Foundations — Thermal control — multi-layer insulation (MLI), heaters, heat pipes, radiators

3.6.23 · D1 · Physics › Spacecraft Structures & Systems Engineering › Thermal control — multi-layer insulation (MLI), heaters, hea

Parent note mein koi bhi formula padhne se pehle, tumhe usmein har letter ki ownership chahiye. Yeh page har symbol ko ek baar introduce karta hai — plain words mein, ek picture ke saath — aur explain karta hai ki yeh topic us symbol ke bina exist kyun nahi kar sakta. Neeche kuch bhi line one se upar kuch assume nahi karta.

0. Stage: hot box in cold nothing

Figure dekho. Ek warm spacecraft hai (yellow box) aur uske chaaro taraf black space hai. Earth par, ek hot cup of coffee teen tarakon se cool hoti hai: conduction (heat touching solids ke through crawl karti hai), convection (moving air heat ko le jaati hai), aur radiation (glowing energy light ke roop mein leave karti hai). Space ke vacuum mein koi air nahi hai, isliye convection gone hai, aur spacecraft thin isolating struts par mounted hai, isliye conduction bhi nearly gone hai.

1. Temperature — $T$

Plain words: koi cheez kitni garam ya thandi hai, kelvin (K) mein measure kiya jaata hai.

Picture: ek thermometer jo absolute zero se start hota hai — sabse thanda possible, jahan atoms ki saari jiggling ruk jaati hai. $0\text{K}$ woh floor hai; paani $273\text{K}$ par freeze hota hai (jo $0°\text{C}$ hai), room temperature $293\text{K}$ hai ($20°\text{C}$).

Topic ko kelvin kyun chahiye, celsius nahi: ek surface jo light radiate karta hai woh $T^4$ par depend karta hai (yeh §5 mein milega). Kisi number ko fourth power tak raise karna tabhi physically sense deta hai jab number absolute zero se measure kiya gaya ho — agar tera zero arbitrary hai toh "twice as hot" ho nahi sakta. Kelvin ka ek real zero hai, isliye $T^4$ ka kuch real matlab hai.

2. Area — $A$

Plain words: ek surface ka size, square metres (${\text{m}}^2$) mein measure kiya jaata hai.

Picture: spacecraft ki ek wall ko table par flat unfold karo aur uski length times width measure karo. Ek $2{\text{m}}^2$ panel ek door ke size ke barabar hai.

Topic ko yeh kyun chahiye: radiation ek surface ke through leave hoti hai. Bada window zyada heat lose karta hai, chhota window kam — sab kuch scale karta hai is baat se ki kitni surface radiating kar rahi hai. $A$ double karo toh heat flow double ho jaata hai.

3. Heat flow — $Q$ aur power $P$

Plain words: $Q$ hai kitni heat-energy har second ek boundary cross karti hai, watts (W) mein measure ki jaati hai. Ek watt matlab ek joule energy per second.

Picture: energy ko paani ki tarah imagine karo. $Q$ flow rate hai — litres per second — tank mein total amount nahi. Ek $60\text{W}$ lightbulb har second 60 joules heat pour out karta hai.

Topic ko yeh kyun chahiye: thermal control flows ka balance hai. Andar leak hone wali heat bahar pump hone wali heat ke barabar honi chahiye, warna temperature drift karti hai. $Q$ aage ke har equation ki currency hai.

4. Emissivity — $\epsilon$

Plain words: $0$ aur $1$ ke beech ek number jo batata hai ek surface kitna achha radiate karta hai ek perfect radiator ke comparison mein.

Picture: same temperature par do surfaces.

Black-painted panel (right) almost utni saari heat glow karta hai jitni woh possibly kar sakta hai — $\epsilon \approx 0.9$. Shiny aluminium foil (left) barely glow karta hai — $\epsilon \approx 0.03$. Same temperature, wildly different radiated power, purely surface finish ki wajah se.

Topic ko yeh kyun chahiye — aur yeh master knob kyun hai: thermal control ki har strategy aslaan mein $\epsilon$ ki ek choice hai:

• MLI chahta hai $\epsilon$ jitna possible ho low (heat ko trap karo) → shiny aluminium.
• Radiators chahte hain $\epsilon$ jitna possible ho high (heat dump karo) → black paint.

Ek number, opposite goals. Dekho Materials Science — Kapton & Mylar.

5. Stefan–Boltzmann constant — $\sigma$

Plain words: nature ka ek fixed number jo temperature ko radiated power per square metre mein convert karta hai.

$\sigma =5.67\times 10^{-8}\frac{\text{W}}{{\text{m}}^2{\text{K}}^4}$

Picture: $\sigma$ ko ek exchange rate samjho. Tum ise ek temperature (K mein) aur ek area (m² mein) dete ho, aur yeh batata hai ki kitne watts of light stream off hote hain.

Fourth power $T^4$ kyun, aur $T$ ya $T^2$ kyun nahi?

Steep curve dekho. Temperature double karo toh radiated power $2^4=16$ se multiply ho jaata hai. Yeh poore topic ka sabse important single fact hai:

$\sigma$ ki units mein chhota ${\text{K}}^4$ theek wahan hai taaki $T^4$ cancel ho sake, aur clean watts bachein.

6. Do temperatures ke beech net heat — $T_1^4-T_2^4$

Plain words: jab $T_1$ par ek hot surface $T_2$ par ek thandi surface ke saamne hoti hai, dono ek doosre par radiate karte hain. Net flow woh hai jo hot wala bhejta hai minus woh jo cold wala wapas bhejta hai.

Picture: ek doosre ke saamne do campfires. Dono ek doosre par light daalta hai; tum sirf difference feel karte ho. Agar dono equally hot hain, toh koi net heat move nahi hoti.

Topic ko yeh kyun chahiye: MLI aur radiators hamesha ek andar aur ek bahar temperature ke beech gap ke baare mein hote hain. Jab cold side space hai ($T_2=3\text{K}$), $T_2^4$ itna tiny hai ki vanish ho jaata hai, aur $Q\approx \epsilon \sigma A{T}_1^4$ — "one-way" radiator formula.

7. Effective emissivity — ${\epsilon }_{\text{eff}}$ aur layer trick

Plain words: jab do shiny surfaces ek doosre ke saamne hoti hain, ya jab tum bahut saari stack karte ho, toh combined poorness-at-radiating ek lumped number ${\epsilon }_{\text{eff}}$ mein capture hoti hai.

Picture: har shiny layer ek toll gate hai jo ~97% heat wapas bounce karta hai. $N$ stack karo aur heat ko har gate par pay karna padta hai; leak roughly ${\epsilon }_{\text{MLI}}\approx \epsilon /(N-1)$ ki tarah drop hoti hai.

Topic ko yeh kyun chahiye: yahi MLI ka poora secret hai — bahut saari cheap shiny sheets milke ek enormous insulator bana deti hain. Tumhe ${\epsilon }_{\text{eff}}$ aur ${\epsilon }_{\text{MLI}}$ ko series mein bahut saare $\epsilon$'s ka ek bookkeeping samajhna chahiye, naya physics nahi.

8. Heaters ke liye electrical symbols — $V$, $I$, $R$, $P$

Plain words:

• $V$ = voltage, electrical "push" (volts).
• $I$ = current, charge ka flow (amps).
• $R$ = resistance, wire current ke against kitna fight karta hai (ohms).
• $P$ = electrical power jo heat mein turn hoti hai (watts).

Picture: electricity ko ek stubborn wire se push karo aur woh warm ho jaata hai — exactly toaster coil ki tarah. Saari electrical energy heat ban jaati hai.

Topic ko yeh kyun chahiye: heaters sirf active way hain heat add karne ka jab sunlight nahi hoti. Bus se powered, yeh precious electricity ko warmth mein convert karte hain. Heater ka jalaya hua har joule ek joule hai jo batteries kahin aur spend nahi kar sakti — isliye unka sizing ($P$ ke zariye) matter karta hai.

9. Heat capacity — $c$, $C$ — aur cooling ki rate $\frac{dT}{dt}$

Plain words:

• $c$ = specific heat: $1\text{kg}$ ko $1\text{K}$ warm karne ke liye zaroorat joules.
• $C=mc$ = poore object ki heat capacity (mass $m$ times $c$): joules per kelvin.
• $\frac{dT}{dt}$ = temperature change ki rate: thermometer har second kitna move karta hai.

Picture: ek bada thermal mass (bada $C$) ek heavy flywheel hai — slow down (cool) hone mein bahut time lagta hai. Minus sign ka matlab hai heat lose karna temperature lower karta hai.

Topic ko yeh kyun chahiye: yeh batata hai ki ek battery eclipse survive karegi ya nahi heater ki zaroorat se bhi pehle. Bada $C$ → slow drift → shayad koi heater nahi. Chhota $C$ → rapid drop → heater essential.

10. Heat pipe ke symbols — $L_v$, $\dot{m}$, $\Delta P$, ${\sigma }_s$, $\theta$, $r$

Ek heat pipe heat carry karta hai hot end par fluid boil karke aur cold end par condense karke. Iske symbols:

• $L_v$ = latent heat of vaporisation: $1\text{kg}$ liquid boil karne ke joules bina uska temperature change kiye. Dekho Phase Change Heat Transfer.
• $\dot{m}$ = mass flow rate: vapour ke kilograms jo per second move kar rahe hain (dot ka matlab "per second" hai).
• $Q=\dot{m}{L}_v$: carry ki gayi heat equals fluid boiled per second times energy per kilogram to boil it.
• $\Delta P$ = pressure difference jo vapour ko hot end se cold end ki taraf drive karta hai ($\Delta$ ka matlab "mein difference" hai).
• ${\sigma }_s$ = liquid ki surface tension; $\theta$ = contact angle; $r_{\text{pore}}$ = wick pore radius. Milke yeh capillary suction set karte hain $\Delta P_{\text{cap}}=\frac{2{\sigma }_s\cos \theta }{r_{\text{pore}}}$ jo liquid ko wick ke through wapas suck karta hai. Dekho Capillary Action.

Topic ko yeh kyun chahiye: ek heat pipe ek solid rod se ~1000× zyada heat move karta hai kyunki latent heat $L_v$ bahut bada hota hai — boiling enormous energy store karta hai. Lekin yeh tabhi kaam karta hai jab wick liquid wapas pump kar sake, jo ek race hai $\Delta P_{\text{cap}}$ (suction) aur $\Delta P$ (flow resistance) ke beech. Bahut thande end ke liye dekho Cryogenics.

Prerequisite map

Ise top to bottom padho: har box ek symbol hai jo ab tumhara hai, aur arrows dikhate hain kaunse milke char thermal-control tools banate hain.

Equipment checklist

Right side cover karo aur khud ko test karo.