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

True. Kyunki ${\dot{q}}_{\text{conv}}\propto 1/\sqrt{R_n}$, ek bada nose radius $R_n$ flux ko kam karta hai, aur woh shock ko door rakhta hai toh zyaadatar heat hawa mein jaati hai, wall mein nahi.

True. Ablator ka kala char aur kale-coated tile dono glow karte hain aur re-radiate karte hain; farq yeh hai ki ablator alag se phase change aur blowing se bhi heat remove karta hai, jabki tile sirf radiation aur insulation par depend karta hai.

False. Bahut zyada flux par equilibrium wall temperature $T_w=(\dot{q}/\epsilon \sigma {)}^{1/4}$ kisi bhi solid ke melting point se upar chali jaati hai; sirf ablation ka phase change aur blowing us energy ko dump kar sakta hai, isliye PICA wahan survive karta hai jahan tiles vaporize ho jaati hain.

False. Zyaadatar energy hawa ko heat karti hai (shock layer aur wake); sirf ek chhoti convective aur radiative fraction wall tak pahunchti hai — wahi fraction hai jo TPS manage karta hai.

True. $T_w=(\dot{q}/\epsilon \sigma {)}^{1/4}$ se, denominator mein bada $\epsilon$ $T_w$ ko kam karta hai; isliye tiles par white silica rehne ke bajaye kali high-emissivity coating lagayi jaati hai.

False. Woh sabse hot hote hain — chhota nose radius $R_n$ matlab zyada $\dot{q}$ — isliye unhe sabse zyada temperature wala reusable material chahiye, carbon–carbon jo 1600 °C se upar rated hai.

True. $\dot{s}=({\dot{q}}_{\text{net}}-\epsilon \sigma T_w^4)/({\rho }_{\text{abl}}{Q}^{\ast })$ se, ek bada $Q^{\ast }$ denominator mein hota hai, isliye recession rate $\dot{s}$ kam ho jaata hai aur kam material consume hota hai.

False. Glowing shock gas se radiative heating sirf roughly 10 km/s se upar important hoti hai; ek ~7.8 km/s LEO reentry convective heating se dominate hoti hai, jahan ${\dot{q}}_{\text{conv}}\propto v^3$.

Error physics ko ulta karta hai: chhota $R_n$ ek thin shock deta hai jo wall se chipki rehti hai aur maximum heating deta hai ($\dot{q}\propto 1/\sqrt{R_n}$). Blunt bodies thandi hoti hain, isliye capsules round hote hain.

Thickness total heat load $Q=\int \dot{q}dt$ par depend karti hai, sirf peak flux par nahi. Ek chhoti spike aur ek lamba moderate soak same peak $\dot{q}$ hit kar sakte hain lekin bahut alag material thickness maangenge.

Yeh $v^2$ nahi, $v^3$ ke saath scale karti hai: mass per energy ek $v^2$ deta hai, aur us energy ko wall tak deliver karne wala mass flux ek aur $v$ deta hai, toh $v^2\times v=v^3$.

Re-radiation ($\epsilon \sigma T_w^4$) teen mechanisms mein se sirf ek hai aur aksar high flux par sabse chhota hota hai; dominant sinks hain pyrolysis/phase change ($\dot{m}{Q}^{\ast }$) aur blowing jo boundary layer ko block karta hai.

Ye near-zero conductivity (≈94% air) ki wajah se kaam karti hain toh heat back tak pahunch nahi sakti, aur saath mein re-radiate karne ke liye high emissivity — heat store karne ki wajah se nahi. Isliye ek glowing tile ko furnace ke baad seconds mein apne edges se pakda ja sakta hai.

Mars entry comparatively moderate flux hai, isliye sasta SLA-561V iske liye matched hai; PICA sabse zyada high-flux cases ke liye reserve hai jaise deep-space aur lunar return jahan $\dot{q}\propto v^3$ bahut bada hota hai.

Shield energy destroy nahi karti balki use redirect karti hai: blowing hot gas ko wake mein door dhakelta hai, aur ablated mass apni absorbed energy vehicle se le jaata hai, toh woh energy structure mein kabhi enter hi nahi karti.

Ek blunt nose flow ko spread karta hai aur boundary layer ko motaa karta hai, jo wall ko insulate karta hai, aur shock ko door rakhta hai toh heat hawa mein dump hoti hai — dono $1/\sqrt{R_n}$ factor mein capture hote hain.

Outgassing (transpiration) physically hot shock-layer gas ko wall se door dhakelta hai, insulating layer ko motaa karta hai aur convective heat ko surface mein conduct hone se pehle block karta hai.

Kam density matlab ek diye gaye thickness ke liye halka shield, aur kyunki deep-space return mein huge $\dot{q}\propto v^3$ hota hai, tumhe ek high-$Q^{\ast }$ material chahiye jo mass budget ko bhi punish na kare — PICA dono deta hai.

High $\epsilon$ ek diye gaye temperature ke liye re-radiated power $\epsilon \sigma T_w^4$ ko maximize karta hai, jo equilibrium wall temperature ko kam karta hai aur material ko survive karne deta hai.

RCC bhaari, mahenga, aur oxidize hota hai; vehicle ke zyaadatar hisse par flux itna kam hai ki halki silica tiles ya ablators mass- aur cost-efficiency mein bahut behtar hain, isliye RCC sirf extreme leading-edge hot spots ke liye reserve hai.

Yeh roughly ek kilogram steel ko boil karne ke liye chahiye energy se char guna zyada hai, toh yeh soch ke ki agar iska ek chhota hissa bhi unmanaged structure tak pahunch jaye toh unprotected metal vaporize ho jaayega.

Woh zero hai — flow wahan completely ruk jaata hai — phir bhi yeh highest heating ka point hai, kyunki saari kinetic energy compressed gas mein thermal energy ban jaati hai wall ke bilkul paas.

Numerator ${\dot{q}}_{\text{net}}-\epsilon \sigma T_w^4\to 0$, toh recession $\dot{s}\to 0$: radiation akela load carry karta hai aur ablator essentially eroding band ho jaata hai.

Kyunki $T_w=(\dot{q}/\epsilon \sigma {)}^{1/4}$, ek chhota $\epsilon$ wall temperature badhata hai — ek local hot spot jo material limit se exceed kar sakta hai, yahi wajah hai ki coating integrity safety-critical hai.

Yeh vanish ho jaati hai, kyunki ${\dot{q}}_{\text{conv}}\propto v^3\to 0$; heating ki problem trajectory ke high-speed portion tak confined hai, final subsonic descent tak nahi.

Yeh underlying structure ko koi protection ke bina peak flux ke samne expose karta hai, jo exactly Space Shuttle Columbia Accident ka failure mode tha — ek reminder ki sabse chhote $R_n$ wali jagah ka margin sabse kam hota hai.

Zyada lambi wali, kyunki total heat load $Q=\int \dot{q}dt$ equal peak flux par bhi zyada hoti hai, aur thickness/mass $Q$ ko track karta hai, sirf peak ko nahi.