Foundations — Thermal protection systems — ablators (PICA, SLA), metallic tiles, RCC

3.4.22 · D1 · Physics › Rocket Flight Mechanics › Thermal protection systems — ablators (PICA, SLA), metallic

Yeh page toolbox banata hai. Parent note mein $v$, $\rho$, $\sigma$, $Q^{\ast }$ jaise symbols aur "stagnation point" aur "$\sigma T^4$" jaise phrases uchhaale gaye hain. Agar koi bhi cheez magic runes jaisi lagi, toh yeh page unhe pictures mein badal deta hai. Hum order mein jaate hain, taaki har idea uske pehle waale par tikaa rahe.

1. Speed $v$ — kitni tez, seedhi line mein

Picture: flight path ke saath ek arrow. Arrow ki lambai speed hai. Lamba arrow matlab clock ki har tick mein zyada distance.

Topic ko iska kya kaam: har heating formula $v$ par depend karta hai — aur khatarnaak tarike se. Jab hum section 6 mein convective heating law tak pahunchenge, hum dekhenge ki heat $v^3$ ke saath scale karti hai, yani speed double karne par heating aathguni ho jaati hai.

2. Kinetic energy aur specific energy $e_k$

$\frac{1}{2}{v}^2$ kyun aur sirf $v$ kyun nahi? Kyunki energy woh hai jo motion ko rokne mein lagti hai, aur kisi cheez ko jo do guna tez ho rokne mein chaar guna kaam lagta hai — yeh $v^2$ usi ko capture karta hai. Try karo: diwar ke khilaf do guna zyada push karna do gune zyada distance par chaar guna effort hai.

Picture: socho vehicle ko 1-kg ke cubes mein kaat diya. Har cube same $e_k$ carry karta hai. Isse hum "per kg of shield" heating ki baat kar sakte hain bina total mass jaane.

Worked number (parent se match karta hai): $v=7800$ m/s par, $e_k=\frac{1}{2}(7800{)}^2=3.042\times 10^7\text{J/kg}\approx 30\text{MJ/kg}.$ Prefix M (mega) ka matlab ek million hai, toh $30$ MJ $=3.0\times 10^7$ J. Yahi woh "terrifying" number hai: ek kilogram steel ko boil karke udaane mein lagti energy se lagbhag chaar guna.

Yeh "per kilogram" bookkeeping kahan se aati hai, uske liye dekho Specific Impulse and Energy Budgets.

3. Density $\rho$ — kitni hawa packed hai

Picture: ek metre side waala ek box. Usme hawa ke molecules gino aur unhe tolo — woh weight $\rho$ hai. Bhara hua box = high density; lagbhag khaali box = low density.

Topic ko iska kya kaam: vehicle ko jo heat feel hoti hai woh depend karta hai ki har second kitni garm gas uski taraf feki jaati hai, aur yeh $\rho$ se set hota hai. Jaise hum section 6 mein dekhenge, convective heating $\rho$ aur $v$ ko ek product $\rho v^3$ mein combine karta hai jo ek middle altitude par peak karta hai — zyada upar bahut kam hawa hai ($\rho$ chota), neeche vehicle pehle hi slow ho chuka hai ($v$ chota). Hum uss product ko theek se milte hain jab uska formula table par aa jaata hai.

4. Nose radius $R_n$ — front kitna "round" hai

Picture: neeche, ek mota rounded nose (bada $R_n$) versus ek pointy nose (chota $R_n$), dono ke saath sabse bada circle jo fit ho sake.

Topic ko iska kya kaam: stagnation-point heating $1/\sqrt{R_n}$ ke saath scale karti hai — bada, blunter nose heating ko kam karta hai. Section 6 mein hum batayenge kyun, bilkul convective-flux symbol define karne ke baad. Yeh ek fact batata hai ki reentry capsules round bowls kyun hote hain, arrows kyun nahi. Details mein hain Blunt Body Aerodynamics.

5. Bow shock aur stagnation point

Picture: nose ke saamne ek curved shock khadi hai jaise naav ke aage bow-wave. Shock aur wall ke beech chalta raheega glowing shock layer. Centreline wall se stagnation point par milti hai.

Zyada Bow Shock and Stagnation Point aur Reentry Aerothermodynamics mein.

6. Convective heat flux ${\dot{q}}_{\text{conv}}$ — wall mein aane waali heat

Picture: shield ke stamp-sized patch par ek heat lamp jalao. $\dot{q}$ yeh hai ki woh patch kitna intense pak raha hai; ${\dot{q}}_{\text{conv}}$ us cooking ka woh hissa hai jo surface ke khilaf ragadti garm gas karta hai.

7. Temperature $T$, radiative flux ${\dot{q}}_{\text{rad}}$, aur exponent-four law

Fourth power $T^4$ kyun aur sirf $T$ kyun nahi? Kyunki radiated power temperature ke saath ferociously badhti hai — yeh measure kiya gaya hai, choose nahi kiya, aur Stefan–Boltzmann law isko capture karta hai. Isliye ek glowing tile bahut badi heat waapas bahar dump kar sakti hai: $T$ double karne par radiated heat $2^4=16$ guna ho jaati hai.

Picture: rising temperature par wahi tile dull red, orange, phir white glow karti hai — aur nikaalne wale light ke arrows temperature badhne se bahut tez mote hote jaate hain.

Poori kahaani Radiative Heat Transfer and Stefan–Boltzmann Law mein hai.

8. Ablator symbols: $\dot{m}$, $Q^{\ast }$, $\dot{s}$, ${\rho }_{\text{abl}}$

Picture: char surface dheere dheere peel back ho rahi hai jaise ek dheere pighalta hua candle; $\dot{s}$ woh speed hai jis par surface peechhe hat rahi hai, $\dot{m}$ woh mass hai jo "smoke" ke roop mein nikal raha hai.

Pieces topic ko kaise feed karte hain

Har box ek symbol carry karta hai jo ab tumhara hai, text mein jaise likha waise hi: ${\dot{q}}_{\text{conv}}$ "q_conv" hai, ${\dot{q}}_{\text{rad}}$ "q_rad" hai, aur $\dot{m}$ "m dot" hai. Parent note ke formulas sirf yeh boxes ek saath wired hain.

Equipment checklist

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