My Rocket Engine Design

[Apoapsis150km]

Hi, im new here i come from the Orbiter forums, I encourage you to go check it out, its a realistic space simulator, simulates the shuttle, saturn rockets, and any rocket you could want :D http://orbit.medphys.ucl.ac.uk/orbit.html

Some time ago I set a goal to design a small rocket engine. I want it to be small, working on gaseous hydrogen and gaseous oxygen. Have read much, and done alot of calculations, and since I want to make it small, I cant get enough pressure inside the combustion chamber to produce much thrust. The obvious solution would be to add more gas, but I want to work with that amount, in order to keep it safer. Then I thought of adding air to the mix inside the combustion chamber. I think the air would keep the pressure high, and I would take advantage of the enormous heat the H2 and O2 reaction produces to expand the air and therefore produce more thrust, at the same time making the engine cooler. Has this been done? is this nonsense, or a good idea? How could I do the math if i added the air? Thankyou for your replies.

[NickC]

I dunno, it sounds like it probably won't work. the whole air concept is kinda off too because when you add air you also add nitrogen (78% of air) which slows down burning. you also burn the ~21% oxygen that's in the air.

[marksteamnz]

Aye...............Well................Hmm.............

Your idea is mired by the whole thing a rocket motor lives and dies by. Specific impulse ie the mass fired out the nozzle and it's velocity. Dumping stuff into the chamber cools it so the mass increase is clobbered by the drop in ejection velocity. Yes Yes I know the air is presurised so just vent it in the same direction as the rocket motor nozzle. Better thrust overall but it's still a heavy miserable performance booster.

Of course you will have to presurise the air and to do this you could use a turbine in the exhaust to drive the compressor turbine. And as there is plenty of O2 in the air leave that out and as gaseous H2 needs VERY heavy tanks use a liquid fuel..........opps we now have a turbojet engine. So grab a turbo charger and have at it.

[Apoapsis150km]

Your idea is mired by the whole thing a rocket motor lives and dies by. Specific impulse ie the mass fired out the nozzle and it's velocity. Dumping stuff into the chamber cools it so the mass increase is clobbered by the drop in ejection velocity. Yes Yes I know the air is presurised so just vent it in the same direction as the rocket motor nozzle. Better thrust overall but it's still a heavy miserable performance booster.

I see.. More mass but less velocity. But the thing that I thought is that in a normal engine theres all that excess heat energy produced by the H2 and O2 that goes to waste, being taken off by the coolant; this way a small part of that wasted heat would go into acelerating that air. So the excess trhust would come from using that heat that otherwise would be wasted, not from nowhere. More nonsense or does it make sense?

Of course you will have to presurise the air and to do this you could use a turbine in the exhaust to drive the compressor turbine. And as there is plenty of O2 in the air leave that out and as gaseous H2 needs VERY heavy tanks use a liquid fuel..........opps we now have a turbojet engine. So grab a turbo charger and have at it.

Yeah I already thought of leaving out the O2. So it would resemble a turbojet engine, but the air coming from a tank not a compressor turbine.

Thanks!

[Ray]

I see some difficulties with using gaseous Oxegen and gaseous Hydrogen. It'll be hard to get enough gas to make a useful motor.

Motors work by sending high velocity gases out the nozzle. They rely on mass flow to work.

If you pressurize the O2 and H to high pressures, you can get a good amount of mass to burn, but then the tanks get very heavy, if you don't pressurize it to high pressures, you don't get much mass. That's why the pro's use liquids...the density is higher.

ISP is a measure of the efficency of the propellant. Its measured in pound seconds per pound. Low mass combustion products get you higher ISP...O2 and H give you the lightest compound out the back, so you get the highest ISP. The only way to get higher ISP's than O2/H motors is to go nuclear...I've heard numbers as high as 800 seconds from them. O2/H motors will be in the high 200's to low 300's.

Adding air will lower the ISP and mess up your chemical ratios. You want the heat in the combustion chamber, don't cool it. Most of the motors we make don't burn long enough to worry too much about heat. Ablative materials for liners and Graphite for the nozzles take care of most of the heat problems we have.

O2/H motors can be explosive, so be careful. I've heard that you can detonate H in ratios of 4%-96% concentrations...you'll be running at stoiciometric, so the chance of detonation is HIGH.

Look into Nitrous Oxide motors. They are much safer, although you give up ISP.

You should really spend some time studying...You don't understand some of the very basic principals of motors...before you go into designing them you should really have a clue what you are doing. There are some excellent books out there, some excellent websites...spend some time there. The sticky link post in this forum has some good places to start. Don't be put off by the solids stuff, all the prinicipals of rocket motors are the same, execution of liquids, solids and hybrids differ, but they all run on the same physics.

[Apoapsis150km]

Thanks Ray. I have asked of my crazy idea, and it may seem I have no idea what Im talking about, but i do, at least enough to make a design. I know the big drawbacks of using gas, but I want to design it within my fisical building capabilities so i wanted it to be with gas anyways. I know that if I want it working with gas I should add much more gas to get more pressure difference, more heat and more mass, but I had to accept the second big drawback of not being able to produce enough fuel. So I thought that there could be a possiblity that I could use air instead of pure oxygen, that way adding much mass and pressure, and maybe the extra inert gases present could take advantage of some extra heat that would otherwise be wasted to the surroundings, and that way produce more thrust. But it seems it doesnt, so I will have to design a different engine. I will research onto the Nitrous Oxide motors, and see if its within my building capabilities. Thanks again

[Ray]

I think you'll find that Nitrous Oxide/solid fuel will be easier to construct.

You can use almost anything for fuel. PVC, Nylon, Paper, HTPB rubber, Wax, and even sausage...hehe

Much safer to work with, not too much risk of detonation if you size the injector corectly.

Of course, I prefer solids myself...

[NickC]

what about liquid rockets? like there are 3 tubes, one with nitrogen or other inert gas, one with a liquid hydrocarbon fuel, and one with Nitrous. The inert gas tube is pressure regulated and flows into the liquid hydrocarbon tube and pressurizes it to flow out of however many nozzles , then the Nitrous tube sprays out of however many nozzles and into the combustion chamber. The whole thing is started similar to a hybrid. Or the 2 tube design, which would be something like liquid methane or liquid propane to achieve self pressurization. These would be cheaper and safer lower impulse fuels.

[Ray]

I've seen some of these...The guys from West Coast Hybrids have done it...pretty cool.

[Apoapsis150km]

Thanks for the guidance. I think Ill start up solid, although it seems somewhat less interesting for me than liquid or gas. But I simply dont have the capabilities for building those. Gonna start low with candy propellants. Alot of reading ahead! :)

[Ray]

The hardware isn't that much different between a hybrid and a solid motor. Aluminum casings with snap rings grooves. The hybrid motor is longer, and has a bulkhead in the middle. Other than the bulkhead, all the parts are the same...

Aerocon sells paper grains...any hot rod shop will have nitrous oxide. Home depot sells PVC...etc.

Ground support equipment is the biggest difference, you need to have the capability to remotely fill and dump the nitrous in the motor.

Solids can be a lot of fun. I enjoy the chemistry (not that I am good at it) and also the data you can gather...its the data that drives me. Flights are lots of fun too.

In the community that I run in (northwest US), hybrid motors are affectionately known as "fart motors" for the distinctive sound they make...