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This picture shows a couple of Mk-76 25lb (11kg) Inert Training Bombs on a McDonnell Douglas AV-8B Harrier II; it's clear that the physical characteristics of the dummy bomb are quite different to actual munitions.

Mk-76 dummy bombs source

The manufacturer says these bombs are "used to simulate the MK-82 in low drag configuration". The venerable Mk-82 is typically 500 lb (227 kg): here's an F-16 with Mk-82s for scale comparison:

F-16 dropping Mk-82 bombs source

One obvious advantage of the smaller, lighter bombs is that they're vastly cheaper to operate, both in terms of handling on the ground and the amount of fuel you'd be using in the air. However, the dummy bomb is 20 times less massive than the bomb its designed to simulate: its small size and weight surely mean their aerodynamic characteristics are very different to the bombs they're supposed to be simulating. (I know that speed of fall in a vacuum is not dependent on weight, but a falling object in air is affected by shape, weight, drag etc.)

My question is - since their purpose is presumably to help train the pilots, how can tiny dummy bombs usefully simulate real bombs?

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  • $\begingroup$ "surely mean their aerodynamic characteristics are very different" really? $\endgroup$ – user3528438 Jan 15 at 21:11
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    $\begingroup$ This goes way back Galileo's famous gravity experiment (later quantified by Newton). I'm surprised that people still don't know that things of different sizes all fall at the rate of 9.8m/s/s. If you're worried about aerodynamics characteristics that has nothing to do with size or weight - a grain of corn is roughly the same size and weight as a feather but their aerodynamics characteristics are completely different. On the other hand a model 747 is hundreds of time smaller and lighter than the real plane but their aerodynamics characteristics are basically the same $\endgroup$ – slebetman Jan 16 at 4:54
  • $\begingroup$ @slebetman - Thanks - as I said in the question I'm aware of the very basics of gravity, but "things of different sizes all fall at the rate of 9.8m/s/s" is just not true (in Earth's atmosphere), e.g. a beachball and a cannonball, or a penny and a skydiver. For all these things their initial acceleration decreases to zero at different rates, and their terminal velocities are different too. Hence the question. $\endgroup$ – Party Ark Jan 16 at 9:01
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The actual mass alone is nearly irrelevant to its a trajectory - what mostly matters is the mass/drag ratio - so if you scale the drag correctly, it feels quite the same - at least from the operational point of view. As one does not drop bombs by sight only, a small adjustment on the computer will do the other trick.

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  • $\begingroup$ I guess this is common, when munitions are rather expensive [citation needed]. There is a sub-caliber training device for the RPG-7, which fires a 7.62 mm bullet, from a dummy grenade. Here is a discussion from the ForgottenWeapons YouTube channel. $\endgroup$ – Dohn Joe Jan 16 at 8:39
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Provided the bomb has the same ballistic characteristics, the mass of the bomb would be irrelevant to this. Now the weight of the actual MK82 series 500lb bombs may have a distinct effect on the maneuvering characteristics of the attacking airplane during a bombing run as opposed to carrying a load of MK76 ‘Smurf Killers’ to a target.

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