Materials Science Experiment

[Ted]

Typical whole orbit data for the panel and chassis temperatures are as follows:

Black panel Max 25 Min -15
Silver panel Max 29 Min -12
Black chassis Max 25 Min -12
Silver chassis Max 25 Min -12

The black panel should absorb and radiate heat faster than the silver panel, if these changes are only due to radiation. The temperature of the black panel should then rise and fall more quickly than that of the silver panel.

Similarly, the silver panel should reflect heat more effectively than the black panel. Therefore the temperature of the silver panel should rise and fall more slowly than that of the black panel, assuming temperature changes are only due to radiation

The results show that the black panel falls to a lower temperature than the silver panel, which is in line with heat loss due to radiation only. However, the silver panel rises to a higher temperature than the black panel, which cannot be explained, if changes are only due to heat absorption. Surely, the black panel would be expected to rise to the higher temperature? Both panels are attached to the chassis. If the black panel is losing heat to the chassis by conduction faster than the silver panel, this could explain why the silver panel rises to the higher temperature. However, this argument does not appear to hold up, since the panels appear to be offset from the chassis and each is only attached by two small screws. Loss of heat to the chassis by conduction would be slow. The range of temperature changes suffered by the black and the silver panels differs by only one degree, 40 for black and 41 for silver.

If anybody is able to provide me with a convincing explanation for the recorded temperature changes, I would appreciate it as I wish to explain the results to a class of children.

Ted, G3YWA

[wa8sme]

Ted,

Please contact me direct, I'd like to share some data and observations.

Mark, WA8SME mspencer@arrl.org

[g0mrf]

Hello Ted / Mark.

Typical whole orbit data for the panel and chassis temperatures are as follows:

Black panel Max 25 Min -15
Silver panel Max 29 Min -12
Black chassis Max 25 Min -12
Silver chassis Max 25 Min -12

The black panel should absorb and radiate heat faster than the silver panel, if these changes are only due to radiation. The temperature of the black panel should then rise and fall more quickly than that of the silver panel.

Similarly, the silver panel should reflect heat more effectively than the black panel. Therefore the temperature of the silver panel should rise and fall more slowly than that of the black panel, assuming temperature changes are only due to radiation

Looking at your examples above, hopefully the black and silver chassis sensors are essentially the same temperature because the chassis sections are physically connected. This demonstrates that conduction is the dominant form of energy transfer. i.e. the difference between the sections ability to absorb and emit heat energy by radiation is not seen in the data because the temperatures are equalised by conduction.

Re the black and Chrome plated samples. I would be interested in your views on this. My personal view is:
1) The black panel reaches a lower temperature in eclipse because it radiates energy at a higher rate than the 'silver' panel.
2) The silver panel reaches a higher temperature in sunlight because it does not radiate energy as efficiently as the black panel.

Okay, No 2 disagrees with Teds comments but it's worth a discussion. My reason for saying this is that while the black sample absorbs a lot of Infra red in sunlight, it also emits it very well (see answer 1). The final temperature will depend on the balance between absorption and emission. Black does both very well and so in sunlight will not be subject to temperature extremes.
The silver panel does not absorb as much energy as the black sample, but it is an extremely poor emitter. On balance more energy is retained in sunlight and so its temperature exceeds the temp of the black sample.

Ground based tests can show this. Take identical Aluminium sheets. Spay one matt black and polish the other. If you place them on an insulated surface facing the sun, (preferably on a calm hot cloud free day) after 20 minutes you will find the black sheet is hot, but the silver sheet is even hotter...as it does not radiate much of the energy it has absorbed. - It needs to be a calm day as airflow across the surfaces ruin the results.

Thanks for your interest.

David G0MRF

Incidentally, have you noticed how the solar panel that supports the MSE samples drops to a lower temperature in eclipse than the black MSE sample. - I've puzzled over this one. The only theory I have is that the solar panel is fabricated from a metal substrate and not the usual FR4 PCB material. (To protect the solar cells and add some radiation protection to FUNcube's electronics) The solar panels have a black finish. My contention is that the panel drops to a lower temp in eclipse because it has a greater surface area to mass ratio than the Black MSE sample. - Reasonable??

[Ted]

David G0MRF,
The black and silver panels spend twice as long in the Sun as they do in the dark, so I prefer to concentrate on the maximum temperatures reached by these two panels, rather than their minimum temperatures. Mat black surfaces gain heat faster than silver reflective surfaces. As we have all seen, the panel labelled silver rises to a higher temperature than the panel labelled black. This is contrary to what one would expect if heat is gained almost exclusively by radiation from the Sun, which is highly likely.
The builders of large commercial satellites cover them with reflective material to minimise the heat that they gain when in sunlight. They expect that their satellites would rise to higher temperatures if they were not covered with reflective material. Why should FUNcube-1 be any different? The simplest explanation for the observed maximum temperatures of the panels is that suggested by Matt KC8YLD in an e-mail message copied to me, namely, that they are merely labelled the wrong way round in the Dashboard software! Black is actually silver and silver is black. I should like the software writers to tell us that this is not the case, so that we can eliminate this simplest of explanations. The only alternative that I can think of, is that one or both of the thermocouples on the panels is/are delivering erroneous results.
If we wait long enough, an expert who really knows what he is talking about, will come along, give us the correct explanation and completely ruin this debate!
Kind regards,
Ted G3YWA

[g0mrf]

Hi Ted.

As you say, less speculation and more evidence.
I believe the sensors themselves are as close to identical as possible. - Same production batch, tested etc etc. This is probably borne out by the chassis sensors producing essentially identical results. (conduction) but is easy to check against preflight thermal testing results.
The possibility of reverse connection could be looked at. The engineering model may provide some answers. Also, data handling before display on the ground.
Re the numbers. There are some online calculators that look at final equilibrium temperature, and they agree with you, black should reach the higher temperature.
FYI This one is quite good and looks at many different materials and distances from the sun.
http://www.alternatewars.com/BBOW/Space ... _Temps.htm

Thanks

David

[Ted]

Hi David,
Thank you for the useful reference that you gave me. It suggests that the ratio of the equilibrium absolute temperatures of silver to black panels on a satellite could be approximated by the following equation.

Ts:Tb = (As/Es)^0.25:(Ab/Eb)^0.25

Ts is the absolute temperature of the silver panel
Tb is the absolute temperature of the black panel
As is the solar absoptivity of the silver panel
Es is the emissivity of the silver panel
Ab is the solar absorptivity of the black panel
Eb is the emissivity of the black panel

I assume that the absoptivities and emissivities of black and silver panels could be measured for the engineering model. If we knew the figures, this might give us an estimate of the absolute temperature ratio for the highest temperatures reached by the panels on the flight model. The above equation refers to equilibrium temperatures, whereas we are interested in maximum temperatures. However, some absorptivity and emissivity figures might give us a qualitative answer to our problem.
Regards, Ted