Awesome but Impractical/Real Life/Military

"A flashy feature that has limited usability for victory."

Watercraft

 * The real life USS Enterprise: Eight. Nuclear. Reactors. They figured this one out before they built another one, fortunately. Why eight? Conventional large aircraft carriers have eight boilers, so logically the "Big E" should have eight nuclear boilers, right? Also, the size of the boilers matched up nicely with the reactors that the Navy had already been building for submarines, meaning it was much simpler to adapt than to create a massive new reactor (which they later did). After drastic cost overruns nixed the five sister ships she was supposed to have (and resulted in the next two carriers being conventionally-powered), engineers realized that even a ship that big only needed two reactors (though Nimitz-class reactors are much larger and more powerful).
 * It's not quite that bad. To drive an aircraft carrier at speed requires X megawatts of power, the S5W nuclear reactor only produces Y megawatts, so you need 8 of them to move a ship the size of the Enterprise. (Well, you need somewhat less than 8, but you want redundancy in case one breaks). As to why they didn't use larger nuclear reactors, they hadn't invented them yet.
 * In general, most non-carrier nuclear powered surface ships, though in the 21st century, some have argued that nuclear cruisers and destroyers might actually be cost-effective in an era of extremely high oil prices. Gas turbine-powered vessels are, generally speaking, much more efficient than steam-powered (even nuclear steam-powered) ones due to their higher operating temperatures; carriers only get a pass in this regard due to the need for steam to power their aircraft-launching catapults.
 * There's also that for non-nuclear ships, you have to deliver the fuel to them somehow. Fleet oilers are very vulnerable targets during time of war. If you don't want your fleets tethered to bases during a major conflict, you'd better have at least some nuke ships in the complement. Indeed, one of the tasks of the aircraft carrier is to carry fuel oil for its escorts, a task made possible only by its not needing to use any of that fuel for itself.
 * There's also that the amount of fuel needed to move a ship at speed does not scale linearly with size, or in plain English twice as big a hull means more than twice the fuel requirement. An amount of fuel supply practical to drive a gas-turbine frigate would be absurdly impractical for 30,000 tons of fleet carrier.
 * In the 1700's, warships were growing bigger. Spain decided to build the largest of all... the Santísima Trinidad, a ship LARGER than the already massive 1st rate ship of the line. This is a picture of the already massive HMS Victory, Which is a First Rate, for comparison. The Santísima Trinidad carried 122 CANNONS, compared to only 102 for HMS Victory. But it was so huge that it crawled at a snail's pace (and was nicknamed el Ponderoso (The Ponderous) as a result.) So many men were required to man it that its supplies ran out very quickly unless it was near a friendly port. The Santísima Trinidad only saw battle once (where it was too slow to make an impact on events), and then later capsized in a storm due to its high center of gravity. But 122 CANNONS!. It was Awesome but Impractical incarnate.
 * Before the Santísima Trinidad, there was the Swedish Vasa, at its time the largest warship ever. In 1628, on its maiden voyage, it majestically left Stockholm harbour, and majestically....... sank.
 * The Yamato-class battleships from World War II. Yes, they were armed with colossal guns and massively armored, but they were were considerably slower than they could have been and were astounding fuel hogs, so they usually got left in Truk, and the faster and more efficient Kongo class battlecruisers were used instead despite their World War One era vintage.
 * The Yamatos were actually rather fast considering their massive size and relatively weak engines. They carried nearly 20,000 tons more displacement than the Iowas on only 70% of the ship horsepower, and managed to get 27 knots compared to the Iowas' 31-32 knots.
 * Actually, the Yamato's main problem was fire control - without American advancements in radar and computing power, most of the capability of its magnificent 18-inch guns was wasted because anything over the horizon might as well have been on the moon for all the Yamato was going to hit it. An Iowa-class's 16-inchers, on the other hand, didn't have a maximum range quite as far as the Yamato's but had considerably more effective range because they could reliably score kills pretty much as far as the shell could reach.
 * Battleships in general, at least eventually. There's still debate over exactly when the battleship class became obsolete (The rise of air power? Modern anti-ship missiles?), but it did. And when those weren't a large concern for the US Navy (e.g., off the coast of Lebanon or the first Gulf War, where the 16-inch gunnery of Iowa-class battleships proved very useful), they were still impractical for being extremely expensive to operate and manpower-intensive.
 * The military function of battleships (killing enemy ships of all lesser capabilities without them being able to do anything about it) was taken over by the aircraft carrier pretty mush as soon as it became feasible to build them. It did, however, take rather a lot to convince some of the Admirals of this.
 * Battlecruisers. Combining the speed of a cruiser with the firepower of a battleship, they were designed to hunt down enemy cruisers, and were effective in that role. Unfortunately, the combination of speed and firepower made it tempting to use them in place of battleships... a role which quickly exposed the light armor that enabled them to have that combination of speed and firepower. Such matchups were a case of Glass Cannon versus Lightning Bruiser.
 * The People's Republic of China's Kuznetsov-class aircraft carrier, purchased from Russia. As one Chinese officer was quoted, "All of the great nations in the world own aircraft carriers -- they are symbols of a great power." But whether China will able to operate an aircraft carrier as an effective weapon of war, as opposed to just a symbol of prestige, remains to be seen. The United States Navy has nearly a hundred years experience in carrier aviation, something that can't easily be matched overnight.
 * The Tessarakonteres is an ancient example from the Hellenistic period. The largest human-powered vessel ever built, it had 4,000 rowers... and roughly the maneuverability of the Parthenon.
 * The Submarine aircraft carrier - such an awesome idea that the Germans, British, French, Americans, Italian and Japanese all pursued the idea. Only the Japanese ever actually built one, though. The Japanese actually built 47 submarines with the capability to carry seaplanes (between one and three, depending on the model). But the aircraft carried were limited in capabilities (with potential strike damage essentially limited to propaganda value), the process of surfacing, launching, recovery and submerging took a lot of time, and the submarine itself was vulnerable to attack while doing so. They were also very large -- the I-400 class boats were among the largest non-nuclear submarines ever built. It's still an awesome idea, though... and one which just might have a shot at becoming practical in the 21st century, in the form of submarine-launched UAVs.
 * The USSR's "Alfa" class submarine. It set the record (still held to this day) for the fastest and deepest diving military submarine in the world, and knowledge of its production greatly alarmed the West, to the point that the US and Britain both designed torpedoes for the specific purpose of hunting down Alfas. Unfortunately the Alfa had small and powerful but very maintenance-intensive lead-cooled nuclear reactors that couldn't normally be turned off, as doing so would let the lead solidify and essentially turn the whole thing into a solid inert lump. Entire maintenance facilities had to be constructed at Alfa homeports simply to keep the reactors hot when they weren't being used - but, in typical Soviet fashion, the facilities themselves weren't properly maintained and often didn't work. As a result Alfa reactors had to be kept running at all times, which they hadn't been designed for and which resulted in several expensive failures. While the reactors could remain active for 15 years they also could never be refueled and were intended to be replaced at the end of their life; despite this the Alfa hadn't been designed with quick reactor replacement in mind, so the process would have been expensive and slow, potentially more than refueling a traditional submarine. In addition, while the Alfa reportedly had a crush depth of over 1300 meters, deep dives did permanent damage to the submarine's onboard equipment, so that impressive diving ability was largely wasted in practice.

Landcraft

 * Double Barrel Tanks are undeniably cool, but:
 * The Soviet T-28 tank, with Finnish nickname Postivaunu (Stagecoach). Three turrets, three cannons and up to five machine guns, but horribly unmaneuverable and slow, and easily defeated with improvised anti-tank weaponry. Finnish troops captured seven of these monsters during the war.
 * The Soviet T-35 heavy tank deserves special mention here; it looked impossibly cool, had 5 turrets and 6 machine guns, weighed 45 tons and took 11 crew members to operate. It was also slow, incredibly expensive, and far too mechanically complex for the rigors of war. Only 61 were built, and most of those were lost due to mechanical failure rather then German Panzers. The T-34 was half as big, and only had one turret. This tank won World War II, and 84,000 T-34s were eventually built.
 * What could be more awesome than War Elephants? Unfortunately, they tend to panic in battle, trampling friend or foe with indifference. During Timur's invasion of India, his forces faced 120 armored Indian war elephants with (for even more awesomeness!) poisoned tusks. In an act of genius, insanity, or both, he ordered all his camels lit on fire and sent the screaming animals towards the advancing elephants. The massive beasts panicked and trampled over their own forces. Timur's army then easily ran down the fleeing enemy troops. Timur then picked up the Idiot Ball himself, incorporating the elephants into his own army, perhaps figuring that no one else would figure out his strategy.
 * War elephants had been made obsolete in Europe by the Roman ways (yes, ways) of dealing with them, that include such things as ox-driven chariots equipped with huge spikes to wound the elephants and pots on fire to scare them (these ultimately failed, but provoked many losses among Pyrrhus war elephants), insane numbers of flaming arrows (scary enough to make Pyrrhus war elephants panic and stomp his own troops), extremely loud horns (that made run part of Hannibal's elephants over his own army) and simple axes. When the Romans started chopping enemy war elephants with axes, their enemies finally got the memo and stopped using them (Parthians and Sasanid made use of war elephants, but never deployed them against the Romans).
 * The Soviet KV-1 and KV-2 tanks. They were so heavily armored that no early German tank could hope to punch through their frontal armor, and they had good firepower. Unfortunately, they were also extremely heavy, and had no snorkeling equipment, which meant they were too heavy for many bridges and didn't have the option of simply fording a shallow river.
 * The German Panther and Tiger tanks were very effective in terms of firepower, protection, and mobility, and quite feared by the Allies. Frequently regarded as the best tanks of WWII. The problem was, they weren't cost effective. While superior to the U.S. Sherman and (depending on who you talk to) Soviet T-34, they were also expensive and difficult to maintain, so the Allies could easily outmatch the Germans in quantity, if not (perhaps) quality (at least until the M26 Pershing came along, but by that point the war was nearly over).

Aircraft

 * The B-1 "Lancer" was originally conceived as a nuclear bomber that would roar in at supersonic speeds to defend itself against missiles and enemy aircraft, and would, if hit, eject the entire cockpit as a survival capsule that would parachute to earth. By the time its production version, the B-1B, ended its operational life it was used as a conventional bomber operating almost all the time at subsonic speeds. And no capsule. Ironically, despite it less glamorous mission, the B-1B has become quite effective in its new role, the "Roving Linebacker" for urgent-need air support
 * Some awesome Atomic Age aircraft were rendered impractical not so much by inherent design problems as by advances in missile technologies:
 * The XB-70 Valkyrie was a six-engined high-altitude strategic bomber designed to travel at Mach 3 (which would allow it to outrun Soviet interceptors). All very impressive -- before the development of surface-to-air missiles that could effectively target and destroy high-altitude supersonic bombers. Furthermore, bomber designs like the XB-70 were made obsolescent by advances in intercontinental ballistic missile technology. ICBMs that could accurately hit a target half way around the world in 45 minutes increasingly marginalized the role of strategic bombers.
 * The MiG-25 Foxbat was a blisteringly-fast high-altitude interceptor designed to intercept bombers like the XB-70. Despite its short range and primitive but rugged avionics, it might have been effective in that role. But it also had terrible maneuverability and a limited payload (four missiles, no cannon) which made it rather useless when its intended mission disappeared. While useful in a reconnaissance role, its combat record (in the service of Egypt, Syria, and Iraq) is poor. And its engines would melt if it pushed to around Mach 3 (what it was designed to do in the first place), eliminating its cost effectiveness as well.
 * The Foxbat did get a limited, short-term use as a propaganda item as it was by far the fastest and highest flying jet fighter at the time (one incident had a Soviet Foxbat in Syrian colors saunter up to an Israeli F-4 running flat out at operational ceiling altitude, let the F-4 crew get a good look, and then accelerate and climb away), but even this backfired when the US developed jet fighters designed to defeat the plane that the US THOUGHT the Foxbat was, namely the awesome AND practical, as well as uber-successful F-15 Eagle. Then, the US got hold of one through Viktor Belenko's defection in 1976, and discovered it was mostly rubbish.
 * The B-17 Flying Fortress. While nowadays the B-17 is lauded as a weapon that led the charge in the Air War for the Allies, back in the day it was considered a deathtrap. While it was quite tough, they were blasted out of the sky by the dozens in the early years of the war and it only got marginally better further on. While festooned with machine guns, they were only intended for ranges that were rather close and fighters flew by so fast that the machine gunners would rarely hit anything (though there were plenty of instances of friendly fire in the massive bomber formations) and machine guns did nothing to help against flack cannons. There was also very little in the way of crew comfort or protection. The B-17 was almost immediately retired when the B-29 Superfortress was brought into service.
 * The Messerschmitt Me 321. It began it's life as a glider, the biggest glider ever made. The mission for this glider was to rapidly transport large amounts of troops and medium or light tanks into the battlefield. The first problem was how to make something that big glide. Thus, it was made largely of hollow steel tubing, doped fabric and wooden spars. Then came actually getting it up into the sky. Normally, a tow aircraft would be used to drag it up into the air, but two towing aircraft would have to do the job, which would be impossible to synchronize safely. The solution? They just stuck two Heinkel He-111's together with a third engine between. Originally it was intended to be used for the scrapped invasion of Britain, but then was used for Operation Barbarossa. After that, feedback from the people who "flew" them led to a big design change. Sticking six powerful engines on it, they turned it from a glider into the Me 323 transport plane, and it still needed the damn Fraken-Heinkel to take off (or RATO or three airplanes working in synch) if it was fully loaded. It arrived just in time to support Rommel in his collapsing North African campaign. Where they were shot down in droves, because they were slow, ungainly and massive targets loaded with fuel, ammo and other things that went boom. In one famous incident, 22 were shot down in just one flight. It saw service for little under a year before being retired.
 * The Convair B-36 "Peacemaker" long-range atomic bomber, which kept the balance of power during the cold war and looked positively badass. When everyone was switching to jets Convair used six huge radial engines - which turned out to be maintenance nightmares, both for their inherent complexity (the ground crews loathed sparkplug replacement duty) and because they were never meant to be mounted in a pusher configuration, resulting in many failures (when your plane losing an engine is considered so routine that the mission is allowed to continue as if nothing happened, you know you have a problem). And for all that the plane was still underpowered, so they eventually fitted four additional jet engines to compensate. The B-36 also featured an innovative control-by-wire system for the engines... but no mechanical backups, so if the electrics failed you were screwed. And the electronics were mounted in delicate housings that would shake themselves apart under the vibrations caused by the turret guns. Even after the design was tweaked and bludgeoned into some kind of functionality, it still had a tendency to spring oil and fuel leaks all over the place. It got retired ten years after its creation, while its Soviet counterpart the TU-95 is still in use today and is predicted to stay in production for several more decades.
 * Fighter aircraft have been getting progressively more complex and more expensive, to the point that a joke got started that pretty soon the Air Force, Navy, and Marine Corps are going to have to share one plane (the Air Force gets it on even days, the Navy on odd days, and the Marines on Feb. 29). The F-35 Lightning II was partly intended to offset this in that it was supposed to have a much lower unit price tag than the F-22 Raptor. As luck would have it, it ended up being about a third again more expensive.
 * While at the same time not bringing the same capability for aerial combat to the table that the F-22 did, which would've left the US military lacking in aerial combat capability. Of course, it could be said that it already has since the F-22 itself is proving to be plagued with some rather unfortunate problems.
 * Those problems are turning out to be largely exaggerated. (For example, the fact that the stealth coating doesn't function entirely during rain is mitigated by a) 100% stealth is pretty much needed only for ground-attack missions in contested airspace, which are never scheduled during bad weather anyway because you need to be able to see the ground and b) rain is largely a problem only for low-altitude missions, and the F-22 is not only capable of flying well above weather its damn near capable of flying above the stratosphere.)

Artillery, Missiles, and Small Arms

 * The Nock Volley Gun, a smoothbore flintlock small arm with seven barrels, designed to be fired from the rigging of Royal Navy warships during the Napoleonic Wars. Unfortunately, it turned out most men weren't big or strong enough to fire it without a) being thrown violently backwards by the recoil, b) falling off whatever high place they were firing it from, c) having their shoulder shattered, or d) all of the above. It also took bloody ages to reload, even by the standards of the period.
 * The Japanese Type 97 20mm Anti-Tank Rifle. Fielded by the Imperial Japanese Army during WWII, and operated by nobody that was sane. The recoil on the Mk-97 was likely to produce the same self-inflicted injuries as the Nock Volley Gun.
 * The rubber-band Gatling gun. The ultimate in rubber band small arms technology, it can fire over a hundred bands in a matter of seconds. Unfortunately it costs $500.00 (not including shipping), takes around half an hour to load, has a tendency to jam if not loaded very carefully, and is horribly inaccurate.
 * Ironically, the actual Gatling Gun was viewed as being this when it was first developed, although it has since been Vindicated by History. With the American Civil War underway, military quartermasters already had their hands full trying to develop logistical standards for weapons and ordinance. They simply did not want to deal with another weapon with its own unique set of ammunition and upkeep needs.
 * The Gyrojet gun fired rocket propelled bullets and was cool enough to showcase in the Bond film You Only Live Twice. Rocket propulsion caused problems, though: rather than starting fast and slowing down, it started slow and built up speed. This mean that within a certain range, a bullet would not be moving fast enough to do any significant damage. They were also both more difficult to manufacture and much more expensive then conventional bullets. Finally, air turbulence resulting from the transition from subsonic to supersonic speed effectively destroyed its accuracy. So, lack of power at short range, and lack of accuracy at long range. While later designs have ameliorated some of these problems, they remain more curiosities than practical weapons.
 * Nuclear weapons are the most awesome (in the truest sense of the word) weapons ever created, but practical only for their considerable deterrent and political value, not in terms of strict military utility. Some specific nuclear weapons were even more impractical:
 * The Tsar Bomba hydrogen bomb took this trope Up to Eleven. While massive and impressive in yield, none of the planes in the Soviet air force was large enough to carry it without special modifications, and the bomb's explosion could destroy the bomber that dropped it.
 * In a similar category, many American interceptors from the 1950s and 1960s were equipped with nuclear-tipped air-to-air missiles to shoot down incoming bombers. However, the F-101, when used in this role, was so unmaneuverable that if it managed to destroy the bomber, it was guaranteed to also destroy itself, along with both its crew members.
 * The M-388 "Davy Crockett" recoilless rifle, formally named the M65 Nuclear Rifle, was designed to fire a .01 kiloton atomic weapon a maximum distance of three miles, which left the crew within range of fallout from the bomb. It was a failure as a nuclear launching platform, even in the nuke-friendly 1950s, but come on -- it's an atomic bazooka! It doesn't get much more awesome than that.
 * The actual nuclear device in the Davy Crockett was the W54, the smallest acknowledged nuclear device the US ever manufactured. One of the other uses it was put to was the Special Atomic Demolition Munition, which was intended for nuking any ground target you might want to attack with a small, hand placed nuke. Yes, that's right, this bad boy was meant to be schlepped in on foot by a soldier (actually two specially-trained US Army Engineers Demolitions specialists, who might go in by parachute, boat, swimming, foot or ground vehicle, depending on the precise target) in a special backpack, then set of with a mechanical timer.
 * Other types of WMDs fall victim to this as well. If you use chemical weapons on the battlefield, you automatically allow counter use... and there are a LOT of countries with covert or open stocks of chemical weapons. The standing policy of the United States (which no longer has chemical weapons in its arsenal) is that any WMD attack will be responded to in kind - i.e., with nuclear weapons, the only WMDs America still has.
 * Full automatic fire, particularly with assault rifles and submachine guns. While full auto can be useful in its place (chiefly for suppressive fire), during battles in Iraq and Afghanistan, you can tell the sides apart by sound alone. One side's dominated by full-auto "spray and pray" fire, the other by single shots and 2-3 round burst fire. Guess which side tends to win.
 * The advantages of controlled bursts over "spray and pray" is exactly why the casualties in Black Hawk Down were only 18 Americans, and 1000-2000 Somalis (though training and taking cover was a big part of it as well).
 * The U.S. military now favors assault rifles that don't even have a full-autofire setting. Instead, the selective fire switch on modern assault rifles lets the shooter switch between semi-automatic (1 round per trigger pull) and burst fire (3 rounds per trigger pull, like RoboCop's gun).
 * The first iterations of the Sidewinder heat-seeking missile. So strong was the belief in this technology that many NATO fighter and interceptor designs of the 1950s and 1960s era were missile-only (completely without guns.) However, in the Vietnam war, pilots of F-4 Phantom II's equipped with Sidewinders only would sit and watch with frustration as the missiles would fail to launch, launch but miss the enemy airplanes completely, or switch tracking the heat of the enemy's jet exhaust for the heat of the jungle and scream into the ground. The Vietnamese--whose MiGs had cannons--could just get in close and blow them out of the sky. While later versions of the Sidewinder were much more effective and reliable, this experience led to almost all future U.S fighters and attack aircraft being equipped with guns.
 * The AN-94 assault rifle, intended to replace the AK-74 as Russia's general issue rifle. It is extremely accurate thanks to a 2-round burst system that puts two bullets in nearly the same location. However, it is prohibitively expensive and its internals are much more sophisticated than the AK-74, relegating it to Special Forces use.
 * The VSS Vintorez Russian sniper rifle. Its built-in suppressor makes it nearly silent, and its 9mm rifle round is hard-hitting despite its suppressor-friendly subsonic speed. However, said ammunition is heavy and rarely issued in the Russian military, it has a prohibitively short range for a designated marksman rifle, and it's extremely expensive.
 * The German Schwerer Gustav and Dora Gun were railway siege guns, and the two biggest artillery weapons ever. Designed specifically to destroy France's Maginot Line forts, the guns weren't ready at the time of the Battle of France. The Schwerer Gustav was used in the siege of Sevastopol, but only fired 48 rounds before wearing out its barrel. The Dora was only deployed briefly against Stalingrad, but quickly withdrawn when Soviet encirclement threatened.
 * The V-2 rocket, a single-use weapon that cost a fantastic amount of resources per shot, and, thanks to careless use of slave labor, still the only weapon that killed more people on its own side than enemies.
 * The Karl Device. Have a look. The largest calibre weapon ever fired in war, one shell weighed two tons, and each launching platform weighed 124 tonnes.
 * One of the weapons the Ottomans brought with them during their siege (and eventual conquest) of Constantinople was Basilica, a 27-foot long cannon that could launch cannonballs as heavy as 600 pounds up to a mile away. It probably didn't see much action due to it taking three hours on average to reload, a lack of effective ammo to get the desired results and required 60 oxen to drag it from place to place.
 * The XM-29 OICW. It was a standard 5.56mm assault rifle with a 20mm grenade launcher that had programmable airburst grenades. Issues came about due to weight, cost, and the ineffectiveness of the 20mm grenade. The grenade launcher part spawned the XM-25, using a 25mm grenade, but it never got off too.
 * Arguably, a lot of the weapons in the US's Advanced Combat Rifle program could fall into this by the sound of it. Varieties include stuffing two bullets in a single cartridge, making cartridges shorter by packing the gunpowder on the side, flechette rifles, and caseless ammunition weapons. Unfortunately, none of these weapons produced results that were significantly better than what was available at the time. It didn't help that this program was obsessed with smaller ammunition that didn't perform any better all for the sake of More Dakka.
 * The XM-214 Microgun. Intended to be a man-portable version of the M-134 Minigun, it was scrapped when someone figured out two simple facts: 1) the combined weight of the gun, battery and ammo pack were too much for any soldier to carry, and 2) such a huge amount of firepower is rarely actually needed - normal machine guns provide more than enough for the vast majority of purposes.
 * Chemical gases has been so effective at killing (in very gruesome ways) during WW 1 that has been banned from warfare. However, the only thing keeping the cloud of deadly gas over the enemy was the wind, which could and often did start blowing it towards your own lines.
 * The Nazis lacked any longarms that could be dropped with paratroopers due to limitations of their parachute harness, this forced them to drop guns separately from paratroopers and resulted in them being quickly slaughtered. In response the Nazis developed the FG 42: A rifle capable of automatic fire, have a scope, have an integrated bipod, be magazine fed, have a bayonet mount, be capable of firing rifle grenade and being dropped with a paratrooper. The resulting rifle was incredibly fragile (not helped by war time manufacturing limitations) and would destroy itself if fired on automatic for any length of time. Japan had the same problem. They took a normal rifle and gave it a folding stock.

Experimental Weapons

 * The book My Tank is Fight! is all about impractical inventions of World War II.
 * Adolf Hitler loved this trope. Here are just a few of the awesome and extremely impractical weapons that never quite made it:
 * If the Tiger and Panther could have been considered Awesome but Impractical, what can one say about the enormous Panzerkampfwagen VIII Maus?
 * The Landkreuzer P. 1000 Ratte would have been even more awesome and impractical. Dwarfing even the enormous Maus in size, not only would bridge crossings have been completely impossible for the 1000 ton vehicle, it would have destroyed any road it attempted to travel on! This tank would have carried a naval gun as its main armament.
 * The Landkreuzer P. 1500 Monster took the concept from impractical to nearly-certifiable insanity. This 1500 ton aptly-named Monster would have a crew of over 100 and use the Dora Gun as its main armament! While it's true that nothing on land could have stopped it, it would also be an almost unmissable target for Allied aircraft.
 * The Me-163 Komet, a rocket-powered airplane that had a seven minute flight time, flew too fast to effectively target bombers (its intended target), used fuel that would erupt into flame on contact with human flesh, and flew so high that pilots were required to eat a special diet in order to reduce intestinal gas. Furthermore, the rocket fuel would explode if jarred, and if any remained in the fuel tanks on landing it tended to explode, so that no pilot ever scored more than one victory with it. It was also made of wood to keep the weight and costs down and while it had wheels when it was going up, they broke away from the aircraft as it took off. What did you land with? A single skid. The best part was that at this stage of the war, flat even runways were in short supply, so the pilots had to land them in bumpy, rocky fields. And since they turned into gliders to boot, they were completely defenseless from enemy fire. More pilots died trying to land these things than in combat. Oh, and the engines were considered more valuable than the pilots. What does that tell you?
 * The whole idea was so poorly worked out that there were multiple incidents of these aircraft destroying themselves on takeoff with their own launch wheels. Because of the poor fields they were taking off from, it was entirely possible for the rubber-tired take-off wheel assembly to bounce higher than the altitude it was dropped from...and smash into the plane, often resulting in a catastrophic explosion.
 * The Natter (unrelated to Natter), a vertically launched rocket propelled interceptor that carried its offensive armament of more rockets directly in front of the pilot. Endurance was even shorter than the Komet, but missions ended with the aircraft breaking apart and the pilot being thrown free. So impractical it was never used operationally.
 * Blohm & Voss BV 40 Glider fighter - sort of a one shot deal.
 * The Silbervogel bomber, a semi-orbital intercontinental bomber. Too impractical to build at the time, and would have had a disappointing payload in any case. The inventor figured out that the thing would never be able to work half way through designing it, but continued to work on it anyway for the insane amount of money the Nazis were giving him.
 * The ne plus ultra of impractical Nazi inventions was the Sun Gun, an orbiting mirrored space station intended to be a solar Kill Sat. Oddly enough, also counts as Mundane Utility: in addition to establish Nazi superiority, the designers recognized its usefulness as weather satellite and communications platform.
 * The Northrop XP-79 was intended to be a rocket-powered flying wing fighter that would destroy incoming bombers be ramming them. (No, they thought of that, it was designed to take the impact without significant damage.) Three prototypes were built, two were abandoned after the rocket engine failed to perform adequately, and the project was entirely cancelled when the remaining prototype, fitted with conventional jet engines, was lost (and the pilot killed) after an unexplained loss of control during test maneuvering.
 * The Soviet Union/Russia, like the Nazis mentioned above, came up with some impractical gems for battle. In fact, it sometimes seems that the only reason they would design and build these weapons was just to prove that they could:
 * Cast in 1586, the Tsar Pushka/Tsar Cannon is recognized as the largest cannon ever made (excluding those used on battleships) by caliber. With a barrel nearly 18ft. in length, it had the capability to fire cannonballs nearly 3ft. in width. The cannon would have been great in battle... if it wasn't so, well, impractical. First, the cannon weighs over 40 tons, so actually getting the thing to the battle would need its own army. Second, you'd need a lot of gunpowder and manpower to load one shot into it. Third, aiming would be a pain. Finally, the recoil would be so powerful, that some major damage would be done to the cannon itself, assuming it wasn't outright destroyed. The cannon has never fired a single shot (and never will). Today, it can be seen in Moscow, where it acts as a monument/symbolic weapon (though a duplicate was made and can be found in Donetsk, Ukraine).
 * The Antonov A-40 "Krylya Tanka" is... a Flying. Tank. It's a tank with wings and a propeller. If anyone remembers making paper airplanes as a child, you can understand where the problem comes in. Tanks weren't/aren't designed to fly for good reason: they're heavy and aerodynamically unsound. To actually make it possible for it to fly this stinkin' thing, it was designed with no ammunition, no armaments, and no headlights. It was everything a tank is not, making it one heavy plane. One was made, and in its solitary test, it did in fact fly... with some assistance. The project, not surprisingly, was scrapped afterwards.
 * The SLAM. Imagine a locomotive. Now, imagine that locomotive with a nuclear ramjet engine, flying at three times the speed of sound at low level, lobbing nuclear bombs at things. Even without the nuclear bombs, the shockwave, exhaust, unshielded reactor, and fission fragments would destroy, kill and irradiate whatever it flew over. Unfortunately, the problem with building a weapon that spews nuclear waste everywhere is that nobody will give you permission to test-fly the damn thing, and your allies might disapprove of it flying over their countries to get to the USSR. Finally, nuclear tipped ICBMs are a lot cheaper, and get to their targets faster.
 * The Convair X-6, a nuclear powered bomber. The X-6 had potential if it was practical, such as being able to stay aloft in the air for WEEKS at a time without refueling. But to shield the crew at a minimal safety level required 12 tons of lead and rubber. While test flights with an operational nuclear reactor on board were conducted on a similar aircraft as a testbed, the X-6 never got off the drawing board.
 * The Soviets had their own nuclear powered bomber, the TU-95LAL, which did partly solve the weight problem of the Corvair X-6... albeit with the added impracticality that the plane tended to kill its own crew by radiation poisoning.
 * The Nuclear pulse propulsion rocket concept made even the SLAM and Corvair X-6 look sedate and practical by comparison. This design used the periodic detonation of nuclear bombs for launch and forward propulsion. Surprisingly, it might have been quite practical for space travel from an engineering perspective, but it was hideously impractical politically (the nuclear detonations would violate the Partial Test Ban treaty, and that's before even considering public opinion).
 * So far, Railguns are much more Awesome than they are Practical. If problems of power supply and rapid wear can be resolved, however, they will be vastly superior to chemically-powered guns in terms of range, and much cheaper to fire than rockets with similar range.