Rockets + gasoline?
July 27, 2008 1:17 AM   Subscribe

What's-the-worst-that-can-happen-filter: Say I douse the protective wadding in a model rocket in gasoline... what's the worst that happens?

I'm not thinking a TON of gas... but I think it'd be a cool mid-air explosion.

Or the ejection charge doesn't go off until it hits the ground, and then explodes on the ground... Bad idea?
posted by disillusioned to Sports, Hobbies, & Recreation (14 answers total) 3 users marked this as a favorite
This is a terrible idea. Gasoline doesn't explode so much as burn, so it would just be a bunch of flaming wreckage coming done. Aside from obvious risks to people and buildings in the area, you live in a pretty dry place, so there would be risks of causing fires elsewhere.
posted by !Jim at 1:24 AM on July 27, 2008

I'm not even sure it would get very far off the ground, though it's been a long time since I flew model rockets. Isn't the point of the protective wadding to allow the rocket to maintain its physical integrity long enough for a flight?

If you're specifically looking to do stupid pyro tricks, perhaps you could run a fuse up through the wadding to some black powder - just make sure the fuse is long enough to give it time to go up, but not so long that it will descend before blowing. On second thought, rockets are finicky enough that it might blow at too low an altitude anyway.
posted by spaceman_spiff at 1:35 AM on July 27, 2008

You want to embed a firework higher in the fuselage, above the engine, with a secondary fuse that is lit by the rocket's engine in flight. Not gasoline.

(Adventurous childhood.)
posted by rokusan at 1:39 AM on July 27, 2008 [2 favorites]

Spaceman, I actually suggested black powder instead.

The purpose of the wadding is to protect the parachute from the ejection charge that ignites.

And yeah, gasoline doesn't explode...
posted by disillusioned at 1:42 AM on July 27, 2008

You can also use powdered artists paint, added above the wadding to add a puff of color to the sky.
posted by Gungho at 5:10 AM on July 27, 2008

Consider the 'law of unintended consequences' and realize that there are infinities of possible outcomes you can't predict.

Personal or community injury are certainly among the worst.

Balance those against the thrill of the event and see if it's worth it.

Rocket engineers spend a lot of effort on making things 'go off' when they are supposed to, and include destructive mechanisms in their products to destroy them in flight if they don't. You are hoping for a good outcome and that's the most thought you are putting into this plan.

Hoping for a good outcome is not science or engineering. It's faith.
posted by FauxScot at 6:05 AM on July 27, 2008 [1 favorite]

Gasoline soaked wadding is a bad idea. The wadding itself is fire resistant and gasoline isn't explosive. The fumes are explosive when they're in the right concentration of air which you probably won't manage to achieve in your model rocket. Most likely you'll end up with a lightweight gasoline doused wadding burning and drifting on the wind.

I haven't used a model rocket engine in ages but I recall that they had a relatively good chance of failure in manners ranging from boring but expensive (because the engine burned through the casing mid flight turning your model rocket into a bunker buster) to exciting but expensive (engine goes boom in model rocket)

If you're really interested in pyrotechnics, which is what you're attempting to do here, don't mix it with model rocketry. Find a pyrotechnics arts guild and learn how to make mortars, shells, bursts, stars etc. They'll have property and protocols that maximize safety and minimize liability.
posted by substrate at 6:10 AM on July 27, 2008

Just to get a little pedantic, here, but gasoline/petrol can be made to be explosive. It's not really about the fuel as such, as an explosion can technically be made with anything that burns - it's about the expansion of the fuel in relation to the ability of that expansion to escape from that which is holding it - a far greater rate of expansion than the means of escape will produce an explosion.

Petrol explosions are perfectly possible, but it is unlikely you will hit any happy result without lengthy experimentation. The most likely result is a vaguely disappointing falling flame, as mentioned. Burying the firework in there sounds like an excellent idea and more inclined to produce the desired effect*.

*VIDEO it...
posted by Brockles at 7:18 AM on July 27, 2008 [1 favorite]

I actually don't even think that the ejection charge on the motor is going to ignite the gasoline.

You definitely want something more like a firework than a molotov cocktail.
posted by Netzapper at 9:04 AM on July 27, 2008

you want to get one of those super sparkly and colorful fireworks (giant bottle rocket or roman candle) and carefully take it apart. make a wick out of gunpowder wrapped tightly in a little bit of tissue paper to form a core. the core sits on top of the engine and the stuff you scavegend from the fireworks goes around. it will look like little balls (that you might have to break in half) and little bits of stuff that look like firecracker fuse.

cheapo rocket made out of a layer or two of paper wrapped around a dowel and glued, you might want to test launch a few times to make sure.

dangerous boy built rockets out of aluminum tubing and launched from under water.... strapped rocket engines to anything that would move, launched them with a piece of coat-hanger heated with a blowtorch.

oh, pool chlorine and sulfur and gasoline will spontaneously combust on you. don't do that, it melts aluminum and you get scars.
posted by zengargoyle at 10:00 AM on July 27, 2008

OK. (Thank God I grew up in the country-side.)

Gasoline is just going to give you a brief fireball that really isn't going to be that spectacular because the rocket's moving to fast, so its kind of a long (~10 feet behind the fuselage at best), thin trail of flames. And that's only when you've marinated the entire freaking rocket in gasoline, not just the internal wadding. You won't get much back but the melted nose cone and fins maybe some still-smouldering wreckage if you do that, however, and as noted above, don't do it when the fields are dry (you should really be conducting these experiments only in wide-open fields, btw, where you can see there is no one around that your stuff can fall on and hurt so they can sue you etc.).

Adding the gas, in a water balloon (easiest to use a turkey baster to fill it, but can still get messy) in place of the protective wadding, works slightly better, in my recollection, but it wasn't so much an explosion as a shorter, brighter burn that started sooner (100-200 foot range maybe). Whatever you do, DO NOT try to attach an external mechanism filled with any kind of combustible fuel, to add to the effect - i.e. duct-taping on additional water-balloons anywhere on the external parts of the fuselage. Even with a lot of work, you're not going to get the aerodynamics down very good, and the reverse effect you have on them will have your rocket going in heavens-knows-what direction, ready to blow at any time, but most likely on impact. Bad idea.

The firework idea is pretty good, the best way to ensure it detonates is to run the fuse internal to the fuselage so that in-flight wind won't extinguish or destroy it. To do this you either need to hollow out the top end of the booster and insert the wick in there, or use a slightly smaller booster in a larger-circumference rocket so that you can run the fuse from the base and connect it to the same fuse that lights the booster. This method generally gives you a higher detonation as the fuse by itself doesn't burn quite as fast as the booster and then the fuse, but that's all based on the type of fuse and how you set up the firework (you can insert it fuse-side up and run fuse up the body of the firework, or upside down and then there's less fuse to ignite it. You need to think really hard about what kind of fireworks you want, as different ones will of course have different effects - variety is the spice of life, and all that. Also, depending on the size of the firework, you may be able to cut out the fuselage and use duct tape to replace that portion of the fuselage with solely the firework, if its tubular in shape and can act as rocket-body on the way up.

Where we really had the most fun though was where we got most irresponsible. I will not go into greater detail than to talk at a high level to both categories:

A) Alternate launching vectors. These involved angular launchpad setups for long-distance low-orbit flight. These take a lot of planning to do right, we generally tried to use the railroad tracks and good aim to keep it in the swath cut for the tracks. These also ventured into the ballistic use of practically perpendicular flight, generally towards the back of the barn, although there may have been a couple of other times that it was aimed at...something else.

B) Multiple boosters / varying booster volume to overall fuselage size. This is where we really focused our experimentation. I recommend starting with the cluster formation for some really impressive initial blast-offs, if your timing is right. You do the standard internal booster and then boosters attached externally between each tail fin, and then fuse them all to the same primary fuse. If you don't get the timing right, you're going to have serious drag on the side that doesn't light and will get an arching effect. You can play around with the number of boosters but I'd recommend starting small and aiming for aerodynamic balance at the outset. Once you've played around with the clusters, you can always due the multiple-internal, where you fuse a second (or even third) internal booster just above the primary one, through the top of the primary (similar to the firework method mentioned above). The internal method will usually result in burning the tail section off so watch out for that sucker when it's coming down sans-parachute.

If you get really good at method B, you'll have rockets leaving visibility, even if you're trying to track them with binoculars (which is kind of needle in a haystack, even for the bigger rockets, anyway). Take that into consideration, as well as weather conditions - that is, what wind factors are likely to be for every 500 feet you climb, etc.. You might not retrieve all your launches, but then if you were concerned with that than you wouldn't have asked this question. It got to be kind of a fun game for us, seeing how big of a fuel-payload we could send up and still find some evidence of the craft returning to land.

Now that I'm an adult I would probably add that you should keep a fire-extinguisher on hand. Once you get some nice launch combinations that you like, be sure to try one at night, it really adds to the effect. Best to do this in winter...and don't forget that its harder to see descending wreckage in the dark.

If I was a kid nowadays, I'd definitely be experimenting with delivering some sort of GPS / RFID tracking device via a ridiculously-propelled rocket and seeing if you can track it down electronically afterwards, I'm sure there's the low-cost tech out there available for that by now.

One last tip - you can save a lot of money going with the lower-cost cardboard tubing that can be found at a lot of craft stores. Don't burn all your money on the tubing that hobby stores sell, those are designed for custom assemblies and multiple uses. If you do this smart you can use the same nose-cone and tail assemblies multiple times on different tubings, assuming you can find them after each launch.

I think that's pretty much it. Have fun.
posted by allkindsoftime at 10:26 AM on July 27, 2008 [2 favorites]

Also. IANAModelRocketScientist. I am not recommending you try any of this. The answer to your original question is that something you are responsible for is falling from the sky at an extreme rate of speed, or on fire, or both. Do the math with the possible things it could land on in a liberal circumference of calculation, and what effect it might have on such things (dry fields, neighbors' roofs, moving vehicles, livestock, a gas station, etc. etc. etc.). You're probably the only person that can answer your original question, and you can probably only answer it when the worst is actually happening.

Experiment accordingly.
posted by allkindsoftime at 10:38 AM on July 27, 2008

You could follow my method for my rocket in jr high physics - substitute the protective wadding with kleenex. Guess it depends on the size of your rocket, mine couldn't have been more than a foot long.
posted by Orrorin at 12:42 PM on July 27, 2008

OK my PETA-filter got the better of me, so I'm back.

We sent a mouse up, in an enclosed container above some wadding, so he wouldn't get burned. This was one of the rare rockets that we did consider the parachute-deployability heavily into our calculations.

I can report that he safely returned to earth and Squeek Armstrong was released into the barn from whence he came. I will admit nothing further on this thread.
posted by allkindsoftime at 1:26 PM on July 27, 2008

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