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

2. 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

3. 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

4. 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

5. 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

6. 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)

7. 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

8. 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

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

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

11. 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

12. 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

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

14. Index Variation in front of a blunt body (boresight)

15. Temperature Effects The compressed air behind a shock wave is very hot
For a blunt body:
𝑇 2 𝑇 ∞ =(7 𝑀 ∞ 2 −1)( 𝑀 ∞ ) / 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

16. Temperature Effects

17. 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

18. Temperature Effects

19. 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

20. Temperature Effects

21. Temperature Effects

22. Window Materials for Hypersonics

23. 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

24. Sources
_1987_Tropf.pdf
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