What if you had a laser solar sail and in front of the sail was a small thruster (either chemical or itself a laser)...thrusting constantly at a right angle to the forward direction of the craft so that the sail's trajectory would bring it back towards the first (main/stationary) laser that would then provide another push. you could keep doing this until the sail's velocity was near c. So let me just explain it another way. Laser pushed solar sails are a well discussed and thought out idea, the problem is that the further you go away from the laser the less push from the laser you will get...so you need a big laser and a tough material to withstand the high initial temperature...but if you could bring the sail back to the laser by circling it's trajectory you could keep pushing the sail faster and faster. Just like you would spin a wheel faster and faster by constantly providing pushes. If you used a small thruster at the front of the solar sail (really where ever you want but to keep it simple put it at the front) and you had it pushing at a 90 degree angle to the forward trajectory of the sail you could have the craft return obliquely back towards the first most powerful laser which could push it again and increase the velocity. You could do this as many times as needed or until the small front thruster runs out. Now I anticipate three objections. The first is that the front thruster would cancel out the velocity provided by the main laser. But this is not so because the velocity of the front thruster pushes the craft at 90 degrees the actual velocity would be changed and increased (look up a basic 101 physics book about addition of velocities). The second is that as the craft was accelerated faster by the main laser the small front thruster would not return it in a 'circle'. The return need not be in a perfect or near circle so long as the angle is constantly applied and/or controlled (on/off) at 90 degrees you can return the sail back towards the location of the laser. In other words it is not necessary that the sail return travelling in the same direction as it was previously pushed since you could rotate the main laser. The last objection could be that the energy needed to rotate the sail would be very large, but this is also not so since the energy isn't against the forward trajectory. In fact, by my calculations, a craft travelling at .1c (3*10^7 m/s) would require 134 days of a constant 10 m/s impulse to return back to the main laser. And this is without the addition of the impulse from the front thruster which would shorten the time. So...what would be the problem of using a laser push sail and having the craft 'do laps' to be accelerated again by the laser?