The MSL SkyCrane Landing Architecture A GN&C Perspective


The MSL SkyCrane Landing Architecture
A GN&C Perspective

Miguel San Martin
Jet Propulsion Laboratory, California Institute of Technology

International Planetary Probe Conference Barcelona, Spain
June 14-18, 2010

A Few Definitions

• The Delivery System/Phase

– Starts during the Parachute Descent phase and ends at first ground contact

• The Touchdown System/Phase
– Starts at first ground contact and ends at 6-DOF full stop

• The Safing/Egress System/Phase

– Starts at full stop and ends when the payload is power, thermal, and telecom safe and ready to start the science commissioning phase

© 2010 California Institute of Technology. Government sponsorship acknowledged.


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Terminal Descent Architectures

The Role of GN&C

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Beagle Airbag

MPF/MER Airbag

Legged Lander


Delivery System/Phase

• Subsonic Chute
• Altimeter (for airbag inflation)

• Altimeter
• RAD Rockets

• Altimeter/Velocimeter • IMU
• Thrusters
• Leg Deploy

• Altimeter/Velocimeter • IMU
• Thrusters
• Mobility Deploy

Touchdown System/Phase

• Airbags
• Shock Absorbing Structure

• Airbag

• Shock Absorbing Legs • Touchdown Detection sensors

• Altimeter/Velocimeter • IMU
• Thrusters
• Mobility System

Egress-Safing System/Phase

• Petal Actuation

• Airbag-Retraction
• Petal Actuation
• Rover Stand-Up … • Rover Egress-Aids

• Ramps • Cranes

• None


Delivery System TD Velocity Performance

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30 m/sec

Beagle Airbag

20 m/sec

10 m/sec

2.5 m/sec 0.75 m/sec

0 m/sec



Viking/Phoenix Legged Lander MSL SkyCrane

1.5 m/sec 0.5 m/sec

10 m/sec

20 m/sec Touchdown Horizontal Velocity

30 m/sec


Touchdown Vertical Velocity

Terminal Descent Architectures

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Beagle Airbag

MPF/MER Airbag

Increased GN&C Performance/Complexity

Legged Lander


Decreased Touchdown-Safing-Egress System Performance/Complexity


Terminal Descent Architectures

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Beagle Airbag

MPF/MER Airbag

Legged Lander


Increased Development Risks

Decreased Mission Risks due to increased robustness to environmental uncertainties

• Slopes • Rocks • Winds


Load and Stability Limitations of Viking Type Landers at Touchdown

  • Ground thruster plume interaction requires large vertical approach speeds to deal with:
    • –  Trenching effects
    • –  Dust effects
    • –  Ground pressure effects
  • Due to the coupling between translational and attitude control (and vice versa) GN&C/thrusting must stop immediately at first contact with the ground
    • –  Improves lander stability (let the g-vector stabilize the landing)
    • –  Requires touchdown sensors

• Millisecond sensor response

– On slopes the lander will drop resulting in increased loads
• The combination of these two effects impedes further reduction of

touchdown velocities and resulting loads resulting in:

  • –  The need for more capable shock absorbing legs
  • –  Lower touchdown stability in the presence of slopes and rocks
    • Stricter landing site selection requirements
    • Active Hazard Detection and Avoidance

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Advantages of the SkyCrane at Touchdown

  • Two-body architecture and rocket plumes distance to the ground enables continued GN&C operation during touchdown event
  • GN&C is part of the Touchdown System
  • The Descent Stage softly offloads the rover on theMars surface
    • –  Reduces touchdown loads
    • –  Improves touchdown stability
    • –  Can land on mobility
      • No need for specialized touchdown gear
      • Solves rover egress problem
  • Minimum touchdown vertical velocity is limited by fuel

– Driven by altitude knowledge errors

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SkyCrane Touchdown Detection Flat, mean above threshold

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Non-flat, mean below threshold

Flat, mean below threshold


Throttle Throttle Throttle

Sky Crane Maneuver Description

20.5 m

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DS Velocity 0.0 m/s

Damp-Transients Phase

18.6 m

Rover and Mobility Deployment Phase

Radar Beams (4)

Post-Deploy Settling Phase

Ready for Touchdown Phase

Nominal Touchdown

Radar Beams (6)

13.3 m 11.8 m

8.8 m

Last Altitude Fix


Throttle- Down


Rover- Sep

Latest Snatch

TD Logic Enabled

Fuel usage = ~4kg/sec

0 sec

2.5 sec

9.5 sec

11.5 sec

15.5 sec



Altitude Above Ground Level (m)

0.75 m/s

The Sky-Crane GN&C Problem Definition

• Avoid Re-Contact => Rover Separation Requirements

  • –  Off-nadir attitude error < 3 deg (3-sigma)
  • –  Attitude rate < 3 deg/sec (3-sigma)

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  • Rover Loads & Stability => Rover Touchdown Velocity Requirements
    • –  Vertical velocity = 0.75 m/sec, +/- 0.1m/sec (3-sigma)
    • –  Horizontal velocity < 0.5 m/sec (3-sigma)
  • Avoid Rover Pluming => Descent-Stage Touchdown Requirements

– Horizontal velocity < 0.1 m/sec (3-sigma)

• Disturbances

– Throttle Down
• CG Offset, MLE thrust errors

– Rover Separation
• CG Offsets, Rover off-load

  • –  Rover Deployment snatch
  • –  Touchdown

• Half of the weight off!

• Fuel consumption rate = ~4kg/sec => Time is of the essence!!


Descent Stage Configuration

Entry RCS 8x

Descent IMU (DIMU) 2x

Terminal Descent Sensor (TDS) 1x

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Descent Stage Entry Configuration

Descent Stage Skycrane Configuration

Mars Landing Engines (MLEs) 8x

Terminal Descent Sensor (TDS)


Control Law Architecture

  • Descent Stage Reference Trajectory
    • –  Horizontal velocity = 0 m/sec
    • –  Vertical velocity = 0.75 m/sec
    • –  Attitude = z-axis aligned with local vertical
    • –  Note: This Reference Trajectory applies to the full Sky Crane Phase including the full duration of the touchdown event (I.e. until the full rover weight has been transferred to the ground, as detected by the Touchdown Logic)
  • The purpose of the Position and Attitude control loops is to follow the Reference Trajectory position and velocity as close as possible in the presence of disturbances
    • –  Throttle down (of MLE’s) at start of Sky Crane maneuver
    • –  Rover deploy, Mobility deploy, and bridle snatch
    • –  Wind, Slosh, others
    • –  Touchdown event

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Traveling ground (Reference Trajectory)

0.75 m/sec



Control Design Design for Robustness

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Robust stability is achieved by phase stabilization of

appendage modes (pendulum and bridle-axial modes)

1.0 Hz spring, 0.7 damping

~1000 kg – Make the control system behave as springs and dashpots mass

0.2 Hz spring, 0.7 damping

• Collocated sensing (IMU) and control (MLE) – 16 Hz structure

• High sampling rate and a single sample delay – 64 Hz sample rate

  • High bandwidth sensing and control
    • –  IMU bandwidth > 30 Hz
    • –  MLE bandwidth > 15 Hz
  • All modes below 5 Hz are phase stabilized
    • –  Fuel slosh modes < 2 Hz
    • –  Bridle mode axial 0.4 to 1 Hz
    • –  Pendulum mode < 0.2 Hz
    • –  System robust to uncertainty in mode » Frequency

» Damping
Structural modes are gain-stabilized

– 8th order roll-off filter

0.75 m/sec



Pendulum Mode: 0.1Hz

-180 deg

0 DB

Axial Mode: 0.9 to 1.5Hz

Control Block Diagram Sky Crane Phase

accMagPath verticalPosPath


accMagCorrection commandedAttitude


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Reference Trajectory


error attitudePath



+ error + -+


horizontalPosEstimate verticalPosEstimate








Vertical Position Controller

Horizontal Position Controller

Attitude Controller


Engine Mixing Logic

NAV Filter

Simulations: Sky-Crane Behavior

40.20 Throttle-Down
42.69 Rover Separation
49.43 Deployment Done
59.70 Touchdown
61.22 Fly-Away Start




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Terminal Descent Sensor

  • The low touchdown velocities requirements dictates the Terminal Descent Sensor (TDS) to be capable of measuring terrain relative velocity with high precision even in the presence of uncertain terrain slopes
  • To satisfy this requirement, MSL has:

– Specified a narrow beam, Doppler TDS sensor that measures terrain relative S/C velocity in the S/C reference frame

• This makes the sensor highly insensitive to terrain slopes

  • –  Placed the TDS in the Descent Stage and configured the beams to allow continued velocity measurements through touchdown
  • –  Specified a vertical Sky Crane trajectory in combination with vertical beam that minimizes the effect of terrain on altitude measurement error



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Terminal Descent Sensor

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• The Radar Terminal Descent Sensor produces line of sight range and velocity measurements on 6 independent beams

  • –  Single antenna per beam, with 3 beams canted 20o off nadir, 2 beams canted 50o off nadir, and 1 nadir beam
  • –  Ka-band (35.75 GHz) center frequency
  • –  Pulse doppler radar design

18 Q


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Key MLE Features

  • MLE design based on Viking descent thruster design
    • –  Updated catalyst bed retention design and updated materials
    • –  New cavitating Throttle Valve Assembly (TVA) makes flow independent of downstream pressure drop
  • Key performance characteristics
    • –  Delivered thrust at maximum throttle setting ≥ 3060 N / engine at 600 psia inlet pressure
    • –  Nominal throttle range 400 N to 3060 N
    • –  Minimum specific impulse is a function of thrust level:
      • 221 lbf-s/lbm @ 3060 N
      • 213 lbf-s/lbm @ 1500 N
    • –  MLEs are capable of operating at “near shutdown” (about 1% of maximum flow):
      • Following initial ignition (prior to warm up)
      • From the start of the sky crane phase to the end of flyaway
      • There is no fully closed position of the throttle valve
  • Bandwidth (TVA + reactor) = ~15 Hz
  • Accuracy = 5%

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Requirements on Mechanical

• Structure Flexibility Requirements – First Mode > 20Hz

• Bridle Stiffness Requirement
– Free-free axial mode = from 0.9 to 1.5 Hz

• Confluence Point & CG Offsets Requirements Descent Stage

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• Sensor/Thruster Placement and FOV Requirements


Fly Away

  • Descent Stage shall land at a distance > 100 meters from Rover
  • Profiled-Closed-Loop pitch-rate profile
  • Control law hosted in the Descent Stage MLE/TVA Motor Controller
    • –  Has access to MLE control
    • –  Added interface to IMU. Acts as a relay to the Rover Computer
  • No explicit transfer of state data

• Fly-away controller assumes that the Descent State state is

  • –  Zero velocity
  • –  Zero attitude rate
  • –  Aligned with vertical

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  • The SkyCrane landing architecture enables low velocity touchdown on the rover wheels
    • –  Simplifies Touchdown and Egress Systems
    • –  Reduces Mission Risks by making the system more robust to terrain
  • GN&C plays a major role in reducing the velocity prior and during the touchdown event

– GN&C is part of the Touchdown System

  • No additional hardware required to sense touchdown
  • Two-Body pendulum and bridle-flex modes are phase-stabilized
  • Structural modes are gain-stabilized
  • High bandwidth control for high disturbance rejection and fast transient response during major events– Throttle-down – Rover-Sep
    – Touchdown

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