Motorjet

[Zippiot]

First off it was a paper towel roll sized ramjet that made 80 pounds, a mugg cant expect to make mroe than 10 at 300+ mph airspeeds.

Ponder this:
You say that outside air has a higher pressure than fanned in air, this is true but!!!
think about the 200 mph minimun airspeed
that means that you can RAM in enough air to get pressure up. RAMJET, GET IT? OOPS CAPS LOCK
So what if the air is at a lower pressure, you fan in enough you can still sustain RAM operation. What you need to know is the fans exit velocity and gpm of flow, then design a ramjet around that. Keep in mind it is going to be a tiny ramjet prolly, most likely not the size of a muggs, soup can should be about right.

Some of you may think, why does he need to know exit velocity and gpm instead of just gpm? GPM is the amount of air flowing, but velocity will help design the intake and the rest of the ramjet, as each ramjet has a design speed.

ALso to ponder:
what is the optimal fuel to air mixture? Find the gpm and divide to find the optimal amount of fuel, work from there.





MATH!!!!!!!!!!!!!! I know very little but this crap seems to be important in jet engine design


Correct me if any of the above is wrong, I am hungry and tired...

[Zippiot]

I said 10 b/c I had 2 ramjets, one a modified muggs and the other a scratch build. They had the same internal area but the scratch was much longer, it made just over 8 pounds of thrust. With some better math and a nice piece of stainless (mine melted!!!) along with some good fuel power can be boosted to over 10 pounds.

Best fuel I found that was easy to find was white gas mixed with bacon grease. I suppose other grease will work, filter mcdonals stuff might turn out quite nice. It gave the highest thrust measurement of just over 8 pounds, whilst regular gasoline avg'd under 5 pounds. I finally grabbed some kerosine and mek, I will experiment with those when I rebuild/design a new ramjet.

But my scratch build did melt using the whitegas-bacon grease fuel.....wonder how hot it was burning. I retired it after than and it was accidentally thrown out since it looked like scrap :(

[Najm]

If you want to protect your ramjet/motorjet from heat then make it out of aluminium and anodize it. Aluminium oxide can withstand temperatures upto 2300C.
Anyone else can please verify if this method will work because making ramjets out of aluminium is much much easier.

[Zippiot]

Rustolium (check spelling, all right reserved) high heat for bbq's. It can take up to 1200 degrees f, but donnot expose to direct flame. Just plate the outside with it and it'll look like the inside of a bbq forever!

Aluminum has a very very low fail temp, the fail temp of steel may be up to 1500 degrees (again, someone verify this). Even gold has a higher fail point than aluminum.
I would say that you should make an insulated shell if you want it to look pretty forever. Some fiberglass insulation between the jet and an aluminum shell can work wonders.

[redneck1]

hey yall new to the tread but i got a colling idea. if you put a thrus augmentor shroud around the engine from intake to extend a bit past the exhaust the exhaust should create a low pressure in the shroud causing cool air to flow through the augmentor , over the hot engine and cool it. not to mention increase static thrust a bit as well depending on how far the shroud extends past the exhaust. just my 2 cent. dont even know if the tread is still alive.[/b]

[larry cottrill]

Something that was said above makes me think that there is a strong belief that air pressure will be reduced any time you increase velocity. Nothing could be further from the truth! It is true that a nozzle or venturi (which is just a special type of nozzle) will reduce pressure as flow speed increases, but this is not true of an axial or centrifugal compressor, nor even of a fan.

A compressor (of any type that we might use for jet work) increases pressure and density at the same time that it boosts velocity. Of course, this comes at the cost of a lot of horsepower. The main difference between a fan and a jet engine compressor is that the fan will only have a compression ratio of about 1.0, while a high-efficiency compressor will have a reasonably high compression ratio at the outlet. My old turbine design book shows a centrifugal compressor as being able to deliver a ratio of nearly 2.0 at the impeller tip.

If the pressure of air going into our diffuser were greatly reduced, we would be hard pressed to get a working engine, since the diffuser would probably never recover enough energy to get us back up to and slightly beyond the outdoor air pressure! But a ramjet operating at high speed near sea level has normal pressure air going into the front end, so that the diffuser can produce moderately high pressure at the front of the chamber, and that is what we must do with our motorjet! Fortunately, the fan will give us air at normal pressure and increased velocity.

The diffuser is really the key to good operation (assuming a steady supply of high-speed air, of course). In the textbook graph I'm looking at, the gradual increase in cross-sectional area moves the compression ratio from the almost 2.0 already mentioned up to better than 4.0 at the chamber entrance! Of course, this is a very long, gradual diffuser much lengthier than we are likely to build on a low-speed ramjet, but it makes a good illustration of what can be accomplished.

Diffusers of extreme ratio (as someone mentioned in relation to some tests with experimental vehicles) are only applicable to very high speed engines - at near-sonic or supersonic speeds, a pretty tiny inlet area represents a HUGE mass flow! But this cannot be so at model airplane speeds. The best we can hope for is a moderate pressure rise at the point where velocity comes down to a reasonable level for the flameholder design used. Remember that the point is to get a stable flame envelope in the chamber - too fast and we lose it out the tail, too slow and combustion will creep forward into the diffuser. The reason the "blocking washer" worked in the one example of Maggie cited above would have to be that the air input velocity was unreasonably high, and therefore a greater ratio produced the right speed at the flameholder for stable operation. The thing that makes ramjet design as tricky as it is, is that the air velocity at the chamber entrance MUST fall between fairly tight limits (although of course, this can be broadened significantly by good flameholder design).

To reiterate: To make the ramjet work, at the chamber entrance we MUST have two characteristics: moderate flow speed (well-matched to the "flame speed" of the air/fuel mixture used); and elevated static pressure (i.e. significantly higher than the outside air at the nozzle exit). And, of course, we would like good mixing and fairly smooth flow going into the chamber as well.

As far as insulation is concerned, I don't think you want it. Far better would be a shell spaced 1-2 cm outside the engine wall, with plenty of air available to flow through. There should also be airflow outside this reflective shell. Ordinary insulation can surround that, right up against the structure of the vehicle. Good examples are found in 1950s scale ships designed for the Dynajet - the engine, an air space, sheet aluminum cone, another air space, then a thin coating of asbestos paste insulation on the inside of the carved balsa shell, with a coating of aluminum foil glued onto the inside surface of that. Of course, something other than asbestos has to be used today, but you get the idea. There is ALWAYS airflow immediately around the engine shell, and it works - once you're moving! That's the trick, to start up and get into motion right away. Of course, you should be able to use the exhaust flow to draft cooling air in, even at zero forward speed. Some experimentation required.

L Cottrill

[redneck1]

well my friend asking questions only gets you so far, give it a try, and looks like the fellow who just typed another chapter to what looks like is quickly becoming a how to- not to book had the same idea. try it all and see what works, that will answer a few of your questions.

[redneck1]

hate to be the skeptic here but do you think you can get it right in one try? no doubt you could get a working one but without wearing things out, blowing things up (thats always fun) especially from possiable detonation of a fuel air mixture of sertain amounts as apposed to deflagration process which you want. be careful my frind less your hat wind up in one tree and you in another.

[redneck1]

i dont know how much an augmentor will help cool but i emagine it would be significant if designed properly. the heat just needs to be blow/sucked out of the pipe faster than it can radiate upwards to the duct/augmentor itself. and not only that but sure the augmentor will get hot but i dont think it will get hot enough to discolor if you see where i am going with this.fuel delivery shouldnt be to hard with a liquid. with minimum modification to a small model 2 stroke engine carb you could probably get a wonderful spray of fuel into the airstream.

[redneck1]

YOU LIKE THAT? THANKS...WELL if you have a test bed be prepared to make radical changes to it.and rreally my friend i think an augmentor is the way to go, between trhust increase, and cooling it will do more than its job. how do you plan to rig a turbo fan unit without robing power from your motor, and have a clean airflow from both fans as they will be stacked on eachother?i have a jet turbine that i am planning on building that uses a stacked fan idea and lemme tell ya it aint gonna be easy to build.

[redneck1]

i agree a custom spray nozell will be better but it will be harder. now dont think they are all plastic, i have onei got off an engine for 50 bucks and it is metal. dont forget to search ebay for used items my friend. and anything can ddt if you mix stuff right, or wrong , depends on what you are trying to do. now here is another thing, dont forget that pressure is relative, to what it is being compared to. yes you probably want higher pressure than ambient air, but dont neglect exhaust pressure.you can boost relitive compression by simply increasing exhaust speed to a degree which may give you a boost in thrust, without to much of a degree in heat increase and front end design. a way to do that would be increasing your exhaust area but if done to much the drag it creates will start to counter balence the binifits.

[redneck1]

its a 3 stage fan desinged to shorten conventional jet engines. my problem is that i cant seem to find a way to fit the individual blades togeather on such a small scale.

[redneck1]

well your fan idea i like, plus i see no reason why you couldnt make the outside shroud bigger in diameter to make it searve a dual purpose. by the way my turbine also uses and augmentor.

[redneck1]

it combines acial and centrifugal in an inverse way, instead of trowing air outward and compressing it against a wall, it pushes air inward, using its own conectic energy to compress the shoots it to the diffuser at a high speedand volume where it stops almost dead still. i dont have pics and cant even work on it right now as i am in japan for the next few months.

[redneck1]

hell your idea sounds good to me i have no way of critizing your measurments, i caint even spell right now its almost 4 am over here and ive been up since really early. just so long as the fuel is injected at the narrowest spot.