18 replies to this topic

[SirBlastalot]

So my friends and I decided that having an actuall rocket-launcher would be cool, instead of the nerf rockets. (That are not technically rockets, just extra-large darts.) We're going to make some large, styrofoam rockets and stick a C02 cartridge in the back. The thing is, we lack the mathematical know-how to figure out how fast one of these rockets would get going. Would anyone with a physics background be willing to help us out? My dad calculated that the CO2 will be expanding from about 50ml to around 5L. Anymore variables you need me to observe, just let me know

I wouldn't normally post this here, but before I started I wanted to make sure this isn't going to be going so fast as to be dangerous.

[flameboy]

It depends on how big/heavy the missile is if it's like heavy sort of then it will probably fly pretty good if it is kind small and light then it is probably gonna hurt someone

[CaptainSlug]

Too much mass will be flying around and it would be very dangerous. Definitely a bad idea to try making your own rocket launcher using a pressurized vessel as the projectile.
Bad idea.

Edited by CaptainSlug, 04 June 2007 - 09:33 PM.

[Prometheus]

SirBlastalot, on Jun 4 2007, 03:22 PM, said:

So my friends and I decided that having an actuall rocket-launcher would be cool, instead of the nerf rockets. (That are not technically rockets, just extra-large darts.) We're going to make some large, styrofoam rockets and stick a C02 cartridge in the back. The thing is, we lack the mathematical know-how to figure out how fast one of these rockets would get going. Would anyone with a physics background be willing to help us out? My dad calculated that the CO2 will be expanding from about 50ml to around 5L. Anymore variables you need me to observe, just let me know

I wouldn't normally post this here, but before I started I wanted to make sure this isn't going to be going so fast as to be dangerous.

Are you gonna shoot the cylinder to make it go off? Also, P1V1=P2V2, where P is pressure, and V is volume.

[anders]

SirBlastalot, on Jun 5 2007, 01:22 AM, said:

So my friends and I decided that having an actuall rocket-launcher would be cool, instead of the nerf rockets. (That are not technically rockets, just extra-large darts.) We're going to make some large, styrofoam rockets and stick a C02 cartridge in the back. The thing is, we lack the mathematical know-how to figure out how fast one of these rockets would get going. Would anyone with a physics background be willing to help us out? My dad calculated that the CO2 will be expanding from about 50ml to around 5L. Anymore variables you need me to observe, just let me know

I wouldn't normally post this here, but before I started I wanted to make sure this isn't going to be going so fast as to be dangerous.

Have you figured out how to open/activate a Co2 cartridge? And are you going to use a 12gram regular Co2 cartridge or a bigger paintball type of tank?

I made a rocket by drilling up the release hole on a empty one and filling it with gunpowder.

[SirBlastalot]

It's a little 12g tank. The rocket will be around 600-700g of styrofoam, somewhere around 8 inches long and with a diameter of 4in. We can, fairly easily, make a device similar to the air-restrictors in stock nerf guns, except on the back of the rocket. Playing with that and the aerodynamics should be able to get it down to a resonable speed, but I was hoping someone would happen to have a convienient formula I could use.

Also, Prometheous; I've already calculated all the PV=nRT stuff and whatnot, but I need to figure out something like the velocity, force, work, etc of the gas coming out of the end of the cylinder, which I can't do after just taking a chemestry class. I'm taking high-school physics and calculus next year, so I'll be able to figure it out then, but I'd rather not wait till next summer.

Capt. Slug:I'll keep your view in mind. And don't worry, I'm not about to do something really stupid, like fire the first test at myself or something.

Even if this dosen't work out for Nerf, it will still be like, model-rocket fun.

[bpso86]

Well, off the top of my head there's:

Bernoulli's equation:

P + 1/2*rho*V^2 + rho*g*h = constant.

In your case, it's safe to assume that there is no potential energy quantity (rho*g*h), so it's only the first two items. There are assumptions that go along with this, but I think we can safely and generally apply the concept. Basically, you'll need the pressure inside the CO2 tank, the density of air and the atmospheric pressure. Using this, you'll be able to solve for the velocity of the exit air. When you do solve for the exit velocity of the air, you can use the mass of the air (volume of CO2 canister and apply specific volume of air) to calculate the kinetic energy of the air, which can subsequently be used to calculate the kinetic energy of the rocket (1/2*m*V^2) by equating the two kinetic energies and solving for Vrocket.

Cliffs:

Grab pencil/calcumalator/physics book
Write down Bernoulli's equation
Lookup CO2 tank pressure, density of air, atmospheric pressure
plug/chug, solve for air exit velocity (equating values pre-release and post-release)
use air exit velocity to find K.E. of exit air (with mass of air)
use K.E. of exit air to find Vrocket (K.E. = constant)

[Prometheus]

Okay, here is the most basic version of the amount of force you will have (* means multiply). Pressure * surface area = force in pounds. Convert to kilograms by dividing by 2.2 . Force in kilograms * 9.81N/kg (gravitational constant), gives force in Newtons. Measure mass of rocket in kilograms. F=ma will give you the acceleration of the rocket. Minus about 10%, to somewhat consider friction. Anybody find any flaws in this? I'm looking over it, and it seems okay.

[Jergling]

Before you attempt this, I will warn you (Yes, CS is right, this is altogether a BAD idea, but...)

The force required to make this thing shoot horizontally would be nearly impossible. With spiraled fins, shooting it at the desired target would require so many calculations that by the time you had it positioned to coordinates, your target would be hundreds of feet away.

Military rocket launchers have enough power and flight technology to shoot on a horizontal plane. Real rockets that are more advanced need room for computers, and so those are the ones your would have to model after.

Non-horizontal, hellfire rocket launchers use a precisely delayed timer to first shove the rocket lightly out of the tube, and then (the gyroscopes and timers work here) it waits until just the right moment in falling out of the tube so that it's angled upwards and launches from there.

There is almost no way to have that kind of gyroscopically delayed rocket made of foam. Even if you tried to make it launch horizontally, the weight of the rocket and CO2 tank together would stay in balance to keep it from simply dropping out of the barrel (and maybe drifting five feet)

And, yes there's more, it's really dangerous. Not a particularly good idea to be playing with unregulated co2 tanks.

(I thought about this at one point too, I just don't think it's safe or practical to do)

[bpso86]

Prometheus: Yes, I found flaws. First of all, the pressure force is always changing as the air is exiting the chamber. Less air in the chamber=less pressure, so he would have to use an integral to calculate it your way. Therefore, force is also always changing. And he's asking for the speed, not acceleration.

Jergling: What the F#$k are you talking about? Do you remember how rocketry first started? People putting explosives in containers and adding nozzles. No computers needed. Hell, the Chinese had a crude form of rocketry down to a science much before most of the modern world knew what they were. In WWII, aircraft (particularly P-38 Lightnings) carried rockets that were basically an engine attached to some explosives. There was no guidance system, and there was no need. How do you think those estes model rockets fly straight? I built a rocket out of a paper towel roll, the end of a plastic easter egg and cardboard. Trust me, it's possible. And, I can't understand half of what you're saying buddy. You should do some research and maybe go build and fire a rocket before telling someone else that they can't do it.

[SirBlastalot]

That last paragraph was well stated, bpso86...I'm not sure I would have been as civil. No offense Jergling, but that post didn't make much sense at all.

As for your orinal equation, bpso86, what are each of the variables there? That is, whats r, h, o, and P?

[bpso86]

My apologies. P is the Pressure in whatever control volume you are looking at. RHO is a greek letter commonly used to denote the density of a substance.

[Prometheus]

bpso86, on Jun 5 2007, 02:29 PM, said:

Prometheus: Yes, I found flaws. First of all, the pressure force is always changing as the air is exiting the chamber. Less air in the chamber=less pressure, so he would have to use an integral to calculate it your way. Therefore, force is also always changing. And he's asking for the speed, not acceleration.

Tru enough, I overlooked that. Also, a*t=v , as int acceleration muliplied by time, assuming you have somewhat uniform acceleration, will give you a rough speed. Although, this will be a bitch to calculate. I'd just say, it will go fast. Faster that some bottle rockets that I've seen launched.

Hey, you'd need a reinforced pressure chamber, as the intense pressure could cause some damage, thus rendering the rocket useless, and an altered projectory.

[bpso86]

Prometheus, on Jun 6 2007, 11:27 AM, said:

Tru enough, I overlooked that. Also, a*t=v , as int acceleration muliplied by time, assuming you have somewhat uniform acceleration, will give you a rough speed. Although, this will be a bitch to calculate. I'd just say, it will go fast. Faster that some bottle rockets that I've seen launched.

Hey, you'd need a reinforced pressure chamber, as the intense pressure could cause some damage, thus rendering the rocket useless, and an altered projectory.

Yes, a*t=v, but as previously stated, the acceleration is constantly changing. You also do not know how long the thing will be in the air.

And where would you need a reinforced pressure chamber? The air is leaving the CO2 cartridge (which damn well better be able to hold the pressure) and consequently instantly ejecting the styrofoam rocket from the launch tube. No reinforcement necessary. That last statement doesn't make much sense to me.

Oh, and just so everyone knows, I'M NOT ADVOCATING THE CREATION OF A NERF RPG. I'm just saying it's possible, but in no manner of reality would I ever do it. Well, maybe. But I wouldn't shoot it at anyone.

[SirBlastalot]

I think Prometheous assumed that the back of the launch tube will be closed...a resonable assumption, given that this is a Nerf forum, after all.

I need to know what units these need to be in, as well.

So what I have so far is this:



Now, I know something's wrong here. Help?

[bpso86]

Okay, here goes.

Units need to be in either metric or english. I prefer metric. PSI=pounds per square inch, an english unit. Convert that to N/m^2 (Newtons per meter squared) for metric. Or, you could convert the kilograms/meters into pounds and inches. Up to you.

Now, since your initial state is going to be just potential energy in the form of pressure, you have no initial velocity. All that should be on the left side of the equation is your 422psi.

Once the air is expelled, you have the final velocity of the air adding a pressure force (which is what we're dealing with) along with atmospheric pressure.

Basically, your initial pressure turns into atmospheric pressure and pressure from the air exiting. So, just change your units, get rid of the second term in your initial state and you should be good.

Keep in mind this will give you the exit velocity of the air, which isn't the velocity of the rocket.

And for some reason, I keep getting weird numbers when I work it out. Let me try again in a little while.

[Prometheus]

bpso86, on Jun 6 2007, 08:37 AM, said:

And where would you need a reinforced pressure chamber? The air is leaving the CO2 cartridge (which damn well better be able to hold the pressure) and consequently instantly ejecting the styrofoam rocket from the launch tube. No reinforcement necessary. That last statement doesn't make much sense to me.

Oh, and just so everyone knows, I'M NOT ADVOCATING THE CREATION OF A NERF RPG. I'm just saying it's possible, but in no manner of reality would I ever do it. Well, maybe. But I wouldn't shoot it at anyone.

I sure hope the cartridge will hold the pressure, but the immense pressure being released, as well as the super-low temperature created, might cause some damage to the foam. I am not totally sure, as I don't normally worry about stuff like that. So, I was thinking a small insert (similar to a thimble) should be used, to help reduce damage, if any would occur.

[bpso86]

Oh, I see what you're saying now. Put some reinforcement between the co2 canister and the foam, not to reinforce the mechanism that is launching it. So the co2 canister won't shoot through Styrofoam. Gotcha.

[Prometheus]

bpso86, on Jun 6 2007, 05:05 PM, said:

Oh, I see what you're saying now. Put some reinforcement between the co2 canister and the foam, not to reinforce the mechanism that is launching it. So the co2 canister won't shoot through Styrofoam. Gotcha.

Exactly. We don't want a Nerf-version of the Challenger launch...