O'Dell, Stephen L.

Swartz, Douglas A.

NASA Technical Reports Server

Motivation:
The accelerated accumulation observed since ~2012 cannot be reproduced 
under the old assumptions of one (or more) gradually depleting 
contaminant source(s) -- any new paradigm must consider source(s) with 
rates increasing with time (but consistent within S/C trends). 
Chandra Contaminant Migration Model 
Methods: (the usual)
(1) Simulate migration (vaporization/deposition) of potential 
contaminant(s) by solving a set of 1st order ODEs through explicit 
(forward) difference scheme and within thermal & geometric model 
constraints.
(2) Compare time evolution of material mass column density to 
observations at select locations on OBF filters to identify best candidate 
contaminant properties.
Doug Swartz & Steve O’Dell
Monday, April 13, 2015
https://ntrs.nasa.gov/search.jsp?R=20150022498 2019-08-31T05:02:17+00:00Z
Monday, April 13, 2015
Simulation Constraints:
Chandra Contaminant Migration Model 
 Spacecraft geometry (surface areas & view factors) and time-averaged 
surface temperatures as determined by Neil Tice (LM) using flight data 
inputs to Thermal DesktopTM RADCAD (geometric) & SINDA FLUINT 
(radiative/conductive thermal) modules; accounts for effects of DH heaters 
being turned off ~2008.4 (t~9 yr)
 X-ray observations of contamination effects throughout the mission as 
documented in CALDB contamination model N0009 (June 2014); 
constrains time evolution and spatial distribution of contaminant optical 
depth, τ(t,x,y), proportional to the mass surface density, µ (t,x,y)
 Temperature dependence of vaporization rate follows Clausius-
Clapeyron relation  ρ(T) = ρ(T0) (T/T0)1/2exp(-ΔH(1/T-1/T0)/R) 
where ρ(T) is the mass vaporization rate at temperature T, ΔH is the 
vaporization enthalpy, and R the universal gas constant
Monday, April 13, 2015
Simulation Parameters:
Chandra Contaminant Migration Model 
  Material (enthalpy, ΔH, and reference temperature, T0, fixed); 
vaporization rate normalization ρ(T0) is a free parameter
 Declining source rate: S(t)=A(1+B*exp(-t/C)) where A, B & C are 
chosen to follow the early build-up (t<2008) at the center of S3 
 Rising source rate: Sʹ′(t)∝Aʹ′exp(-Bʹ′(1/T-1/T0) where the normalization, 
Aʹ′ is a free parameter and Bʹ′ is given by the enthalpy ΔH of the supposed 
contaminant. Physically, Aʹ′ includes a multiplicative scale factor 
representing the outgassing ‘escape efficiency’.
Monday, April 13, 2015
TOBA(t) = T0 [ 1+0.006 exp((t-8.1)/4.0) ]
S ʹ′(t) = S ʹ′(T0) (TOBA(t)/T0)1/2
                       * exp[-ΔH/RT0(T0  - TOBA(t))/TOBA(t)] 
...the Clausius-Clapeyron equation for  a time-dependent TOBA 
( ΔH and T0 are those of the contaminating material)
Chandra Contaminant Migration Model 
Monday, April 13, 2015
Chandra Contaminant Migration Model 
Monday, April 13, 2015
Chandra Contaminant Migration Model 
Monday, April 13, 2015
Chandra Contaminant Migration Model 
Monday, April 13, 2015
Chandra Contaminant Migration Model 
T oC Octadecane 0.1*Hepta-decane
-60 1.6x10-10 5.6x10-9
-40 2.5x10-7 2.4x10-6
+10 0.27 0.21
+20 2.4 1.3
Monday, April 13, 2015
Chandra Contaminant Migration Model 
Monday, April 13, 2015
Chandra Contaminant Migration Model 
Monday, April 13, 2015
Chandra Contaminant Migration Model 
Monday, April 13, 2015
Chandra Contaminant Migration Model
SUMMARY 
 Volatility of contaminants can be tightly constrained by 
simulations.
 Accelerated buildup since 2010 has 30* higher volatility at 
-60oC ; likely to ‘clean’  readily at elevated temperatures.
 Current exponentially increasing trend in OBA temperature is 
predicted to lead to extremely rapid buildup of ‘second’ 
contaminant (5*source rate for 10oC increase in 3 yrs; 30* in 5 yr; 
but polynomial fit implies only 3* in 5 yr).
 At t~16 yrs, 0.4 grams of contaminant is within the S/C with 
only 5% (20mg) on OBFs (~270 µg/cm2).
Monday, April 13, 2015
backup slides
Monday, April 13, 2015
Monday, April 13, 2015
Collimator
Camera Body
Optical Bench 
Closeout
ACIS DoorSnoot
OBF-I & S
 (ACIS Housing)
From Neil Tice/LMC
Thermal Desktop (finite element): 
RADCAD 
   to calculate geometric view factors
SINDA FLUINT 
   to calculate temperatures
738 nodes, 121 OBF-I, 203 OBF-S
   186x186p/node (I),  146x212p/node (S)
Geometry Model
Monday, April 13, 2015
Geometry Model
Monday, April 13, 2015
Chandra Contaminant Migration Model
Results 
Mass Column of Octadecane (C18H38) at t=9 years
low volatility (0.10) high volatility (2.50)
“deposition” dominated: central 
regions have highest accumulation 
because center views more nearby 
cold surfaces, pattern is asymmetric
“thermal” (vaporization) dominated:  warm 
central regions begin to clean, pattern 
follows  local temperature distribution with 
more material near cold edges
Monday, April 13, 2015


Chandra Contaminant Migration Model

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