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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
Shear wave
Expansion waves
Vehicle Body
Optical Components
Boundary layer
Vehicle Body
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
6(𝑀𝑠𝑖𝑛ϴ)2
=
Ρ∞
(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
Subsonic Region
Optical
Seeker
Blunt Body Shock Structure
Shock standoff distance
3.24
𝑑𝑜 = 0.143𝑅 ∗ 𝑒
2
𝑀∞
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:
𝑇2
2 − 1)(𝑀 2 + 5) / 36𝑀 2
=
(7𝑀
∞
∞
∞
𝑇∞
• 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