Landing on Mars

Landing on Mars is a difficult problem.

To date over 60% of the missions [to the Martian surface] have failed. The scientists and engineers of these undertakings use phrases like "Six Minutes of Terror," and "The Great Galactic Ghoul" to illustrate their experiences, evidence of the anxiety that's evoked by sending a robotic spacecraft to Mars — even among those who have devoted their careers to the task. But mention sending a human mission to land on the Red Planet, with payloads several factors larger than an unmanned spacecraft and the trepidation among that same group grows even larger.

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 course 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.

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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.