5.4.7
Jab ek spacecraft atmosphere mein ~7–11 km/s ki speed se re-enter karti hai, toh uske aage ki hawa hat nahi paati. Woh compress hokar ek shock layer banati hai jo thousands of kelvin tak pahunch jaati hai. Aap ise ek passive blanket se hamesha ke liye "insulate" nahi kar sakte — heat eventually aaar-paar ho jaayegi. Toh engineers ek jugaad lagate hain: woh ek aisi heat shield banate hain jo ek controlled tarike se khud ko destroy karne ke liye design ki gayi ho . Material apna khud ka mass sacrifice karta hai taaki heat door ho sake. Yeh ek ablative (Latin ablatus , "carried away") thermal protection system (TPS) hai.
Intuition Shield jalane se vehicle kyunthanda hota hai
Ek saath teen tricks hoti hain, aur har ek energy churaati hai iससे pehle ki woh thandi structure tak pahunche:
Endothermic pyrolysis — polymer (phenolic resin) decompose hota hai, bonds todne ke liye heat absorb karta hai.Pyrolysis gas injection (transpiration/blowing) — produce hue gases char ke through bahar ki taraf flow karte hain, hot boundary layer ke against push karte hain aur use thicker banate hain, toh andar kam heat conduct hoti hai.Char layer formation — ek porous carbon char heat ko wापas space mein re-radiate karta hai (∝ T 4
Ablation surface material ka removal hai — pyrolysis, melting/sublimation, chemical reaction, aur mechanical erosion ke combination se — jahan mass loss thermal energy ko saath le jaata hai aur underlying structure ko protect karta hai.
WHAT we want: ek aisa single number jo hume bataye ki shield ka har kilogram kitni heat absorb kar sakta hai.
Surface par time d t q ˙ in m ˙ ′′ Effective heat of ablation Q ∗ m ˙ ′′ Q ∗
Jo energy ablation se nahi hati, woh re-radiated hoti hai ya conduct hoti hai:
q ˙ in = carried away by mass loss m ˙ ′′ Q ∗ + re-radiated ε σ T w 4 + into structure q ˙ co n d Yeh step kyun? Wall par energy conservation: jo aata hai woh store, radiate, conduct, ya jaate hue mass ke saath le jaaya jaana chahiye. Protective benefit ke liye solve karna:
m ˙ ′′ = Q ∗ q ˙ in − ε σ T w 4 − q ˙ co n d
Worked example Kitni moti layer lost hoti hai?Maano q ˙ n e t = 2 MW/m 2 t = 100 Q ∗ = 20 MJ/kg ρ = 270 kg/m 3
Mass lost per area: m ′′ = q ˙ t / Q ∗ = ( 2 × 1 0 6 × 100 ) / ( 20 × 1 0 6 ) = 10 kg/m 2 Kyun? total absorbed energy ÷ energy per kg.
Recession depth: s = m ′′ / ρ = 10/270 ≈ 0.037 m = 3.7 cm Kyun? mass/area ÷ density = thickness.
Toh aap shield ko 3.7 cm se zyaada mota design karoge plus ek insulating margin.
Intuition Phenolic resin workhorse kyun haiPhenol–formaldehyde (phenolic) resin ek cross-linked aromatic network hai. Garm karne par yeh melt hokar behta nahi — yeh char karta hai, ek high-carbon skeleton peeche chhod jaata hai. Zyaada char yield (~50–60%) exactly yahi chahiye: zyaatar resin bekar boil off hone ke bajaaye protective carbon mein convert hoti hai.
Pyrolysis (simplified):
Phenolic resin > 300 – 800 ∘ C C (char) + pyrolysis gases (blowing) H 2 O , CO , CO 2 , CH 4 , H 2 Yeh halke gases bahar ki taraf percolate karte hain — transpiration cooling . Bahut zyaada T C + 2 1 O 2 → CO C ( s ) → C ( g )
Definition PICA — Phenolic-Impregnated Carbon AblatorEk low-density carbon fiber preform (FiberForm) jo phenolic resin se impregnated hai . Density ~270 kg/m³ (bahut halka). NASA Ames mein develop kiya gaya. Stardust (sabse tez Earth re-entry ever, ~12.9 km/s) aur SpaceX Dragon (PICA-X variant) par use kiya gaya.
WHY accha hai: rigid carbon skeleton resin pyrolyze hone ke baad bhi survive karta hai → apni shape rakhta hai; extremely light → kam mass penalty; high Q ∗
Form: machined tiles , gap-filled. High-heat-flux, lower-pressure regimes ke liye sabse accha.
Ek epoxy–novolac (phenolic-type) resin silica fibers aur phenolic micro-balloons se filled , fiberglass–phenolic honeycomb matrix mein inject kiya gaya. Density ~500 kg/m³ . Apollo aur phir Orion (lunar-return shield) par use kiya gaya.
WHY honeycomb: honeycomb cells individually fill (gun) ki jaati hain → mechanical robustness aur bade, curved heat shields par accha charring behavior. Glass/silica ek glassy, viscous melt layer deta hai jo shear resist bhi karta hai.
Definition SLA — Super Light-weight Ablator (SLA-561V)Ek silicone (RTV) elastomer matrix cork, silica/phenolic microspheres se filled , phenolic-glass honeycomb mein pack kiya gaya. Density ~260 kg/m³ . Mars Viking, Pathfinder, MER, MSL entries par use kiya gaya.
WHY Mars ke liye: Mars heating moderate flux hai par significant aeroshell area & duration ke saath ; SLA ultralight aur flexible hai → CO₂ atmospheres mein great mass efficiency. (SLA-561V MSL par famously ek usage limit rakhi thi kyunki high shear ise waqt se pehle spall kar sakta hai.)
Mnemonic Kaun sa kaun sa hai yaad rakhne ke liye"PICA is Pure-Carbon, AVCOAT is Apollo-honeycomb, SLA is Silicone-Light-for-Areography (Mars)."
Density order (kg/m 3 SLA ≈ PICA (~260–270) < AVCOAT (~500) . Mnemonic: "Mars aur comets travel light karte hain; Moon ki ride zyaada bhaari hai."
Material
Matrix / filler
Density (kg/m³)
Flagship mission
PICA
Carbon-fiber preform + phenolic
~270
Stardust, Dragon
AVCOAT
Epoxy-novolac + silica + honeycomb
~500
Apollo, Orion
SLA-561V
Silicone + cork + microspheres + honeycomb
~260
Viking, MSL
Intuition Ek trade-off jo yaad rakhna chahiyeAblative TPS = single-use lekin extreme heat handle karta hai (deep-space return, ~MW/m²). Reusable TPS (Shuttle silica tiles, RCC) = kaafi flights survive karta hai lekin sirf lower LEO-return heating ke liye. Woh 20% jo 80% explain karta hai: agar heat flux brutal hai aur mission one-shot hai, ablate karo; agar heating mild hai aur tum dobara fly karo, reusably insulate karo.
Common mistake "Char layer ko fresh material expose karne ke liye hata dena chahiye."Kyun sahi lagta hai: char "jalaa hua" hai, toh clearly hum chahte hain ki clean material cooling kare.
Fix: Char sabse valuable part hai — yeh ek porous refractory carbon hai jo ε σ T 4 bura hai kyunki yeh char ko hatata hai iससे pehle ki woh apna radiating/insulating kaam kar sake.
Common mistake "Zyaada density wala material better protect karta hai."Kyun sahi lagta hai: zyaada mass = zyaada thermal capacity, intuitively tougher.
Fix: Jo matter karta hai woh hai Q ∗ s = m ′′ / ρ low density ka hai phir bhi superb hai kyunki uska Q ∗ Q ∗
Common mistake "Ablation = simple burning/combustion."Kyun sahi lagta hai: heat aur gas hai, aag jaisi lagti hai.
Fix: Dominant process endothermic pyrolysis + transpiration cooling hai, jo energy absorb karte hain. Oxidation secondary hai aur kabhi kabhi undesirable hoti hai (yeh protective char consume karti hai). Ablation mostly cooling hai, combustion nahi.
Common mistake "Mars par PICA use karo aur lunar return par SLA."Kyun sahi lagta hai: yeh sab ablators hain, freely swap karo.
Fix: Material ko heating regime se match karo. PICA → high flux / low pressure (deep space return). SLA → low flux, large area, mass-critical (Mars). AVCOAT → high integral load over big curved shields (Apollo/Orion). Galat choice → spallation ya excess mass.
Recall 12-saal ke bachche ko explain karoSocho tum ek darwaaze se guzar rahe ho jo aag mein hai. Metal suit pehanne ke bajaaye jo dheere dheere itni garam ho jaaye ki tum pak jao, tum marshmallow-foam ka coat pehante ho. Jab tum se guzarte ho, coat ka bahar wala hissa jalta hai aur smoke phoolta hai . Coat ko jalana aag ki heat kha jaata hai , aur smoke bahar ki taraf dhakelta hai aur lahon ko tumse door karta hai. Jab tak tum paar ho jaate ho, coat patla aur crispy ho jaata hai — lekin tum thande rahe. Yahi ek ablative heat shield hai: yeh tumhe ek smart, controlled tarike se khud ko destroy karke protect karta hai. PICA super-light carbon coat hai (sabse tez, sabse hot trips ke liye), AVCOAT tough honeycomb coat hai (Moon coat), aur SLA feather-light rubber-aur-cork coat hai (Mars coat).
Heat shield ke liye "ablation" ka matlab kya hai? Surface material ka controlled removal (pyrolysis, char loss, sublimation) jiska mass loss thermal energy ko saath le jaata hai, structure ko protect karta hai.
Ablation mein teen energy-removal mechanisms ke naam batao. Endothermic pyrolysis, pyrolysis-gas injection (transpiration/blowing), aur char re-radiation/insulation.
Q ∗ Effective heat of ablation (J/kg); bada Q ∗
Surface energy balance likho. q ˙ in = m ˙ ′′ Q ∗ + ε σ T w 4 + q ˙ co n d
PICA ka full form kya hai aur uski density kya hai? Phenolic-Impregnated Carbon Ablator; ~270 kg/m³.
PICA kis missions mein use kiya gaya? Stardust (record-fast re-entry) aur SpaceX Dragon (PICA-X).
AVCOAT ki structure kaisi hai aur key mission kaun sa hai? Epoxy-novolac/silica filler fiberglass-phenolic honeycomb mein gun kiya gaya; Apollo aur Orion.
SLA-561V kis cheez se bana hai aur kahan use kiya gaya? Silicone elastomer + cork + microspheres phenolic-glass honeycomb mein; Mars (Viking, MSL).
Phenolic resin kyun use ki jaati hai? High char yield (~50–60%) — yeh melt hone ke bajaaye char karta hai, protective carbon plus blowing gases chhod jaata hai.
Char layer valuable kyun hai, waste nahi? Yeh ek porous refractory carbon hai jo ε σ T 4
Ablative vs reusable TPS — kab kaun sa? Ablative extreme, one-shot heating ke liye (deep-space return); reusable (silica tiles/RCC) milder repeated LEO returns ke liye.
Recession depth kya hai aur uska formula kya hai? Material lost ki thickness: s = ∫ ( m ˙ ′′ / ρ ) d t = m ′′ / ρ
Thermal Protection Systems (TPS) Re-entry Aerodynamics & Shock Heating Phenol-Formaldehyde (Phenolic) Resins Stefan–Boltzmann Radiation Law Pyrolysis & Char Yield Carbon-Carbon Composites & RCC Mars Entry Descent Landing (EDL) Apollo & Orion Heat Shields
absorbs heat breaking bonds
Atmospheric re-entry 7-11 km/s
Ablative TPS designed to self-destruct
Ablation removes surface material
Endothermic pyrolysis of phenolic resin
Pyrolysis gas injection / blowing
Energy stolen before reaching structure
Effective heat of ablation Q*
Recession depth s = integral m''/rho
