The Mamba folks told me in Toledo that their turbine is more fuel efficient than ano other 11-12 pound thrust turbine on the market today. That sounds good to me, but would there be any benefit in fitting another compressor stage to get the pressure ratio higher. Would there be a good fuel economy benefit? I would be willing to pay a higher initial turbine price for reduced fuel flow.
Also I hate the idea of a starter right infront of the turbine due to FOD. Has anyone tried a starter mounted inside the front cover?

What are your thoughts?

Are you considering modifying an existing turbine engine to have an additional compressor stage, or are you thinking of designing a completely new engine with a two stage compressor? Either way you are in for a lot more work than you seem to realize!!!
I am no expert on the subject, but I would guess that (with centrifigal compressors, which are what is used on every model turbine engine on the market today) the additional mass and complexity added to an engine in order to have a multiple stage compressor will almost certainly cancell out any performance gain you might achieve with such a setup.
Adding an additional centrifigal compressor stage to and engine would require considerable machining for internal ducting and would probably lengthen the engine by around 30%. The longer engine would require a longer main shaft which in turn would require more bearings to support it. This is assuming of course that you were able to get by with only a single shaft (i.e. both compressors driven by a single turbine wheel at the same rpm) which is doubtful. Using available compressors (automotive turbo wheels) it is more likely that in order to achieve maximum boost you would have to select compressors with different pitches and drive them at different rpm's which would then require multiple turbine wheels and multiple shafts, along with more than twice as many bearings, which will all require lubrication. You are 'as they say' building a boat in the basement!


As for your other concern, why does the starter mounted in front of the turbine cause you more concern for FOD? Are you worried about the starter motor itself coming apart?
If this is your concern then don't worry. In all of the designs I have seen there is very little chance that anything like this could happen. Two of the major manufacturers use an aluminum tripod mounted to the front cover that supports a machined aluminum "pod" that houses the starter motor. The legs of the tripod are either thick walled tubine with a bolt running down the center from inside the pod to the front cover, or they are threaded with a screw at each end. In either case if one or more of the leg securing bolts were to loosen and let go I doubt the remaining leg(s) would allow it to move enough to go into the compressor, and the hardware that came off of it would be captured inside of either the pod or the front cover, and it is likely that if one leg started to loosen you would have plenty of warning before it let go completely in that the engine wouldn't start because the starter was not able to engage the compressor spinner. The only other starter motor mounting design that I am aware of is what is used on the Jetcat engines. They use a one piece molded nylon mount/starter pod. It is one piece, and will not fit into the compressor. As far as I know there has never been a failure of either design during normal operations.
By far the most FOD that any model turbine will ever see is from insects, grass, sand, and gravel. The only FOD that has ever done any damage to my turbine was a bubble gum card sized candy wrapper that zipped into my intake during a full throttle run up on the ground.
The added complexity of gearing a starter from the side (or inside the front cover) would significantly add to cost fo producing an engine, and your concerns about a front mounted starter causing FOD are mostly invalid. There is no need to fix what is not broken.

Kevin Whitlow

Hi Kevin,

That was a bloody good answer.

BRG,
Chris at TJT.

Kevin, what was the extent of the damage from the candy wrapper? Thanks.

Hey Mark,

From what I could gather the candy wrapper could not be re used.

BRG,
Chris.

Guess Kevin had to wrap his "candy" in something else. Probably got it at Showgirls near Mississippi Afterburner!

Centrifugal compressors go hand and hand with poor fuel consumption, especially at the altitudes we fly at. Turbines are very in efficient until you get them up to less dense altitudes, and we don't fly there.

Hi guys
on a serious note, you should see what afinger will do to an engine if ingested, never mind a candy wrapper, I have some very good pics for a guy who fed his finger into a KJ66 compressor.

BRG,
Chris.

Lets see the pics of the finger

yeah, lets see them finger pics!

Kevin W said:
It sounded expertly to me!!!

Yikes, I got a little long winded there didn't I? Just goes to show that I shouldn't post on the forum after having more than 3 beers.
The candy wrapper could not be re-used!!!!
The father of the child that dropped that candy wrapper (a fellow jet pilot) was made to feel rather guilty about it!!! "LOL" (I will not mention any names)
The wrapper was made of mylar and it shredded into a zillion pieces when it hit the compressor, but many of those pieces were still too large to make it past the diffuser. The leading edge of several of the compressor blades and the diffuser vanes were slightly deformed, the pieces of mylar were clogging more than 80% of the passages in the diffuser.
The wrapper went into the engine just as I had reduced the throttle to idle, so the turbine was spooling down from 106,000 rpms at the time it clogged. It shut down and there were some flames out the tailpipe, but no permanent damage was done to either the airframe or the engine. It ran fine after being cleaned out.

Kevin Whitlow

This fod screen from JETCAT would have saved your engine.

I thought there was going to be some sort of "electric motor on a stick" starting wand available for the Mamba but at Toledo they said it would be strictly an air started turbine.

Dustflyer,
I think your confusing the Mamba with the SimJet 700 which will have a wand on a stick for electric start...
Todd

Hey Chris,

Are you (TJT) working on a 44mm size turbine?? That elec thing next to me in the pic could sure use two of `em.

Thanks,
Vin...

Is it under powered, or just sound like a vacume cleaner?

Anyone ever give the thought of taking one of our engines and maybe doing a fan section on one.if someone can do a turbo prop conversion I don't think it would be too far of a stretch to do a fan.......Perry

Several manufacturers are actively working on a fan model.

who's working on the turbo fan's?

Lars from Simjet had some pieces at Fla Jets.

You know, if the design is anything like the larger turbofans, you'll have the fuel economy and thrust that everyone is craving for!!
Only problem I forsee is sizing of the fan unit at the front of the turbine. Just like the big boys, the turbine used will still hold it's standard dimensions. The fan will have a larger OD than the turbojet itself, if this design is going to be the route of standard turbofan designs. May be suitable only for larger airframes. Still would love it and have been waiting for someone to pull it off, figured it would only be a matter of time before someone does.

Most of the designs I have seen have called for a power turbine, extracting energy from just behind the turbine wheel, with the fan placed radially outboard of this power turbine. So the fan is in the back and not very large at all in diameter. The overall dimensions of the engine do not increase too much.

Woketman

I take it then, that the fan will still be a direct axial attachment to the main turbine shaft? The configuration you speak of is similiar to what Boeing did with their turbofan engines in a small handful of experiments. Fan blades (J-Blades) where axially attached at the Aft end of the Turbofan engines. In this case, the fan blades would be at the aft end of our little turbojet configurations with a power turbine wheel instead of a thrust wheel? How much blade tip clearance past the OD of the engine casing are we talking about? If it's not much, they won't get a significant amount of output directly from those blades due to the lack of cross air flow. Of course though, if the turbine were to continue to operate within it's orginal designed rpm parameters this would increase overall thrust. The large turbofans can move very high volumes of air with less fuel consumption (lower RPM's) vs the turbojets. But, the fans have a considerable size increase on the engine's overall dimensions (diameter). Another obsticle will be an increase in throttle lag. More axial mass and drag from the fan blades pulling on the external air surrounding the turbine casing. Should be interesting. Look forward in seeing what they come up with.

Here's a cross-sectioned picture of a Pratt and Whitney 2000 Turbofan engine. The picture is just to help some of the folks that may not be overly familiar with the Turbofan configurations.