Landing on Mars

A typical mission to land on Mars consists of atmospheric entry phase using an aeroshell, a parachute descent phase, a terminal descent phase and ends with the spacecraft touching down somewhere within a predetermined landing ellipse. The spacecraft can enter the atmosphere either directly from the Earth-Mars transfer orbit or it can descend after first being captured into Mars orbit. The parachute descent phase begins in the supersonic flight regime and normally utilizes ring-sail or disk-gap-band supersonic parachute. The terminal descent phase has been achieved with retrorockets and lanyards/skycranes, and spacecraft have been cushioned on touchdown with foam and airbags.

To date there have been 11 successful soft landings: Mars 3, Vikings 1 and 2, Pathfinder, Beagle 2, MER A and B (Spirit and Opportunity), Phoenix, MSL (Curiosity), InSight and Perseverance. Mars 3 and Beagle 2 have been declared as successful soft landings though the spacecraft did not operate correctly once on the surface. There have been 4 failed landing attempts: Mars 2, Mars 6, Mars Polar Lander, and the Schiaparelli EDM lander.

Wind tunnels, rocket sleds, and drop tests have been used to test equipment for entry, descent, and landing. Work continues on new technology such as supersonic retropropulsion, ballutes, low-density-supersonic-decelerators (and other expandable entry shields), biconic heat shields (and other entry vehicle shapes), and new ablative or non-ablative heat shield materials.

Supersonic retro-propulsion is the proposed landing method for the Spacex Starship

Why Landing On Mars Is Difficult

It is hard to land on Mars because of its large size and its thin atmosphere. If Mars had a thick atmosphere, it would be straightforward to land with aerobraking, then parachutes, as on Earth or Venus. If Mars had no atmosphere, there would be no concern about surviving the heat of reentry, you could simply use rockets to land. But the thin atmosphere is thick enough to require managing the heat of reentry, and rockets are needed to land large craft. The requirement of multiple system for a landing adds complexity, mass, and increases the chances of failure.

Concepts

Powered landing

An entirely powered landing would be possible for a futuristic high thrust vehicle. This would require a deltaV of about 4.5-to 6 km/s, close to the deltaV required for liftoff from Mars. Such a powered landing would be a requirement for an airless body such as the Moon. However, it makes sense to use the Martian atmosphere to help with deceleration and save on propellant, as in the following proposals.

Supersonic retro-propulsion landing

SpaceX and NASA have proposed supersonic retro-propulsive landing as a method to land large payloads on Mars. SpaceX will be using supersonic retro propulsive landing for Starship and has done simulations of the landing path. This method has been tested and validated on Earth, using SpaceX Falcon 9 first stages in the high atmosphere to simulate Martian conditions. The flight path is a follows:

1. An initial speed reduction using aerodynamic breaking in the upper Martian atmosphere.
2. Retro propulsion in the supersonic flight regime is used to slow the vehicle during part of the flight path. This element of the flight path compensates for the low Martian atmospheric density, that otherwise would be insufficient to slow down the vehicle adequately, preventing a high velocity crash on the surface.
3. More aerodynamic breaking and conventional retro propulsion is used to land.
4. Precise landing within a few meters of target (rather than a km sized landing ellipse) using the capability of very precise landing developed by SpaceX for landing on Earth that could be transposed to Mars.

This flight trajectory provides the capability of landing very large payloads on Mars, in the hundreds of tonnes. The required retro propulsive deltaV depends on the entry velocity. The SpaceX simulations show that the deltaV might be as low as 1% of the entry speed, or 75 m/s, a tiny fraction of the deltaV required for an entirely powered landing.

Large diameter parachutes

Marspedia contributors have supplied the following concepts for EDL technology:

If we need a four hundred foot diameter parachute manufactured in space out of aluminum oxide fiber and sent to Mars in stiff deployed condition instead of being packed, we will not learn about it unless we see a need to experiment. Such a parachute might merit investigation. It would avoid opening shock and might be sufficiently heat resistant to maintain structural integrity during the entire descent in Mars' low gravity well. The larger the diameter of the parachute, the less the max g loading. So let us be honest with ourselves about all necessary colonization technology.

The expected max temperature for ballistic entry into Mars atmosphere is expected to be a thousand or more Kelvin degrees above the melting point of aluminum oxide so coating coarse aluminum oxide fibers with potassium oxide which decomposes at 490 Centigrade might protect the fibers through atmospheric entry by ablative cooling or it might not. A mixture of potassium and sodium oxides as a coating or Teflon as a coating are things that are conceivable. Engineers in this specialty would have a better idea.

High Lift Vertical Landing Vehicle

Another alternative with a greater probability of working, but possibly high cost, is a delta winged entry vehicle or lifting body with insulation like that on the space shuttle. The insulation would be somewhat cheaper because Mars atmospheric entry is less demanding than Earth reentry. After losing most of its orbital velocity to the atmosphere by heating the atmosphere in passing, this vehicle would fly supersonic close to the ground then ignite its rockets for landing. Then it would perform a Pugachev's Cobra maneuver losing horizontal velocity by drag and by rocket thrust. It would then touch down on its tail. Rocket thrust directly into the supersonic slipstream of Mars' atmosphere will not work to safely land on Mars because the supersonic slipstream that the lander flies into would carry the noise of the rocket exhaust right back to the lander. The potential for the chaotic forces of this rocket noise to destabilize the lander's orientation and damage its structure rule out this technique. In the Pugachev's Cobra maneuver, rocket thrust is never directed directly into the supersonic slipstream. The rocket thrust always has a vertical component while the slipstream moves horizontally until the slip stream velocity is reduced to a negligible value.

This sort of vehicle might approach the point of entering a Pugachev's Cobra maneuver by flying horizontally near Mars' surface while increasing angle of attack to maintain lift while killing velocity. At a pitch attitude of 45 degrees there is little lift left to be gained by increasing angle of attack. This should occur at about Mach 2.5, which is about 600 meters per second on Mars. Then the rockets are ignited generating two Mars gravities of acceleration and the angle of attack is further increased past 90 degrees to generate negative lift and keep the vehicle in horizontal flight. As the speed decreases and negative lift generated by the wings decreases, the pitch angle is increased to reduce the component of rocket thrust in the vertical direction and increase the component of rocket thrust directed to braking. As the vehicle eventually slows to a stop in horizontal motion, a combination of throttling and thrust deflection reduces thrust to about 1 Mars gravity, the vehicle moves to a 90 degree pitch angle and settles on its tail. A guesstimate of the required rocket delta V for killing the last 600 meters per second and landing in this way is about 850 meters per second. This includes the amount of speed lost to atmospheric drag and very substantial gravity losses.

The Sky Crane

the 2009 Mars Science Laboratory (MSL) rover, weighing 775 kilograms (versus MER at 175.4 kilograms each) requires an entirely new landing architecture. Too massive for airbags, the small-car sized rover used successfully a landing system dubbed the Sky Crane. "Even though some people laugh when they first see it, my personal view is that the Sky Crane is actually the most elegant system we've come up with yet, and the simplest," said Manning. MSL used a combination of a rocket-guided entry with a heat shield, a parachute, then thrusters to slow the vehicle even more, followed by a crane-like system that lowers the rover on a cable for a soft landing directly on its wheels. Given the success of the Sky Crane with MSL, it's likely that this system can be scaled for larger payloads, but probably not the size needed to land humans on Mars. (See Ref #1)

A Sure Way to Land on Mars

A sure but expensive way to land on Mars with a ten metric ton vehicle is to build a heat shield in orbit around Earth and send it to Mars as part of the spacecraft. After the heat shield slows the spacecraft, rockets bring it to a safe stop on Mars. Since Mars' atmosphere at the surface is one hundredth the density of Earth's atmosphere at the surface, make the heat shield proportionally bigger. Considering that the 12,250 pound Apollo command module was 12.8 feet in diameter, a ten metric ton Mars lander should have a 52 meter diameter heat shield. Assembled from 127 roughly hexagonal pieces about 4 meters in diameter, this would be a hexagonal heat shield instead of a round one. That should do. Each hexagonal piece would have a layer of ablative material on one side of a hexagon of aircraft grade aluminum. Aluminum t cross section extrusions would be fastened to the Aluminum sheet as stiffeners. In orbit, two hexagon sections would have their ablative sections butted against each other, protrusions fitting into cavities. A small gap would remain between the aluminum sheets. A 2 inch strip along the edge of each aluminum sheet would be pre-coated with brazing material. A 4 inch wide strip of aluminum to join them would likewise be coated with brazing material on one side. A ridge on the joining strip would fit in the gap between the aluminum hexagons. Then an iron heated to the right temperature would be placed on the joining strip and left for the right time. When the iron is removed and the piece cools the two hexagons make one piece with brazing material partially filling the gap between the two hexagons and rounding out the corner where the hexagons meet the joining strip. Likewise, a trusswork joining the stiffeners of all of the hexagons would be assembled by the same brazing technique and make the whole heat shield one strong rigid light weight piece. Some work has already been done considering robotic truss assembly on orbit. Light-Weight Mobile Robot for Space Station Trusswork A cone section for the sphere-cone reentry vehicle would also need to be built or a somewhat different shaped section serving the same aerodynamic function, avoiding excessive, uncontrolled and chaotic side slip.

The advantage of sending up a ten ton vehicle, many pieces of heat shield and a robotic assembly station two make a big heat shield as compared to sending up a vehicle with heat shield and parachutes on an Ares V is that the big assembled-on-orbit heat shield would allow a 10 ton vehicle to land cargo safely on Mars while the Ares V scheme would not land cargo or people safely on Mars. Mars direct would do no better. See The mars landing Approach: Getting Large Payloads to the Surface of the Red Planet

Just as all economic activity in orbit so far has been done by robots, assembling a spacecraft to go to Mars should be done by robots and setting up the infrastructure for people to survive on Mars should be done by robots. There are some technical difficulties with this approach that must be addressed, but they seem likely to be amenable to solution.

Terminal Velocity of the Big Heat Shield Vehicle

For a rough estimate take as a starting point the estimated terminal velocity for the Apollo reentry vehicle. Estimate that the square of the terminal velocity is directly proportional to mass and the force of gravity and inversely proportional to air density and cross sectional area. The 10 metric ton vehicle with a 52 meter diameter heat shield should then have a terminal velocity less than 56 meters per second (125 miles per hour) at Mars' surface. If near the end of this descent a hole is burst through the bottom of the heat shield in the center right under the payload and hinges swing that portion of the heat shield out of the way, then retro rockets can fire at an altitude of 160 meters for 5.1 seconds with an acceleration of 11 meters per second squared and bring the payload to a stop about 17 meters above the heat shield that crashes into Mars. A few more seconds for horizontal maneuvering and throttling the rocket motor brings the payload safely to Mars on landing legs. A guesstimate of the required rocket delta V for this maneuver is about 68 meters per second.