Thermal Design Part 2

Question 1

Why does a greenhouse feel slightly warm. What properties of glass allow for this state?

Answer 1

The glass is opaque to IR whilst it is transparent to the UV radiation from the sun

This means the glass can absorb and re-emit the IR

Question 2

What is the absorptivity of white paint?

Answer 2

0.2

Question 3

What is the absorptivity of black paint?

Answer 3

0.9

Question 4

What is the absorptivity of silver-coloured metals?

Answer 4

0.1 - 0.2

Question 5

What is the absorptivity of coloured metals like gold?

Answer 5

0.5

Question 6

What is the emissivity of paints?

Answer 6

0.9

Question 7

What is the emissivity of polished metals?

Answer 7

0.05 Al and 0.01 for silver

Question 8

what is the emissivity of oxidised metals?

Answer 8

0.2

Question 9

For planets with no atmosphere and slow rotation, how is the equilibraium temperature different than for earth?

At what time is this modified equation true?

Answer 9

the factor for is removed:

T=4RT((a/e)(flux/sigma))

At local noon time when the sun is directly overhead

Question 10

Fot a satellite in GEO during eclipse, what is the dominant source of heat?

Answer 10

EArth’s IR emissions

Question 11

Why cant we predict the temperature of a spherical spacecraft in LEO?

Answer 11

there are internal sources of heat for the satellite (onboard electronics etc)

Albedo radiation from earth

Question 12

When does viewing factor become relevant?

Answer 12

When the heat source can no longer be considered as a point source

Question 13

Why does sputnik never actually achieve its calculated eclipse and irradiated equilibrium temperatures?

Answer 13

Because it does not spend enough time in the respective phases of its orbit, it therefore oscillates at some intermediate temperature point

Question 14

For a LEO orbit, what is eclipse time and sunlight time?

Answer 14

60 minutes sun, 30 minutes eclipse

Question 15

What are the four sources of heat for sputnik?

What is the earth’s emissivity in this calculation?

Answer 15

Albedo
insolation
Earth IR
Internal

The earth is treated as greybody where a_IR=e_IR

Question 16

Describe how a TMM works and its purpose

Answer 16

It is used to model the spacecraft thermal behaviour which is complex as a result of the varying space environments

the spacecraft is modelled as a number of descrete regions called isothermal nodes

-each node is characterised for its thermal properties

-nodes that can ‘see’ space will include the corresponding radiative characteristics in the nodes

Question 17

What is fourier’s law of conduction

Answer 17

Q_c=h_c.deltaT

Question 18

How is the stored heat energy calcualted for a node

Answer 18

energy stored = energy from edxternal source + energy internal - heat radiated to space - heat conducted to neighbouring nodes - heat radiated to other nodes

Question 19

what are the external heat sources in the TMM

Answer 19

solar, planetary and albedo radiation

Question 20

What is the purpose of the TVAC test

Answer 20

the thermal vacuum test tests the accuracy of the TMM by checking the thermal performance of the spacecraft

The chamber has a shroud which can be electrically heated, and cooled using LN

A platten can also be used to stimulate the spacecraft thermal response

Question 21

What state does the spacecraft need to be in?

Answer 21

It needs to be operated as normal to capture the full thermal behaviour of the spacecraft resulting from operation of on board systems

Question 22

What is the purpose of the intial hot cycle?

Answer 22

The initial cycle is known as the bake out where volatiles are driven off that could later result in outgassing and redeposition impacting over spacecraft equipment

Question 23

What can be done with outgassing products?

Answer 23

They can be analysed by capturing them on a ‘cold finger’ and then analysed on mass spectrometer

Question 24

What must be done before the spacecraft can be returned back to room temperature and pressure?

Answer 24

It must be warmed up in the vacuum such that when repressurised, water does not condense on the spacecraft

Question 25

What does QCM stand for?

Answer 25

quartz-crystal microbalance, this is the cold finger

Question 26

What does TVAC NOT do?

Answer 26

It does not account for response to solar radiation. Instead a solar simulation test must be carried out by illuminating the spacecraft under vacuum with a powerful light source