I realize that the reason the U-235 projectile rings in Little Boy were going at immense speeds was to prevent a predetonation, but how fast were they actually going? Is the speed directly related to the amount of fissions per amount of time the projectile takes to cross the distance a released neutron can travel? Thank you in advance, this question has been plaguing me for a couple weeks.

EDIT: Grammar

  • $\begingroup$ B. Cameron Reed's paper on 'Predetonation probability of a fission-bomb core' at American Journal of Physics 78, 804 (2010); doi: 10.1119/1.3367757 is worth a read (as are his other papers on the physics of the Manhattan Project). $\endgroup$
    – Jon Custer
    Jun 2, 2022 at 14:06

1 Answer 1


They were going at about 1,000 feet per second. The projectile and target were designed so that they would not go critical until they were almost completely aligned.

Because the plutonium bombs were far more sensitive to the effects of background sources of neutrons, there was no way to statistically guarantee that a random cluster of neutrons would not be released sometime during the assembly process which would then trigger a chain reaction before the (nominal) critical mass was assembled, thereby causing a predetonation or"fizzle".

The background neutron source was in the plutonium itself (which had been manufactured in the Hanford breeder reactor) in the form of other plutonium isotopes which could not be chemically removed from the as-irradiated plutonium/uranium mixture.

  • $\begingroup$ @neils - I thought the projectile had to be much faster than a typical bullet, i.e., at least 2500-3000 ft/s. This is why it was difficult to engineer (i.e., how to launch such a large mass within a confined space to such speeds without the recoil causing structural issues... not to mention just how to get such a large mass to such speeds). Is my memory really that far off (I am leaning toward yes since it's been 18+ years since I read the gory details of this stuff)? $\endgroup$ Jul 5, 2022 at 21:36
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    $\begingroup$ Just grab a copy of Richard Serber's book The Los Alamos Primer, it's got all the cool details like that in it. $\endgroup$ Jul 5, 2022 at 22:43
  • $\begingroup$ "From the beginning the plutonium gun with its nearly unattainable muzzle velocity of 3,000 feet per second had been a gamble..." The Making of the Atomic Bomb page 541. So it seems I was recalling the wrong gun, whoops ;) $\endgroup$ Jul 6, 2022 at 12:47

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