Orion EFT1 uncrewed martian flybye.

[Cassel]

49 minutes ago, kerbiloid said:

The "day" side of the car freely rotating in the space is getting heated up to ~+100°C (and expanding), while its "night" side is getting cooled down to ~-100°C (and shrinknig).

 

I wasn't talking about sending car into space, only put it on the ground in vacuum chamber. Why would you want car in space? My point is that half of things he said won't happen there. Sure some materials aren't designed to survive in vacuum, but if you are making space craft you do design them in right way.

As for day/night temperature, how this car in space (vacuum) would radiate so much heat so fast?

Edited September 3, 2018 by Cassel

[YNM]

4 minutes ago, Cassel said:

As for day/night temperature, how this car in space (vacuum) would radiate so much heat so fast?

By reaching equilibrium temperature.

 

You need quick rotation to make it a uniform body wrt solar irradiation though.

[Cassel]

3 minutes ago, YNM said:

By reaching equilibrium temperature.

 

You need quick rotation to make it a uniform body wrt solar irradiation though.

How metal from car radiate heat in vacuum? And how long does it take to cool down from 100 C to 0 C?

[kerbiloid]

26 minutes ago, Cassel said:

Sure some materials aren't designed to survive in vacuum,

All of them.
Metal parts tend to weld to each other in vacuum, organic parts tend to sublimate in vacuum. Space materials just do this a little slower.

Salyut-7 has problems with extending of a solar panel because its tether got welded to the brackets after three years in space.

On Quantum module a telescope detector to be replaced got welded to the telescope hull.

During Gemini-4 flight EVA was lasting for ~30 minutes, so the door was staying open in vacuum.
Next 30 minutes they were trying to close the door with four hands because the hinge spring got welded.

26 minutes ago, Cassel said:

As for day/night temperature, how this car in space (vacuum) would radiate so much heat so fast?

If a spacecraft rotates fast (several minutes per turn), its crew will have headache,
If it rotates slowly (more than several minutes per turn), it's heated not uniformly.

In both cases it still is in vacuum (see p.1.)

16 minutes ago, Cassel said:

And how long does it take to cool down from 100 C to 0 C?

How long does it take for your roof, car, or window to heat under sun?

Also let's remember that both cars and spaceships are tin cans, made of thin layer of something. So they get warm/cold much faster than a several tonne heavy piece of metal.

Edited September 3, 2018 by kerbiloid

[Cassel]

49 minutes ago, kerbiloid said:

Salyut-7 has problems with extending of a solar panel because its tether got welded to the brackets after three years in space.
 

It was problem with temperature or structure of metals?
 

Quote

Also let's remember that both cars and spaceships are tin cans, made of thin layer of something. So they get warm/cold much faster than a several tonne heavy piece of metal.

I was asking about radiating heat from 100 to 0 C. How fast metal can do that in vacuum? And how does it work? In air it is simple, but in vacuum it is something I don't understand.

Edited September 3, 2018 by Cassel

[Green Baron]

Heat is radiated by every body whose temperature is higher than 0 K. It is mostly radiated in the infrared spectrum and clearly measurable. In a vacuum it is radiated freely. If there was a medium in between (like air), conduction and/or convection would come into play. But that wasn't the question.

tl:dr: radiation takes the heat away from each and every body whose temperature >0 K. Mostly in the IR spectrum.

 

Edit: how fast depends on surface area, temperature difference, conduction inside the body, etc. In case of a metal sheet of submillimeter thickness cooling from 100°C to 0°C in -160°C environment i'd say pretty fast. Like tens of seconds or a minute or 2 Like tens of minutes ? Ready for correction :-)

Edit: corrected after @kerbiloid's hint below.

Edited September 3, 2018 by Green Baron

[linuxgurugamer]

12 minutes ago, Cassel said:

I was asking about radiating heat from 100 to 0 C. How fast metal can do that in vacuum? And how does it work? In air it is simple, but in vacuum it is something I don't understand.

There are two ways to get rid of heat on earth.  One is by convection, where the air gets warmed by the hot metal and move away, to be replaced by cooler air, etc.  This is what you can easily observe

The second is by radiation.  Just as a light bulb radiates light,  it also radiates heat.  Light and heat are both radiation, and get radiated the same way.  It's slower than the convection, though.  Try this.  Turn on an incandescent light bulb, and hold your had below it, but close to it.  Your hand will get warm, that's from the radiation.

In space there is no atmosphere, so the ship is reliant on radiation

[Cassel]

11 minutes ago, Green Baron said:

 

Edit: how fast depends on surface area, temp. difference, conduction inside the body, etc. In case of a metal sheet of submillimeter thickness cooling from 100°C to 0°C in -160°C environment i'd say pretty fast. Like tens of seconds ? Ready for correction :-)

If that is true, why not use Stirling engine instead of solar panel to power up ISS? This engine is very effective at large temperature differences.

Anyway I don't want offtopic too much :-)

 

10 minutes ago, linuxgurugamer said:

There are two ways to get rid of heat on earth.  One is by convection, where the air gets warmed by the hot metal and move away, to be replaced by cooler air, etc.  This is what you can easily observe

The second is by radiation.  Just as a light bulb radiates light,  it also radiates heat.  Light and heat are both radiation, and get radiated the same way.  It's slower than the convection, though.  Try this.  Turn on an incandescent light bulb, and hold your had below it, but close to it.  Your hand will get warm, that's from the radiation.

In space there is no atmosphere, so the ship is reliant on radiation

Thanks, I like this explanation.

[kerbiloid]

22 minutes ago, Cassel said:

It was problem with temperature or structure of metals?

It is a problem with diffusion of atoms. In vacuum it runs easier.

24 minutes ago, Cassel said:

I was asking about radiating heat from 100 to 0 C. How fast metal can do that in vacuum?

1 m² of tin aluminium can surface, say 1 cm thick.
Mass = 2700 kg/m³ * 1 m² * 0.01 m = 27 kg.

Heat capacity per mass = 900 J/(kg * K)
To change the temperature for 100 K you need add/remove = 27 * 900 * 100 ~= 2.5 MJ of energy.

Say, at the moment temperature = 0°C = 273 K.
Luminosity = 1 m² * 5.67*10^-8 W/(m²*K⁴) * (273 K)⁴ = 315 W.

Time to absorb/emit = 2.5*10⁶ / (315 * 3600) = 2.2 hours.

So, at reasonable values of you craft rotation its day and night side will have up to tens degrees of temperature gradient.

(And diffusion still runs whether this gradient exists or not).

10 minutes ago, Cassel said:

If that is true, why not use Stirling engine instead of solar panel to power up ISS? This engine is very effective at large temperature differences.

This ideal engine maybe would, but the heat exchange rate is limited. 
It would be very bulky.

[Green Baron]

17 minutes ago, Cassel said:

If that is true, why not use Stirling engine instead of solar panel to power up ISS? This engine is very effective at large temperature differences.

Anyway I don't want offtopic too much :-)

 

Thanks, I like this explanation.

The stirling engine is a pump. It would still need a motor and energy to get working ;-) Solar panels otoh panels are terribly effective for the use case.

Actually, radiation can be much more effective than convection if the radiation area is large and differences huge. And the cooling curve is parabolic. That is what would make the thin metal sheet of the "car in space" radiate the first 100°C very quickla.

9 minutes ago, kerbiloid said:

This ideal engine maybe would, but the heat exchange rate is limited.

Well, if you let it rotate and expose the one and other side to the sun in a perfect cycle of expansion and compression. But you won't get much power out of it ...

A car's skin isn't a cm, it is less than a millimeter ;-)

Edited September 3, 2018 by Green Baron

[Cassel]

3 minutes ago, Green Baron said:

The stirling engine is a pump. It would still need a motor and energy to get working ;-)

You have energy from the Sun, some said here it is 100 C in very short time. If cooling is so fast as you said this engine would give you enormous amounts of power.

[Green Baron]

25 minutes ago, Cassel said:

You have energy from the Sun, some said here it is 100 C in very short time. If cooling is so fast as you said this engine would give you enormous amounts of power.

I said a sheet of submillimeter. And the engine is a chunk of metal, that one huge difference.

Edited September 3, 2018 by Green Baron

[kerbiloid]

11 minutes ago, Green Baron said:

A car's skin isn't a cm, it is less than a millimeter ;-)

Depends on a car.

Spoiler

Though, I was thinking about fueltank-based hulls.

P.S.
But now I worry about the marsotesla.
With < 1mm hull it would be heating/cooling in just 15 minutes.
Does it still have anything except hull, or wheels , chairs, and glasses are already orbiting it.

Edited September 3, 2018 by kerbiloid

[Green Baron]

Yeah, that's a different thing :-) We'll ask Elon if he would if he could.

Be it as it may, i hope we have transported the notion that heat is carried away by radiation in vacuum, and that pretty effectively. This fact is actually (Science & Spaceflight !) an underlying working base of Astronomy :-)

[Cassel]

29 minutes ago, Green Baron said:

I said a sheet of submillimeter. And the engine is a chunk of metal, that one huge difference.

I know what you said, but you misunderstand what I said. You do not have to heat and cool the entire engine. One metal panel may heat up (the red one elements), and the blue panel may be cooled in shadow.

[Green Baron]

Yeah, and if the thing it is mounted to is rotating in the sun, cooling and heating the sides alternating ? Because that was the original scenario which triggered the heating heating discussion, or am i wrong ?

Edited September 3, 2018 by Green Baron

[kerbiloid]

The gif would be slowed down for hundred times.

[MinimumSky5]

Also, while sterling engines are very efficient in terms of converting solar energy into electrical energy, solar panels are better in terms of power per kilogram

[Cassel]

1 hour ago, MinimumSky5 said:

Also, while sterling engines are very efficient in terms of converting solar energy into electrical energy, solar panels are better in terms of power per kilogram

I not sure about this. Lockheed Martin built the Stirling engine, they called it an advanced Stirling radioisotope generator. It generates 130W and weights 32kg.
How much solar panels would weight?

 

[YNM]

8 hours ago, Cassel said:

How metal from car radiate heat in vacuum ?

Radiation.

As Stefan-Boltzmann law gives

And the incoming solar radiation is, say for example, for Earth :

(you can adjust the data with other things)

So there will be a temperature where the irradiated heat is equal to the solar-radiation heat.

8 hours ago, Cassel said:

And how long does it take to cool down from 100 C to 0 C? (or vice versa -ed)

That depends on the imbalance that exist at a given time. You could try numerical methods for approximayion or differential methods for true value.

[Green Baron]

1 hour ago, Cassel said:

130W and weights 32kg.
How much solar panels would weight?
 

A ~300Wp panel weighs a fraction (1/3 to 1/2) of that actually, has >double the power output under optimal conditions, with framing and wiring. And costs little. And it has no moving parts and lasts many years without maintenance ;-)

Edit: i mean the consumer grade mass product ...

 

Editedit: You are not completely on the wrong track, @Cassel, wherever there is a temperature gradient a stirling engine would principally make sense. This would be the case in small geothermal (the hydro variant above) applications, it would be my first choice there because it is cheap then, small, easy to repair and maintain. But not in spacey applications, i guess. Solar power is the best choice there.

Edited September 3, 2018 by Green Baron

[YNM]

12 hours ago, Cassel said:

One metal panel may heat up (the red one elements), and the blue panel may be cooled in shadow.

Just use thermocouples.