Developing
Stealth Technology - Lessons from the American XST/F117 programme
Prof. Prodyut Das M.Tech, MIE, PGCGM M.AeS.I.
Forty
years ago the Americans developed a very serviceable stealth aircraft , the
world’s first, in a remarkably short
time and at very modest costs. As India discusses its own AMCA programme, the
honesty, simplicity and common sense that guided the American programme is
worthy of review.
The Americans, geographically isolated, has
always had an unique requirement of “air superiority over hostile airspace”.
The Air war over North Vietnam showed that this requirement could no longer be
met by the traditional US combination of “superior” aircraft and ECM. North
Vietnam war was a static defensive action. The Ramadan/Yom Kippur War showed
that the same carefully co- ordinate
defense system using low technology mobile AAA, tracked SAMs and aircraft could
also be used for offensive operations. The American requirement of air
superiority over hostile airspace was clearly under threat.
The
Americans, despite considerable skepticism, identified stealth as a possible
solution to the challenge. Stealth meant reduction of Radar, Aural, IR and
visual signatures. Since the threat came mainly from radar and radar guided
weapons the reduction of radar cross section (RRCS) was given primacy. The
details of the American programme to develop stealth technology is of interest
because an urgently needed technology was developed to meet an identified
requirement at low cost to a tight time scale- a management skill that will be
of interest to Indian Aerospace if the AMCA programme is pursued.
The
Stealth development Programme; Step1. Establishing the basics
The
bare bones of the American Stealth development programme are as given below.
The focus on short steps, strict timelines, low costs and the verification and
securing of each technology step before taking the next steps is inescapable.
Within
a few months of the Yom Kippur/Ramadan War, as soon as the Israeli loss
patterns had been analysed, Ken Parko of DARPA requested submissions from the
top five American fighter manufacturing
companies. The DARPA focused on just two parameters.
What
were the signatures thresholds for an aircraft to be essentially undetectable
at an operationally useful radar range?
Did the company have the capability to design
and produce an aircraft with the necessary low signatures?
The
approach at this stage was exploratory. Was stealth feasible?
The
eventual winner, Lockheed, was not even contacted because Lockheed had not
produced a fighter since the Starfighter of the ‘60s .The emphasis on experience because of the urgency of
the programme is to be noted.
Of
the five companies contacted two declined and one proposed continued emphasis on
sophisticated platforms and advanced ECM. Two companies-Northrop and MacDonnell
Douglas showed a grasp of the problem .In the closing months of 1974 i.e..twelve
months after Yom Kippur the two companies were funded $ 100,000 each to conduct
further studies. For convenience of Indian readers the $ values have been converted
to rupees at he then exchange rates and the local purchasing power parity
(PPP) factor of about 3.5 is being also given because the cost of an Indian
Engineering man hour is much less than US. Thus we have $100,000/ 12Lacs/3.5
lacs. The small amounts released, even by 1974 prices, is noteworthy.
At
this stage Ben Rich, President of the Lockheed “Skunk Works” (secret prototype workshop) noted that
Lockheed’s earlier SR 71 Blackbird and D 21 drones had shown low RCS
capabilities and he approached the CIA-which had financed these programmes – for permission to
discuss with DARPA the practical experience that Lockheed had already gained on
RCS.On the strength of these studies Lockheed was permitted to join the
programme and submit a proposal but without any funding.
In
early 1975 Lockheed formed a three man project team comprising of the
following:
Dick
Scherrer- Project Leader
Denys
Overholser Radar Expert
Bill
Schroeder retired
mathematician
The
sub team Overholser and Schroeder concluded that a faceted shape based on
simple geometric optics would be giving the required results at the lowest
risk. They collaborated to set up a mathematical model and asked for six
months. This was brought down to three months and in fact the team had
developed RCS prediction software of reasonable accuracy, the ECHO 1 in five
weeks.
The
simulation software was immediately validated by experiment .The Lockheed
Company funded $25,000 / Rs3 lacs/ Rs 85,000) for two metal foil covered1/3
scale wooden models- one for RCS anechoic chamber and the other as a wind
tunnel model. The RCS test model was a checked at the Lockheed Radar Anechoic Chamber
in 1975. The anechoic chamber was fairly simple. It was found that there was a
reasonable but not perfect match between the spikes as predicted by the ECHO 1
software and as seen in the anechoic chamber tests. The main problem was the
diffraction due to the edges. At this stage the ECHO 1 software was modified to
take benefit of the work done a decade earlier by a Russian physicist Pyotr
Ufimetsyev Chief Scientist, Moscow Institute of Radio Engineering, whose 1962 work
“Method of edge waves in the physical theory of diffraction” permitted the
prediction of RCS using curved surfaces and therefore a more advanced airframe
could also be considered. Indeed there were was a powerful group amongst them the
formidable legend Kelly Johnson who would have liked to see the Ufimetsyev
equation applied for a more efficient airframe but this lobby was firmly over ruled
in the interest of programme certainty. There was so much skepticism over the faceted
design that the programe was nicknamed in typical American manner as the
“Hopeless Diamond” the hopeless being for the predicted/presumed flying quality
and the “Diamond” referring to the faceted shape.
The
model was then sent to the Mojave Desert where the flat featureless terrain was
like a giant natural anechoic chamber. Amongst the many discoveries that comes
from an experiment driven approach the engineers found that designing a non
reflecting pole to mount the model for radar reflection studies was almost as difficult as designing the aircraft shape!
The Mojave tests further boosted confidence in the ECHO 1 modified software.
Based
on its own experiments Lockheed submitted its proposals to DARPA for an experimental
survivable Test vehicle (XST) in the summer of 1975- i.e. about 18 months
from DARPA’s first enquiries.
DARPA
now had three proposal from which to select for funding. The two from Lockheed
and Northrop were quite similar .The MacDonnell Douglas team were the first to
establish the minimum level of stealth required but according to reports they
were not able to give a suitable submission-possibly by being up to their eyebrows in the F 15 programme!
The
Lockheed Northrop (LN) XST technology
menu
The
LN XST focused on maximizing total stealth
i.e. Radar, IR, Aural and visual.
Everything else was compromised to achieving this end. The logic was that
stealth was a substitute for conventional good performance which in any case
was and still is unachievable with total stealth.
Given
below is the menu of stealth features in the programme.
High
wing leading edge sweep. The XST had a LE sweep of 72oand
the F 117 had a LE sweep of 67o. By comparison the highly swept MiG
19 and Su7 wings had 57 0 LE
sweep. Modelled on geometrical optics, the extraordinary high LE sweep reduced
radar returns. Lockheed/Northrop’s previous experience in lifting body dynamics
gave the team the confidence. High LE sweep is aerodynamically inefficient and
current LE sweep is around 40o.
Sharp
edged LE sweep. Chosen for low frontal area this feature
gives a “sharp” stall. Current practice
is not known to the author.
Faceted
Aerofoil Pictures
of the F117 shows an aerofoil that appears to be made up of three flattish
segments each that make up the upper and lower curved surfaces. This would give
poor lift and uncertain handling. Current designs do not appear to use this
feature.
Hopeless
Diamond faceting The
American team chose this ‘inferior’ technology despite knowing
its problems of uncertain handling and despite knowing of Ufimetsyev’s work in
the interest of timely completion of project and because it “worked”. Current
designs, particularly the B2, use increasing amounts of Ufimetsyev’s theory.
Inlet
meshing gave excellent stealth of the inlet which is a
stealth “hot spot”. However it is poor in terms of propulsive efficiency and is
prone to blockage in icing conditions. Current designs use DSI ((Diverter less
Supersonic Intakes) or use prominent ‘S” intakes.
Fishtail
exhaust Excellent in terms of IR
and Aural stealth it is propulsion wise bad with a rumored twenty percent loss
in efficiency. This was last used on the F 22 Raptor and current design go for
as high a BPR turbofan as possible along with exhaust shielding by the
empennage.
The
resulting aircraft, the XST/F117 had excellent stealth characteristics but the
current thinking is that like Variable Geometry the solution is as bad as the problem!
Full stealth is “mission profile sensitive i.e. it is good for long range
subsonic aircraft but not possible for fighters.
Step
2 proving the basics .The XST
DARPA
studied the submissions of Lockheed and Northrop and based on their grasp of
the problems as shown by their proposals they were asked to collaborate to
produce the following quanta of advance:
1.
A large scale ground test model i.e. a
structural test model.
2.
The construction of two flight models
The
funding for the above was released on 26 April1976. The amount was $10.4 M/12.5
crores/3.56 crores. The aircraft
produced would be used to validate the following:
Validate
in flight the four stealth features designed-Radar, IR, Aural, and Visual.
Demonstrate
acceptable flying qualities.
Demonstrate
that accurate computer modeling of stealth has been achieved.
It
will be noted that the task requirement was focused on just stealth, handling
and confirming a maturity of the RCS prediction and stealth engineering
capabilities. Confirmation of the predictions at the earliest opportunities was
the persistent aim. The XST was not required to have ANY operational
capabilities. The XST prototypes were practically hand built- in metal (sic)
–stealth being achieved by shape, faceting and RAM. There was no attempt at
“optimization” which would be a waste at this stage of the development. The
aircraft was “cobbled” together using many standard aggregates off the shelf.
For example the engines GE J 85 4 came
from the T 2 Buckeye Trainer, the landing gear was salvaged from a written off
Northrop F 5 and the flight controls came from Lear Siegler and the cockpit
instrumentation came from a F 16. The time saved by such an approach was
reflected in the fact that the prototype flew on 1st December 1977
i.e. within 18 months of funding approval which had come on 26 April1976. The
“cut and try” approach can be seen from the fact that the aircraft had to be
grounded to fit bigger fins which had been found necessary. This prototype was
lost on 17 January 1978 on its 36th flight. The second prototype was
flown on 20 July 1978 and lost on 20 July 1979 on its 52nd sortie
due to systems related problem. It was found that the prototypes’ handling and
flying qualities were so bad that it was officially reported that “the only
(bad) thing that the XST does not do is to tip on its tail when parked! Despite
such adverse comments the programme had generated sufficient confidence and
data as to go in for the actual combat version, the F 117, was a substantially
different machine from the XST technology demonstrator.
Step 3 Converting to a weapon: XST to F 117
The
project had demonstrated that the limited objectives set out above had been
clearly achieved the Americans moved on to the next stage. They funded on 1st
November 1978 $ 340 M (Rs. 408 crores/ Rs.117 crores) for 5FSD airframes, support equipment etc with
a target date of 21 months to first flight i.e. July 1980. At this time
the design specified that maximum stealth
was to be in the “Head on - +/- 25 degrees” segment” .i.e. during the
penetration phase. There was no attempt even at this stage produce “all aspect
stealth”. The IOC was to be March 1982. The F 117 used the ‘full stealth”
treatment already proved on the XST. There is now increasing evidence that
these features were overkill and later designs have moved away from ‘full
stealth” in the interest of restoring aerodynamic efficiency.
The XST/F 117 programme management policy.
The
management policies followed for the programme laid down fourteen rules
covering programme management, organization, contractor customer relations, documentations,
customer reporting, specifications engineering drawing, funding, cost control,
sub contractor inspection, testing security and management compensation. The
following expansion of some of the rules showed the result oriented approach.
1.
It is essential that the programme manager have authority in all aspects of
managing the programmes.
2.
The Customer programme manager must have similar authority to the prime
contractors managers. Incidentally the Customer was an equal financial stake
holder with one third of the stake.
3.
The number of people involved must be reduced to a vicious minimum i.e. between
10 to 25% of “normal”. In development programmes mediocre numbers is no
substitute for talent. You either have the “right stuff” or you only have a
high wage bill.
4.
Very simple drawing and drawing release systems with great flexibility to make changes.
The People
The
profile of Dick Scherrer, the team leader who was 54 when he took charge of the
project is typical of the people involved. He was a BS in Engineering and from
1942 to 1959(he was at NASA Ames Wind Tunnel and had much practical experience
in Tunnel testing. From 1955 to 1959 he was also working on the side on the
Disney land “rides” such as “Dumbo the flying elephant”. In 1959 he moved to
Lockheed California making project studies for about 5 or 6 important programmes
including winning Lockheed submissions such as the P3 Orion, The S2 Viking and
the Tristar. Though by our rules he would be “low” on formal qualifications the
emphasis on hands on experience is to be noted.The degree of ‘hands on”
experience can be gauged by the fact that when the Lockheed shop floor workers
went on strike during the assembly of the prototype the engineers were
confident enough to do it themselves.
Analysing the management of the programme
Rene
Descartes (1596- 1650) had observed “If you begin with certainties you will end
up in doubts but if you are content to begin with doubts you will end up in certainties.
The Americans were unconsciously following Descartes.
Their
starting point was not an aim or an identified Technology but a search for what
technology – at that point unidentified- would enable the USAF to survive and
dominate hostile air space. Having identified Stealth as a possible solution
they then tried to establish a minimum level of stealth required to
survive. The technological difficulties were respected as only engineers and
practical men can and no impossible goals were set.
Recognizing
that “stealth” would have an adverse effect performance they focused on getting
to know the Stealth technology. The XSF was a pure technology demonstrator with
no combat capability and even the final F 117 was decidedly subsonic and of
limited war load capability.
The
Americans at the start, focused on just two common sense questions:
a)
Is it possible to reliably predict RCS
/stealth characteristics?
b)
If so, would the resulting aircraft be
flyable?
In spite of their
enviable depth of experience , or perhaps
precisely because of it , the Americans lost no time in backing each
theoretical projection with hard data n matter how “crude” the data was at that
point of time. The Americans, again because of their depth of experience,
avoided making the mistake of developing the combat F 117 in one go. The Table
1 shows how different the XST was from the F 117. Translated in to India it
would mean that the LCA TD 1should have been not KH 001, the LCA prototype , but
a HF 24 or even a HJT 16 or MiG 21 but
having the four technologies – composites , FBW, BVR and the Glass cockpit needed
in the LCA programme. The resulting collapse of “to flight” times and the
refinement hard data would have brought to the LCA programme can be imagined. In
the case of the AMCA it would mean a subsonic aircraft using aggregates from
concurrent programmes but incorporating all that we have learnt so far on
stealth airframes so that all our detail design is based on proven technology
and not on hypothesis and validated before we proceed to the infinitely more
difficult job of designing a supersonic stealth
aircraft. It is the problems that we cannot know theoretically that need to be
unearthed at the earliest. No amount of computer studies can solve problems
that one cannot foresee and can only be seen in flight.
The
very short lives of the two XST
prototypes – 52 and 36 sorties respectively shows the emphasis that has to be placed
on generating data to validate the theoretical projections. The prototypes were
there to generate data and they were hard driven. It may be mentioned that the
equally skilled Russians had ten crashes in their SU 24 programme-four
apparently due to airframe related problems and six due engine related
problems. Crashes are an occupational hazard in developmental flying and there
is no merit in a crash free programme. The risk taking informal approach can be
seen by the fact that the F 117 prototype was test flown after just 4 taxi
tests.
The Americans achieved
remarkable fiscal and time control by slicing up the development into salami slices. At no point was the development
Target beyond the time horizon. Since DARPA officials stay on the job for about
two years before moving out most of the horizons were in months and we in India
can do so too.
Whilst the differences
between the American and our way of doing things is obvious they Americans were
not doing anything that is impossible to
do in India-save the way we think. The
HJT 36 and the HTT 40 programmes underline that we can put up a prototype in 36
months and these figures can improve. The only correction will be to slice up a
project, no matter how complex, into a one, two, or maximum three years slices.
Any project with a target ten years away will, to use a current phrase, go BVR.
No one- the “funder” and the “fundee” will be around to answer or pick up the
pieces when the time to call to account comes ten years hence! This is the
fundamental weakness in our management.
Lessons
for our AMCA programme
Given the above our AMCA
programme should be reviewed thus:
1. Stealth technology is
important enough for India to work on this field. The caution is that advances
in RAM may radically change the way stealth aircraft are designed. Thus timeliness
of completion is of even more importance than is usual.
2. At the proposal stage
it is essential to have more than one submission even if it means releasing 1-2
crores each to various selected (private sector) vendors. Multiple competitors
generate ideas which can all be included in the final winner. Multiple entries will also avoid “the only son”
performance syndrome which solitary organizations have shown.
3. Developing a stealth
AMCA “in one go” over ten years as is being proposed is a bleak prospect.
4. The first step should
be a stealth technology demonstrator an AMCA-ST a subsonic stealth aircraft
which will feature everything that we think we have learnt about stealth
aircraft. This like the XST will maximize use of aggregates- engines systems
undercarriage etc from the existing programme so that the only major design
load is the stealth technology airframe.
5. The target time to
flight should be thirty months and about three hundred crores. If the team
cannot meet this target it will also not be able to do meet a ten year, ten
thousand crore plan.
6. The AMCA XST should
complete its flight tests – focusing mainly on stealth signatures and handling,
in about a year.
7. If the programme turns
in encouraging results it should lead to a small subsonic nocturnal intruder –
not a supersonic AMCA as a first aircraft. A detection
range
of one tenth of the Canberra could be a possible target.
Small steps, tight
control, continuous Government Interest (which seems now to be happening) can
work wonders. Funding big plans and distant promises will be folly repeated.
Table
1
Parameter
XST F117
Wing
span 6.58 13.21
Length 14.41 20.09
Wing
Area 35.86 105.9
Empty
Weight not known 13,154
MTOW 5670 23,181
Engines GE-J85-4A GE
F404F1D2
2x13.15kN 2x48.51kN
Vmax not known M0.9
approx.
The
difference between the tech demonstrator and the Final product is obvious.
All
dimensions in Mts. Sq.Mts, Kgs,
Prof. Prodyut Das
9007434226
Kolkata
26 August 2016
Appendix 1 Note on the
sketches D 7 & D9
1. Stealth is a subject
we cannot ignore and yet it may prove to be another dead end in Airframe design
like VG wings.
2. If stealth s useful
it’s most likely application is in subsonic bombers and intruders. A stealth
supersonic fighter is a technical contradiction.
3. A stealth supersonic
fighter like the proposed AMCA is at the moment beyond our capabilities,
certainly in terms of time and possibly in terms of our
capabilities.
Given the above our focus
should be on stealth aircraft like UAVs or an Intruder.
4.A
stealth intruder with 30% more range than a Canberra B(I)58 with one tenth the
detection range of a Canberra represents both a “do-able”
challenge
to our present engineering capabilities and also the resulting warplane would
be a useful addition to our armoury.
5.
The two sketches are provisional but show the emphasis on “robust” (i.e. non
RAM) stealth
Stealth
is dependant largely on small size, high L.E. sweep, prominent “s” -ing of the
inlet etc. The IR signature is reduced by fairly high BPR turbofans i.e. 2 to 4
BPR and masking of the exhaust by the empennage in the D7 layout. The Flying
wing design is naturally stealthy but both layouts-“tailless”-“ V-tailed” need to be considered
carefully as the Flying wing has its CG limitations and cannot be “grown”. The
wing sweep of the Flying wing can be varied on the ground for
stealth research.
6.
The small size of the aircraft notwithstanding the bomb bay size of 4.00xmts
1.8mtsx 0.60 mts will accommodate three of all current IAF 500 kgs stores with
LG kits. We do not want a situation when the service
introduction is delayed by having to develop especial SDRMs
7. The aircraft is a two seater. Human stresses of flying long
missions in strict radio silence are not known to us and a second crewman may
be useful. The flying wing has tandem seating and the V tail design has side by
side which despite stealth difficulties has better crew co-ordination.
Programme wise it is easier to take out a cockpit than to introduce a stealth
cockpit.
8. The design empty weight varies between 5500kg to 6500 kg with an
MTOW of around 13,000kgs. Two Turbofans of around 30.00kN are proposed e.g. CF
700, TFE 731,3xFJ44 etc. The HTF 2500 can be a prospective candidate freeing
dependence on a “foreign” engine. The NIS (I) –XST can use the Adour or the
AI55 or the unreheated F 125.
9.
The management of the programme would like the XST programme be in short
steps.-i.e. stealth models, stealth anechoic chamber results, an XST and then a
final design based on experience.
Note: First published in Indian Defence Review, 2016
