Re: 2 vs 4 engines: R&D costs too much ?

Date:         16 Sep 99 16:42:51 
From:         James Matthew Weber <>
References:   1
Next article
View raw article
  or MIME structure

>I am starting to wonder if the extra cost of pushing  the "state of the
>art" limits to get the required performance on a twin is really worth
>all the effort ?

The problem is the R&D cost for any large project is huge now, and
whether it is worth doing or not is a complex economic analysis. GE
apparently thought it was. P&W thought it wasn't, on the other hand
P&W has been late out of the starting blocks before, they didn't do a
replacement for the JT8, and the CFM delivered their 10,000th engine not
long ago as a result. RR took a middle of the road path, Trent isn't
really all new, but it was certainly a big push for a derivative.

>Are we not getting to a point where conventional jet engines are
>reaching the top of the bell curve and any improvements will cost more
>and more ?

Certainly improvements in SFC are going to be very difficult from now
on, but that's been the case for a long time. It seems to be an
investment that GE has been more willing to make. Generally the CF6,
CFM56 (and the core is made by GE), and the GE90 were fuel econonomy
leaders in the class when they were introduced. The Best SFC's outside
the GE90's have been pretty much stuck in the .31 range for a decade.
By comparison most JT8's are around .6 At the end of the day, I doubt
Boeing, Airbus, PW, RR or GE would be doing this if they didn't believe
it made economic sense.

>On the other hand, by using 4 engines (or even 3), it allows you to use
>much more conventional technology which is not only cheaper  for R&D but
>has greater market and hence more competition.

While you may save on R&D, you may spend on  maintenance. One very large
engine will generally cost a lot less to maintain than two smaller
engines, and engine maintenance expenses run hundreds of dollars per
hour per engine, and the parts that are not there (like another engine)
cannot break either. The result is mechanical dispatch reliability on a
twin is likely to better than on a tri or a twin just by reduction in
the number of parts that can break!

The other issues are weight and drag.  An RR Trent 556 at 56,000 pounds
will weigh about 10,400 pounds, or 5.4 pounds of thrust per pound of
engine, with a frontal area of about 51 square feet, or about 1100
pounds thrust per square foot.

A trent 895 has 95,000 pounds thrust, and weighs13,100 pounds, or about
7.2 pounds of thrust per pound of engine, and has a frontal area of 66
square feet or about  1440 pounds per square foot.  The bigger engine
gives you less drag, and more payload.

Over the life of the aircraft a few thousand  pounds is worth a truly
amazing amount of money. The SFC on these two engines are almost
identical, and it it likely that the maintenance costs for the two
engines will be very similar.

(By comparision the GE90-115 is expected to weigh about 17,000 pounds,
so it  will be about 4000 pounds lighter than a pair of Trent 556, and
about 2500 pounds less than a pair of 56,000 pound thrust CF6's.) In
addition, you don't need all of the extra plumbing and wiring and
electrical/fuel, instrumentation, controls, nacelles and  hydraulic
systems that goes with the extra engines, so the true weight savings are
likely to substantially larger.

The A330/A340 gives some idea. An A340-200 weighs about 13,000 pounds
less than an A330-300, of that 13,000 pounds , about 2000 pounds is
engine weight.

>Also, in the theroretical A3XX plane, what sort of power is expected out
>of each of the 4 engines ? Would they be using 777-class engines (90k
>and above) or 747 class engines (about 60k pounds, right ?) ?

No one wants to use a 777 class engine because the smallest 777 engine
is in fact  large for the A3XX, which implies a substantial weight and
probably fuel economy penalty.  For example the largest CF6, the 80E
variant weighs 10,700 pounds, The PW4060 weighs 9400 pounds.  The
smallest GE90 is over 16,000 pounds for 8000 pounds more thrust, and the
PW4084 is 15,000 pounds, Tren 875 is 13,100 pounds.  the problem is
these engines were all designed for higher thrust, and it is like the
aircraft themselves.  You usually don't pay a big weight penalty to
stretch, but you don't get much weight back when you shrink them!

One of A3XX's and 747-400X's  problems is they both  need an engine
where no one really builds one today, 72,000-75,000 thrust. It is
sufficiently painful that GE and PW have teamed to offer engines for
both projects, the GP7170 for the 747 and the GP7275 for A3XX. Neither
GE or PW wants to carry the risk alone, or individually carry the R&D,
and infact both engines are hybrids built from technology developed for
the PW4000 and GE90 families.

James Matthew Weber   (623) 587 7514 .  Fax  (623) 434 7598