Lunar Landing GN&C and Trajectory Design

Go For Lunar Landing: From Terminal Descent to Touchdown Conference

Panel 4: GN&C

Ron Sostaric / NASA JSC

March 5, 2008

National Aeronautics and Space Administration

Slide 25 March 2008Ron Sostaric NASA Johnson Space Center, Aeroscience and Flight Mechanics Division

Introduction

• ALHAT is a NASA project developing technologies needed to improve landing capability– Autonomous Precision Landing and Hazard Detection and Avoidance

Technology Project

• The objective of the project is to develop and deliver an autonomous GN&C hardware and software system and certify it to Technology Readiness Level (TRL) 6 through analysis and testing – Functional on robotic, cargo and human missions

– Place humans and cargo safely, precisely, repeatedly and autonomously anywhere on the lunar surface under any lighting conditions within 10’s of meters of certified landing sites

– Detect surface hazards with the capability to re-designate to hazard free landing areas

– Extensible to other missions

Slide 35 March 2008Ron Sostaric NASA Johnson Space Center, Aeroscience and Flight Mechanics Division

Approach Phase Hazard Detection and Avoidance

Hazard Relative Navigation

Braking Maneuver

Terminal Descent Phase

Pitch-up Maneuver

Deorbit maneuver

Powered Descent Phase

Powered Descent Initiation (PDI)

~1

5 k

m

~3

0 m

~300 to ~600 km

NOTE –

Not to scale

Transfer Orbit Phase (coast)

Hazard Detection

Human Interaction

Hazard Avoidance

Parking Orbit ~100 km

Braking Phase Terrain Relative Navigation

~1

to

~2

km

Orbit1

Coast2

Braking3

Approach4

Vertical Descent5

TRAJECTORYPHASE

#

---

~ 55 min

~ 6 - ~10 min

~30 - ~180 sec

~ 30 sec

TIME ALLOCATION

De-orbit Maneuver

Powered Descent Maneuver

Pitch Up Maneuver

Vertical Descent Maneuver

Touchdown~ 1 hourTotal Time Allocation

Descent Trajectory

Slide 45 March 2008Ron Sostaric NASA Johnson Space Center, Aeroscience and Flight Mechanics Division

Trajectory Design Drivers for Approach and Landing

• How to shape the approach and landing trajectory, and why?

• Trajectory design drivers during Approach

– Minimize propellant usage – Trajectory design must be

representative of what crew would be willing to fly

– Provide reasonable operating conditions for sensor (and/or crew member) to scan landing area for hazards

– Allow time for interpreting sensor scan information and crew decision making

– Allow enough margin for maneuvering to avoid hazards

– Provide enough margin to account for dispersion control

Approach Phase Hazard Detection and Avoidance

(HDA)

Terminal Descent Phase ~

30

m

Hazard Detection

Human Interaction

Hazard Avoidance

~1

to

~2

km

Slide 55 March 2008Ron Sostaric NASA Johnson Space Center, Aeroscience and Flight Mechanics Division

Trajectory Interaction With Conditions for Hazard Detection

Too shallow for sensor

Too shallow for sensor

Too steep for window view

Too steep for window view

Too far for sensor scan

Too far for sensor scan

Trajectory path

Meets constraints

Meets constraints

Slide 65 March 2008Ron Sostaric NASA Johnson Space Center, Aeroscience and Flight Mechanics Division

Trajectory: Slant Range for Hazard Detection

• Need to be within range of landing site for sensor scan, crew viewing

• Spending more time sensing/viewing closer to the landing site is preferred for sensing and viewing

• This has a trade-off with propellant usage

• The relationship of time during approach and landing with propellant usage is about 10 kg for each second – Assuming low throttle, Altair-size

lander

Slide 75 March 2008Ron Sostaric NASA Johnson Space Center, Aeroscience and Flight Mechanics Division

Trajectory: Path Angle During HDA

• The trajectory path angle directly affects the angle for sensing/viewing

• Shallower approach ideal for window viewing

– Landing area moves “up” in the window as path becomes more shallow

– Apollo flew ~16 deg approach • HDA sensor performance degrades at

shallow approach angles– Shallow approach causes stretching of

samples, partial or complete obstruction of small and medium size hazards behind large ones

• ALHAT working to fully characterize the trade space and better understand path angle effects

• Other considerations– Lighting conditions– Cameras, light tubes, or augmentation

systems may affect the path angle constraint– These things (and others) under investigation

Slide 85 March 2008Ron Sostaric NASA Johnson Space Center, Aeroscience and Flight Mechanics Division

Hazard Avoidance

• Hazards must be detected early enough that they can be avoided – for a reasonable amount of propellant and – without exceeding tipover limits or other vehicle constraints

• The required divert distance capability can be sized by relating it to the size of the hazard scan area

– The hazard scan area is determined by a probalistic terrain analysis to determine the amount of area needed to ensure a safe landing

• The required divert distance drives the point at which divert must be initiated

30 m

@ -1 m/s

Hazard Avoidance (HA)

Last point with “full” HA redesignation capability

Final DescentDivert to edge of

scan area

Scan area

Scan area

180 m

80 m

The maximum divert for a 180 m scan area is 80 m

Vehicle footprint assumed to be 20 m

(10 m radius)

Slide 105 March 2008Ron Sostaric NASA Johnson Space Center, Aeroscience and Flight Mechanics Division

Introduction to Safe and Precise Landing

• Safe Landing– A controlled touchdown within tolerance on vehicle state

while avoiding any hazards• Hazards are defined as rocks, craters, holes, slopes, or other

obstructions that exceed the vehicle hazard tolerance

– Safe Landing is by primarily accomplished knowing about all hazards prior to the mission, or by providing a real-time method of hazard detection, and by having the capability to avoid hazards

• Precise Landing– Landing accurately enough inertially as required for mission

design and also precisely enough locally to achieve a safe landing (avoid any hazards)

– Precision Landing is primarily accomplished by providing accurate enough state knowledge early enough to fly out dispersions, and accurate enough state knowledge near touchdown to avoid hazards