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Following up on this answer...

@mark, in the answer, writes that "[i]n the event that the crew of Enola Gay had to abort their mission, they had no intention of trying to land with the bomb still aboard".

That sounds like there was a plan to drop the bomb somewhere else if they were forced to abort the mission. Was there such a plan?

Kryten
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There was discussion of options. The "gadget" was an atomic bomb.

From the minutes of the Target Committee for 10 May 1945:

  1. Gadget Jettisoning and Landing

A. It was agreed that if the aircraft has to return to its base with the gadget and if it is in good condition when it has reached there, it should make a normal landing with the greatest possible care and with such precautions as stand-by fire equipment being held in readiness on the ground. This operation will inevitably involve some risks to the base and to the other aircraft parked on the field. However, the chance of a crash when the aircraft is in good condition and the chances of a crash initiating a high order explosion are both sufficiently small that it was the view of those present that the landing operation with the unit under these circumstances was a justifiable risk. Frequent landings with inert and H.E. filled units have been made in the past. Training in landing with the unit should be given to all crews who carry an active unit.

B. In case the aircraft returns to its base and then finds that it cannot make a normal landing it may be necessary to jettison the bomb. In the case of the Fat Man this can probably best be accomplished by dropping the bomb into shallow water from a low altitude. Tests on this will be carried out with both inert and live units. In the case of the Little Boy the situation is considerably more complicated since water leaking into the Little boy will set off a nuclear reaction, and since the American held territory in the vicinity of the base is so densely filled that no suitable jettisoning ground for the Little Boy has been found which is sufficiently devoid of moisture, which is sufficiently soft that the projectile is sure not to seat from the impact, and which is sufficiently remote from extremely important American installations whose damage by a nuclear explosion would seriously affect the American war effort. The best emergency procedure that has so far been proposed is considered to be the removal of the gunpowder from the gun and the execution of a crash landing. In this case there is no danger of fire setting off the gun and the accelerations should be sufficiently small to prevent seating of the projectile by the impact. Tests on the feasibility of unloading the gun powder in flight will be conducted.

C. It was agreed that prior to actual delivery some form of instructions should be prepared as a guide to the senior man on the aircraft as to procedures to be followed in cases of different types of disasters.

Captain Parsons, the weaponeer aboard Enola Gay in the actual mission, was present at this meeting.

Full minutes may be found at https://nsarchive2.gwu.edu/nukevault/ebb525-The-Atomic-Bomb-and-the-End-of-World-War-II/documents/011.pdf

Community
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R Leonard
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    "since water leaking into the Little boy will set off a nuclear reaction" - I'm curious why this is the case - I know there was a war going on, but it seems unlikely the engineers would have designed a nuclear bomb that could be set off by rain... wasn't there an arming mechanism that would have rendered it safe under most conditions like modern bombs do? – Dai May 09 '19 at 19:40
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    @Dai, when they say "water leaking into Little Boy", they're referring to the level of leakage you'd expect from dropping the bomb into the ocean. To maximize the explosive power of the bomb, the two halves of the core are each nearly of critical mass in open air; water acts as a neutron moderator, so if you fill the bomb with water, those two subcritical halves are now critical masses, producing a great deal of radiation and possibly a small explosion. – Mark May 09 '19 at 19:59
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    Little Boy was of an extremely crude design: inefficient, probabilistic chance of fizzling, and as the above mentions, unsafe. That said, it was extremely likely to work, such that the designers didn't even bother testing it (the "gadget" of the Trinity test site was of the Fat Man design). Modern weapons use explosive lenses that compress a subcritical mass, which is a) more efficient because you don't actually need a critical mass at standard conditions, b) safer as the lenses need to go off with very tight timings. – Nick T May 09 '19 at 20:59
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    @Dai The issue would be with the bomb flooding, not with it merely getting wet. We talk about "the critical mass" as if there's only one but, really, the amount you need depends on the shape of the material and what's around it. Subcritical configurations have enough neutrons escaping from them that a chain reaction can't be sustained. However, if you fill the bomb with water, the formerly subcritical pieces of uranium will have more neutrons bounced back into them, so they may become critical in that environment. – David Richerby May 09 '19 at 21:34
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    First they called it a gadget, but then they called it a bomb and mentioned nuclear reactions - why? – user253751 May 09 '19 at 22:25
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    @immibis I was wondering the same thing. They started off cryptic, but then ... forgot? – Shadow May 09 '19 at 23:57
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    @immibis The usage "bomb" appears only in paragraph B, "gadget" elsewhere. This suggest either (or both) of different authors or time of writing for the two paragraphs. – Pieter Geerkens May 10 '19 at 01:43
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    In both The Making of the Atomic Bomb (Richard Rhodes) and the The Los Alamos Primer (Robert Serber), it was explained that the "gun-type gadget" was designed to pass a sub-critical 'bullet' through a nearly critical doughnut, not a "smash two halves together" approach. This makes the introduction of water as a moderator in the bomb cavity all the more likely to start a suboptimal chain reaction, as well as sudden trauma to cause the bullet to assemble slowly. – Vince May 10 '19 at 02:18
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    @Vince" Technically, the doughnut was the projectile, and the solid part the target (for technical reasons). – Vikki May 10 '19 at 03:18
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    I'm curious whether "gun powder" is a euphemism. – chrylis -cautiouslyoptimistic- May 10 '19 at 04:10
  • @immibis There's also the possibility of anticipation of redaction. There's a clear hierarchy of sensitivity from the section heading to paragraph A to paragraph B. – chrylis -cautiouslyoptimistic- May 10 '19 at 04:11
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    @Sean This is known now but was deliberately obscured for several decades, as figuring out how to deal with neutron reflections off the backing plate (attached to the plug) was one of the tricks that could make the difference between success and fizzle. – chrylis -cautiouslyoptimistic- May 10 '19 at 04:13
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    @chrylis: No more a euphemism than the use of the term "gun powder" to refer to the propellant in actual firearm cartridges - Little Boy used cordite as its propellant, just like most firearms and artillery of the time. – Vikki May 10 '19 at 04:24
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    @Mark Little boy carried almost exactly double critical mass of enriched uranium, with the hollow projectile being made subcritical by its shape (hollowness) only (obviously the not hollow part couldn't be of critical mass). Filling the void in the projectile with water would mean the projectile itself would go critical regardless of the other mass of uranium. – Pavel May 10 '19 at 07:40
  • @Mark mind it wouldn't go supercritical and set off a nuclear explosion, but it would still release serious amounts of radiation. And if the shock of impact were to dislodge the other Uranium mass, the two might potentially combine and form a small nuclear detonation (though not a full yield one) and serious fallout. – jwenting Nov 19 '19 at 06:33