Is a somewhat practical rebar crossbow possible?

March 28, 2006 11:10 PM

A crossbow that fires rebar. AND: Superheating the rebar.

First:

In Half-Life 2, my favorite weapon happens to be what looks like a homemade crossbow that fires rebars heated by a lantern battery. Obviously this is pretty farfetched, but would a crossbow that fires the smallest rebar (Imperial #3 at .376) be all that farfetched?
Bolts weighing 10.5 grains per inch are mentioned on this page. This comes out to .0015 pounds per inch, with the rebar being .031333... pounds per inch.

The rebars in the game are longer than six inches by far, but for the purposes of this question assume that the crossbow would be firing six inch rebar.

Secondly:

I'm guessing a lantern battery isn't enough power to heat up rebar enough that it glows (as it does in the game). My knowledge of electricity is somewhat limited, so I must ask: How much Wattage/Amperage/Voltage would it take to heat up some rebar to that point? Assume six inches again.

posted by cellphone to grab bag (22 comments total)

Well, to answer your second question:
First we need to know how hot the rebar gets, lets assume about 1000C (1)
Imperial #3 has a diameter of about a centimetre, let's assume it's perfectly cylindrical, and 6 inches (15 cm long)
This gives us a volume of V =(cross sectional area) x (height)
V = ( (pi)(0.5^2) x 15)
V = 12 cm^3

The density of mild steel is 8 g / cm^3 and so the mass of the bar is about a 100 grammes.

Using the specific heat capacity of mild steel, c = 0.47 J /g deg C, and assuming that the bar starts out at a balmy room temperature of 30 degrees Celcius, we have that the energy that needs to be put into it is:
E = mcT where T is the temperature change.
E = (100)(0.47)(1000-30)
E = 45590 Joules

Let's say a car battery can provide a maximum current(I really don't know about this, anybody care to correct me?) of 200 amps at 12 volts, this is 2400W.

One Watt is a Joule per Second, so at this rate, assuming no heat is lost, it would take about 20 seconds for enough energy to flow to heat the rebar. Of course, this assumes that all that energy is turned into heat, which isn't the case for a conductor like steel... but you could alloy it with whatever they use in space heaters to get a reasonable approximation.

Assuming that you're looking for a power source that can heat your projectile more quickly - let's say a quarter of a second - you'd need a much higher power rating.

For 0.25 seconds, you'd need a power of 200,000 Watts.
That's quite a lot.
at 5000V, you'd need 40 Amperes of current. In theory.
posted by atrazine at 11:51 PM on March 28, 2006

Also, the largest crossbow bolt diameters I see are 5/16' , is that 5/16 of an Inch? If so, you'd probably have to build something custom to fire projectiles that are so much bigger.
And heavier. Hmm. I don't think a crossbow would be a very effective way of propelling 100g of steel come to think of it.

Take into account, that in Half Life, lantern batteries last but a few minutes, even when powering wimpy little flashlight...
posted by atrazine at 11:58 PM on March 28, 2006

atrazine has already covered most of what I was going to say, however I can fill in the few remaining details.

Using the numbers from your crossbow page, a 22" bolt weighing 10.5 grains per inch with a 37 grain nock and a 125 grain head has a mass of approximately 25 grams. Let us assume that a crossbow that fires 6" pieces of #3 rebar stores a comparable amount of energy in its arms when drawn and is as efficient in converting that potential energy to a projectile's kinetic energy as a standard crossbow. A little math shows that the relative veloctity of the rebar compared to a bolt at launch will be (25/100)^-2, or 1/2. In a vacuum the range of the rebar about half of that of the bolt.

With regards to the battery, a reasonable estimate for the amount of energy stored in a high capacity battery of the approximate size of the one in your image (perhaps 6" x 4" x 2") is 72 watt-hours, or about 250,000 joules. If it takes about 50000 joules to heat a 6" piece of #3 rebar to "red hot", then it should be possible to do this with a portable battery, although you'd need a heating mechanism with at least 20% efficiency.
posted by RichardP at 1:14 AM on March 29, 2006

Wow. This is why I love this place!
posted by gage at 2:17 AM on March 29, 2006

this assumes that all that energy is turned into heat

although you'd need a heating mechanism with at least 20% efficiency

heating mechanisms are usually close to 100% efficient. what else is the rebar going to do apart from get hot?
posted by andrew cooke at 4:25 AM on March 29, 2006

The type of battery will also matter. In order to heat up the rebar like this you would essentially be shorting out the battery across the rebar, which would have an extremely low resistance, and hence draw a huge amount of current. But to do this will require a battery chemistry with a very low internal resistance. Lead acid and nickel cadmium are both good choices here, and they are often chosen in cases where brief but very high currents are required. Alkaline, dry cell, and particularly lithium and nickel metal hydride cells will all likely have too high an internal resistance however. This is why battery powered hand tools such as drills and saws still use good old NiCd cells and not the more modern fancy chemistries -- because NiCds can generally survive being shorted for short periods of time, which is essentially exactly what happens in a drill when you pull the trigger but the chuck can't rotate (such as with a stuck bolt or at the end of screwing in a screw.)
posted by Rhomboid at 4:34 AM on March 29, 2006

A real car battery is 12V and has about 50 A-h of charge. Per rhomboid, you'd be unlikely to get it to discharge all at once, but even if you took 20 minutes to do it, you'd only get 150 A, for 1800 W. Even then, I imagine you'd see a pretty severe voltage depression at the end of that cycle. Higher voltage will work better, so you'd need some sort of DC-DC step-up circuit to make it work really well.

Oddly enough, I remember seeing one here... which is twelve kinds of cool itself. Something like that would help accelerate the heating. That one is going from 3V to 54V. You'd want to go from 12V to, well, probably as high as you can get it. 240V or more. Handle with care.

BTW, no normal battery will put out 2400W. A standard electrical outlet normally has a 15A breaker/fuse on it, so sticking your finger in a wall socket only gets you 1800W (in North America). Most hair dryers are close to the legal limit for current draw (1500 to 1800 W), so that's about the best you can hope for in real life.

Something fuel-based would have a much higher energy density. e.g. a cell with high-pressure propane, or whatever gas is used in welding (propane??). Which is why we still drive gas cars - poor battery energy density.
posted by GuyZero at 5:07 AM on March 29, 2006

i don't think conversion to higher voltage would work better (with a battery).

basically, you have a fixed amount of energy - the capacity of the battery. at the end of the day, when you finish discharging, that energy has turned into heat. no matter how long it takes or what the voltage/current. and the only things that are going to get hot are the rebar and the battery (including any voltage conversion) itself (ignoring the environment - the surrounding air, for example - which assumes you do things relatively quickly).

the rebar and battery are effectively in series, so the same current goes through both. so the relative amount of energy dumped in each is proportional to the resistance of that component.

again: you are simply dividing the energy up between heat in the battery/converter and heat in the rebar. to a first approximation, it's that simple.

so to get the most energy possible into the rebar, you need as low an internal resistance as possible in the battery/converter. and you are not going to find much that beats lead acid. adding a dc to dc voltage converter is going to increase the internal resistance terribly, and the end result is that you end up heating the converter, not the rebar.

the best practical fix i can think of is to cut regular notches in the rebar. that increases the resistance at each notch which helps both reduce the instantaneous current and increase the proportion of energy dumped into the rebar. the higher the resistance of the rebar, the better (because you're dealing with a fixed amount of charge - this would not be the case if you were using a mains supply, which is fixed voltage, and where lower resistance would be better).
posted by andrew cooke at 5:29 AM on March 29, 2006

Another aspect of this little project you might want to consider: since you'll more than likely be constructing a custom cross bow in order to handle the unique diameter and weight of the rebar, keep in mind that red hot metals are very good at melting synthetics (depending). So you'll need the cradle for the rebar bolt to be a heat resistant material as well as the cross bow's "string." This could potentially increase the overall heft of the cross bow to a not-so-fun-lifting weight. Perhaps you may want to look into a siege platform as opposed to hand artillery. A ballista perhaps.
posted by Smarson at 5:41 AM on March 29, 2006

Fastest way to heat a lump of rebar with a battery would be to use a big lithium battery and set fire to it.

If you want to make workable flaming bolts of death, make your bolts out of 9mm steel tube with the front inch or two packed with thermite. Use a sparkler or a piece of magnesium ribbon for a fuse.
posted by flabdablet at 6:04 AM on March 29, 2006

Another aspect of this little project you might want to consider: since you'll more than likely be constructing a custom cross bow in order to handle the unique diameter and weight of the rebar

I was thinking about that too. You'd probably want to use a ceramic slot and little thing to actually push the rod, obviously the wire can't get too hot or it will lose tension.
posted by delmoi at 7:17 AM on March 29, 2006

The point of highest resistance is going to be the battery contacts, most lantern batteries have little wimpy springs made of 1/16" wire. Those springs would heat to liquid and melt off before the rebar ever got hot. A car battery might work with its 3/4" lead post, however the acid inside may boil causing it to explode.
posted by 445supermag at 7:32 AM on March 29, 2006

I had wondered this post myself. (sidetrack:) But I had also thought to mimic the insanely high damage onto enemies that this does (eg. going through armor from a distance, spiking them into a wall) it would have to be shot from a cannon.
(/sidetrack)
posted by uni verse at 8:09 AM on March 29, 2006

atrazine writes "Let's say a car battery can provide a maximum current(I really don't know about this, anybody care to correct me?) of 200 amps at 12 volts, this is 2400W."

Some where around there. The winch on my powerwagon can pull 200+Amps off of two batteries in parallel when it stalls.

A deep cycle automotive battery would do the job easy though you'd want to include a current limiter of some sort. Remember that a car battery has about the same internal resistance as your steel bar. I've seen a 1/2" combination wrench that jammed between the poles of a car battery get glowing hot in seconds. Right before the battery exploded.
posted by Mitheral at 8:50 AM on March 29, 2006

assuming no heat is lost

Unfortunately, a red-hot glowing lump of steel in air is going to quickly lose a lot of heat to the surroundings. You'll need to fire off those bolts quickly and consider some kind of insulated quiver to keep them in while heating. A blowtorch might be much more practical than electrical heating, for both speed and portability.
posted by normy at 10:13 AM on March 29, 2006

Why stop at this level of silliness? I suggest using DU bolts.
posted by five fresh fish at 10:41 AM on March 29, 2006

Electrical is going to be way faster than flame if you can supply the energy which something like a car battery can. One of the key advantages of electrical welding is it goes quicker.
posted by Mitheral at 10:47 AM on March 29, 2006

You've got a license for this sort of thing, right?
:D
posted by Baby_Balrog at 7:42 AM on March 31, 2006

If you're going to heat the thing up electrically, and it's a ferrous projectile, why not make a railgun?
posted by phrontist at 6:31 PM on April 1, 2006

phrontist, railgun projectiles must have low resistance, but need not be ferromagnetic. Follow your link for more info.
posted by ryanrs at 8:04 PM on April 1, 2006

What if we impregnate the bolt? Temperature transfer is a function of surface area and time of exposure; perhaps we could implant magnesium strips into the rebar that could be set to dump their stored energy a few seconds before launch. In this situation, fire/electrical stimulation would just be the blasting cap that sets off the actual energy release.

The implanting could be done during bolt manufacturer, or there could be some sort of Just-In-Time embedding of the high temperature material. Thinking about it, magnesium does need oxygen to operate, and I don't know nearly enough about melting temperatures to know whether setting the stuff off would melt the bolt, but the basic idea -- embed something with an exothermic but non-explosive reaction thats set off right before bolt fire -- seems sound.
posted by effugas at 5:20 PM on April 2, 2006

If you shot a piece of rebar without mounting fins or anything else on it to stabilize it, wouldn't it end up pinwheeling in the air? That would transfer a lot of its momentum into angular momentum and throw it way off course, as well as ruining the ballistics of impact.
posted by Mitrovarr at 1:08 PM on April 7, 2006