On April 17, the SpaceX company was on the verge of completing one of the great milestones of space exploration: the first orbital flight test of the Starship ship 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 meant the cancellation of the flight until the next launch window, scheduled for April 20.
If everything goes as expected, the Starship will be boosted by the Super Heavy for approximately 3 minutes. From there, both will uncouple. The rocket will land in the sea near the Gulf of Mexico, while the aircraft will continue its trajectory until reaching an 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 arriving, but also returning to Earth, which represents a technological challenge never faced in the history of space exploration.
The important thing is to return from Mars
One of the main challenges is the design of its heat shield. The main requirement that must be met, apart from guaranteeing the survival of the payload or the astronauts it transports, is to allow the reuse of the ship for the return to Earth. To date, all space vehicles that have used a similar heat shield have been exposed to a single re-entry maneuver into Earth’s atmosphere. In this case it would have to endure two: the Martian one and the return to the Earth’s surface.
Space programs have employed reentry capsules with similar designs for decades, including the Mercury, Gemini, Apollo, Orion capsules and SpaceX’s Dragon capsule. 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 degrade to dissipate heat transmitted to the capsule by the stream of high-velocity gases surrounding the vehicle. During reentry, temperatures of up to 3,000ºC can be reached on the surface of the shield, something incompatible with the life of astronauts inside. The combined response of the ablative material and radiation heat dissipation should prevent the capsule structure and its interior from overheating.
Could reentry capsules be useful for a round trip? Not really. These are not reusable, due to the high degradation they suffer during flight.
The Space Shuttle program during the 1970s marked the first step in the development of reusable reentry 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 a supported flight, less thermally demanding, reaching lower temperatures.
The design of the shield was based on the use of various varieties of thermal insulators, mostly in the form of tiles, which allowed easy replacement after reentry. These tiles consisted of a very low-conductive silica fiber filling (practically made up of 90% air) which were provided with the necessary rigidity through a coating whose properties also allowed maximizing 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 an entry maneuver into the Martian atmosphere, it is expected that part of the thermal 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 spaceship with double skin
The first idea that crossed the minds of SpaceX engineers was absolutely revolutionary: a completely exposed stainless steel ship, with no trace of a heat shield to protect the ship during entry into the atmosphere. How was it intended to be achieved?
Starship test vehicle under assembly will look similar to this illustration when finished. Operational Starships would obv have windows, etc. pic.twitter.com/D8AJ01mjyR
— Elon Musk (@elonmusk) January 5, 2019
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, water, to evaporate, needs energy that it takes from our own body, thus managing to maintain its temperature. This process is called evaportative 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 spacecraft with a double skin could be developed. The outermost one would be porous, so that during reentry a flow of liquid methane would circulate between them, for example, given the ease of obtaining it on Mars. The methane would absorb a large amount of heat during reentry, evaporating and exiting the vehicle through the pores. But it’s too complex.
Ceramic type tiles
The complexity of this type of solution made SpaceX opt for a passive heat shield that bears many similarities to the Space Shuttle on a conceptual level.
In the latest tests it is observed that practically two thirds of the surface of the spacecraft will be covered by more than 18,000 tiles (in this case ceramic type) with a hexagonal shape, arranged on the stainless steel structure. A type 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.
These tiles are fixed through three joining points, maintaining a certain relative movement between them except in the most critical parts of the warehouse, 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 the need to have specific 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 paves the way for travel to Mars and other long-distance space exploration, which could prove key to humanity’s future in space.
This article has been published in ‘The conversation‘.