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OCR for page 28
6
End-of-Life Disposal
At some point in the future, the ISS will have to be
decommissioned, deorbited, and returned to Earth. The
return of the ISS will require stringent safety standards to
minimize third-party damage and avoid casualties. NASA
has performed, and must be commended for, its detailed
analyses of the requirements for and methods of end-of-life
disposal of the ISS in a manner consistent with NASA's
stated safety requirements (i.e., less than a 1 in 10,000 chance
of a casualty from reentry operations).
NASA has performed a number of analyses related to ISS
end-of-life disposal, including an analysis performed for the
International Space Station Alpha (ISSA), a space station
configuration that preceded the current ISS design (NASA,
1995~. The analysis consisted of a risk assessment, a con-
trolled deorbit analysis, a debris dispersion analysis, and a
disposal area assessment. Additional analyses were also
performed based on the use of a U.S. propulsion module
(Thorn, 1999~.
The disposal risk assessments concluded that the risk to
human life from an uncontrolled ISS reentry would be unac-
ceptable, ranging from a 0.024 to a 0.077 chance of a single
casualty (i.e., 2 in 100 to 8 in 100~. On the assumption that
a failed deorbit mission would result in an uncontrolled
reentry, with about a 0.05 chance of a casualty, then even a
1-percent chance of a failure exceeds NASA's stated safety
objective. According to NASA's safety guidelines, the
casualty risk must be limited to a 0.0001 chance of a single
casualty (i.e., 1 in 10,000~. The risk assessment, therefore,
included other alternatives, such as: boosting the ISS to a
much higher orbit to prolong its on-orbit lifetime; disassem-
bling and returning ISS components to Earth via the Space
Shuttle; and controlling and targeting reentry to a safe loca-
tion in the oceans.
In one analysis conducted for the ISSA, an end-of-life
deorbit maneuver could begin from a circular orbit after the
two Soyuz vehicles had been separated from the station.
After separation of the Soyuz vehicles, the Progress
vehicles) and the service module, both fully loaded with
28
propellant at the beginning of the maneuver, would be used
for the deorbit burn (Thorn, 1999~.
A more recent study was done with end-of-life disposal
performed by a U.S. propulsion module. This study estab-
lished deorbiting criteria based on the U.S. propulsion
module directing the reentry to a remote ocean area to ensure
that the dispersed surviving debris "footprint" would not fall
within 370 km (200 nmi) of any land mass. The ISS deorbit
trajectory would be designed so that natural orbit decay
would lower the orbit to the point where excessive attitude
control propellant would begin to be needed (i.e., about 241
to 185 km [130 to 100 timid. Solar arrays would be posi-
tioned to minimize aerodynamic torque. At 222 km
(120 nmi) altitude, the U.S. propulsion module would lower
perigee (the lowest point in the orbit) to an altitude of ap-
proximately 140 km (75 nmi) with orbit adjustments made
over several orbits because of the long burn times needed to
achieve the required change in orbital velocity (/\v). A final
deorbit burn would then lower perigee from 140 km to
83 km (75 to 45 nmi) altitude reaching at least a 16.8 m/s
(55 fps) /\v in a period of 35 minutes. Solar arrays would
collapse at about 130 km (70 nmi) altitude as the ISS begins
its atmospheric reentry profile.
As a result of the reentry and subsequent breakup of the
ISS, surviving debris would be scattered over the surface of
the Earth. About 80-percent of the debris would vaporize in
the atmosphere. The debris impact footprint is affected by
many factors, including deorbit maneuver accuracy, range
of debris ballistic coefficients, breakup altitude, breakup /\v,
atmospheric density, winds, and debris aerodynamic lift.
Assuming a successful reentry mission, NASA analyses have
concluded that the ISS dispersed debris footprint could be as
large as an ellipse measuring 300 km by 5,370 km (162 nmi
by 2,900 nmi). Therefore, ocean disposal will be necessary.
The largest disposal region, in the eastern Pacific Ocean,
would allow the initiation of deorbit maneuvers on either of
two consecutive orbital passes over the area.
NASA has concluded that the U.S. propulsion module, as
OCR for page 29
END-OF-LIFE DISPOSAL
presently designed, will not be able to meet the performance
requirements of the ISS end-of-life deorbiting mission
(Thorn, 1998~. To achieve the deorbiting mission require-
ments, the U.S. propulsion module would have to burn more
than 142 kg (5,000 lbs) of propellant, developing 3,556 New-
tons (800 lbs) of thrust for a duration of 35 minutes to com-
plete the final deorbiting burn.
NASA concluded that boosting the ISS to a higher orbit is
not an option because of insufficient propellant and because
the orbit would gradually decay to a lower orbit, and the ISS
would ultimately deorbit in an uncontrolled reentry. Another
alternative, disassembly of the ISS, was considered too
expensive (the ISS is not designed for disassembly). The
committee concurs with NASA's conclusion that the only
viable solution for ISS end-of-life disposal is controlled
deorbiting of the ISS.
Based on these assessments, a controlled ISS reentry to a
remote ocean area would be the safest disposal option.
Therefore, ISS end-of-life disposal requirements will have
to be incorporated into the U.S. propulsion module design
requirements. NASA believes that a deorbiting mission must
have at least a 99-percent reliability. The committee believes
that even this reliability level would not meet NASA' s safety
goal of a less than a 1 in 10,000 chance of a casualty.
Recommendation. End-of-life disposal should be accom-
plished by a controlled deorbiting of the International Space
Station. Sufficient onboard propulsion must be provided for
this operation. The National Aeronautics and Space Admin-
istration should consider upgrading the U.S. propulsion
module to provide the required deorbiting capability.
Recommendation. Because of the potential hazards asso-
ciated with the reentry of relatively large objects, the safety
requirement for International Space Station reentry should
be more stringent than the requirement for other National
Aeronautics and Space Administration operations (i.e., the
chance of casualties should be much less than 1 in 10,000~.
NASA's calculations of the probability of success of the
29
final deorbit burn do not make enough allowances for the
fact that these operations will take place in the very stressful
environment of reentry, which will include the heating,
vibration, and collapse of subsystems. Therefore, NASA
cannot ensure that the U.S. propulsion module will have a
greater than 99-percent probability of success. In fact, the
committee believes that the reliability of the U.S. propulsion
module will have to exceed 99-percent to achieve NASA's
stated safety objectives.
Recommendation. The National Aeronautics and Space
Administration should undertake a thorough analysis of
International Space Station reentry operations, including
ranges of uncertainty associated with the multiple variables
of reentry operations. The analysis could take the form of a
Monte Carlo simulation of reentry operations and projected
impact areas to characterize the hypothetical potential for
property damage or casualties. The analysis should include
the sequence of operations, possible failures, and conse-
quences of failures, from the initiation of reentry operations
to final impact. Uncertainty variables should include, but
should not be limited to, reliability characteristics, duration
of burn, atmospheric density, ballistic coefficients of frag-
ments, population densities, and the characterization of
acceptable impact areas.
REFERENCES
NASA (National Aeronautics and Space Administration). 1995. End-of-
Life Disposal Assessment International Space Station Alpha. May 8,
1995. Engineering Directorate, Houston, Texas, NASA Johnson Space
Center, Aerospace and Flight Mechanics Division.
Thorn, V. 1998. ISS End-of-Life Disposal. Presentation by V. Thorn, Mis-
sion Integration Office, International Space Station, to the Committee
on the Engineering Challenges to the Long-Term Operation of the Inter-
national Space Station, National Aeronautics and Space Administra-
tion, Washington, D.C., September 18, 1998.
Thorn, V. 1999. Personal communication from V. Thorn, Mission Integra-
tion Office, International Space Station, to J. Greenberg, member of the
Committee on the Engineering Challenges to the Long-Term Operation
of the International Space Station, March 24, 1999.
OCR for page 30
failures.
The SR&M program would benefit from more trend analyses of ad data that
coma have a bearing on the long-term failure rate, maintenance capabilities, and
spare Parr; requirements of the ISS. The Hubble Space Telescope (MST) program, for
example, now considers "trending" as its primary SR&M analysis methocI.
Trend analysis for the ISS could include the foBow~ng elements:
.
· · ~
1ncommg inspection reports
· in-process test reports
failure reports from on-orbit segments
~ maintenance records for ground and on-orbit operations
Recommendation. Analyses of the incoming inspection and in-process testing data
should be used to establish a s~x-sigma environment In which failures wiD be
extremely rare. Analyses of failure reports and maintenance records should be used
to improve on-orbit procedures and Me quality of replacement items.
SPARE PATS PHILOSOPHY
NASA provided the committee with information on plans to provide spare parts
and logistics for the ISS (NASA, 199Sb). The committee also reviewed the same
information for the HST, He onIv other On-duration U.S. sDace Rehire that hat
Involved crew servicing.
_ ~s ~_ T Tic ~ a ~
~ J ~ O ~ ~ ~ - Id___ ~ ___ ~~ new
The opportunities for component replacement on the ISS
and the Em amer tour or five visits per year for the ISS and three or four years
between v3 sits for the EAT. In terms of manufacturing lead-time for producing spare
parts, however, the biggest difference is that significant lead-time is available
between repair visits to the HST to secure replacement parts.
~.
For the HST, solar panels, mechanical relays, and rotating devices (e.g., wheels,
_ _ e -_1_ 1~ 11 ~ ~ - ~ ~ ~ ~ ~ ~ ~
gyrOS, ~D~, ma servos) were stocKpHea, DUt ~ b~1 S me yews on orbit, He
electromechanical devices and the purely electronic devices have had only moderate
failure rates (Styczynski, I999~. Experience win the HST project revealed that NASA
could not afford to stock and maintain the extensive depot facilities and the large
numbers or spare pares required Tor tne net oasea on tne ~~o~- measure of statistical
mean time between failures of the hardware (KeDey, I999~. Another method of
providing spare parts would be to introduce upgrades during the operational life of
the ISS to reduce failure rates and thereby reduce maintenance requirements (~is
1_ ~ ~& _ ~a ~T To . ~ ~. ~ or
30
Representative terms from entire chapter:
space station
