Cryotechnics
Since 1987, the 'Laboratoire de Chimie industrielle' has had contractual
and research activities in the space field and especially with the manipulation
of liquid oxygen often used in rocket propulsion. This work led to the development,
the construction and the improvement of a cryotechnic test bench, allowing
to use liquid oxygen under pressures as high as 50 bar. This bench is remotely
controlled by a dedicated software able to manage control sequences with a
precision of less than one millisecond. This software is also in charge of
data acquisition from up to 128 channels, with frequencies higher than 10
kHz.
This bench, actually very versatile, has allowed the realisation of various
studies related to the use of liquid oxygen in space cryotechnic engines,
among which the following are the most important:
- study of the ignition sequence of the gas generator of the HM7B engine,
which is a part of the third stage of the Ariane IV rocket launcher,
- characterization of a liquid oxygen spray in the gas generator of the
HM7B engine,
- behaviour of the flooded ball bearings of the oxygen pump used in the
HM7B engine.
Since our experience is mainly in the field of liquid oxygen, we will describe
the researches made in this area.
Study of the ignition sequence of the gas generator of the
HM7B engine
The aim of this study was to determine a valid sequence for the starting
up of the gas generator of the HM7B engine. To start up the gas generator,
a pyrotechnic device is ignited. Eventually, the released gases activate the
engine's turbine allowing to drive the pumps which inject first liquid oxygen,
and thereafter hydrogen, in the combustion chamber of the gas generator. The
oxygen reacts with the hot powder's gases, and provides a flame which will
ignite the hydrogen. After ignition, the gas turbine runs autonomously. In
our work, we studied the complete starting up sequence, except for the injection
of hydrogen. We investigated the parameters of this sequence, in order to
find valid one, that is a sequence where the powder's gases burn with oxygen
inside the combustion chamber without generating a peak of pressure.
The test bench was fitted in order to respect to the configuration of the
actual gas generator and its equipment:
- even if the pyrotechnic device used is not the same as the one which
is used in the HM7B engine, it is equivalent and provides the same dynamic
response,
- the combustion room has the same volume and shape as the actual one;
the only difference is the wall thickness, which is more generous on the
bench in order to allow the insertion of probes,
- the increase of pressure due to the starting up of the liquid oxygen
turbopump is reproduced rather well by filling and emptying fast remote
controlled buffer containers.
The testing bench and the studied material are equipped with a high number
of fast temperature and pressure probes. The combustion room is also equipped
with very fast photodiodes, used to detect the flame front when the powder's
gases burns with oxygen, and with a sampling probe connected to a mass spectrometer.
This one allows to determine the chemical composition of the gases.
Many experimental parameters were varied during the experiments : the length
of the pyrotechnic device, and then the duration of its combustion, the pressure
and duration of the preventing of the combustion room with helium, the oxygen
injection pressure, the moment at which this injection is made, the duration
of the cooling of the liquid oxygen injection system,...
research duration: 3 years
Characterization of a liquid oxygen spray in the HM7B gas
generator chamber
These experiments were done in order to characterize the dynamic evolution
of the liquid oxygen spray in the combustion room of the gas generator of
the HM7B engine, after the opening of the injection gate. For these experiments,
the injection head of the gas generator was fitted on a display box equipped
with perfectly plane windows. This box was filled with pressurized helium,
to simulate the combustion gases, and an helium stream could be injected convergently
with the liquid oxygen spray, to simulate the hydrogen flow.
The main parameters which were varied during these testings were the pressure
in the combustion chamber, the cooling time used for the liquid oxygen injection
system, and the flow of helium, simulating the hydrogen injection in the combustion
room. Qualitative and semi-quantitative information was obtained with fast
camera and high definition snapshots, using direct spray visualization or
shlieren technique. Some pictures taken by these methods were analyzed
by computer. More quantitative information were obtained using the PDPA technique
(Phase Doppler Particle Analysis ). It was the first time that such a technique
was used to characterize the size and the velocity of liquid oxygen droplets.
With such techniques, we characterized the dynamic evolution in size and velocity
of the oxygen droplets in the spray since the opening of the injection gate.
The data obtained by the PDPA technique were also analyzed using the neural
networks computation methods. By this way, we were able to model the behaviour
of this liquid oxygen spray, and to reproduce the influence of each operating
parameter. The results obtained by the neural networks were relevant with
the observations taken during the experiments, and gave valuable information
about the phenomena occurring during the starting up of the liquid oxygen
injectors. Neural network computation seems to be a promising technique to
model such kind of highly complex, strongly non-linear phenomena.
research duration: 2 years
Behaviour of the flooded ball bearings of the HM7B oxygen
pump
The HM7B engine is equipped with turbopumps, used to feed the cryotechnic
engine with liquid oxygen and gaseous hydrogen. The gear of the liquid oxygen
pump is supported a ball bearing located in the liquid oxygen flow, so the
liquid oxygen cools and lubricates this bearing. This configuration is simple
and light, but if some hot spot appears on the bearing, it will vaporize oxygen
near it, so it will no longer be cooled and lubricated. The hot spot will
extend and, if its temperature reach a critical value, the metal will react
with oxygen and burn. We had to determine a safe range of operating conditions
for this ball bearing.
In order to realize these experiments, a special testing box has been built
for the Université de Liège. This one allows to study the influence
of the axial and radial loading of the ball bearings, induced in the actual
engine by pressure of the liquid oxygen on the pump gear, the flow of this
fluid inside the bearing, the pressure, the temperature, and the dimensional
and metallographics characteristics of the bearings.
The test device is fitted with a high number of temperature, pressure, strength
and flow probes. The ball bearing itself is fitted with two temperature probes
on its inside ring, an three other ones on the outside ring. The data measured
on the rotating part are sent from the rotating shaft by a modulated frequency
induction transmitter.
The experiments determined the maximum loading that the original ball bearing
can sustain, and to qualify the new surface-treated ball bearing which is
used at the present time in the HM7B engine, since the failure of flight 63.
At the present time, comprehensive tests are made in order to understand the
thermal divergence phenomenon for the ball bearings.
research duration: 3 years
