Jet turbine Observations for Pulse Jets

[Jutte]

Some observations from working with Jet Turbine engines.
1. They are incredibly expensive
2. They are incredibly complex.
3. Modern electronic fuel systems are incredibly complex . The older fuel systems are incredibly complex as well.
4. Modern Jet turbines are designed for many different niches.
For example turbo prop for commuter type situations, modern
high bypass turbines for economic transport of high numbers of passengers or freight , modern military jet turbines for ‘economic’ eye watering performance of fighters – to providing power for transport aircraft, helicopters, tanks , ships etc.
Also the use of turbine engines for fixed power generation

You probably all know these observations anyway – but it is good to bring them up.

Things/ideas that could be crossed over to pulse jets from turbines…
Pulse jets ...
1. Range from ridiculously cheap to relatively expensive ( ie: stainless steel)
2. Fairly simple. This statement does not include the relatively
complex physics formulae in how they actually do what they do. Suffice to say Pulse Jets will provide hours of fun for the garage tinkerer to the University Professor and his students.
3. OK - here is where it gets interesting.
The Jet turbine engines I have worked on have a dual nozzle system. One ‘set’ for starting – another for main running. However in saying that the nozzles are one piece – the secondary flow still is gong when the primary is on.
So perhaps a dual or double nozzle for pulse jets could a worthwhile idea. ( eg: Roscoe for starting and another type for main running thrust etc). On the fuel economy side of things modern turbines use electronic fuel systems and variable compressor stator vane angles to gain maximum fuel economy. However they get their main fuel economy at their maximum height for cruising - the passenger airline that can get a 1% better fuel economy will save millions of dollars in fuel bill over the long haul.
So while Pulse jets can’t really do the above( the garage lab just isn‘t going to get a pulse jet to 36,000 feet)- we could aim at good fuel ‘economy’ at a certain thrust value. We could do electronic fuel delivery via the thrust / intake cycle – but that would involve a mechanical or complex electronic solution ; and at the end of the day at the ‘garage level’ is it really worth it ?( plus some university data doesn’t seem to stack up in the real world -ie: Eric’s comments re the AIAA Pulse fuel paper ).So basically it is an open field here. I see that one Boeing patent seems to be trying to control the flame shape by introducing intake gases into the later part of the combustion chamber . So how important is the nozzle shape construction? It should be noted that a ‘simple’ fuel nozzle change in one of the F104 engine series saw an increase in thrust and ‘economy’.
4. Also another obvious question is what do you want your pulse jet for?( this can also a fuel economy question)
Garage heater, demonstration model, something for the ‘hell of it’, propulsion for some
aerial/ground machine (manned or unmanned) or what ever. Form can dictate construction so again it is an open field here to experiment in.

Finally some ideas for all us experimenters…

* Shaped intakes – treated as exhaust pipes ( bustles etc) - + internally shaped intakes ala the Russian Enics engines
*Choked tail pipe nozzles ( some of Chadlys engines exhibit this )
*Internal choked exhaust nozzles.( some of the early military jets do this before the coming of the variable jet nozzle)
*Multiple fishmouths ( eg. pressure jet) to reduce sound signature. If you want to get some good ideas
for sound reduction check out what the Jet Turbine manufacturers do( air mixing to reduce gas shearing, sound proofing materials etc). But be aware it does reduce thrust – and that’s just one of the reasons they have got bigger .Mr Cotterills outer shells beg for experimentation in this regard ( remember anything you hang off or on a Pulse Jet needs to be ‘tuned’ for it to work).
*Electronic ‘hydro dynamic valves’ (the valve when you’re not having a valve) – anyone done this yet?
* Valveless Pulse Jet to Ram Jet hybrids. Would involve some tricky supersonic intake problems – not what the garage lab would be capable of but still nice to ponder about. (But should be still doable on a small scale)
*Propeller driven by a Pulse Jet. Yes – turbines hate unsteady flow but pulse jets don’t use turbines.
How about an exhaust impeller type arrangement with an exhaust valved system for the exhaust return flow ( prop overcoming the reduced back flow via inertia?) Light weight ultra light engine anyone? ( which of course brings us back to # 4)
*Use of cool intake flow to induce cooling, or pressurising a fuel tank ( plus the inherent valves needed for the tank. Note: WW2 V1 )
These are just some ideas - comments anyone?
Finally to everyone here I wish you all a Merry Christmas and New Year.

[Johansson]

Interesting observations!

But I cannot agree with you that pulsejets are simpler than gas turbines, a GT is all about temps and flow which can be (roughly) understood simply by looking at the parts. Try the same on a pulsejet with its frequences and waves...

[metiz]

which can be (roughly) understood simply looking at the parts. Try the same on a pulsejet with its frequences and waves...

Ha works for me!

[Jutte]

Works for me too!
Although in saying that I did wish for the Pulse Jets simplicity after spending heaps of hours
inspecting and blending out pitting damage of the 12th stage compressor of a JT8!