# How close can we get to heated blades?

In many sci-fi scenarii, most often when mechs are involved, it is not uncommon to see such a technology, usually called heated blade. Supposedly, it would make the blade even more efficient at cutting through anything; usually designed to cut through or pierce metal plating, melting it in the process.

Now the thing is, I love this kind of idea (if only for its sheer awesomeness, just like vibrating blades), but there is no way for it to work the same in real life; the temperature would need to be absurdly high to melt any metal that fast. Besides, the blade needs to have a higher fusion point, and with a temperature that high, I doubt there is anything that would work.

But I started wondering: given we are sufficiently technology advanced (assume at least common space travel, maybe even FTL-level if needed), how close can we get to it? As of today, the closest we have that I can think of are what is essentially a blade-shaped soldering iron, but that's because we have no need for heated weapons.

What if we can put it Up To The Next Level?

Restrictions and notes:

• The blade has to be solid; otherwise I could easily imagine lightsabers or futuristic blowtorches to be the ideal solution.
• It has to significantly increase the blade's cutting power. Glowing knives are cool but I want something that can do more than slice and toast bread at the same time.
• Any alloy or technology is allowed, as long as it's scientifically explainable. Does your sword uses the power of a nuclear plant in order to work? Alright no problem, as long as you can explain how to miniaturize all that in the blade (although I accept external solutions like powered suits).
• Of course we can handwave that, if such a material that can withstand such a temperature while maintaining a decent hardness is found, it is not often used for armor plating; maybe because of the cost.
• Size does not matter; if the blade has to be 10 feet long to accommodate for the needed technology, and as such can only be used by a mech, that's fine. All the better if you can make it human-sized, however.

Of course, I'm interested in the How, not in the Why. Never in the Why.

• One problem would be that heating the blade would make the material a lot softer, even when still far away from the melting point. So if anything, it would reduce the cutting power. Heating would only be a performance improvement when the material is not just harder than the enemies armor but also loses much less hardness when heated (by orders of magnitude). Jan 12 '15 at 16:05
• Rhenium might be interesting to look into. Feb 3 '15 at 13:39
• @Linkyu, from what I found during some quick searches, it seemed a lot less brittle than tungsten. But I'm absolutely no expert, I just googled some elements with high melting points. Feb 3 '15 at 14:09
• Just throwing in this video for a real-world here and now analogue: youtube.com/watch?v=JI5whCBV0bk Apr 14 '15 at 21:41
• @MBurke Thermal lances have a tremendous advantage just by having such an awesome name. "Captain, this won't work, the enemy armour is just too thick for the vibroblades and sonic hammers!" "We have no choice then, we have to use the thermal lances!" cue epic music Jun 23 '15 at 16:01

The blade is acually a metal sheath housing a series of prisms and optic fibers designed to guide the laser beam to exit in the direcion of the blade's cutting edge, and in the hilt is a powerful laser generator.

Sensors in the blade would detect the iminence of a slash, and trigger a series of actuators that would direct the laser beam along the blade edge, to cut the enemy's armor plating.

Once the laser created a gap, the blade would serve as a wedge to widen the breach.

Some glare from the laser's diffraction would make the blade glow, giving the likeness of being "hot"

• So these 'heated blades' are essentially laser energy optimizers. The best place to spend this laser energy is right before you hit the target, right where you're going to hit the target. Either your power technology or your mech armor technology(or focusing. Maybe there's no way to collimate such powerful lasers because they ionize the air or something.) means that stationary laser emplacements aren't effective. Good answer. Apr 14 '15 at 16:56
• Use semicunductor nanolasers along the edge that either emit UV to melt flesh, or use another stage to double the frequency through 2-photon conversion. No collamination needed, but operates at near-contact range. Apr 15 '15 at 0:42

I'd like to expand on @DoubleDouble's answer a bit, as cutting torches was my first thought reading the question as well.

Industrial cutting of steel is can be done using a couple of different techniques, for a sword type weapon your best bets are likely something along the lines of a an oxy-fuel torch or a plasma cutting torch.

In oxy-fuel (fuel like propane or acetylene) cutting you heat up a metal with torch, and once it's hot enough you hold a lever that adds additional high pressure air to "cut" the metal. Cut is a bit of a misnomer here since the excess oxygen jet is actually oxidizing the metal and air pressure is blowing away oxide (rust for steel), so it's slightly closer to high-temperature-rapid-directed-rusting =P. Drawbacks to this method will be fuel and usually a concentrated source of oxygen, generally this means high pressure gas tanks which can be pretty heavy not ideal for a person to lug around but probably acceptable on a mech. However the issue you'll see here is the "preheat" time before the cutting can start.

Plasma cutting is the arc welding equivalent to the oxy-fuel approach, a gas mixture is directed at the metal and an electrical arc is established across this gas flow which creates a plasma. The plasma heats up the metal and again the excess gas blow away molten metal. This process is faster and only requires one gas tank but also a good source of electrical energy for the arc.

Ideally you could have each blow of the sword "smartly" create several plasma cutting arcs along the edge do "burst" plasma cuts perhaps charged by high density capacitors and gas release valves.

One other quick thought though, both of these would require eye protection. Oxy-fuel would require welding lens shades of #2-3 and arc plasma cutting is normally in the range of ~#8 but for higher energy (capacitor bursts or other "high energy" hand waving) the lens may need to be #11 or #12. These are very dark shades and so the armor or mech may need to have a built in auto-darkening welding helmet type lens.

• I suspect that in this case, it would be pretty darn difficult to create a "long" arc that would follow the length of the blade, as electricity has the habit of choosing the path of least resistance. In the end, you would probably only get the arc at a few points where the air resistance happened to be the lowest. Also, It seems to me that you're really just building a very short range plasma weapon rather than a sword:) Jan 12 '15 at 18:12
• Not entirely, it's a sword "frame" with plasma cutting vents near (but not along) the edge, if the plasma cutting is disabled (or hits low/non-conductive targets) then you just slice them =D Jan 12 '15 at 19:20

This is an induction heater. The amount of heat you can get out of it is, essentially, only dependent on how much power you have available, and being able to prevent the heated object from destroying the induction coil.

And here is a 2 minute attempt at building the sword.

Problems abound. Ceramics are hard and brittle, but this is a common problem with all swords. The weapon will remain somewhat usable as long as the jacket on either side mostly holds together. A steel induction core would allow a temperature just below it's own melting point - depending on alloy, this is likely around 2500 degrees Fahrenheit, or about 1370 degrees Celsius.

This particular design could never be held by an unshielded person, so we're probably talking about mechs or powered armor. Either way, hitting one with this would take an appendage with it, but not why you might think - we're not talking about "hot knife through butter" so much as "I annealed the thing I just hit and made it a little softer so I could destroy the inner workings." However, if you could subdue a victim and hold this thing to it's cockpit for a while, you'll have subdued the pilot; and certainly pressing it to something with force will eventually make it through. But I wouldn't expect something like this to work after more than, say, 10 swings against an armored target - 1 if your powered armor is particularly strong.

• It does solve the problem of the edge becoming too soft to be useful anymore. Ceramic really is very brittle though. Wouldn't a Rhenium or Tungsten superalloy be a better fit? Apr 14 '15 at 23:05
• @Linkyu, absolutely. I only used materials that I could think of quickly. Tungsten is a great choice. Another option would be to use alternating layers - a very hard sharp shell bonded to a stronger substrate. Thats actually a common method for building metal cutting edges in lathes and such. Apr 14 '15 at 23:09
• Carbon nanotubes conduct heat amazingly well, and remain stong until they melt if its a monofilament. Apr 15 '15 at 0:38
• @JD, you are absolutely right but those can't be manufactured like this today - I just went with what would work without additional research. Apr 15 '15 at 1:23
• I'm okay with an answer that uses any technology as long as it's scientifically explainable; it doesn't matter if it's not possible yet. After all I did say "assume near-FTL technology advancement if needed". Apr 15 '15 at 10:57

Apply nuclear waste vitrification technology to "hot" nuclear material and voila! Something which handles like a ceramic knife but is self-heating. This is bonkers, but in the same league as the polonium laser rifle concept circulated by an actual defence contractor.

Advantages: inflicts radiation burns through steel plate, you just have to wave it around near the victim. Surface can be hot enough to burn skin.

Disadvantages: it's impossible to turn off, has a fairly short half-life, and is lethal to unshielded wielders. It's made of ceramic or glass and is therefore fragile.

• While not exactly what I was looking for, I really like the idea of this as a kind of one-time use weapon. Feb 3 '15 at 13:22
• As a "terror" weapon wielded by a nonhuman or a mech it would have quite a psychological impact. Feb 3 '15 at 13:48
• Ohhh that's even better! Feb 3 '15 at 13:49

A possibly modern approach that immediately comes to my mind, is that you have your blade, and you have many "cutting torch" flames built into the sword, so that they come out on either side of the cutting portion of the blade, angled so the flames create an "edge" in front of the blade. These probably have to be built in a way which, when the blade cuts, doesn't get them in physical contact with anything. (Wish I could draw a picture, but I can't at the moment)

Hopefully, since our blade itself shouldn't be getting heated "too" much, it will retain most of its strength.

I could see this working well for swords designed for cutting, if the sword was allowed to "hit" and stay there a moment to heat the enemy, and then use the slicing motion. As opposed to swords more designed for "crushing" where the majority of their damage comes from how hard and fast you swing it.

The obvious drawback is fuel, and whether all the space taken up by this sword and the fuel would be better used with more guns. ;)

Using this sword, I would be afraid of coming across enemies which use normal swords, or just large spiked/blunt weapons, that are designed to just be heavy and solid. Blocking a direct blow from these weapons would probably break my sword...

In the way the sword in used, where prolonged contact is more beneficial, something like a chainsaw blade might make more sense in the overall design of how the weapon works.

• Interesting approach! I like the idea. Jan 12 '15 at 16:29

The cutting effect of a blade is entirely down to force per unit area. A broadsword doesn't need to be particularly sharp if you've got a lot of weight (e.g. 6 ft of steel) behind it.

Within that context - being hot is going to be of negligible use. You won't 'cut' with a soldering iron or blowtorch, ever.

But what you might find as a reason for the hot-blade is cauterization - it'll stop bleeding if it's really hot, which'll mean someone isn't going to bleed out as quickly... but they might also never heal the wound afterwards.

The approach I could see working is similar to a thermal lance - which is essentially a pressurised oxygen flow, allowing metal (like magnesium) burn extremely hot. This won't cut particularly fast (although it will cut through basically anything).

But it's benefit in sword might be to heat up the armour your opponent is wearing, making it more malleable.

About the best I can give for the 'burning sword' approach is a self-sharping blade. E.g. heat the edge of the blade to lots, and then use a magnetic field to reshape it and re-align the atoms in it to make is really sharp. Impacts to a magnetic material will cause field reshuffling, and a sword that you've just hit something solid with is going to be nicked and dented in short order.

On a related point though - you've also got a water knife - using high pressure water as a cutting surface. I could envisage a 'water knife' type sword, but again it suffers from the same problem as the heat-blade, in that it's no longer an 'impact weapon' but rather a rather elaborate can-opener.

http://www.flowwaterjet.com/en/waterjet-cutting/accessories/waterknife.aspx

My approach is to use nanotechnology. The edge is covered with nano-disassemblers that take apart the material they are presented with. It doesn't have to reduce it to atoms, but separate enough to make dust partcles separate from the bulk.

The blade can have a selection of nanomachnes along the edge, and can deploy whatever is needed for metals, organics, minerals, etc.

For organic material, particular uv laser frequency can destroy bonds without heating neighboring tissue; this is used for Lasik and laser scalpels. Putting nanolasers on a mechanical scalpel (rather the weilding a fiber with bulky machines) is a natural continuation to make a medical tool. That can be beefed up to make a weapon.

• Nanotechnology: Always the answer to science-based questions. I like this one though; you get my vote. Apr 15 '15 at 5:39

You could have a light-saber-like weapon.

The main problem with this type of weapon in the real world is the containment of the "blade." If you weren't stuck on having a free-standing column of plasma (i.e. your sword would have a point) a light saber is theoretically possible if you could generate the energies it would require to create such a super-hot plasma. The "blade" portion wouldn't actually be touching anything due to magnetic containment, so it could actually be a feasible weapon.

• I think the lightsaber "blade" more or less turns back at its end, making it some kind of elongated plasma loop. My memory might be wrong. Oct 25 '16 at 11:36

Y'all are pikers. The handle of the sword is a power source. The core of the blade contains antimatter in a containment field. The antimatter is fed to the edge of the blade by use of mumble-mumble nanotechnological faster-mumble magnetic monopoles hand-waving superconductor more-mumbling sapphire/fullerene construction. The result is that the edge of the blade emits a narrow band of antimatter at low density, with the flow rate tailored locally to the proximity and velocity of approaching matter. The result is a powerful local energy release which vaporizes the target in a line which coincides with the edge of the sword, allowing it to cut through any material with ease. By the time the sword is in a position to make contact with the target, a channel the width of the sword has been cut into the target.

Eye protection is recommended.

• Right. But that does make use of an awful lot of handwavium; I think I understand the approach you're taking here, but I'm no expert on "mumble-mumble nanotechnological faster-mumble magnetic monopoles hand-waving superconductor more-mumbling sapphire/fullerene construction". Could you provide a source or two about the theory of it, at least? Apr 15 '15 at 5:35

• Hi Connor. Welcome to Worldbuilding.SE. What I miss in your otherwise interesting answer, is an estimation of the temperature your flame may attain. As the Original Poster (OP) specified, it needs to be high enough to melt other metals. Are you sure your propane mixture can attain such temperatures with this configuration? Jul 2 '15 at 6:58

Plasma cutter: a knife shaped device with edge that shoots hot accelerated plasma. We already use plasma cutters to cut metal plates.

Induction cutter: instead of using induction to heat the blade itself, the blade has a induction coil inside capable of causing inductive current in a surface that it's about to cut. This means instead of the blade, the surface of armor heats up, reducing heat stress and damage on the blade.

A plasma sword would be nice. I'm using those and vibro blades for a sci-fi book I'm writing. The plasma sword works by using the fusion core of a suit of powered armor as a source for the blade. Using a carbon nanotube tube to funnel the plasma from the core to the hilt and super conductive magnets in the hilt to stabilize the plasma and there you go.

• Please note the [science-based] tag. How much of this is based in real science (and ideally engineering) of today?
– user
Nov 14 '16 at 7:42
• Most of it is. It's like a blow torch. It focuses a hot concentration of plasma into a high powered electromagnetic field in the shape of a sword. When the field is disrupted the plasma bleeds out and comes into contact with whatever material you want to destroy super heating it. The carben nanotubes we have today. As for super conductive magnets we have those today. And you can even make a super strong magnetic field even with super small magnets. Dec 25 '16 at 4:06

Here is a little twist you might play with. The common, most effective armor going around is something like a composite, carbon fiber yada yada armor that is effective against most melee weapons, a host of projectile weapons, and so on. Every-one has it and it has become something that creates stalemates in ground battles. It's effective against most energy weapons. Anything strong enough to get through is impractical in small scale or precise battles..

How to get through it? It has a problem with high heat and steady pressures. Not something that is super easy with the current crop of weapons. Energy weapons that might get the heat won't produce the pressure and melee weapons and projectile weapons get the pressure but not the heat. Even things like current anti-tank rounds that inject molten copper into a tank are defeated by the ablative properties of the armor. You need something that will apply consistent heat and pressure over a relatively long period of time.

You can use a heated blade made of Tungsten/unobtanium alloy that will enable your guys to hit the armor with both heat and pressure in close engagements. Use induction powered by the proton pack on your back to heat the blade.

Maybe use it as some sort of assassination weapon. Cover your traces after the engagement. Once the enemy gets the idea that the armor can be beat in this way, they will begin looking for ways to either fix the weakness in the armor or try to make their own hot blades. Once the secret is out, the arms race just jumps to another level of stalemate.

sounds like fun. I know you asked for the what, not the Why, but the why often provides the keys to the what :)