MSL Picture of the Day: T-1 Days: Entry, Descent and Landing

MSL Picture of the Day: T-1 Days: Entry, Descent and Landing

In the above depicted scene, thrusters on the backshell of the spacecraft’s aeroshell are firing to adjust the orientation of the spacecraft during the guided entry manoeuvres

One set of instruments carried on the heat shield of the spacecraft’s entry vehicle serves specifically to gather data about the Martian atmosphere and performance of the heat shield for use in designing future systems for descending through planetary atmospheres: the Entry, Descent and Landing Instrument (MEDLI) Suite.
F. McNeil Cheatwood is the principal investigator,NASA’s Langley Research Center, Hampton, Virginia.
Michael Wright is the Deputy principal investigator, NASA’s Ames Research Center, Moffett Field, California.

The MSL Entry, Descent and Landing Instrument (MEDLI) Suite consists of a set of sensors attached to the heat shield of the Mars Science Laboratory.
MEDLI will take measurements eight times per second during the period from about 10 minutes before the vehicle enters the top of the Martian atmosphere until after the parachute has opened, about four minutes after entry.

The measurements will be analyzed for information about atmospheric conditions and performance of the entry vehicle.
MEDLI was installed to learn for future landings on Mars. NASA’s Exploration Systems Mission Directorate (which has responsibility for planning human missions beyond Earth orbit) and Aeronautics Research Mission Directorate (which invests in fundamental research of atmospheric flight) have funded MEDLI.

The heating and stress on the heat shield will be the highest ever for an entry vehicle at Mars. This is due to the mass of the entry vehicle (2,431 kilograms, or 5,359 pounds after jettison of the spacecraft’s cruise stage), the diameter of its heat shield (4.5 meters, or 14.8 feet) and the speed at which the vehicle will enter the atmosphere (6.1 kilometers per second, or 13,645 miles per hour),

Experience gained with this mission will aid planning for future missions that could be even heavier and larger, such as would be necessary for a human mission to Mars.

Models of the Martian atmosphere, heating environments, vehicle aerodynamics, and heat-shield performance, among other factors, were employed in designing the Mars Science Laboratory entry vehicle.

Ofcourse we don’t know everything or precisely. To account for those uncertainties, the design incorporates large margins for success. However, that margin comes at a cost of additional mass. The goal of MEDLI is to better quantify these atmospheric entry characteristics and possibly reduce unnecessary mass on future Mars missions, by collecting data on the performance of the Mars Science Laboratory entry vehicle during its atmospheric entry and descent.

MEDLI consists of seven pressure sensors (Mars entry atmospheric data system sensor, or MEADS), seven plugs with multiple temperature sensors (Mars integrated sensor plug, or MISP) and a support electronics box.

Data from the entry vehicle’s inertial measurement unit, which senses changes in velocity and direction, will augment the MEDLI data. Each of the temperature-sensing plugs has thermocouples to measure temperatures at four different depths in the heat shield’s thermal protection tiles, plus a sensor to measure the rate at which heat shield material is removed due to atmospheric entry heating.

Analysis of data from the pressure sensors and inertial measurement unit will provide an altitude profile of atmospheric density and winds, plus information about pressure distribution on the heat shield surface, orientation of the entry vehicle and velocity.

Data from the temperature sensors will be used to evaluate peak heating, distribution of heating over the heat shield, turbulence in the flow of gas along the entry vehicle’s surface, and in-depth performance of the heat shield material.

This sounds all as if landing on Mars is 1+1=2. It will not surprise you to learn that this is not the case. The span of time from atmospheric entry until touchdown is not predetermined. That timespan hinges on atmospheric density (more density gives more drag, means faster slow down of the spacecraft. How long you have for touchdown obviously also depends on the elevation of the Mars terrain you are going to land on. When you land on a high plateau you will meet the ground sooner than when you plan to land on a deep vallis.

As it is impossible to send commands to the landing spacecraft while it is landing (due to commands not being able to fly to space faster than the speed of light and the fact that Mars is at best over 3 minutes away at that speed of light) the complete landing sequence has to be preprogrammed, but also be highly adaptable to the circumstances of the landing

The Mars Science Laboratory has been fitted with a guided entry technique which enables the spacecraft to respond and adapt to the atmospheric conditions it encounters more effectively than any previous Mars mission. The span between the moment the spacecraft passes the entry interface point and a successful touchdown in the target area of Gale Crater could be as short as about 380 seconds or as long as about 460 seconds.

Times for the opening of the parachute could vary by 10 to 20 seconds for a successful landing.
The largest variable during EDL is the length of time the spacecraft spends on the opened parachute. Curiosity could be hanging below a fully inflated chute as briefly as about 55 seconds or as long as about 170 seconds.

Times given in the EDL description below (as well as on the image of the EDL sequence) are given for one typical landing with a touchdown 416 seconds after entry.

The intense period called the entry, descent and landing (EDL) phase of the mission begins when the spacecraft reaches the top of the Martian atmosphere, traveling at about 13,200 miles per hour (5,900 meters per second).

EDL ends about seven minutes later with the rover stationary on the surface. From just before jettison of the cruise stage, 10 minutes before entry, to the cutting of the sky crane bridle, the spacecraft goes through six different vehicle configurations and fires 76 pyrotechnic devices, such as releases for parts to be separated or deployed.

The top of Mars’ atmosphere is a gradual transition to interplanetary space, not a sharp boundary. The atmospheric entry interface point — the navigators’ aim point during the flight to Mars — is set at 3,522.2 kilometers (2,188.6 miles) from the center of Mars.

That altitude is 131.1 kilometers (81.46 miles) above the ground elevation of the landing site at Gale Crater, though the entry point is not directly above the landing site. While descending from that altitude to the surface, the spacecraft will also be travelling eastward relative to the Mars surface, covering a ground-track distance of about 630 kilometers (about 390 miles) between the atmospheric entry point and the touchdown target.

Ten minutes before the spacecraft enters the atmosphere, it sheds the cruise stage. The Mars Science Laboratory Entry, Descent and Landing Instrument (MEDLI) Suite begins taking measurements.

A minute after cruise stage separation, nine minutes before entry, small thrusters on the back shell halt the two-rotation-per-minute spin that the spacecraft maintained during cruise and approach phases. Then, the same thrusters on the back shell orient the spacecraft so the heat shield faces forward, a maneuver called “turn to entry.”

After the turn to entry, the back shell jettisons two solidtungsten weights, called the “cruise balance mass devices.” Ejecting these devices, which weigh about 75 kilograms (165 pounds) each, shifts the center of mass of the spacecraft. During the cruise and approach phases, the center of mass is on the axis of the spacecraft’s stabilizing spin.

Offsetting the center of mass for the period during which the spacecraft experiences dynamic pressure from interaction with the atmosphere gives the Mars Science Laboratory the ability to generate lift, essentially allowing it to fly through the atmosphere. The ability to generate lift during entry increases this mission’s capability to land a heavier robot, compared to previous Mars surface missions.

The spacecraft also manipulates that lift, using a technique called “guided entry,” to steer out unpredictable variations in the density of the Mars atmosphere, improving the precision of landing on target.

During guided entry, small thrusters on the back shell can adjust the angle and direction of lift, enabling the spacecraft to control how far downrange it is flying. The spacecraft also performs “S” turns, called bank reversals, to control how far to the left or right of the target it is flying. These maneuvers allow the spacecraft to correct position errors that may be caused by atmosphere effects, such as wind, or by spacecraft modelling errors. These guided entry maneuvers are performed autonomously, controlled by the spacecraft’s computer in response to information that a gyroscope-containing inertial measurement unit provides about deceleration and direction, indirect indicators of atmospheric density and winds.

During EDL, more than nine-tenths of the deceleration before landing results from friction with the Mars atmosphere before the parachute opens. Peak heating occurs about 75 seconds after atmospheric entry, when the temperature at the external surface of the heat shield will reach about 3,800 degrees Fahrenheit (about 2,100 degrees Celsius). Peak deceleration occurs about 10 seconds later. Deceleration could reach 15 g, but a peak in the range of 10 g to 11 g is more likely.

After the spacecraft finishes its guided entry maneuvers, a few seconds before the parachute is deployed, the back shell jettisons another set of tungsten weights to shift the center of mass back to the axis of symmetry. This set of six weights, the “entry balance mass devices,” each has a mass of about 25 kilograms (55 pounds). Shedding them re-balances the spacecraft for the parachute portion of the descent.

The parachute, which is 51 feet (almost 16 meters) in diameter, deploys about 254 seconds after entry, at an altitude of about 7 miles (11 kilometers) and a velocity of about 900 miles per hour (about 405 meters per second).

About 24 more seconds after parachute deployment, the heat shield separates and drops away when the spacecraft is at an altitude of about 8 kilometers (5 miles) and traveling at a velocity of about 125 meters per second (280 miles per hour).

As the heat shield separates, the Mars Descent Imager begins recording video, looking in the direction the spacecraft is flying. The imager records continuously from then through the landing. The rover, with its descent-stage “rocket backpack,” is still attached to the back shell on the parachute.

The terminal descent sensor, a radar system mounted on the descent stage, begins collecting data about velocity and altitude.

The back shell, with parachute attached, separates from the descent stage and rover about 85 seconds after heat shield separation. At this point, the spacecraft is about 1,6 kilometers (1 mile) above the ground and rushing toward it at about 80 meters per second (about 180 miles per hour).

All eight throttleable retrorockets on the descent stage, called Mars landing engines, begin firing for the powered descent phase.

After the engines have decelerated the descent to about 0.75 meters per second  (1.7 miles per hour), the descent stage maintains that velocity until rover touchdown.

Four of the eight engines shut off just before nylon cords begin to spool out to lower the rover from the descent stage in the “sky crane” manoeuvre. The rover separates its hard attachment to the descent stage, though still attached by the sky crane bridle and a data “umbilical cord,” at an altitude of about 20 meters (about 66 feet), with about 12 seconds to go before touchdown.

The rover’s wheels and suspension system, which double as the landing gear, pop into place just before touchdown. The bridle is fully spooled out as the spacecraft continues to descend, so touchdown occurs at the descent speed of about 0.75 meters per second (about 1.7 miles per hour).

When the spacecraft senses touchdown, the connecting cords are severed and the descent stage flies out of the way, coming to the surface at least 150 meters (492 feet) from the rover’s position, probably more than double that distance.

Soon after landing, the rover’s computer switches from entry, descent and landing mode to surface mode.

This initiates autonomous activities for the first Martian day on the surface of Mars, Sol 0. The time of day at the landing site is mid afternoon — about 3 p.m. local mean solar time at Gale Crater.

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