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<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'><font size="3">What about using ION engines in stages?</font></DIV><br /><br />Dryson, using four stages will not multiply the final velocity by four.</p><p>If V is the change in velocity of a single stage, and P is the wet mass to dry mass ratio, the final speed is V1 + V2 + Vn and the necessary initial mass is P1 * P2 * Pn.</p><p>Example</p><p>Stage 1:<br />V1 = 3 km/s<br />P1 = 10 (including mass of stage 2 + payload)</p><p>Stage 2:<br />V2 = 3 km/s<br />P2 = 5 (including payload)</p><p>Stage 3 (payload)<br />V3 = 0<br />P3 = 1</p><p>Total change in velocity = 3 + 3 + 0 = 6 km/s<br />Total initial mass = 10x5x1 = 50 x payload mass</p><p>Concerning the laser beam pointing at the destination star, this cannot be of use in any way. If you want to thrust to Alpha Centauri, simply point your vehicle in the right direction and thrust away.</p><p>Concerning the original post on this thread, if you're interested in what nuclear fusion could do for us in something like a century from now, check out Borowski's 2005 Piloted Spherical Torus Nuclear Fusion Propulsion. To summarize, this manned ship to Jupiter and Saturn attains a peak velocity of "only" 156 km/s on the Saturn scenario. This is 0.05% of light speed. Such a ship could get to Alpha Centauri in just over 8,000 years...</p><p>Adding many more fuel tanks and dropping them off as the ship accelerated could probably reduce trip duration to 1,000 or 2,000 years but anything less would require more advanced fusion or antimatter. </p><p>I would say anything beyond that is unfortunately pure science fiction, today.</p><p> </p><p> </p> <div class="Discussion_UserSignature"> <p><em>“An error does not become a mistake until you refuse to correct it.” John F. Kennedy</em></p> </div>