"Thunderchine" 55 lb thrust pulsejet plans

[milisavljevic]

Greetings Builders --

While I'm glad you like that bit, you really should take a closer look at the plans.

And apparently, so should I. "Oh noes!"

We will fix this in the first update, but for now, please be advised that the combustor cover
need only be "dished in" a few mm...just enough to give the cover strength against warping.*

Thanks for bringing this to our attention, Larry!

Cheers,
M.

* Note: a 3-4 mm "dish" will do it; more seems excessive.

[larry cottrill]

Thanks for bringing this to our attention, Larry!

* Note: a 3-4 mm "dish" will do it; more seems excessive.

You're welcome. I was under no illusion that the "sombrero" was meant to be a large effect. I just thought it did a little something internal, like getting air or blast gas out "into the corners" or some such.

Unfortunately, details like that always turn out to be a good "monkey trap" for me. I once posed the question of why a continuous spark from a model T spark coil was able to draw in tiny red-hot cinders from a burning cotton ball placed nearby -- whether the attraction was electrostatic or electromagnetic. Ha.

L Cottrill

[milisavljevic]

Hello Larry --

You're welcome. I was under no illusion that the "sombrero" was meant to be a large effect.

It would be an interesting experiment. Thunderchine has excess thrust in the core (ie., Fmax > 55 lbf),
so it could tolerate a "proper" sombrero, thrust-wise. And given the volume of the combustor (5 liters),
anything to promote circulation would be a boon. Design-wise, I have done what can be done: notice it
has a shorter combustor (L/D) than a "classic" chinese, ie., as per the Laird drawing (1.95 vs 2.125:1).

That said, given Thunderchine's intake arrangement, I'm not sure a central deflector is the best choice.
We will introduce a build option to taper the intakes down to 60 mm; this definitely impacts efficiency.*

"Monkey trap" -- I like that.

Cheers,
M.

*The optional tapered intakes improve combustor and fuel efficiency, with a slight drop in core thrust.

[GRIM]

Hi M ,
How critical is the bead blast roughening of the cc and transition cones , bearing in mind that this particular engine is mild steel and will scale very quickly,

[milisavljevic]

Hello Joe!

Mmm... 5.7 liters of goodness. The surface treatment is optional: more potential than a proven benefit.

However... I encourage you to consider this step, perhaps following it up with your WD-40 treatment, to
both the inside and outside of each piece? If this does not appeal, no worries. Btw., don't the plans start
at the other end of the pulsejet? Thunderchine is a "tailpipe-centric" design (thanks to integral ejectors).

Looking good, mate!

Cheers,
M.

PS: Love the movie! Any chance "Kill Bill" will be making an appearance on these pages? Please say "Yes"!

[No. 44]

A shame there doesn't seem to be much room for wrapping fuel lines around the exhaust section for regenerative cooling with those thrust augmentors mounted but as I'm still a bit of a noob on pulsejets I'm not sure if that would be such a good idea with the thermal stress it can cause. Wouldn't want your engine to crack during a test or a demonstration.

[Ghrey]

And now from left field....


What is the maximum allowable radius for each joint?

Picture the jet made from a contiguous piece of material if that helps...


Now franticly clearing shop space...

[larry cottrill]

A couple of interesting (to me, at least) observations about this design:

Like the Lockwood, the total intake area here is far larger than the rear-end "nozzle" throat, although I think the ratio is not as great as in the classic Lockwood (which I have always thought of as pretty extreme, though obviously effective). The intakes operate at fairly high velocities for such a large bore, due to the extreme aft transition location.

The tailcone as a whole is highly efficient. The tail-end (exit face) velocity swings are about 3/4 the amplitude of the corresponding velocities in the intake throat (taken just inside the flared end), but the massflow amplitudes are almost four times as great, even though driven by that proportionately small "nozzle". Outside air intake at the tail during the "breathing" phase of the cycle manages to form a cool air slug that occupies almost 1/4 of the length of the cone. That's a lot better than it might sound, because the diameter is so large at the rear -- it represents a LOT of piston mass ready to be kicked out in the next cycle.

I think that (even without the way cool augmentor) this is a FAR better engine to build than a Lockwood, for the time, effort and material involved. AND, you end up with a truly compact and lightweight engine that will be easy to work into a lot of reasonable mounting situations. The only thing I see "missing" from an engineering standpoint (at this time) is a decent set of tuned vibration-damping engine mounts.

This should be an excellent choice for that "first advanced" engine attempt for anyone, if they want/need a relatively big motor.

L Cottrill

[milisavljevic]

Hello Larry -- Thank you for the kind words!

( your check is in the mail )

The tailcone as a whole is highly efficient.

I have to assume that you meant to write "tailpipe", as the tailpipe has four (4) distinct cones:

[1] tail couple (length = 91 mm)
[2] entry cone (length = 217 mm)
[3] super cone (length = 420 mm)
[4] exit cone (length = 203 mm)

Am I right? Or by "tailcone", do you mean the "exit cone"? The ambiguity continues throughout.

Outside air intake at the tail during the "breathing" phase of the cycle manages
to form a cool air slug that occupies almost 1/4 of the length of the cone.

Again, do you mean 1/4 the length of the tailpipe, or the exit cone? This would only be 51 mm.

(even without the way cool augmentor) this is a FAR better engine to build than a Lockwood...

Thank you! We hope so!

The only thing I see "missing" from an engineering standpoint (at this time)
is a decent set of tuned vibration-damping engine mounts.

Given the wide variety of possible mounting situations, it was best to leave this to the builder;
however, as you may have noticed, the plans promote this website as a source of information.

So, Larry ... Have at it!

It is noted here that while the fundamental resonance frequency of this pulsejet is ca. 160 Hz,
the great majority of acoustic power developed is radiated from the upper harmonics (ca. 320,
480, 800 Hz). Peak radiated power will shift between F2 and F3 in response to thermal output.

These harmonics are a tad high for significant coupling into more likely mounts, eg., go-karts.

Edit: the previous sentence means "higher frequencies are less prone to mechanical coupling
than lower ones"; the Lockwood-Hiller 55 lbf pulsejet runs at a much lower frequency than
Thunderchine and radiates strongly around its fundamental frequency. Thunderchine was
designed (in part) to mitigate this vibration problem. I apologise for any ambiguity. -- M.

That said, I would not want to stand anywhere near the business end of one of these beasties!

Cheers!
M.

[larry cottrill]

Hello Larry -- Thank you for the kind words! :D

You're welcome. I wouldn't say it if I had any serious doubt.

The tailcone as a whole is highly efficient.

I have to assume that you meant to write "tailpipe", as the tailpipe has four (4) distinct cones:

[1] tail couple (length = 91 mm)
[2] entry cone (length = 217 mm)
[3] super cone (length = 420 mm)
[4] exit cone (length = 203 mm)

Am I right? Or by "tailcone", do you mean the "exit cone"? The ambiguity continues throughout.

Well, I apologize for that. I meant the quadruple cone assembly taken as a whole. "Tailpipe" just doesn't seem right somehow.

Outside air intake at the tail during the "breathing" phase of the cycle manages
to form a cool air slug that occupies almost 1/4 of the length of the cone.

Again, do you mean 1/4 the length of the tailpipe, or the exit cone? This would only be 51 mm.

No, again I mean the whole thing.

(even without the way cool augmentor) this is a FAR better engine to build than a Lockwood...

Thank you! We hope so! :D

I see no comparison, for the cost and effort involved. The only difficulties for a good amateur builder are:
- The precision of the pieces needed to achieve full power potential (but that's going to be true of any "advanced" design)
- The welding of the rear edge of the transition pieces to the rear chamber cone -- and you've specifically detailed a reasonable approach to that problem.
(Of course, I still don't think it's as nice as my Lady Anne Rev 07 intake, but hey ... ;-)

The only thing I see "missing" from an engineering standpoint (at this time)
is a decent set of tuned vibration-damping engine mounts.

Given the wide variety of possible mounting situations, it was best to leave this to the builder;
however, as you may have noticed, the plans promote this website as a source of information.

Understood. I didn't think I should rave without quibbling somewhere along the line ;-) More seriously, I was cluing prospective builders into the fact that the vibration (essentially, repeated impact loads) of an engine like this is going to be a MAJOR part of the mounting problem (and also the thrust measuring problem, of course).

It is noted here that while the fundamental resonance frequency of this pulsejet is ca. 160 Hz,
the great majority of acoustic power developed is radiated from the upper harmonics (ca. 320,
480, 800 Hz). Peak radiated power will shift between F2 and F3 in response to thermal output.
These harmonics are a tad high for significant coupling into more likely mounts, eg., go-karts.

EDIT: The following response is a misunderstanding of what M just said in the quoted passage; he is saying that vibration should be NO problem for something the size and mass of a kart frame, and this is correct. Please read on through next few posts for clarification! L Cottrill
Exactly. Mounting, especially on a light-frame vehicle, is going to be a CRUCIAL issue. Of course, that's exactly the kind of thing that amateurs are good at innovating, and there are many ways to accomplish it. However you do it, though, it will need to be thought through with great care. You do NOT want one of these breaking loose unexpectedly (due to say, metal fatigue), at any power level! It is NOT the same situation as mounting a Dynajet size engine in a scale or speed model plane (an utterly trivial problem by comparison).

L Cottrill

[milisavljevic]

Let's try this again...

It is noted here that while the fundamental resonance frequency of this pulsejet is ca. 160 Hz,
the great majority of acoustic power developed is radiated from the upper harmonics (ca. 320,
480, 800 Hz). Peak radiated power will shift between F2 and F3 in response to thermal output.

These harmonics are a tad high for significant coupling into more likely mounts, eg., go-karts.

Exactly. Mounting, especially on a light-frame vehicle, is going to be a CRUCIAL issue.

Umm... Larry, you have completely misunderstood what I have written.
What I have said is that vibration coupling is not likely to be a problem.

Thunderchine will present LESS of a problem than the Lockwood-Hiller,
for the reason I provided, above. You really need to do the calculations,
and not simply offer opinion; this is wandering dangerously close to FUD.

Think before you post another reply.

Cheers,
M.

[milisavljevic]

Hello Larry --

Of course, I still don't think it's as nice as my Lady Anne Rev 07 intake, but hey ...

Yours IS a very nice intake, but it is also not fully specified. You left bits to the builder's interpretation.
And that I could not afford to do with Thunderchine. I am held to a different standard of accountability.

Cheers,
M.

[larry cottrill]

It is noted here that while the fundamental resonance frequency of this pulsejet is ca. 160 Hz,
the great majority of acoustic power developed is radiated from the upper harmonics (ca. 320,
480, 800 Hz). Peak radiated power will shift between F2 and F3 in response to thermal output.

These harmonics are a tad high for significant coupling into more likely mounts, eg., go-karts.

Exactly. Mounting, especially on a light-frame vehicle, is going to be a CRUCIAL issue.

Umm... Larry, you have completely misunderstood what I have written.
What I have said is that vibration coupling is not likely to be a problem.

Well, you're right of course, I DID misunderstand. What you're really saying is something like what I have said before:
If the object you bolt your engine to is massive enough so that its natural frequency is far BELOW the engine frequency(ies) in question, then frequency coupling will be no problem for the mass as a whole. I have stated that this is essential to good thrust measurement, for example. So I concede my immediate error on that point.

HOWEVER, the problem of pulsejet engine mounting is not just how the WHOLE driven mass reacts. Engines tend to be mounted at POINTS -- in reality, small assemblies of objects with relatively small area and mass. It is just as important that these "little pads" not respond in tune to any of the frequencies in question, and be designed practically non-fatiguing. So, in that "micro" sense, I think what I said still applies.

L Cottrill

[milisavljevic]

Hello Larry --

Engines tend to be mounted at POINTS - in reality, small assemblies of objects with relatively small area and mass.
... It is just as important that these "little pads" not respond in tune to any of the frequencies in question,
... and be designed practically non-fatiguing. So, in that "micro" sense, I think what I said still applies.

Agreed. However, this IS NOT more of a problem for Thunderchine than the Lockwood-Hiller it compares to.
As Thunderchine is lighter than the Lockwood, it may be a lesser problem. IOW, yours is a general comment.

That means it applies equally well to your Lady Nancy Astor, or wakmat's "Astor M special" or the Kentfield.

Btw., I amended my previous post to clarify the apparent ambiguity.

Cheers,
M.

PS: Have a nice day... I need to sleep now.

[larry cottrill]

For when you awaken:

Engines tend to be mounted at POINTS - in reality, small assemblies of objects with relatively small area and mass.
... It is just as important that these "little pads" not respond in tune to any of the frequencies in question,
... and be designed practically non-fatiguing. So, in that "micro" sense, I think what I said still applies.

Agreed. However, this IS NOT more of a problem for Thunderchine than the Lockwood-Hiller it compares to.
As Thunderchine is lighter than the Lockwood, it may be a lesser problem. IOW, yours is a general comment.

That means it applies equally well to your Lady Nancy Astor, or wakmat's "Astor M special" or the Kentfield.

Oh no, I didn't mean to imply (or believe I said) that this was some Special Property of your engine at all.

It's just that a lot of hobbyists don't give too much thought to the details of mounting really large engines. Consider the photos we've seen of fairly big Lockwoods mounted on spindly trusses of 1/2-inch square steel tubing, etc. All I was trying to do was remind folks out there that you had better not think you can take something this size and just strap it onto your hastily converted kart right behind your head with a couple of hose clamps and hope for the best. Just trying to preclude something like that kind of approach.

The general tendency seems to be to mount small engines with way more "beef" than needed and large ones with not nearly enough (part of the reason being that the hardware stresses don't "scale", in the usual linear sense of the term). That's bad enough on the test bench, but potentially deadly when you're actually getting pushed around by your motor. How many amateur builders even consider the large degree of extra strength needed for crash loading at reasonably attainable speed? ("Oh, I'll NEVER let that happen.")

L Cottrill