How to make a slow but powerful futuristic bow and arrow

Question

Obviously, guns are pretty superior to bows and arrows, nearly no matter how good you make a bow (within realistic measures). But in this case, I need a weapon that can have a slow projectile that can be tracked with the eye (it doesn't have to be super slow, but it does need to be traceable by the eye).

I figure a depleted uranium arrow or something similarly dense would be needed to achieve good impact force when a slower speed is required. As for the bow itself, it would need to be relatively easy to draw, yet still have good release velocity. I think that's possible with some pulleys or mechanisms that "activate" at full draw, creating a greater pullback after reaching full draw, but I don't know the specifics.

In any case, I'm wondering the specifics to making a bow and arrow (or similar projectile-launching device) that is traceable with the eyes, yet viable in terms of potential damage. If there's a better solution, I'm open to it. A bow and arrow is preferable, but not necessarily the only viable answer. If more info is needed, lemme know.

Answer 1

As for the bow itself, it would need to be relatively easy to draw, yet still have good release velocity.

This requirement categorically rules out bows (as well as weapons operating under similar principles, like slingshots), because the energy contained in an arrow is entirely supplied by the user's strength. Powerful bows require strong wielders and practice to effectively exert that strength.

There's a technology contemporary to bows that alleviated this.

Crossbows

Specifically, Arbalests get around the issue of being limited to the user's ability to exert force by distributing that force with a winch. Using modern or science-fictional materials, you can have an arbalest with a very favorable gear ratio that can store a tremendous amount of energy while being easy to wind. The downside is that the easier it is to wind, the longer it will take to do so.

However, regardless of how this arrow or bolt is launched, your fundamental problem is that a slow-moving, heavy projectile is not effective as a weapon. Kinetic energy is linearly proportional to mass, but quadratically proportional to velocity. Higher velocities also increase range and accuracy, requiring less compensation for gravity.

For reference, where hunting bows tend to have around 50ftlbs of energy, common 9mm handgun ammunition has around 300ftlbs, giving it much greater range and armor-piercing ability. In order for this slow projectile to be a practical weapon in a sci-fi context, it needs to derive its killing power from something other than its kinetic energy. Thankfully, we already have the perfect solution.

Grenade Launcher

Or a derivative. A 40mm grenade launched from a handheld launcher travels at about 75m/s, comparable to crossbows, while being easy to load and fire and having softer recoil than its caliber would suggest. A variety of projectile types are available, ranging from dumb slugs (training rounds) to high-explosive to HEAT to buckshot to flechettes.

If a single-shot weapon isn't effective enough, try pump-action or revolving. You can deliver a much higher practical rate of fire than a bow, with much greater lethal potential.

Answer 2

You projectile needs to be heavy to slow it down. Your impact will be:

F s = 1/2 m v^2 
F - force
s - slowdown distance
m - mass 
v - velocity

Since we are talking bow and arrow here, we can disregard the slowdown distance (assuming it's always the same, something close to 0), therefore to keep the force the same but lower the speed you need to increase the mass of the projectile. Obviously this becomes untrackable pretty quickly since the speed is squared.

Say you want to cut your speed by a factor of ten, from 10 to 1. F initial = 1/2 m * 10^2 = 1/2 *m *100 = 50m F desired = 1/2 m * 1^2 = 1/2 * m * 1 = 1/2 m

So to cut your speed from 10 to 1 (whatever units), you need to increase your mass 100 times.

Physics dictates you have to move fast to deliver a punch.

Answer 3

It depends on what you are shooting at, but Modern compound bows will meet your needs as stated.

There are a number of things an archer has to consider when doing the bow and arrow thing. Each contribute to how the arrow is going to perform.

First thing everyone thinks of is the bow. Draw weight and draw length determine a lot of how fast the arrow is going to go. Long draw length means the arrow is accelerating for a longer period and Draw weight is how much force is being applied to the arrow. In this question a compound bow (with pulleys) is mentioned, so we can go with that. For a normal human male around 1.8 m tall (6 feet or so) the recommended draw length from a compound bow is about 29 inches. The Draw weight can be anywhere from 15lbs for kids up to 75lbs for burly, manly men. A good guideline would be around a 40 to 50lb draw weight, because that's what is usually recommended for hunting medium to large game like deer. Here is where a compund bow has an advantage though, once you get to a certain point, it "lets off", meaning what it takes to hold the drawn bow is much less than the full draw weight. A 50lb bow with a 70% let off means you only have to hold 15lbs on target creating greater accuracy.

Next, the weight of your arrow. This has 2 influences on what you can do, Momentum and Energy. The Energy is how hard the arrow hits initially, but Momentum is how far that arrow will carry through the target. A light arrow is faster, and therefore has more energy, but a heavy arrow will have more momentum.

Next you have to consider the arrowhead. Simple target points are fairly blunt while Broadheads consist of multiple razor sharp blades. There are a variety of other types but these are the most common. You aren't going to kill a deer with a blunt tip even with a light arrow with a heavy draw, there isn't enough momentum to penetrate. A slower moving heavy arrow with a razor broadhead will cut through deep enough for a clean kill.

You can then combine these things to have a weapons system capable of bringing down Moose, Cape Buffalo, or just about anything else on the planet from a couple hundred yards, at least.

Other answers have mentioned explosives. Add impact or proximity based explosives for even more fun and carnage. These would be lightweight affairs, but they would have enough power to splatter everything in a small sphere. This mitigates the semi randomness a timed fuse might have.

Answer 4

Slings are even less futuristic than arrows.

http://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1168&context=nebanthro

A sling projectile travels more slowly than an arrow. But slings get the job done very well, and rocks are a lot cheaper than arrows. The reference for this table has some good back reading. The table shows the damage done as you increase projectile weight and speed. A normal arrow is about 90 m/s and so the fastest sling speed here was still slower; from the reference normal sling speed is probably 40 m/2.

You could scale it up, with heavier projectiles. With a really heavy projectile you need a counterweight as you spin it up to speed. Hammer throwers use their bodies as the counterweight.

It occurs to me that if you want to throw a very heavy projectile, you might be better off throwing a group of lighter projectiles. The cloud will spread out in flight. It will be less easy to step out of the way.

Answer 5

You get power from speed and mass. If you decrease the speed, you have to increase the mass. This is OK if you are targeting a satellite or other non moving object. In fact the increased mass may mitigate countermeasures since it will take more force to knock the weapon off course.

However, if the target can maneuver and can see the weapon (or just knows it is in danger and is moving randomly), you will likely not hit it.

A slow moving massive object cannot be used in a strong gravity well unless the target is at the bottom of the gravity well. Shooting a slow & heavy from a bow on Earth will result in an arrow hitting the ground not far from you. However, dropping a crowbar from orbit onto a building on the Earth will be quite effective.

Answer 6

You don't need a superior bow and arrow.

You need superior archers.

If you replace the limbs of your commandos with powerful, enhanced prosthetics and enhance their torso with more resistant materials and synthetic muscles, you can end up with archers that put way more power into each shot, using bows with far higher draw strength and heavier arrows.

Alternative, if mutilating your soldiers for the Greater Good(tm) isn't an option, you can go the power armor route:

This has the added benefit being.. not so silly, when compared to battle prosthetics, and also doubling as very powerful armor.

Answer 7

While the question as asked does not seem to have a reasonable answer, I would suggest that (assuming firearms are not allowed) torsion based weapons might be considered rather than tension weapons like bows.

Ancient ballista were designed to transfer the draw energy into skeins of rope or sinew, which could then store the energy for a short time while the crew loaded and aimed the weapon. Ballista could range in size from Roman Scorpions, which would be analogous to a modern crew served machine gun, to massive devices for flinging stones at enemy fortifications.

Seige Ballista

Scorpion

Like crossbows, they have slow rates of fire, given the need to mechanically draw the device, and they eventually fell out of use due to the expense of building them and the need for experienced engineers to build and maintain them (the skeins of rope or sinew needed constant attention, since stretching due to weather or overuse would reduce the performance of the device. It is also thought that the larger devices would be built on the spot using local materials, with the engineers only providing critical metal components for assembling and tensioning the skeins).

While it seems possible to build a torsion powered device the size of a man portable crossbow, it will be very expensive compared to a steel crossbow with a similar draw weight, so it is hard to see what the advantage is at that scale. A scorpion could throw a javelin far longer and harder than any human being, which was the big advantage for Roman armies, but even then, the expense of such devices caused them to disappear from the Roman armoury after the crisis of the Third Century, if not before.

A ballista shrunk to hand held scale

Counterpoise devices like the trebuchet have similar issues with size, speed of action and requiring talented engineering crews to build and operate.

Trebuchet. An example of a counterpoise device

Frankly, the requirement to see the round in flight can be far more easily achieved by using a tracer on the end of the round. Tank crews use tracers to observe the fall of shot when firing APDS-FS rounds at 1200 m/sec, which would seem to meet your need for a DU "arrow"...

120mm APDS-FS round. You can see this in flight as the tank gunner...