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KSP inspired me to design a liquid-fueled rocket engine

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29 minutes ago, totalitor said:

I opened throat area of RUD-8 engine. Couple new pictures in imgur. See how cooling works.

 

https://imgur.com/a/3LZNOEf

 

Good to see progress! I haven't been following this thread for very long, what is the purpose of the cooling rods? Is it to keep the cooling jacket centred?

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That also but the main reason is decrease cooling volume and thus increase coolant flow speed. It is still not enough yet.

Edited by totalitor

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On 10/14/2018 at 10:01 AM, totalitor said:

That also but the main reason is decrease cooling volume and thus increase coolant flow speed. It is still not enough yet.

Correct me if I'm wrong, but whilst that would increase flow speed (according to Bernoulli's equation) doesn't that actually decrease mass flow rate? It seems a little counterintuitive that a smaller cooling volume should correspond to a higher rate of cooling.

It's been a while, but I (think) I've finished the design equations for my hybrid motor. I would post my math here but I did it all on paper and it looks a mess :rolleyes:, so I'll just give you the numbers I got.

Specific impulse: 111s

Total impulse: 9.625 N/s

Thrust: 19.25 N

Area at throat: 3.17mm^2

Area at exhaust: 9.47mm^2

 

These calculations were based on the assumption that:

Chamber pressure: (20 bar - perhaps a little conservative)

Combustion temperature: 1750K

Burn time: 0.5 seconds (this allowed me to calculate mass flow rate, as I already knew the mass of my propellants)

 

Here's the general design that I'll be following. The problem with using nitrous cartridges as an oxidiser is that the container must be physically pierced to allow the gas to escape. The system I came up with here uses a wing bolt which, when tightened, forces the cartridge into a sharp and airtight pin valve, but the gas is still prevented from escaping by a small plastic disc. Then, when it is time to light the motor, an electronic match can be used to melt the plastic burst disc in the injector, releasing the nitrous and igniting the engine in the process.

k283H9Y.png

I hope to begin building a prototype soonish, but I have my final exams coming up :0.0: so I'm not sure how much time I will have for this project.

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3 hours ago, Ol’ Musky Boi said:

It's been a while, but I (think) I've finished the design equations for my hybrid motor.

That looks great! Are you using a straight, parabolic approximation, or exact de laval nozzle?

I love the idea of using N2O cartridges for mini hybrid engines.

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5 hours ago, Mad Rocket Scientist said:

That looks great! Are you using a straight, parabolic approximation, or exact de laval nozzle?

I'm using a straight approximation with an ~15° exit angle, it'll make manufacturing the nozzle easier.

5 hours ago, Mad Rocket Scientist said:

I love the idea of using N2O cartridges for mini hybrid engines.

Thanks! I'm not the first to think of it, there are commercially available hybrid motors that also use N2O cartridges, like this. I took a few cues from their design, like the burst disc ignition system, but I'm using a fuel that'll be easier to cast into different grain types, and it doesn't look like they had a deleval nozzle on theirs either.

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"Correct me if I'm wrong, but whilst that would increase flow speed (according to Bernoulli's equation) doesn't that actually decrease mass flow rate? It seems a little counterintuitive that a smaller cooling volume should correspond to a higher rate of cooling. "

I am not sure if I can make cooling work like this but I will try.

I liked that idea of using rods inside the engine but that is adding extra mass and I want to make engine as light as I could. So now I am planning to use aluminium tubes and electro etch channels to inner tube. First test of etching were good. Steel is easy, copper is not and aluminium is easy too.

Aluminium rocket engine? Sounds crazy but it has been done. Check this out.

http://watzlavick.com/robert/rocket/index.html

And if I give up using gasoline and try 75% alcohol flame temperature would decrease about 600 C. But then I need more fuel to achieve same thrust. More mass. If I can make this motor working I want also to make a flying rocket some day.

Edited by totalitor

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10 hours ago, Ol’ Musky Boi said:

I'm using a straight approximation with an ~15° exit angle, it'll make manufacturing the nozzle easier.

Thanks! I'm not the first to think of it, there are commercially available hybrid motors that also use N2O cartridges, like this. I took a few cues from their design, like the burst disc ignition system, but I'm using a fuel that'll be easier to cast into different grain types, and it doesn't look like they had a deleval nozzle on theirs either.

I've heard ABS or PLA plastic can work as a fuel, which would mean you could 3d print the fuel grain. I don't know if that works with N2O oxidizer though.

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On 11/4/2018 at 9:59 AM, Mad Rocket Scientist said:

I've heard ABS or PLA plastic can work as a fuel, which would mean you could 3d print the fuel grain.

Seems risky. Your print quality has to be top notch. The kind of cracks I get occasionally in my prints would cause a hybrid engine to over-pressure for sure.

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Hello all! Construction of my first prototype hybrid (named Mr. Whippy, because the N2O cartridges are used to make whipped cream) has begun! I had a friend come over today and give me a hand, and in about an hour and a half of work this is what we got done:

https://imgur.com/a/MQGTK7R

Unfortunately due to a lack of tools the parts ended up being quite crude and wonky, and they took forever to cut with a rusty hacksaw. We will see if we can get permission to use the school lathe to make production easier.

Casting the wax was a little tricky, because paraffin shrinks as it cools. So we had to top it up several times to get a complete cylinder (we then twisted a drill bit into it to make a hole, but that ended up being quite wonky). Next time I'll probably put a rod or something in the actual mould so we can get it more consistent, and we might try more sophisticated grain shapes.

(note: the nozzle dimensions changed since last time because I found a few errors in my calculations, the throat diameter is now 3mm and the exhaust diameter is 8.2mm based on 25 bar and 2000K chamber conditions - chamber temp was calculated with help from the book "Ignition!" and chamber pressure is just the half the pressure of the N2O cartridge).

So some progress has been made, we hope to get a motor test fired before Christmas, winter isn't particularly harsh in the UK so that shouldn't be a problem.

Edited by Ol’ Musky Boi

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So last weekend I finished assembling Mr Whippy using the parts that I machined on the school lathe (that made the whole process a whole lot easier) - and I then attempted a test fire. I encountered a couple of problems, the first being that my assembly wasn't airtight around the pin valve (which you can see in this album), so leaked when I attempted to pierce the cartridge. Given that all of the gas escaped out of the front end of the rocket I narrowed down the leak to where the pin broke the seal, and I've attributed this failure to an incorrect o-ring size (I didn't have one that fitted snugly around the pin, so I used one that was much too large and didn't plug up the cartridge after piercing properly).

The second problem was that my ignition system failed to melt the burst disc. I tried to use a firework sparkler that I slid through the nozzle and rested against the disc in the hopes that it would be hot enough to melt it, and this was supported by some testing I did outside of the rocket. However this didn't end up working, and I think this is because the contact time between the disc and the flames is so low and the exit diameter so small that no meaningful amount of heat can be transferred to the plastic. I will try to remedy this by widening the exit hole and possibly using some kind of pyrotechnic grain or slug in place of the sparkler, I was thinking nitrocellulose "flash paper" rolled up into a tube or some sort of gunpowder pellet, any other ideas there would be great.

Failures are good for improvement, so I'm not discouraged by this and still hope to get something working by Christmas, but only time will tell  ¯\_(ツ)_/¯

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Nice work! Without failures you don't learn. It is very rewarding to think and design all systems. Food for brains.

Remember to take videos from many angle because everything happens so fast when an engine finally start.

Take care when you ignite, remember it could explode.

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Posted (edited)

@totalitor

Take a leaf out of the book of goddard. Run your engine fuel rich ;)

 

Or perhaps you could use steel brakeline for cars and coil that around the nozzle and combustion chambre, then you can run your fuel through it. To test this you could pump water through the lines first to prevent explosions.

Edited by Flying dutchman

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Hi!

New test coming, this time with cooled rocket engine. I am using gaseous oxygen and alcohol-water (about 80 % alcohol).

I am planning to do at first only few second firing but I have two ways to do that

1. Fill propellant tank full and count seconds and then close all valves. In cooling jacket there remains propellant.

2. Fill propellant tank only a little and let it run empty. Nitrogen purges system after that.

 

Which way is better to engine survive?

I look videos after test and judge should there be more or less oxygen.

Granted, engine meltdown is highly propable.

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44 minutes ago, totalitor said:

Hi!

New test coming, this time with cooled rocket engine. I am using gaseous oxygen and alcohol-water (about 80 % alcohol).

I am planning to do at first only few second firing but I have two ways to do that

1. Fill propellant tank full and count seconds and then close all valves. In cooling jacket there remains propellant.

2. Fill propellant tank only a little and let it run empty. Nitrogen purges system after that.

 

Which way is better to engine survive?

I look videos after test and judge should there be more or less oxygen.

Granted, engine meltdown is highly propable.

Hmm, filling the propellant tanks to full does increase the chance of an explosion and is a little wasteful, but I've heard from some sources that letting a rocket engine run until it's depleted all of it's propellant can damage the engine from the sudden changes in pressure. You might want to look into that.

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I have been two years designing and building a small rocket engine. Now I've switched fuel from gasoline to 80% alcohol.

I think I got it now. Four test today with same cooled engine, no melting.

 

Test 1 - too much fuel. The engine was almost cool after test. No significant thrust.

Test 2 - less pressure for fuel, good and stable combustion. There was thrust, 300 g metal plate moved (look at the video). Hot engine after test.

Test 3 - same pressure, more fuel in tank.

Test 4 - more pressure for fuel and oxygen, mach diamonds visible!

 

I am not sure if this engine can run longer without melting becouse  it seems I have too little tank for fuel. Cannot run longer until I change it.

What do you think?

https://www.youtube.com/channel/UCf2lLsBevPguEq7i2YKQ4Sg/videos

 

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Wow, yeah, those are Mach diamonds. What's the expansion ratio on the nozzle you've got fitted? Mach diamonds are indicative of over-expansion (i.e. the pressure at nozzle exit is lower than ambient pressure), which may or may not be significant.

Also, it's interesting to see the history of propellant chemistry play out in small here with the switch from gasoline to 160-proof vodka. Von Braun would have been proud.

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Ok to be clear: fuel is a mixture which contains 80 % alcohol and 20 % water. Better?

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Just now, totalitor said:

Ok to be clear: fuel is a mixture which contains 80 % alcohol and 20 % water. Better?

I wasn't criticizing your choice of language. I knew perfectly well what you mean. But, to be pedantic, an 80% alcohol, 20% water mixture is precisely what 160-proof vodka would be, if anyone were crazy enough to make vodka that strong. It's to this that I was referring - I was actually making a (small) joke about how boring vodka is.

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What is the best test run? 2-3 or number 4? Mach diamonds looks nice but is there too little fuel and so more temperature in combustion chamber?

 

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