Rrerku

Giant coiled spring.

It would uncoil itself and drive the chain. It would provide loads of power on demand. Storing that kind of energy as a coiled spring is dangerous, but I see that as a positive thing for your scenario.

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.

Pyrotechnics, in the form of twin spinning rockets rotating the saw axle. You have jet fighters, missiles would look like an obvious option. And as a bonus, a faulty device would explode throwing shrapnel into the enemy.

I would go for a pneumatic solution. It can support easily a 40 pound high pressure container storing sufficient energy for 15 seconds.

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.

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 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.

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.

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.

I would go for a chemistry based solution. Think of current engines; they are all based on the concept of explosions. Now think of a giant explosion happening on release of the chainsaw. It would create enough pressure to power the chainsaw for quite some time.

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.

Dual liquid explosive, the force would trash the mechanism but its a one use weapon.