Mythbusters' Rocket

[hinote]

Did anybody else see "Mythbusters" last night, on Discovery Channel?

They put together a hybrid rocket, using paraffin and nitrous oxide.

Now, I'm not a rocket guy, but it sure looked like an easy combo to work with. In particular the paraffin (which they mixed graphite into, to act as a buffer) would be very easy to cast into the necessary shape--and is not poisonous, volatile or expensive.

In addition the NOx would be easy to tank and supply at the correct rate, for most efficient oxidation. Also available OTC at auto performance centers, at competitive prices.

I'm sure there was lots of experience and sophistication that wasn't made evident during the hour-long program (about 1/3 was commercials) but it certainly seems like a viable alternative to this PJ guy.

Bill H.
Acoustic Propulsion Concepts

".......some day soon we'll be flying airplanes powered by pulsejets."

[hinote]

Hybrid rockets are very simple, which is why they've seen such an upsurge in recent years. I'm currently working on a partly reusable/partly disposable hybrid engine, made almost entirely out of plastic.
.

Presumably, the recent success of Spaceship One (powered by ground-up tire rubber/NOx) belongs to the same family. Maybe this is a window of opportunity to develop a safe and cost-effective rocket concept.

Personally, I'm chicken.

The pressures necessary to create an efficient rocket engine are too dangerous for a hobby-level participant, IMHO.

Pulsejets, OTOH, operate on very low levels of pressure--at least until somebody develops the compression augmentation concept.

Bill H.
Acoustic Propulsion Concepts

".......some day soon we'll be flying airplanes powered by pulsejets."

[ed knesl]

Hybrid motors are lot safer than solid boosters.
Almost any solid combustible matter can be used as a fuel grain.
I.e. rubber or paper tubes, standard ABS and PVC plumbing drain
pies, asphalt - just name it., high density PVC is the best.

I helped to launch 8 ft tall rocket on ABS 3 inches pipe and it was
a sight to remember... Beautiful purple flame, long burn and very
realistic slow lift off.

I am working on easy to use system, different from hobby outlets
available types, mostly using sliding piston that has number of
difficulties.

Ed

[Mark]

Going off on a tangent, I was watching Myth Busters a few weeks ago and they speculated whether a fully pressurized steel scuba tank would explode if shot in the bottom with a bullet which in fact ripped a neat hole in the bottom. Well it didn't exoplode and then they shot another new tank in the side of the tank the second time with a rifle and remote triggering. The tank flew around the concrete reinforced room in a strange way but it didn't explode.
I imagine it might be fun to strap a tank to a backpack and then have rifle shoot a hole in the bottom of the tank and then you could fly like Buck Rogers. Minor details to be worked out.
Mark
http://www.pyramydair.com/blog/2005/10/ ... art-1.html

[hinote]

Yes, the pressure can be scary. But hydrostatic testing is not expensive or complicated, so the parts can be tested and shown to be safe before they ever come into contact with the nitrous. And the entire thing can be fueled and launched from a distance, if you're particularly paranoid.

Kind of reminds me of my steam car days. I used to drive around town in my steam car, feeling perfectly safe. The operating pressure of the superheated steam could be adjusted, and we occasionally ran at 1200 psi.

Of course, we had hydro'ed the coil stack at something like 2.5 or 3x (I forget after 28 years)-and besides, we knew that boiler failure would only result in a minor tubing perforation. I actually experienced one, after a control failure.

In knowledge there is power--and safety.

Bill H.
Acoustic Propulsion Concepts

".......some day soon we'll be flying airplanes powered by pulsejets."

[paul skinner]

Jeff Jakobs hybrid motors are probably the most impressive on the market for amateurs right now

http://www.rockethigh.com/shopping/enter.html

(I've posted the link before).

His 150mm hybrid produces 1,500 to 2,000 pounds of thrust (10 sec burn time). Enough to launch a moderate space vehicle, if you're so inclined.

There's also an excellent article at Wikipedia regarding hybrids, and several links to companys producing them in quantity.

http://en.wikipedia.org/wiki/Hybrid_rocket


I started with the Rene Caldera design, and have launched it probably close to a hundred times. Rene shut the business down in '02, but it carries on at

http://groups.yahoo.com/group/microhybr ... d=94748789


**it's nice to be able to edit~ You can scale the Caldera design way up. It works really well with the large NO2 cylinders used in paintball.

[Greg O'Bryant]

I have a question about hybrids because I was thinking of making one too. From what I have seen they seem to be core burning engines,right?
Anyway if I had a tank of nitrous set at a certain flow rate are there any problems encountered as the engine burns and changes the area of the burning surface of the grain? Do you need to somehow increase the amount of nitrous as the engine burns to keep up with the increased grain area or is the burn more preasure dependent, or perhaps there is a simple trick to over come this problem like a grain that is burned from the inside-out and the outside-in at the same time?

[Greg O'Bryant]

Thanks Ben; so I guess you would just make shure that the ratio of grain to nitrous wouldn't be too lean at the start and it should be fine? makes sense

[Eric]

I saw most of that show.

I dont know if they were intentionally trying to sabotage the project so people wouldnt go building big 200 pound rockets that could fly a considerable distance, but didnt their rocket nozzle look horribly horribly wrong?

From what I saw when I was returning with food it looked like they basically just drilled a hole through some graphite with absolutely no expansion.

So many times they make these huge elaborate things, and then screw it up on such a basic level.

Eric

[Anders Troberg]

Kind of reminds me of my steam car days. I used to drive around town in my steam car, feeling perfectly safe.

Could you please tell us more about this?

I've been thinking about steam cars since I noticed that Swedish law defines a motor vehicle as a vehicle with an electric or internal cumbustion engine. That should mean that a steam or sterling car should fall straight through the loopholes in most of the traffic related laws, including such details as speed limits, tax requirements, annual safety inspections and so on. Kind of cool in some discreet, low level outlaw kind of way...

[hinote]

Kind of reminds me of my steam car days. I used to drive around town in my steam car, feeling perfectly safe.

Could you please tell us more about this?

I've been thinking about steam cars since I noticed that Swedish law defines a motor vehicle as a vehicle with an electric or internal cumbustion engine. That should mean that a steam or sterling car should fall straight through the loopholes in most of the traffic related laws, including such details as speed limits, tax requirements, annual safety inspections and so on. Kind of cool in some discreet, low level outlaw kind of way...

Steam power is fun--but not very efficient. It's also complex, when you try to install it on a motor vehicle with automatic control and recycling of the condensate for feedwater.

Here's a link to the best of the current modern steam car efforts:

http://www.firedragon.com/~kap/Barrett/

--this has taken years of development, but it's reliable and probably the most fuel efficient unit on the market.

I'll be happy to answer any specific questions you may have.

Bill H.
Acoustic Propulsion Concepts

".......some day soon we'll be flying airplanes powered by pulsejets."

[Anders Troberg]

Phew, that was a bit beyond my capabilities. Still, very interresting reading!

[MJD]

Presumably, the recent success of Spaceship One (powered by ground-up tire rubber/NOx) belongs to the same family. Maybe this is a window of opportunity to develop a safe and cost-effective rocket concept.

Personally, I'm chicken.

The pressures necessary to create an efficient rocket engine are too dangerous for a hobby-level participant, IMHO.

The SpaceShip One did not used ground up tire rubber, it used a cast grain made of HTPB - hydroxyl-terminated polybutadiene - which cures with isocyanate curatives. This is the same system used as the binder for most composite solid rocket propellants.

Paraffin grains are cute and seem to be the soup de jour these days for tinkerers - Stanford was messing with them recently. One of the tricks is melt casting large diameter grains with all the shrinkage problems. Cast rubber is much easier in that regard.

Typical nitrous oxide pressures in the flight tank of hobby hybrids are 700-1000 psi. With the pressure drop through the injector the chamber pressure usually peaks at 125-200 psi below the initial tank pressure. The pressure drops continuously through the burn too. These are not scary pressures whatsoever especially for smaller diameter motors. The nitrous oxide tanks are required to be designed for a minimum of 3000psi and are required to be hydrostatically tested to 2000 psi. The motor chambers are good for wayyy over 1000 psi. A 1-1/2" OD 6061-T6 aluminum tube with 1/16" wall for example has a yield pressure well over 2000 psi.

The other factor is that with rocket motors, unlike hobby jet engines, you don't ignite them while you're standing next to them - they are filled and fired remotely. So with the use of primarily ductile materials and remote filling and firing, safety is fine. Probably safer than starting a large pulsejet up close, esp. on gasoline.

MJD

[WebPilot]

Please don't try anything that you are about to see us do, at
home .... ever!

This is the Hail rocket launch, edited from the vid.

The file is a .avi and requires the DivX codec for viewing.

Episode is a cool 42 minute watch, but they "bleeped" the actual constituent how-tos from time to time.

-fde

[Jim Berquist]

Just a nubee Q:

If the gain is toasted from the inside and out side ? What stops it from becoming lodged in the nosel and creating a bomb???

Dose it float in the gas stream?

I too don't like the boom thing!!