Science desk
< June 12	<< May \| June \| Jul >>	June 14 >

Welcome to the Wikipedia Science Reference Desk Archives
The page you are currently viewing is a transcluded archive page. While you can leave answers for any questions shown below, please ask new questions on one of the current reference desk pages.

June 13

Enola Gay traveled 18.5 km in 44 seconds after dropping the atomic bomb over Hiroshima

Atomic_bombings_of_Hiroshima_and_Nagasaki#Bombing_of_Hiroshima says: Little Boy containing about 64 kg (141 lb) of uranium-235 took 44.4 seconds to fall from the aircraft flying at about 9,400 meters (31,000 ft) to a detonation height of about 580 meters (1,900 ft) above the city. Enola Gay traveled 18.5 km (11.5 mi) before it felt the shock waves from the blast. That is over 940 mph, but the top speed of Enola Gay is 339 mph. Is that a mistake? Vpab15 (talk) 15:27, 13 June 2022 (UTC)[reply]

"before it felt the shock waves" is the key statement -- the shockwave has its own travel time. So, 44 seconds of head start, plus relative position head start, plus distance travelled by the bomb (can be part of that relative position, but remember to include it), plus the relative overtake speed of shockwave versus plane-in-a-dive-to-add-speed. It seems plausible. This page says the goal was to achieve at least 7 miles of separation. (editing to add: 509th_Composite_Group#Atomic_bomb_missions says it was "a minute", no firm precision, after the blast before the shockwave reached the aircraft -- point being, more time elapsed post-blast than pre-blast) — Lomn 15:39, 13 June 2022 (UTC)[reply]

Yep, this is an important point. The blast wave propagates outward at speeds roughly comparable to the speed of sound, at least initially: about 700 mph, or about 300 m/s.

So the aircraft gets a head start of 44 seconds from the time it takes for the bomb to fall, plus the time-of-flight for the blast wave. Even if the Enola Gay were hovering in place directly over the detonation, that's another 30 seconds or so for sound (and the blast) to climb back up again. Assuming the pilots were travelling at a brisk pace away from the detonation, they'd be moving close to half the speed of sound. It would take a while for the blast to overtake them.

11.5 miles at 339 mph is almost exactly 2 minutes of flight time. I can't bring myself to do the pencil-and-paper work, but it strikes me as plausible that they had sufficient head start. TenOfAllTrades(talk) 16:21, 13 June 2022 (UTC)[reply]

The blast wave propagates outward at speeds roughly comparable to the speed of sound, at least initially: about 700 mph, or about 300 m/s. Is that correct? A blast wave starts with a shock wave which can go (much) faster than the speed of sound. Whether that was the case for those bombings, I do not know (the speed depends on how much energy was released and the gas properties). The later, non-shock wave does propagate at the speed of sound with a relatively linear behavior (= the speed does not depend on the amplitude). Tigraan^{Click here for my talk page ("private" contact)} 09:10, 15 June 2022 (UTC)[reply]

Thanks for the reply and the link. However, it says As it turned out, with the first bomb, which was dropped on Hiroshima, they did get approximately 6 to 7 miles away from the blast, following this procedure. That is significantly less than the 11.5 miles mentioned in the wiki article. Any idea which one is correct? Vpab15 (talk) 16:26, 13 June 2022 (UTC)[reply]

Two different statements. "Away from the blast" suggests distance from the point of explosion at the time of explosion, which is relevant for surviving the flash. But the initial question is about surviving the shock wave, which has the additional travel time (and therefore distance). — Lomn 18:05, 13 June 2022 (UTC)[reply]

(edit conflict) A careful reading of the escape tactics link shows that the bomb traveled onwards for 21⁄4 miles before detonating. In the meantime Enola Gay had performed a diving turn to go as fast as possible in the other direction. The bomber was therefore nearly 7 miles away from the detonation. As TenOfAllTrades pointed out, Enola Gay continued to fly at around half the speed of sound away from the expanding blast front which was itself traveling at around the speed of sound. By the time the blast reached the 6 or 7 mile point, Tibbits had already put another 3 miles or so between him and the explosion. The blast finally caught up with him at 111⁄2 miles from the point of detonation. Martin of Sheffield (talk) 18:13, 13 June 2022 (UTC)[reply]

That makes sense. Thanks all for your replies. Vpab15 (talk) 21:26, 14 June 2022 (UTC)[reply]