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APRIL 2015
AEROSPACETESTINGINTERNATIONAL.COMz Environmental trials
THRUST GOES CRYOGENIC
In 2009 ESA and Arianespace started the Ariane 5
Midlife Evolution (ME) program to prepare the Ariane 5
launcher for future market demands. The upgraded
launcher will be capable of carrying heavier satellites.
It features a completely new upper stage, equipped
with a re-ignitable cryogenic engine, in order to fulfill
a broader range of missions. The maiden flight is
scheduled for 2018.
The Engine Thrust Frame (ETF) of the new
upper stage is required to go through a thorough
qualification test program, which includes a static
strength test to rupture at a representative
temperature level. Since the operational temperature
of the ETF top ring is 90-120K, this called for a
cryogenic test setup. This requirement triggered
the cold testing developments at NLR.
The common test principle at that time was to place
the adaptor cone (attached to the ETF top ring) on top
of a cylindrical heat sink. It was thought unthinkable
to put the entire test setup in a cabinet filled with cold
nitrogen as it was too large, too cumbersome and
had containment problems. Within this heat sink a
contained flux of liquid nitrogen is pumped. This
controllable flow ensures that the bottom of the
adaptor cone has a temperature of 77K. Toward the
end cap, at the top of the test setup, there will be a
temperature gradient, depending on the size of the
adaptor cone, the heat flux in the cone and the
temperature of the end cap.
To analyze the temperature gradient, the conical
structure was modeled as an axially symmetric shell
with a uniform thickness of 5mm. For the simplified
model, the relevant material properties of aluminum
7075 were used. They included the temperature-
dependent material density and the conductivity.
An uncoupled heat transfer analysis was
performed to determine the temperature distribution
in the ETF as a function of time. In the heat transfer
analysis, natural convection and conduction were
taken into account. Radiation was not accounted for.
The heat transfer coefficient was prescribed with a
convective film condition, by defining the heat
transfer coefficient h (=1,000W/m^2 /K) and the sink
temperature of -77K. Also, film conditions were
defined far from the other outside and inside surfaces
of the ETF for natural convection h (=2W/m^2 /K)
between the air and the ETF.
The following preliminary conclusions were drawn,
based on the (simplified) analyses:- The heat transfer through the adapter cone is of
major importance for reaching the prescribed
temperatures at the top ring within a reasonable time. - When insulated perfectly, the prescribed
temperature at the top ring can be reached. - Due to natural convection, however, the prescribed
temperature at the top ring can not be achieved.
The long cooling times are a obstacle to a
competitive full-scale test setup. Local cooling with
contained use of nitrogen, would not suffice;
something drastically different needed to be
developed. It was decided to submit the entire test
article to a forced cooling mechanism and thereby
solve the containment problem on a full-scale level.
75
NLR’s annual
turnover in
millions of
euros2
The number of
laboratory
aircraft: a
Cessna Citation
and a Fairchild
MetroThe development of cold testing at
NLR continues. A possible application
is the full-scale testing of space
structures down to -180°C. To prepare
for this, a prototype setup has been
built to test an aluminum plate with
a thickness, size and geometry
equivalent to a segment of the Ariane
Engine Thrust Frame (ETF). The test
was conducted successfully in October
2014, demonstrating that the system
is capable of cooling a representative
structural component down to -180°C
in a reasonable time. NLR is are now
confident that it can affordably perform
the actual full-scale static strength test
on the ETF, or any similar structure, at
this very low temperature. zHotze Jongstra is the principal project engineer,
structures testing and evaluation department, at
the National Aerospace Laboratory in
the Netherlandsreaching -170°C within minutes and
have been developed in cooperation
with experts from a local cryogenic
engineering company.
Some developmental problems have
been encountered. Since the endurance
test is to run 24/7 and unattended at
night, a fully automated safety system
is in place. One particular night this
system was triggered and the test was
automatically shut down. The next
morning the operator on duty had
great problems restarting the test. The
circulation of the hydraulic oil in the
shutdown condition turned out to be
insufficient to prevent it from freezing
and blocking the flow. Additional oil
heating elements were therefore
included and the hydraulic actuators
are monitored and electrically heated
when necessary. This enables an easier
restart after a cold shutdown and it
also smooths daily operation.
“THE CIRCULATION OF THE HYDRAULIC OIL
IN THE SHUT-DOWN CONDITION TURNED OUT
TO BE INSUFFICIENT TO PREVENT IT FROM
FREEZING AND BLOCKING THE FLOW”RIGHT: Two experts
flush a 33,000 liter
liquid nitrogen tank