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Nb: This question was posted in sandbox for proposed questions!

Backgrounds

Design Goals: The weapon should be capable of delivering a powerful impact, but would be super easy to turn. Not to mention that the sword should be powerful enough to cut through most solid things. Despite its ability to cut through most materials, the blade should be safe to use, that is, when facing another ordinary metal sword, the opponent's sword wouldn't just be sliced and the sliced opponent's blade flown with its remaining momentum toward the user. In another word, when detecting a potentially dangerous situation as outlined, the sword should be able to repel opponent's metal sword than just slicing it.

Current Design Solution: If summarized, current solutions provide these features:

  1. The sword is hollow, inner parts would be for counter masses and machinery.

    • The blade is an empty hard shell of strong yet light material (perhaps diamondoid, graphene, or even carbon-nanotube reinforced steel).
    • The handle is filled with computer control units, magnetic actuators to move the counter masses around the rails and an energy source.
  2. The cutting edge is composed of microscopic "curtain-held"s arranged in a row along the sharp edge. The "curtain-held"s would erect curtains of some kind of monomolecular wire along the path of the edge when in use (yes, you could see that this is partly inspired by this answer). The monomolecular wire would be either graphene as the answer linked suggest, or TMDC ribbons. The curtains could be superheated to aid in cutting power. As a safety measure it could also produce strong magnetism or eddie currents that repels an opponent's metal sword when in close proximity.

  3. Dynamic mass counterbalance system inside the sword that could be used to shift the center of mass of the sword, and is generally heavy, comprising at least half of the weapons whole mass. The inner side of the blade's "shell" would be a rail system extending from near the tip of the blade to near the end of the handle, of which a number of heavy counter masses would be shifted to change the whole sword's center of mass in battle according to need. Practically there are at least two different configurations in use during battle:

    • center of gravity moved to near the tip of blade for swinging and slashing, or
    • center of gravity moved to near the handle for easy and quick turning.
  4. Has sufficient battery power and strong capacitor that must be able to supply the system with enough energy to move the counter masses and power its integrated control circuits. Also to heat the cutting strand if heating is necessary.

  5. The weapon is about 1.5 meters in length (1.2 meters in blade and 0.3 meters in handle). The mass could be anywhere between 5 kg to 7 kg (perhaps the upper limit could be up to 10 kg).

Modus of Operation: Basically to operate it, when it is swung, the center of mass would be changed to be near the tip of the blade to increase the hitting mass, while to rotate the blade the center of mass would be moved near the handle.

Question

What I would like to know is whether or not this arrangement is conceivable or even plausible.

Ps: Assume that the weight of the sword is not an issue for the wielder.


Edit 20170716

Thanks for everyone that contribute in this questions! With so many comments and broad range of topics discussed in it I feel like it would be better to address it with a statement in the question instead. With this edit I would like to clarify some points:

  • Apparently some answers address the fact that this sword may be difficult to wield, or might not always be in the favor of the wielder. This sword is designed to be wielded not by man but by some other creature that could micromanage the sword on the fly, and yes every mechanism of the sword is manually controlled by the user.
  • Another thanks for those that mentioned the problem with heated sword, especially (@John) the fact that TMDC or monolayer materials may burn in oxygen rich environment (and our atmosphere is!). Other than that, heat may weaken the sword even more (@John, and PipperChip).
  • Apparently user stated that shift-able center of mass would not add any benefit, that even a solid sword with no dynamic center of mass would be more or less the same in term of efficiency (@John), and center of mass near the handle wouldn't help in rotating or manipulating the blade any faster either (@Aify).
  • Also qualification I use to decide which of the answers is the best answer would be those that could answer the actual question (in bold), and there are many to choose, most qualifies as such. Therefore I'll wait for some days before I would pick one.
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    $\begingroup$ Well your first problem is both graphene and TMDC burn when heated in an oxygen environment. Second moving parts are never a good idea in a melee weapon since they bend and flex considerably. $\endgroup$ – John Jul 14 '17 at 20:45
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    $\begingroup$ I'm almost 100% sure this is a duplicate but I can't find the dupe. Either way, heating a sword does not in any way make it any better in cutting. $\endgroup$ – Aify Jul 14 '17 at 21:13
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    $\begingroup$ Related: worldbuilding.stackexchange.com/questions/48677/… Answers addresses point 3, and worldbuilding.stackexchange.com/questions/31968/… addresses heat $\endgroup$ – Aify Jul 14 '17 at 21:17
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    $\begingroup$ They are stiffer as well more brittle but very stiff is not the same as inflexible. Additionally an internal shifting weights in a sword also don't do anything to improve striking force. The force remains the same, you still need to supply the additional force that moves the shifting weight, so you don't gain anything more than you would if it were solid, if anything you are losing a small amount as friction. You could make a good question about this alone, good for the physics thread. $\endgroup$ – John Jul 14 '17 at 21:33
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    $\begingroup$ Is this just a [reality-check] of a lighsabre? $\endgroup$ – Mathmagician Jul 15 '17 at 4:23
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As a practitioner of (mostly) Eastern martial arts, your sword does not work for many of our styles.

First of all, you do not want a hollow blade whatsoever. A hollow blade reduces the strength of the blade, especially since you'll be taking mass away from the core. Your blade will be much easier to shatter or otherwise mangle in combat, and a mangled blade is a useless blade.

Having mechanism inside the hollow section is even worse, as your blade is likely to be constantly banged from various directions, leading to your mechanisms possibly malfunctioning.

Heating a metal almost always makes it weaker (actually I'm not sure that there's any metal that's stronger when heated).

I don't quite understand why you'd want to repel the opponent's metal sword when in close proximity; wouldn't the same mechanism repel the opponents armor (and as such the entire opponent), resulting in you never cutting any metal items?

Dynamic counter mass sounds like a mess to practice with, and it doesn't really help with our martial arts system. Eastern martial arts (related to swords and sabers) typically focus on speed and accuracy over strength and power; as such, many of our swings aim to do damage using the tips of the blades. We rely on cutting edge and cutting speed instead of hitting mass. Therefore, there is no reason to increase the hitting mass at all; instead, a better design choice for this (with regard to our style) would be to remove the dynamic mass completely.

The weight near the handle doesn't help us rotate or manipulate our blade any faster either.


To address the edit: Your sword as described is "conceivable", as in, "yes, you can make this sword with technology in the future" - I would suggest the removal of the heated edge though. However, I have my doubts as to the effectiveness of the blade in combat. I can already think of several tactics to abuse the mechanisms of the sword against the wielder.

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    $\begingroup$ Always good to see an eastern martial arts practitioner chime in! $\endgroup$ – PipperChip Jul 14 '17 at 21:39
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    $\begingroup$ @PipperChip You covered the European section, how could I not cover the Eastern side? $\endgroup$ – Aify Jul 14 '17 at 21:44
  • $\begingroup$ Just FYI there are metals that get stronger when heated but only up to a certain temprature then they start getting weaker again as the temprature rises past that. $\endgroup$ – John Jul 14 '17 at 21:46
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    $\begingroup$ Good points on: eastern martial arts' focus on speed and accuracy, there's no point on shifting the center of mass, and the fact that heating metal makes the sword weaker. About magnetic repel mechanism (I'm not sure what to call it) is designed for sword on sword, so other kind of metals like armor is dismissed by the mechanism. Thanks for your insight as a sword practitioner (you are, arent you?). I never wield any sword before, it is an invaluable insight for me to know. $\endgroup$ – Hendrik Lie Jul 15 '17 at 18:47
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    $\begingroup$ @HendrikLie I've edited the answer to "answer" the bolded question. $\endgroup$ – Aify Jul 23 '17 at 18:13
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The Problems I See

As a HEMA practitioner, I don't think this is very practical for a few reasons:

  1. It is common in many styles of swordsmanship to block other weapons with your own weapon. If you block edge-on-edge and successfully cut through the other weapon, you may find the part you cut off flying into your face! Yes, I know edge-on-edge blocking is looked down upon by certain people, but it's common enough that I should mention it. You cannot reliably enter into a bind or parry with this blade against a weapon.

  2. A changing center of mass is very tricky. There are situations where you may want to very quickly turn and cut, or you rely on the blade's speed (such as German longsword's duplieren and mutieren) and the changing center of mass can make these maneuvers more difficult. So this weapon must always guess if you're going to need the speed or the center of mass to change, and it may guess wrong and cost you.

  3. The magnetic field idea is interesting, but sometimes you really do want to block the opponent's weapon with your own and enter into a bind. Also, this magnetic field must be really, really strong to have much of an effect in combat. That magnetic field could actually work to the wielder's disadvantage: instead of entering into a bind and controlling the fight, the magnetic field could prevent it, and then the opponent's blade is free to offend! So you can't control your opponent's blade with this weapon.

  4. Heating a blade usually does not help with cutting. For one thing, if your weapon is held together with atomic bonds of just about any kind (which you plan to do), it takes less energy to break it. Hot things fall apart more quickly than otherwise. (To see an example of this, think of frozen butter vs. room temperature butter!)

  5. If you cut with a blade hot enough to burn flesh, you will cauterize the wounds, preventing bleeding. Bleeding is important to killing people in a timely fashion! So cutting with this weapon doesn't cause bleeding, even if you do land a hit. Oh yeah, no using your sword as a big ol' lever in wresting for half-swording without a heat glove. Halfswording may not be needed because this thing is so sharp... ish...

In Summary

This weapon:

  1. Cannot reliably bind or parry
  2. It may disagree on you if you should thrust or cut
  3. You cannot enter a bind
  4. It heats up to become less durable
  5. It heats up to prevent your opponents from bleeding to death
  6. You can't half-sword with it unless the blade can sense the wielder's hand.

So this weapons sounds terrible to me. It won't work with European swordsmanship and very likely will not work with eastern swordsmanship.

This is possible, but not very practical. There are certainly engineering challenges for this, but there is nothing physically preventing such a thing from existing. Is it useful? Likely not.

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  • $\begingroup$ Good points on inability of the sword to bind or parry, and the fact that heat makes the sword less durable. For control mechanism I forget to mention that it is safe to assume that the sword would know when to shift the center of mass 100% of the time. Thanks for your perspective, I never try to wield any sword so this insight is invaluable! $\endgroup$ – Hendrik Lie Jul 15 '17 at 18:38
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    $\begingroup$ @HendrikLie The real problem with ultimate swords is, perhaps, not obvious: there may be no ideal sword. How you use one is just as important as to what it is. Additionally, battlefield weapons were made to solve specific problems: how can you quickly kill without getting killed? How do you get around or through the armor of the enemy? The arms races between different cultures show the myriad of ways these questions get answered. $\endgroup$ – PipperChip Jul 16 '17 at 0:05
  • $\begingroup$ @HendrikLie I should also mention that (1) I've edited my post to answer the highlighted question and (2) the "ideal sword" is really context based as well, because other properties, like blade shape and length, have different ideal properties depending on what you do with it. Fighting spear men? Get a 6 ft sword. Fighting in close quarters? Get a cutlass or falchion. Wanna have amazing cutting ability? Get a curved sword. Want to be good in a thrust? Get a straight sword. $\endgroup$ – PipperChip Jul 23 '17 at 21:01
  • $\begingroup$ Thank you. I am sorry for my week-lonh absence, i will have your updated answer considered. $\endgroup$ – Hendrik Lie Jul 24 '17 at 10:49
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I think dynamic behavior of this "super-sword" (if and when you could actually build it) would surprise you quite a lot, possibly in a non-favorable way.

Main problem is during a swing linear and angular momentum need to be preserved (unless actively acted upon by exerting an appropriate force on the handle). This means that, when "counterbalance" system shifts the inner weights (presumably accounting for a large part of sword total weight) toward the tip the handle would push hard in wielder hands and, if not countered by muscle force, this would result in the tip of the sword to move where the handle was (i.e.: the inner weights would remain in the same place and the blade would move). Worse yet when counterbalance weights are suddenly retracted in the handle, the sword will try to leave wielder hands.

Similar problems about momentum: if you start a fast swing (weights in the handle) when weights go up the blade would stop. In the reverse situation (retracting weights with a blade already in motion) would result in a blade accelerating uncontrollably its motion.

The whole problem arises from fact to change momentum a force is needed and that force must be anchored to something solid. This is reason why "real" swordsmen are taught to initiate all movements from the hips, which solidly rest on the most muscular potion of our body, able to transmit the force down to the unmoving ground with ease.

To obtain an effect similar to what you need you can think about an active gyroscopic system, detecting intentions with sensors in the handle and enhancing the movement by up-spinning the correct gyroscope (this would increment its momentum in a certain direction and thus the sword would acquire the same momentum difference in the opposite direction). With this setup most of the weight can be in the gyro system and sword wouldn't loose its power, if it's rigid enough. Keep in mind such a weapon would mostly turn around its center of mass (i.e.: the gyro system) and would offer no enhanced translation capability.

Please note: what follows is not 100% scientifically correct, but I believe the specifics I left out re not relevant in current discussion.

In an "isolated system" (i.e: a system not subjected to external forces) there are three "quantities" that are preserved: linear momentum, angular momentum and energy.

You need not worry about the latter because You are storing a sizable energy amount (in the form of chemical potential energy) to be converted to other forms (in this case kinetic).

You have no way to modify linear momentum (i.e.: to move sword center of gravity) without exerting force from outside (i.e.: to actively use wielder muscles to transmit momentum from sword to ground) so that the "isolated system" is not just the sword, but includes the whole planet the wielder is standing on. To say it in another way: you have no way to do that with something contained within the sword itself.

The same applies to sword momentum (i.e.: rotation around sword center of gravity), but, in this case, you can "cheat". If sword is not rigid (i.e.: a single piece of steel) then what is conserved is the "total amount of momentum". If you have a (heavy) flywheel (gyroscope) that starts turning in a certain direction in order to conserve total momentum the rest of system (i.e.: the blade) will start to turn in the opposite direction. This means that in order to have the sword turn (e.g.) clockwise "on its own" you "just" need to start a flywheel in the handle to turn counter-clockwise until the sword is in the expected direction and then to stop it. Having three orthogonal flywheels enables to rotate the sword in whatever direction needed (and to impart it enough "momentum" to cut, even if the actual blade is quite light).

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  • $\begingroup$ Thanks to mention difficulty on wielding it and problems it may cause. The fact is, this sword is not designed to be wielded by human, so assume a wielder that could (either) perfectly predict the sword's behavior, or manually controlling the mechanism (by thought or brain-to-computer interface). Also, are you sure gyroscope could do just that (your last paragraph)? Because in original version of the sword there is gyroscope system that do just that, but my concern were that system might be unrealistic. $\endgroup$ – Hendrik Lie Jul 15 '17 at 18:50
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    $\begingroup$ @HendrikLie: I will update the answer to try to explain better. $\endgroup$ – ZioByte Jul 15 '17 at 21:58
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I think your best bet is more of a laser/lightsaber type weapon. It would on the molecular level slice the atoms apart. You would probably require a secondary magnetic field generator to repulse the severed blade.

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  • $\begingroup$ The trouble is, it's really hard to repel metal with a magnet - no matter which pole you use the metal will still want to stick to it. Your best bet is to try to force the opponent's blade perpendicular to its current motion then turn it off at exactly the right time, but that would require a ridiculous amount of energy. A "propel a squirrel into temporary orbit" amount of energy. $\endgroup$ – wizzwizz4 Jul 15 '17 at 15:31
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    $\begingroup$ I think the best way is to avoid needing to repulse the blade, by timing your attack. If the attacker blade is not in motion, it will simple drop to the ground. So the key is to attack before the opponent can add any significant moment to his/her blade. Either that or simple develop a dodging technique. $\endgroup$ – cybernard Jul 15 '17 at 15:35
  • $\begingroup$ Also, I assume that you mean "on the molecular level separate the atoms" not "on the molecular level split the atoms", but metal doesn't form molecules - rather giant metallic lattices. Unlike giant ionic lattices you can't crack metal - your "slicing" idea would not work unless it also physically separated the parts of the opponent's sword because the metal ions are attracted to negatively charged space around them and are not bonded like molecules, so wouldn't really "notice" the molecular separation. If the sword did separate them, it would probably bond to the pieces. $\endgroup$ – wizzwizz4 Jul 15 '17 at 15:35
  • $\begingroup$ I think of it as a highly focused laser beam just wide enough to force breakage. The laser would vaporize a thin slice of metal as it goes through the opponents blade. $\endgroup$ – cybernard Jul 15 '17 at 15:39
  • $\begingroup$ Define "vaporize": are you evaporating the ions? They wouldn't just disappear unless handwavium. (I'm probably picking too many holes in this - Star Wars pulled it off!) $\endgroup$ – wizzwizz4 Jul 15 '17 at 15:49
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An alternative way of doing this was invented by SF writer Graydon Saunders, on Usenet in 1997:

"Yes, I know it looks like a sword, see, but the edge really has all these small holes, and we've stuck an atom of anti-hydrogen into a fullerene molecule, which takes forever, and the small holes flex under the impact and eject the fullerene grease they're packed with, and the rebound flex affects these piezoelectrics in the spine of the sword, and those spark the grease, which is very reactive, and that breaks down the fullerenes, and then whoosh, gapping rent through anything."

"Drawbacks? Well, you need to wear a really good adaptive eye shield, because dark enough to be safe when you hit is too dark to see what to hit, and you need lead undies to stop the gamma, and the concussion is fairly noticeable in air, and if the grease isn't completely off the sword edge when it sparks, well, that's bad. That's really bad. And to be refractive enough, the sword has to be this osmium-rubidium alloy, so it weighs eight kilos. But other than that it works fine."

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None of this make sense

"The weapon should be capable of delivering a powerful impact"

The whole point of a sword is that it's an edged weapon. It's designed to cut and stab, not bludgeon.

"but would be super easy to turn."

To make it easy to turn, you make it lighter but making it lighter lowers the impact

"Not to mention that the sword should be powerful enough to cut through most solid things."

Ok so something like a mono filament blade or a lightsaber

"Despite its ability to cut through most materials, the blade should be safe to use, that is, when facing another ordinary metal sword, the opponent's sword wouldn't just be sliced and the sliced opponent's blade flown with its remaining momentum toward the user."

How can you cut most thing but not a plain metal sword? How will a sword tell the difference between a sword and a metal railing?

"In another word, when detecting a potentially dangerous situation as outlined, the sword should be able to repel opponent's metal sword than just slicing it."

You can't make a sword like this without some magic sentient sword hoodoo as the effects you ask for are contradictory. The sword would need to work out what it's about to hit and change it's effect accordingly.

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I encountered such a device in a story - I think it was a fanfic based on Out of the Dark by David Weber (I'll probably ask about it on scifi.stackexchange, I'd love to read it again).

The sword - it wasn't a sword, but it would work in sword shape - was based on a pretty advanced technology and wasn't even ordinary matter. Ordinary matter is ultimately held together by the electromagnetic force, from its weakest form as London force to its strongest form as covalent bond. But it's still electromagnetism. This "sword" was held together by a form of nuclear strong force, about one hundred forty times more powerful (albeit at much closer range).

The surface of the sword would have been covered in minuscule force projectors acting as very fast chainsaws, capable of tearing apart diamond as if it was butter. The speed of the effect being higher than the speed of the swing means that the sword never actually hits anything, because everything in its path gets cut by a disintegrating effect milliseconds before any impact can occur - if the "nuclear blade" is activated. If it is not, then the opponent's blade hits a sword-shaped object that's literally unbreakable, since it is harder and tougher than any ordinary matter.

The "unfeasible" element in the original story - and in this sword - is the possibility of harnessing and projecting the strong nuclear force using nano-scale devices. While impossible for the foreseeable future, it is not strictly against any physical law ("color confinement" is an observation and is not based on any theoretical model).

In the story, the disintegrated matter was sucked in and converted in more nuclear-strong matter as feedstock. In your case you'd want instead the matter expelled on the other side of the blade, still as ordinary matter.

The blade's source of power is controlled low-energy nuclear fusion; the blade would be capable of "eating" oxygen from air expelling only a weak sulfur smell (you could "suffocate" it by plunging it into a molten iron crucible).

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  • $\begingroup$ I believe this is not very conceivable in near future technology on my setting, but the idea is interesting to look at! $\endgroup$ – Hendrik Lie Jul 24 '17 at 10:53

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