2023-04-19 13:57:05
On April 17, the SpaceX company was on the brink of completing one of the great milestones in space exploration: the first orbital flight test of the Starship spacecraft and the most powerful space rocket ever created, the Super Heavy. Just 40 seconds before launch, a freezing problem in one of the rocket’s pressurization valves led to the cancellation of the flight until the next launch window, scheduled for April 20.
If all goes as expected, the Starship will be powered by the Super Heavy for approximately 3 minutes. From there, both will be decoupled. The rocket will land in the sea near the Gulf of Mexico, while the aircraft will continue its trajectory until it reaches Earth orbit between 150 and 250 km altitude. After an hour and a half of flight (without completing the 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 before faced in the history of space exploration.
The important thing is to return from Mars
One of the main challenges is the design of your heat shield. The main requirement that must be met, apart from guaranteeing the survival of the payload or the astronauts it carries, is to allow the ship to be reused for the return to Earth. To date, all spacecraft that have used a similar heat shield have been exposed to a single reentry maneuver into the Earth’s atmosphere. In this case, it would have to endure two: the Martian one and the one back to the terrestrial surface.
Space programs have used reentry capsules with similar designs for decades, including the Mercury, Gemini, Apollo, Orion and SpaceX Dragon capsules. All of them need a thermal shield to dissipate the heat generated during reentry, which can represent up to 50% of its structure.
Heat shields use ablative materials, which break down to dissipate heat transmitted to the capsule by the high-velocity stream of gases surrounding the vehicle. During reentry, temperatures of up to 3,000ºC can be reached on the surface of the shield, something incompatible with astronauts living inside. The combined response of the ablative material and radiation heat dissipation would have to prevent the capsule structure and its interior from overheating.
Could the reentry pods be used for a round trip? Not really. These are not reusable, due to the high degradation they suffer during the flight.
The Space Shuttle program of the 1970s marked the first step in the development of re-entry vehicles. The objective of the heat shield in both cases was the same: to minimize heat transfer to the interior of the vehicle. However, the means used differed significantly because the space shuttle followed a trajectory based on sustained flight, less thermally demanding, reaching lower temperatures.
The design of the shield was based on the use of various varieties of thermal insulation, mostly in the form of tiles, which allowed easy replacement after re-entry. These tiles consisted of a very low conductive silica fiber filling (practically made up of 90% air) which was given the necessary rigidity by means of a coating whose properties also allowed it to maximize heat dissipation by radiation.
The Space Shuttle was reused after the corresponding maintenance between flights and, if necessary, the damaged tiles were replaced with exactly the same ones.
On a trip to Mars, in which the Starship will have to make a maneuver to enter the Martian atmosphere, it is to be expected that part of the heat shield will deteriorate due to the high temperatures to which it will be exposed. A repair process would be necessary before returning to Earth.
However, this was not in Elon Musk’s plans. In his words: “The Starship needs to be ready to fly again immediately after landing. Zero remodeling.”
A double-skinned spaceship
The first idea that came to the mind of SpaceX engineers was absolutely revolutionary: a completely exposed stainless steel ship, with no trace of a heat shield to protect the ship during re-entry. How was it intended to be achieved?
If we take nature itself as a reference, we could ask ourselves how the human body cools itself. Sweat, basically water, evaporates when it comes into contact with a dry environment. However, to evaporate water, it needs energy that it takes from our own body, thus managing to maintain its temperature. This process is called evaporative cooling, something that has been used for decades in industry and in thermal and nuclear power plants as a cooling mechanism.
If we transfer this to the design of the Starship, a spaceship with a double skin could be developed. The outermost would be porous, so that during reentry a flow of liquid methane circulated between them, for example, given how easy it is to obtain it on Mars. The methane would absorb a great deal of heat during reentry, evaporating and exiting the vehicle through the pores. But it is too complex.
ceramic type tiles
The complexity of this type of solution led SpaceX to opt for a passive heat shield that bears many similarities to the Space Shuttle on a conceptual level.
The latest tests show that practically two thirds of the surface of the spacecraft will be covered by more than 18,000 tiles (in this case ceramic) with a hexagonal shape, arranged on the stainless steel structure. A kind of blanket made of silica or alumina fibers will be placed between the tiles and the structure to insulate the internal structure from the outside, exposed to higher temperatures.
Twenty Tiles – Image Taken: January 12, 2023 pic.twitter.com/6vleWbC7hZ
— Starbase Surfer (@cnunezimages) January 13, 2023
These tiles are fixed through three joining points, maintaining a certain relative movement between them, except in the most critical parts of the building, where an adhesive is used to reduce the risk of detachment.
The homogeneity of the shield design makes it relatively easy to replace damaged tiles with others, without having to have specific spare parts for each of them as was the case with the Space Shuttle.
Related news
Along with other innovations in engineering and technology, the Starship’s heat shield paves the way for travel to Mars, and for other long-distance space exploration, which could prove key to humanity’s future in space.
This article was originally published on The Conversation. Read the original.
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