h a l f b a k e r yQuis custodiet the custard?
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In fighter planes, performance requires minimal cross-section area. But space must be found for fuel, weapons, landing gear.
Between the high-pressure compressor stage and the combustors, a jet engine is very slim. If we push the compressor towards the front of the aircraft and the hot section to
the very back, we open up 'free' internal volume in the plane. There would be an airpipe and a shaft connecting the two bits of the engine, but their area is much less than the frontal area of the compressor fan. The F35 liftsystem transmits 28,000 HP forward to its liftfan, so we would have a similar shaft driving the compressor. Less frontal area = more performance.
Note: Bypass air ratio is small in fighter engines. Maybe there are two air pipes for pressure air and bypass air, but the idea should still work.
Coanda effect
http://en.wikipedia.../Coand%C4%83_effect
[normzone, May 03 2011]
Lightning
http://en.m.wikiped...+electric+lightning
A beast … [8th of 7, May 04 2011]
Interesting reading -
http://www.sci.fi/~fta/Day-1.htm
[normzone, May 04 2011]
F104 Starfighter
http://www.916-star...F104_cutaway_AI.jpg
Cooler looking than a lightning. [DIYMatt, May 04 2011]
[link]
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[DamianH], welcome to the Halfbakery. |
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For some reason I keep wanting to say "turbo lag", or "doppler effect". But since those things don't apply, I will resist. |
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The turbine shaft carries a lot of torque. A long shaft is going to
have to be very stiff; either a large diameter thin walled tube, or
intermediate support bearings. The long air ducts won't be much
of a problem. |
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Remember that because of Newton's First Law, there is equal
torsional force on both rotor and stator. In a conventional engine
this is distributed via the outer casing, and the central frame.
You can't just decouple the two ends of an axial cflow gas turbine
without substantially strengthening the airframe to compensate. |
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The biggest objection is that it makes maiontainance by swap-out
more difficult, and the spare units probably bulkier. |
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[norm...], you could say things like: boundary layer effect, coanda effect, navier-stokes inequalities. It is all the same stuff, namely PV=nrT, as applied to different (funnily enough) cross sections of flow. |
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// performance requires minimal cross-section area.// Err, not quite. Minimum cross section, or something that approaches it (minimal, conserving what you are trying to do (in this case manouver a fighter plane)), is not related to performance. In fact it seems to be an inverse ratio. The smaller the cross section the less the performance (for fighters, anyway). This does not work for high altitude, or high speed (> mach 7) planes, but then you can't classify them as fighters, in the classical sense. |
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The "free" space you want to create between the compression vanes and the combustion stage does not really exist (in three dimensions), and that is why they are as close togeher as possible in fighters. They exist in exotic proulsion systems, purely because they have, by definition, vectors in the plane (sorry, horrible joke). |
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There should be a torque tube around the drive shaft. It could contain intermediate bearings. Space between the the drive shaft and the torque tube could be used for the compressed air transfer maybe... |
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Even forward of the combustion cans, the air is pretty hot- and
the cooler bearings can run, the better. That's implying a high
rate of flow of lubricant, and more external cooling. |
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So, if we put the compressor stage at the front of the plane (not out on the wing) and the combustor section right at the back doesn't that mean it's going to be a tad windy in the cockpit? |
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I'm suggesting some super-strength adhesive for the Post-It notes and a sterling engine on the turbine shaft, so the pilot can recharge their ee-pad in flight. |
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no like a dumbell: compressor > (relatively) skinny tube > combustor. You could put the pilot on top, and the wings underneath, the tube... Lots of pitch momentum though and the airframe would have to be beefed up. |
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Sounds eerily similar to the ol' English Electric Lightning, [FT] |
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Did they put a compressor of some kind behind the nacelle in the Lightning ? |
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Wow. That "Lightning" was quite a project. |
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"The English Electric Lightning is credited with a single kill, ironically a British aircraft – a Harrier pilot ejected and the pilot-less aircraft continued to fly. The order was given to shoot down the aircraft and the Lightning did." |
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Actually, the policy was concisely summarised by US President
Theodore Roosevelt, who is quoted as saying,"Speak softly,and
carry a big stick". |
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On a distantly related note, there's something I've been trying to follow up on. |
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The story goes that in Gulf War One a spotter plane was set upon by an Iraqi fighter plane in the early days when there were such things in the sky. |
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The spotter dived for the ground saying his prayers and following procedure (chaff, flares) and pulled up just above the deck. |
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He heard a boom and discovered that the fighter jet had misjudged his turn and flown into the ground. |
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They were trying to decide whether to give the spotter credit for a kill or not. |
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I never heard how it all turned out. You? |
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I had not intended to start a policy discussion. Oh well. And [DamianH] had not intended to start a dogfight thread, so my apologies. |
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I think I found it (link). |
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"- 1 x Mirage F-1 downed by suicide attacking an EF-111A |
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... 390th ECS of the 48th TFW(p) |
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... Aircraft 66-0016, from RAF Lakenheath |
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... Capt James A. "JD" Denton (pilot), and Capt Brent D. "Geat Brandini" |
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... Brandon had 400 hours in the EF-111A flying 150 combat hours in 28 missions in both the F and EF models |
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... Mirage gave chase to EF-111A over Western Iraq, fired 1 missile, as the Raven headed for the deck, but an F-15 was after it |
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... EF-111A evasive break to low altitude using chaff/flares |
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... hard right turn to supersonic speed |
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... Mirage tried to follow and crashed |
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... but, the KILL, was awarded to Graeter |
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... John Deur reports that the USAF now feels that the F-1 that crashed is the same one awarded to Graeter in a maneuvering suicide" |
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The Lightning does not fit the description in this idea. Its engine sits behind the pilot, only the air intake is upfront. Same goes for the F86, F100, MiG15, etc. The reason this idea wouldn't work is because the section of a jet engine that Damien is talking about isn't actually that thin. Most fighter aircraft *are built around the minimum diameter of the engine and have some other stuff sticking off. Look at the F104 for a good example. |
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We wish to point out that the mention of the Lightning was
prompted by [FT]'s annotation, not the original idea, and we
concur with your critique. |
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Actually the Lightning's two engines are staggered
fore and aft and nested together to minimise frontal
area - this IS the same idea! |
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//Actually the Lightning's two engines are staggered fore and aft and nested together to minimise frontal area - this IS the same idea!// |
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Oh, in that case, baked - English Electric Lightning :) |
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The Lightning had very little volume for fuel, avionics etc because the fuselage is full of ducts carrying bulky air at atmospheric pressure. They ended up fitting a big conformal belly tank to get some range. A two-bit engine would pass that air down the fuselage at high pressure and the fueltanks etc could nestle into the resulting cavity. |
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But would the power saved by a slimmer fuselage make up for the pumping losses in the pipe and shaft friction losses, weight etc? And how would we handle a two-spool engine? Two shafts? Geared fan? Anybody want to volunteer to do the maths? |
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Most of the aerodynamic drag arises from the wings, not the
fuselage. Some aircraft gain a little lift from a" lifting body"b
fuselage shape, but it's mostly the lift/drag losses from the
airfoils. |
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Ah, but if we can accommodate more fuel and main landing gear in the fuselage, we may use more slender wings yes no? Apparently the Lightning (always talking about the Lightning!) used special skinny tyres stored in the wing, again because there was no space in the fuselage. |
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I was going to be mention the Lightning, but didn't get around to it. Anyway, it'd be a bugger to get the Sterling engine magnets to stick onto a shaft rotating that quickly. |
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I agree. Silver isn't very magnetic. |
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Make the shaft from magnetised pound coins. |
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K, we got weight distribution and shaft precession/vibration... maybe something to do with backpressure as well. On the upside you might be able to cool the air(/warm the fuel) after the compressor in the tube as well as the aforementioned cross-section shaving. |
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The shaft problems could be mitigated by not having one: use a recip engine ducted fan to drive a ramjet. |
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Like an Argus pulsejet, the recip-driven feeder won't give the
ramjet the cojones for takeoff, because of low specific impulse
until you get some decent forward speed. |
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You're going to need JATO boost to get airborne and out of the
ground effect. |
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The gap between the compressor and the combustion cans could
be used for a charge cooler, but since the whole point of the idea
is to free up volume within the airframe, that seems pointless. |
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Unless, of course, you do away with the coupling shaft altogether
and link the turbine to the compressor with psychic telepathy… |
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Or an electric driven compressor run from batteries until the speed is high enough for the ramjet to kick in by itself. Then recharge from the "compressor" blades, now a generator. |
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Care to do the math on the weight of all that kit? A combined
motor/generator in the half-Megawatt range, at a minimum, plus
the batteries and the control switchgear … wethinks thou
extracteth the urine. |
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You'd only need a 400mph airstream to get the ramjet going (albeit rather inefficiently at that speed). |
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If it's that simple, then why not use a fission reactor as your
power source? |
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You sayin' you couldn't make a 400mph airstream out of say a 50kw electric motor ? |
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Mind I'm not talking "fighter jet"; I think there'd be balance issues: a proof-of-concept rather. |
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It's possible to make a 600km/h airstream with a much smaller
motor than that; the issue is" how big does the unit have to be to
feed a ramjet effectively" |
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The mass flow rate needs to be substantial to produce enough
thrust to propel a man-lifting airframe. |
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If you can build a V1- sized unmanned demonstrator we will be
more convinced. You may use ground power to spin up the
compressor and fire the ramjet but no rockets or catapults- that
would be cheating. |
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//The mass flow// needs only be enough to feed the throat of a ramjet that thinks it's going 400mph. So a 2' engine intake: 3sq ft or 1/3m2, moving at 180m/s... call it 60m3/sec. |
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Yeah I could fit that onto a V1. |
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