Latest Puzzler answer is wrong. Here’s the question and supposed answer:
"Sometime in the late 1930’s, German aeronautical engineers were working on a device. Perhaps engineers from other countries were doing the same thing.
So this device did the following. It took water vapor, which is one of the products of a gasoline engine combustion, and it would condense it into water and save it.
You might say, well, not such a big deal. However, it would save the water in an interesting way. It would save it in such a way that the amount of water saved would be exactly the equivalent in weight as the amount of fuel that the engine burned.
So as the engine burned a pound of fuel or kilogram of fuel, this device would save a pound of water and
discard the rest. This way, the plane would always weigh the same amount.
And the puzzler is, why would you want to do this?
Answer time.
So, why did they want to plane to always weigh the same amount? And because of this, they created this water saving device?
So what would happen is, as the engine burned a pound of fuel, a pound of water would be saved, so the weight remained the same. The rest would be discarded.
Why?
It turns out that this device was used for many types of vehicles. When a plane was carrying weapons, it was important for the weight to remain the same for accuracy in dropping bombs. And in submarines it was also used, as weight was extremely important to staying at the right depth. When they burn fuel, they replace that weight with water, so that the submarine does not unintentionally head up to the surface and stays weighted. "
No bomber captured exhaust and condensed out water, very slow changes in airplane weight had no impact on accuracy. And the only submarines that might have used it were the small number of U boats late in the war that used a snorkel to run the diesel engines. All other subs ran on electricity while submerged.
The obvious answer, dirigibles, was ignored.