REG NASA-LLIS-0679--2000 Lessons Learned Meteoroids& Space Debris.pdf
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1、Best Practices Entry: Best Practice Info:a71 Committee Approval Date: 2000-03-09a71 Center Point of Contact: JSCa71 Submitted by: Wil HarkinsSubject: Meteoroids from NASA Technical Memorandum 4322A, NASA Reliability Preferred Practices for Design and Test.Benefit:Reliability is greatly enhanced beca
2、use the likelihood of serious mission degradation or spacecraft loss is significantly reduced.Implementation Method:Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-Prepare design requirements which specify mean velocity, mass density, and mass distri
3、bution for the impacting particles in terms of the integral fluence. This fluence (sample units m-2) represents the expected number of impacting particles per unit area, above several different mass thresholds for the mission (using the worst case trajectory if more than one is contemplated). The de
4、sign must then satisfy two separate requirements: (1) that the smallest penetrating particle have a probability of impact below 5%, using the product of fluence with vulnerable area and a Poisson distribution, and (2) that for smaller particles, of which many will impact any given spacecraft surface
5、, the resulting degradation of surface properties (e.g., optical, thermal, dielectric) does not exceed allowable ranges for surface performance (considering, e.g., pitting, spallation, contamination, etc.).In practice, the first of these refers to a sum of probabilities over a variety of vulnerable
6、spacecraft surfaces (each having specific values for area and threshold penetrating mass), allocated so as to make effective use of resources (e.g., shielding mass) and to achieve the desired probability for mission success. For this purpose, experience dictates that a two-surface configuration, of
7、which the outer surface serves as the thermal blanket as well, provides the least massive meteoroid protection.Technical Rationale:For a given mission (specified in terms of geocentric and heliocentric positions as functions of time, for example), the environments comprising impacting solid particle
8、s are both independent of mission control and rather uncertain. The flux and fluence of such particles can be evaluated from suitable numerical models (here for space debris and for interplanetary meteoroids, although others may occur, e.g., for Saturn ring particles). The integral fluence typically
9、 decreases as mass increases according to a power law, illustrated here using the exponent a:refer to D descriptionD (1)Here F and F1represent the integral fluences (sample units m-2) for particles with masses greater than m and m1respectively, accumulated over the mission. Table 1 provides examples
10、 of such distributions (where the exponent is not necessarily constant over the range of masses of interest) and additionally specifies mean density and impact velocity.For large particles, the distributions represented by equation (1) or Table 1 imply that the exposed Provided by IHSNot for ResaleN
11、o reproduction or networking permitted without license from IHS-,-,-surface area Asof a spacecraft subsystem has a probabilityrefer to D descriptionD (2)that no particle larger than the mass ms(corresponding to the fluence F) will hit, where equation (2) is obtained assuming Poisson statistics for t
12、he particle impacts. If the surface is designed so that no particle of mass msor larger, impacting at the mean velocity, can penetrate or lead to other component failure (e.g., by spallation), then the probability of no failure is also Ps(assuming that penetration leads to component failure with uni
13、t probability). When Psis small for each subsystem (as is the case when the area-fluence product in eq. 2 is much less than unity), the sumrefer to D descriptionD (3)represents the probability of failure (Pt) of the system, where psis the conditional probability that the system fails when subsystem
14、s fails. If the values of psare not independent then equation (3) must be replaced by the appropriate combination of probabilities. Finally, the probability of mission success, considering particle impact alone, becomes (1-Pt), and design must proceed to ensure that this quantity exceeds the 95% pro
15、bability cited above.To protect a subsystem against those large particles for which equation (2) applies, and for impact velocities larger than a few km/s, hypervelocity impact experiments show that a two-surface configuration (often named a bumper shield) prevents penetration far more effectively t
16、han a single surface of the same mass. This is so because the kinetic energy of impact leads to the vaporization (or liquefaction or disintegration) of the projectile when it hits the outer target surface; the momentum is thereby dispersed over a large area as the vapor expands in the space between
17、the surfaces, and becomes less capable of rupturing the second surface than had the latter been hit directly. Typically a thickness of a few tenths of a millimeter, and a standoff distance of a few centimeters, suffice to prevent penetration of a spacecraft structural wall by a milligram particle ar
18、riving normally at 15 km/s. For a sample configuration, Figure 1 displays the threshold penetration mass as a function of impact velocity. Such a figure can be used to verify by analysis that the design does not fail for the mass necessary for equations (1) through (3) to provide the required probab
19、ility; and a parametric set of such figures spanning a suitable design space can be used to select the design Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-appropriate for a given spacecraft assembly. In this design process, the uncertainties in pe
20、netration threshold and the variability thereof with angle of incidence (Fig. 1 and related data are commonly presented for normal impact, oblique impacts being less well characterized) must be considered, possibly by application of margin to some measure of shield effectiveness (the use of Poisson
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