no payload.

[qft123]

1340896506[/url]' post='686950']

If I turn on a laser in space, will it accelerate ?

Usually rocket engine send mass to the back to accelerate in the opposite direction. Light has no mass, but has momentum and by conservation of momentum law the laser should accelarate...

I am not sure and that is why I am asking.

Yes, it will accelerate. Photons have do have momentum and they also do have energy, but very little. E=PC P=e/c Substituting E=hv P=hv/c But the amount of momentum will be less and thus it will take a large amount of time to reach to your destination..

 

 

[pmb]

Now rework the problem with a payload of mass M, and a sail of mass m, where M >> m.

 

I mean, there's no point of discussing a practical propulsion system with no payload.

Yep. I know. What I had in mind was to look at the upper bound of the acceleration and if the upper bound was insufficient then a real sail wouldn't work. The upper bound of acceleration is with no pay load. With the payload you want the sail to be as large as it can.

[D H]

In reverse order of the last couple of posts,

 

What I had in mind was to look at the upper bound of the acceleration and if the upper bound was insufficient then a real sail wouldn't work. The upper bound of acceleration is with no pay load.

What do you mean by an "insufficient" acceleration? There's a lot to be said for a continuous acceleration, even if it is very small. Example: A solar sail with an acceleration of 2 mm/s² at 1 AU and a high temperature tolerance could fly past Neptune in a bit over 2.5 years. Why the high temperature tolerance? The sail doesn't spiral out to Neptune. It dives in toward the Sun. It needs the high temperature tolerance so it can fly in close to the Sun before escaping the solar system. The idea is to launch the sail (by conventional rockets) on an orbit with aphelion between Earth and Mars. Once deployed, the sail is tacked so the photonic acceleration is directed against the velocity vector. That mm/s² deceleration will gradually decrease the perihelion distance from 1 AU to 0.1 AU by the time the vehicle reaches aphelion. The sail is turned edgewise to the Sun at this point, making it just another falling rock. The vehicle is turned normal to the Sun at perihelion, once again making it a solar sail. End result: The photonic equivalent of a gravity assist, and a very big one. The vehicle eventually attains solar system escape velocity and flies by Neptune.

 

That's not quite realistic (yet), for a couple of reasons. 2 mm/s² sounds small, but it limits the mass to 4.5 grams per square meter of sail area, including structure and payload. Another problem is that 0.1 AU. That's no problem for a perfect reflector, but it's a big problem for a realistic material. It would be another Icarus. Nonetheless, the concept of these weird sundiving orbits is still quite valid for lesser accelerations, greater minimum distance to the Sun. It just takes longer and requires multiple such photonic assists.

 

With the payload you want the sail to be as large as it can.

No, you want the payload to be as small as possible and still achieve something useful. We don't know how to make big solar sails.

 

 

Now rework the problem with a payload of mass M, and a sail of mass m, where M >> m.

That's the wrong way to look at it. Solar sail payloads, like conventional rocket payloads, need to be small. A solar sail with a big payload ain't going nowhere¹.

 

I mean, there's no point of discussing a practical propulsion system with no payload.

Scientists and engineers can make some very small but still quite useful payloads these days. Just look at your cellphone.

 

 

 

¹Don't harangue me about using ain't and double negatives. Ain't is a word in Texas and in the farm country where I grew up. Texans, like Shakespeare, are quite adept at using double (and even triple) negatives to emphasize negativity. (Most languages use double negatives to emphasize negativity. It's those puissant logicians who just can't understand this concept.) This makes passage perfect sense in Texas:

 

"Well I ain't never used no toothbrush!" exclaimed the gap-toothed cowboy.

 

"So is that why you're missing those teeth?" asked the bartender.

 

"Heck no. I lost this'n in a barfight in Amarillo, and that'n, I ate some chili with beans, only one of them beans was a rock that chipped my tooth. From then on, I swore by the adage “real chili ain't got no beans”."

Edited July 1, 2012 by D H

[Spyman]

I am not speaking of solar sail although it is very interesting.

Sorry, I did not intend to bring the thread off topic, what I probably should have clarified is that even though a solar sail and a laser thruster are different concepts they both involve momentum conservation of photons and this kind is of a confirmed phenomenon.