It is clear that we may want to send a battlestar Galactica type of spacecraft to Mars, but we can’t (yet). Which means that we will have to make do with what is possible. And what is possible is determined by the engineering constraints of our Mars landers. Simply put: we can only send to Mars, what the engineers can land on Mars.
Landing is a mission critical event, for if you don’t land safely, you end up with nothing. And the engineering problem of landing the Mars Science Lab starts with the fact that it is a much heavier lander than the previous Mars exploration rovers. That ruled out the bouncing principle of the 24 man-sized airbags, and called for a new system.
None of the scientist or engineers are willing to risk a billion dollar mission on having an inadequate landing system, so they had to get very creative. And Curiosity is big. 12 times heavier than Spirit or Opportunity, even twice as heavy as the Phoenix Lander.
When designing a landing system engineers are always looking for a trade off between science payload and landing safety.
Airbags as used by Pathfinder, and both Spirit and Opportunity, add weight to a landing system what than has to be taken from the weight allowance for (read: the number of) science instruments.
For that reason Phoenix in 2007 used a landing system comparable to the twin Viking landers in 1976. Phoenix used descent thrusters in the final seconds down to the surface and was set down onto three legs.
However, compared to the Vikings, Phoenix used leaner components, such as thrusters controlled by pulse firing instead of throttle-controlled, and more complex interdependence among the components. Which gave Phoenix 59 kilograms ( pounds) of science equipment on a 410 kilogram (904 pound) lander, that started its Entry, Descent and Landing with 110-kilogram (242-pound) back shell and a 62-kilogram (137-pound) heat shield.
Ofcourse landing of a three legged vehicle has its own engineering challenges. The following wisdom on landing systems, either three-legged, airbag protected or skycrane assisted, I found in Frontiers of Engineering: Reports on Leading-Edge Engineering from the 2004 NAE Symposium on Frontiers of Engineering (2005) ; NAE stands for National Academy of Engineering
Basic landing-leg technology was developed for the lunar Surveyor and Apollo programs in the early 1960s.
In conjunction with a variable-thrust liquid propulsion system and a closed-loop guidance and control system, legs represented an elegant solution to the touchdown problem. They are simple, reliable mechanisms that can be added to an integrated structure that houses the scientific and engineering subsystems for a typical surface mission. The legs of the 1976 Viking mission lander represent the first-generation landing system technology.
The first challenge for a legged system is to enable the lander to touch down safely in regions with rocks.
For this the legs must either be long enough to raise the belly of the lander above the rocks, or the belly of the lander must be made strong enough to withstand contact with the rocks. Neither solution is attractive.
Either the lander becomes top heavy and incapable of landing on sloped terrain or a significant amount of structural reinforcement must be carried along for the remote chance that the lander will directly strike a rock. The decreased stability because of the high center of mass is exacerbated if a mission carries a large rover with it.
A second major challenge of the legged-landing architecture is ensuring safe engine cutoff. To prevent the guidance and control system from inadvertently destabilizing the lander during touchdown, contact sensors have been used to shut down the propulsion system at the moment of first contact. On sloped terrain, this causes the lander to free fall the remaining distance, which can significantly increase the total kinetic energy present at touchdown and, in turn, decrease landing stability and increase mission risk. Implementation and testing of fault protection for engine cutoff logic has been, and continues to be, a difficult problem.
A third major challenge with a legged landing system occurs when the mission calls for a rover to drive of the lander. Once the lander has come to rest on the surface, the rover must be brought to the surface. For legged landers, a ramped egress system is the most logical configuration. Because rovers are bidirectional, the most viable arrangement has been considered two ramps, one at the front and one at the rear of the lander. Spirit and Opportunity both had three ramps to choose for their drive off the landing platform.
In the Mars Pathfinder mission, one of the two ramps was not able to provide a safe egress path for the Sojourner rover, but the second ramp did provide safe egress.
The second-generation landing system was developed for the Mars Pathfinder mission and subsequently improved upon for the Mars Exploration Rover (MER) missions. These second-generation systems have a combination of fixed-thrust solid rocket motors and air bags to perform the touchdown task. The solid rocket motors, which are ignited two to three seconds prior to impacting the surface, slow the lander down to a stop 10 meters above the surface, from an initial velocity of approximately 120 meters/second. The lander is then cut away from the over-slung rockets and free falls for the remaining distance.
The air-bag system, which was developed to reduce cost and increase landing robustness, is designed to provide omnidirectional protection of the payload by bouncing over rocks and other surface hazards. Because the system can also right itself from any orientation, the challenge of stability during landing has been completely eliminated.
Because the lander comes to rest prior to righting itself, the challenge of rock strikes has been reduced to strikes associated with the righting maneuver, which are significantly more benign.
The challenge of thrust termination, in this case cutting the lander away from the rockets, remains but has been decoupled from the problem of landing stability. The problems of rover egress were addressed systematically on the MER missions; a triple ramp-like system provided egress paths in any direction, 360 degrees around the lander.
Although the air-bag landing system has addressed some of the challenges and limitations of legged landers, it has also introduced some challenges of its own. Horizontal velocity control using solid rockets and air-bag testing were significant challenges for both the Mars Pathfinder and MER missions.
As Mars surface explorations mature, roving is becoming more important in the proposed mission architectures. Spirit and Opportunity demonstrated the value of a fully functional rover not reliant on the lander to complete its surface mission.
Fort the Mars Science Laboratory rover Curiosity the rover’s capabilities and longevity are even more important. Were Spirit and Opportunity had both a nominal mission of 90 days, Curiosity has a nominal mission of 98 weeks. If we want to access larger areas of the planet, we need more robust landing systems that are tolerant to slope and rock combinations that were previously considered too hazardous to land or drive on.
This lead to a big rover, which needed a new landing system to put her safely on the ground: the sky-crane landing system (SLS).
This landing system was developed for the MSL mission to overcome all of the major challenges presented by the three-legged or the airbag based landing systems. Obviously it will also eliminate the problem of driving the rover off a landing platform.
The Skycrane Landing System (SLS) eliminates the dedicated touchdown system and lands the fully deployed rover directly on the surface of Mars, wheels first. This is possible because the rover is no longer placed on top of a lander. In the SLS, the propulsion module is above the rover, so the rover can be lowered on a bridle, similar to the way a cargo helicopter delivers underslung payloads.
Sky-crane landing sequence showing the three main phases and events. Nominal touchdown velocities are 0.75 meter per second vertically and 0.0 meter per second horizontally.
The landing sequence for the Curiosity will be similar to the Viking landings, except for the last several seconds when the sky-crane maneuver is performed.
After separating from the parachute, the SLS follows a Viking-lander-like propulsive descent profile in a one-body mode from 1,000 meters above the surface down to approximately 35 meters above the surface. During this time, a throttleable liquid-propulsion system coupled with an active guidance and control system controls the velocity and position of the vehicle.
At 35 meters, the sky-crane landing maneuver is started, and the rover is separated from the propulsion module. The rover is lowered several meters as the entire system continues to descend. The two-body system then descends the final few meters to set the rover onto the surface and cut it away from the propulsion module. The propulsion module then performs an autonomous fly-away maneuver and lands 500 to 1,000 meters away.
The central feature of the SLS architecture is that the propulsion hardware and terrain sensors are placed high above the rover during touchdown. As a result, their operation is uninterrupted during the entire landing sequence. One important result of this feature is that the velocity control of the whole system is improved, and, therefore, the rover touches down at lower velocity. Thus, there is no last-meter free fall associated with engine cutoff, and, because dust kick-up is minimal, the radar antennas can continue to operate even while the rover is being set down on the surface.
The lower impact velocity has two effects.
First, the touchdown velocities can now be reliably brought down to the levels the rover has already been designed for in order for it to be able to travel over the Martian surface.
Second, the low velocity, coupled with the presence of bridles until the rover’s full weight has been transferred to the surface, results in much more stability during landing.
Because the rover does not have to be protected from the impact energy at landing and because there is no need to augment stability at landing, there is no longer a need for a dedicated touchdown system. This, in turn, eliminates the need for a dedicated rover ramp drive off system.
The SLS takes advantage of the fact that the rover’s mobility system is inherently designed to interact with rough, sloping natural terrain. Rovers are designed to have high ground clearance, high static stability, reinforced belly pans, and passive terrain adaptability/conformability. These are all features of an ideal touchdown system.
The Skycrane landing system chosen for Curiosity uses up less weight of the overal landing / rover vehicle, which allowed the scientists to mount more kilograms of instruments on the rover.
Such are the engineering choises when aiming to put a system on Mars.