(Peter Stickney) wrote in message ...
In article ,
Cub Driver writes:

Pete Stickney wrote in another thread:

(The Aliies, after all, succeeded in 1943 in
producing what the Germans could not - practical, reliable jet engines
that could be flown for more than a day before needing to be
overhauled, and which could be worked on by typical mechanics. Before
the Me 262 appeared in 1944, both the Americans and the British were
running engines with more than twice the thrust, and 10 times the life
of the best realized German efforts.)

Pete, the rough figures I carry in my mind is 10 hours TBO for the
Jumo engine in the Me 262 and 25 hours for the GE? engine in the P-80.
I have also seen 50 hours mentioned in a 1945 briefing about the P-80.

The 10 hour TBO for a Jumo 004 is rather misleading. According to
Aircraft Engineering's analysis of the Jumo 004B, you had to drop the
engine set somewhere between 6 and 10 hours for a teardown and
mandatory turbine wheel replacement. (Which was a shop job, and
couldn't be done in the field. Balancing the compressor-turbine rotor
was critical, adn was a hand-fit job). The burner canse were also
checked and replaced as needed at the same time. You ran it for
another 5-10 hours, and threw it away.

The BMW003 was a far more impressive engine in terms of service than
the Jumo 004 despite being introduced into service latter.

There were also several variants of the Jumo 004 with different
serviceabilities.

The first engines captured by the allies (An Arado 234 of III.KG76)
were the Jumo 004B1 which although the main production engine didn't
have the hollow turbine blades yet and also had compressor vibration
problems)

The next production series that entered service, the jumo 004B4, (B2
and B3 never entered mass production) introduced hollow turbine blades
and changes in cooling bleed air and combustion system as well as
solved compressor vibration problems. These changes were capable of
increasing thrust via a greater exhaust temperature but the the
potential improvements were eschewed to improve service life
considerably.

The combustion chambers were made of ordinary steel treated with an
aluminium oxide to reduce corrosion. They were being continiously
inproved with new gas flows and cooling film systems.

Had they been made of even ordinary stainless steel they would have
had a vastly increased service life.

The engine used two types of hollow turbine blades of the alloy
tinadur or cromadur. Tinadure Bladed engines used ony 6.5kg of
nickel and 4.6kg of chromium and 0.2kg of molydenum (for the whole
engine). cromadure bladed Jumo 004B4s used only 3.5kg kg of nickel
and 4.7kg of chromium and 0.2kg of molydenum (for the whole engine)

There must have been nearly a hundred kg on an allied engine!

The Jumo 004C made further improvements in service life as well as
sericeabillity and was in mass production when the war ended.

One thing that also has to be remembered was that the confusion and
paucity of raw materials often led to inproper substitution that
reduce engine life. In the case of one raw material on the BMW 003 it
was changed seven times such was the confusion at the end of the war.

The Jumo 004B had accesibility problems to the combustion chambers
which meant that the engine had to be dropped down from the aircraft
to be replaced. The life nvertheless reached 60 hours at the end of
the war.

The Powerjets (whittles company) analysis of the first Jumo 004B1
engines indicates that if a higher level of supply of nickel could be
achieved these engines would improve. One important technique was to
pull a violin bow over the turbine blade to make sure they had been
soldered into the roots properly (this was a critical point of
failure)

The BMW003A1/A2 had a very reliable combustion chamber which lasted
200 hours. The turbine entered service with a MTBO (service of
replacement was in the field and on the wing) of 20 hours and ended up
with 50 hours without trouble: the limitation being the attachments of
the turbine to the disk.

The Anular combustion chamber must have been the most impressive of
any WW2 engine as it ahieved a gas velocity of only 110 m/s.

It appears that BMW was about to receive an order to develop the BMW
P3006; a scaled up BMW003 of 1700kg thrust.


The early J33s get hot section inspections every 25-50 hours. You
split the airplane and looked at the turbine blades with a borescope.
(P-80 tails came off as a unit to give full access to the engine -
It's not a big job - I've seen T-33s (The same airplane, at that
point) done in about an hour. TBO was originally set for 100 hours,
and that was bumped up to 400 hours after the war. In 1950-51,
according to the AIAA Yearbook, they bumped it up to 1,000 Hrs.

The J35, the Axial that GE also developed in the 1944 timeframe,
started with about a 25 Hr TBO, which was gradually incresed to 500
Hrs before the Korean War. (Save for one batch that used farm
machinery bearings, and those didn't last long). Post Korea, J35 TBO
was increased to more than 1000 Hrs.

The Rolls Nene/Pratt & WHitney J42 started with a 250 Hr TBO, and
this was increased to 1000 Hrs in 1949.

It ought to be noted that the big recips, especially those used on
fighters, were usually pulled for overhaul every 100-150 Hrs.

I'd be grateful if you could flesh out the parenthesis. The only
engine I know anything about is the Whittle turbojet as modified by GE
for the Bell YP-59A. I never followed up on what changed before the
P-80 got running.

Basically, the engines got bigger, and ran hotter. The biggest
changes on the Allied side were in the areas of the fuel controls.
Originally, the pilot directly controlled fuel flow with the
throttle. That's doable, but the engine required a widely varying
fuel flow depending on speed and altitude. (The faster you go, the
more air you're using, and the more fuel you have to burn. The higher
you go, the less fuel you burn becasue of the lower density.) The
pilot also had to moniter the turbine temperature (Gas temperature
after the turbine, usually (TET - Turbine Exit Temperature) which
stays pretty fixed with the critical TIT (Turbine Inlet Temperature),
adn the engine RPM to make sure he wasn't exceeding any
limits. (Exceeding limits would casue the turbine wheel to come
apart). Fast throttle movements could very easily lead to an
overtemperature, or flame the engine out. Improved fuel controls were
little hydromechanical analog computers that monitored the various
parameters, adjusted things to meed the pilot's demands for power, and
controlled the fuel flow accordingly. The Brits actually ended up
with a lead in this area until the early '50s.

And what about that P-80? It seems to have had an unwonted number of
crashes for an airplane that turned into the longest-serving jet ever
built (still in service, as I understand, as the T-33 in recce and
light-attack roles for various air forces).

I've just done some poking around, and I don't see that the P-80's
accident rate was any worse than that of any contemporary fighters.
P-61s had an accident rate of about 120/100,000 hrs, and they wer the
safest of the bunch. (The worst were P-39s, with a rate of
249/100,000 flt hrs).

There were accidents with the early jets that were certainly due to
their "jet-ness", if you will. A case in point was Richard Bong's
crash. He took off in a P-80 without turning on the alternate fuel
booster pump. (A checklist item) just after takeoff, the fuel control
packed it in, and he ended up flaming out. As I've alluded to before,
jets don't decelerate like recips. Even the early jets were very
clean (Almost like sailplanes) and, until you got into the transonic
range, there wasn't a whole lot of drag. With prope it's different -
when you pull back on the power, you get a big increase in drag as the
slipstream tries to drive the prop. That makes formation flying, or
entering the landing pattern, a lot easier. The low drag also meant
that when you pointed the nose down, the jet accelerated like a rock.

The early jets also were quite underpowered at low speeds (That
thrust/horsepower thing) and the engines didn't respond quickly - 8-16
seconds from Flight Idle to Max Power wasn't unusual. So they didn't
accelerate well at all. Paradoxically, though, once they got going,
it didn't take long before they were pushing into the transonic range,
and things would really get squirrely. (Roland Beamont established
the maximum Mach Number for the Meteor IV's speed record by entering
the Meteor's Mach Tuck (Meatboxes would nose down) at roughly 610 mph
and 50 ft altitude.) Vampires would porpoise, banging on alternating
postive and negative G until something broke. Venoms would lose all
elevator control. P-80s would buffet like mad, and the elevator
effectiveness would decrease. F-84s (The straight winged ones) would
pitch up at something like 7 or 8 Gs. Unlike piston engined
fighters, these airplane could reach that region in level flight, on a
good day. So yo couldn't just hop into one and fly it - you had to
develop a whole new set of reflexes. That was one of the reasons that
Lockheed stretched the F-80 into the TF-80C/T-33.
Jets can be deceptively easy to fly - they're very smooth, there's no
torque, and there's only one power lever to worry about, rather than
the handful of stuff that goes with, say, a Merlin or R2800. But they
can also get you into trouble fast, and booouncing around in teh Mach
Buffet at 10,000' with the nose 45 degrees below the horizon is no
place for On the Job Training.