Valvless Turbine Idea

[Mike Everman]

I promise I’ll make something someday and post actual photos. For now, I am spinning freely trying to decide on what to try first. Naturally, I should build a tried and true design to gain experience (thanks, Bill ;-D) but it’s just not my way…
I’m posting this idea with some obvious problems for discussion. I like the idea of a spinning engine, if we can capitalize somehow on the centripetal forces for compression(?) or allowing shorter exhaust length(as shown?)
This one is roughly 10â€

[Bruce]

This is not too dissimilar to a design that NASA patented a few years ago -- except that theirs uses constant-combustion.

Unfortunately (and there has to be a reason) it's never been turned from a concept into a reality.

[Bruno Ogorelec]

My problem with the design is the temperature at which a typical pulsejet operates. This thingy will spin at great rpm and generate huge centrifugal forces. The material will be heated to very high temperatures, yet expected to withstand those forces. Not a good combination.

[Mike Everman]

This thingy will spin at great rpm and generate huge centrifugal forces. The material will be heated to very high temperatures, yet expected to withstand those forces. Not a good combination.

As with all my turbine ideas, the rpm must be limited by a variable pitch prop, fan or other power take-off, with or without speed reducer; here more than a ramjet would be. I think high rpm's on this one will tend to extinguish it, but there may be crossing curves as to the benefit of centrifugal compression (it will be modest rpms to obtain 2atm in the CC) and that same force preventing the exhaust from igniting the CC in a sustaining cycle. (My back-of-the-envelope calcs show you can get 5atm on a 200mm radius at only 2,150 rpm.)
Also, this design and any follow-ons are creatable in any material by conventional machinery that I have, even machinable ceramic (Macor). Though I beleive this type of design can cool itself well enough to be aluminum alloy(?) But titanium is easy to machine, too, which would make a lot of strength and thermal issues moot.

This is not too dissimilar to a design that NASA patented a few years ago -- except that theirs uses constant-combustion.

Unfortunately (and there has to be a reason) it's never been turned from a concept into a reality.

I remember seeing that in Tech Briefs, but didn't save it. I'd love a project name or link, if you've got it... My philosophy Bruce, is to research all that has come before me and use my fairly good internal randomizer (he said modestly :) to propose configurations for evaluation, pick the most promising, and develop. I therefore have my room of shattered dreams; a hundred devices actually built of 1000 ideas, whose best application might be "boat anchor", but one in 200 is completely new, and has benefits. Every new thing, old news to you guys or not, can spark a wild idea, mostly bad, but the dream is that one will be good over the next few years. For now it's just a glimmer that pulsing combustion has the potential answer that continuous just can't create.

It's heartening to think that if you had equivalent of the thrust of only 25 lb tangentially at a 1m diameter, you'd have the performance of a popular ultralight aircraft powerplant....

[Bruce]

Yeah, I long ago discovered that "good ideas" are simply the collision of a problem with a solution. If you know what your problems are, then fill your head with enough solutions (even ones that seem totally unrelated) then eventually you'll have such collisions and the resultant "good ideas" that they produce.

To this end, reading as much prior-art as you can is always a good thing -- especially if you simply assimilate the information without allowing it to blinker you other options. I've seen many academics who simply dismiss ideas out of hand because they've been taught that such things are "impossible" -- they're blinkered by their knowledge.

The really *clever* people never believe anything is impossible until they've proved it to themselves -- and even then they're not completely sure ;-)

You mention a figure of (does mental aritbmetic) about 115 Newton metres of torque -- but at what RPMs? After all, it's *power* not force that is the important measure when you're talking about shaft-driven devices.

[Mike Everman]

The really *clever* people never believe anything is impossible until they've proved it to themselves -- and even then they're not completely sure ;-)

People believe a lot of things just because they are in print. Too many things are even taught in school that are theory, not fact at all. The Big Bang theory for one. I make a point of asking any physics student I meet if they believe in the Big Bang. Usually, it has been taught to them as fact, not theory, not taught that there are competing theories.

You mention a figure of (does mental aritbmetic) about 115 Newton metres of torque -- but at what RPMs? After all, it's *power* not force that is the important measure when you're talking about shaft-driven devices.

Quite true, regrettable omission. The motor I chose for comparison was a Hirth 3203, 625CC, 65HP@6,500 RPM; but a survey of typical properties gets you 50-60 ft-lb @5-6,000 RPM. Typical two blade ultralight prop is 65" dia turning at 27-2800 RPM so usually a belt reducer is used to get the motor in it's sweet spot, say 2:1.
If 60 ft-lb is desired at 6,000RPM, then your two tip jets would be delivering only 12 lb thrust each on a 30" radius; and a 30" radius has the jets (if they are actually out at the tips, not my favorite place considering the forces involved) moving through the air at about 19,000 ips, (1,570 fps or 1,000 MPH.) So, ideally, you'd design for twice that thrust and get your velocities down to 500 MPH, or iterate as necessary for whatever blows your skirt up in the way of ideal speed... ;-)

[Bruce]

> If 60 ft-lb is desired at
> 6,000RPM, then your two tip jets
> would be delivering only 12 lb
> thrust each on a 30" radius; and
> a 30" radius has the jets (if
> they are actually out at the
> tips, not my favorite place
> considering the forces involved)
> moving through the air at about
> 19,000 ips, (1,570 fps or 1,000
> ? MPH.) So, ideally, you'd design
> for twice that thrust and get
> your velocities down to 500 MPH,
> or iterate as necessary for
> whatever blows your skirt up in
> the way of ideal speed

But don't forget that the 12 or 24lbs thrust must be the excess of thrust over drag at 1000/500 mph

This would nean a significantly larger engine would be required to produce x/ft-lbs at 6,000 RPMs compared to that which produces a static torque of x/ft-lbs.

Then there are the effects of the huge centripetal accelerations associated with tip-jets -- there's a great NACA document that goes into this in some depth.

I think the downsides outweigh the upsides -- hence we see very little of this technology being used beyond the 1950's and 60's.

Perhaps the only commercial use was the Fairly Rotodyne in the late 1950's but it had a very limited life and was hardly a run-away success :-)

[Mike Everman]

But don't forget that the 12 or 24lbs thrust must be the excess of thrust over drag at 1000/500 mph

This would nean a significantly larger engine would be required to produce x/ft-lbs at 6,000 RPMs compared to that which produces a static torque of x/ft-lbs

Then there are the effects of the huge centripetal accelerations associated with tip-jets --

Too true, remember I said I didn't like the jet out at the tips, it was by way of example, an easy way to look at the math... ;-) I'm thinking more a completely seperate unit, as depicted, which has less the aerodynamic and centripetal problems, like a real turboshaft. Pehaps (4) .5m diameter units as shown or similar on the same shaft, one or two stage belt reduction to a prop depending on the power band, though the pulse jets have quite good static performance and you won't have to spin this type of turbine up fast to get the torque out of it! Waitaminnit! It pretty much has constant torque at a wide range of rpm's, so therein lies the advantage of a pulse cycle turbine...

[Bruce]

No matter how you do it though, fuel efficiency is going to be a real problem since:

a) you're using a pulsejet

b) it's acting as a pure reaction turbine which is less efficient than the impulse/reaction turbine used in conventional gas-turbine engines by quite a significant degree.

Then there are other problems such as re-breathing the exhaust gases of the engine in front.

This latter problem could be reduced by mounting the engines, not at a tangent to the arc of rotation, but with the exhaust facing slightly outwards -- blowing the exhaust gases away from the center of the rotational disk. However, that will effectively waste some of the thrust and reduce efficiency even further.

So why not build the pulsejet so that it has its intake at the center of the disk and its exhaust at the edge (like your turbine disk). Well that engine's going to have more drag than a system that uses thin or aerofoil shaped arms to hold the pulsejets away from the center -- which means bigger engines and less efficiency again.

Now that we have cheap, reliable, relatively efficient small turbojets available for the model airplane market (capable of 30lbs of thrust or more) have you noticed how nobody (that I've seen) has suggested using these as tip or arm-mounted engines for a personal helicopter?

Makes you wonder why not eh? ;-)

[Bruno Ogorelec]

Bruce is right, as far as I'm concerned.

I love the pulsejet cancept and have devoted a nice chunk of my life to its study, but it has not made me blind. (Hair has not grown on my palms either.) To me, it is obvious that the turbojet and the turboshaft are the best two ways to use internal combustion of hydrocarbons for propulsion.

Some distance in the future, PDEs might perhaps take over in one segment of turbojet applications. Until then, the turbine will reign supreme.

I see the pulsejet as a clear alternative in two distinct areas. One is lightweight and (especially) cheap applications. Namely, purpose-built turbines will always be expensive. Turbocharger-based turbines will always be inefficient. Both will always be more massive than pulsejets and their efficiency will diminish with size.

The other area of promise is in gas generation for turbines – historically the very first practical application of pulsating combustion (by Karavodine in 1906).

The pulsejet still suffers from problems in both areas and I think the only fruitful course for any pulsejet developer is in the effort to remove those problems.

[Mike Everman]

Now that we have cheap, reliable, relatively efficient small turbojets available for the model airplane market (capable of 30lbs of thrust or more) have you noticed how nobody (that I've seen) has suggested using these as tip or arm-mounted engines for a personal helicopter?

Probably because you don't have to dig far on the subject to find out that the extra drag and mass on the rotors makes them a deathtrap when power is lost! (nearly impossible to autorotate in emergency.) Point taken, though; once I put together 4 or 6 of these to do some reasonable task, I've forked out enough to buy a real turbojet! (Though I'm trying like hell to make the elements cheap)

Some distance in the future, PDEs might perhaps take over in one segment of turbojet applications. Until then, the turbine will reign supreme.

Ah. But how best to harness detonations to create torque?

[Bruno Ogorelec]

Ah. But how best to harness detonations to create torque?

Um... I can't think of an obviosu way, even of something wildly improbable. But, you have turboshafts for torque. Use PDEs for pure jet propulsion. That's the only realyl good thing they are likely to be good for (apart from shaking walls loose with noise).

Bruno

[Mike Everman]

(apart from shaking walls loose with noise).

I believe in the possibility that you can cancel most of this noise. I've done quite a bit of research into cancellation methods for less energetic noise, and clearly the hard part is going to be using the engine's own noise energy for the cancellation; amplifiers and speakers are flat out!

And Bruce, before you say phase shifting of two engines, it can't work for several reasons:
1. their noise signatures would have to be exactly sinusoidal, which they are not. Out of phase engines could reduce some of the cyclic noise, and will amplify some of it to titanic proportions!
2. they would have to be the exact same frequency and amplitude which they will never be
3. and the most important reason is that they are some distance apart. Simply put, the distance between noise source and cancellation source places the fundamental limit on the frequencies you will be cancelling; literally, if the exhausts are 10" apart, the only frequencies cancelled to any degree are of a wavelength of 10" or longer. Since the audible range for us humans is 20 Hz to 20 KHz, the wavelength cutoff at 10" is 1304 Hz or lower affected, leaving the fat part of the nastiest end of the spectrum. Granted the pulsejets are making most of the noise at the very low end, so at that, out of phase tubes will help more than hurt, I think.

I'm sure someone else here could expound more knowledgably than me! These are the basic scratchings of which I am capable, and surprise, I do have a design to address this! Want to see? ;-D

[Mike Everman]

No matter how you do it though, fuel efficiency is going to be a real problem since:

a) you're using a pulsejet

b) it's acting as a pure reaction turbine which is less efficient than the impulse/reaction turbine used in conventional gas-turbine engines by quite a significant degree.

Then there are other problems such as re-breathing the exhaust gases of the engine in front.

This latter problem could be reduced by mounting the engines, not at a tangent to the arc of rotation, but with the exhaust facing slightly outwards -- blowing the exhaust gases away from the center of the rotational disk. However, that will effectively waste some of the thrust and reduce efficiency even further.

So why not build the pulsejet so that it has its intake at the center of the disk and its exhaust at the edge (like your turbine disk). Well that engine's going to have more drag than a system that uses thin or aerofoil shaped arms to hold the pulsejets away from the center -- which means bigger engines and less efficiency again.

I'll give you that it's low efficiency, but maybe not as much as you might think. Turboprops are much more fuel efficient than turbo-jets, even with all the parasitic mass of gearboxes and other things to do with transmitting torque, mainly because you are converting the jet's energy such that you are moving more air slower. I am most interested in VTOL, so a pure jet solution will be horrificly inefficient, not just at the pulse jet end, but because of the disparity in exhaust velocity to velocity of my craft. I do not want to make a helicopter, I draw the line at ducted fan, but if we can do a pure jet that is augmented to a rediculous level, then you're talking.

As to your comments at the end of the quote, I've already addressed most of your concerns with the design, low(ish) drag, center-out, no exhaust breathing, etc, so I agree with all you are saying! ;-)

[Mike Everman]

Found a Japanese pulse jet turbine.