NACA Investigation of a Jet-Propulsion System Applicable to Flight Page: 4 of 17
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REPORT NO. 8 0 2-NATIONAL ADVISORY COMMTEE FOR AERONAUTICS
,.Exhust boilers 1 Primcry fire /nterchoageable
e uipmenf circular nozzles
i Primary ba-ner ement of different -reas
C Gasoline manifold , Spiral second
,ate Main boiler
\ Mownting. 23gas \lMixir
\ rings-" vcporje s
Y .Flexible mounting couplings
"Engine mo rnt ring
FIGURE 1.--Ground-test mock-up.
or superheating part of the boiler, each of the 24 tubes is
wrapped into two flat coils, which are connected in series
and mounted radially in the, duct. The tube ends are led
out through the Inconel shell to jets located in the mixing-
duct entrance. The air-fuel mixture at the end of the mixing
duct is ignited by a flame from a ring burner. This annular
igniter is fed vapor from one of the 24 main boiler tubes and is
initially ignited by two sparks 1800 apart.
The black-iron combustion chamber was designed to
provide a blanket of air on both the inside and the outside
of the chamber wall and the exit nozzle. The several exit
nozzles used for the ground tests were interchangeable and
of various areas.
For the purpose of measuring the static thrust, the entire
ground-test mock-up is mounted bn three ball-bearing wheels,
which roll on sections of steel track. The thrust is indicated
by a sensitive dial gage that measures the deflection of a
calibrated U-spring dynamometer.
TEST RESULTS AND DISCUSSION
In accordance with the original purpose of the investi-
gation, the test procedure consisted of a series of observations
of burning under various conditions. Many such qualitative
observations were accomplished with model burning experi-
ments and led to the conclusion that a blue flame would be
advantageous. These experiments also indicated the most
promising methods, which were later used in the burning
experiments with the full-scale apparatus.
It may be said that the results of the full-scale burning
experiments generally exceeded expectations. The main
fire was restricted to an intense, small, and short annular
blue flame burning steadily and under control in the intended
combustion space. In fact, in the last series of experiments,
burning runs lasting 7 to 9 minutes were consistently made
with hands-off operation. The results exceeded expectations
in that satisfactory flames were obtained up to fuel rates
corresponding to burning approximately one-half the air
passing through the entire system. Under these conditions,
the temperatures in parts of the jet must be very high and
even if complete mixing .-with all the cooling air-an impos-
sible condition-were assumed, the, mean temperature
would be almost 22000 F. Even this fictitiously low
temperature corresponds to bright yellow black-body
radiation. In the presence of the burner flames and jet air
at 22000 F and much higher temperatures, the black-iron
liner forming the actual combustion chamber and nozzle
wall, which was expected to require the use of stainless
steel or other heat-resistant material, became only hot
enough to blue the iron in a few spots. These spots were
probably the result of only transient or locally defective
conditions. Under these conditions, the outside shell
became only slightly warm.
From the burning experiments, it was concluded that,
with proper conditions, a blanket of cool air can be main-
tained between the hot gases and the walls. In the presence
of suitable combustion, furthermore, adequate cooling air
may readily be provided to carry away any radiant heat and
to maintain the walls and structure at normal temperatures.
It is believed that the foregoing conclusions, together with
the information that has been gained about combustion,
constitute the new and really significant results of the
The operation of the burning system was satisfactory in
all respects with the possible exception of one detail. During
one of the burning experiments, it was noticed that the flow
had stopped through one of the boiler tubes. An inspection
of this and several other tubes indicated that the inner
surfaces of the tubes were generally clean. A plug of carbon,
which was removed by probing and blowing out the tube,
had apparently collected, however, in the radial superheat
unit at the end of the defective tube. Air was subsequently
passed through all the boiler tubes while they were kept at
red heat by means of the primary fire, with the object of
burning out any carbon deposits in the rest of the tubes.
During this process, hot spots were seen to develop on some
of the tubes, which indicated that other carbon deposits were
burned out by the process. It may be that some such
simple carbon-removing process would be required as part
of the service on these boiler-type burning systems.
Engine cooling aduct.
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Ellis, Macon C. & Brown, Clinton E. NACA Investigation of a Jet-Propulsion System Applicable to Flight, report, September 17, 1943; (digital.library.unt.edu/ark:/67531/metadc64950/m1/4/: accessed December 16, 2017), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.