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

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Hard to say. It looks like tests 2-3 were very under expanded and test 4 was a over expanded, which seems counterintuitive because 2-3 were conducted at lower pressure. Maybe for tests 2-3 the throat size was too large to properly choke the flow so the exhaust exited at higher than ambient pressure, but for test 4 the pressure was high enough to choke the flow and drop the pressure. Seems like there's a sweet spot somewhere inbetween 7-8 bar for the fuel pressure and 8-9 bar for the oxygen for ideal performance. Have you considered building some sort of thrust measuring rig? Might be easier to compare performances that way.

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Yes I have to design thrust measuring system. Then I know better.

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• 3 weeks later...

Great day today! I test fired my homemade rocket engine four times and it is still alive.

My goal is to have total 10 seconds thrust without melting.

Last run no. 8 was very good. Tank wasn't full becouse I ran out fuel mixture but still I got a record 7 seconds.

After test the engine wasn't too hot, I think cooling works well (sure, it doesn't melt...)

Here are videos:

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• 3 weeks later...

I installed a thrust measurement system and did two test run with bigger fuel tank.

Thrust is 29 N (3 kg). I expected more but this engine may have low isp or there is not enough pressure in combustion chamber.

11 seconds burning time with 190 grams of fuel. Engine weight is 180 grams (without any fittings) so TWR=16,7.

Fuel is alcohol 70 % water 30 %.

Videos here (RUD-10 test 9 and RUD-10 test10).

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That would give you an isp of 173 seconds, not too bad for something that you build yourself.

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Not quite.

Isp=F/W total. If I assume O/F ratio was 1 then W total=2*0,038 lb/s=0,076 lb/s

So Isp=6,61/0,076=87.

That is bad. I try to raise combustion chamber pressure next time.

Edited by totalitor
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• 2 weeks later...

New test today. More pressure - more thrust?

Look what happened. Same engine, same test platform. Thrust in earlier test 29 N (3 kg)

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• 2 weeks later...

New engine: RUD-11 with regenerative AND film cooling. How much film cooling eats thrust? Answer: a lot.

Videos here:

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I wonder how your engine would handle diesel fuel. Of course it is much harder to atomize so probably would require a different engine entirely. Just a thought though.

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• 4 weeks later...

New engine RUD-12, new test.

Better injector, does it increase thrust?

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• 1 month later...

Is the engine still being developed? Last update was 2017.

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I think creator of this post have gave up, for many reasons I think. But he inspired me and I kept going. Now I have a small working rocket engine and next year I try to build a flying rocket. Hopefully.

ap0r - are you still there?

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34 minutes ago, totalitor said:

I think creator of this post have gave up, for many reasons I think. But he inspired me and I kept going. Now I have a small working rocket engine and next year I try to build a flying rocket. Hopefully.

ap0r - are you still there?

I can't blame him, I gave up on my hybrid rocket project for a year. I've restarted development though, should see a few more tests and the construction of a second prototype in the coming weeks (hopefully). Seeing everyone's work on this thread was a big inspiration.

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I'm alive and well, thank you! I have abandoned the project, the design is almost done but I realized there's very little chance of actually building it given my personal and financial situation. You should check Totalitor's youtube channel (I'm a subscriber) for your DIY liquid fueled rocketry needs :)

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Here's some testing from last week, no ignition but I think we're pretty close, just need to get the burst disc the right thickness, we went too thin for 2 of the tests and too thick for the other, so the sweet spot must be somewhere in the middle. The fuse is a 20cm or so length of plastic ignitor cord (PIC), which is coiled up in a cavity under the burst disc, hopefully that'll serve to both melt the burst disc and to decompose the N2O. The injector piece was 3D printed by a friend (largely because I had no access to metal working tools at the time) who's been helping out with the project, and held up fairly well under the heat, although there is a little bit of wear. We're going to do some more testing this weekend probably, and see if we can achieve ignition.

If all goes well we can start construction of a second prototype, where we will try to cut down on weight and fix any other issues (current design weighs 126g wet and 117g dry, so there's a lot of trimming to do). I've already done some preliminary nozzle calculations:

Throat diameter: 3 mm

Exit diameter: 7 mm

Specific Impulse: 112 s

Average thrust: 41.6 N

Impulse: 10.4 Ns (D size motor)

For the second motor we will likely switch to using acrylic fuel over paraffin, as paraffin wax has proven to be a right PITA. And we may switch from using 8g N2O cartridges to using 16g ones, which will give us double the burn time and impulse (current motor is about 5 Ns). But we're not certain yet, 16g cartridges are a lot more expensive.

Edited by Ol’ Musky Boi
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Average thrust 41,6 N? Is that right? TWR=33? Sounds incredible.

Use more cameras when you are testing. This IS science!

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28 minutes ago, totalitor said:

Average thrust 41,6 N? Is that right? TWR=33? Sounds incredible.

Use more cameras when you are testing. This IS science!

That's correct, I assumed the mass flow rate would be 38 g/s based on how long it took for one canister to empty. But it may be that when the engine is firing and under pressure the mass flow rate goes down somewhat, I'm not sure. A TWR of 33 would be amazing!

You're right about the cameras, next time we'll set up some tripods and try to get a few different angles.

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1 hour ago, Ol’ Musky Boi said:

That's correct, I assumed the mass flow rate would be 38 g/s based on how long it took for one canister to empty. But it may be that when the engine is firing and under pressure the mass flow rate goes down somewhat, I'm not sure. A TWR of 33 would be amazing!

Yes, mass flow will be lower when you have choked flow at the throat.

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3 minutes ago, sevenperforce said:

Yes, mass flow will be lower when you have choked flow at the throat.

Ah, thanks for clearing that up.

I did think 41.6N sounded a bit much, I've not studied fluid mechanics in my physics course yet so I'm basically learning all this stuff off the internet. Guess I'l have to redo the calculations then...

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43 minutes ago, Ol’ Musky Boi said:

Ah, thanks for clearing that up.

I did think 41.6N sounded a bit much, I've not studied fluid mechanics in my physics course yet so I'm basically learning all this stuff off the internet. Guess I'l have to redo the calculations then...

In a converging-diverging nozzle, the fluid velocity must increase with increasing pressure (Bernoulli), but the fluid density must decrease at the throat with increasing velocity (Venturi). Initially the mass flow goes up with pressure, but eventually Venturi overcomes Bernoulli and the mass flow decreases because the density at the throat drops precipitously. When fluid velocity reaches Mach 1 at the throat, you have a steady state choked flow where Venturi and Bernoulli are dancing together in sync.

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• 4 weeks later...

The testing from 3 weeks ago didn't go quite as planned (high pitched language warning):

Spoiler

What happened here was probably something I should've anticipated, the lump of hot glue used to stick the ignitor in place came loose upon ignition, clogged the nozzle, and the engine then burnt through. Despite that, for a fraction of a second it was actually working, and there were visible mach diamonds, so I'm pretty happy with that part of the test.

Obviously, there are several things that need improvement. First and foremost are my safety procedures, currently testing is done behind some glass doors with manual ignition, but this presents a few hazards. One is if the engine explodes shrapnel could fly straight through the glass and into myself and anyone else who's watching, no shrapnel was produced in this failure, but it'd be naive to assume that this could never happen, and I'd rather not be a cautionary tale. The second is that the engine fails whilst I'm lighting it, and flies right into my face. To mitigate both of these issues future testing will be done at distance (perhaps ~30m) with remote ignition, and around the test stand thick plywood walls will be placed to catch or slowdown any escaping debris. Better safe than sorry. Cameras are more expendable than humans so they can remain close to the test site.

The second thing that needs improvement is evidently the injector.

The old injector was very much inspired by those of similar micro-hybrid engines, but I believe it has several flaws. One is that it intentionally puts a barrier between the burst disc and the ignitor, which directs most of the fire away from the burst disc, and makes the whole thing unnecessarily difficult to light. It also provides no means of support for the ignitor, meaning that it can come loose before ignition, which results in a dud launch, or can come loose after ignition and clog the nozzle, which provides quite the unexpected firework show. Fixing this looks to be fairly simple, just shift down the barrier so it directs the flames towards the burst disc and holds the ignitor firmly in place, improving ignition reliability and reducing clogging risk.

The third improvement to be made in the next engine iteration is weight saving, the old engine had a measly propellant mass fraction of 0.07. Now due to the small size and use of a heavy commercial pressure vessel a low propellant mass fraction can't be helped, but based on some rough calculations I reckon I could get the whole thing down to around ~80-90g with a propellant mass fraction of about 0.1. This will be achieved by trimming down some of the unnecessary aluminium on the internal components, using smaller bolts for cracking the canister, and switching from a stainless steel tube to an aluminium one. Still a long shot from the 0.5 propellant mass fraction of similarly sized solid motors, but better than nothing.

To make designing the next engine a little easier I also wrote a few bits of code to calculate nozzle dimensions, minimum material thicknesses, and maximum apogee in flight for me. The current specs of the next engine are:

Expansion ratio: ~4

Throat diameter: 1mm (unfortunately, the ignitor cord used for ignition is 2mm in diameter, so all nozzle dimensions will have to be doubled, I'm not sure what effect this'll have on performance?)

Exit diameter: 2mm ideal, 4mm practical

Apogee in flight: 100m

Thrust: 34N (this was after calculating mass flow rate across the 1mm injector orifice at a 10 bar pressure difference, this is still about 3 times higher than what other hybrid engines of similar size achieve, so I assumed ~11N for my flight calculations)

Fuel: For cost reasons the next engine will probably use a paper fuel grain (this is what was used in the above test), one thing I'd like to try is to soak the fuel grain in liquid wax to see if this increases performance.

Isp: ???, this is entirely dependant on chamber temperature, and the only thing I know about that is that it's greater than 1640K, because it melted through the steel body no problem. A reasonable guess is between 2000K-3300K, or a performance of 130s-220s.

To assist in future design and testing work I thought it'd be a good idea to buy a few more textbooks, so I've bought and read this one, which contained quite a lot of useful data about regression rates and chamber pressures. The next thing I need to research more is flying model rockets, because this will actually be the first rocket I've ever flown, ignoring the deathtrap of a rocket-candy-car I built about 5 years ago, which managed to move a whole 3 feet.

I've got about 3 weeks left of school till the holidays, and parts will take another week to arrive, so I have about a 2 week window where I have access to tools to make the next engine. Hopefully that'll be enough time. if not further testing will have to be delayed to next year.

Edited by Ol’ Musky Boi
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• 3 weeks later...

Yes better to be safe than sorry. In my tests I am 10 m away behind 15 cm thick wood and still I look rocket engine from mirror. It is nice to see what happen when you are testing but you sure look many times videos after tests. That is more important to analyze what happend during test.

Good luck to your next version.

I have collected parts to my rocket, I am planning if I can get it to fly next year. Now I have 3500 grams together including engine, pressure regulators, fuel. gaseous oxygen and other sfuff. Not including stability system and recovery system. Best thrust is so far 3400 grams so there is still work - this rocket can not fly yet. Cutting weight is important but I think I have done everything I can. Ok, I have "not-so-safe-system" weight 3300 grams but there is no check valves. I am hoping to get more thrust with more pressure.

Next real test begin when spring arrives - on May I think.

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• 4 months later...
Posted (edited)

Welcome to follow small rocket engine testing and rocket building on my youtube channel.

Any suggestions to get more thrust or saving weight on rocket are welcome!

Check it out.

Edited by totalitor

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