(EST PUB DATE) ABOUT AN AIRPLANE

• Document Type: FOIA
• Collection: FOIA Collection
• Document Number (FOIA) /ESDN (CREST): 00806939
• Release Decision: RIFPUB
• Original Classification: U
• Document Page Count: 23
• Document Creation Date: December 28, 2022
• Document Release Date: February 9, 2017
• Case Number: F-2015-02619
• Publication Date: January 1, 1982

Introduction
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ABOUT AN AIRPLANE
Twenty-five years ago, with the U-2 still new, work on its successor was
.under way. The effort would produce a revolutionary airplane. In this issue of
:Studies in Intelligence, the Editorial Board Pre:::c
.account of that accOMplishment, a technological triumph for intelligence with
a bittersweet twist.
The account begins with -Development of the Lockheed SR-71 Black-
bird,- by Clarence L. (Kelly) Johnson, who was in charge of that development
Next is another first-hand recollection, -158/SR-71 Propulsion Integration,- by
1William H. Brown, in authority on the engine. Both articles were originally
-published in Lockheed Horizons, Issue 9, Winter 1981/82, Copyright (c)
:1981�Lockheed Cerporation, Burbank, California 91520. The Editorial
:Board gratefully acknowledges permission from Lockheed Horizons, its editor,
Roy A. Blay, and the authors to reprint the articles and accompanying
'illustrations.
As the articles discwis the Nirious vetsions of the airplane under devel;;-"---
ment, the nomenclature expands. A glossary
� A-II was the designation Mr. Johnson gave to his initial design as
submitted tol CIA. It was frequently used thereafter, as for example in
the President's announcunent.
- YF-12A was' the designation given to a two-seated interceptor version
of the A-11, three of which were built for the Air Force. Two of these
three were Clown to Edwards Air Force Base for display after the
President's announcement. Unclassified.
� SR-71 became the designation for a two-seated reconnaissance version
produced for the Air Force. Unclassified.
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.1/..4 � 24, Nt). .2 ,
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2
Lockheed SR-71 at altitude.
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Recollections from the "Skunk Works"
DEVELOPMENT OF THE LOCKHEED SR-71 BLACKBIRD
Clarence L. Johnson
This paper has been prepared by the writer to record the development
history of the Lockheed SR-71 reconnaissance airplane. In my capacity as
manager of Locklieltd's Advanced Development Division (more commonly
known as the "Skink Works-) I supervised the design, testing, and construc-
tion of the aircraft referred to until my partial retirement five years ago.
Because of the very tight security on all phases of the program, there are very
few people who were ever aware of all aspects of the so-called "Blackbird"
program. Fortunately, I kept as complete a log on the subject as one individual
could On a program that involved thousand, of people, over three hundred
subcontractors and partners, plus a very select group of Air Force and Central
Intelligence Agency people. There are still many classified aspects of the
design and operation of Blackbirds but by my avoiding these, I have been
informed that I can still publish many interesting things about the program.
In order to tell the SR-71 story, I must draw heavily on the data derived
on two prior Skunk Works programs�the first Mach 3-plus reconnaissance
type, known by our design number as the A-12, and the YF-12A interceptor,
which President Lyndon Johnson announced publicly 1 March 1964. He
announced the SR-71 on 24 July of the same year.
Background for Development
The Lockheed U-2 subsonic, high-altitude reconnaissance plane first flew
in 1955. It went operational a year later and continued to make overflights of
�the Soviet Union until 1 May 1960. In this five-year period, it became obvious
to those of us who were involved in the U-2 program that Russian develop-
ments in the radar, and missile fields would shortly make the U-Bird too
vulnerable to continue overflights of Soviet territory, as indeed happened
when Franois Cary Powers was shot down on May Day of 1960.
Starting in 1956, we made many studies and tests to improve the
survivability of the by attempting to fly higher arid faster as well as
reducing its radar ,cross-section and providing both infrared and radar
jamming gear. Very little gains were forthcomi!ig except in cruise altitude so
we took up studies of other designs. We studied the uw. of new fuels such as
boron slurries and liquid hydrogen. The latter was carried into the early
manufacturing phase because it was possible to prochzce an aircraft with
cruising altitudes well over 100,000 feet at a Mach number of 2.5. This design
was scrapped, however, because of the terrible logistic problems of providing
fuel in the field.
Continuing concern for having a balanced reconnaissance force made it
apparent that we still would need a manned reconnaissance aircraft that could
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SR-71 alockbird
be dispatched on wcrldwide missions when required. From vulnerability
studies, we derived certain design requirements for this craft. These were a
Cruising speed well over Mach 3, cruising altitude over R0,000 feet, and a very
low radar cross-section over a wide band of frequencies. Electronic counter-
measures and advanced communications gear were mandatory. The craft
should have at least two engines for safety reasons.
Getting a Grasp on the Problem
Our analysis of these requirements rapidly showed the very formidable
problems which had to be solved to get an acceptable design.
The first of th, se was the effect of operating at ram-air temperatures of
over 800�F. This immediately ruled out aluminum as a basic structural
material, leaving only various alloys of titanium azd stainless steel to build the
aircraft. It meant the development of high-temperature plastics for radornes
and other structures, as well as a new hyGraulic fluid, greases, electric wiring
and plugs, and a whole host of other equipment. The fuel to be used by the
engine had to be stable under temperatures as low as minus 90�F in subsonic
cruising flight during aerial refueling, and to over 350�F at high cruising
speeds when it would be fed into the engine fuel system. There it would first
be used as hydraulic fluid at 600�F to control the afterburner exit flap before
being fed into the burner cans of the powerplant and the afterburner itself.
Cooling the cockpa and crew turned out to be seven times as difficult as
on the X-15 research airplane which flew as much as twice as fast as the SR-7I
but only for a few minutes per flight. The wheels and tires of the landing gear
had to be protected from the heat by burying them in the fuselage fuel tanks
for radiation cooling to save the rubber and other.systems attached thereto.
Special attention had to be given to the crew escape system to allow safe
ejection from the aircraft over a speed and altitude range of zero miles per
hour at sea level to Mach numbers up to 4.0 at over 100,000 feet. New pilots'
pressure suits, gloves, dual oxygen systems, high-temperature ejection seat
catapults, and parachutes would have to be developed and tested.
The problems of taking pictures through windows subjected to a hot
turbulent airflow on the fuselage also had to be solved.
How the Blackbird Program Got Started
In the time period of 21 April 1958 through 1 September 1959, I made a
series of proposals for Mach 3-plus reconnaissance aircraft to Mr. Richard
Bissell of the CIA and to the U.S. Air rome. These airplanes were designated
in the Skunk Works by design numbers of A-1 throu4 A-12.
We were evaluated against some very interesting designs by the General
Dynamics Corporation and a Navy in-house design. This latter concept was
proposed as a ramjet-powered rubber inflatable machine, initially carried to
altitude by a balloon and then rocket boosted to a speed where the ramjets
could produce thrust. Our studies on this aircraft rapidly proved it to be totally
unfeasible. The carrying balloon had to be a mile in diameter to lift the unit,
which had a ,proposed wing area of one-seventh of an acre!
4'211!}41
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Surface temperatures at design cruising speed and altitude.
Convair's proposals were mud) more serious, starting out with a ramjet-
powered Mach 4 aircraft to be carried aloft by a B-58 and launched at
supersonic speeds. Unfortunately, the B-58 cou!dn't go supersonic with the
bird in piJ,-T, and even if it could, i;ie survivability of the piloted vehi:le
would be very questionable due to the probability of ramjet blow-out in
manPuvers. At the time of this proposal the total flight operating time for the
Marquardt ramjet was not over 7 hours, and this time was obtained mainly on
a ramjet test vehicle for the Boeing Bomarc missile. Known as the Y-7, this test
vehicle was built anci operated liy the Lockheed Skunk Work!
The final Convair' proposal, known as. the Kingfisher. was culminated by
Air Force and Der.artment of Defense technical experts, wlio were given the
job of evaluatirg all designs.
On 29 Augmt 1959 our A-12 design was declared the winner and Mr. Bis-
sell gave us a iimited. go-ahead for a four-month period to Conduct tests on
certain models and to bui!nl a full-scale mock-op. On 30 ;Arnim 1960 we were
given a full go-ahead on the design, manufacturing, and testing of 12 aircraft.
The first one flew 26 April 1962.
The next version of the aircraft, an Air Defense long-range fighter, was
discussed with Genera! Hal Estes in Washington, D.C. on 16 and 17 March
1960. He and Air Face Secretary for Research and Development, Dr.
Courtlandt Perkins, were very pleased with our proposal so they passed me on
for further discussion's with General Marvin Demler at Wright Field. Ile
11�;,.=!if;i.
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SR-71 Blackbird
directed us to use the Hughes ASG 18 radar and the GAR-9 missiles which
were in the early development stages for the North American F-108 intercep-
tor. This we chd, and when the F-108 was eventually cancelled Lockheed
worked ,with Hughes in the development and flight testing of that armament
system. The first YF-12A flew 7 August 1963.
In early Jariu9ry 1961 I made th, first proposal for a strategic reconnais-
sance bomber to Dr. Joseph Chan c, Secretary of the Air Force, Colonel Leo
Geary, our Pentagon project officer on the YF-I2; and Mr. Lew Meyer, a high
financial officer in the Air Force. We were encouraged to continue our
company-funded studies on the aircraft. As we progressed in the development,
we encounLered very strong opposition in certain Air Force quarters on the
part of those trying to save the North American B-70 program, which was in
considerable trouble. Life became very interesting in that we were competing
the SR-71 with an airplane five times its weight and size. On 4 June 1962 the
Air Force evaluation team reviewed our design and the mock-up�and we
were given good grades.
Our discussions continued with the Department of Defense and also, in
this period, with General Curtis LeMav and his Strategic Air Command
officers. It was on 27 December 1962 that we were finally put on contract to
build the first group of six SR-71 aircraft.
One of our major problems during the next few years was in adapting our
Skunk Works operating methods to provide SAC with proper support,
training, spare parts, and data required for their special operational needs. I
have always believed that our Strategic Air Command is the most sophisti-
cated and demanding customer for aircraft in the world. The fact that we
have been able to support them so well for many years is one of the most satis-
fying aspects of irr, career.
Without the total support of such people as General Leo G'eary in the Pen-
tagon and a long series of extremely competent and helpful commanding
officers at Beale Air Force Base, we could never have jointly put the
Blackbirds into service successfully.
Basic Design Features
Having chosen the required performance in speed, altitude, and fange, it
was immediately evident that a thin delta-wing platform was required with a
very moderate wing loading to allow flight at very high altitude. A long,
slender fuselage was necessary to contain most of the fuel as well as the
landing gear and payloads. To reduce the wing trim drag, the fuselage was fit-
ted with lateral surfaces called chines, which actuall,K converted the forward
fuselage into a fixed canard which developed lift.
The hardest design problem on the airplane was making the envine air
inlet and ejector work properly. The inlet cone moves almost three feet to
keep the shock wave where we want it. A hydraulic actuator, computer
controlled, has to provide operating forces of up to 31,000 pounds under
certain flow conditions in the nacelles. To account for the effect of the
fuselage chine air flow, the inlets are pointed down and in toward the fuselage
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The use of dual vertical tails canted inward on the engine nacelles took
advantage of the chine vortex in such a way that the directional stability
improves as the angle of attack of the aircraft increases.
Aerodynamic Testing
All the usual low-speed and high-speed wind tunnel tests were run on the
various configurations of the A-12 and YF-12A, and continued on the SR-71.
Substantial efforts went i'tfto optimizing chine design and conical camber of
the wing leading edge. No usefri lift increase effeci was found from the use of
wing flaps of any type so we depend entir..4y on our low wing-loading and
powerful ground ef t to get satisfactory takeoff and landing characteristics.
Correlation of wind tunnel data on fuselage trim effects was found to be
of marginal:value because of two factors: structural deflection due to fuselage
weight distribution; and the effect of fuel quantity and temperature. The
latter was caused P'Y fuel on the bottom of the tanks, keeping that section of
the fuselage cool, v. tile the top of the fuselage bec..me increasingly hotter as
fuel was binned, tending to Push the chines downward due to differential
expaasion of the top and bottom of the fuselage. A ;till-scale fuel system test
rig was .1;eci to test fuel feed capability for various flight attitudes.
By far the most tunnel time b. as spent optimizing the nacelle inlets, bleed
designs, and the ejector. A quarter-scale model was built on which over
250,000 pressure readings were taken. We knew nacelle air leakage wou;d
canse high drag so an actual full-size nacelle was fitted with end plugs and air
leakage carefully measured. Proper sealing paid off well in flight testing.
With the engines located half way mit on the wing span, we were very
concerned with the very high yawing movement that would develop should an
inlet stall. We therefore installed accelerometers in the fuselage that immedi-
ately sensed the yaw rate and commanded the rudder booster to apply 9
degrees of correction within a time period of 0.15 seconds. This device worked
so well that cur test pilots very often couldn't tell whether the right or left
engine blew out. They knew they had a blowout, of course, by the bad
buffeting that occurred with a "popped shock." Subsequently, an automatic
restart device was developed which keeps this engine-out time to a very short
Period.
Powerplant Development
Mr. Bill Brown of Pratt & Whitney presented a fine paper on this subject
13 May 1981 to the American Institute of Aeronautics and Astronautics in
Long Beach, California. Mr. Brown's paper is reproduced following this
article.
I have little to add to Mr. Brown's fine paper except to record an
interesting approach to the problem of ground starting the J-58. We learned
that it often required over 600 horsepower to get the engine up to starting
RPM. To obtain this power, we took two Buick racing car engines and
developed a gear box to connect them both to the 1-58 starter drive. We
operated for several years with this setup, until more sophisticated air starting
systems were developed and installed in the hangars.
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seared to a common shall drive to rotate the OS engine. This roe produced 04 elf ago
horsepower tor starting.
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Structural Problems
The decision to use various alloys of titanium for the basic structure of the
Blackbirds was based on the following considerations:
1. Only titanium and steel had the ability to withstand the operating
temperatures encountered.
2. Aged B-120 titaniu-n1 weighs one half as much as stainless steel per
cubic inch but its ultimate strength is almost up to stainless.
3. Conventional construction could be used with fewer parts involved
than with steel.
4. High strength composites were not available in the early 1960s. We did
develop a good plastic which has been remarkably serviceable but it
was not used for primary structure. .
'Having made the basic material choice, we decided to build two test units
to see if we could reduce our research to practice. The first unit was to study
thermal effects on our large titanium wing panels. We heated up this element
with the computed heat flux that we would encounter in flight. The sample
warped into a totally unacceptable shape. To solve this problem we put
chordwise corrugations in the outer skins and reran the tests very satisfactorily.
At the design heating rate, the corrugations merely deepened by a few
thousandths of an inch and on cooling returned to the basic shape. I was
accused of trying to make a 1932 Ford Trimotor go Mach 3 but the concept
worked fine.
The second test unit was the forward fuselage and cockpit, which had
over 6,000 parts in it of high curvature, thin gauges, and the canopy with its
complexity. This element was tested in an oven where we could determine
thermal effects and develop cockpit cooling systems.
We encountered major problems in manufacturing this test, unit because
the first batch of heat-treated titanium parts was extremely brittle. In fact, you
could push a piece of structure off your desk and it would shatter on the floor.
It was thought that we were encountering hydrogen embrittlement in our
Leat-treat processes. Working with our supplier, Titanium Metals Corporation,
we could not prove that the problem was in fact hydrogen. It was finally re-
solved by throwing out our whole acid pickling setup and replacing it with an
identical reproduction of what TMC had at its mills.
We developed a complex quality control program. For every batch of ten
parts or more we processed three test coupons which were subjected to the
identical heat treatment of the parts in the batch. One coupon was tensile
tested to failure to derive the stress-strain data. A quarter-of-an-inch cut was
made in the edge of the second coupon by a sharp scissor-like cutter and it was
then bent around a mandrel at the cut. If the coupon could not be bent 1800 at
a radius of X times the sheet thickness without breaking, it was considered to
be too britth. (The value of X is a function of the alloy used and the
stress/strain value of the piece.) The third coupon was held in reserve if any
reprocessing was required.
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SR-7I Blackbird
For an outfit that hates paperwork, we really deluged ourselves with it.
Having made over 13 million titanium parts to date we can trace :tie history of
all but the first few parts back to the mill pour and for about the last 10 mil-
lion of them even the direction of the grain in the sheet from which the part
was cut has been recorded. On large forgings, such as landing gears, we
trepanned out 12 sample coupons for test before machining each part. We
found out the hard way that most commercial cutting fluids accelerated stress
corrosion on hot titanium so we developed our own.
Titanium is totally incompatible with chlorine, fluorine, cadmium, and
similar elements. For instance, we were baffled when we found out that wing
panels which we spot welded in the summer failed early in life, but those
made in the winter lasted indefinitely. We finally traced this problem to the
Burbank water system which had heavily chlorinated water in the summer to
prevent algae growth but not in the winter. Changing to distilled water to
wash the parts solved this problem.
Our experience with cadmium came about by mechanics using cadmium-
plated wrenches working on the engine installation primarily. Enough cad-
mium was left in contact with bolt heads which had been tightened so that
when the bolts became hot (over 600�F) the bolt heads just dropped off! We
had to clean out hundreds of tool boxes to remove cadmium-plated tools.
Drilling and machining high strength titanium alloys, such as B-120,
required a complete research program to determine best tool cutter designs,
cutting fluids, and speeds and feeds for best metal removal rates. We had par-
ticular trouble with wing extrusions which were used by the thousands of feet.
Initially, the cost of machining a foot out of the rolled mill part was $19.00
which was reduced to $11.00 after much research. At one time we were
approaching the ability 6t our vendor's plants to roil parts to net dimensions,
but the final achievement of this required a $30,000,000 new facility which
was not built.
Wyman Gordon was given $1,000,000 for a research program to learn
how to forge the main nacelle -rings on a 50,000-ton press. Combining their
advances with our research on numerical controls of machining and special
tools and fluids, we were able to save $19,000,000 on the production program.
To prevent parts from going undergauge while in the acid bath, we set up
a new series of metal gauges two thousandths of an inch thicker than the stan-
dard and solved this problem. When we built the first-Blackbird, a high-speed
drill could drill 17 holes before it was ruined. By the end of the program we
had developed drills that could drill 100 holes Ind then be resharpened
successfully.
Our overall research on titanium usage was summarized in reports which
we furnished not only to the Air Force but also to our vendors who machined
over half Of our machined parts for the program. To use titanium efficiently
required an on-going training program for thousands of people�both ours in
manufacturing and in the Air Force in service.
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NJ, \,�
rewsweime.
17-1ZA test pilot in full preAure suit with walk-around oxygen kit.
Throughout tl.is and other programs. it has been crystal dear to me that
our country needs a 250.000-ton metal forgi-g press�five times as large as our
biggest one avaiLsh.e today. When we have to machine away 90 percent of our
rough forgings today both in titanium (SR-71 nacelle rings and landing gears)
and iluminum (C-5 fuselage side rings) it seems that we are nationally very
stupid! My best and continuing efforts to solve this problem have been
defeated In many years. Incidentally. the USSR has been much smarter in
this field IA. mat it has more and larger forging presses than we do.
Fluid Systems
Very difficult problems were encountered with the use of fuel tank
sealants and hydraulic cil. �Ve worked for years developing both of these,
drawing as much on other industrial and chemical companies as they were
willing to devote tr, a very limited market. We were finally able to produe- a
sealant which does a reasonable kJ- over a temperature range of minus 90�F to
over 000�F. Our experience with hydraulic nil started out on a comical
situation. I saw ads in technical Journals for a -material to he used to operate
up to 900�F in service.- I contacted the producer who agreed I. send me some
for testing Imagine my surprise when the maletral arrived in a large cansas
bag It was a white pcavder at room lems*.-rature that you certainly, wouldn't
put in a hydraulic system If you did, one would have to thaw nut all the lines
and other elements with a blow f web! We did final!) get a petroleum-based
ad develorrd at Penn SIsitr University In which we he I to add several other
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chemicals to maintain its lubricity at high temperatures. It originally cost $130
per gallon so absolutely no leaks could be tolerated.
1,
Rubber 0-rings could not be used at high temperatures so a complete line
of steel rings was provided which have worked very well. Titanium pistons
working in titanium cylinders tended to gall and seize until chemical coatings
were invented which solved the problem.
The Flight Test Phase
The first flight of the A-12 took place 26 April 1962 or thirty months after
we were given a limited go-ahead on 1 September 1959. We had to fly with
Pratt & Whitney 175 engines until the J58 engine became available in January
1963. Then our problems really began!
The first one was concerned with foreign object damage (FOD) to the
engines�a particular problem with the powerful 158 and the tortuous flow
path through the complicated nacelle structure. Small nuts, bolts, and metal
scraps not removed from the nacelles during construction could be sucked into
the engines on starting with devastating results. Damage to the first-stage
compressor blades from an inspector's flashlight used to search for such foreign
objects 'amounted to $250,000! Besides objects of the above type, the engine
would suck in rocks, asphalt pieces, etc., from the taxi-ways and runways. An
intensive campaign to control FOD at all stages of construction and opera-
tion�invciving a shake test of the forward nacelle at the factory, the use of
screens, and runway sweeping with double insvections prior to any engine
running�brought FOD under reasonable control.
The hardest problem encountered in flight was the development of the
nacelle air inlet control. It was necessary to throw out the initial pneumatic
design after millions of dollars had been spent on it and go to a design using
electronic controls instead. This was very hard to do because several elements
of the system were exposed to ram-air temperatures over 800�F and terrific
vibration during an inlet duct stall. This problem and one dealing with aircraft
acceleration between Mach numbers of 0.95 to 2.0 are too complex to deal
with in this paper.
Initially, air temperature variations along a given true altitude would
cause the Blackbird to wander up and down over several thousand feet in its
flight path. Improved autopilots and engine controls have eliminated this
problem.
There are no other airplanes flying at our cruising altitude except an
occasional U-2 but we were very scared by encountering weather balloons sent
up by the FAA. If we were to hit the instrumentation package while cruising
at over 3,000 feet per second, the impact could be4eadly!
Flight planning had to be done very carefully because of sonic boom
problems. We received complaints from many sources. One such stated that
his mules on a pack-train wanted to jump off the cliff trail when they were
-boomed.- Another complained that fishing stopped in lakes in Yellowstone
Park if a boom occurred because the fish went down to the botton for hours. I
had my own complaint when one of my military friends boomed my ranch
and broke a $450 plate glass window. I got no sympathy on this, however.
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Operational Comments
The SR-71 first flew 23 December 1964. It was in service with the
Strategic Air Command a year later.
In-flight refueling from KC-135s turned out to be very routine. Over
18.000 such rebelings have been made to date by all versions of the
Blackbirds. The SR-71 hasjlown from New York to LOTI�lon in 1 hour 55 min-
utes then returned nonstop to Beale Air Force Base, including a Londo.if Los
Angeles time of 3 hours 48 minutes.
It has also flown over 15.000 miles with refueling to demonstrate its truly
global range.' t is by far the world's fastest, highest flying airplane in service. I
expect it to be so for a long time to come.
The suthor about to By in an early A-I2 night test.
'WOW I
iiiessea
CLARENCE L. (KELLY) JOHNSON is serving J3 senior advisor to
Lockheed corporate management and the firm's advanced deve1.1.-nent
projects (Skunk Works). He retired as senior vice president of the corporation
In January 1975 and from ihe board of directors in May 1980.
Johnson joined Lockheed in 1933 as a tool designer. Alley assignments
as flight test engineer. stress analyst. aerodynamictst, weight engineer, and
wind tunnel engineer, he became chief research engineer in I93N In 1952.
Johnson was named chief engineer Of Lockheed's Burbank. California Plant.
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now the Lockheed-California Company. When the office of corporate vice
president-resea rch and developmen! was established in 1956. he was chosen
for the post. He became vice president-advanced development projects in
1958, a member of the board of directors in 1964 and a senior vice president
of the corporation in 1969.
Johnson has played a leading role in the design of 40 world renowned
aircraft�among them the F-80, America's first production jet; the high
altitude U-2; the double-sonic � F-104 Starfightcr; and the spectacular
2000-m.p.h. YF-12A and SR-71.
A native Of Michigan, Johnson was born in Ishpeming on February 27,
1910. He later moved to Flint, was graduated from Flint Junior College, and
completed his education at the University of Michigan, where he received
his bachelor Of science degree in: /932 and his master of science degree in
aeronautical engineering in 1933.
Many honors have come to him for his unioue contributions to aerospace
decdopment through the years, and to the defense of the United States. He
has won the Collier Trophy twice and has also received two Theodore von
ICarman and two Sylvania Albert Reed Awards. In 1964. President Lyndon
B. Johnson presented to him the Medal of Freedom, the highest civil honor
the President can bestow. He was elected to the Aviation Hall of Fame in
1974 and is dile 1981 recipient of the Daniel Guggenheim Award.
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Adventure into the technical unknown
J58/SR-71 PROPULSION INTEGRATION
William H. Brown
Successful integration of the 758 engine with the SR-71 aircraft was
achieved by:
� Inherently compatible engine cycle, size and characteristics.
� Intensive and extensive design/development effort.
Propulsion integration involved aerodynamic compatibility, installation and
structural technology advances, development of a unique mechanical power
takeoff drive, and fuel system tailoring. All four areas plowed new ground and
uncovered unknowns that were identified, addressed, and resolved. Interact-
ing airframe systems, such as the variable mixed compression inlet, exhaust
nozzle, and fuel system were ground tested with the 758 engine prior to and
coincident with flight testing. Numerous iterative redesign-retest-resolution
cycles were required to accommodate the extreme operating conditions.
Successful propulsion operation was primarily the result of.
� Compatible conceptual designs
� Diligent application of engineering fundamentals.
� Freedom to change the engine and/or aircraft with a minimum of
contractual paperwork.
� A maximum of trust and team effort with engineer-to-engineer
interchange.
The centerline of the basic 758 engine was laid down in late 1956. It was to be
an afterburning turbojet rated at 26,000-lb maximum takeoff thrust and was to
power a Navy attack aircraft which would have a dash capability of up to
Mach 3 for several seconds. By the time Pratt & Whitney Aicraft, along with
Lockheed and others, began to study the SR-71 "Blackbird" requirements
several years later, we had completed approximately 700 hours of full-scale
engine testing on the J55.
In the "Blackbird" joint studies, the attitude of open cooperation between
Lockheed and Pratt & Whitney Aircraft personnel seemed to produce better
results than if a more "arms-length" attitude were adopted. This open
cooperation resulted in a more complete study which identified the enormous
advances in the state-of-the-art and the significant amount of knowledge
which had to be acquired to achieve a successful engine/airframe integration.
The completeness Of this study was probably instrumental in Lockheed and
Pratt & Whitney Aircraft winning the competition. The Government stated
that the need for the -Blackbird- was so great that the program had to be con-
ducted despite the :risks and the technological challenge. Furthermore, the
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Government expected the risks to be reduced by fallout from the X-15 and
13-70 programs. Unfortunately, there was no meaningful fallout.
COMPARISON OF )58 DEVELOPMENT OBJECTIVES WITH THEN-CURRENT
PRODUCTION ENGINES
)57 and J75 MID-20
Mach number 2.0 for 15 mm (175 only) 3.2 (continuous)
Altitude 55.000 ft 100.000 ft.
Compressor Inlet Temperature 250�F (J75 only) 800�F
Turbine Inlet Temperature 1750�F (takeoff) 2000�F
1550�F (cruise)
Maximum Fuel Inlet Temperature 110-130�F 300�F
Maximum Oil Inlet Temperature 250�F 550�F
Thrust/Weight Ratio 4.0 - 5.2
Military Operation 30-min. time limit Continuous
Afterburner Operation Intermittent Continuous
Increased Requirements
The table shows some of the increased requirements of the 'Blackbird"
engine compared to the requirements for the previous J75 engine. As it turned
out, even these requirements didn't hold throughout the -Blackbird's- actual
mission. For example, the engine inlet air temperature exceeded 800�F under
certain conditions. The fuel inlet temperature increased to 350�F at times and
the fuel temperature ranged from 600�F to 700�F at the main and after-
burner fuel nozzles. Lubricant temperatures rose to 700�F and even to 1000�F
in some localized parts of the engine.
Because of these extremely liostile environmental conditions, the only
design parameters that could be retained from the Navy J58-P2 engine were
the basic size and the comptessor and turbine aerodynamics. Even these were
modified at a later date.
The extreme environment presented a severe cooling problem. It was vital
to cool the pilot and aircraft electronics; but this left little or no heat sink in the
fuel available to cool the rest of the aircraft or the engine. Because of this, the
only electronics on the engine was a fuel-cooled solenoid which was added
later and a trim motor buried inside the engine fuel control. To keep cooling
requirements to a minimum, we even had to provide a chemical ignition
system using tetraethyl borane (T.E.B.) for starting both the main engine and
the afterburner. A new fuel and a cEemical lubricant had to be developed to
meet the temperature requirements. Pratt & Whitney Aircraft together with
the Ashland, Shell, and Monsanto Companies took on the task of developing
these fluids.
Early in the development, we found that a straight:turboiet cycle did not
provide a good match for the inlet nor the required net thrust at high Mach
number operating conditions. To overcome these problems, we invented the
bleed bypass cycle with which we could match the inlet airflow requirement.
Another advantage of this cycle was that above Mach 2, the corrected airflow
could be held constant at a given Mach number regardless of the throttle
position. The bleed bypass cycle also provided more than 20-percent addi-
tional thrust during high Mach number operation.
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J58 Engi.:e under test.
Fabrication and materials technology presented one of the greatest
challenges. We had to learn how to form sheet metal from materials which
previously had been used only for forging turbine blades. Once we had
achieved this, we had to learn how to weld it successfully. Disks, shafts, and
other components also had to be fabricated from high-strength, temperature-
resistant turbine-blade-like materials to withstand temperatures and stresses
encountered. I do not know of a single part, down to the last cotter key, that
could be made from the same materials as used on previous engines. Even the
lubrication pump was a major development. The newly developed special fuel
ss as not only hot, but it had no lubricity. A small amount of fluoro-carbon
finally had to be added to allow the airframe and engine pumps and servos to
work
Fuel was used as the engine hydraulic fluid to actuate the bleeds,
afterburner nozzle, etc. Because there was nothing to cool the fuel, it nisi made
one rass through the hydraulic system and then was burned.
Instrumentation for Testing
If the foregoing were not enough, develoomental testing 'problems also
had to be overcome. There were no test kcilities which had the Capabilities to
provide steady-state temperature and pressure conditions required for testing
at ITIWCIMUM operating conditions nor could they provide for performing
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transients. On a test stand built as a partial solution. the exhaust of a JT5 engine
was run througji and around the J5S to simulate transients of the temperature
environment.
In addition, there was essentially. no instrumentation rugged enough to
obtain accurate real-time measurements. As Pratt & Whitney Aircraft devel-
oped more rugged instrumentation and better calibration fac,....,, improved
data were gradually obtained. Lockheed, of course, was kept up-to-date as we
obtained better data. A good part of the time Lockheed and Pratt & NA'llitney
Aircraft jointly ran fuel system rigs, inlet distortion, rigs, etc., as well as sonic
engine calibration tests and wind tunnel testing of the ejector.
Its important to remember that this all started nearly a quarter of a
century ago. Although Pratt & Whitney Aircraft had a very large computer
system for its day (the IBM 710), it was no more sophisticate,., than some of the
hand held calculators now available. Consequently, the 158 engine, in effect,
was a slide-rule design. Despite all of the testing and faired curves, we knew.
we had to solve many- of our mutual integration problems through flight test.
Approximately three months before Pratt & Whitney. finished the Pre
Flight Bating Test, which was 3 years and 4 months after go-ahead (the Model
Qualification Test was completed 14 months later), the first -Blackbird- took to
the air. It was powered by two afterburning J75 turbojet engines to wring out
the aircraft subsonicallY. As soon as Lockheed felt comfortable with the aircraft.
a 158 was installed in one side. After several months of subsonic flight tests. J5S
engines were installed in both sides, and we started flight testing for real.
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Naturally there were problems. Here are a few notable ones and the
solutions.
The first problem happened very early�the engine wouldn't start! The
small inlet wind tunnel model did not show the iniet being so depressed at the
starting J58 airflows. In fact, instead of air flowing out of the compressor
fourth stage through the bleed ducts into the afterburner, it flowed the other
way! As a temporary fix, Lockheed removed an inlet access panel for ground
starts. They later added two suck-in doors and Pratt & Whitney Aircraft
added an engine bleed to the nacelle. These two changes eliminated the
ground starting problem.
Originally, the blow-in door ejector or convergent-divergent nozzle was
built as part of the engine. It was subsequently decided johitly by Lockheed
and Pratt & Whitney Aircraft that it would save weight if it was built as part
of the airframe structure. This was deemed appropriate, particularly as the
main wing spar structure had to go around the throat of the ejector. Pratt &
Whitney Aircraft, however, would still be responsible for nozzle performance
in conjunction with the engine primary nozzle. In addition, we would perform
all of the wind tunnel testing. In exchange, Pratt & Whitney Aircraft would
build the remote gearbox because Lockheed's gearbox vendor had no experi-
ence with gear materials or bearings and seals that would withstand the
temperatures required. As a matter of fact neither did we, but we were
already committed to learn.
A problem partially related to the ejector was that the airplane burned too
much fuel going transonic. To help solve the problem, thrust measurements
were taken in flight, movies of ejentor operation in flight were made, local
Mach numbers were measured, etc. Two fundamental mistakes were uncov-
ered. The back end of the nacelle (the ejector) went supersonic long before the
airplane did, and the fairing of the aircraft transonic wind tunnel drag data
was not accurate. While we were puzzling out the solution, some pilot decided
to go transonic at a lower altitude and higher Keas. This for all intents and
purposes solved the problem. From this we learned not to run nacelle wind
tunnel tests unless the model contains at least a simulation of the adjacent air-
craft surfaces. We also learned to take enough data points so that transonic
drag wind tunnel data does not have to be faired.
A Shifting Gearbox
As flight testing increased to the higher Mach numbers, new problems
arose. One, which today may be considered simple with our modern computer
techniques, concerned the remote gearbox. The gearbox mounts started to
exhibit heavy wear and cracks, and the long drive shaft between the engine
and the gearbox started to show twisting and heavy spline wear. After much
slide-ruling, we finally decided that the location of the gearbox relative to the
engine was unknown during high Mach number transients. We resorted to the
simple test of putting styluses on the engine and mounted a scratch plate on
the gearbox. We found, to our astonishment, that the gearbox moved about 4
inches relative to the engine. This was much more than the shaft between the
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Pratt & Whitney .158 0111D-20) engine.
engine and the gearbox could take. The problem was solved by providing a
new shaft containing a double universal joint.
Another problem arose when the aircraft fuel system plumbing immedi-
ately ahead of the engine started to show fatigue and distortion. Me...surements
with a fast recorder showed that pressure spikes at the engine fuel inlet were
going off scale. This overpressuring was found to be caused by feedback from
the engine hydraulic system. This phenomenon did not show up either durinz
Lockheed's or Pratt & WhitneV Aircraffs rig testing nor during the engine
ground testift because of the large fluid volumes involved. To solve the
problem Lockheed invented a "high-temperature sponge" (proMptly named
"the football") which they installed in an accumulator ahead of the engine.
This reduced the pressure spikes to a tolerable level.
A mounting-related problem occurred under certain conditions of down
load on the wing. At these conditions, the outer half of the nacelle would
rotate into the engine and crush the engine plumb a and anything else in the
way. Originally, the engine was mounted on a stiff rail structure at the top of
the nacelle with a stabilizing link from the top of the engine rear mount ring to
the aircraft structure. To solve the crushing problem Pratt di Whitney Aircraft
redesigned the rear mount ring so that a tangential link could be installed
between the engine and the outboard side of the nacelle. This maintained a
finite distance between the nacelle and engine under all conditions.
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As mentioned previously, there was a minimum of electronics in the
engine control system because electronics would not survive the environment
and the fuel was Arcady too hot to provide cooling. Consequently, control
adjustments normally made automatically had to be made manually. For
example, the pilot operated a vernier trimmer to make fine adjustments in the
Ear (Exhaust Gas Temperature) as conditions varied from standard (one such
device was used succa411y. in the U-2). The pilot was provided with a curve
of EGT versus enginelnlet temperature to make the required manual
adjustments. However, unexpectedly sharp atmospheric changes were encoun-
tered. These, in combination with the speed of the aircraft, resulted in changes
too fast for the pilot to handle. By the time he read the engine inlet
temperature and adjusted the EGT, the inlet temperature had changed. This
caused some inlet =tarts (highly reduced inlet airflow) and other undesirable
results. To correct this unacceptable state of affairs, Pratt & Whitney Aircraft
proposed to revise the aircraft EGT gauge by feeding in an engine inlet
temperature signal and adding some additional gadgetry to trim automatical-
ly. The digital EGT readout was retained as was an override manual trim in
case of failure. This modification has worked well ever since.
Trouble on the Wrong Side
The most sensational and most confusing problem at the high Mach
number condition uzs inlet unstarts. These occurred without warning and
were seemingly inconsistent. To add to the confusion, the pilots consistently
reported the unstart occurring on the wrong side of the airplane. This anomaly
was solved rather quickly when Lockheed found that the Stability Augmenta-
tion System (SAS) slightly overcompensated for the sudden one sided drag.
This led the pilot to believe that the wrong side had unstartecl, and
consequently, his corrective action usually resulted in worsening the problem.
Oddly enough, the engine did not blowout. It just sat there and overheated
because the inlet airflow was so reduced that the engine minimum fuel flow
was approximately twice that required. Worst of all, the inlet would not
restart until the pilot came down to a much lower altitude and Mach number.
A great many tests and investigations were conducted including the possibility
of engine surge being the initiator. This was not the case. Three major causes
were finally isolated:
I. Manual trimming of eligine.
2. High, inconsistent nacelle leakage at the approximately 40:1 pressure
ratio.
3. Alpha Fignal (angle of attack from noseboom) to inlet control subject to
G-loading.
The following improvements were incorporated by Lockheed and Pratt &
Whitney Aircraft essentially as a package:
1. Improved sealing of the inlet and bypass doors.
2. Auto-trimmer of engine installed.
3. Derichment , valve with unstart signal installed on engine to protect
turbine.
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4. Increased area inlet bypass doors and addition of an aft inlet bypass
door which bypassed inlet air direct to ejector.
5. Added a "G" bias on inlet control.
6. Automated inlet restart procedure on both inlets regardless of which
unstarted.
The foregoing six items essentially eliminated inlet unstart as a problem. An
additional benefit was also realized by the ability to use tht aft inlet bypass
door in normal flight instead of dumping all inlet bypass air overboard. As this
air became heated as it passed over the engine to the ejector instead of going
overboard, drag was substantially reduced. Also better sealing of the nacelle
reduced drag further.
As you have probably noticed, I have had difficulty in differentiating
between "we" Pratt & Whitney Aircraft and "we" Lockheed. But that is the
kind of program it was.
In any complicated program of this magnitude we all do something dumb
and we both did our share. Here is one from each of us: "We" (Pratt & Whit-
ney) became so obsessed with the problems of hot fuel and hot environment
that we neglected the fact that sometimes the fuel was cold when the
environment was hot and vice versa. When this occutred, the engine fuel
control did not track well. To correct this, we had to insulate the main engine
contro! body from the environment and make all the servos, etc., respond only
to fuel temperature. Eventually, we had to make a major redesign of the
control.
Lockheed and Pratt & Whitney Aircraft spent many hours coordinating
the inlet and engine arrangement so that doors, bleeds, air conditioner drive
turbine discharge, etc., would not affect any of the engine control sensors in
the engine inlet. In fact, the air conditioner turbine discharge was located 45
degrees on one side of the vertical centerline and the engine temperature bulb
was located 45 degrees on the opposite side. To save design time,. Lockheed
built one inlet as a mirror image of the other. It is now easy to conclude where
the I200�F air conditioner turbine discharge turned out to be!! For a while the
fact that one engine always ran faster than the other was s big mystery!
Management Philosophy
That this complex, difficult program was successful is attributable, in
large part, to the management philosophy adopted by the Government people
in charge. Their approach was that both the engine and airframe contractors
must be free to take the actions which in theirludgment were required to
solve the problems. The Government management of the program was
handled by no more than a dozen highly qualified and capable individuals
who were oriented toward understanding the problems and approaches to
solutions, rather than toward substituting their judgment for that of the
contractors. Requirements for Government approval as a prerequisite to action
were minimal and were limited to those changes involving significant cost or
operational impact. As a result, reactions to problems were exceptionally
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Lockheed SR-7I in flight.
quick. In this manner, the time from formal release of engineering paperwork
to the conversion to hardware was drastically shortened. This not only
acceleiated the progress of the program but saved many dollars by incorporat-
ing the changes while the .number of units was still relatively small;
On this program, the Government fully recognized that many of the
problems involving either the engine or airframe manufacturer, or both, could
be solved most effectively by a joint engineering effort and the contracts were
written to allow this activity without penalties. As a result, an extremely close
working relatioi.ship between the engineering groups was developed and
flourished until the S1-71 became fully operational. This method of operation
led to prompt solutions of many problems which, under a more cumbersome
management system, could have severely impeded the program b4 introduc-
ing very costly delays or forcing inappropriate compromises because of
contractual interpretations.
In summary, the method of managing this program by the Government
resulted in shorter development time, faster reaction to field, problems,
reduced retrofit costs, and earlier availability of production systems incorpo-
rating corrections for problems uncovered by operations in the. field. The
result was an operating system incorporating a magnum step in the state-of-
the-art at an earlier time and at less cost to the Government than would
otherwise have been possible.
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