Critique this Plasma Weapon Design

Question

From the answer of another question, the plasma launcher design from UFO: AI (this is a game title, guns are not necessarily UFO mounted) creates the plasma inside a replaceable canister and directs it through the firing apparatus upon triggering. To prevent dissipation, spinnerets on the front of the gun weave a plastic membrane, which is warped by the plasma upon firing, encapsulating the pulse for the remainder of the flight to the target. As a comparison, imagine the plastic as suspended detergent on a bubble wand, and the air blown through it as the plasma.

For the fictional people I am entrusting this technology, they have access to complex and advanced bio-mechanical technology and precise genetic engineering (most of their tech - machines, vehicles, tools, weapons - are semi-organic, so they basically 'grow' cars and guns). This tech potentially allows the spinnerets to weave a much stronger organic membrane (like that of spider's silk or hag-fish mucus - read up on that it's interesting) for capturing the pulse. Carbon being the base of organic material (this is carbon based life), a lattice like graphene could be plausible. Chamber surfaces that come in contact with the plasma will obviously most likely be metal alloy and not organic material.

With this (some-what vague) design, what kind of draw-backs and problems might I encounter that hinder its plausibility? I ask because my knowledge on plasma behaviour is less than expert to say the least.

Note: should this be too broad, narrowing to the largest problem/s and hand-waving the less-significant is acceptable.

Edit: though hand-held ground weaponry is favourable, the equivalent of a tank might also use this, so vehicle-mounted plasma launchers aren't out of this questions reaches - this question is more about the launching design than what holds the launcher.

Answer 1

Important Note

Monty Wild pointed out to me in the comments below that this question is actually about terrestrial, hand-held weaponry. (I had thought it was about ship-to-ship space weaponry.) I will do a thorough edit of my answer a little later on, but in the meantime, I expect the main difference will be that the power source will be a limiting factor, and that friction and conductive heat losses in the atmosphere will further limit the weapon's effectiveness. Unless I think of something fairly major, the main takeaways will still be broadly the same.

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Disclaimer: while all of this is based in real science, there are a few hypotheses here that I won't be able to test they let me up into orbit with my plasma cutter and tungsten forge.

It's hot. It's real hot.

The main issue will be temperature. Plasma is hot, like close to the surface of the sun hot. Plasma is created by heating a gas past $5\ 000^\circ \text{C}$. About your best bet for a metal container would be tungsten (W), which melts at around $3\ 400^\circ \text{C}$. As it turns out, the vacuum of space is a good insulator, so your only losses are going to be radiated visible and infrared (heat), so the tungsten will heat up.

The semi-good news is, tungsten doesn't boil until $5\ 930^\circ \text{C}$, and in the absence of other acceleration or strong gravitational fields, the tungsten and plasma will stay together in a spherical shape.

The bad news is, the plasma will exert considerable outward pressure (conventional and radiation pressure) on the tungsten vehicle, so it'll likely blow apart in fairly short order. If that doesn't happen, then because the liquid tungsten is far more dense than the superheated gas (plasma), it will "sink" to the center of the sphere as soon as there is any sort of surface irregularity in the tungsten.

Be quick about it.

The maybe good news is that heat transfer (be it via conduction, convection, or radiation) takes time, so if your plasma bolts are moving quickly enough and the target is close enough, they might survive. (This also assumes that they can be created quickly and fired instantly; no pre-loading, here!) This will limit the range and rate of fire of your weapon, obviously. Let's have a look at conduction. There, the heat transfer, $\text{Q}$ (in Joules), is given by:

$$Q = \frac{kA\Delta \text{T} t}{d}$$

$k$ is the object's thermal conductivity. Higher conductivity means your plasma bolt fizzles out more quickly.

$A$ is the cross sectional area between the hot and cold surfaces. $\Delta \text{T}$ is the difference in temperature between hot and cold (in C or Kelvin; they're equivalent). $d$ is the diameter of the material.

Thus, rearranging for $t$ (time): $$t = \frac{dQ}{kA\Delta \text{T}}$$

As you can see, going with a larger bolt will mean it will take longer to transfer its heat, as will decreasing the surface area (a sphere is ideal). Getting exact values for some of these variables would take some more research, but you're probably reasonably safe with tens of seconds if your projectile is at least a few centimetres across. Hopefully that's enough time for a ship-to-ship impact.

Where does that leave us, then?

Plasma actually isn't all that amazing as a weapon. We already use it in plasma cutters today, and if you've ever seen a fabricator cut a piece of steel with a plasma torch, you'll know it takes a lot of sustained application of the plasma jet to melt and remove the material to make a 2mm-wide cut through even a 1cm piece of non-tempered ("soft") steel. That sort of hull breach could be plugged with bubble gum. Plasma, while very hot, is simply not dense enough to make much sense as a projectile payload with a more-or-less instantaneous impact. Everything that helps your projectile survive the travel time from your ship to theirs hurts its effectiveness at dumping all of its heat into the bad guys' hull. The tungsten container will do far more kinetic and thermal damage on impact than the plasma, unfortunately.

Even with advances in materials and science, this relationship isn't likely to change much, since the laws of thermodynamics are dominating the discussion at this point.

Is there an alternative?

At extremely close range (as in, you could reach out your airlock and write graffiti on their hull), a much larger version of a plasma torch might do some decent thermal damage, but its power falls off very quickly with distance.

Honestly, regular old kinetic projectiles (think: conventional guns) would be more effective. In the vacuum of space, friction won't slow them down, and if you're relatively far from massive celestial bodies, gravity won't bend their trajectory much. Also due to the lack of friction, with a very long barrel, you can accelerate the projectiles much much more than you could on Earth, without any additional energy input.

If you want some kind of energy weapon that does thermal damage, lasers are probably your best bet, unless you want to cross over into the realm of speculative/fantasy sci-fi to come up with your own version of phasers, photon torpedoes, etc., but that's clearly a different sort of question.

Answer 2

My first thoughts regarding this is that the membrane will need to be created extremely quickly, and the plasma - if sufficient is present for use as a weapon - will also burn through the membrane very quickly.

The issue is that most polymers have a relatively low melting/combustion point, and a weaponised plasma - by definition, being very, very hot, on the order of millions of degrees - will inevitably expand to impinge on this membrane and will burn through it in a few microseconds.

However, if propelled sufficiently quickly, a polymer membrane could delay the bursting of a plasma charge long enough that the launcher could project it a little way - on the order of ten metres - before bursting.

However, "plastics" aren't necessarily the best material for such a membrane. Graphene has a very much higher melting point - though it burns at a quite low temperature, so it would be necessary to coat it with something to keep oxygen away - and is a good conductor of electricity. By adding a capacitor and arranging the graphene in bands separated by some high-temperature non-conductor, it might be possible to provide some magnetic containment of the plasma that would prevent it from impinging on the membrane for a few microseconds, perhaps even as long as a millisecond, before the charge ran out, the magnetic field collapsed, and the plasma vaporised the membrane anyway, though given the membrane's material, it might last quite a few more microseconds than "plastic" might.

This gain in flight-time of perhaps a millisecond or so would significantly increase the range of the weapon, by a factor of ten, so the "plastic"-wrapped plasma weapon with a range of ten metres might now have a range of one hundred metres when the plasma is wrapped in graphene and accompanied by a capacitor to generate a magnetic containment field.

However, such a weapon would be bulky, heavy and power-hungry, capable of firing only one or at most a small handful of expensive shots before exhausting its power, graphene and fuel reserves, and would either have a violent recoil, or would produce a large back-blast like a recoilless rifle.

The effects of such a plasma weapon would be similar to that of a tiny fuel-air explosive.

Considering that it would be far more efficient to build a launcher for small thermobaric weapons, in that the resultant weapon would be smaller, lighter, much longer ranged, potentially with a greater magazine capacity and a greater destructive effect, I can't see why any sensible military would want to use a plasma weapon such as this, when the alternative with the most similar effect is so much more practical.