Solid oxidiser, liquid fuel hybrid rocket.

[shynung]

That's for you to find out, unfortunately. Most amateur rockets that use KNO3 mix it with sugar rather than silicone sealant. Hybrid rockets are also commonly designed with solid fuels and liquid oxidizers, rather than the other way around.

So, what you're making is a kind of reverse-hybrid rocket. Which AFAIK no one has ever developed before, at least none that got through my news feed. You're practically in an uncharted territory here.

But don't let that stop you. Go on with your project; find the optimal ox/binder ratio, then the ox/fuel. Then burn away.

After that's done, It'd be wonderful if somebody else tried it with hydrogen and PTFE. Not as a binder, but as the primary oxidizer.

[Frozen_Heart]

Luckily the ox/fuel ratio can be worked out with some chemistry calculations. Time to dust off my text books to remember how its done.

I'm guessing the less binder the better to allow the oxidiser to react. Too little binder though and it won't hold together.

In rocket candy its simply mixed at the correct ratios to react. The fact that the sugar acts as a binder is just a convenient side effect.

[sgt_flyer]

the only homemade reverse hybrid rocket engine concept i've seen used compacted ammonium nitrate protected by a thin plastified nitrocellulose layer (to prevent water from ruining the ammonium nitrate - the nitrocellulose will completely combust with almost no residue during ignition) with LPG as fuel. - though he didn't gave much more informations beside that. here you use KNO3, so i don't know if simply compacting the KNO3 together like he did with ammonium nitrate would be enough.

[shynung]

Luckily the ox/fuel ratio can be worked out with some chemistry calculations. Time to dust off my text books to remember how its done.

I'm guessing the less binder the better to allow the oxidiser to react. Too little binder though and it won't hold together.

In rocket candy its simply mixed at the correct ratios to react. The fact that the sugar acts as a binder is just a convenient side effect.

Ox/fuel ratio depends on the combustion temperature you're aiming for. Basically, the lower the O/F (more fuel), the cooler it burns, easing the hardware requirements at the cost of energy produced. If a high performance motor is the objective, you'd run it close to, or at, the stoichiometric O/F. Though, that'll put more stress on the hardware, so you'd probably need a sturdier casing.

EDIT: I messed around with an online calculator, and found this reaction:

4 KNO3 + C3H8 = 2 K2CO3 + CO2 + 4 H2O + 2 N2

Don't take my word for it, test it before attempting an O/F of 4.

Edited December 7, 2014 by shynung

[lajoswinkler]

Hmm these google searches for these various chemical reactions are likely to get a knock at the door from men in suits by themselves...

No, it wouldn't. Don't exaggerate.

[Frozen_Heart]

KNO3 + C3H8 = 2 K2CO3 + CO2 + 4 H2O + 2 N2

None of those byproducts seem nasty so that is good.

[lajoswinkler]

I can get potassium nitrate very cheaply and easily. I can't use it for solid motors because they aren't legal in the UK. That why I'm going hybrid.

If you don't live in the city, and have easily accessible (not by highway or far away) wide, spacey meadows, why not? Who will see you? Honestly, why would you care about that?

[Frozen_Heart]

If you don't live in the city, and have easily accessible (not by highway or far away) wide, spacey meadows, why not? Who will see you? Honestly, why would you care about that?

Because if I have a non illegal option that also provides a fun development challenge why would I just make a solid. I'm also doing this for the engineering challenge rather than just what sets on fire fastest.

[shynung]

Any progress so far? No explosions, I hope?

[Frozen_Heart]

Any progress so far? No explosions, I hope?

Got the silicone and KNO3. I'm sorting out the workshop atm to get some useable working space. Once I've done that I can assemble the fuel / combustion chamber. I also need a connecter to go between the propane and the combustion chamber.

Not bothering with a nozzle yet. Simply testing to see if it even works.

[SmartS=true]

Ethanol/KNO3 might work, but this is just a quick, educated guess. It also probably would be inefficient.

[K^2]

Got the silicone and KNO3. I'm sorting out the workshop atm to get some useable working space. Once I've done that I can assemble the fuel / combustion chamber. I also need a connecter to go between the propane and the combustion chamber.

Not bothering with a nozzle yet. Simply testing to see if it even works.

Just another reminder that it will explode. Ideally, you'll eventually figure out how to stop it from happening, and you might even get lucky on the first run, but your assumptions going in should be that it will explode. Make sure your casings aren't metal and in general, try to minimize use of metal/ceramic parts. Replace with plastics wherever possible.

I really hope you don't have to worry about blowback with this one, but just in case, Make sure your propane tank is in no danger of injuring anyone if it also blows up. Preferably, dig a hole in the ground for it. You'll need a remote way to close the valve. Then make sure that anyone in the viscinity has good cover an no line of sight. If you need to see what's happening, your best bet is probably a cheap web cam and a laptop.

Goggles, gloves, clothing that covers all of you (no synthetics), and something to put out fires. At very least, a wet towel.

[Frozen_Heart]

I really hope you don't have to worry about blowback with this one

Surely there isn't any oxygen inside the propane tank?

Also pretty much every rocket engine I've ever seen is metal, including hybrids. Why can I not make the nozzle and combustion chamber out of them?

I've got large metal screens and cameras I can set up first. I was planning to stay well out of line of sight.

Btw this is not going to be a powerful rocket. A couple of kg of thrust at the very max.

[K^2]

Surely there isn't any oxygen inside the propane tank?

There shouldn't be. At least, not enough for sustained reaction. But if heat causes the ox to powder, and blowback pushes some of that back, who knows? The more realistic scenario is pressure simply causing the valve failure on the tank, turning it into a missile. Either way, likely or not, I would not want to take chances with this. If you can burry your fuel tank during tests, it's probably a good idea.

[Technical Ben]

Is sugar not used at times? Candy... Toffee... yum!

http://youtu.be/12fR9neVnS8

(The "Kitty Litter" less Yum)

[Tommygun]

You might want to go to a welding shop and see about getting an inline flashback arrestor for your propane line.

They're used for Oxy-fuel welding setups, but I think it might help.

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

[Frozen_Heart]

Is sugar not used at times? Candy... Toffee... yum!

That's for solid propellant which is illegal in the UK.

You might want to go to a welding shop and see about getting an inline flashback arrestor for your propane line.

Cheers. I'll look into that.

[Technical Ben]

Ah... I knew it would fit the "solid" part. Sad it's "illegal" as I eat the stuff (Toffee, not rockets!) all the time.

So what makes this build "solid"?

[shynung]

Ah... I knew it would fit the "solid" part. Sad it's "illegal" as I eat the stuff (Toffee, not rockets!) all the time.

So what makes this build "solid"?

If the fuel and oxidizer is in the same solid mix, it'll burn to completion by itself. Frozen_Heart said UK banned those rockets for amateur projects, and plan to get around the ban by separating the oxidizer and fuel. As per his previous posts, the oxidizer will be potassium nitrate, and the fuel will be propane, presumably gaseous.

[Frozen_Heart]

If the fuel and oxidizer is in the same solid mix, it'll burn to completion by itself. Frozen_Heart said UK banned those rockets for amateur projects, and plan to get around the ban by separating the oxidizer and fuel. As per her previous posts, the oxidizer will be potassium nitrate, and the fuel will be propane, presumably gaseous.

^This

Also its a lot more of a fun engineering challenge to do it this way rather than mixing two things and putting it in a cardboard tube. Hybrids are throttleable so I can start off slow to make sure things are working. Much safer than the light it and run method. Also the fuels aren't so dangerous on their own.

At the moment I'm simply trying to find non-reactive binding agents for the oxidiser. Silicone sealant is what I'm trying next.

[lajoswinkler]

Fire reaching into the propane tank and causing it to explode? ROFL

That can not happen and will not happen. There is nothing inside the tank except propane and a smelly warning compound. The only way propane tanks can explode is if they are caught in fire and the pressure inside gets so high that the forces are high enough to overcome the structural integrity of the steel.

Flashback arrestors are needed when you expect a gas+air/oxygen mixture in pipes, to make sure they don't detach themselves from the tank.

No need to bury the tank. What you need to take care of is the combustion chamber. Don't even try fooling around with hot ethanol and potassium nitrate. Always work with tiny amounts.

Chemical reaction is one thing, actual engineered device is something else.

[shynung]

Actually, the silicone sealant is for the oxidizer's binding agent. The goal is to find one that doesn't react with potassium nitrate in such a way that the binder does not become fuel.