Thermonese Thrust Test Troubles

[metiz]

Some time ago I made the 5 pound thermonese. It performed great, above expectations so I decided to make a couple of upscale engines on paper to about 50 pound. I recently made the engine designed for 20 pounds. James ran it through NUDiS for me and it stated it expected up to 27 pounds of thrust. Awesome right?

So yesterday when I decided to thrust test it, I was fully expecting a reading at 22+ pounds. It was TEN So I have only one question: why

The plans for this engine are posted below and so are some photographs of the engine itself. There's also a video of the thrust test. The intakes had a weird heat pattern - I do not know if it has any correlation with the performance but I made some pics of them anyway

This is a video of the first test. near the end you can see me putting my hand behind the exhaust and pulling it away quite fast because it was burning my hand, even a very low throttle
http://www.youtube.com/watch?v=UTlXjtif ... annel_page

The injectors were 5mm external D, 3mm internal, Rossco's and positioned 30% in the intakes. The injectors were kind of malformed but that didn't stop the engine from running great in my first test so I didn't alter anything. see plans for more on that

I used vapour to get the engine up to about 80% and the remaining 20% I did with liquid.

In the video, the coughing you hear and see are my injectors not liking the liquid/ not liking vapour+liquid at the same time.

Ambient temperature was about 15 degrees

I can not see any obvious design flaws other then a couple of things I could tweak the engine with.

Does anyone have any ideas or suggestions as to why this engine performed like I was feeding it fairies instead of high energy fuel and what I can do about it?

[milisavljevic]

.

So I have only one question: why?

I'm on it!

I previously validated your 5-lber, so this one should pose no problem for my model.
I will post again once the model converges. And then we shall see what can be seen.

Cheers,
M.

[milisavljevic]

.

I will post again once the model converges.

Well, turns out there was no need to converge a model. Static thrust was available early on
and so I decided to "pull the plug" on this exercise until more information is made available.

If I consider the stated dimensions only, then sea level static thrust at 15 C is 14.8-15.3 lbs.
Let's call these "ideal world" results at net 15 lbs. Nothing more is possible with this design.

Not ever. Well, not ever, unless Fedde immerses it in his hypothetical 4 bar testing cell.

On the other hand, no build is perfect; there are always dimensional variances. You need to
carefully audit the "as-built" dimensions, especially the inlet and tailpipe internal diameters.

If you look closely at the second photo you supplied, you will notice that one of your intakes
is pinched just after the inlet flare. If this is not an optical effect and both intakes have an
inlet constriction of the magnitude seen here, the predicted thrust falls to about 11-12 lbs.

Limitations or problems in fuel delivery could easily take 11-12 lbs down to the observed 10
pounds, so I would ask you to carefully measure the internal diameters of your twin intakes.

If you intakes are not pinched, and the other dimensions are to spec., then you will need to
look closely at improving your fuel delivery system. I am happy to check the "as-built" dims.

Just post them here.

Good luck,
M.

[ace_fedde]

M.,

Not ever. Well, not ever, unless Fedde immerses it in his hypothetical 4 bar testing cell.

Not 4 bar, 4 atm=3 bar (in fact 3080mbar), at least in technical terms: Zero bar (overpressure) is still 1 atm. Oh dear, how people can make life difficult…
But entirely not important, the idea stays the same.

I previously validated your 5-lber, so this one should pose no problem for my model.

So you have the .qsav for the 5lb thermonese?
Could you, would you, please, maybe, perhaps, mozda,… see what happens at 4 atm (or 3 bar)?
Or could you pass on the .qsav to Metiz?

Thanks!!

Fedde

[Eric]

Looks short for its size, and way small for 20+ lbs.

Needing liquid for 10lbs thrust is also not a good sign.

[metiz]

Well, crap. 15 pounds max is, well, sh*t. Thanks for running it through its paces, M.

Yeah the intakes are not perfect. I found a dent or 2 here and there, probably because the engine bumped into the workbench or something.

With fuel delivery and (minor) construction faults I do see this engine not pull its weight. BUT I have a trick up my sleave! The tailcone has an angle (on paper) of 4.15 degrees and I think it could be bigger, maybe closer to 4.8 or even the full 5. With those mods, I'd say the tailpipe should be cut shorter by about 30% to. back to the drawing board!

[metiz]

Looks short for its size, and way small for 20+ lbs.

I don't nececarily agree. this engine is an upscale of my 5 pounder and that one performed above expectations

Needing liquid for 10lbs thrust is also not a good sign.

I agree. that said, I have a 15 pound chinese here, one of yours, that needs quite a bit more liquid to even get up to 15 and that's a chinese and not a fuel hungry thermo

[milisavljevic]

.

Not 4 bar, 4 atm=3 bar (in fact 3080mbar).

Sorry. It is 4 bar, total pressure, as I used the word "immerse". Please insert coin and try again.

So you have the .qsav for the 5lb thermonese?

Sorry. I do not use NUDiS. I have never used NUDiS. I will never use NUDiS. No .qsav is available.

And yes, if I had the .qsav file for this pulsejet design, then I would gladly share it with you both.

Cheers,
M.

[ace_fedde]

M.,

Sorry. It is 4 bar, total pressure, as I used the word "immerse". Please insert coin and try again.

O.k. you win

Sorry. I do not use NUDiS. I have never used NUDiS. I will never use NUDiS. No .qsav is available.

Makes me wonder. Uflow it is? Or is there more in this wonderous world?

Fedde

[metiz]

alrightly couple of quick mods. Ramped the tailcone angle right up to 5 degrees and lenghtend the tailpipe to 270. Other dimensions remain unchanged.

[milisavljevic]

Hello Metiz,

Thanks for running it through its paces, M.

No problem. I was happy to do it. The 5-lber was a real treat to model. It "punches above its weight".

Yeah the intakes are not perfect.

Keep in mind that 100% of the oxygen for combustion comes through the intakes. "Handle with care".

Back to the drawing board!

Good luck with that! Oh! You have already changed your design (new post). Will check after sleeping.

Cheers,
M.

[milisavljevic]

Hello Fedde,

Makes me wonder. Uflow it is? Or is there more in this wonderous world?

"There are more things in heaven and earth, Horatio, than are dreamt of in your philosophy."
-- Hamlet Act 1, Scene 5

UFlow it is certainly not. Not with green eggs and ham, not in the rain, nor in an Air France plane.

Cheers,
M.

[Eric]

What kind of injector were you using? I've gotten over 17 lbs out of the '12' lb chinese with vapor only and it wasnt particularly warm out and wasnt able to flame out. With a proper injector the flame out the intake of the chinese should just be a tight skinny flame at max throttle, if its a big fat flame the injector needs to be placed deeper or changed to a different type.

If the engine is too short, or not fat enough, the combustion wont have enough resistance to expansion to do any meaningful work.

Bigger intakes are not always better. If you want big intakes its better to go with a single intake, there are less losses as far as breathing in and out, if breathing in and out is what you want to do. With the typical chinese or bent engine, you want it to breath in and out of the intake.

Multiple intakes are generally for controlling flow in and out of the intakes. Small intakes, with propane as fuel, act as a one way valve compared to more even in and out flow bias of a chinese.

When done properly, the intakes expel so little exhaust that they may as well be a valve. The combustion products want to go out of the tail, which is where you can efficiently generate thrust. When scaled up real large, the thermo's need more than 2 intakes to maintain this effect.

You're small one probably worked better because the intakes were smaller relative to the fuel jet, giving less back flow. But there is no reason the little one lengthened a bit, with a fatter tail, and smaller intakes with larger flares, should put out any less than 8 lbs. I wouldnt consider 5.2 lbs thrust a big increase for a chinese type tail, when a straight tail thermo of the same size can put out 4.5.

Short fat engines maybe able to produce a lot of thrust, but often at significantly higher fuel consumption costs. A longer engine will be better able to harness the expansion to produce thrust efficiently.

[Irvine.J]

Metiz It seems I infact ran nudis on your 45/50 lb picture. My bad.
I also told you that with nudis when you turn two intakes into 1 your error margin grows widely. Theres nothing wrong with nudis, its a useful tool when you know how to read it.

[Eric]

Extrapolating from your dimensioned drawing, the engine is approximately the size of my 10lb thermo.

Once you get injectors set up properly to inject enough fuel I would expect it to put out about 11.5-12.5 lbs thrust with the expanded tail, which would correlate to the increase in thrust from your smaller version compared to a straight tail thermo. Could be more or less depending on your exact dimensions.