TECH III: POWERPLANT & PERFORMANCE SECTION

A Qualitative Analysis: Jumo 004 versus the J-85.

 

 

RELIABILITY

 

JUMO  The Junkers Jumo 004 is often remembered as a  temperamental and failure-prone powerplant.  Despite its advanced design, engine life was only between 10 and 25 hours, with the mean being at the lower end of this range.  These failures were anticipated to some extent and the Me 262 was designed to permit extremely rapid engine changes.

Contrary to popular belief, the 004A was a  fairly sound performer when premium steels were used, and early versions were known to achieve a 200-250 hour service life.  However, the diversion of critical materials into U-boat production and other projects late in the war forced Junkers to produce the 004B model with only 1/3 of the high grade steel  that had been used in the 004A.  It was to be a disastrous concession for the Me 262.

The introduction of inferior metals compounded an already problematic situation with the turbine blade design.  These blades were rigidly mounted, contributing to severe root stress relief problems.  The weaker metals simply could not withstand this kind of abuse and regular compressor failures were an inevitable consequence.

 

J-85  The General Electric J-85/CJ-610 series turbojet engine is a benchmark in the advancement of pure jet technology.

The engine was originally designed in the 1960s for use in military applications.  Shortly thereafter, civil certification and production followed under the CJ-610 designation.

The CJ-610 was quickly selected to power the popular Gates Learjet; meanwhile, its military cousin was called into service with such noteworthy combat aircraft as the F-5 Freedom Fighter and A-37 Dragonfly

The resilience and forgiving qualities of the engine also made it a natural choice for training aircraft, and the J-85 was adopted for both the T-38 Talon and T-2J Buckeye.

The J-85/CJ-610 engine has a reputation for extreme reliability, allowing wide variations of inflow distortion.  It also places a minimal maintenance burden upon ground crews.  Proven in war and in peace over three decades, the engine is ideally suited to power this classic warbird well into the next century.

 

POWER

 

JUMO  In aircraft applications, engine power is characteristically measured in terms of thrust versus weight.  The Jumo 004 was typical of early jet engines in that it was rather heavy, and not especially efficient.

Production model 004s produced 1,980 lbs. of thrust, and weighed in at about 1,800 lbs.  Because of this, the engines were not extraordinarily effective at low airspeeds or altitudes or at reduced power settings.

Long takeoff rolls (>3,000') were evidence of this phenomenon and, once aloft, power management became critical.  Abrupt throttle changes or rapid maneuvering often resulted in a flameout, or worse, a complete compressor failure.

 

J-85  Each J-85 produces 2,850 lbs. of thrust, yet weighs only 395 lbs.  In simpler terms, the new engines offer nearly twice the power for less a quarter of the Jumo's weight penalty.

The design dynamics of the Jumo engine castings are expected to reduce the thrust available by about 300 lbs. per engine.   Our current engineering estimates call for an actual power output in the vicinity of 2400-2500 lbs. per engine.

Integration of the J-85 will bring many noticeable improvements.  Takeoff distances will be significantly shortened (<2,000'), and time-to-climb rates vastly improved.  Also, the J-85 responds well to varying power demands (including low power settings) and is highly tolerant of the kind of airflow disruptions that gave the Jumo such difficulty.

 

SPEED

 

JUMO   The Jumo-powered Me 262 was capable of level flight speeds in excess of 540 miles per hour at altitude; a trait that made it all but invulnerable to Allied escort fighters.

Higher airspeeds were recorded under certain circumstances but, in general,  compressibility-related aerodynamic factors prevented the airframe from ever pushing into the high transonic range.

Postwar tests in the West confirmed that at very high airspeeds airframe vibration levels and buffeting grow increasingly worse until the jet enters into a shallow dive and becomes all but completely uncontrollable.  Recently revealed Soviet documents demonstrate that this was also a major finding in Red Air Force flight testing of the Me 262.

 

J-85  In purely theoretical terms, the added power of the J-85 should give the new production Me 262s a speed advantage of at least 75 miles per hour over any previous generation Me 262.

The fact remains that the airframe was never designed to handle the stress loads encountered at speeds in the 600 mile per hour range.  To push the aircraft into this environment simply because additional power "happens to be available" is a highly dangerous and ill-advised move.

In the interest of safety, the Me 262 Project will be placing a placarded airspeed limitation upon the jets in the vicinity of 500 MPH.  The official position of the project is that there is simply no need -- or benefit -- in flying these aircraft any faster.

 

ENDURANCE

 

JUMO  Specific Fuel Consumption (SFC) values provide a quantifiable and uniform means of measuring a turbojet engine's efficiency.  All jets have an associated SFC, and for the Jumo 004, the correct figure is 1.39.

In practical terms, this means that for each pound of thrust provided, the Jumo will burn 1.39 pounds of fuel per pound of thrust per hour.  With a typical fuel capacity of 1,800 liters, the range of the original Me 262 was approximately 600-650 miles (at altitude).

 

 

J-85  The J-85 has a Specific Fuel Consumption value of .99, meaning that it will burn slightly less than one pound of fuel per pound of thrust per hour.  When compared to the Jumo, the J-85 is obviously some 40% more efficient.

This improvement will have a marked impact upon both the range and endurance of the of the new Me 262s.  A new Me 262 should be able to travel well over 1,000 miles on a single fuel load.

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