by Boeing under contract to NASA Marshall Space Flight Center for use on the Space Station Freedom program in 1986, the original BUMPER code was designed for use on VAX computers and is still used on workstations for space shuttle and ISS assessments. In 1991, the BUMPER code was updated to the BUMPER II code, and configuration control was established at NASA Johnson Space Center in 1994. BUMPER II (hereafter referred to simply as BUMPER) is now clearly considered the standard by which other MMOD risk assessment tools are measured—even the European and Russian space agencies have used versions of it. Two versions of BUMPER have been maintained, one for the ISS program and one for the space shuttle program. The primary differences between the two versions are in the impact damage subroutines related to different exterior materials and failure criteria.
From the perspective of a particle–spacecraft interaction that could lead to spacecraft failure, risk from a single MMOD impact may be considered to be a product of the following three terms:
These terms can be combined in a number of ways to determine PK for a spacecraft. For example, the first two terms (PH and PP/PH) can combine to form the probability of penetration, or PP. This is essentially what BUMPER was originally designed to do—to determine the probability of penetration of the space station—because a penetration was (conservatively) equated with a crew or station loss. It is because of this original (highly conservative) assumption that the last term—the probability of “kill” given a penetration (PK/PP)—while clearly an integral part of the total risk equation, was never designed for inclusion within BUMPER. However, the ability to quantify the PK/PP term—the vulnerability of spacecraft to loss following penetration—allows spacecraft designers to examine the entire probability of loss, thereby shifting some of the focus on increasing safety with respect to orbital debris from the external portion of the spacecraft or the ISS to the entire spacecraft design envelope, including internal equipment design, crew procedures, and other factors that contribute to potential failure modes and the overall probability of loss.
BUMPER is used as an in-line requirements compliance verification tool by the ISS and was previously used for the space shuttle, as well. It is also being used for development of the Orion Multipurpose Crew Vehicle.3 It has been used by ISS contractors and international partners to design shielding to protect station crews and meet lifespan requirements. BUMPER has also been used to identify ways of reducing the risk posed by MMOD to within established NASA risk levels (through operations, shielding, or other means). 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). During the mission, a relatively large orbital debris particle did, in fact, impact one of the closed payload bay doors. Had it not been decided to close the door following analysis and interpretation of BUMPER data, the resulting damage to the space shuttle would have been significant. In addition, several modifications to the space shuttle were developed following analysis of BUMPER risk assessments, such as adding isolation valves to the coolant lines on the payload bay door radiators. If one of the two redundant coolant loops was penetrated by an MMOD particle, it could be isolated without affecting the operation of the remaining coolant loop.
Although BUMPER is a powerful tool, it does have some limitations. The major limitations of BUMPER are (1) that it calculates only a portion of the MMOD risk to a spacecraft (the probability of a penetration, however
3 Larry Price, Orion Deputy Program Manager, Lockheed Martin, “Orion Spacecraft MMOD Protection Design and Assessment,” presentation at the Workshop to Identify Gaps and Possible Directions for NASA’s Micrometeoroid and Orbital Debris Programs, March 10, 2011, National Research Council, Washington, D.C.
4 J. Williamsen, Review of Space Shuttle Meteoroid/Orbital Debris Critical Risk Assessment Practices, Report No. P-3838, Institute for Defense Analyses, Alexandria, Va., November 2003.