View Full Version : Which is better for the task? Rocket or Cannon?

11-18-2005, 08:40 AM
In another post (http://boards.straightdope.com/sdmb/showthread.php?t=345050) (which is still active) I proposed a question about propellant in a rocket and also in a cannon. The larger question I have now regards a rescue operation.

Lets say that I needed to rescue a person from another building. I can't walk across the groud (because lets say there are zombies. For this post, I'm going to have to ask people to regard zombies as real) and I can't fly over to this person. The only way I could get to him is by sending over a safety line, having him attach it to his building and shimmy across.

Question: What would work better for delivering the safety line? A rocket or a cannon?

It would need to go atleast 1000 feet but I wouldn't necessarily have to do it multiple times. (The line of sight is very clear and the wind is very low) As long as I was able to go over his head once, it would be fine.

11-18-2005, 08:51 AM
* Is he on top of the building, or do you have to get it through a window on his side?

* Are you on top of the building, or are you inside a room a few stories above/below your friend?

* What kind of cable do you have at your disposal: steel? nylon rope? cotton rope?

11-18-2005, 09:28 AM
Rockets have a decided advantage in being more portable. If there happens to be a cannon on the roof of your building, sure, go for it; but if the munitions are stored in the basement, and the power is out, as it often is in these scenarios, you're far better off lugging an RPG launhcer up 40 lights of stairs than a 300 pound cannon.

11-18-2005, 09:41 AM
A couple of problems with the cannon:
-Paying out the line rapidly; your projectile has only the kinetic energy it acquired at launch; you don't want to introduce any more drag than necessary.
-Keeping the line from breaking during launch; you've got to launch at high velocity in order to impart enough energy to make the whole trip dragging a line, but you've got to make sure that this high velocity launch doesn't simply destroy the apparatus.

With both methods, 1000ft is far enough that you'll have to launch a lightweight line (I want to call it a drogue line, but I'm not quite sure if that's the correct term) and have him pull the proper cable over; I begin to doubt that it is even possible for a human to pull 1000 ft of any strong cable across a void without it sagging to the ground.

11-18-2005, 10:27 AM
Something like this is done in the Navy for underway replenishment (aka UnRep) - basically, refueling at sea. Here's how I recall it:

Two ships steam along side by side, about 150' apart (rather intimidating, until you're used to it). The first step involves a line gun, which uses a blackpowder cartridge to shoot a largish plastic-tipped projectile dragging a small line (about like parachute cord). This line is grabbed by crew on the other ship.

The next step involves using the small line to haul across a larger line (about 1/2" in diameter) called the messenger. The messenger is strong enough to haul across a seriously strong cable, which is attached to a strong point and tensioned by a winch. This cable is then used to suspend a large fuel hose, that can deliver something like 8,000 gallons/minute of fuel.

The Navy line gun has a maximum range of around 500' - something much more substantial would be needed if the separation is 1000'. OTOH, you could start with a very thin line (fishing line?) and work your way up to something strong in several steps.

But Mangetout is right - a 1000' cable strong enough to carry a person would be heavy and almost certainly require a winch to give it the necessary tension. (Note that when tensioned it must carry a load much greater than the weight of the person.)

Paul in Qatar
11-18-2005, 11:54 AM
I presume this OP was in part brought about by my bad-mouthing of rockets in another thread.

A rocket is the only way to go in this situation, as a cannon has to be really big to handle the recoil involved. The rocket is bigger than the cannon shell + propellant charge but is still much smaller than the cannon shell + charge + honking huge cannon.

On the other hand, as other posters have pointed out, in real life situations of firing a rope, people almost always prefer to use a cannon. Why is that?

With a cannon, you shoot it, and the shell goes where you aim it. Sure win, humidity, barrel wear and your zodiac sign makes a difference, but only at long ranges.

Now, with a rocket, a lot goes on. Consider a straight one-stage rocket. You shoot it, it comes out of the tube still on fire, it sets your hair on fire, and then some neat stuff happens. The rocket is still being propelled downrange. It hits the wind and the pointy end points upwind, encouraged perhaps by the rocket's fins. This means the fiery end is pointing downwind. So you can see, the rocket tends to fly upwind.

Now as a practical matter, nobody like having their hair on fire. (Richard Prior: "People get out of your way.") So we will use the Acme Two-Stage Rescue Rocket. Fire it, a small booster rocket kicks the thing out of the tube and burns out in the tube. leaving your hair intact. The second stage ignites at some safe distance, and the rocket proceeds upwind as before.

Consider what that means. It goes out of the tube, it falls down (as it is unpowered), it ignites the second stage, it flies upwind. As I said, sort of a loopty-loop sort of action as you see on the RPG family (well the early ones like the RGP-7 family.) Great fun to fire.

OK, so what else can we do with a rocket? We use a one-stage rocket that burns out inside the tube. All the bazooka sort of things you see in movies work like this. A single, very strong rocket kick the thing all the way to the target, but burns out before it leaves the tube, sparing your hair.

Since it is moving very fast, it does not drop much before getting where it is going. Since it is unpowered, it does not fly upwind. Problem? All the propulsion has to be applied in the short length of the launch tube. The rocket is in fact quite like a recoilless rifle. It can carry any amount of weight you like as long as you make the rocket much more powerful that the motors used in the types I described above.

Anything else? It is near bedtime here.

11-18-2005, 01:03 PM
If the wind is favorable, you could also use a kite. As I recall, that's how the first strand was sent across for the first bridge over Niagra Falls (heavier cables, of course, followed).

11-18-2005, 06:13 PM
Paul in Saudi I'm not trying to angry about your post. I'm just needed to see which would be better for the task.

The original line would be something lightweight. The power it would take to drag a 1000 foot steal line would be huge. We would be at about level with each other. I would want to shoot some kind of cord over and then attach the steel cable. He could pull it, creating tension, that way it wouldn't drag.

So what I'm gettin here is
"Rocket would be good( although its hard to aim) but if you had a cannon, that would be better"

11-18-2005, 06:45 PM
"... but if you had a cannon, that would be better"
You certainly wouldn't need (or want) a cannon if the task is to shoot fishing line 1000'.

Paul in Qatar
11-18-2005, 08:56 PM
Gee Surbey, no offense taken,I hope none was given.

Yes exactly. Since we can see people who do this for a living, like the navy, invests in a cannon despite its weight and cost, a cannon would be better. A rocket is tough to aim, but is lighter.

Stealth Potato
11-19-2005, 12:54 AM
One thousand feet? I guess we should assume that these are tall buildings then; otherwise, the tension needed just to keep the rope from touching the ground would be quite large. The zombies could just pluck you like low-hanging fruit when you tried to cross. I think your best bet is to try to use your hypothetical cannons and/or rockets to clear a path through the zombies. ;)

Bryan Ekers
11-19-2005, 01:33 AM
I actually gave this some thought, and I'll assume the question was inspired by the same source - the recent Dawn of the Dead remake and the characters wondering how to get food to Andy, trapped some distance down the street. I thought if the mall had a hobby store, there should be an adequate supply of hobby rockets they they could launch at an angle, trailing a light line that Andy could use to haul over a series of gradually heavier lines. Andy would have to rig up some kind of winch and I doubt they could ever pass a cable strong enough to hold his weight, but I can imagine small "care packages"of energy drinks and powdered food could be sent, with Andy sending small quantities of ammunition back.

Of course, a well-stocked hobby store might also have fancy-schmancy remote-controlled model airplanes, which might make delivering the first light line even easier. A big enough remote-controlled helicopter might even be able to carry small payloads (this monster (http://www2.towerhobbies.com/cgi-bin/wti0001p?&I=LXLDV7&P=0) should be able to carry a pound or so) without the need for a line. Andy would have to use a series of hand signals to guide the copter in for landing. Ten airlifts a day should be enough to keep him alive and shooting zombies. Andy could take out at least 500-1000 zombies a day, rigging up a comfortable sniper's perch with as much shade as possible to save energy.

For that matter, would it be possible to get a big remote-controlled car, rig up a big wire circle on top of it, lower it to the ground with a string, send it over to Andy's and let him hook the circle with some kind of improvised fishing rod and pull it up? The remote-control would need a big antenna and possibly more power to transmit the longer distance, but any good Radio Shack should have the equipment and batteries for that.

Given the resources of the mall, there was no logical reason to ever leave it and a ton of stuff that could be experimented with to help Andy. Within a month, assuming no rescue ever arrived, Andy and the mall survivors could kill tens of thousands of zombies with methodical shooting. I guess that wouldn't be as exciting a movie, though a time lapse (black screen with fade-in-fade-out "...three months later...") and the fall of the last zombie would've been fun (with an autumn metaphor). The mall people go to get Andy, thinking the streets are safe, and then run into Zombie Wave Two, Electric Boogaloo. Hilarity ensues.