Augmented Solar Sail. Would this work?

[mistermack]

I'm wondering if it would be theoretically possible to give a huge boost to the solar sail principle, by using tuned mirrors?

Imagine the craft with the solar sail, reflecting the Sun's photons back towards the Sun, and being propelled as a result. What if you positioned a massive object, with a mirror, directly opposite your required direction of travel. The mirror is tuned to reflect the light accurately back at the solar sail. And the solar sail reflects it back to the mirror. And the light ends up going back and forth until its lost almost all of it's energy. 

So instead of using the light just once, it's used possibly thousands of times, and instead of extracting a small amount of energy from each photon, you are getting nearly all of it. 

The only drawback would be that you would be sending the mirror and it's massive ballast in the opposite direction, eventually losing it, unless you can figure out a use for it. 

I realise the practicalities would not be simple, but would it work in principle?

[zapatos]

1 hour ago, mistermack said:

The only drawback would be that you would be sending the mirror and it's massive ballast in the opposite direction, eventually losing it, unless you can figure out a use for it. 

 

Overall I have no idea if your plan will work (sounds interesting to me though!) but for your mirror could you simply put a mirror on both sides of your object so that it reuses the photons as you suggest, but also gets pushed along with your craft. Having a larger mirror facing the sun than facing the craft could allow it to keep up with the craft.

[mistermack]

9 hours ago, zapatos said:

could you simply put a mirror on both sides of your object so that it reuses the photons as you suggest, but also gets pushed along with your craft. Having a larger mirror facing the sun than facing the craft could allow it to keep up with the craft.

That sounds like it's worth exploring. It would probably help but not keep up, if the system did actually greatly enhance the propulsion, because the first mirror would only be hit once by the photons. It might be more efficient in practice, to use that extra material on making the solar sail bigger or the reflecting mirror bigger, if they were getting a really significant gain in propulsion from the system. 

I was thinking that you could add mass to the mirror unit with Moon rock, once the Moon was colonised. It won't be costly to lift off heavy objects from the Moon, once it's colonised. The more massive the mirror, the less it would be propelled away from the craft. It might even be possible to use a mirror on the Moon, although it would be complicated because of it's orbit. Maybe two or three mirrors on the Moon would do the trick though.

Or, if the mirror was getting too distant, you could arrange for a slingshot orbit around the various planets to force them closer together again. 

 

[mistermack]

I'm assuming that a photon must experience some amount of red shift, when it bounces back off a solar sail or mirror, to account for the energy that it's adding to them. I wonder if there are any figures for the amount or proportion of energy that it loses with each reflection? If it's tiny, then the scope for enhancing it with a mirror like this would be quite high. 

[mistermack]

Just found the same idea has already been put forward. They make some big claims for it, claiming it could be up to 1,000 times more effective.

 

 

https://web2.ph.utexas.edu/~mwguthrie/p.solarsails.pdf  

https://arc.aiaa.org/doi/abs/10.2514/2.3807?journalCode=jsr

[swansont]

On 10/31/2021 at 9:33 AM, mistermack said:

I'm assuming that a photon must experience some amount of red shift, when it bounces back off a solar sail or mirror, to account for the energy that it's adding to them. I wonder if there are any figures for the amount or proportion of energy that it loses with each reflection? If it's tiny, then the scope for enhancing it with a mirror like this would be quite high. 

It's more likely that you are limited by the reflectivity of the mirrors, but you can do the calculation:

The photon imparts a momentum of ∆p = 2E/c with each reflection. The target is massive, so it gains KE of p^2/2m from the first photon (this is quite small, so this will continue to be a good approximation at the beginning of travel). That energy has to come from the photon.

The energy loss is therefore 2E²/mc²

Keep in mind that a visible photon has energy of order 1 eV, and a single proton has a mass energy of order 1 GeV; so mc^2 is going to be something like 10^39 eV for ~1000 kg (10^9eV x 10^27 atoms per kg, and 1000kg. Alternately, you can say that c^2 is ~10^17 and the conversion from J to eV is about 10^19, which also gets you to 10^36 eV per kg)

So your energy loss is tiny. (You need to scatter millions of photons to slow a single thermal atom down to rest, which is how I'm familiar with this)  

Meanwhile, the losses in the mirrors will limit you to perhaps millions of scatters. 10^6 <<10^39

(the engineering implication here is that almost all of your photon energy goes into heating the mirrors)

 

[mistermack]

I suspected that the loss for one reflection would be very low, as there is no noticable red shift when you look in a mirror. Of course, that means that solar sails that work on a single reflection are only using a tiny fraction of the available energy in each photon, so if you can get the light to reflect thousands of times you have a great potential for multiplying the effectiveness, so the estimate of 1,000 times by Meyer et al is probably a reasonable guess. 

The losses initially would be to heat, but as the sail and mirror got farther apart, you would start to lose photons due to inaccuracy of the aiming of the beam, and eventually it would stop working. What sort of distance you could send a usably tight beam over, I have no idea. But in any case, you would initially at least get a hugely increased thrust, to start the sail on it's way. With the speed of light being so high, each photon would give up it's available energy very quickly. 

The Meyer article that I linked is just a summary, I just noticed, the full article has to be paid for. But the summary makes it clear that it's exactly the same idea.

[swansont]

"system such as they describe would be able to reach Pluto in a time of approximately 6.5 years. This gives an average velocity of 21.5 _km/s"

This suggests the speed when they get to Pluto is more than 21.5 km/s (if you have constant acceleration, it's twice the average speed, but you won't have constant acceleration since the sun's intensity will drop as 1/r^2))

So: how do you slow down when (or better yet, before) you get what you're going?

[mistermack]

On 11/2/2021 at 3:14 PM, swansont said:

So: how do you slow down when (or better yet, before) you get what you're going?

That's the six billion dollar question. However, the same problem applies to ordinary solar sails, and also conventionally powered craft. 

A device like this won't add to the problem, but possibly be part of the solution. If you can get decent acceleration without using carried fuel, then the fuel that you saved could be carried to slow the craft on arrival. One thing about slowing a craft with rockets is that it would get more effective as the fuel was burned off. The stored fuel would be dropping, and you could ditch fuel tanks as they emptied. 

The whole thing would be the usual compromise, trading one thing off against another. Unless somebody comes up with a new way of slowing. If it's possible to get a gravitational assist from a massive body, it should be possible to use it in reverse to get some slowing effect. Maybe a planet with a moon could be "slingshotted" repeatedly till the speed dropped to a useable level. 

[zapatos]

If you are headed to another planetary system you could of course turn your solar sail around to use it for deceleration.

[swansont]

59 minutes ago, mistermack said:

A device like this won't add to the problem

In a way it does, because you’re going faster, so it’s harder to come to a stop.

[geordief]

If the craft had  a very large mass it will take longer to accelerate to very high speeds  (unless ,perhaps it can be given an initial  launch using a land or moon based laser)

 

But when it needs to decelerate, could something like 99% or more of the total mass  be disconnected from the spacecraft and used to slow down the very small payload quite quickly?

 

Could the disconnected mass be  made to emit a laser  beam at the sails?

 

Edit:might it make sense for the craft to  be launched from  a  conventional spacecraft that was already travelling at its maximum speed relative to the Earth?And for that spacecraft to fire lasers at the sails of the departing  craft...

Edited November 4, 2021 by geordief

[mistermack]

The best I can come up with, is to have many many mirrors, packed together as layers, comprising the craft. Each time a mirror gets out of range of an effective exchange of photons with the main craft, it jettisons another mirror layer and starts again from zero range. Then, when you are half way to the destination, you start jettisoning the mirrors in the opposite direction, the way you are going, and you turn the craft around so that the solar sail is facing away from the sun, at the current mirror. 

Obviously, a lot depends on how accurate you can make the reflected beam over long distances.