Re: [MV] water injection

From: cgarbee@ipass.net
Date: Tue Aug 19 2003 - 09:34:37 PDT


I forwarded the message quoted below to a friend who is not on this list who
used to spend a lot of time with the BUFF's hoping that he would repsond with
some comments as to the thrust increase. I got the following in reply and
thought that I would share it...

"The poster is right about water/walc injection in B-52s.

Any time a turbojet-powered BUF took off near max gross takeoff weight or on
a hot day water injection was used. You could tell from a mile away that
they were "burning water" because the aircraft trailed thick plumes of black
smoke on takeoff, and the (incredibly loud) engine noise on takeoff had a
ripping-canvas quality on top of the usual shrieking roar.

Of course, feeding eight J-57s enough water to make a difference results in
a lot of water going through the engines. Deionized water was used to
prevent mineral deposits. There were flightline water tank trucks (I
remember mid sixties vintage Chevrolet or GMC straight trucks painted bright
yellow with a legend on the tank identifying them as containing non-potable
deionized water) which hauled the water from the deionizing plant to the
aircraft.

As with the WWII recips, alcohol was added to the water to prevent freezing
in the plumbing when the weather threatened it. In the later years, I don't
recall this being a common practice on the D and G models, which were the
last turbojet BUF models in service. They burnt water on takeoff regularly,
but only as a takeoff-roll-shortening thrust supplement on hot days. I
suspect that corrosion and seal embrittlement were problems with the
alcohol, so they avoided it as much as possible.

It is worthwhile to note, however, that water injection on a turbojet has a
somewhat different function than it has on an Otto cycle engine. The recip
uses water to slow the combustion event down in the cylinder and prevent
abnormal combustion (knock, ping, detonation, etc.) under high MEP
operation. In the WWII combat aircraft, it allowed the use of more
supercharger boost and fuel to generate a little more power than was safely
attainable without the cooling effect of water in the cylinder.

On a turbojet, however, water is injected simply to reduce the maximum
temperature in the compressor stage, which makes the air charge being fed to
the combustor denser, and results in more thrust being generated. It acts
as a sort of quick-and-dirty compressor stage intercooler.

Modern airplanes don't use the technology because it adds another complex
system to an aircraft, and simply because water is heavy, and the
infrastructure required to supply that much DI water isn't cheap.
Sufficient thrust just hasn't been a problem on airplanes since the mid
fifties, so the technology has disappeared."

Thanks for giving me an excuse to bug an old friend about his past life,
C. Garbee
'53 USMC M37 with Cummins 4BT
'51 USA M38 stock
www.garbee.net/~cabell
www.ncmvpa.com

Quoting william f cox <grntrks@juno.com>:

> The early B52 bomber used water and later models used water/alcohol
> injection in the turbojet engines during take-off and for extreme
> emergencies. That's what caused the heavy black exhaust trails during
> take-off. I don't remember the percent of thrust increase ( jeez, it's
> been 40 years already? ) but, it was substantial.
>
> Frank



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