This slide presentation discusses the application of testing aerospace materials to the environment of space for flammability. Test environments include use of drop towers, and the parabolic flight to simulate the low gravity environment of space

Hirsch, David B.

NASA Technical Reports Server

Slide 1
Current Trends on the Applicability
of Ground Aerospace Materials Test
Data to Space System Environments
David B. Hirsch
NASA JSC White Sands Test Facility
Las Cruces, New Mexico
USA
International Organization for Standardization
Technical Committee 20, SC14 on Space Systems and Operations
Plenary Meeting
London, England
May 2010
Slide 2
• Current approach to spacecraft fire safety
• Long duration space mission fire safety
challenges
• Correlation of ground flammability test data with
spacecraft environments
https://ntrs.nasa.gov/search.jsp?R=20100021112 2019-08-30T09:45:07+00:00Z
Slide 3
• Major flammability tests – ISO 14624-1 and -2
• Upward flame propagation
• Conducted under worst-expected spacecraft
conditions, usually in 30% oxygen at 10.2 psia
• Pass/fail test logic
Discuss:
materials passing criteria (pass/fail approach) - qualitative
ignition source - simulates a worst case
testing in the "worst" case expected, usually 30% oxygen, 10.2 psia - explain (pre-EVA
operations)
note that the pass/fail logic assumes a direct correlation between 1-g data and flammability
behavior in spacecraft environments.
JSC 29353 – Flammability Configuration Analysis for Spacecraft Applications
Slide 4
• Long duration; unplanned events more likely
• Limited options for mission termination
• Variety of spacecraft, surface habitats, planetary
landers
• Spacecraft environment and its importance
• Flexibility and adaptability critical for success
Current safety design challenges
ECLSS designed to handle a CM leak equivalent to 1/4-in. diameter for an hour
Unplanned transitory events: D N EVA C MIR
• Crew Module (CM) depressurization - real event in ISS during late 2002
• Loss of nitrogen - will drive the need to know what max oxygen concentration we can expose the CM to
and still manage fire risks;
• Frequency and duration of EVAs will increase, so there may be an increase in emergency rescues of an
astronaut in EVA distress; need the capability to respond quickly to EVA emergencies
• Increasing communication delays
The MIR fire of 1997
Limited options for mission termination:
a salvageable event in Earth orbit could be catastrophic during most of the transitory Earth-Mars voyages; the result
could be loss of crew and mission.
Flexibility in selecting different environments for various spacecraft, surface habitats, planetary landers, or
pressurized extraterrestrial rovers will allow flexibility in space system design and operation. The complexity of
space system design often requires changes to mitigate various safety issues, financial impacts, and so on. Design
and operational flexibility could result in increased ability to mitigate hazards or develop superior systems with less
financial burdens. For example, retesting of materials will not be required if new spacecraft conditions are selected.
Slide 5
• Air velocity magnitude contours at aft-forward mid-section of the
U.S. Airlock Module
– Courtesy: Son, C.H., et al, Integrated Computational Fluid Dynamics
Ventilation Model for the International Space Station, SAE Technical
Paper 2005-01-2794, Society of Automotive Engineers, 2005
Space system environments
• Range of gravities - large at launch, microgravity during interplanetary travel, reduced
gravity on the Moon and Mars
• Ventilated environment, for mixing and spacecraft skin temperature control; for
example, a velocity magnitude of up to 20 cm/s is encountered in 75% of the U.S.
Airlock. Velocities up to 100 cm/s are also encountered.
• Range of oxygen concentrations and total pressures for various operations (i.e., pre-
EVA acclimatization to prevent decompression sickness) - with the oxygen partial
pressures kept above the hypoxic levels and oxygen concentrations below what is
believed to be the materials’ flammability level (subject to epistemic uncertainty with the
current pass/fail logic)
Slide 6
• 1-g materials flammability qualification tests conducted
at NASA WSTF determine the oxygen concentration
flammability extinguishment limits
• NASA WSTF also conducted a series of tests to
evaluate total pressure effects on the flammability
threshold
The tests conducted at WSTF will allow flexibility in spacecraft environment selection. For
example, we are contemplating having environments of up to 34% in the Lunar surface
habitation modules; the oxygen-enriched environments require less nitrogen and would minimize
the acclimatization time required for astronauts prior to an EVA.
A practical application example of knowing the pressure effects on the flammability threshold is
establishing the internal pressure limit for landing spacecraft at KSC. The WSTF data in
corroboration with flammability threshold data in spacecraft environments will allow us to
determine fire safety factors. American microgravity test facilities initiated flammability
threshold testing following the WSTF approach. In the long term, we will be able to have
accurate fire risk assessments, which will allow proper mitigation. As a consequence, we will be
able to achieve increased spacecraft fire safety.
Slide 7
• A more accurate assessment of the margin of
safety of the materials in real space system
environments
• Improved management of spacecraft fire risks
• Allows option of selecting better or best
materials as opposed to what would just be
considered acceptable
Unexpected space mission events could challenge space system fire safety design boundaries.
Correlations of 1-g flammability test data relies on many assumptions, some of which may not
stand up to future findings.
Slide 8
• The knowledge afforded by this technique allows
for limited extrapolations of flammability
behavior to conditions not specifically tested and
that would result in significant cost and time
savings
• Understanding fire risks in transient spacecraft
environments will allow flexibility in spacecraft
design and operability.
Operational flexibility is of critical importance for advanced space programs relying on
integrated designs extending across multiple vehicle atmospheres and focusing strongly on crew
safety and operational effectiveness.
In the future, we’ll need to work on reducing the epistemic uncertainty related to space system
fire safety factors and fire risks in various space vehicles, Lunar and Mars surface habitats,
planetary landers, pressurized extraterrestrial vehicles, and other space systems.
Slide 9
Partial gravity centrifuge in the NASA GRC Zero Gravity
Facility Drop Bus
ITop camera
Sense of rotation
^• •••	 Side camera
To study the partial gravity effects on ground, a centrifuge was developed for use in drop tower
experiments. It consists of a turntable and a chamber dome (since the drop tower is evacuated
before a drop). Note that the rotating chamber needs time to spin-up all the internal gas so that it
rotates as a solid body. Only then can the analogy of centrifugal force to gravitational force be
made.
The centrifugal force F centrifugal = m x omega2 x R, where m is the mass of the object, omega is
the angular velocity, and R is the distance of the object from the axis of rotation. Omega 2 x R is
called the centrifugal acceleration.
An important limitation may be imposed by the generation of the Coriolis force, which is due to
the fact that the local acceleration depends on the distance from the axis of rotation, R. F coriolis =
-2omega x u, where u is the velocity vector of a particle moving inside the reference plane.
Another limitation arises when establishing a flame in normal gravity before a drop: the flame
must swing down 90 degrees for the transition to occur, which simply takes too much time.
Unquestionably, the limitations inherent in this experiment must be validated in micro or reduced
microgravity airplane flights and space experiments.
Slide 10
NASA GRC and WSTF proposed a
comprehensive program on correlating 1-g
flammability threshold data with a variety of
spacecraft-specific environments (response to a
NASA Research Announcement)
– Oxygen concentration and pressure effects
(prevalent, transitions, extremes)
– Ventilation effects (0 to 1 m/s)
– Microgravity or partial gravity effects
Slide 11
Materials with oxygen concentration thresholds in the
25 to 35% range will be tested
Most oxygen concentration flammability thresholds will
be evaluated at the spacecraft prevalent pressure
(14.7 psia)
Further 1-g tests will focus on the immediate
post-ignition and flammability extinguishment behavior to
attempt determination of a pass/fail criteria amenable to
the short duration of ground microgravity tests
• The NASA GRC
microgravity wind tunnel
will be modified for
parabolic flights
Slide 12
• Most reduced gravity
and microgravity
testing will be
	
4
conducted in
parabolic aircraft
flights
	
•I	 P
• Approximate test
time: 22 s	 — :.'— •t "	 '._^"'
Slide 13
Slide 14
• Flow velocities of 0 to 30 cm/s while maintaining the total
test chamber pressure constant
Total pressure range: 0-14.7 psia
Oxygen concentration range: 0-100% oxygen
Slide 15
• Under set conditions (prevalent spacecraft
pressure, commonly encountered surface flow
velocities of
30 cm/s), determine the oxygen concentration
flammability threshold in reduced gravity and
microgravity
• Current limited data indicates that somewhere
between zero and 30 cm/s flow velocities the 1-g
data becomes less conservative than data in
microgravity and reduced lunar gravity
Slide 16
Continue oxygen concentration flammability
threshold measurements at lower velocities until
a flammability velocity threshold is obtained
where the oxygen concentration thresholds in 1-
g and microgravity coincide
Build a flammability threshold map and compare
microgravity and 1-g threshold limits. The data
will allow evaluation of the level of
conservativeness (or the lack of it) of 1-g data
(“safety factors”)
Slide 17
• Computational Fluid Dynamics (CFD) of
spacecraft flow velocities will identify zones
where 1-g materials qualification data is not
conservative and if ignition occurs, flame
propagation would be possible. This will provide
information for ventilation design to improve
spacecraft fire safety.
• Knowing parametric effects on the flammability
threshold will allow effective spacecraft fire risk
management.
Slide 18
• Verification flight experiments in the International Space
Station Combustion Module will be conducted
• Flammability threshold maps will be determined to
validate the ground microgravity testing approach for
several materials
• The information obtained will be correlated with materials
characteristics to predict flammability thresholds for other
aerospace materials
• In the long term, we’ll have comprehensive mapping of
spacecraft flammability hazards and associated risks.
The approach to testing has been described in detail in a Technical Specification which has been
proposed as a new work item


Current Trends on the Applicability of Ground Aerospace Materials Test Data to Space System Environments

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