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I've been learning about Multi Layer Insulation (MLI) blankets for spacecrafts. I'm trying to figure out how to estimate the number of MLI layers that will be required at a particular orbit.

MLI principles and structure are discussed in detail at Why aluminum is added in Multi Layer Insulation? and at https://www.elecdiv.kaneka.co.jp/en/products/heat-measures/insulation.html (mentions KOUNOTORI and KIBO) and in Ratermann's Your Guide to Cryogenic Insulation, but is there any method that I could follow while determining the number of layers that will be needed for this purpose?

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    $\begingroup$ It would be helpful to define what MLI blankets are. $\endgroup$
    – Galerita
    Commented Apr 18 at 9:09
  • $\begingroup$ Multi Layer Insulation- used to shield the satellite from external heat flux and prevent the loss of internal heat to space. $\endgroup$
    – Arjun Krishnan
    Commented Apr 18 at 9:15

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Choosing MLI configuration is part of the larger topic of Thermal Control System design.

Thermal design of a spacecraft in vacuum is basically about power equilibrium. Some heat (thermal power) will be generated inside the spacecraft and needs to be radiated away. Heat from the sun (and other external sources such as the Earth) generally needs to be rejected.

Characterizing MLI specifically in these types of models is typically done by effective emissivity. Note that the details of setting up the MLI system are important and go beyond number of layers, though that's a common initial approximation. Note the curve departing from the data points. Emissivity and emittance are used interchangeably in this source.

Graph from the linked source showing effective emittance / emissivity versus number of MLI aluminized layers

Since you ask about working it out for a typical orbit, you might start with this Introduction to On-Orbit Thermal Environments. I chose the following picture since it depicts several of the important factors that play into how differing orbital height eclipses you for more or less time (though that particular slide is focusing on calculating how much albedo energy is received from the planet the spacecraft orbits). Other parts of the presentation go into how that changes with season (beta angle) and the importance of spacecraft pointing and configuration (will one side always be sun-facing? then it gets more solar flux).

Slide from the linked presentation showing calculations for heat radiated from the planet changing around an orbit

Unfortunately it's not a simple design discipline. There isn't a simple correspondence between one orbit and one amount of MLI layers--the entire design of the mission is going to play in to your answers.

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    $\begingroup$ Good response, interesting looking references too. As a small detail, I don't know what you think, there is something quite hard to digest about the y axis of the effective emissivity plot: the upper part jumps two orders of magnitude in the same vertical change that the lower part jumps one order of magnitude; I'm not saying its wrong, just hard to read! $\endgroup$
    – Puffin
    Commented Apr 18 at 18:46
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    $\begingroup$ @Puffin huh, I think you're right. So it's not a straight logarithmic scale...is that log-log? I agree, it's not a great graph. In general the references might not be spectacular--I chose the first ones that seemed to fit, without doing much of a survey. With luck someone with more thermal design expertise will come in with better ones. I should probably dig out SMAD and see what it has to say on the topic too $\endgroup$
    – Erin Anne
    Commented Apr 18 at 19:11
  • $\begingroup$ Well, I did think it was positive that the two NASA links each stayed with one limited aspect of the subject and went through it chapter and verse. A while ago the bible on the subject was the Gilmore book, blackwells.co.uk/bookshop/product/… probably long superceded in terms of new materials, MLI surface finishes etc. $\endgroup$
    – Puffin
    Commented Apr 18 at 19:43
  • $\begingroup$ Thank you @Erin Anne. While reading the Thermal Control System design paper, it mentions that the adhesive tapes and other passive systems are preferred over MLI blankets in the case of small space craft. I'm also currently researching on small space craft. In your expertise, for a communication satellite with the maximum payload weight of 100 kg, can we actually not have MLI blanket? $\endgroup$
    – Arjun Krishnan
    Commented Apr 21 at 15:01
  • $\begingroup$ @ArjunKrishnan I'm not an expert on thermal systems, but I worked on two satellites that weighed about 25kg each in college. Both had MLI blankets. Both functioned in space until their batteries wouldn't recharge anymore. $\endgroup$
    – Erin Anne
    Commented Apr 21 at 17:39
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I would like to add to Erin Anne's points about the mission design.

It helps to have a guiding thermal strategy (which will depend upon satellite pointing requirements etc) for the "primary heat loss path".

This is because, typically, one wants limited uncertainty over operating temperatures and, just by random example, a 50% uncertainty in something meant to be a good conductor will give a much lower temperature uncertainty than a 50% uncertainty in something meant to be an insulator. One probably wouldn't want insulators (e.g. MLI) all round a satellite unless the heat dissipation is particularly low.

Once you have worked out what your heat loads are and how to get rid of heat its easier to think in terms of insulating areas that are off the main heat loss path. Consider the sketch below. The temperature at the node at the top is a balance of the two paths.
enter image description here

I suggest trying to see the thermal problem in big picture terms like this, even if you have begun modelling and have a thermal model with hundreds of detailed links.

It will help to see some simplified worked examples, i.e. if you can see the overall strategy at satellite level for other designs.

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