Starship Shield: how it will survive a flight to Mars and back

On April 17, SpaceX was on the verge of completing one of the greatest milestones in space exploration: the first orbital test of the Starship spacecraft and the most powerful Super Heavy space rocket ever built. Just 40 seconds before liftoff, a problem with one of the rocket’s pressurization valves freezing caused the flight to be canceled until the next launch window, scheduled for April 20th.

If all goes as expected, the starship will be powered by the superheavy for approximately 3 minutes. From there, both will be separated. The rocket will land in the sea near the Gulf of Mexico, and the plane will continue on its trajectory until it reaches Earth orbit at an altitude of 150 to 250 km. After an hour and a half of flight (without reaching orbit), it will fall into the Pacific Ocean about 400 km from the island of Kauai, Hawaii.

However, the ambition of SpaceX and its CEO Elon Musk is to reach Mars with a manned mission. Not only to reach, but also to return to Earth, which represents a technological challenge never seen before in the history of space exploration.

SpaceX spacecraft promotional video.

The main thing is to return from Mars

One of the main concerns is the design of your heat shield. The main requirement to be met, beyond guaranteeing the survival of the payload or the astronauts it carries, is to allow the ship to be reused to return to Earth. To date, all spacecraft that have used such a heat shield have undergone a single re-entry maneuver. In this case, he would have to survive two: the Martian and one back to the earth’s surface.

Space programs have used reentry capsules of a similar design for decades, including the Mercury, Gemini, Apollo, Orion, and SpaceX Dragon capsules. They all need a heat shield to dissipate the heat generated during re-entry, which can be up to 50% of its design.

The heat shields use ablative materials that break down to dissipate the heat transferred to the capsule by the high-speed flow of gases surrounding the vehicle. Upon entering the atmosphere, temperatures up to 3000ºC can be reached on the surface of the shield, which is incompatible with astronauts living inside. The combined reaction of the ablative material and heat dissipation by radiation should prevent overheating of the capsule structure and its interior.

Can return pods be used for round trip flights? Not really. They cannot be reused due to severe degradation during flight.

The Space Shuttle program of the 1970s was the first step in the development of reentry vehicles. The task of the heat shield in both cases was the same: to minimize heat transfer to the vehicle interior. However, the means used varied considerably because the Space Shuttle followed a trajectory based on sustained flight, with lower thermal requirements and reaching cooler temperatures.

The design of the shield was based on the use of various types of thermal insulation, mainly in the form of tiles, which made it easy to replace it after re-introduction. These tiles consisted of a very low conductivity quartz fiber core (almost 90% air) stiffened with a coating that also maximized heat dissipation by radiation.

The Space Shuttle was reused after proper maintenance between flights and, if necessary, damaged tiles were replaced with exactly the same ones.

On a mission to Mars, in which the Starship would have to maneuver to enter the Martian atmosphere, part of the heat shield would be expected to deteriorate due to the high temperatures it would be subjected to. A repair process will be required before returning to Earth.

However, this was not part of Elon Musk’s plans. According to him: “The starship should be ready to fly again immediately after landing. Zero repair.

double skinned spacecraft

The first idea that came to the mind of SpaceX engineers was absolutely revolutionary: a completely open stainless steel ship without traces of a heat shield to protect the ship during re-entry. How was this to be achieved?

If we take nature itself as a benchmark, we might ask ourselves how the human body is cooled. Sweat, mostly water, evaporates on contact with a dry environment. However, in order to evaporate water, it needs energy, which it takes from our own body, thereby managing to maintain its temperature. This process is called evaporative cooling and has been used for decades in industry and in thermal and nuclear power plants as a cooling mechanism.

Transferring this to starship design, a double-skinned spacecraft could be developed. The outermost ones need to be porous so that a stream of liquid methane circulates between them when entering the atmosphere, given how easy it is to get on Mars, for example. The methane will absorb a lot of heat as it enters the atmosphere, vaporize and exit the ship through the pores. But it’s too difficult.

ceramic tiles

The complexity of this type of solution prompted SpaceX to opt for a passive heat shield, which is conceptually similar to the Space Shuttle in many ways.

Recent tests show that nearly two-thirds of the spacecraft’s surface will be covered by more than 18,000 hexagonal tiles (ceramic in this case) on top of a stainless steel structure. A kind of silica or alumina fiber coating will be placed between the tiles and the structure to insulate the internal structure from the outside exposed to higher temperatures.

These tiles are fixed through three connection points, maintaining a certain relative movement between them, except for the most critical parts of the building, where adhesive is used to reduce the risk of delamination.

The uniformity of the shield’s design makes it relatively easy to replace damaged tiles with others, without the need to have special spare parts for each of them, as was the case with the space shuttle.

Along with other innovations in engineering and technology, Starship’s heat shield is paving the way for travel to Mars and other deep space exploration that could hold the key to humanity’s future in space.

David González-Barcena, ETSIAE Associate Professor of Fluid Mechanics and Aerospace Propulsion and Research Fellow, Institute of Microgravity, Ignacio da Riva University, Polytechnic University of Madrid (UPM)

This article was originally published on The Conversation. Read the original.

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