It's not twice as many bombs, it is (using your numbers) six time as much bomb mass as payload mass, so 50Kt payload, 300Kt bombs for slowing down, which gives

It's not twice as many bombs, it is (using your numbers) six time as much bomb mass as payload mass, so 50Kt payload, 300Kt bombs for slowing down, which gives you 350Kt payload for the acceleration phase, which would require another 2100Kt of bombs to get up to speed, for effectivley seven times the launch mass