Consider the SR-71 Blackbird. It was meant to be used as a high altitude, high velocity recon aircraft. It was, and still is, one of the US Air Force's greatest accomplishments, technologically speaking.
Military technology, like any other technology, becomes outdated when newer systems greatly improve upon them, either in capability or in resource requirements to use and equip, or are rendered irrelevant by other technologies and techniques.
The Blackbird was really expensive, though, and had it's limitations. Many of these were solved by the U2.
The U2 provided many of the same capabilities and filled the role of the SR-71 without such a heavy cost. Though it was technologically inferior in many ways to the SR-71, it was a much more efficient way to get the same job done. What was interesting? The U2 was older than the SR-71, but improvements in the development of stealth for the Blackbird improved the U2's performance to where it was actually a superior all-round plane for the job, economically speaking.
Of course, now airplanes are being replaced altogether for many recon missions.
This is the RQ-4A Global Hawk. It is one of the best UAV reconnaissance vehicles in the world. It outperforms the U2 in many ways. Let alone that there are no pilot risks, it can stay in the air far longer than any human piloted plane could hope to.
In many ways the U2 improved upon SR-71 capabilities and was a better strategic fit. Now the Global Hawk is improving on the U2 for a better strategic fit in the area of aerial reconnaissance. Of course, there are now fewer, and fewer improvements being made in piloting mechanisms because those technologies haven't been improved upon, they've been made obsolete.
Of course, there is no reason that a weapon system has to be shelved. Some very old systems are still in use today.
This is the M2 Browning .50 calibre machine gun. It's a monster and so fun to shoot...
Especially considering it is a near-unchanged weapons system since being built in 1933. That's the same weapon, almost identical in operation, posing next to the with the leader of the UK during World War II, Winston Churchill.
The key feature about the weapon is that it still fulfils its role of being a heavy weapon used for taking down mechanised units, vehicles, and fortified locations. None of those basic defences have advanced to a point that the weapon doesn't still work against it. The weapon is relatively cheap compared to other newer versions that might replace it. It's also pretty easy to use, so long as you remember headspace and timing; that could be improved, but besides that, the weapon hasn't need a change since it was used back in Germany during the war.
You would think that it would be better by now, but honestly, it was a really good system to start off with, and there is very little that could economically do a better job. (For now.)
Then there are those technologies that never truly reached the height of how far they could go, so to speak, but were simply rendered strategically unfit by technologies in other areas.
The Zeppelin, there was a beautiful machine. As a wartime weapon it was cutting edge at its time. It wasn't that it was really worse at its job than other alternatives: it's just that as soon as rocketry and attack airplanes became a thing, the zeppelin was done for. Technologies not really related to them made them irrelevant in warfare because they simply couldn't defend themselves. Want another example?
For thousands of years castles made perfect sense. Then some guys found out that you could stick some powder in an iron tube, stuff some more iron in there, add fire and boom (literally) those walls which have evolved over the course of thousands of years have just been outdated by non-competing technology that fundamentally changed the landscape of warfare.
The number of years a technology has been replaced doesn't matter. The questions that matter are:
- Is the mission/role of the weapon still relevant?
- Does it outperform other competitors?
- All things equal, is option A more or less logistically practical than option B?
At the point where a new technology either outperforms an older one completely, or that other technologies make the weapons system completely useless, then that tech is considered outdated.
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