Andrew Mossberg (aem@hypertek.com) recently posted this chart to
    rmr, which includes approximate propellant weights for maximum impulse
    motors for each class:

      P
      O
      W      Low       High        Low         High      200 ISP Propellant
      E     Limit      Limit      Limit        Limit            Weight
      R    (NtSec)    (NtSec)    (lbsSec)     (lbsSec)    (grams)   (lbs)
      =    =======    ========   =========   =========   ========   =======
      A        1.26        2.5       0.28         0.56        1.3    0.0028
      B        2.51        5.0       0.56         1.12        2.5    0.0056
      C        5.01       10.0       1.13         2.25        5      0.0112
      D       10.01       20.0       2.25         4.5        10      0.02
      E       20.01       40.0       4.5          9          20      0.04
      F       40.01       80.0       9           18          41      0.09
      G       80.01      160.0      18           36          82      0.18
      H      160.01      320.0      36           72         163      0.36
      I      320.01      640.0      72          144         326      0.72
      J      640.01    1,280.0     144          288         652      1.44
      K     1280.01    2,560.0     288          575       1,305      2.88
      L     2560.01    5,120.0     575        1,151       2,609      5.75
      M     5120.01   10,240.0   1,151        2,301       5,219     11.5
      N    10240.01   20,480.0   2,301        4,602      10,438     23.0
      O    20480.01   40,960.0   4,602        9,204      20,875     46.0
      P    40960.01   81,920.0   9,204       18,409      41,751     92.0

    Currently, consumer rocketry stops at rockets with a total of no more
    than 81,920NS of total impulse. Anything larger than that is by
    definition an amateur rocket.