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Two-bit jet engine

Create 'free' aircraft volume by stretching engines
  [vote for,

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.
DamianH, May 03 2011

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]


       [DamianH], welcome to the Halfbakery.   

       For some reason I keep wanting to say "turbo lag", or "doppler effect". But since those things don't apply, I will resist.
normzone, May 03 2011

       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.   

       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.   

       The biggest objection is that it makes maiontainance by swap-out more difficult, and the spare units probably bulkier.
8th of 7, May 03 2011

       [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.   

       // 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.   

       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).
4whom, May 03 2011

       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...
DamianH, May 04 2011

       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.
8th of 7, May 04 2011

       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?   

       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.
not_morrison_rm, May 04 2011

       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.
FlyingToaster, May 04 2011

       Sounds eerily similar to the ol' English Electric Lightning, [FT]   

8th of 7, May 04 2011

       Did they put a compressor of some kind behind the nacelle in the Lightning ?
FlyingToaster, May 04 2011

       Wow. That "Lightning" was quite a project.   

       "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."
normzone, May 04 2011

       Actually, the policy was concisely summarised by US President Theodore Roosevelt, who is quoted as saying,"Speak softly,and carry a big stick".
8th of 7, May 04 2011

       On a distantly related note, there's something I've been trying to follow up on.   

       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.   

       The spotter dived for the ground saying his prayers and following procedure (chaff, flares) and pulled up just above the deck.   

       He heard a boom and discovered that the fighter jet had misjudged his turn and flown into the ground.   

       They were trying to decide whether to give the spotter credit for a kill or not.   

       I never heard how it all turned out. You?
normzone, May 04 2011

       I had not intended to start a policy discussion. Oh well. And [DamianH] had not intended to start a dogfight thread, so my apologies.   

       I think I found it (link).   

       "- 1 x Mirage F-1 downed by suicide attacking an EF-111A   

       ... 390th ECS of the 48th TFW(p)   

       ... Aircraft 66-0016, from RAF Lakenheath   

       ... Capt James A. "JD" Denton (pilot), and Capt Brent D. "Geat Brandini"   

       Brandon (30) (EWO)   

       ... Brandon had 400 hours in the EF-111A flying 150 combat hours in 28 missions in both the F and EF models   

       ... 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   

       ... EF-111A evasive break to low altitude using chaff/flares   

       ... hard right turn to supersonic speed   

       ... Mirage tried to follow and crashed   

       ... but, the KILL, was awarded to Graeter   

       ... 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"
normzone, May 04 2011

       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.
DIYMatt, May 04 2011

       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.
8th of 7, May 04 2011

       Actually the Lightning's two engines are staggered fore and aft and nested together to minimise frontal area - this IS the same idea!
DamianH, May 04 2011

       //Actually the Lightning's two engines are staggered fore and aft and nested together to minimise frontal area - this IS the same idea!//   

       Oh, in that case, baked - English Electric Lightning :)
DIYMatt, May 04 2011

       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.   

       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?
DamianH, May 04 2011

       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.
8th of 7, May 05 2011

       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.
DamianH, May 05 2011

       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.
not_morrison_rm, May 05 2011

       I agree. Silver isn't very magnetic.
Ling, May 06 2011

       Make the shaft from magnetised pound coins.   

       There - I fixed it!
Twizz, May 06 2011

       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.   

       The shaft problems could be mitigated by not having one: use a recip engine ducted fan to drive a ramjet.
FlyingToaster, May 06 2011

       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.   

       You're going to need JATO boost to get airborne and out of the ground effect.   

       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.   

       Unless, of course, you do away with the coupling shaft altogether and link the turbine to the compressor with psychic telepathy…
8th of 7, May 06 2011

       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.
FlyingToaster, May 07 2011

       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.
8th of 7, May 07 2011

       You'd only need a 400mph airstream to get the ramjet going (albeit rather inefficiently at that speed).
FlyingToaster, May 07 2011

       If it's that simple, then why not use a fission reactor as your power source?
8th of 7, May 07 2011

       You sayin' you couldn't make a 400mph airstream out of say a 50kw electric motor ?   

       Mind I'm not talking "fighter jet"; I think there'd be balance issues: a proof-of-concept rather.
FlyingToaster, May 07 2011

       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"   

       The mass flow rate needs to be substantial to produce enough thrust to propel a man-lifting airframe.   

       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.
8th of 7, May 07 2011

       //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.   

       Yeah I could fit that onto a V1.
FlyingToaster, May 07 2011


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