DG cooling

[martins]

As part of the Delta-glider update for the next version, I want to make the radiator deployment a bit more meaningful by adding a cooling system module.

Given that the radiator is quite small and maybe not always convenient to deploy, I want to use it as a secondary system, while the primary system uses a cooling loop through the wings.

My question: is it feasible to use the wings as radiators, or is it a ludicrous idea? I can see at least two problems:

1. The wing has thermal protection (at least on the underside), so getting the heat to the surface may be a problem.
2. In sunlight, the wing will heat up and reduce its effectiveness as a radiator. In the worst case, you may end up actually pumping heat from the wings back into the fuselage.

The first problem essentially adds a time delay: it will take time for the heat to diffuse through the insulation, before the surface can do its duty as a black body radiator.

The second problem means that it may only work in Earth shadow, or by maintaining a low sun incidence angle on the wings. In the worst case (vertical incidence), assuming Earth orbit (solar constant W_0=1368W/m^2), an albedo of, say, a=0.2, and the wing to be a black body, I make the equilibrium temperature to be
[math]
T = \left[ \frac{W_0 (1-a)}{2 \sigma} \right]^{1/4} = 313K
[/math]
with \sigma=5.67e-8 J s^-1 m^-2 K^-4 as per Stefan-Boltzmann law.

That doesn't sound too excessive. Is it realistic to get the Freon temperature in the coolant loop significantly higher than that? What were the typical coolant temperatures for the Space Shuttle at the inlet and outlet of the radiators?

 

[statickid]

I'm not qualified to QC your work there, but I think this would be really cool *budum-tisss~~*

I actually like to pretend that my ships need thermal management ever since I saw some documentary about the space shuttle, and an astronaut mentioned having to keep the cockpit windows in shadow and the sun/wing profile small. So when flying the various space planes I will usually orient the craft in such a attitude just for kicks.

I'd be excited to see this kind of mechanic mandatory for successful flights!


p.s. I think on that same documentary they said that facing the cockpit in shadow and exposing the bottom tiles which were insulated better actually helped manage the internal temperature.

with that in mind, maybe the top of the DG wing has thinner insulation than the bottom, and the cooling system takes advantage of this.

 

[Urwumpe]

http://spaceflight.nasa.gov/shuttle/reference/shutref/orbiter/eclss/atcs.html

The radiator outlet temperature can be regulated to 38°F (276 K, 3.3°C) or 57°F (287 K, 14°C), I would need to search for actual performance data. At 41°F (278 K, 5°C) the flash evaporators automatically activate for increased cooling performance, evaporating water from fuel cells and ECLSS.

The ISS radiators are also operating at about 3-4°C outlet temperature.

Also thermal protection during re-entry is really a small problem, since you can expect some radiation from the plasma vortex over the wings, if you are trying to be realistic. But it is no show stopper, you could assume that the radiator surfaces just need some cleaning after flight for preventing degraded performance.

 

[Ravenous]

The first problem essentially adds a time delay: it will take time for the heat to diffuse through the insulation, before the surface can do its duty as a black body radiator.

I'm not sure if it's a time delay as such - when first switching "on" the hot coolant, wouldn't it start to dump heat into the cooler structure? (Not sure.)

Also I found a ref, where they go on about radiators:
http://www.nss.org/settlement/nasa/spaceresvol2/thermalmanagement.html
...but does the above have an incorrect number for the stefan-boltzmann constant? (Confused now.)

EDIT: Nah, Urwumpe's link is far more relevant by the looks of it!

 

[martins]

I'm not sure if it's a time delay as such - when first switching "on" the hot coolant, wouldn't it start to dump heat into the cooler structure? (Not sure.)

But the heat capacity of the insulating material will be quite low, so not much heat will get absorbed.
In fact, come to think of it, the insulation is not just a delay problem: it would reduce the heat flow from the coolant to the surface, so the amount of heat reaching the surface will be small compared to what can be radiated off. The insulated wing surface will have its own low-temperature thermal equilibrium which is much lower than the temperature of the coolant circulating inside. So I guess the only hope is an upper wing surface without insulation.

Edit: I guess one term missing in my above equation is the heat conductivity of the radiator itself. The wing will have a relatively low conductivity, so will only work as a radiator at high coolant temperatures. The dedicated radiator will have a higher conductivity, so will work at lower coolant temperatures.

Also I found a ref, where they go on about radiators:
http://www.nss.org/settlement/nasa/spaceresvol2/thermalmanagement.html
...but does the above have an incorrect number for the stefan-boltzmann constant? (Confused now.)

Yes, they seem to be missing a fairly significant factor of 1e-8.

 

[jedidia]

while the primary system uses a cooling loop through the wings.

Where are we going to store all the propellant, then?

assuming Earth orbit (solar constant W_0=1368W/m^2), an albedo of, say, a=0.2, and the wing to be a black body

The problem you're facing here is that basically, the more efficiently a surface radiates, the more efficiently it also absorbs. This isn't constant over different wavelengths, however, so a surface can reflect a decent ammount of the visible spectrum while still being able to radiate (but also absorb) heat in the IR-spectrum. Modelling this would be somewhat complicated, though.

Is it realistic to get the Freon temperature in the coolant loop significantly higher than that?

Well, the coolant temperature is kind of dependant on the operating temperature of what it's cooling (but look who I'm talking to, you already know that :lol. What I'm getting at is, the sun influx really only matters up to somewhere around 400 to 500 K (from my expierience in IMS, where I could gather some expierience with simulating the thermodynamic processes of a spacecraft). I.e. I would expect the wing to be a pretty useless surface to cool the DGs cabin (at least when it's facing the sun)... but cooling its unobtainium reactor should be perfectly possible.

 

[statickid]

Where are we going to store all the propellant, then?

The Delta Glider doesn't store propellant in the wings, it is powered solely by the unadulterated ardor and moxie of the pilot, as collected by the helmet apparatus.

 

[RisingFury]

The wing has thermal protection (at least on the underside), so getting the heat to the surface may be a problem.

No, not just a time delay! It would actually reduce the amount of heat you can dump!

Remember that P = (K / d) * dT / dx <--- If you have a certain maximum temperature difference, the thicker your insulation (higher d) or the more effective your insulator (lower K), the lower the heat flow through the material.

You want the wall of the radiator to be as thin as possible and as conductive as possible.

 

[martins]

Yes, this is what I meant to convey by my second post

Anyway, it seems that using the wings as radiators is probably not such a clever idea, so I'll stick to the actual radiator. Maybe I can redesign it to have a larger surface area.