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On August 6, 2012 (GMT), the Mars Science Laboratory Spacecraft will attempt a Landing on the Red Planet’s Gale Crater – making the first ever guided re-entry and Propulsive Landing on Mars – Without a doubt the most complex maneuver ever attempted in Planetary Space Flight. Entry, Descent and Landing is the term for this crucial and risky mission phase, that begins when the Spacecraft is about to hit the Martian Atmosphere and ends when the Curiosity Rover is standing on the surface of Mars – going from 21,000 Kilometers per Hour (13,000 Miles per Hour) to Zero in just under seven minutes. These will be ‘Curiosity’s 7 Minutes of Terror.’
7 Minutes of Terror
This is Spaceflight101’s exclusive Minute-by-Minute Breakdown of those seven minutes:
Entry, Descent & Landing Sequence Overview
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The Entry, Descent and
Landing Mission Phase – short: EDL – begins when the Mars Science Laboratory
(MSL) has nearly completed its 567-Million Kilometer (352-Million Mile) Trip
from Earth to Mars. Having completed the launch phase (24 hours prior to launch
to L+1 Day), the long Cruise Phase (207 Days and 14 Hours) and being at the end
of the Mars Approach Phase (the final 45 Days leading up to EDL), the MSL
Mission transitions to the EDL Phase at the Point of Cruise Stage Separation.
The Cruise Stage was needed during the Cruise Phase to control the vehicle,
perform engine burns and communicate with Earth. It is separated by
pyrotechnics marking the end of the Pre-Entry Phase and the start of the
Exo-Atmospheric Entry Phase. At that point, the Aeroshell, the Descent Stage
and the MSL Rover perform a de-spin maneuver and make a re-orientation to
achieve the proper Entry Attitude. Also, the vehicle jettisons two weights to
generate a Center of Gravity Offset. Entry Interface occurs at an altitude of
125 Kilometers and MSL begins a guided Entry Process going through Peak Heating
and Peak Deceleration. Once the Hypersonic Aero Maneuvering Portion of the
Descent is complete and the Center of Gravity Offset is eliminated, the vehicle
deploys its large parachute to slow it down further. Once reaching a safe
velocity, MSL drops its Heat Shield exposing the Landing Radar. Once at a Speed
of 120 Meters per Second, the Descent Stage and Curiosity Rover separate from
the Backshell and begin the Powered Descent Portion of the EDL Phase. The
vehicle uses its Mars Landing Engines to reduce its Horizontal Velocity to Zero
and make a vertical descent using its Landing Radar and slowing down to about
0.75 Meters per Second. At an Altitude of 20 Meters, the Curiosity Rover
Separates from the Descent Stage and is lowered on bridles.
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Image: NASA - JPL
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This Sky-Crane
System is used to avoid any contamination by the Mars Landing Engines and to
prevent a large dust cloud around the vehicle. While being lowered, Curiosity
deploys its Mobility System (its Wheels which also function as Landing Gear).
When Curiosity achieves contact with its Wheels on the surface of Mars its
computers make sure that contact has indeed occurred and initiate the
separation of the Descent Stage. The Descent Stage then throttles up its engines
and performs a fly-away maneuver to retreat to a safe distance from the landing
site for its crash landing. At that point, Curiosity is switching from its EDL
Mode to the Landed Mission Mode starting Sol 0 Operations which marks the end
of EDL and sets the stage for MSL’s two-year landed mission.
MSL Spacecraft and Rover - At a Glance
Image: NASA - JPL
Spacecraft Components:
>>> Detailed Spacecraft Information |
Image: NASA - JPL
Curiosity Rover Facts:
>>> Rover Information >>>Instrument Information >>>MSL Sampling System |
MSL Landing Site
Image: NASA - JPL
MSL Landing Ellipses - Smaller Ellipse = Landing Target
Photo: Ryan Anderson
Gale Crater Topography
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NASA’s
Curiosity Rover is planned to land inside Gale Crater targeting a 6.4 by
19.3-Kilometer Landing Ellipse on the base of Mount Sharp, a 5.5-Kilometer hill in the center
of the Crater. Mount Sharp is MSL’s primary science target. The Rover targets
an area of flat terrain for its landing and moves to its first science
targets shortly after landing. Gale is believed to be 3.5 to 3.8
billion years old. The crater has a span of 154 Kilometers. An unusual feature of Gale
that played a large role in its selection is an enormous mound of debris that
is slightly taller than the southern rim of the crater itself. This material is
expected to have a history of 2 billion years conserving a large record of
Martian Evolution, both geologically and atmospherically. Through the mound's layers, a number of channels track along which are the features that have formed most recently. These channels are up to 250 meters deep and 2 Kilometers from side to side presenting different layers on orbiter photography. These channels might offer a chance to explore the past of the Planet’s evolution. The channel is lower and lies on a gently sloping pile the are suspected to have washed down the channel.
MSL traverses to Mount Sharp from its Landing Site in order to start operations. Moving the landing target as close as possible to the mountain, but still keeping error rates in mind, cuts about 6 Kilometers of traverse time. Engineers determined that MSL’s Attitude Control System was working better than expected, narrowing the original 20 by 25-Kilometer Ellipse down to 6.4 by 19.4 km. A landing 6 Kilometers closer to this target would eliminate 4 months of traverses across the base of the Crater increasing the time that can be spent with science operations. Gale holds a diversity of features and layers for investigating changing environmental conditions which will provide extensive information on the Planet’s habitability, past, present and future. The landing site selection process for MSL occurred from 2006 to 2011 and included numerous proposals and workshops. Over 60 potential landing sites were considered and eventually narrowed down to a list of four. The final landing site was announced after the final workshop and when analyses by the MSL mission team were complete. The MSL rover was designed without a particular landing site in mind so that the rover can access more of the Martian Surface. The vehicle can tolerate a broader spectrum in environmental conditions and the process of selecting a landing site could be extended and occur later in the mission design procedure. |
Detailed EDL Description
Overview Animation
EDL Phases
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Image: NASA - JPL - Caltech
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Final Approach and Pre-Entry
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The last 5 Days of the
MSL Mars Approach Phase are considered to be the Final Approach Phase that ends
at EI (Entry Interface). The appropriate Flight Software for the EDL Process
will have been loaded into the vehicle’s computers well before the Final
Approach Phase, but the final Navigation Updates will be sent to MSL during
this mission Phase. During Final Approach, Deep Space Network Coverage
increases to obtain more detailed navigation data. Data used for navigation
analysis includes two-way doppler tracking, two-way ranging and Delta
Differential One-Way Range Determination (DDOR). Navigation data is used to
precisely calculate the Entry Interface Point of the MSL Vehicle. The exact EI
Position has to be correct for MSL to reach its landing site. Up to three
Trajectory Correction Maneuvers are available to the Vehicle to adjust its path
in order to fine-tune the exact point of Entry Interface.
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Image: NASA - JPL - Caltech
Cruise Stage Thruster Burn for TCM
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One of these
maneuvers is a nominally planned Engine Burn that occurs on EDL-2 Days. The
other two maneuvers are placeholders for Contingency TCMs at EDL-1 Day and
EDL-9 Hours. 120 Minutes prior to EDL, the final Navigation Data Update is sent
to the MSL Spacecraft. This update includes MSL’s exact position at EI-9
Minutes. The Flight Computers will use this information and modify it based on
measurements taken by its own navigation sensors such as Inertial Measurement
Units from that point on to create a real time navigation profile that is essential for all upcoming navigation tasks until acquisition of Landing Radar Data after Heatshield Jettison.
Timeline
| EI +/- | Event | Range | Relative Velocity | |
| -02:00:00 | Final EDL Navigation Update | ~23,500km | ~3,996m/s | |
| ~-01:24:00 | MSL 'crosses' Orbit of Deimos | |||
| ~-00:33:00 | MSL 'crosses' Orbit of Phobos | |||
| -13:30 | HRS Propellant Vent | |||
| Duration: 3 Minutes |
Exo-Atmospheric Entry
Photo: NASA - JPL - Caltech
Photo: NASA - JPL
MSL - Aeroshell Configuration
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This EDL
Phase begins 15 Minutes ahead of Entry Interface and ends at the Point of EI
which is defined as a position at a radius of 3522.2 Kilometers. 15 Minutes
prior to EDL, the Flight Control System issues the ‘Do EDL’ Command which
initiates the Transition to EDL Mode from the previously used ‘Do PEDL’ Mode.
Shortly after EDL Initiation, about 13.5 Minutes prior to EI, the Cruise Stage
vents its Heat Rejection System. Exactly 10 Minutes before Entry Interface, the
Cruise Stage is jettisoned by initiating 10 pyrotechnic devices that
physically separate the the Cruise Stage from the Backshell. (More about the
Cruise Stage can be found here.) Immediately after Cruise Stage Separation, the
MEDLI Instrumentation Suite is enabled. The MSL
EDL (Entry, Decent and Landing) Instrument suite is a set of engineering
sensors and electronics to measure atmospheric properties and heat shield
performance during the crucial Entry and Landing Phase of the MSL Mission. At EI-9 Minutes, the Rover
Guidance, Navigation and Control System is activated and begins a series of
maneuvers lasting 3 Minutes and 6 Seconds to place the Entry Vehicle in its Entry Orientation. MSL performs a De-Spin Maneuver
reducing the rotation of the vehicle from 2rpm to zero. The spacecraft was
spin-stabilized during its cruise phase, but for EDL, the vehicle has to be in
a configuration without any rotation. To generate a Center of Gravity Offset,
two 75-Kilogram solid Tungsten Weights mounted on the outer area of the Aeroshell
are jettisoned. With a perfectly centered vehicle, a guided re-entry would be
impossible since there is no chance of generating any lift, but for the Cruise
Phase and its maneuvers, the Center of Gravity had to be along the spin-axis so
that the vehicle could ‘fly’ smoothly. The two weights are separated by
pyrotechnics. This produces a lift-to-drag ratio of 0.24 at a Velocity of Mach
24 and results in an angle of attack of around 18 degrees. This angle creates
the lift necessary to fly a guided entry and the making small maneuvers, the
angle can be controlled from 16-20 degrees in order to constantly modify the flight profile based
on real-time navigation data. With the two weights jettisoned, MSL is enabled
to make its final Pre-EI Maneuver, using the Descent Stage Reaction Control
System to dampen any transients. At EI-5:19, a so called ‘Quiescent
Period’ begins.
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During this time, the Spacecraft makes no maneuvers and
calibrates its Inertial Measurement Units. The MSL Descent Stage is outfitted
with two IMUs that are the primary Source of Navigation Data during the
Atmospheric Entry Portion of the Flight.
Timeline
| EI +/- | Event | CSS +/- | |
| -10:00 | Cruise Stage Separation | 00:00 | |
| -09:30 | Transition to EDL Communications: | +00:30 | |
| X-Band: Tones | |||
| UHF: 8kbps (Bent-Pipe Relay via Odyssey) | |||
| -09:00 | Descent Stage Reaction Control System Enable | +01:00 | |
| De-Spin from 2rpm | |||
| Maneuver to Entry Attitude | |||
| Total Maneuver Duration: 3min 6sec | |||
| -05:49 | Entry Guidance Enable | +04:11 | |
| Achieve Center of Gravity Offset | |||
| (Jettison 2 Weights) | |||
| Maneuver Duration: 30 Seconds | |||
| -5:19 | Quiescent Period | +04:41 | |
| Inertial Measurement Unit Calibration |
Atmospheric Entry
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This portion of the Mission begins at the
Point of Entry Interface and ends with Parachute Deployment. The Atmospheric
Entry itself consists of four different phases.
Once reaching and detecting Entry Interface, the Guidance System is switched to Entry Mode and the Reaction Control System is Pressurized. As soon as Entry begins, Guidance Computers start to modulate the lift vector by rotating the vehicle to achieve the required angle of attack to reach the desired downrange and cross range target for Parachute Deployment. The Entry Controller of the vehicle generates roll, pitch and yaw torque commands based on real-time navigation data. These commands are translated to on/off commands for the Reaction Control System that consists of eight thrusters installed in pairs outside the aeroshell. At the Point of Entry Interface, MSL is at an Altitude of 125 Kilometers above the Martian Surface (131.1km above Gale Crater) traveling at a velocity of 5,900 meters per second. Once Entry Guidance is active, the Guided Entry Portion of the Process begins with MSL holding its pre-bank attitude until sensing 0.5G (Earth Gs). This marks the start of the Range Control Phase during which the bank angle is modified in a way to minimize the predicted downrange error at parachute deployment. Bank reversals are conducted as required with cross-range error margins being maintained at a manageable level. During this Guidance Phase, MSL experiences Peak Heating at about EI+85 Seconds. At that point, the PICA Heatshield of the vehicle has to withstand a thermal load of more than 5,700J/cm² with a Turbulent Air Flow around the Entry Vehicle. Peak Heat Rate will be greater than 200W/cm². Eleven Seconds after Peak Heating, the Vehicle passes Maximum Deceleration experiencing a G Force of approximately 12.9. Once MSL’s Velocity drops below 900m/s, the Heading Alignment Phase of the EDL Process starts. Residual Cross-Range Error is minimized during this phase with bank angles being adjusted so that the vehicle achieves a direct-flyover of the Parachute Deploy Target. Approximately 15 Seconds prior to Parachute Deployment at about EI+230 Seconds, six 25-Kilogram weights mounted on the inner portion of the Aeroshell are jettisoned with two-second intervals to eliminate the Center of Gravity Offset. This maneuver is known as SUFR – Straighten Up and Fly Right. Also part of the maneuver is a 180-degree azimuth turn of the vehicle to align the Terminal Descent Sensor for proper Ground Acquisition. SUFR is velocity triggered. Transients occurring due to SUFR will be dampened by the Reaction Control System and the Angle of Attack will be reduced to nearly zero. |
Image: NASA - JPL - Caltech
Image: NASA - JPL - Caltech
Image: NASA - JPL - Caltech
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Image: NASA - JPL - Paper 1467
Center of Gravity Offset Elimination
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Image: NASA - JPL - Paper 1467
Descent Stage & Rover Configuration inside Aeroshell
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Timeline
| EI +/- | Event | Altitude | Velocity | |
| 00:00 | ENTRY INTERFACE | ~125km | 5,900m/s | |
| Pressurize Propulsion System | ||||
| Guided Entry Phases: | ||||
| Pre-Bank (Initiation at 0.1G) | ||||
| Range Control Phase (Initiation at 0.5G) | ||||
| Heading Alignment (Initiation at 900m/s) | ||||
| +01:25 | Peak Heating - 2,100°C | |||
| +01:36 | Peak Deceleration - Up to 12.9 G | |||
| +03:00 | Hypersonic Aero-Maneuvering | |||
| +03:50 | Eliminate Center Of Gravity Offset | |||
| Jettison 6 Inner Weights | ||||
| Damp CG-Offset Elimination Transients |
MEDLI - MSL EDL Instrument Suite
The MSL EDL (Entry, Decent and Landing) Instrument
suite is a set of engineering sensors and electronics to measure atmospheric
properties and heat shield performance during the crucial Entry and Landing
Phase of the MSL Mission. It is not a part of the core science payload of MSL. Its
goals are to provide information to help the improvement of systems that will
be used on future planetary missions. MEDLI was designed, developed and built
by NASA’s Langley Research Center and NASA Ames Research Center.
Photo: NASA - JPL
MEADS
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MEDLI
includes 7 MEDLI Integrated Sensor Plugs (MISPs) and 7 Mars Entry Atmospheric
Data System (MEADS) Pressure Sensors that are installed on the heat
shield of the spacecraft. Inside the heat shield and not exposed to the
entry environment is the Sensor Support Electronics (SSE) Unit. It
provides power, signal conditioning and signal conversion for digital
processing. Data acquisition is initiated when the entry system is separated from the spacecraft bus approximately 10 minutes prior to
entry. It will be taking data at a frequency of 8 Hz until after the
main chute is deployed about two minutes after entry interface. A
portion of MEDLI data will be included in the real time telemetry stream
during the entry phase. The full data set that is collected, is sent to
the Rover Computing Element for storage and downlink. It is expected that
the complete data collection will be downliked during the first month of
landed operations should downlink time be available. Each MISP plug consists of four type-K thermocouplers that are installed at different depths in the Thermal Protection Material of the Heat Shield to measure the heat profile inside the TPS. The couplers are installed at depths of 0.25, 0.5, 1.14 and 1.78 centimeters.
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These instruments measure
the temperature of the TPS as a function of time during entry. Each plug
also contains a single Hollow Aerothermal Ablation and Temperature
(HEAT) Sensor. This particular sensor measures the propagation of a
single isotherm through the material. Data of both instrument types will
provide an exact history of the performance of the MSL TPS. The nominal
data frequency is 8Hz, but some sensors will gather data at 2Hz. HEAT
will also determine a loss in surface material due to ablation as a
function of time during entry. The MISP plugs are concentrated in the
area of the highest thermal loads due to turbulent air flow. 3.3 centimeters
in diameter, the plugs are enclosed in the same material that is
utilized on the TPS and inserted into 3.33-centimeter holes in the heat
shield. Smaller holes are drilled through the aeroshell structure to
direct the wires of the plugs to the SSE. The seven MEADS sensors
include a pressure sensor that is installed on the interior of the heat
shield. A small 0.25-centimeter hole is drilled through the TPS to allow
accurate surface pressure readings. These holes will not impact TPS
performance during entry. MEADS sensors form a cross pattern in the
low-heating – high pressure portion of the aeroshell. This pattern will
enable scientists to deduce exact vehicle orientation data from
comparing MEADS data to predicted values.
Photo: NASA - JPL
MISP
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Photo: NASA - JPL
SSE on Vibration Isolators
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Objectives
It is anticipated that MSL will enter the Martian Atmosphere at a velocity of 6.1m/s. In combination with its size and mass, the air flow around the vehicle will become turbulent fairly early into re-entry. Heat flux and shear stress on the Thermal Protection System will be greater than on any previous Mars Mission. Uncertainties in simulations prompted large margins in the design of MSL’s heat shield at the cost of mass and science payloads. Reducing these margins for future missions requires more accurate simulations based on actual data obtained in the entry environment. MEDLI will provide data concerned with atmospheric properties and heat shield performance that will be compared with pre-flight predictions to evaluate the level of uncertainty and the margins used for this mission. MEDLI’s data collection will be the largest set of data acquired in a non-Earth Entry. Inertial Measurement Unit Data of the entry phase will be combined with MEDLI information to provide data on surface pressure distribution, vehicle attitude, dynamic pressure on the structure, Velocity, and the atmospheric density and winds. With MEDLI data, peak heat flux, distribution of heating on the heat shield, map transition to turbulence, and TPS performance will be acquired.
It is anticipated that MSL will enter the Martian Atmosphere at a velocity of 6.1m/s. In combination with its size and mass, the air flow around the vehicle will become turbulent fairly early into re-entry. Heat flux and shear stress on the Thermal Protection System will be greater than on any previous Mars Mission. Uncertainties in simulations prompted large margins in the design of MSL’s heat shield at the cost of mass and science payloads. Reducing these margins for future missions requires more accurate simulations based on actual data obtained in the entry environment. MEDLI will provide data concerned with atmospheric properties and heat shield performance that will be compared with pre-flight predictions to evaluate the level of uncertainty and the margins used for this mission. MEDLI’s data collection will be the largest set of data acquired in a non-Earth Entry. Inertial Measurement Unit Data of the entry phase will be combined with MEDLI information to provide data on surface pressure distribution, vehicle attitude, dynamic pressure on the structure, Velocity, and the atmospheric density and winds. With MEDLI data, peak heat flux, distribution of heating on the heat shield, map transition to turbulence, and TPS performance will be acquired.
Supersonic Parachute Descent
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The Parachute of the Vehicle features a
Velocity triggered deployment with PD occurring when a Relative (navigated) Speed of 450 meters per second is detected by the Guidance System. This speed will be
achieved at an altitude of about 11 Kilometers above the surface of Mars and
the biggest of the 76 pyrotechnic devices that are used during EDL will be
fired. To deploy the 19.7-meter Disk Chute at a navigated speed equivalent to
Mach 2, a mortar deployment system with the power of a stick of dynamite is
necessary. The Chute can be opened at velocities of Mach 1.8 to 2.2 to allow
some margin for unpredicted Entry Events. The Parachute burns off 95% of the
remaining kinetic energy of MSL in just 55 to 170 Seconds (depending on atmospheric conditions) decelerating the vehicle
from 450 to 100 m/s generating a drag force of up to 289 Kilonewtons. The parachute has 80 suspension lines with a length of more than 50 meters.
A major concern for Parachute Descent is a possible oscillatory behavior of the vehicle suspended underneath the parachute. This phenomenon has been observed on previous missions and is of importance for this flight due to MSL's increased PD Velocity and the mass of the Entry Vehicle. Large loads of energy could be transferred to the capsule causing rotation underneath the Parachute around the Center of Gravity. A wrist mode dampening mechanism has been implemented for MSL using its Reaction Control System to reduce wrist mode frequencies to acceptable levels when limits are exceeded. Keeping the vehicle stable also ensures a safe Heatshield Jettison. As the Parachute decelerates the Vehicle quickly from transonic to subsonic conditions, MSL completes a rapid list of preparations for powered descent. The first of those is the Separation of the Heatshield to expose the stowed Rover and the Descent Stage to free-stream conditions. 
Image: NASA - JPL
The heat shield is jettisoned
by pyrotechnics. Heatshield separation has to fulfill two objectives, the
first being a clean separation without re-contacting the descending Entry
Vehicle and second being a separation without obscuring more than one beam of
the Terminal Descent Sensor – MSL’s Landing Radar and primary Navigation Sensor
from that point onwards.
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Image: NASA - JPL - Paper 1467
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The first objective is met by jettisoning the Shield
at a point at which there is a sufficient difference in the ballistic
coefficients of the Heatshield and the Entry Vehicle ensuring that a positive
separation velocity is achieved. For that, an accurate Velocity Trigger is used
to initiate Heat Shield Separation at a max speed of Mach 0.7. (Using IMU
Navigation makes it hard to measure exact velocities and Flight Computers rely on
Navigated Velocities until the moment of TDS acquisition.) At the point of Heatshield Jettison, the MARDI Instrument becomes active (refer to the section below). With the Heat Shield
out of the way, the TDS (Terminal Descent Sensor) could begin to collect data
immediately after release, but it could receive false data by acquiring the
heat shield instead of the ground with more than one of its beams. The minimum
distance of the Heatshield for TDS activation is 17 meters and the shield is
expected to cover that distance relative to the Entry Vehicle in about 8
Seconds. An 8-second delay has therefore been implemented into the TDS
Activation Sequence making sure at least 5 beams will acquire the ground. The
Vehicle will take data for about 30 seconds measuring its speed with a 3-axis
Doppler Velocimeter and calculating its altitude with a Slant Range Altimeter.
At that point, a velocity trigger will initiate Backshell Separation. Just
before Backshell Separation, the 8 Mars Landing Engines are primed by opening
fuel valves with 8 pyrotechnic devices to allow fuel to flow to the engines
which will start up at 1% Thrust prior to Backshell Separation.
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Timeline
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MARDI - Mars Descent Imager
Photo: NASA JPL / Malin Space Science Systems
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MARDI is a
fixed focus full color camera fixed-body-mounted to the fore-port-side
of the Rover. Its optical axis points in the +Z direction toward the
ground. The camera will take high resolution images during the rover’s
final descent to the Martian Surface. It will be active between heat
shield separation until a few seconds after touchdown taking 5 images
per second over a period of about 2 minutes. The images will be
1600x1200 pixel in resolution. Activation of the camera is done by the
rover software which will send a start imaging command, the camera will
operate autonomously writing data into the permanent flash memory in
real time during acquisition before receiving a stop imaging command
from the rover’s software when is has detected a successful landing.
Image downlink will be performed later when communications and time are
available.
The field of view of the detector is providing a 70° x 55° frame with the long axis transverse to the direction of motion. At a range of 2 Kilometers in altitude, the resolution will be about 1.5m per pixel, at 2m it will be 1.5mm. Over the detector is a Bayer RGB filter. |
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An
exposure time of 1.3 milliseconds will be used and it is expected that
many images will be blurred despite the short exposure due to
vibrations when the landing engines are firing and angular rate motions
while the rover is on the parachute.
8 Gigabytes of internal buffer are available. This enables the camera to acquire 4,000 images which is the equivalent to 800 seconds of descent with actual descent lasting only 120 seconds. The photos will be stored in raw data formats however, real time compression (lossless and lossy JPEG) is available. Small Thumbnail images will be generated prior to downlink to make the process of selecting images to downlink easier. MARDI’s objectives include determining the exact landing spot on Mars and providing a geological&engineering framework of the landing site that will help decide on the first targets on Mars and possible transition paths. Vehicle Horizontal offsets between images within the descent sequence may be applicable to generating a digital elevation model. Additionally, ground referenced motion deviations from inertial measurement unit positioning during descent will be examined by using the images in order to extract lower boundary layer winds and to help in the development process for future automated landings and hazard avoidance systems. |
Photo: NASA - JPL
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Powered Descent
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The Powered Descent Portion of the Flight
begins with Backshell Separation and ends when the Curiosity Rover initiates
the Sky Crane Process.
Backshell Separation occurs at a velocity of 120 Meters per Second at an altitude of 1600 to 2000 meters above Ground Level as detected by the TDS Sensor. This sensor is capable of measuring terrain relative velocity with high precision utilizing pulse doppler radar. Six independent beams (3 beams canted 20 deg. Off nadir, 2 beams canted 50 deg. Off nadir, and one nadir beam) which provide range and velocity data for the area below the vehicle so that an exact landing zone profile can be created in real time. Backshell Separation is initiated by firing pyrotechnic separation nuts freeing the Powered Descent Vehicle (PDV). |
Image: NASA - JPL
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After dropping out of the Backshell, the PDV is in a near free
fall condition (except for drag and the 8 Mars Landing Engines at 1% Throttle).
This free fall increases velocity to about 125 Meters per Second and creates
enough separation between the descending Backshell and the PDV. After 0.8
Seconds of free falling, the eight Mars Landing Engines are warmed up by
throttling up to 20% for 0.2 Seconds after which the Engines are ready for
operation. 2.2 Seconds of Attitude Rate Dampening follows and at Backshell
Separation +3.4 seconds, the first Phase of the Powered Descent Process begins.
Image: NASA - JPL
Photo: NASA Kennedy
MSL - Curiosity and Descent Stage during final Processing
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The PDV follows a 3-D polynomial trajectory that is generated at the point of
Backshell Separation. The Powered Descent has two objectives, the achievement
of the proper altitude and velocity for the Sky Crane Phase to start, and the
separation from the Parachute and Backshell that can not land in the same area
as the Rover. The Powered Approach follows its polynomial trajectory and brings
the PDV’s Horizontal Velocity down to zero in a smooth fashion. Also during
Powered Approach, the vehicle reduces its vertical velocity to 20m/s to set up
for the Constant Velocity Accordion. The desired end point of the Powered
Descent is at an altitude of 100 meters and 300 meters perpendicular to the
plane of the Entry Trajectory. This 300-meter diversion is sufficient to ensure
that MSL lands at a safe distance to its Backshell and Parachute which actually
land before Curiosity does since the PDV is actively slowing its descent. The
Constant Velocity Accordion is implemented in the Landing Sequence due to
potential errors in altitude knowledge at Backshell Separation (due to limited
Time on Radar and IMU Navigation errors). At BSS, the vehicle still has a
fairly large amount of horizontal velocity which causes the Terminal Descent
Sensor to acquire data from an area that is not the actual landing zone which
could result in deviations of +/-50 meters at the end of the Powered Descent
Phase. This error is flown out at a constant vertical velocity of 20 Meters per
Second. (Should the surface be 50 meters closer, the CVA will be zero. For the
surface being 50 meters lower than expected, 100 meters of distance have to be
covered, resulting in a 5-second CVA.) The Constant Velocity Accordion ends at
an altitude of 50 meters when the vehicle is still descending at 20 meters per
second. At that point, the Constant Deceleration Phase of the Powered Descent
begins. The PDV throttles all eight Mars Landing Engines up to 90% to maintain
a constant deceleration level to reduce its speed from 20 meters per second to
0.75 meters per second which essentially is the landing speed of the Curiosity Rover.
The Constant Deceleration Phase ends at an Altitude of 21 meters.
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At that
point, the Throttle Down Sequence begins. More than half of the 390 Kilograms
of Descent Stage Propellants will have been consumed at that point. MSL seeks
to maintain a constant Thrust to Weight Ratio which would require the eight
engines to be throttled down to about 20-25%, but the MLEs are not as fuel
efficient at low-thrust-levels so that four of the Engines will be switched to
near Shutdown Level (1%). Also, propellant consumption margins are incredibly
tight since the Descent Stage needs to save fuel for its Fly-Away Maneuver. The
four active engines are operated at about 50% during the Throttle Down
Sequence. Shutting down four Engines causes Attitude Disturbances that have to
be corrected and 2.5 seconds are reserved to complete that operation and place
the PDV in a stable position 18.6 meters above the surface to begin the Sky
Crane Phase.
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Powered Descent Timeline
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At a Glance: Mars Landing Engines
Source: NASA - JPL - The MSL Sky Crane Landing Architecture - A GN&C Perspective - Miguel San Martin
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Source: NASA - JPL - The MSL Sky Crane Landing Architecture - A GN&C Perspective - Miguel San Martin
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Photo: NASA Kennedy
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At a Glance: Terminal Descent Sensor
Source: NASA - JPL - The MSL Sky Crane Landing Architecture - A GN&C Perspective - Miguel San Martin
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Sky Crane
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At an Altitude of 18.6 meters and a stable
orientation, the Guidance, Navigation and Control System transitions to the Sky
Crane Mode and initiates Rover Separation from the Descent Stage. This is
expected 42.5 seconds after Backshell Separation and about 5 minutes and 45
seconds after Entry Interface. Rover Separation is accomplished by firing
pyro-separation devices. After Separation, the MSL Rover’s only structural
connection to the Descent Stage is a triple bridle system with an umbilical
cord providing power to the Rover and Data from the Rover to the Descent Stage
and vice versa. While the Sky Crane System is in motion, the Descent Stage
continues to move at a vertical velocity of 0.75 meters per second. Immediately
after Rover Separation, the three nylon Bridles start to lower Curiosity. Three
Seconds after Rover Separation, Curiosity starts to deploy its Mobility System
– the six wheels that also function as landing gear.
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Image: NASA - JPL
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Data: NASA - JPL -- Diagram: Spaceflight101
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After 7 seconds, the
Bridles are fully extended at 7.5 meters and motion stops. This snatch causes attitude
disturbances which need to be corrected by the Descent Stage within 2 seconds.
The bridles are stopped by an electromagnetic brake that is connected to the
spool that contains the bridles. All three bridles pass through the same
confluence point on the Descent Stage which is nearly collocated with the
vehicle’s Center of Gravity to limit any disturbances to a minimum. Two Seconds
after the snatch, 9 seconds after Rover Separation, the Touchdown Logic is
enabled (refer to the Diagram to the left). Rate of Descent remains at 0.75
meters per second and the Flight Computers monitor Thrust Levels of the four
active MLEs in one-second intervals. When touchdown occurs, the Rover’s weight
will be supported by the Martian Surface and the bridles will be offloaded
causing the Descent Stage Thrust Level to go down. As per the Touchdown Logic,
the Thrust Level has to be within a certain threshold and it needs to be a flat
profile indicating that all four wheels have settled on the surface. It is
expected that all wheels will be down 1.7 seconds after first contact with
Touchdown Detection occurring at TD+2.7 seconds. As soon as Touchdown is
declared, the Bridle Cut Command is sent. It takes the pyrotechnics about 0.3
seconds from CMD to Bridle Cut. Just 0.2 seconds after the Bridle Cut Command
is issued, the Umbilical Cord Cut Command will be sent and the cord is cut
0.015 seconds later. At TD+3.2 Seconds, 0.5 seconds after Touchdown Declaration,
Curiosity should be on its own standing somewhere near Mount Sharp inside Gale
Crater with its Descent Stage right above its head thundering to 100% throttle
for the Fly Away
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Image: NASA - JPL -- Paper: MSL EDL System Overview
MSL Rover Deployment
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Image: NASA - JPL -- Paper: MSL EDL System Overview
Bridle and Umbilical Device
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At a Glance : MSL Descent Stage
Image: NASA - JPL
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Descent Stage - Stowed Configuration
Image: NASA - JPL
Descent Stage - Sky Crane Configuration
Image: NASA - JPL
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Sky Crane Timeline
| ~EI +/- | Event | RS +/- | Rover Alt. | Velocity | |
| +06:28 | Rover Separation | 00:00 | 18.6m | 0.75m/s | |
| Deploy Front Rocker | 1.82m/s | ||||
| +06:31 | Mobility Deploy | +00:03.0 | <12.0m | 1.82m/s | |
| +06:33.5 | Release Bogie | +00:05.5 | 1.82m/s | ||
| +05:35 | Snatch | +00:07.0 | <8.8m | 0.75m/s | |
| +06:37 | Enable Touchdown Logic | +00:09.0 | <3.0m | 0.75m/s | |
| Differential Release | 0.75m/s | ||||
| +06:39 | Touchdown | 0.00m/s | |||
| Total EDL Duration (From EI to TD): 380-460sec |
Sky Crane Flight Profile
Source: NASA - JPL - The MSL Sky Crane Landing Architecture - A GN&C Perspective - Miguel San Martin
Fly Away
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After all connections with Curiosity are
cut, the Descent Stage Controller detects a successful landing sequence
completion and initiates its Fly Away Mode. After assuming control over itself,
the Descent Stage controlled by the Descent Stage Controller waits for 187msec
hovering in position above the Rover to allow some margin for bridle and
umbilical cord cut. When the waiting period is over, the Descent Stage
throttles two of its Mars Landing Engines back up to 100% with the other ones
throttling up to a little bit less than 100% to allow the vehicle to pitch over
to about 45 degrees along the Y-axis. Once the turn duration is complete, all
engines come back to 100% and the Descent Stage is on a patch away from the
Landing Site. The exact duration depends on the amount of fuel that is
remaining. Once all tanks are empty, the Mars Landing Engines shut down and the
vehicle is on a ballistic trajectory. It impacts at a minimum distance of 150
meters to Curiosity. With the impact of the Descent Stage and the transition of
the Curiosity Rover from its EDL Mode to the SOL 0 Procedures, the most complex Planetary
Landing Operation ever attempted will be completed and the largest Mars Rover to date will
hopefully be standing somewhere near Mount Sharp inside Gale Crater.
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Image: NASA - JPL - Caltech
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Image: NASA - JPL - Caltech
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Communications Delay
On Landing Day, Mars and MSL will be 13 Minutes and
51 Seconds of Light Travel Time from Earth which is equivalent to the
one-way signal travel time from the MSL Vehicle to Earth. This obviously means
that there is no possibility of human intervention should anything wrong show
up in the Telemetry Stream during EDL. It also means that MSL will already be
on the surface of Mars – dead or alive – when we will be looking at Telemetry
Data sent during the Exo-Atmospheric Entry. MSL lands at Gale Crater on August 6, 2012 at about 5:17 GMT with earliest Signal Arrival at 5:31 GMT. At the earliest, confirmation of
mission success or failure will come 14 minutes after the actual landing time,
although it is anticipated that it could take much longer to establish initial
contact with Curiosity. (See the Entry Communications Section Below)
Image: NASA - JPL Solar System Simulator
EDL Communications
Photo: NASA
Deep Space Network Station Goldstone, California
Image: NASA - JPL
Mars Reconnaissance Orbiter
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During Pre-Entry, nominal Cruise
Communications are used. The Cruise Stage provides X-Band Communication with
the Deep Space Network Stations on Earth at a rate of about 500 bits per second
during Pre-Entry via its Medium Gain Antenna. At the point of Cruise Stage Separation, Communications switch to the EDL Mode during which Low Gain
Antennas on the Backshell, Descent Stage and Rover are used. The Backshell is equipped
with a regular Low Gain Antenna and a Tilted Low Gain Antenna that is used when
the nominal antenna is blocked by the Parachute. The Descent Stage is outfitted
with a single Descent Low Gain Antenna that is active during powered descent. Aboard
the Rover itself is a High Gain Antenna which is not used during EDL and a Low
Gain Antenna that is used immediately after EDL or during final Descent should
the Descent Stage Communication Assets fail.
These low gain antennas send a Direct To Earth X-Band Frequency that will be monitored by the Deep Space Network. Data sent via this link is of a very low rate and only consists of MFSK Tones also called semaphores. Each timeline segment of EDL has a unique set of nominal and off-nominal tones assigned to it. There are a total of 256 different tones that can be sent to Earth. These tones can be converted to useful information about the basic status of the vehicle and the completion of EDL Events, but can’t be considered real telemetry data. MSL’s antennas will also provide a UHF Signal during EDL which has a data rate of up to 8kbps and includes basic vehicle telemetry that is sufficient for failure analysis, but is still no complete EDL Engineering Data Set. A UHF Antenna with 8 patch antennas is mounted on the Backshell and provides a solid data stream during EDL. The Descent Stage has a single UHF Antenna and the Rover is also outfitted with a UHF Antenna that can be used during EDL. The UHF Frequency will be picked up by the Mars Reconnaissance Orbiter and the Odyssey Orbiter as well as ESA's Mars Express that will be attempting to record the UHF Signal. Odyssey also has the capability of directly relaying the UHF Telemetry Stream to Earth via X-Band and its more powerful antennas. UHF Relay will feature a short additional delay due to Orbiter Capabilities. ODY is the only Orbiter capable of providing bent-pipe communications. Also, the UHF data stream and the X-Band Tones could experience several drop-outs during EDL. Loss of Signal is expected for a 1-second period at Cruise Stage Separation and the switch to the Low Gain Antennas of the Backshell during the Maneuver to the Entry Attitude. During the Peak Heating Phase, a plasma envelope forms around the MSL Vehicle that causes an UHF Blackout of 25 to 100 seconds. X-Band tones are uninterrupted at that point. |
After parachute deployment, X-Band Tones are no longer received on Earth because MSL is no longer visible from Earth and its DSN Stations. Tones are sent throughout EDL, but won't be heard since Earth will have set at the Landing Site.
UHF should be solid at that time. At the Point of Backshell Separation, Communications are changing to the Descent Stage UHF and Low Gain Antennas which are blocked by the Parachute for 1 to 6 seconds. The final expected Communications Drop Out comes at the moment of Rover Separation when UHF Communications switch to the Rover UHF Antenna. After touchdown, MSL sends a Landing Sequence Complete Signal and switches to pre-planned communication modes which involve Direct To Earth Communications and Mars Orbiter Communication Phases. All Communication windows of the initial Surface Operations Phase are loaded into the Flight Computers during the Cruise to Mars so that initial contact opportunities are not mandatory. Confirmation of a successful Landing could reach Earth in ‘real time’ with the nearly 14-minute delay should communication angles be favorable after touchdown. In case of any blockages due to geological features (Rocks, Craters, or MSL landing on a slope facing away from Earth), confirmation of a successful touchdown could take several hours if a communications pass with one of the Orbiters is needed. About two hours after landing, Odyssey has a planned Communications Pass over Gale Crater which would be the next chance of getting a signal from the Rover after EDL Communications. Should the UHF Signal be interrupted for any reason, this is the earliest confirmation of the EDL outcome after touchdown. ESA's Mars Express Spacecraft relays the stored UHF data at Landing +1 to 2 Hours which is the earliest insight into EDL events not depending on Odyssey. MRO transmits stored UHT Telemetry to Earth at EDL+4 to 5 Hours. Sol1 communication passes have also been loaded into MSL's Computers so that the vehicle could theoretically afford to miss all Communications following EDL-120 Minutes since all aspects of EDL are automated. The first HazCam images acquired by MSL after landing could reach Earth via the EDL+2-Hour ODY Pass.
Because real time telemetry is limited during EDL, the MSL Spacecraft will store a full EDL Telemetry and Engineering Data Set in its on-board memory. This data will have a rate of 100Mbit/s and include all vehicle parameters from EI-10 Minutes to Touchdown +1 Minute. This data set is downlinked to Earth during the first few Sols of Landed Operations.
Detailed: EDL Communications
UHF should be solid at that time. At the Point of Backshell Separation, Communications are changing to the Descent Stage UHF and Low Gain Antennas which are blocked by the Parachute for 1 to 6 seconds. The final expected Communications Drop Out comes at the moment of Rover Separation when UHF Communications switch to the Rover UHF Antenna. After touchdown, MSL sends a Landing Sequence Complete Signal and switches to pre-planned communication modes which involve Direct To Earth Communications and Mars Orbiter Communication Phases. All Communication windows of the initial Surface Operations Phase are loaded into the Flight Computers during the Cruise to Mars so that initial contact opportunities are not mandatory. Confirmation of a successful Landing could reach Earth in ‘real time’ with the nearly 14-minute delay should communication angles be favorable after touchdown. In case of any blockages due to geological features (Rocks, Craters, or MSL landing on a slope facing away from Earth), confirmation of a successful touchdown could take several hours if a communications pass with one of the Orbiters is needed. About two hours after landing, Odyssey has a planned Communications Pass over Gale Crater which would be the next chance of getting a signal from the Rover after EDL Communications. Should the UHF Signal be interrupted for any reason, this is the earliest confirmation of the EDL outcome after touchdown. ESA's Mars Express Spacecraft relays the stored UHF data at Landing +1 to 2 Hours which is the earliest insight into EDL events not depending on Odyssey. MRO transmits stored UHT Telemetry to Earth at EDL+4 to 5 Hours. Sol1 communication passes have also been loaded into MSL's Computers so that the vehicle could theoretically afford to miss all Communications following EDL-120 Minutes since all aspects of EDL are automated. The first HazCam images acquired by MSL after landing could reach Earth via the EDL+2-Hour ODY Pass.
Because real time telemetry is limited during EDL, the MSL Spacecraft will store a full EDL Telemetry and Engineering Data Set in its on-board memory. This data will have a rate of 100Mbit/s and include all vehicle parameters from EI-10 Minutes to Touchdown +1 Minute. This data set is downlinked to Earth during the first few Sols of Landed Operations.
Detailed: EDL Communications
Tight Margins
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The Entry Sequence as described above
does not include a lot of margin for any problems and unforeseen Events.
Timeline Margins are not a real factor since the MSL Entry Timeline depends on
several points of reference that are triggered by actual EDL parameters (Velocity
and Altitude) rather than a fixed timeline.
One margin that is important for a safe EDL Process is the so called Time on Radar. This is the time period spent under the Parachute with a good Data Stream coming from the Terminal Descent Sensor. A minimum of 5 seconds of Radar Time are required to obtain an initial profile of the landing zone, but teams would prefer at least 10 seconds of Time on Radar to allow the vehicle to get a full profile of its landing zone prior to computing its Powered Descent Trajectory which occurs at the moment of Backshell Separation. Because Heat Shield Jettison and Backshell Separation are velocity and altitude triggered events, there is no real chance of confirming that the time in between the two events is sufficient. HS Separation occurs at 238 meters per second and the TDS becomes active 8 seconds after HS Jettison, but needs correct off-nadir angles to obtain valid data. Backshell Separation occurs at 120 meters per second. |
Diagram: NASA
MSL Descent - No Room for Error
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In the predicted Entry Timeline,
there are about 25 to 30 seconds of Time on Radar, but in actuality, this time will
be shorter due to the capabilities of the Radar and a time offset generated by
atmospheric conditions, so that the statistical margin goes down to 20 to 25
seconds of time of radar, but still giving a sufficient margin for a near-nominal EDL.
The biggest Limiting Factor of the EDL Portion of the MSL Mission is Fuel. At the point of Cruise Stage Separation, the Descent Stage starts out with 390 Kilograms of Monomethylhydrazine. During Exo-Atmospheric Entry, Propellants are used for the De-Spin Maneuver, attitude settling after Cruise Stage Separation and the Maneuver to the Entry Attitude. During Entry, the RCS System is used for Entry Guidance Maneuvers, Wrist Mode Management and propellant is consumed during Mars Lander Engine Priming. The Powered Descent consumes the largest amount of Propellants with an partly active RCS and eight burning Mars Landing Engines. After touchdown, the Descent Stage requires enough Propellant to conduct the minimum Fly-Away Maneuver of 4 seconds. Statistical Values obtained during simulations have shown that fuel margins are relatively tight. The models did not take propellant use during Exo-Atmospheric and Atmospheric Entry, Parachute Descent and MLE Priming into account and came up with a margin of about 91 Kilograms of Hydrazine. Assuming that the actual margin is somewhat lower, MSL has about 20 seconds of margin during powered Descent and Fly-Away because propellant consumption during Constant Velocity and Sky Crane Operations is 4 kilograms per second.
The biggest Limiting Factor of the EDL Portion of the MSL Mission is Fuel. At the point of Cruise Stage Separation, the Descent Stage starts out with 390 Kilograms of Monomethylhydrazine. During Exo-Atmospheric Entry, Propellants are used for the De-Spin Maneuver, attitude settling after Cruise Stage Separation and the Maneuver to the Entry Attitude. During Entry, the RCS System is used for Entry Guidance Maneuvers, Wrist Mode Management and propellant is consumed during Mars Lander Engine Priming. The Powered Descent consumes the largest amount of Propellants with an partly active RCS and eight burning Mars Landing Engines. After touchdown, the Descent Stage requires enough Propellant to conduct the minimum Fly-Away Maneuver of 4 seconds. Statistical Values obtained during simulations have shown that fuel margins are relatively tight. The models did not take propellant use during Exo-Atmospheric and Atmospheric Entry, Parachute Descent and MLE Priming into account and came up with a margin of about 91 Kilograms of Hydrazine. Assuming that the actual margin is somewhat lower, MSL has about 20 seconds of margin during powered Descent and Fly-Away because propellant consumption during Constant Velocity and Sky Crane Operations is 4 kilograms per second.
EDL Propellant Budget
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Photo: NASA - JPL - Caltech
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One additional margin that is worth
noting is the maximum stress the vehicle can handle. MSL’s maximum tolerable G
Force during Entry is 15G. During a nominal EDL Sequence, Peak Deceleration
occurs at Entry Interface +96 seconds and reaches a maximum of 11 to 12 Gs. In
simulations, slightly off-nominal EDL Parameters such as an off-target Entry
Interface lead to G Forces of up to 13G. The Entry Logic does not have a G
Limiting feature built into it.
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Touchdown margins for the MSL Rover are
fairly tight as well. MSL can land with a maximum horizontal velocity of 0.5
meters per second (planned: 0m/s) and a vertical velocity of 0.85 meters per
seconds (planned: 0:75m/s). The vehicle can safely land on slopes of up to 15
degrees. Steep slopes cause the potential of direct plume impingement or
contamination by combustion products while the Descent Stage hovers
horizontally. The worst case scenario would be MSL Landing on a 15-degree slope
and a 0.55-meter tall rock. In such a case, the Rover would be in danger of
suffering damage by direct exposure to MLE Plumes. In a nominal landing
scenario on flat terrain, plume clearance is sufficient, even with small
attitude settling maneuvers made by the Descent Stage.
Systems Redundancy is also an item with a very tight margin. There are not many systems of the entire EDL Design that provide redundancy. One of the key-elements in succeeding at a propulsive landing on Mars is weight. Adding redundancy to the system would add more weight to the vehicle and tighten other margins such as the already critical fuel consumption. For MSL’s EDL Operation to be successful, all Systems have to work 100%. All 76 pyrotechnic devices have to work at the exact millisecond they are planned to, all 8 Mars Lander Engines have to start up properly, all 3 bridles have to lower as expected and the single parachute has to open for Curiosity to arrive on Mars in one piece. |
Source: JPL - Caltech - Paper: MSL EDL System Overview
MSL - Worst Case Plume Clearance
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Dynamic Architecture
Many of
MSL’s EDL Operations are triggered by time-stamped commands which in turn are
triggered by a series of Events. These individual Events depend on parameters
such as Velocity, Atmospheric Pressure and Altitude. This enables MSL to
respond to dynamic conditions such as atmospheric disturbances and winds in the
lower atmosphere without sticking to a fixed, pre-programmed timeline which
would increase error rates. The duration of the EDL Sequence from EI to Touchdown can vary from 380 to 460 seconds depending on dynamic conditions of the Martian Atmosphere.
MSL Points of Reference:
Entry Interface: The Moment MSL encounters the first traces of the Martian Atmosphere. All Pre-EI and a number of Post-EI Events are time-triggered based on the moment of EI.
Parachute Deploy: PD is triggered by the vehicle’s Velocity and following Events depend on the timing of PD.
Heat Shield Separation: The Separation of the Heat Shield is velocity triggered to ensure the Heat Shield has a positive separation velocity as a result of a sufficient difference in ballistic coefficients. Terminal Descent Sensor activation is triggered based on the timing of HS.
MSL Points of Reference:
Entry Interface: The Moment MSL encounters the first traces of the Martian Atmosphere. All Pre-EI and a number of Post-EI Events are time-triggered based on the moment of EI.
Parachute Deploy: PD is triggered by the vehicle’s Velocity and following Events depend on the timing of PD.
Heat Shield Separation: The Separation of the Heat Shield is velocity triggered to ensure the Heat Shield has a positive separation velocity as a result of a sufficient difference in ballistic coefficients. Terminal Descent Sensor activation is triggered based on the timing of HS.
Image: NASA - JPL - Caltech
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Backshell
Separation: This Event is one of the most important steps in the EDL Sequence.
At the moment of Backshell Separation, the Flight Control System computes its
powered descent trajectory that is essentially unchanged until Touchdown
occurs. The initial Sequence of events after BSS is time triggered and is part
of a 3.4-second operation. All other events from BSS to Rover Separation are
altitude and velocity triggered.
Rover Separation: Rover Separation initiates a series of time triggered events such as the deployment of the Rover’s Mobility System and the snatch of the bridles as well as the activation of the Touchdown Logic which is a sequence with 1-second intervals that detect the actual moment of touchdown. Touchdown: After touchdown is declared, a series of time-stamped events occurs that culminate in the initiation of the Flyaway Maneuver. |
Calculated Risk
Image: NASA - JPL - Caltech
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MSL’s EDL Sequence is the most complex Planetary
Landing Operation ever attempted. It features tight margins and complex systems
that all have to function correctly. 510,000 Lines of Code are needed for EDL.
All the Entry Software has to work correctly and be executed by the associated
hardware properly. However, the entire EDL Sequence is based on engineering
reason and analysis, and teams are very confident that it will work, but some
doubt remains since there are elements that have never been tested before.
These elements include the powered descent and sky crane sequence. Tests of
these key features are impossible on Earth because the environment on Mars is
completely different. Individual components such as the Mars Landing Engines and
the Landing Radar have been tested extensively, but the complete sequence will
be executed for the first time when MSL makes its actual descent.
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Initial Surface Operations
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Calming down from the fast moving events
from EDL, Initial Surface Operations are moving at a much slower pace. Since
MSL’s primary Mission duration is much longer than that of previous Mars Rover
Missions, teams can afford to take a longer time period for initial operations
to make sure all set-up tasks and Rover health checks are completed
satisfactory.
Immediately after landing, Curiosity switches from the EDL Mode to its Sol 0 Mode which marks the start of MSL Surface Operations. The focus of the first few hours and days on the Martian Surface will be initial Rover Checks. With the uncertainty of the timing of first contact with Earth and the large Communications Delay, the Sol 0 Operation is autonomous and Curiosity can complete these steps without contact to Earth. After touchdown, the Rover conducts a status check of all essential systems. |
Image: NASA - JPL - Caltech
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Temperature sensors become active and the Thermal Control System adjusts to the new
environment, keeping all equipment within Red-Line Temp Limits. Also, HazCam
Footage is acquired and prepared for downlink. The first larger data package
is planned to be downlinked to Earth about 12 hours after landing when the Mars
Reconnaissance Orbiter passes over the landing site. The first priority for the
mission team is to gain understanding in the immediate surroundings of the
Rover making sure that the landing zone is known and that the situation the
Rover is in (Systems Status, Landing Zone, Hazards in the vicinity, etc.) is
fully understood with Curiosity being in a secure configuration. Also being
looked at, is the surface directly underneath and around the rover to make sure
there is no immediate threat. It could
take up to five days to complete initial checks and validations. To achieve
solid communications, MSL deploys its High Gain Antenna some time after
landing. After that, Curiosity is given the Green Light to deploy its large
Mast and Sampling System including the Robotic Arm. The navigation cameras take
images of the sky to calculate the position of the sun and the High Gain Antenna
is pointed towards Earth for more stable communications. After the first 5
Sols, Subsystem and Instrument checks are in progress. The first drive of the
vehicle is expected about 1 week after landing. As the Science Mission gets underway,
more tests of the instruments are performed before actual sampling operations can begin.
MSL - Full Mission Animation
Editor's Note: All information given on this site have been compiled on a best efforts basis, however, some values of EDL Parameters can deviate from the latest planned EDL Sequence due to modifications of the MSL EDL Profile. Parameters are taken from various official NASA sources and papers, but can contain outdated information that potentially results in data being off by a few percent.
Last Update: July 30, 2012
Last Update: July 30, 2012
