New Mars Forums

tahanson43206

Moderator

Registered: 2018-04-27

Posts: 19,416

Heat Management Thermal Balance Spacecraft Deep Space or Local

For SpaceNut ...

NewMars members have included the challenges of managing heat in spacecraft in numerous posts...

This topic is offered in hopes our members will want to build up a collection of useful guidelines for a spacecraft designer, or for a reader who may become a spacecraft designer.

As this topic goes up, humans have launched unmanned probes in great numbers, including some whose travels are taking them out of the Solar System.

In addition, humans have put astronauts on the Moon, and several Nations have launched and run successful space stations.

All these devices and systems had to deal with heat management.

This topic provides an opportunity for members to post links to relevant articles or web sites, or text quoted from one or more of them.

(th)

tahanson43206

Moderator

Registered: 2018-04-27

Posts: 19,416

Re: Heat Management Thermal Balance Spacecraft Deep Space or Local

This post is reserved for an index to posts that may be contributed by NewMars members over time.

(th)

tahanson43206

Moderator

Registered: 2018-04-27

Posts: 19,416

Re: Heat Management Thermal Balance Spacecraft Deep Space or Local

I'd like to lead off with this quote from Calliban. Calliban was responding to a question from FriendOfQuark1, about the conditions that would govern a spacecraft that makes a cave for itself inside a rubble pile asteroid, to wait out a severe radiation storm.  In his first post on this situation, Calliban had expressed concern about poor thermal conductivity through the asteroid material. In this post, Calliban provides a strategy for dealing with the problem:

That would depend upon the thermal conductivity of the rock, the thickness of the rock and the temperature difference between the surface and the spacecraft.  It also depends upon time - does the system reach thermal equilibrium, i.e a stable temperature gradient?  Lets assume it does.

This reference discusses thermal conductivity of crustal rocks.  This ranges between 2.5 - 4.5W/m.K.  Lets assume the latter.
https://onlinelibrary.wiley.com/doi/10. … 21/6630236

Suppose the surface of the asteroid is at a temperature of 200K (-73°C).  Suppose your spacecraft is at a temperature of 300K.  How much heat would a 10m thick layer of rock conduct away?

Q=kAdT/dX = 4.5 x (300-200)/10 = 45W/m2.

So I would guess it could work, provided you are not too deep within the asteroid.  If the rock thickness can be reduced to 1m, then thermal conduction increases to 450W/m2.  It does impose limits on the practical power generation for the spacecraft.

The problem disappears altogether if the spacrcraft can transfer heat to a radiator on the surface of the asteroid, via a fluid containing pipe.  If that can be done, then limits imposed by thermal conductivity of rock are obviated.

(th)

RobertDyck

Moderator

From: Winnipeg, Canada

Registered: 2002-08-20

Posts: 7,934

Website

Re: Heat Management Thermal Balance Spacecraft Deep Space or Local

If someone in this group can answer the original question, I have a further question. I would like to work out details of temperature management for a greenhouse on Mars. Two basic designs, but very similar temperature management.

Inflatable
As wide as a double car garage, twice as long as a double car garage. Garage dimensions (double car) can be 20' x 20' (6m x 6m) for a small one, or 28' x 24' (8.5m x 7.3m) for a more typical North American one. I'm guessing based on an image from a painting, but no one has defined a specific size. One website for greenhouses claims a hobby greenhouse can be 8' to 10' wide. Another website says a small greenhouse is 6' wide, medium 8' to 10' wide, large is anything over 25'. Notice the gaps in size. For a Mars Direct mission, the greenhouse is not expected to provide all food, or recycle oxygen or water. It's an experiment to prove that food can be grown on Mars. So take dimensions for a double car garage, double length, and use that. Yes, the image shows two greenhouses, but I'm suggesting just one. So let's use that.

Polymer film is PCTFC sold by Honeywell. Their brand name for commercial applications is Aclar; they used to use the brand name Clarus for military and aerospace applications, but they may not use that name anymore. The film will be treated with a spectrally selective coating to absorb UV but let visible light through. NASA used to use vacuum deposited nickel, gold, and silver oxide. (Only silver oxidized.) The commercialized version reflects 90% of short wave IR from extremely hot things like the Sun, 55% of long wave IR from lukewarm things like the floor or furniture. This has the net effect of cooling buildings, used in hot areas like Navada or Arizona, or can be reversed to warm buildings in cold areas like Canada. Obviously we want to warm the greenhouse on Mars. The greenhouse would have 2 layers of film, with a gap filled with argon gas to reduce heat transmission. The gap would be pressurized less than greenhouse interior, but more than Mars ambient. This holds the inflatable roof up for both layers. The gap can be monitored to detect a leak: if the gap increases in pressure, there's a leak in the inner layer. If the gap pressure drops, a leak in the outer layer. The floor must be insulated from cold ground; possibly bubble wrap with the bubbles filled on Mars with argon harvested from Mars atmosphere. A hard plastic mesh walking surface would go over the bubble wrap, perhaps with stand-off posts to hold the floor above the ground so workers don't walk on the bubble wrap. The film will be held down with hold-down straps over the outside of the greenhouse. The straps held by large tent pegs pounded into the Mars ground. Note: Mars InSight had a temperature probe that was supposed to pound itself into the ground, but didn't work. The ground was too hard. The ground just a fraction of an inch below the surface is not loose sand or loose dirt, it appears to be permafrost. So pounding in tent pegs may require heating them to melt the permafrost. Will that require a hot drill? Or would a hot peg be able to push dirt beside itself out of the hole?

So here's the question: could this be engineered to maintain temperature just from sunlight? Mars atmosphere is cold, but very thin. Very little heat is carried by atmosphere that thin. Most of the cooling will be done by conduction through the ground. Could we insulate the floor enough to engineer the system to be self-heated? A curtain oriented flat across the ceiling could help. Aluminized Mylar would reflect almost all radiant heat, keeping heat within the greenhouse. This could be drawn across the ceiling at night, opened during the day. One question is whether this would have much effect if the enclosure has a spectrally selective coating? Another option is to close that curtain during the day if the greenhouse gets too hot. Another option is a curtain that isn't opaque, but more reflective of radiant heat while being mostly transparent to visible light. That could be used to reduce heating during the day, while allowing plants to get needed sunlight. Another option is to create an air gap between the bubble wrap and enclosure film where it contacts the ground. A fan could be turned on or off to blow cold air into the greenhouse.

Another detail: when erecting the greenhouse, all stones would be raked away leaving a smooth layer of fine dirt. The dirt would be levelled and smoothed. A tarp would be drawn across the smooth ground. The inflatable greenhouse then laid on the tarp. That is to avoid anything puncturing the pressure layer.

Permanent, built using in-situ materials
Tempered glass. Soda-lime glass, but heat treated to temper. Tempered glass is stronger, but more importantly harder. It's harder than minerals of Mars dust storms, so the dust storms won't craze the glass. The glass panes won't be perfectly flat, they'll be curved like the windshield of a car. This is done while heat treating; it's heated enough to allow the glass to sag while the glass is supported by the edges. How to do this has already been perfected by the windshield industry. Again double pane. Place each pane in an aluminum frame with rubber pressure seal holding the glass pane in place. Commercial greenhouses on Earth made of glass have flat angled roofs for rain, and flat vertical walls. Ones made of polymer film have a curved "barrel arch" roof if made of polymer film. This just makes them easy to make. One for Mars will have flattened cylinder shape with rounded ends to contain pressure. The glass will have the same spectrally selective coating, and two panes with the gap filled with argon.

If the permanent greenhouse is larger, do we need to bury Styrofoam boards vertically under the perimeter of the greenhouse? Styrofoam in the ground would allow the ground directly beneath the greenhouse to remain warm. Would that be beneficial? What kind of insulation do we need in the floor of a permanent greenhouse?

For a permanent greenhouse, crops that thrive in shade on Earth can grow in what is described above. However, crops that require direct sun would benefit from a mirror. I have suggested a long-narrow greenhouse oriented perfectly east-west, with mirrors on both of the long sides. The mirrors would be full length of the greenhouse, with the bottom of the mirror the same height from the ground as the bottom of the wall windows. No need for walls to be transparent lower than soil trays. Hard opaque walls could protect against wind-blown sand or dust, and protect against a sand dune forming against the greenhouse. The top of the mirror the same height above ground as the apex of the roof of the greenhouse, or the tallest plant grown in that greenhouse. The flat mirrors would not track the sun, but would be adjusted by season. Angle changed 1° every 14 Mars solar days. If a greenhouse is built at the equator, then at spring or autumnal equinox the mirrors will be 45° from horizontal. Adjust mirror angle for latitude and season. If the greenhouse is twice as wide as height, this will double illumination. The reason for the greenhouse to be long and narrow (relatively) is so that at dawn the sun rises in the east, reflecting sun toward the west but still within the greenhouse. At dusk the sun sets in the west, reflecting light toward the east but again still within the greenhouse. This avoids the need to track the sun. I mention this because additional illumination (sometimes called insolation with an 'O') will affect heat calculations. An inflatable greenhouse will not have any mirror.