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The collapsible sword in question would be:

  • 3 feet / 90 cm long total length of handle + blade. The blade would be 2 feet / 60 cm long, and the handle would be 1 foot / 30 cm long. (Handle could be a couple inches longer, or blade a couple inches shorter if necessary)
  • The blade would expand out of the handle with the push of a switch or latch located on the handle.
  • The blade must be prevented from collapsing back on itself for thrusts.
  • The blade would be made of a hard/strong material such as a metal, polymer, etc. that could keep a sharp/durable edge.
  • The blade should be strong enough to block other weapons such as swords (at least like a saber or other short sword) without breaking.
  • Single-edged. (Likely similar to a short samurai sword).
  • The mechanism couldn’t be a telescoping system.

Some potential mechanisms:

  • Multiple separate metal pieces that latch/hinge together?
  • Powerful electromagnetic field holds pieces together?
  • Nano particles?
  • Shape memory alloy?

I’m open to other creative ideas that would achieve the same result as well.

The sword should use scientifically achievable technology, no magic is involved.

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    $\begingroup$ What you describe presumes that a modern sword has unnecessary material that could be removed so that the sword could fit into the handle. I assure you that this is not the case. $\endgroup$
    – Robert Rapplean
    Commented Apr 19, 2023 at 3:19
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    $\begingroup$ Also, remember that science-based and science-fiction are mutually exclusive. You're allowed to use only one. $\endgroup$
    – JBH
    Commented Apr 19, 2023 at 4:06
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    $\begingroup$ @JBH - the duplicate question is completely different. The stresses and forces on a sword blade are substantially different to those faced by a Rifle barrel. I do not consider these to be remotely similar. $\endgroup$
    – TheDemonLord
    Commented Apr 19, 2023 at 4:30
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    $\begingroup$ @JBH, I removed one of those tags. And I concur with TheDemonLord, the “duplicate” question is entirely different and useless for what I’m asking about. $\endgroup$
    – Kal Madda
    Commented Apr 19, 2023 at 11:41
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    $\begingroup$ @TheDemonLord No, they're not particularly different. Both are constructions suffering forces both perpendicular and parallel. Both introduce weaknesses that compromise the fundamental purpose of the object. And they're so much alike that the one answer that slipped in before I VTC'd (and before the OP made edits to circumvent the issue) introduced the same alteration: telescoping. I've no objection to the community overriding my VTC, but you and the OP are both quite wrong. Indeed, without telescoping, the blade will never be strong enough. Unless a switchblade is all we're talking about. $\endgroup$
    – JBH
    Commented Apr 20, 2023 at 0:09

2 Answers 2

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Soft frame challenge - Cyberpunk street ninjas are not materials science Ph.Ds and they don't care how their hardware works on a molecular level.

Imagine how ridiculous it would be to describe a real technology down to the materials science that makes it work the way it does.

The ninja drew his sword. Iron lattices have a conduction band in which it doesn't take a lot of energy to move electrons around. Thus, because reflectivity is coupled to conductivity by the Hagens-Ruben equation, it gleamed wickedly in the moonlight as the ninja stalked silently towards his victim...

Your fictional sword just works. The fictional materials science that makes it work is irrelevant except that, once you've invented a fictional fact, you're stuck with it. You can just say:

With a flick of his wrist, the ninja unfolded his collapsible sword. A coded pulse of electricity locked its superalloy segments together into a single deadly blade, and he stalked silently towards his victim.

Bam, you're done. The only further thought you need to give to it is that you're now working in a setting in which coded pulses of electricity [or some other science thing of your choice] can cause superalloy segments to lock together (because of materials science that your characters don't care about).

In the long run, the knock-on consequences of what kind of fictional materials science allows for collapsible ninja swords may even be be why you have a collapsible ninja sword in your story in the first place, since it's a convenient way to introduce that fact about the way the rules are slightly different in your setting.

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  • $\begingroup$ @g s Thank you for the answer. You actually provided a plausible explanation in your answer with coded pulses of electricity locking together superalloy segments. This was the kind of thing I was hoping for, I don’t have to describe all of the workings in super-fine detail or anything, but I was hoping for answers like yours that would provide a plausible way for the blade to be formed that could theoretically work. $\endgroup$
    – Kal Madda
    Commented Apr 20, 2023 at 12:44
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Sorry but it won't.

With a handle that is 1 foot long, 2-feet long blade means, unfortunately, that you need a telescopic blade. Without handwavium, it will be structurally too weak for any proper use because at least half of the length of the blade has to be hollow. It cannot be strong enough to block other weapons without breaking.

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  • $\begingroup$ But couldn’t the blade potentially be stored in smaller solid sections that could hinge or lock somehow together? $\endgroup$
    – Kal Madda
    Commented Apr 19, 2023 at 1:31
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    $\begingroup$ You are right, that is an option, too. However, a hinge in the middle of the blade is problematic as well exactly for the same reason: anything that can move compromises structural integrity. $\endgroup$
    – Jani Miettinen
    Commented Apr 19, 2023 at 1:37
  • $\begingroup$ But is there a hinge/locking mechanism combo that would be durable enough for parrying/blocking other weapons? I’m sure it wouldn’t necessarily be as strong as a solid blade could potentially be, but could it be strong enough? $\endgroup$
    – Kal Madda
    Commented Apr 19, 2023 at 1:39
  • $\begingroup$ @KalMadda I think you underestimate how much force a sword swing has. Peak impact can be as high as 27KN - a Hinge or Lock that is sufficiently sturdy to take that kind of impact force is going to be big and bulky - ruining the other attributes of the sword. $\endgroup$
    – TheDemonLord
    Commented Apr 19, 2023 at 1:39
  • $\begingroup$ The problem with moving parts is that all stress from impacts concentrates on very small part of the blade, and the moving parts also introduce discontinuities that make it even worse. If the blade is a more or less homogeneous piece of metal, the stress is distributed evenly, and thus the blade can withstand much more. An extreme example is glass that is scratched: heating up and cooling the glass down repeatedly makes the glass break where the scratch is, and the scratch does not even need to be deep. Slight discontinuity together with stress is all it takes to break it. $\endgroup$
    – Jani Miettinen
    Commented Apr 19, 2023 at 1:57

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