Photon Sails in Interstellar Travel

[MattRob]

I might get a bit of a reputation for space talk, here :P

So, I'm familiar with the concept of Photon Sails, but I was wondering how effective these would be?

To make sure we're on the same page, I'm talking about using a highly reflective surface to propel a spacecraft by hitting it with a ground-based laser. The laser reflects off the "sail", and imparts momentum to the sail.

First off, the application I'm thinking of is Interstellar travel. So my first question is what sort of beam divergence could a large telescopic laser (say, the size of Hubble or larger) achieve? (since this would determine a lot of things, like the size of the sail and the acceleration time.)

I've done some homework, and found this:

...The f/288 field of view was 3.6 X 3.6 arc-seconds square, with resolution down to 0.0072 arc-seconds...

With the f/288 apparently being one of hubble's cameras. Only thing is, this doesn't really apply to lasers very well, unless I can find out what the actual size of the light-recording device in the Hubble is.
I found this here.What kind of power would be required to provide a certain amount of thrust on the sail?
I've seen this equation:

F = P/c

and

P = F * c

Where
F = Force in Newtons
P = Power in Joules
c = Speed of light in a vacuum.

I assume the thrust would be twice this, because the change in velocity of the photon is twice it's velocity, since it's reflected.
But surely that can't be right? According to that, it would take ludicrous amounts of power to move even a very light sail. Is that right?

 

[KingNothing]

But surely that can't be right? According to that, it would take ludicrous amounts of power to move even a very light sail. Is that right?

Why can't it be? How fast do you want your sail to travel, and in how much time?

 

[MattRob]

Why can't it be? How fast do you want your sail to travel, and in how much time?

Really I'm surprised that it's been seriously considered if it requires so much power. That's what I mean.

 

[KingNothing]

Really I'm surprised that it's been seriously considered if it requires so much power. That's what I mean.

I guess I'm a little confused as to why you say it requires so much power. Neglecting inefficiency losses, moving any object of given mass at a given acceleration will require the same amount of power regardless of propulsion method.

Are you saying that it's surprisingly inefficient?

 

[Matterwave]

The trick with a lot of these interstellar propulsion schemes is that usually they provide a small but constant acceleration over a long period of time so that it takes a long time to achieve high speeds, but it doesn't require you to carry 50,000tons of fuel on board (just throwing a random number out there).

 

[MattRob]

I guess I'm a little confused as to why you say it requires so much power. Neglecting inefficiency losses, moving any object of given mass at a given acceleration will require the same amount of power regardless of propulsion method.

Are you saying that it's surprisingly inefficient?

No, it requires a lot more power than any other major propulsion method. Requires. If you want 1 Newton, you have to have an electrical powerplant supply 300 Megawatts of electrical power to a laser.

Meanwhile, if you want 1 Newton from any other propulsion system, it will take a lot less electrical power. Chemical rocket engines don't even need any electrical power, aside from a spark ignition source, and some don't even need that...

I'm thinking in more engineering/practical terms. 300 Megawatts in a chemical rocket engine is much easier to create than 300 Megawatts of laser power.

 

[Matterwave]

The reason for this large discrepancy is that the momentum of a photon is very very small, and so you impart only a very small part of the energy of a photon to the sail, most of that energy is just carried back in the reflected beam.