Imagine when a jet fighter swoops in to drop a 400 pounds giant chainsaw to you just so that you can use it to grind down one problematic enemy into a pulp then discard it a foot away from you. What power source would suit such a weapon with only 1 instant use before breaking itself apart?
Capacitors. These are electric components which store an electric charge for a limited amount of time and can release it very rapidly. They are relatively cheap and easy to manufacture. They are also reusable, but that doesn't matter in this case.
Capacitors can be manufactured in many different ways depending on their desired characteristics. But in general they have a lower energy density than chemical power cells, but they can release their stored energy a lot faster.
Capacitors are usually not designed to hold a charge for longer than a couple hours, so the capacitors either need to be charged before the mission or during the mission from the fighter's internal power supply.
Launching from a plane, that chainsaw is going to waste a lot of nice gravitational energy, which could be used for much more interesting tasks, such as providing motion to the chainsaw itself.
So, the chainsaw itself is tethered to the plane with a very long roller chain, which is connected to a pinion inside the chainsaw (think of a Beyblade launcher).
When launched from the plane, the chainsaw falls, so that the roller chain makes the pinion start rotating. The pinion will then transmit the motion to the chainsaw, so that it will rotate at full speed when touching ground!
Alternatively, a parachute could open and hold a reel with the roller chain tethered to the chainsaw, which is still falling relative to the parachute.
While the question does not explicitly specify this, I think it is reasonable to specify some, er, specifications.
Extreme energy density. The goal of this weapon seems to be to deliver as much destruction as possible in 15 seconds, in a 400lb package which is probably the weight limit for the weapon stores on the jet. It is desirable that the majority of this 400lb is gnawing teeth of mushy annihilation, and only a small part is fuel and power source.
Extreme reliability. If a air-dropped 400lb giant disposable chainsaw seems like the best way out of a situation, you are probably in deep, deep trouble. You don't want to be yanking a starter cord on a dodgy two-stroke while the Tentacles of Kaza'An the Soul Destroyer of Mag-Uhn squeeze the life out of your fellow unlucky party members. You shouldn't have to carefully lubricate the moving parts of your jammed chainsaw while the Teeth of Brig-Wrath the Extirpator of Zw'n-Thragg dismember your group members. You want to have instant destruction available the moment you receive the weapon. You might even want to be able to use the device on Frizhger the Mauling Shark-Fish, submerged in the Sea of Despair.
Obviously, air-breathing engines are no good, what with the underwater Mauling Shark-Fish (also, the torrential rains on Zw'n-Thragg!). Electrics are really not great either - they require careful isolation from the environment, and frankly, the energy density of batteries is quite terrible - you don't want to run out of power halfway one of Kaza'An's Tentacles.
The obvious solution is a hydrazine turbine engine. Hydrazine is an extremely reactive fuel, that does not need an external source of oxygen. It is used as a rocket fuel (where energy density is key). A turbine engine probably has the highest power-to-weight ratio of any rotating engine, and furthermore it only has a single moving part, which can be connected with a single reduction gear to the saw chain. The reaction products of running hydrazine over a catalyst such as iridium are extremely hot nitrogen and hydrogen gas, the latter of which can also combust once it does come into contact with the outside air.
The end result is a screeching whining chainsaw of death spitting blue flames from its exhaust. Chances are the extirpation days of Brig-Wrath are soon over.
As an alternative to coiled spring you can have your own gyro core (or flywheel).
Make a small and dense wheel to rotate very fast on pins in your chainsaw, virtually without energy loss, and then give it all away to chain blades. After it is discharged it becomes a useless piece of metal without any electronics or stuff. It is also much safer to operate than coiled spring.
If anything it will also stabilize delivery of this chainsaw.
Well if you only need it very briefly I'd recommend a power cell.
Where batteries provide a slow trickle of stored energy over time,power cells can dump all of their energy in an instant. So that chainsaw purpose built to be disposable but very deadly could literally be torn apart from the energy it exerts in its short period of activity.
Also this means that being disposable the chainsaw would be made of cheaper materials. Since the power cell never needs to be replaced as it is a one-off use it can be made non-rechargeable and self-destructing. Not in the way it blows up when used, but it burns out the internal storage from the incredible heat and shock of the energy release.
All in all, the chainsaws shall exist. And they shall be terrifying. Maybe just make the edge out of carbon nano-tubes? Nano-tubes can also be used to store energy within their mass much like batteries or power cells. So you could literally make the chainsaw one giant power cell made of an extremely energy dense but light material that at 400 lbs would have enough power to immolate an elephant entirely most likely.
Your chainsaw would have the power of a bomb, all put towards a few glorious seconds to a minute at most of incredibly destructive power as the blade super heats and the chainsaw loses cohesion as the power which simultaneously kept it together suddenly runs out.
I hope this helps.
Googling "battery in missiles" led me to the page of a company manufacturing and selling such solutions.
I am guessing your solution would be "thermal battery". I do not think they are selling a solution just for you but the duration and power can be tailored to application. Also, such batteries are used by military using similar "mil-tech" would probably fit the back story.
Also such batteries are single use, durable, reliable and have long shelf life.
Some of the other battery chemistries they offer might work better for you, so you might want to check them out. They might be rechargeable though.
The parameters of the problem are:
- Single-use ordnance
- Power-to-weight is a significant constraint
- Brief duration of discharge
- Cheap-ish (we're talking about a disposable giant chainsaw here)
So you're probably looking at a chemical energy source that needs to be converted to mechanical energy. That rules out any high-explosive, and anyway, explosives expend much of their energy in brisance.
Fuels like Otto II driving a turbine are one possibility, if you'd like the system to be self-contained (i.e. not subject the user to exhaust fumes). If that is not a serious consideration (e.g. you are okay with a fire-belching disposable giant chainsaw), then the parameters of the problem closely resemble the energy requirements of a rocket.
I'm partial to a powderized-aluminum + ammonium nitrate mixture, myself.
Well possibly the easiest power source to use would be a battery, you can accurately determine how much ‘life’ it could give to a piece of electronics by setting the max capacity and charging it until it is full.
Although, if your problem is just that you need to get the chainsaws down safely, i would recommend using parachutes or heavily padded drop boxes to ensure the contents were undamaged. At that point you could use whatever fuel you liked as it would not be damaged from the fall.
However, i don’t think you’d want to use chainsaws, especially ones that heavy, as weapons. They are not particularly effective, are slow and 400 pounds is about the weight of two humans. At that point you might as well be using a giant maul instead. Here is an answer where i explained the issues with chainsaw weapons. There is also a link to a youtube video in my answer which demonstrates my point.
Since this is the sort of weapon that Coyote would be using against Roadrunner, it must be powered by the future power supply du jour.
In the 1950s that would have meant nuclear power, nowadays everything is battery powered, though some consider that the future is in hydrogen.
It very much depends on the tone of the situation you're creating.
The clean and neat nature of modern hydrogen fuel cells and batteries may not fit the context, so you might want to find some way of making it spew smoke and fire as it's powered up, that means coal or at the least diesel powered.
While not a chain saw, much of the effect could be delivered cheaply and reliably by a 400lb coiled spring, which has saw teeth etched into the edges. This can be deployed from the plane itself (uncoiling and lashing at the target as the plane screams overhead. Alternatively, the hero can be braced with his back to a tree or a wall and pull out the pin or cut the cord holding the coil closed, it lashes out like an Urumi from hell, striking with considerable kinetic energy and razor sharp cutting teeth.
As you can imagine, there is nothing close to what I am describing. Take this, increase the size to account for the 400lbs of spring steel, serrate the edges and have a cord or pin holding it closd against the spring tension
Considering a 400 lb coil of spring steel would actually be quite long, you would also have a considerable advantage of reach over most opponents, who would either think the plane is too low to drop a bomb, or the hero is too far away from then (and whatever is that drum like object he is holding?), so fail to take proper precautions.
Expanding on Willk's answer here:
You have a "lander" and a chainsaw. The chainsaw is spring-powered. The lander has multiple rotors, say 3, that both break the fall and charge spring. It would basically look like a quadrocopter with a chainsaw at the bottom falling down.
A ratchet prevents the spring from discharging. A trigger mechanism in the chainsaw releases the ratchet and directs its power from the rotor coupling to the saw's chain - the chainsaw is running, purely from the energy from the fall.
Coffman (shotgun) starter
The Coffman starter uses a propellant fit in a shotgun shell, with the standard shotgun primer (blasting cap). It generates up to 15 seconds of pneumatic force to start large aviation and tractor engines.
Famously used on the movie Flight of the Phoenix (both versions).
Adjust size for energy need, but it's just the right energy density, weight and burn rate for the job.
Build the chainsaw with a heavy flywheel. Add a heavy duty piston/foot that the chainsaw lands on when dropped towards you. When that piston hits ground at high collision speeds it spins up the flywheel, which directly powers the chain. You run over and grab it, go to work for your 15 seconds of glory.
No power source beyond gravity and the flywheel needed. Being all mechanical and built tough it possibly wouldn't even fall apart, later it gets picked up and reset for another drop.
If a massive chainsaw is deployed from a jet, it should be powered by Mr. Torgue's specialty. The large weight is needed to contain and direct the C4 detonation inside to spin the chain. Maybe also the back side is stronger than the front, creating the possibility for the chain blade to launch through the enemy.
You don't need a power source.
Humans generally have a terminal speed of 53m/s. Your chainsaw, due to weight and profile, will probably have a greater final velocity.
A 400 pounds piece of metal hitting you at 53m/s will impart you with approximately 2.54 x 108 joules, or ~ 70,555.5 wh. That's equivalent to the detonation of around 60~65 kilograms of TNT. It will turn the target to a pulp just by its kinetic delivery, as well as anyone else nearby.