Laser propulsion system - is it possible to tack?

Jun 3, 2020
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Hi everybody,
I read that laser propulsion could, in future, propel spaceships without carrying enormous mass of fuel. And some beams of X-rays coming from space match such a propulsion of a theoretical extraterrestrial civilization (but natural explanations are possible).

Now, as fair as I understand, in layman terms. The theory is that a spaceship traveling between planets A and B carries little fuel itself. A powerful X-ray laser on the planet A accelerates it. Later, a similar X-ray laser on the planet B brakes it. Otherwise 99% of the spaceship mass should be fuel, spent to propel the mass of fuel itself. The arrangement however, requires that the destination B already contains an X-ray laser. So no good for initial space exploration.

My question is: can a theoretical spaceship use external laser propulsion to change direction and possibly brake? The idea would be similar to a normal ship sailing against the wind by tacking. Can this idea advance laser-propelled space exploration? Does it have any effect on interpreting the strange X-ray bursts recorded from space?

best,
Jurek D., Switzerland

 
Jun 29, 2020
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Hi,

A normal sailing ship is able to tack because it can use the keel and rudder of the boat to push against the water it sits on in a different direction to the wind.
I would imagine you could achieve a similar effect in space if you were able to push against the interstellar medium in some way, possibly by using a magnetic field as a physical keel / rudder would need to be pretty big
 
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Mar 5, 2020
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The construction of a solar sail is entirely tensile (pulling).

If the beam were coming from anywhere else except directly behind the mass it is pulling then the lasers impact would generate a lateral or shearing force. Any reflected light would also add another component of shearing force. The material of the sail would want to move to the side. If it gains any significant sideways motion before the lines restrain it, it can wrap around your ship.

The lines from the mass would have to be able to exactly match the shear to keep the sail from folding up like a fan or wrapping around you.

A light sail is very difficult to deploy. Making the sail as adaptable as a sail on a boat where you could pull or extend miles of lines beggars’ the imagination.

Almost any form of propulsion is simpler and more reliable than a solar sail.
 
Feb 1, 2020
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Jurek,

Yes, you can tack with a solar sail. The thrust comes from absorption of the photons of light and then reflecting them away. Each photon has a small amount of momentum. The total thrust is the result of all those photons, both in and out, and the overall thrust is then summed. It's a vector quantity, so there can be two different momentum vectors summed for an overall result.

If you tilt the sail, then there is a angle in from the laser (or the Sun) and an angle out from the reflection. The net thrust then is half way between the two.Though as Catastrophe said above, you do reduce somewhat the thrust in doing this. It's Vector addition, not Scaler, so one plus one is somewhere between zero and two. Just where between depends entirely on the angle of incidence.

It has been done in orbit by the Planetary Society Probe. That however was extremely low thrust. It was really just to demonstrate that the sails work. They do, but our current sails are much too heavy per square meter for effective use.

For high thrust applications, the current plans call for lasers at truly huge power levels to illuminate the sail, providing a thrust level similar to that experienced by the craft inside of Mercury's orbit clear out to the edge of the solar system. Such a probe would have a velocity of a couple percent of the Speed of Light.

Nothing heavier than a single atomic nucleus has ever been so accelerated by human efforts. Particle accelerators do this sort of thing all the time however, but only for small groups of ions or electrons.

The Physics says it's possible, but the engineering is still beyond us at present.

However, solar sail craft can tack to speed up or slow down in orbit. It's a slow process though for anything that has been designed that we could produce at present. Chemical rockets are faster for now, that's why we use them. What is possible in fifty years is a different story however.
 
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Feb 18, 2020
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Yes, but why would you want to tack? As I pointed out, you are only getting the propulsion x cos theta.

Are you saying that you can introduce a lateral component of sin theta?
 
Jun 1, 2020
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Cat said:
Yes, but why would you want to tack? As I pointed out, you are only getting the propulsion x cos theta.
I think "tack" was meant to mean "steer", and not it's normal sailing meaning -- into the wind.

I would guess it would be easy to alter one's course in any direction by simply rotating the light panels along both the longitudinal axis and panel axis to produce the desired directional change, if minor.
 
Feb 1, 2020
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Yes, but why would you want to tack? As I pointed out, you are only getting the propulsion x cos theta.

Are you saying that you can introduce a lateral component of sin theta?
In any orbit you have a velocity for the orbit. You tack by changing that velocity and therefore that orbit. This moves you in or out, just as a chemical rocket would. So yes, you can actually tack across the solar 'wind'. Predicting just what the effect will be requires orbital mechanics. and some other physics. This was all worked out in the 1970's and some earlier even. Some of it was suggested by back in the late 1800's. What you really have is just some adjustment to your overall velocity in a gravitational field. Inertia and gravity still dominate most of your motion. The resistance that makes it tacking is prvided by your inertia.

Generally for the reactions to the thrust of the sail it's back is down, down is forward, forward is up and up is back. It's somewhat counter-intutive until you look at it as orbital mechanics in terms of velocity and orbitals. Larry Niven had an excellent treatment of the overall relations in an old Sci-Fi book, The Integral Trees way back in the mists of time.

That's all for in system. For interstellar (Which we haven't done yet) it's all pretty much straight line with minor steering maneuvers once you get out of the Solar System. Stopping is a problem unless you keep the velocity down to what you can get from the target star. That's why the Breakthrough Starshot program doesn't plan on stopping.

As for the Sin Theta component, remember that Sin squared plus Cosine squared equals one. So yes if you have a cosine function there is an equivalent sin function available. but it's not terribly efficient, as I said above.

Still the fuel is free in system. It's just sunlight after all.
 
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Feb 18, 2020
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"Still the fuel is free in system. It's just sunlight after all."

Yes, but don't forget the light coming from all other stars (of course only the nearest have any appreciable effect).

But, if you are aiming at a nearby star, it is going to send you back more, the closer you get!

Cat :)
 
Feb 1, 2020
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"Still the fuel is free in system. It's just sunlight after all."

Yes, but don't forget the light coming from all other stars (of course only the nearest have any appreciable effect).

But, if you are aiming at a nearby star, it is going to send you back more, the closer you get!

Cat :)
That's why the serious plans call for sails that can be folded back up. Once you get up to speed, you fold the things and coast. You only unfurl the sails when you want to slow or stop. And stars work on an inverse square law after all. The closer you are, the stronger the push away, which slows you down. Once you slow enough, the the tacking we discussed earlier becomes important as a means of getting into stellar orbit, It's how you would stop at your destination, which isn't really some star. Colliding with a star might not be very pleasant.

One major problem is that if you start with a laser push, you don't have that concentrated beam to slow down with, so you may not be able to actually stop at the target star, only slow down a bunch.

Doctor Robert Forward in his SF novel on this wanted to use a two stage sail, one, the larger, was a mirror that got accelerated by the laser back in the Solar System as it reflected the light and pushed the ships sail to slow to a speed that the target star might be able to supply the thrust for final maneuvering for.

It's why the Breakhrough Starshot Project plans only flyby's. They can't stop.

Solar Sails that are limited to the speeds that actual starlight can reach are Slow Ships. Faster than chemical rockets can achieve, but still with trip times of centuries to millennia.
Just what that speed might be varies as the stars do. Proxima Centuri is a red dwarf, very little thrust compared to the Sun or Alpha Centuri A.

So there are drawbacks and limitations to sailing on light.

Besides the glaringly obvious fact that we can't really do it yet, I mean.
 

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