Limiting Future Collision Risk to Spacecraft An Assessment of NASA's Meteoroid and Orbital Debris Programs (2011) / Chapter Skim
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6 Spacecraft Protection in the MMOD Environment
6 Spacecraft Protection in the MMOD Environment
Pages 47-56
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From page 47... ...
, extravehicular activity (EVA) suits, and other spacecraft and also used BUMPER to calculate the probability of an MMOD impact causing critical damage for each space shuttle mission.
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From page 48... ...
. Space shuttle mission profiles and operations were often directly affected by risk predictions based on BUMPER calculations, which resulted in a reduced risk from the MMOD environment to the vehicle.4 For example, BUMPER predictions were essential in determining the proper positioning of the payload bay door on STS-73 to provide MMOD protection to some otherwise lightly protected pressurized tanks within the payload bay (see Box 6.1 to view images of space shuttle damage from debris impacts)
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From page 49... ...
CALCULATING THE PROBABILITY OF SPACECRAFT LOSS As noted previously, NASA currently uses BUMPER to calculate the risk of MMOD impact that would cause mission-limiting or life-threatening damage to the International Space Station, EVA suits, or other spacecraft (and, previously, for the space shuttle) .5 This calculated value for risk, or probability of spacecraft failure, is then com pared against design requirements to determine whether or not a proposed design, or a proposed design change (e.g., number of shields)
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From page 50... ...
FIGURE 6.1.2 Crew module window impact map from space shuttle Discovery's STS-114 mission in July 2005. 6.1.2 STS114 in Box 6.1.eps There were 14 MMOD impacts on the crew module windows, and a total of 41 MMOD impacts sites on Discovery from its 13-day mission.
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From page 51... ...
:3, January 2008. FIGURE 6.1.4 MMOD impact damage to window #6 on space shuttle Endeavour during STS-126 mission in No vember 2008.
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From page 52... ...
The space shuttle and ISS versions of BUMPER use a variety of equations to predict damage to system com ponents in terms of an impacting particle's density, velocity, and angle of impact. Some equations are developed by simply drawing a curve through fail/no-fail test data (the so-called ballistic limit equations, or BLEs)
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From page 53... ...
Current Efforts to Model Uncertainty NASA has recently performed a series of studies aimed at establishing uncertainty bounds for BUMPER cal culations of MMOD risk for the space shuttle and Orion program offices; plans are currently in place to calculate ISS MMOD risk uncertainties using similar procedures. These uncertainty bounds were estimated using a Monte Carlo method wherein the BUMPER code was executed multiple times, while varying the program inputs for the environment, the BLEs, the failure criteria, and the operational parameters.
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From page 54... ...
Orbital debris risk assessments performed by NASA through BUMPER use ballistic limit equations that have been developed using high-speed-impact test data and results from numerical simulations that have used spherical projectiles. However, it has become increasingly evident that consideration of particle shape and impact orientation could produce a pronounced effect in reducing predicted risk of failure from MMOD impacts.
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From page 55... ...
A preliminary review of the upcoming ORDEM2010 release indicates that NASA is continuing the practice of suggesting that the characteristic length of an orbital debris particle can be equated to the diameter of a spherical projectile provided that the mass density of the spherical particle decreases with increasing size. The difficulty with that approach is that when one attempts to perform high-speed-impact tests to characterize the damage that would be sustained by a spacecraft when struck by the larger particles, one is constrained by the availability and choices of naturally occurring materials from which the projectiles could be made -- one would need to use either hollow spheres, solid spheres filled with voids, or an unrealistic material.
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From page 56... ...
Finding: Using aluminum spheres to develop ballistic limit equations for risk assessments for spacecraft may not accurately portray the range of damage likely from impact with an orbital debris particle of any given characteristic size and thus may result in a non-optimum design of the spacecraft's MMOD protection systems. Recommendation: A priority in the next release of the Orbital Debris Environment Model and Standard Breakup Model should be the inclusion of shape characteristics in the particle distributions to more ac curately portray the range of potential damage from an impact with orbital debris.
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