1. International Planetary Probe Workshop 10
Entry, Descent, and Landing Systems Short Course Subject: Trim Tabs Author: Karl Edquist NASA Langley Research Center
sponsored by
International Planetary Probe Workshop 10
June 15-16, 2013
San Jose, California

2. International Planetary Probe Workshop 10, EDL Short Course
Introduction
Aerodynamic lift is beneficial to EDL performance:
Improves landing elevation & accuracy
Decreases entry loads
Allows for more payload mass
The standard method for generating lift on a blunt capsule is by shifting the radial center of gravity (ZCG) off axis
Mars EDL:
Viking I/II (L/D = 0.18, ZCG/D = = 1.83”)
MSL (L/D = 0.24, ZCG/D = = 3.92”)
CG offset is achieved by moving payload (if volume is available) or by adding ballast mass
The following slides discuss trim tabs for blunt entry capsules
Body flaps (e. g. Space Shuttle) are another example of aerodynamic control
June 15-16, 2013
International Planetary Probe Workshop 10, EDL Short Course

3. MSL Entry Ballast System
Ballast vs. Trim Tab
MSL used > 300 kg of ballast (>35% of rover mass) to achieve L/D = 0.24
Ballast ~ payload mass
If payload , ballast  to maintain L/D
Aerodynamic control of trim angle of attack is more mass-efficient
Tab size ~ heatshield diameter
If payload , tab size stays the same to maintain L/D
A trim tab system that gives = L/D is estimated to be ~10% of ballast
Reduced mass = more payload and/or better EDL performance
MSL Entry Ballast System
Cruise Ballast
(~140 kg)
3.92” V∞
Drag
Lift
Entry Ballast (~160 kg)
NASA/TM
June 15-16, 2013
International Planetary Probe Workshop 10, EDL Short Course

4. International Planetary Probe Workshop 10, EDL Short Course
Mars Robotic EDL
Trim Tab
Ballast
Exo-Atmospheric
Deploy Tab (a > 0)
Eject Ballast (a > 0)
Hypersonic
Guided, L/D > 0
RCS Bank
Control
Eject Ballast (a = 0)
Supersonic
Guided, L/D > 0
L/D = 0 at Parachute Deploy
Retract Tab (a = 0)
Powered Descent &
Touchdown
June 15-16, 2013
International Planetary Probe Workshop 10, EDL Short Course

5. Mars Surface Elevation
The mass savings of trim tabs can contribute to making the Mars southern hemisphere accessible to future robotic EDL missions
-1 km MOLA
+2.5 km MOLA
Ref. “Statistics of Mars’ Topography from the Mars Orbiter Laser Altimeter: Slopes, Correlations, and Physical Models”
June 15-16, 2013
International Planetary Probe Workshop 10, EDL Short Course

6. International Planetary Probe Workshop 10, EDL Short Course
Past Studies
AIAA
AIAA
AIAA
NASA TM X-660, 1962
NASA TM X-770, 1963
AIAA
NASA TM X-816, 1963
NASA TM X-579, 1961
NASA TM
June 15-16, 2013
International Planetary Probe Workshop 10, EDL Short Course

7. International Planetary Probe Workshop 10, EDL Short Course
MSL-I Example
The MSL-Improved (MSL-I) study showed that the mass savings of a trim tab transfers to more payload mass
At least 150 kg more payload to 0 km MOLA than other advanced robotic EDL systems (4.7 m aeroshell, 30 m ringsail parachute)
The mass savings could also be used to reach higher elevation with a slightly smaller payload (~1460 kg to km)
“Technology set 5, utilizing the hypersonic trim tab, provides the most payload mass at the relatively lower site elevations, due to the mass savings over replacement of the entry balance masses.” AIAA
June 15-16, 2013
International Planetary Probe Workshop 10, EDL Short Course

8. Recent Supersonic Wind Tunnel Testing*
Langley Unitary Tunnel
Mach 2.5, 3.5, 4.5
38 different configurations
50°/60°/70° cones, Apollo
Trim tab effectiveness (DCm) increases with tab area & cant angle
Tab aspect ratio (W/H) has a negligible effect
60-deg Forebody
3% Tab, 30° Cant, M=4.5
*Ref. Korzun, “Supersonic Aerodynamic Characteristics of Blunt Body Trim Tab Configurations”
June 15-16, 2013
International Planetary Probe Workshop 10, EDL Short Course

9. Sample Result: Trim Tab vs. Ballast
A 3% tab area with a 33° cant angle gives MSL L/D=0.29 at Mach 4.5
No CG offset needed with tab (xCG/D = 0.291, zCG/D = 0)
MSL, L/D ≈ -0.29
notional curve fit
Extrapolated
Tab Cant Angle, deg 20 40 60 80 90 10 30 50 70
(L/D)trim
-0.40
-0.30
-0.35
-0.15
-0.10
-0.20
-0.25
-0.45
Tab Cant Angle, deg 20 40 60 80 90 10 30 50 70
Ratio of Ballast to Entry Mass
0.05
0.06
0.09
0.10
0.08
0.07
0.04
0.03
0.02
MSL
June 15-16, 2013
International Planetary Probe Workshop 10, EDL Short Course

10. Potential Trim Tab Applications
Applications for single deployment or actuated tab(s):
1 tab  One-direction pitch control  L/D magnitude
2 tabs  Two-direction pitch control  +/-L/D magnitude
2+ tabs  Pitch & yaw control  L/D magnitude & direction
One-Direction
Pitch Control
Pitch and Yaw Control
4.3% Area Tabs Shown
(NASA TM )
Two-Direction
Pitch Control
June 15-16, 2013
International Planetary Probe Workshop 10, EDL Short Course

11. Technology Maturation Needs
No NASA missions are actively pursuing EDL missions with trim tabs, but improvements are needed to prepare the technology
Flight Mechanics:
Quantitative benefits of tabs vs. ballast for a range of EDL missions
EDL control strategies using fixed, single deployment, or actuated trim tab(s)
Aerodynamics & Aerothermodynamics:
Databases are needed for a range of tab parameters (area, cant angle) & Mach numbers
Validated CFD tools
Mechanical & TPS Design:
Mechanisms for tab stowage, deployment, and dynamic actuation
Lightweight TPS materials that minimize shape change (e. g. hot structures)
Mass estimation tools for entire tab system
June 15-16, 2013
International Planetary Probe Workshop 10, EDL Short Course