Re: Feb 1945 Me-264 Hybrid Design
- From: eunometic@xxxxxxxxxxxx
- Date: Sun, 20 Sep 2009 07:18:46 -0700 (PDT)
On Sep 20, 8:08 am, guy <guyswetten...@xxxxxxxxxxxxxx> wrote:
On 19 Sep, 22:50, Rob Arndt <teuton...@xxxxxxx> wrote:
http://img24.imageshack.us/img24/7392/me264ritz.jpg
Rob
Centrifugal jets?
Guy
These were twin turbo-propoprops, 'designed' by AEG with a projected
power of 5000shp using "Ritz" style rotating regenerative heat
exchangers which are located along with secondary compressors and
turbines in the nacells outside of the propellor nacelles.
The basic idea is to recover the engines exhaust heat, which though
lacking in pressure still has heat, in a heat exchanger and to use
that to preheat the air prior to combustion to dramatically increase
part load efficiency, a doubleing of efficiciency in cruising flight
would be about right. Such engines have been succesfully built in the
late 40s and indeed have been part of gas turbine work from the very
begining. In fact the world first turboprop was run in Hungary in
1942 had a heat exchanger, a designe by Dr Jendrassik which ran in
1939. By using a heat exchanger Jendrassik managed to keep the
turbine inlet temperature to less than 475C. Unfortunatly he had
been forced to use welded sheet steel in a counterflow design and the
welds etc caused much pressure loss and unimpressive efficiency, it
was OK for a first run especially the way this was managed with
ordinary materials. His later Cs-1 ehich was to power the RNI 1(X/H)
turbo prop bomber never got the time it needed to mature because the
desperate needs of the Hungarian Airforce made them purchase Me 210D.
(the Hungarians managed to bypass the German buerocracy that had
mismanaged the Me 210 program and had extended tails and slats fitted
that the first ever Me 210 pilot had recommended on the aircrafts
maiden flight, these solved the handling issues.)
The schematic you see is from a 1943 study. Expected fuel efficiency
was expected to be 0.3 lbs fuel per equivalent hp per hour at a 600km/
h (373mph) cruise. This would have been exceptional fuel efficiency
exceding even the Napier Nomad 2. The range expected varied
according to whether it was an AEG model or a BBC version of this type
of engine but ranged from over 11,000 miles to almost double the 9375
miles range of the 4 x piston engined Me 264 in its 'overloaded' RATO
assisted takeoff form. The combined power of 10000shp probably being
enough to not require RATO on the Me 264 since the Jumo 211J version
only had about 6000hp total.
These potentially could have run return missions from Europe to the
East Coast USA but also all the way to the West Coast of the USA and
back, polar routes etc.
This was more of a baseline study around the Me 264 than a 'real'
proposal, whose aerodynamics was known, to compare it to piston and
turbo diesel alternatives. The regenerative turbo prop was the best.
Studies on the Me 262 Jet fighter with such turbo props was also made
but the advantages were considered minimal for such a fighter or for
any aircraft undertaking missions under 3 hours.
The Germans did indeed build and fly turbojets equiped not only with
with radial (centrifugal) compressors as well as radial-inflow
turbines. In fact the HeS 003B, which powered the Heinkel He 178 and
was the first jet to everyfly had such a radial turbine while the
latter HeS 008, which powered the Heinkel He 280 also used a radial
compressor and radial turbine design. (The He 280 was, I suspect,
first aircraft to break 500mph ).
Back to the "Ritz" heat regenerators. Basically exhaust gas from
turbine was used to heat a rotating ceramic heat exchanger. The heat
stored in the exchanger then was slowly rotated out of the exhaust
and transfered into "cool" compressed air to run either a second
turbine directly either to preheat air prior to combustion for the
single turbine. The AEG design and the competing BBC design had far
more complicated thermodynamic cyles than the simple one I explained
above. Such rotating heat exchangers engines have been built
succesfully.
These heat exchangers were extensively studied with promising results
and tested by a Dr Ritz at the AVA Goettingen (the AVA was a sort of
NACA for Germany at one of its most presitgious universities).
Testing of labyrinth seals to prevent leakage from the high pressure
air from the compressor to the low pressure gas of the exhaust showed
this could be kept below 1% at pressure ratios of 7:1.
The material was a quartz encassed ceramic fibre woven on rods aligned
in such a way as to create micro-turbulence to increase heat transfer
but had little measurable effect on pressure loss.
The AEG cycle is as follows: exhaust from the Prop shaft Power
Turbine was reheated and used to power the High Pressure turbine which
powered the compressor, the exhaust from this High Pressure turbine
was used to heat a heat exchanger before finally exhausting over an
final Exaust turbine which power auxiliaries. Air from the
compressor was preheated in the heat exchanger before being heated to
higher levels in a combustion chamber and then passing into the Power
Turbine (above)
In the slightly simpler BBC version hot air from the Power Turbine
heats the heat exchanger and is then exhausted. The hot air from the
ceramic heat exchanger is used to preheat compressed air prior to
combustion after which passed onto the High Pressure Turbines which
powered the compressors.
Such recuperative heat exhangers can more than double the efficiency
of a gas turbine at part load and if an open cycle gas turbine engine
was ever used in a car it would use such a heat exchanger.
So why don't we see them in use?
Firstly while they may nearly double the fuel efficiency of an engine
on part load they have lesss effect on full power, hence they do
little to improve takeoff or climb out fuel consumption.
Secondly they possibly add a certain amount of bulk and complexity,
though this is arguablely not true in the simple single and two sppol
designs which don't have seperated sections.
Thirdly modern jets get their range by flying fast (at transonic
speeds) at high altitude where there is little parasitic drag. Prop
aircraft are inefficient at high speed.
In stationary applications or ships its easy just to switch one of
several gas turbines off to keep the remaineders opperating at their
peak efficiency.
The Me 262, with 4 pistone engines jumo 211J or BMW 801, was actually
built, however it started as private Willy Messerschmitt project and
Erhardt Milch and the powers that be did little to support it till it
was too late to find resources to complete it. To reach the east
coast of the USA it would need a 9000ft concret runway (same as a B-52
or 707) or a dozen RATO rockets the failure of anyone of which would
cause the loss of the aircraft.
Basically the powers that be did not plan or want to have a war with
the USA, didn't think it would cause worthwhile damage and thought it
might simply aggravate the USA. Goering though it would be good just
to force blackouts in the USA and latter support long range jet
projects. The resources to run a credible transatlantic bomber force
of sufficient mass and concentration to punch its way through US
defenses were also implausiblly astronomical and it was fairly obvious
bombers could survive without escorts, even the B-29. Technically it
was within the capacity of the German industry to produce a bomber
that could drop a ton or two, perhaps of guided bombs however, the
political will was simply lacking. German in flight refueling
experiments (probe and drogue style between a Ju 352 tanker and a Ju
290 receiver) had also been succesfull. Again Milch put a wet blanket
on suich things by calling it the product of egg head (boffins),
citing their inabillity to appreciate the difficulty of rendezvosu in
bad weather in the middle of the Atlantic. There wasn't anyone
competent in Luftwaffe procurement till Sigfried Kneymeyer came along.
The Me 262 was more of a maritime reconaisance bomber, it was fast
enough to survive over the atlantic but probably to weakly armed, that
had too short a range and too poor an warload for a raid on the USA.
There were thus a myriad of proposals to improve the aircraft, one of
the more creditable proposals was the Me 264/6m also known as the Me
364 which as its name suggest was a 6 engined stretch that had a heavy
tail turret. The aircraft faded away as the prototypes were lost in
allied attacks, the jets came along etc.
The regenerative gas turbines were but one proposal for the Me 264 ,
as I said more as a baseline for comparative perforrmance modelling
for AEG and BBC.
German gas turbine research was massive in its scope, it could fill
several books, They were close to replacing turbine stator blades and
duct linings with ceramics able to opperate at 1200C (though not the
turbine itself), they were planning to use gas turbines to power large
Naval vessels, tanks. Gas turbines were smaller, more powerfull, left
more room and weight for fuel. They were less efficient but in a tank
the saved space and armour makes up for that. More importantly gas
turbines were cheaper to make and most important of all they were
multifuel, able to use low grade fuels that didn't care for octane or
cetane ratings and that were easy and cheap to produce with coal to
oil technology.
Recouperated gas turbines may still show themselves. in concentrated
solar power tower applications and Israeli company uses one. Unlike
steam based units no cooling water is needed.
.
- References:
- Feb 1945 Me-264 Hybrid Design
- From: Rob Arndt
- Re: Feb 1945 Me-264 Hybrid Design
- From: guy
- Feb 1945 Me-264 Hybrid Design
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