The Great Ramjet Challenge!!!

[Eric]

The materials used to be cheap, but unless you are building things strictly for yourself they are never cheap. Stainless has almost doubled in price, and welding equipment / materials arent cheap either, plus theres the time you spend developing something, testing, and all the other things that you have to figure in.

Oh and you let me enter the valveless engines in to compete with the ramjets, the 28 gauge engines are already at a 5.2:1 power to weight ratio. :D

Eric

[Zippiot]

How long does 28 gauge last? I am using 22 at the moment, might switch to 24gauge 440 stainless if the design works.

[Eric]

The 28 gauge engines are strictly for propulsion, like the dynajet if you run them for more than 30 seconds on the ground you risk serious damage / destruction. Especially the advanced chinese, if you put it at full throttle with no cooling air your asking for a rapid destruction and implosion of the main tube after the combustion chamber.

It would work for various components of the ramjet but it is an absolute bitch to weld and if you dont have it perfectly smooth on the inside or your design causes turbulence and heating of the engine casing then it still wont last long.

Dont switch to 440. Or are you switching from mild steel to 440? 440 will be more expensive than mild and not much better. Go with 304, its by far the most cost effective stainless.

Eric

[Zippiot]

I saw 347, would that work? They told me 440 was the best suited for heat, and yes I am using cheap mild steel. Just until I find a working design...

In ramjets you can set up the blower to give a bunch of airflow around the engine, but I still bet it would melt down quite rapidly.

Got all the steel cut up rolled and in place, gonna weld it all up tomorrow and plumb it, hopefully I will have a running jet by wed.

[Zippiot]

And I finally got my mig tuned in to weld thin steel, stopped burning through the 22. I might actually buy some powdered core wire, looks like fun stuff!!

[Irvine.J]

I just ran the micro lance for a few seconds, it was beautiful, couldn't even see the flame out the back but you knew it was there, (you could only see heat distortion) I think its a very pretty, sleek and good design, can't wait for the night time tests. (About 2 hours) Very happy. The fuel injectors are working very well, and I'm quite fond of the flameholder. New properly rolled 2.5 foot cardboard nozzle for the blower is working a treat. The engine weighs 200grms (or very close). You are so going down Zippiot and The Cheat LOL!

[Najm]

I'm also entering this challange!!!
But first let me get finished with my midterm exams....
BTW How to measure the thrust??

[Zippiot]

A spring scale on a skateboard is the most basic way. You cant attatch the blower to the skateboard though, as it would add to the net thrust. If the blower is not strapped to the skateboard then the spring measures net thrust, or the total thrust from the ramjet after drag is accounted for.

[larry cottrill]

A spring scale on a skateboard is the most basic way. You cant attatch the blower to the skateboard though, as it would add to the net thrust. If the blower is not strapped to the skateboard then the spring measures net thrust, or the total thrust from the ramjet after drag is accounted for.

For good diffuser performance, it is VERY important that your inlet rim lie well INSIDE the boundary of the blower flow stream. You must not allow the outer edge turbulence to influence your diffuser, if you want maximum performance out of your engine. Remember that in flight, 100 percent of the air hitting the inlet would be at the same speed! That's hard to duplicate with a small physical size blower.

Because of that, note that you WILL have air (probably a lot of it) moving AROUND your engine. It will behoove you to make the engine exterior AND engine mounts as smooth and streamlined as possible - exactly what would be done for the installation of an actual flight engine.

Also, make sure the pull on your measuring device is truly horizontal and parallel to the thrust line - more than a degree or two out of alignment, and you will be introducing another source of error.

More accuracy could be obtained, theoretically, by using a lever arm to work the measuring scale. For example, a 10-to-1 lever would show you 10 kg on the scale for 1 kg of actual thrust, and the precision would be 10 times as good. BUT, that assumes the lever is set up very accurately, so that a scaling error isn't introduced.

Just some thoughts ... helpful, I hope ...

L Cottrill

[larry cottrill]

Sorry - clicked on 'Quote' when I meant 'Edit'.

L Cottrill

[Irvine.J]

I'm taking a break and thought I'd share some fueling information I have learnt recently. Forgive me if I ramble on a bit

As I've now built 4 little and one big ramjet, I have had the opportunity to experiment with a multitude of fuel line locations, nozzles etc. I have come accross 2 interesting points that might later prove helpful...todays test sucked, I was hopefull on the initial 10 second run, as I didn't turn the gas way up, but again tonight I noticed the flame was slightly infront of the flameholder (Which I couldn't see during the daytime) and I wasn't even close to providing adequite fuel flow...

I think now that regardles of where infront of the flameholder the fuel nozzles are, they must point directly or very closely towards the flameholder. Having the nozzles shoot straight out leaves too much fuel/air mix infront of your flameholder, and can promote the flames creeping foward. I believe this is why in most of the drawings that the fuel nozzles are behind or actually part of the flameholder itself.

I think its hard to strike a balence with fuel nozzle location with these aerospike rams, and if this doesn't work this time I'll cut off the spike, and try the design with he same injector on the previous jet. I'm also thinking that 2 lines, entering inline with the FH, one on either side of the spike, similar to a rosco jector should also work.

[larry cottrill]

Flameholders, regardless of how they are designed, always create some turbulence on the downstream side - one of the many reasons you try to keep them small in area. This turbulence is hugely useful in terms of air/fuel mixing - which is probably why Steve's method of spraying fuel from behind toward the flameholder was apparently so successful. A really good method might be to have the flameholders as radial V-section "struts" and blow the fuel outward along the rear "trough" of the V, where it would immediately get wound up in the heavy vortexing just beyond (behind) the strut edges.

It is easy to overestimate the needed size for the flameholders - they can really be a small part of the total area. Think of what a tiny spoiler does to flow along the wing surface behind - a tiny "glitch" in the surface wipes out a huge area behind it! Same thing here, but acting in three dimensions. You don't need to get a flat "flame wall" right behind the diffuser as it's shown in artist's drawings - it will really be a sort of cone or "tent" flaring rearward from each flameholder location. What's important (it seems to me) is to maintain as much of the heat-driven expansion as possible ahead of the exhaust nozzle region, throughout the operating speed range you're interested in.

L Cottrill

[larry cottrill]

I have tried to show in the other thread that building and testing big ramjets is practically hopeless for amateurs of limited means. That being said, I would suggest a LITTLE more standardization for your competition.

How about specifiying a combustion chamber ID of 2.0 inches / 50mm plus or minus some small fraction (maybe +- 2 percent?)? This would, I think, be small enough that everybody should be able to buy or borrow a leaf blower that would actually power it. All other dimensions and all design details would be up to the contestant. Fuel should be propane for everyone, also, but I think you've sort of tacitly agreed to that.

Also, I think you need a standard thrust measurement rig that everyone can build easily and cheaply and that provides the right level of precision. Even something like James D's "water in a bucket" device is VERY accurate if carefully built and used. Or, I could design a lever system out of standard parts that everyone could easily obtain, and then we could settle on some "standard" fisherman's scale or something as the measuring instrument.

One last thing that should be agreed to is that everyone would be required to post a reasonable drawing with "as built" internal measurements that are as accurate as practicable. That way, you could really end up providing information that others could use in the future.

Just some thoughts that came to mind.

L Cottrill

[Zippiot]

Maybe we will have a great ramjet challenge 2 where everything is standardized, but I do like the freedom allowed at the moment. My 5hp shop vac can run a ramjet with a CC up to 3 inches, and my fuel system is optimized for liquids (But I am getting an adapter for propane and mapp gas use!). The shop vac can be bought for 75-100 dollars (on sale for 50 mine was a steal) and can provide over 250 mph winds!

Look at all the different designs just a few of us have done so far, from J's lances to Eric's aerospikes, not to mention those of us who are still working through a design. If we standardize CC and fuel then, I believe b/c of innefficiencies at subsonic speeds, it comes down to who is the best welder/machinist.

I would say a CC of 1.5-3 inches, freedom of fuel and a standard test rig would do fine. But we may end up at that anyways...

Freedom of fuel b/c there are so many different types out there and it seems that some tanks are starving the jets. If getting enough air wasn't hard enough, now they lack fuel? While propane will give us a more accurate measurement of the effectiveness of a jet design, liquid fueled rams are different dimensions. Kerosine/diesel rams are much longer than propane and gasoline rj's.

Larry can you describe the water in a bucket? Draw up a pic of the lever thrust stand, I learn from pictures.

[Eric]

347 is expensive as hell but if you can get it free or dont mind the price then that will work extremely well.

The water in the bucket is basically just a horizontal track with a pulley that raises a mass, could use sand, water, m&m's or whatever you want.

Another thing, I dont know if any of you guys that play around with the ramjets have really gotten into measuring the thrust, but if you have a blower that provides enough air that a lot is blowing over the engine it is going to create some drag. You could hook the thrust measuring system up in reverse and while its not running measure the drag force.

I wouldnt want to be restricted to any sizes otherwise I would have to go and build a small one specially for this, when I have a perfectly nice one already.