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

3.4.22 · Physics › Rocket Flight Mechanics

TPS ki zaroorat kyun hai?

Energy budget (WHY numbers scary hain). Low Earth orbit ke liye, orbital speed hai $v_{orb} = \frac{GM}{R} \approx 7.8 km/s .$ Specific kinetic energy: $e_{k} = \frac{1}{2} v^{2} = \frac{1}{2} (7800)^{2} \approx 3.0 \times 1 0^{7} J/kg = 30 MJ/kg .$

Heat actually wall tak pahunchti kaise hai?

Do heat-transfer channels dominate karte hain:

Convective heating $\overset{q}{˙}_{conv}$ — hot shock-layer gas wall mein conduct/convect karta hai.
Radiative heating $\overset{q}{˙}_{rad}$ — bahut zyada speed par (lunar/Mars return, >10 km/s) shock gas glows karta hai aur radiate karta hai.

Ablator strategy — "maro taaki bacho"

HOW ek ablator heat shed karta hai (teen simultaneous mechanisms):

Pyrolysis / phase change — chemical bonds toot te hain, latent heat absorb karte hain.
Blowing (blockage) — outgassing hot boundary layer ko door push karta hai.
Re-radiation — black char surface $σ T^{4}$ wapas radiate karta hai.

PICA vs SLA (do famous ablators)

Reusable strategy — "bina mare bachna"

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

TPS choose karna (the 80/20 decision)

Environment	Best choice	Kyun
Bahut high $\overset{q}{˙}$ , single-use (deep space return)	PICA ablator	high $Q^{*}$ , low density
Moderate $\overset{q}{˙}$ , single-use (Mars entry)	SLA-561V	sasta, matched $Q^{*}$
Moderate $\overset{q}{˙}$ , reusable (Shuttle belly)	Silica tiles	re-radiate karte hain, koi mass loss nahi
Extreme localized $\overset{q}{˙}$ (nose/leading edge)	RCC	sabse high survive temperature

Feynman check

Recall Ek 12 saal ke bacche ko explain karo

Jab ek spaceship bahut tezi se Earth par wapas girti hai, toh uske aage ki hawa itni zyada dabayi jaati hai ki woh lava se bhi zyada hot ho jaati hai. Spaceship ko ek heat shield chahiye warna woh jal jaayegi. Iske do tricks hain: (1) ek shield jo aise material se bani hai jo dhire dhire jaan-bujhkar jal jaata hai, apne saath heat le jaata hai jo smoke mein nikalta hai — jaise sweat evaporate hokar tumhe cool karti hai (yeh PICA aur SLA hai). Ya (2) ek shield jo special fluffy tiles ya carbon se bani hai jo laal-hot ho jaati hai aur heat ko space mein wapas glows kar deti hai bina melte, taaki tum isse baar baar use kar sako (Shuttle tiles aur RCC). Round noses isliye use ki jaati hain kyunki woh hot air ko door push karti hain iske bajaye ki usse chipkaayein.

Flashcards

TPS ke do fundamental philosophies kya hain?

Ablate (mass qurban karo, heat absorb karo pyrolysis+blowing+radiation se) vs survive (radiative/insulating tiles & RCC, reusable).

Sutton–Graves stagnation heating relation likho.

\overset{q}{˙}_{conv} = k ρ / R_{n} v^{3}

Reentry capsules blunt kyun hoti hain?

\overset{q}{˙} \propto 1/ R_{n}

, isliye bada nose radius heating kam karta hai aur shock ko door rakhta hai, heat hawaon mein dump karta hai.

LEO reentry ke liye specific kinetic energy approximately kitni hoti hai?

\frac{1}{2} (7.8 km/s)^{2} \approx 30

MJ/kg.

Kaun se teen mechanisms se ek ablator heat shed karta hai?

Pyrolysis/phase change (latent heat absorb karta hai), blowing/blockage of the boundary layer, aur char se re-radiation.

PICA kya hai aur kahan use hota hai?

Phenolic Impregnated Carbon Ablator, low density (~0.27 g/cm³), high heat flux — Stardust, Dragon (PICA-X).

SLA-561V kya hai aur uska niche kya hai?

Super Light-weight Ablator (cork/silicone honeycomb), lower heat-flux, sasta — Mars landers (Viking, MSL).

Shuttle silica tiles kisse bane hain aur kaise kaam karte hain?

~94% air silica fibers, bahut low conductivity + high-emissivity black coating; heat re-radiate karte hain thande rehte hue, reusable.

RCC kahan use hota hai aur kyun?

Nose cap aur wing leading edges par (sabse chhota

R_{n}

→ sabse zyada heating); yeh carbon-carbon hai, >1600 °C mein survive karta hai jahan tiles nahi kar sakti.

Radiative equilibrium wall temperature do.

T_{w} = (\overset{q}{˙}_{conv} / ε σ)^{1/4}

; zyada

ε

matlab kam

T_{w}

Ablator recession-rate equation likho.

\overset{s}{˙} = (\overset{q}{˙}_{net} - ε σ T_{w}^{4}) / (ρ_{abl} Q^{*})

; high

Q^{*}

→ slow recession.

Reusable tiles ablators ki jagah har jagah kyun nahi le sakti?

Bahut high

\overset{q}{˙}

par, equilibrium

T_{w}

kisi bhi solid ke melting point se zyada ho jaata hai; ablation energy mass loss ke through remove karta hai jo koi solid survive nahi kar sakta.

Connections

Reentry Aerothermodynamics
Bow Shock and Stagnation Point
Radiative Heat Transfer and Stefan–Boltzmann Law
Blunt Body Aerodynamics
Space Shuttle Columbia Accident
Mars Entry Descent and Landing
Specific Impulse and Energy Budgets