Lesson 22 Flashcards#
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1. What are the five components of an aircraft IR signature?
Hot parts (nozzle/turbine), exhaust plume, airframe skin emission, reflected sunlight, and reflected sky/earthshine — three emitters and two reflectors.
2. In which band does each of the three emitters dominate?
Hot parts and the plume dominate in the MWIR (3–5 µm); the airframe skin dominates in the LWIR (8–12 µm).
3. State the in-band intensity ranking rule and name each factor.
\(I_{\text{band}} \propto \varepsilon \cdot A \cdot f_{\text{band}}(T) \cdot \sigma T^{4}\): \(\varepsilon\) emissivity (radiating efficiency), \(A\) projected area seen (where aspect enters), \(f_{\text{band}}(T)\) band fraction (set by Wien), and \(\sigma T^{4}\) total output that explodes with temperature.
4. Which factor lets a small hot nozzle outshine the large cool skin per unit area?
The \(\sigma T^{4}\) term — radiated power scales with the fourth power of absolute temperature, so temperature dominates over area.
5. Roughly what temperature do hot parts run, and what band do they peak in?
About 700–900 K, peaking in the MWIR near 3–4 µm. Small area, but the \(T^{4}\) term makes them brilliant.
6. At what wavelength does the exhaust plume radiate, and why?
Near 4.3 µm in the MWIR — the CO₂ emission band of the exhaust gas (shifted by Doppler and pressure). The ~800 K continuum peak sits around 3.6 µm.
7. Why does the airframe skin set the all-aspect signature?
It radiates at near-ambient temperature (up to ~330 K subsonic) in the LWIR over a large area and is visible from every aspect, so it carries the beam and head-on signature. You cannot point it away.
8. When and in which band does reflected sunlight matter?
During the day in the MWIR, where it can rival a ~300 K graybody with \(\varepsilon \approx 0.5\); in the LWIR it is negligible against the surface's own thermal emission.
9. Why is an aircraft a bright MWIR target from the stern but weaker from the beam?
From astern the sensor looks up the tailpipe at the exposed hot nozzle and plume; from the beam those are masked and only the skin (LWIR) remains. Projected area \(A\) toward the sensor changes with aspect.
10. How does look-up versus look-down change detectability?
Look-up places the target on cold sky for high contrast and easy detection; look-down places it on warm cluttered terrain for low contrast and hard detection (the penetrator's friend).
11. What is thermal crossover?
The moments — about an hour after sunrise and after sunset — when skin and background temperatures become equal, so the LWIR contrast \(\Delta T\) goes to zero and the airframe blends into its background.
12. Why is midday negative contrast and night positive contrast in the LWIR?
Midday the sun-baked terrain is hotter than the skin, so the target reads cold (negative \(\Delta T\)); at night the terrain radiates to the cold sky and drops below the skin, so the target reads warm (positive \(\Delta T\)).
13. If you time an ingress at thermal crossover, are you invisible in every band?
No — crossover only zeroes the LWIR skin contrast. The plume and hot parts still shout in the MWIR. You cannot disappear in every band at once.
14. In the demo diurnal model, what are the two crossover times and what do they correspond to?
About 7.5 h and 19.5 h — roughly one hour after sunrise and one hour after sunset.
15. Name one suppression counter for each managed component.
Hot parts: shield the nozzle / avoid afterburner. Plume: mix with bypass air to cool it / route over the airframe. Skin: low-emissivity coatings. Reflections: manage geometry and finish to kill sunglint. For an LO bomber like the B-21 these are baked into the airframe (Lesson 24 / IRCM).