Drag Reduction Methods for Reusable Spacecraft

1
Drag Reduction Methods for Reusable Spacecraft

• Jonathan W. Naughton
• Assistant Professor

2
Acknowledgements

• This work has been supported by NASA Grant
  NAG4-208 and NAG4-167
• Technical Monitor Stephen Tony Whitmore
• This work is the result of efforts by several
  individuals
• Tony Whitmore NASA-Dryden
• Stephanie Sprague University of Kansas
• Weixia Li University of Wyoming
• Robert Decker University of Wyoming

3
Overview

• Motivation and Background
• Base Drag
• What is it?
• Why is it important to reentry vehicles?
• How can it be reduced?
• Objectives for the Current Study
• Current Results
• Base Drag Model Tested at NASA-Dryden Research
  Center
• Effects of Roughness on Turbulent Boundary Layer
  with Favorable Pressure Gradient AIAA Paper
• Upcoming Tests
• What is the Future?

4
Drag

• Several Types of Drag Act on Flight Vehicles
• Simplest case
• Pressure drag (form drag)
• Fore-body
• Base
• Viscous drag
• Fore-body
• Total drag

5
Base DragWhat is it?

• The Boundary Layer on a Vehicle with a Base Area
  Separates
• A Low Pressure Separated Region Forms
• The Low Pressure Causes a Large net Pressure
  Difference
• Drag
• Momentum Deficit

6
Base DragWhy is it a Problem on Launch Vehicles

• Large Base Area to Accommodate Engines
• High drag (D)
• Lifting Bodies Have Relatively Low Lift (L)
• Performance Parameter L/D is Low

7
Why is L/D Important?

• L/D Drives the Unpowered Flight Characteristics
• Glide slope
• Cross range
• Down range
• Landing approach angle
• Rate of descent

8
Base Drag Reduction

• Any Small Decrease in Drag can Increase
  Performance Dramatically
• There is Hope!
• Reduce communication between external flow and
  base area

9
Evidence of Base Drag Reduction

• Base Drag Reduction by Adding Viscous Fore-Body
  Drag
• Early experimental work demonstrated principle
• Subsonic
• Recent experimental work confirmed base drag
  reduction
• Subsonic, transonic, and supersonic!

10
Evidence of Base Drag Reduction

• This Result Leads to the Concept of a Drag
  Bucket
• A drag minimum controlled by addition of
  fore-body drag

11
Overall Objective

• Demonstrate a Base Drag Reduction System
  Applicable to Reuseable Launch Vehicles

12
Specific Objectives

• Establish the Viscous Drag Base Drag
  Relationship Under Controlled Conditions
• Establish CD,base vs. CD,fore-body at various Re
• Low Re model at NASA-Dryden
• Re 123x103 and Re225x103
• Moderate Re model at UWAL
• Re up to 2.5x106
• Manipulate boundary layer using roughness
• Effect on turbulent boundary layer
• Roughness
• Effect of riblets
• Favorable pressure gradient

13
Boundary Layer Manipulation Using Roughness

• To Increase Fore-body Drag
• Add roughness
• Properties of roughness known from previous
  research
• Increases boundary layer thickness
• Increases skin friction coefficient Cf

• Characterizing Roughness
• Roughness type
• d-Type protruding
• k-type cavity
• Roughness height
• k geometrical height
• Rek- roughness Re
• Others
• Roughness density
• ?s ratio of total surface area to roughness
  area
• ?k- ratio of total surface area to roughness
  frontal area

14
Presentation of Results

• NASA-Dryden Base Drag Experiment
• Model Instrumentation
• Results
• UWAL Boundary Layer Study
• Model Instrumentation
• Results

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NASA-Dryden Base Drag Experiment
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NASA-Dryden Base Drag TestModel

• Two Dimensional Model
• Spans test section
• Instrumented with pressure taps
• Removable plates
• Micro-machined overlays create variable roughness

17
NASA-Dryden Base Drag TestInstrumentation

• Pressure Instrumentation
• Static pressure taps on the model
• Traversing Pitot-static wake probe
• Traversing boundary layer Pitot probe

18
NASA-Dryden Base Drag TestAnalysis

• Complex Analysis Performed on the Data
• Details provided in AIAA 2001-0252 (Whitmore et
  al.)
• Non-linear curve fit of the wake measurements
• Momentum thickness determined
• Total drag coefficient determined
• Law of the wake curve fit of the boundary layer
  measurements
• Local Cf calculated
• Integrated skin friction coefficient CF
  determined
• Fore-body pressure curve fit
• Integrated fore-body pressure drag coefficient
  Cd,fore-body determined
• Base pressure curve fit
• Base drag coefficient Cd,base determined

19
NASA-Dryden Base Drag TestResults

• Base Pressure
• Increases with Increasing Roughness
• Decreases for Parallel Grid (Riblets)

20
NASA-Dryden Base Drag TestResults

• Results are the Same for Higher Re Case

Offset from Centerline of Model, y (cm.)
21
NASA-Dryden Base Drag TestResults

• Base Drag Coefficient Decreases with Increasing
  Roughness

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NASA-Dryden Base Drag TestResults

• A Drag Bucket is Observed

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NASA-Dryden Base Drag TestSummary

• Pressure Coefficient on Base Decreases with
  Increasing Roughness
• Results consistent at both Re 1.23x106 and
  2.25x106
• Base Drag Coefficient Decreases with Increasing
  Roughness
• 40 base drag reduction observed between smooth
  surface and coarsest roughness
• Drag Bucket has been Identified
• Initial addition of viscous fore-body drag
  decreases overall drag
• Further addition appears to have little effect

24
UWAL Boundary Layer Study
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UWAL Boundary Layer StudyMotivation

• Why study turbulent boundary layers???
• Boundary layers with dp/dx ? 0
• Widely studied, especially dp/dx gt 0
• Boundary layers with roughness
• Widely studied
• Combination of roughness and dp/dx ? 0 not widely
  studied

26
UWAL Boundary Layer StudyFavorable Pressure
Gradient Effects

• Characterized by Several Parameters
• Clausers equilibrium parameter
• Characteristics of Turbulent Boundary Layers in a
  Favorable Pressure Gradient
• Wake component is smaller
• Thickness is smaller
• Growth rate is lower
• High shear stresses occur
• High velocity gradients
• Relaminarization can occur

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UWAL Boundary Layer Study 2 x 2 Low-Speed Wind
Tunnel

• Unique characteristics
• Reynolds Number
• Max 3 x 106
• Free-stream velocity
• Variable
• Frequency Drive
• 1050 m/s
• Programmable
• New test section
• 4-side access
• Ideal for optical diagnostics

28
UWAL Boundary Layer Study Flat Plate Model

• 1.2 m x 0.61m
• 31 Elliptical Leading Edge
• 4 Interchangeable Inserts
• Aluminum Plates
• Pressure Tap Plate
• Polished Stainless Plate

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UWAL Boundary Layer Study Ramp Model

• 0.76m x 0.61 m
• 31 Elliptical Leading Edge
• 4 Interchangeable Inserts
• Aluminum Plates
• Pressure Tap Plate
• Polished Stainless Plate
• 3 and 5 Ramps
• Low to Moderate Pressure Gradients Created

• Instrumented Base Area
• 24 Pressure Taps

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UWAL Boundary Layer StudyRoughness

• 3 Levels of Sand-Grain Roughness Used
• Calibrated silica sand grains used
• Grain size distribution narrowed by screens
• Applied to aluminum plates using adhesive
• Testing flexibility

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UWAL Boundary Layer Study Instrumentation

• Hot-Wire Anemometry
• Boundary layer surveys
• Typically 51 points
• Points concentrated at the surface
• 512k points sampled at 10 kHz

32
UWAL Boundary Layer Study Instrumentation

• Oil Film Interferometry
• Cf measurements on smooth surfaces
• Very high spatial resolution
• Non-Intrusive measurement

33
UWAL Boundary Layer Study Thin-Oil-Film Theory

• Thin-Oil-Film Equation
• Squires equation (1962)
• Verified traditional oil-flow techniques
• Oil does follow surface streamlines
• Tanners equation (1977)
• Measure h, determine ?
• Use interferometry to determine h

34
UWAL Boundary Layer Study Oil-Film
Interferometry Theory

• Amplitude Splitting or Fizeau Interferometry
• N/2 ? ?constructive interference
• (2N1)/4 ? ?destructive interference
• As Oil Thins, Interference Pattern Moves

35
UWAL Boundary Layer Study OFI Image-Based
Techniques

• CCD Array for Sensor
• Acquire images during run
• I(x,z,t)
• Requires optical access
• Acquire image after run
• I(x,z)
• Does not require optical access
• Illumination Source
• Extended monochromatic source
• Oil Application
• Drops or lines

36
UWAL Boundary Layer StudyOil-Drop and Oil Film
Approaches
Fringes
Flow ?
Oil Film
Cylinder
tt2
tt1
tt3
tt4
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UWAL Boundary Layer StudyTest Cases
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UWAL Boundary Layer StudyResults Boundary
Layer Surveys

• Pressure Gradient
• Flat plate has a large region with dp/dx 0
• Ramps have a large region with dp/dxconst

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UWAL Boundary Layer StudyResults Boundary
Layer Surveys

• All Boundary Layer Data Presented in Wall
  Coordinates

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UWAL Boundary Layer StudyResults Boundary
Layer Surveys

• Flat Plate Results
• Profiles Collapse
• Fully-Developed
• ?0
• Profile shifts downward and to the right
• Even smallest sand effect the boundary layer

41
UWAL Boundary Layer StudyResults Boundary
Layer Surveys

• Zero and Favorable Pressure Gradient Comparison
• Profiles collapse except in wake
• Agrees with previous findings
• 5 case missing
• More later

42
UWAL Boundary Layer StudyResults Boundary
Layer Surveys

• 3 Ramp
• 4 Roughness Levels
• Smooth
• Self-similar
• k0.13 mm
• k0.40 mm
• k1.09 mm
• Self-similar
• Largest Roughness Collapses
• Fully rough
• Slow change of k/?

• Smallest Roughness has a Large Effect
• Relative Roughness Increases

43
UWAL Boundary Layer StudyResults Boundary
Layer Surveys

• 3 Ramp at 1 location
• Smooth case unchanged
• Effect of roughness is much larger
• Smallest roughness
• significant effect
• Largest Roughness
• Shifted downward significantly
• Conclusion
• Favorable pressure gradient significantly
  enhances roughness effect

44
UWAL Boundary Layer StudyResults Boundary
Layer Surveys

• 5? smooth ramp case
• Boundary layer
• not a turbulent boundary profile
• Approaches a laminar profile
• Boundary layer relaminarization
• In favorable pressure gradient
• Transition delayed
• Transition occurs, then BL relaminarizes

45
UWAL Boundary Layer StudyResults Integral
Parameters

• Displacement Thickness
• Roughness has a large effect
• Increases ?
• Pressure gradient has a noticeable but secondary
  effect
• Decreases ?

3? Ramp Case
46
UWAL Boundary Layer Study Results Shear Stress
Flat Plate

• Skin Friction Coefficient
• law-of-the-wake fits
• Cf increases with
• Increasing roughness
• Increasing dp/dx
• Large increase in Cf with pressure gradient
• Factor of 5-6 increase

3? Ramp
47
UWAL Boundary Layer StudyResults Shear Stress

• Skin Friction Coefficient
• Trend continues for 5? ramp

3? Ramp
5? Ramp
48
UWAL Boundary Layer StudyResults Shear Stress

• Oil Film Interferometry Results
• Law-of-the-wall curve fits
• Overpredicting near LE
• BL is not fully turbulent
• Oil-Film Interferometry
• Large number of data points
• Considerably less test time
• Very good accuracy
• Average to reduce

49
UWAL Boundary Layer StudySummary

• Favorable Pressure Gradient Flows are Very
  Sensitive to Roughness
• Smallest roughness has large effect on boundary
  layer
• Base drag control using roughness needs to be
  adaptive
• Relative roughness appears to be the controlling
  factor
• k/d requires more investigation

50
UWAL Base Drag StudyScheduled Tests

• Finish Smooth Surface Cf measurements
• Oil film interferometry
• Base Pressure Measurements
• Determine CD,base
• Cf measurements on Rough Surfaces
• Hot-wire surveys
• Dual Pitot probe
• Add points to the base drag/viscous fore-body
  drag curve

51
Base Drag StudyFuture Work

• Near Term Work
• Build a large base drag model
• Repeat base drag and viscous fore-body drag
  measurements
• Investigate the wake
• What causes the base drag reduction?

52
Base Drag StudyFuture Work

• Far-Term Work
• Alternatives to roughness for BL control
• Can we reduce base drag with less fore-body drag
  penalty?
• Adaptive control
• Roughness adapts to keep vehicle in the drag
  bucket