We know that to move in water we require more energy. Even shooting a gun, the bullet travels a very small distance. But how could torpedoes move so fast inside the water? If they have such an energy then could we use them in air instead of missiles?
2 Answers
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From the wikipedia page on torpedoes:
A modern torpedo is a self-propelled weapon with an explosive warhead, launched above or below the water surface, propelled underwater towards a target, and designed to detonate either on contact with its target or in proximity to it.
(emphasis mine)
Torpedoes can travel further than bullets because they are self-propelled. They have a propeller behind them that keeps them moving in water.
Missiles are already the same concept, except they are designed to self-propel in the air. Self-propelling in water is much easier than in air, because water is heavier and resists movement more, so it is easier to push against for self-propulsion.
We wouldn't be able to use the same mechanism as a torpedo for missiles for several reasons. For one, there's little buoyancy in the air, so the propulsion has to account for a lot more weight in the air. Another reason I already mentioned is that it's harder to push off the air. That is why missiles instead push off of a fuel that they eject from their tanks.
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$\begingroup$ So you're saying having propellers on missiles is a bad idea? $\endgroup$– user234190Commented Jul 4, 2019 at 11:29
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The main reason why torpedos travel further than bullets under water is that they are propelled by an engine. Bullets on the other hand reach their maximum speed at the muzzle and follow a ballistic trajectory after that: they slow down because of air resistance and are attracted by gravity. Torpedos on the other hand are moved through the water in much the same way as the submarine they were fired from. They keep at a certain speed until they hit the target.
Torpedos actually travel at speed of less than 100 km/h, while bullets travel above the speed of sound in air (1234 km/h). Also, the drag increases drastically with speed, i.e. an object traveling at 100 km/h through a fluid experiences much less drag than an object at 200 or 1000 km/h.
It's interesting to note, however, that there are some approaches to reduce the drag on the torpedo by making it go faster. This causes the water around the torpedo to turn into vapor (from the pressure difference), which is a gas and thus offers much less resistance than the water would.
Regarding their use in air: early types of torpedos used propellers to move it through the water. They are effectively just guided, exploding miniature-subs. Nowadays they often use something like a rocket engine, which actually makes them something like a guided missile.
