space environment october 1, 2003 h. kirkici istanbul technical university lecture-6 and 7 vacuum...

Space Environment

October 1, 2003H. KirkiciIstanbul Technical UniversityLecture-6 and 7

Vacuum Environment(in the ionosphere and above range)

Effects and Problem Issues

Thermal effectsContamination (due to outgassing)UV radiationMass loss (due to outgassing)Particulate (no kinetic energy)

Space Environment

October 1, 2003H. KirkiciIstanbul Technical University

Thermal effects Thermal control of spacecraft

Heat transfer processes

Convection not possible in vacuumConduction possible

but may require additional weight andexpense

Radiation Only effective methodHeat absorbed from the Sun is:

SAQ nsin

SAQ nsin

4TAQ totout Radiated energy from satellite

Lecture-6 and 7

Space Environment


Absorbed heat SAQ nsin

4TAQ totout Radiated heat

where s

nA

S

is the material’s absorbtance

is the surface are normal to the solar flux, in m2

is the solar flux (W/m2)

totA

T

is the material emittance

is the Boltzmann’s constant

is the total surface are of the object (m2)

is the object temperature (K)

Lecture-6 and 7

Space Environment


If we assume there are not other heat source presentTo sustain an equilibrium, we have:

Absorbed heat = Radiated heat

This results in an equilibrium temperature

Exercise-1: Drive the equilibrium temperature T for this idea case

Absorptance and Emittance of some of the spacecraft materials will be posted as a Table at a later time

Lecture-6 and 7

Space Environment


Active heat control (thermal radiators)Works good, but adds weight to the spacecraft, may not be an optimum solution)

Passive heat control (use materials having low values of absorptance, and high emittance), Works good, but may need to be cautious that the material thermal properties may change over time due to outgassing and UV radiation

Solution:

Lecture-6 and 7

Space Environment


UV Degradation

UV light: wavelength less than ~300 nm

on orbit, full strength of UV is present

Approximate energy of a UV photon at 200 nm is ~6.3 eV at 300 nm is ~3.8 eV

Photon Energy:

c

hhE Energy sufficient to breakdown chemical bond between molecules

Lecture-6 and 7

Space Environment


Contamination (molecular)

Outgassing during launch or in orbit

Desorption: release of surface molecules, mainly in metals, causes mass reduction

Diffusion: random thermal motion, and sufficient thermal energy, mainly seen in organic materials and causes greater mass loss

Decomposition: Chemical reaction resulting in smaller molecules, less likely problem than the other two

Depend on Activation energyBinding energyAnd other

factors….Lecture-6 and 7

Space Environment


Activation energy Time Dependence

Desorption: 1 – 10 kcal/mole

Diffusion: 5 – 15 kcal/mole

Decomposition: 20 – 80 kcal/mole n/a

21 to~ tt

2/1~ t

Outgassing characteristics are determined experimentallyNo definite analytical equations to predict outgassing ratesMaterial dependent (initial contamination and fabrication)

Lecture-6 and 7

Space Environment


Molecules do not scatter away from the surface

Will adhere to the surface and establish thermal equilibrium

Will arrive from variety of direction (line-of-sight or non-line-of-sight)

May leave the surface when they gain enough energy form the surface

Stay longer on cooler surfaces (most difficult on cryogenic surfaces)

Approximate resident time is:RTEaeT /

0)(

s10~ 130

Lecture-6 and 7

Space Environment


Problems due to outgassingThermal control: solar absorptance change

(increases with film thickness)(higher in UV than IR region)

Solar cell: output degradation (power conversion efficiency drops)

Synergistic Effects: Interactions between the effects can be more problematic than the individual effects totaled

absorptance0.05 0.4

0.1

thickness

0.015 1.0thickness

Output power

1.0

0.8

Lecture-6 and 7

Space Environment


Particulate contamination:

Residual particles on the part during manufacturing and fabrication (not direct space environmental effect)

From the spacecraft design and construction point, it is an important factor, many standards available

Engineering problem!....

Lecture-6 and 7

Space Environment


Questions?

Lecture-6 and 7

space environment october 1, 2003 h. kirkici istanbul technical university lecture-6 and 7 vacuum...

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