# KSP inspired me to design a liquid-fueled rocket engine

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19 hours ago, totalitor said:

I use "How to design small rocket engines" book for insructions. I took there example and made 1/10 size of that (chamber lenght is just guessing).

Example engine thrust is 20 pounds and chamber pressure is 300 psi. I don't know how those scale, maybe thrust in my engine is 2 pounds but I don't know about chamber pressure. Maybe that is the same? Fuel and oxygen flow rates will also be 1/10.

I will make different versions becouse fabrication is so easy, one day for an engine. And it is not expensive.

... That is really not how you should approach engine design.

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You are exactly right. Still, I do it my way...

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It is definitely the opposite of the approach I'm using, and yet it seems really fun too  Can't wait to see RUD-1 belching some flame!

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I would suggest you determine your chamber pressure and expected thrust. Your chamber pressure will be based off the pressure of the fluid you are putting into the system so it will likely be the same as your example. As for thrust if you decreased it by a factor of ten in each dimension you are likely looking at 1/100th the thrust but it should be noted that not everything scales well and combustion processes dont like to scale well. Though so long as you arent hoping for any level of performance it will still probably work. What are the size of your injector holes and your mass flow rates?

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Calculating...tell me if I go wrong.

Expected thrust 2 lb. I use the same book and formulas that ap0r uses.

Isp using gaseous oxygen and 75 % alcohol about 240 so total mass flow 0,00833 lb/sec. Mixture ratio 1.25, chamber pressure 300 psi, so fuel flow 0,0037022 lb/sec and oxygen flow 0,0046278 lb/sec.

Pressure on throat 169,2 psi,

Throat area 0,0043913 inch^2. This was hard. Need info about molecular weight of gas.

Diameter of throat 0,0747741 inch (1,8993 mm so my first RUD-1 throat is too big).

Guessing value L? Make a guess between 50-100? I make 50 so chamber volume is 0,219565 inch^3.

Diameter of chamber should be 3-5 x throat diameter. I use value 3,37 so chamber diameter is 0,25197 inch (6,4 mm)

Lenght of chamber is 4 inch (10,16 cm). Looks weird. RUD-1 is too small I guess.

And hole for fuel injector 0,0113232 inch (0,28761 mm) and if I want two holes even smaller...a big problem.

Oxygen hole 0,0432435 inch (1,0984 mm) - no problem.

Edited by totalitor

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43 minutes ago, totalitor said:

Calculating...tell me if I go wrong.

Expected thrust 2 lb. I use the same book and formulas that ap0r uses.

Isp using gaseous oxygen and 75 % alcohol about 240 so total mass flow 0,00833 lb/sec. Mixture ratio 1.25, chamber pressure 300 psi, so fuel flow 0,0037022 lb/sec and oxygen flow 0,0046278 lb/sec.

Pressure on throat 169,2 psi,

Throat area 0,0043913 inch^2. This was hard. Need info about molecular weight of gas.

Diameter of throat 0,0747741 inch (1,8993 mm so my first RUD-1 throat is too big).

Guessing value L? Make a guess between 50-100? I make 50 so chamber volume is 0,219565 inch^3.

Diameter of chamber should be 3-5 x throat diameter. I use value 3,37 so chamber diameter is 0,25197 inch (6,4 mm)

Lenght of chamber is 4 inch (10,16 cm). Looks weird. RUD-1 is too small I guess.

I have difficulties calculating injector holes. Equation 25 on that book - I can't get the result what is on the book. How do you calculate it?

The numbers still work out somewhat but it's difficult fabricating on such a small scale  I'd say fire this one, see what it does, then build a bigger one (10 pounds is about the absolute minimum without having to involve precision machining and the hassle and expense that goes with it.

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ap0r - that equation no 25. There is a number 6430 on that example. Where does it come from?

I GOT IT! 100 psi is 14400 lbs/ft^2 ... had to convert it.

Edited by totalitor

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The speed of cooling liquid should be the same from nozzle to exit point and to maintain the same speed cooling flow area should be the same. But there is that de laval nozzle. In my design smallest diameter of the outer skin combustion chamber would be 6 mm (about 2,5 mm inside is the throat diameter). Elsewhere diameter would be 8 mm.

Instead of curving outer cooling tube, what if I fill that curve with copper? I would have 3 mm layer of copper there instead of tube thickness 0,8 mm.

The engine would be easier to make becouse no curving of outside cooling tube would be needed.

What would happen?

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At that scale i'm guessing it makes no difference.

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I was drilling 0,3 mm hole to coppertube. It did work, but when I was brazing things together that hole was blocked. Maybe brazing material melt. It was not anymore visible so I have to change way of putting things together. What if I use 0,8 mm hole for fuel (my calculations says it should be 0,3 mm)? Oxygen hole is 1 mm according calculations. Chamber pressure estimated 300 psi. Is it too fuel-rich? What will happen?

Edited by totalitor

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Honestly your tolerances are so extremely loose that I'm not actually even certain you will be able to get high enthalpy supersonic flow. Now the effects of changing the size of your injector holes will depend on a number of things. The mass-flow rate could increase which would throw off your ratios, or the velocity could decrease which would decrease mixing and burn performance and would likely cause large combustion instabilities and coupling of the combustion with the feed system which is not desirable. If you wanted to maintain your mass flow rate you could want to create some way to regulate/choke the flow up stream though that will cause a severe velocity decrease which may prevent you from actually igniting. You could either try with trial and error or re-evaluate your construction techniques.

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I can really now see difficulties of building such a small engine. My injector consist of one oxygen hole and one fuel hole. I calculated that fuel velocity would drop from 1189 inch/s to 153 inch/s if I use 0,8 mm hole instead of 0,3 mm hole. That's bad I guess.

Ok new try with 0,3 mm hole and I definetly need a good fuel filter.

Sorry if I use SI-units and imperial units together...

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If anything else, it should work as a whistle! (Puns aside, yes, it is difficult to build in such a small scale)

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Generally if you are going to deal with lower tolerances you want to have a larger engine so the relative tolerances are closer and use splash plate injector styles. It is fine to build at very small scales but you need to use certain manufacturing techniques.

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RUD-2 construction failed. I decided to build a bigger engine. RUD-3 is ready

Thrust: 14,35 lb (63,9 N)

Isp using gaseous oxygen and 75 % alcohol about 240 so total mass flow 0,0598583 lb/sec (27,151 g/s). Mixture ratio 1.25, chamber pressure 300 psi, so fuel flow 0,0266036 lb/sec (12,067 g/s) and oxygen flow 0,0332546 lb/sec (15,084 g/s).

Pressure on throat 169,2 psi,

Diameter of throat 0,2 inch (5,09 mm)

Chamber diameter is 0,6299 inch (16 mm)

Exit diameter 0,382 inch (9,7 mm)

Lenght about 4,6 inch (11,7 cm)

Fuel orifice diameter 0,031 inch (0,8 mm)

Oxygen orifice 0,116 inch (3 mm)

I guess success in building RUD-1 was just luck. With this engine I had great difficulties in final brazing. I made two attempts and when ready fuel orifice was stucked. I had to dig feeding lines open and try again. Third time was success. My equipment cannot handle building bigger than this, brazing operation needs heating power that I don't have. And designing needs rethink, what looks easy in paper is not in practice.

What next? Where I live winter is coming, I cannot do things becouse I do everything outside. Coldness stops everything. I design other components in winter and do them next summer. Maybe test firing is then.

Please feel free to comment my project.

Edited by totalitor

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AAAAAAAHHHHHHH!!!!! Math!!!!!    TOOO MUUUCHHH!!!!!!!!   WHYYYYYY?!?!?!??!!!!??

I appreciate the winter is coming reference in the above post

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13 hours ago, Rath said:

AAAAAAAHHHHHHH!!!!! Math!!!!!    TOOO MUUUCHHH!!!!!!!!   WHYYYYYY?!?!?!??!!!!??

I appreciate the winter is coming reference in the above post

That was exactly my tought process during most of the design, I have a pretty low math skill so this was a real challenge, but I also learned a ton.

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I also see you are a pilot.  Navigation in the olden times was pretty mathy (More time trying to get FAA guy to understand your flight path than in the air, from what i've heard).  That probably helps?

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It doesn't help. There is a large amount of individual operations required, but most of them are performed during flight planning and consist of simple addition/substracion/multiplication/division, rule of three, etc. Once in the air the amount of math is little and you have an electronic or mechanical calculator to help you. Also most of the time absolute precision is not a crucial requeriment, so rules of thumb and approximations work fine.

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Blah blah blah... Numbers numbers numbers... ROCKETS!

Wow.

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Glad to see I'm not the only one who was inspired to design and eventually build a rocket engine! Except mine is going to be a gaseous oxygen/paraffin hybrid rocket, which I chose to do because a liquid fuel rocket is much more complicated and requires a lot more time, effort, and most importantly, money.

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23 minutes ago, KSPNerd said:

Glad to see I'm not the only one who was inspired to design and eventually build a rocket engine! Except mine is going to be a gaseous oxygen/paraffin hybrid rocket, which I chose to do because a liquid fuel rocket is much more complicated and requires a lot more time, effort, and most importantly, money.

I've actually been designing a rocket engine too (liquid fuel) that runs on Furfuryl and Red Fuming Nitric Acid.

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Why are you using those propellants as the main propellants?! Just use them as a starter or something!

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1 hour ago, The Raging Sandwich said:

I've actually been designing a rocket engine too (liquid fuel) that runs on Furfuryl and Red Fuming Nitric Acid.

Where do you even get that stuff?

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