Minimum usable acceleration for a spacecraft

[MichaelPoole]

e) Using one of the many available acceleration calculators online, it comes up that a constant acceleration 3 milligee boost gets you across an AU of distance in about 52 days. The reality is much more complicated as I indicate above, what will needing to match velocity with your destination and solar sails needing to spiral out from Earth orbit for a good bit before really getting going - but even if all that takes 'years' - it's not really all that different from  various other methods.

Why would 0.003 g take "years" to spiral out of Earth orbit or any orbit at all or to match velocity with the target? Dawn probe was able to enter and leave the orbit of Vesta and then enter Ceres orbit at 0.000007541 G much quicker than that plus the same ProjectRho site has concept studies of fusion spacecraft with about 0.003 g acceleration with much less than a year time to get even to Jupiter.

Page 33 Microsoft Word - Z-Pinch_20101130.docx (nasa.gov) this one takes over a year to get to Jupiter (still absolutely fantastic compared to "high thrust" chemfuel) but can handle Mars in 30 or 90 days depending on how much fuel is one willing to spare. The ship in question has an acceleration of 0.004381 G in the heavier "Mars in 30 days" configuration per info here Realistic Fusion Designs - Atomic Rockets (projectrho.com) , below the supposed 0.005 G limit below which supposedly spacecraft take years just to change orbits. IIRC there is one ion drive probe that did take around a year to just get to the moon (the European SMART probe) but it had an ion drive weak even by modern standards, and the typical ion drive IIRC has accelerations of just a few microgees to tens of microgees so about 100-1000x less than the thrust regime we're discussing here.