With this significantly more powerful engine the
performance was in most respects even higher than
that of the original Ye-6 series and MiG-21F, and
noticeably superior to that of the early Ye-9 variants.
Predictably, this relaxation of pressure on keeping
down weight was immediately put to use—just as,
when a commercial transport is given more power or
more lift, to improve STOL performance, the
customers instantly translate this into greater masses
of fuel or payload!
In the case of the MiG-21 the result was a new subtype
in which the fuel removed from the Type 96
(M/MF) was replaced. The only place to put it was in
the dorsal spine, much of which had been a mere
aerodynamic shape. Most of the spine’s length had to
remain full of nothing but control runs, piping, basic
electrics and avionics, and similar auxiliaries. To get
in the required amount of fuel within a single short
(0.95 m, 37.5 in) length the spine had to be again
enlarged; indeed it was more than doubled in crosssection
compared with the Type 96. This was no
problem at all. Tunnel testing showed that a properly
area-ruled spine with the required cross-section
would make hardly any difference to drag, and (this
was a bigger worry) would not adversely affect airflow
past the vertical tail.
There seems to be some confusion about the
resulting aircraft. Western reports consistently give
its designation as SMT, but the Russians are not
noted for using English (T for tank), and there is
much evidence the actual designation is SMB (B for
bak, tank). Apart from making the fighter look a trifle
hump-backed it even enhanced its appearance (in the
author’s opinion). Apart from the much bigger crosssection
of what has been called a saddle, rather than a
spine, the longer-range model is immediately
identified by the way the spine is so wide it has to
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continue right across the broad fin to merge into the
drag-chute tube at the rear. Of course the extra fuel,
weighing 1,000 lb (454 kg), is located well aft of the
centre of gravity, and destabilizes the aircraft. Pilots
say that there is no problem. With the size of tail
surfaces combined with the advanced flight-control
and autostabilization system, fed with AOA and with
pitch/yaw signals from the nose instrument boom,
the destabilization cannot cause excursions beyond
the safe flight envelope. In theory it ought to make
this variant into a superior dogfighter, though the
author has been unable to find confirmation of this.
The author has never met R A Belyakov, present
head of the MiG OKB, and does not know whether
the ‘SMT’ was ever planned as the definitive
production MiG-21. In the event it proved to be a
rather scarce interim aircraft which merely led to
what actually was the final version, called MiG-2 Ibis.
The Soviet Union has for almost 60 years used the
Latin suffix bis (two, twice, or more precisely an
encore) to indicate what in former years Britain might
have called ‘Mk 2’. In the case of the MiG-21 the bis
is more like ‘Mk 22’, and it has been adopted (so
Russians have seriously suggested) because the
programme had run out of ordinary suffix letters!
Superficially the bis looks like an ‘SMT” that has
tried to lose weight. In practice it is the biggest
modification in the entire MiG-21 family, affecting
structure, systems and powerplant. The airframe was
for the second time subjected to extensive revision to
improve structural efficiency throughout. In going
from the original Ye-9 to the production Ye-96 the
objective had been to introduce local reinforcement,
or increases in gauge of webs and skin, to permit safe
operation at greater weights and higher IAS. With
the bis the decision was taken almost to start again
and come up with a better structure than the endproduct
of almost 20 years of modifications. By the
1970s fighters were expected to have service lives
several times longer than they were in the 1950s, and
this made fatigue, and the accumulation of local
damage, an important new factor. In the bis the
structure was revised both to seek better solutions, by
wrapping a new skeleton round what had become a
different aircraft as far as systems and equipment are
concerned, and also to give a long troublefree life.
The author can vouch for a lot of this, visible on close
walkround inspection, but the only thing that looks
different from the MF from a distance is the upper
mainwheel blister on each side! Closer study shows
that, in fact, the entire spine, or saddle fairing, has
been yet again redesigned.
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