X-ray Missions Delta: Gratings Redux

1. X-ray Missions Delta:Gratings Redux Orbital Debris and End of Mission Plans Ivonne Rodriguez 2 – 4 May, 2012

2. Orbital Debris and EOMP Agenda • Micrometeoroid Damage Assessment • Summary and Comments • Acronym List • Backup Material

3. Reduction in Metering Structure Damage Due to Reduced Area • Perforation of Metering Structures by Micrometeoroids • Failure is defined as penetration of both facesheets. • Honeycomb panels: 2.54 cm total thickness, 0.38 mm facesheets (Original configuration and redux). • Smallest particle capable of penetrating the structures is 0.046 cm diameter. • Significant improvement achieved due to the reduced structure area (all other conditions equal).

4. Significant Increase on Risk to Propellant Tanks Due to Reduced Structural Shielding • The propulsion tanks are selected for micrometeoroid damage analysis because of their particular vulnerability to particle impacts, due to their large area (compared to other components) and internal pressure. Decks and other panels limit the damage in several directions. For the purposes of this analysis only the flux coming normal to the hexagon side is taken into consideration. • Failure is defined as penetration of the tank wall. • Comparison between the original and modified configurations: • Same tank diameter, same number of tanks. • Redux version has a thinner tank wall, which makes the tank vulnerable to smaller particles than in the original configuration. • Closeout panelsin the original configuration provide adequate protection from particle impacts. The lack of closeout panels in the redux version makes the tanks more vulnerable in comparison. • To reduce the risk of explosion due to particle impacts, add structural panels to cover exposed sections or add flexible shields (See Backup Material for more information).

5. Thin Multi-Shock Shields Can Provide Additional Protection to the Exposed Tanks • For the redux configuration only: • A flexible Multi-Shock shield (MSS) with Nextel fabric bumpers and Kevlar fabric rear wall is recommended. • In this case, failure is defined as full penetration of the MSS. • The probability of penetration of a 2-cm thick MSS shield is 0.7% in this case. • The mass of a MSS 2-cm thick covering the exposed tank areas is approximately 1.4 kg for each tank, or 2.8 kg for the spacecraft. • It is assumed that only half of the spherical tank is covered. • The probability of penetration of a wall behind the MSS (the tank wall in this case) has not been defined experimentally. However, the probability of a particle to even reach the outer surface of the tank is reduced to 0.7% (or lower if a 3 cm or 4 cm MSS is used).

6. Probability of Particles Reaching the Instrument from the Telescope Side • See Backup Material section for introduction to FMA analysis. • In both cases a 3-mm glass or filter covering the detector entrance is assumed. The analysis indicates the probability of a particle hitting or penetrating that surface from the telescope side. • In both cases, there is high probability of a few impacts by particles in the 1 micron range. However, the expected damage may be limited to a crater of few microns in diameter on the surface. • The reduction in area produces a modest improvement in damage risk.

7. Summary and Comments • No difference between the original and redux configurations in terms of compliance with orbital debris requirements and disposal plans. • Comparison of original and redux configurations in terms of vulnerability of certain components to damage by micrometeoroid impact:

8. Acronym List • MSS – Multi-Shock Shield

9. Backup Material

10. Orbital Debris Requirements per NASA-STD-8719.14: Most Requirements Do not Apply to L2 Missions

11. If Mass is a Concern, Consider Flexible Shields instead of Solid Shields for Exposed Tank Sections • Multi-Shock shields (MSS) are defined as a combination of four ceramic fabric bumpers followed by either an Aluminum or Kevlar rear wall. • Ceramic bumpers produce higher shock pressures in the projectile than Aluminum, which translates into better projectile breakup. • Fabric ceramic bumpers are more damage tolerant than monolithic (solid) ceramic layers which tend to disintegrate upon impact. • In many cases, fabrics are more suitable for spacecraft shielding applications than solid bumpers. • Increasing the standoff distance (while keeping the layers equidistant between one another) increases the efficiency of the flexible shield. • Reference: Eric Christiansen, Handbook for Designing MMOD Protection, NASA/TM-2009-214785, pp. 66-74.

12. Instrument Damage from The Telescope Side • The probability of damage to the glass or filter from particles coming from the mirrors’ side is a separate case from the hardware assessment for the following reasons: • While in the hardware assessment a failure is defined as penetration of a wall or surface, in this case a particle impact to the surface not always produce permanent damage to the instrument. Depending on the size of the particle, the result may vary from a temporary production of bright pixels to penetration of the surface. • It does not involve a direct hit by the particle, but is the result of scattering through the mirrors. • Note that the results depend on the specific mirror configuration. • The particle reaching the instrument might be ejecta produced by the impact with the mirror foil (the MM vaporizes), or the scattered MM after impacting the mirror shell. • The analysis is limited to the particles that reach the instrument through a mirror assembly, which depends on instrument geometry (next slide). Not all particles striking the mirrors may reach the instrument. • Computation of the mirror effective area is based on Carpenter, et al, Effects of Micrometeoroid and Space Debris Impacts in Grazing Incidence Telescopes, from Space Telescopes and Instrumentation II: Ultraviolet to Gamma Ray, Proc. of SPIE Vol 6266, 62663K (2006).

13. Analysis Assumptions and Equations Mirror effective area based on the probability of an entering particle to reach the instrument: • R1 = front radius of the nth mirror shell’s parabolic mirror. • R2 = radius at the interface between the two hyperbolic and parabolic mirrors. • An = on-axis component of the mirror area. • A = on-axis area of the nest of n mirrors. • a = Minimum scatter angle required to hit the detector or filter. • P(0<θ< a) = probability that a particle is scattered by an angle which is less than some upper limit a. • Phit = probability that a particle will strike the detector or filter. • Anp = on-axis “effective” area for a single mirror shell. • Ap = total on-axis “effective” area for the telescope. • Assumed values: • a = 0.7⁰ (Swift, XMM) Once the total effective area is obtained, the analysis proceeds as in the hardware section (P=FxAxT).