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?

Jurek D., Switzerland

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Jun 29, 2020
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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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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.

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.
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.
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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"Science begets knowledge, opinion ignorance.
"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 :)
"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.
in regards to this concept (which is new to me and sounds great to have a light craft),
does this sound at all feasible? -it's a flight of fancy sure
-- in a test case travel to Mars, and return
our power laser is located on a Mars Moon, and on the Earth's moon
on our craft we have a vast receiver, say a 50m dia plate of photon cell generator
the receiver transforms the laser to electricity,
with dead ballistics required so our distant power sources can hit it
perhaps we can the push our craft with our drive laser, another laser
-a laser or light thruster
*that would mean we could steer as per the OP query

other related ideas are-
- putting an incredibly powerful light on our craft, instead of using a sail
-using the electron thruster. Electrons weigh 1/1800th of a hydrogen atom
to send the electron somewhere seems problematic though,
I mean it seems something of a Tesla coil

the space is the place for wild imagination, while on the home world for sure
I'm just having fun with these thoughts

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"Science begets knowledge, opinion ignorance.
"in regards to this concept (which is new to me and sounds great to have a light craft),
does this sound at all feasible? -it's a flight of fancy sure."

How about light pressure when you want to come back home towards the Sun?

Cat :)
oh hehe
the first thought is that whatever is powering us must defeat any push we are getting from a/the sun

like if our laser receiver from Phobos or Deimos is an electrical solar collector, on one side(Mars) it is cells- on the other(Sol side) {edits}- perhaps receiving electrical energy from the sun too?
- we must have more kinetic energy coming out of our thrusters than the craft is receiving from Sol side

I have actually been quite enthused by this thread because with a laser we have the potential to send energy through space!(- more stationary objects will be better targets to hit)
-thanks Jurek

{edit}- for matter raised by Cat. It is problematic to have a large 'laser receiver', for it is always a sail too
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hmm- when we want to go away from the sun(to Mars), we can use the sun and the laser on the Moon
-to go toward the sun, our thrusters must overcome the solar photon barrage,
so the laser unit on Martian moons will need to do the lot of work, but we can use our large 'receiver' to catch solar electrical power, and put that out the theoretical light/electricity/laser thruster

--the thruster are twin outriggers, enabling us to tack (8D)
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-since our thrusters can be vectored
we point them backwards to brake

mainly the problem here is that the electron,laser or light are pico-masses to thrust
also not blasting anything to pieces with an interplanetary laser power source
as far as I can see
If you can actually hit the target with the laser, the electrical energy might convert ok(ie. not trivial electricity)

the contention is that our laser transmission will defeat the sun in either direction
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"Science begets knowledge, opinion ignorance.
I am all in favour of any viable methods of propulsion, but I do believe that we are in practice limited to the inner Solar System for the time being. Some moons of the gas giants will become accessible but, I believe, will not be viable for colonisation until the Sun expands sufficiently in about 5 billion years time. Just too cold to warm up.

However, I am all in favour of discussion on SF ideas because they are entertaining and may spark ideas for future viable systems.

Keep up the good work!

Cat :)


"Science begets knowledge, opinion ignorance.
FYI This is from Socratic.com:

Answer link
José F.
Mar 14, 2016
4.2×10−5 light years
Light-years is not an adequate measure for distances inside the solar system. Symply a light-year is a humongously large measure of distance.
The distance between Earth and Mars is at the most 400 million km, and could be as small as 55 million km.
Even with the larger distance we would obtain:
400×106km300000km/s~1333s, which is about 22 minutes. So Mars can be at the most 22 light-minutes from the Earth.
Neverthless, here is your answer:
Just divide the time in seconds light takes between Earth and Mars, by the number of seconds within one year:
1333365.25×24×60×60=4.2×10−5 light years
Answer link

Cat :)

That is 0.000042 light years at most.
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I had been all mixed up
I meant it takes 14mins to get to Mars at light speed,
however it actually takes 3 minutes 3 seconds(when I Google)
- judging by your orbital figure that varies 7:1

from 3 minutes to 22 mins

thanks for a more accurate delineation,
my division was over an hour, not a year [blush]

I read there are plasma drives, they too are low thrust, somewhat comparable to ion drives
they also need fuel, not being pure electricity
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"Science begets knowledge, opinion ignorance.
This is a general comment, and is not intended in any way as a criticism of any methods of propulsion described in this thread.

I think we must be very careful in assessing any method and need to bear in mind some points:

Does the weight of fuel or of the propulsion mechanism outweigh the propulsion benefit?
Examples of these might be (A) inefficient fuel combustion, whereby the propulsion effect is barely more than the weight of rocket fuel consumed. (B) weight of mechanism to produce thrust is similarly barely sufficient to lift its own weight, which might be a particle thrust method requiring a heavy generator.

One frivolous example (tongue in cheek) might be shooting peas from a peashooter where the weight of peas needed to be carried would produce inefficient thrust required to accelerate the weight of peas carried and the weight of person(s) blowing the peas.

Cat :)

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