Hypersonic Flight Effect on Optical Sensors Matt Salem OPTI 521 3rd Dec. 2016

Background Hypersonic Flight is currently receiving a lot of attention from NASA and the military. Applications include: Space Shuttles High Speed Aircraft Guided Missiles A variety of sensors are needed for guided flight

The Hypersonic Environment Hypersonic Flight presents extremely difficult and unique environmental conditions that affect the performance of on-board optical sensors. Some of the main concerns are: Air compression Boundary Layers Aerodynamic shockwaves Aerodynamic flow around the vehicle Thermal heating effects Turbulence

Effect on the Optical System These effects can significantly degrade sensor performance if not accurately accounted for Blur and defocus Boresight error Secondary radiation Noise

What is going on? An extreme, highly complicated, hard to predict environment requiring a multidisciplinary team of fluid dynamics, Heat transfer, optics and material science experts Highly Compressed & Heated Air with sharp density gradient Shock wave Interaction Boundary layer Shear wave Shock wave from forward vehicle body Expansion waves Vehicle Body Shock wave from window Optical Window Vehicle Body Optical Components

Shock Waves Shock waves are discontinuities in the supersonic flow of the air. They cause abrupt changes in the density and temperature of the air downstream of the shock. Air compression causes a change in the index (acts like a lens)

Shock Waves Gladstone-Dale Relationship: n-1 = βρ Where β is a wavelength dependent constant (~2.2 x 10-4 m3/kg) And ρ is the mass density of the air Air compression is a function of speed and vehicle structure: ρ2 Ρ∞ = 6(𝑀𝑠𝑖𝑛ϴ ) 2 (𝑀𝑠𝑖𝑛ϴ ) 2 +5 Where ρ2 is the density behind the shock, Ρ∞ is the free-stream (unperturbed) density ϴ is the shock angle And M is the Mach number

Expansion Waves Expansion waves are created by the interaction of multiple shock waves and convex corners They cause a continuous change in the supersonic flow of the air. Density, pressure and temperature ratios decrease through the expansion wave while the Mach number increases Act like gradient index lenses

Boundary Layer Viscosity, thermal conductivity and mass diffusion effects dominate the flow behavior Roughly constant pressure across the boundary layer Boundary Layer

Boundary Layer Density of a flat plate Maximum heat transfer to wall No heat transfer to wall

Effect on Sensors The variation in the index of the air as well as temperature related effects cause a slight focus shift which results in some blurring (generally small compared to the diffraction limit) as well as refraction which results in boresight error. This boresight error can be significant for precise targeting systems

Blunt Body Shock Structure A hypersonic window, particularly mounted in the nose of a vehicle, may be in the form of a blunt body The shock wave is slightly detached from the blunt body A complete flow field and shock-structure depens on body geometry and flight speed. Such solutions are very complex because of the mixed subsonic and supersonic regions Sonic line Supersonic region Optical Seeker Subsonic Region

Blunt Body Shock Structure Shock standoff distance 𝑑 𝑜 =0.143𝑅∗ 𝑒 3.24 𝑀 ∞ 2 Where do is the shock standoff distance And R is the nose radius

Index Variation in front of a blunt body (boresight)

Temperature Effects The compressed air behind a shock wave is very hot For a blunt body: 𝑇 2 𝑇 ∞ =(7 𝑀 ∞ 2 −1)( 𝑀 ∞ 2 + 5) / 36 𝑀 ∞ 2 The Carbon dioxide and water in the air causes it to radiate in the IR Although the thickness of the compressed layer is small the spectral radiance much higher than normal scene background As the optical window heats it will also radiate. This can occur at levels higher than the background scene or the compressed gas behind the shock

Temperature Effects

Temperature Effects The change in the index of the window (dn/dT) and the index of the air due to the density gradient causes a focus shift which results in a slight blurring of the image Generally this blurring is small compared to the optical systems diffraction limited resolution

Temperature Effects

Temperature Effects The bigger concern is related to the thermal expansion of the window. The shock causes a significant temperature gradient on the window This causes reduced resolution and boresight error The larger the aperture the larger the effect

Temperature Effects

Temperature Effects

Window Materials for Hypersonics

Active Window Cooling Techniques Coolant tubes and thin film cooling has been demonstrated as a means of mantaining window temperature. If active cooling is used the optical effects of the coolant must then be considered. The optical effects of the coolant typically cause optical distortions greater than the hypersonic flow alone

Sources http://www.jhuapl.edu/techdigest/views/pdfs/V08_N4_1987/V8_N4 _1987_Tropf.pdf https://www.rt.com/news/211575-china-hypersonic-missile-test/ https://en.wikipedia.org/wiki/Oblique_shock https://en.wikipedia.org/wiki/Shock_wave https://en.wikipedia.org/wiki/Prandtl%E2%80%93Meyer_expansion_ fan