SpaceX has come a long way. After being shocked by the absence of NASA’s concrete plans for a manned mission to Mars in the early 2000s, Elon Musk, the well-known entrepreneur and engineer, founded SpaceX in order to establish affordable access to space.
After spending a couple of years on designing, building and testing the first privately developed orbital rockets, the fourth launch of the Falcon 1 rocket into orbit was successful, marking the dawn of private space transportation. With its proven capabilities, SpaceX was awarded a substantial contract from NASA for supply missions to the International Space Station, providing the funding for a rapid development of new launch vehicles.
Amongst the many examples, the successful reuse of orbital class rocket boosters and the construction of the currently most powerful rocket worldwide, the Falcon Heavy, are some of the outstanding innovations of SpaceX. Elon Musk aims to continue this streak of success with a seemingly bold decision.
After being ruled out in the early years of spaceflight, stainless steel has now been chosen to serve as the structural material for SpaceX’s Starship and its booster called Super Heavy.
The Starship and Super Heavy combination is aimed to be the successor of the current launch vehicles Falcon 9 and Falcon Heavy, including all lessons learned from the extensive experience obtained with the many prosperous Falcon 9 launches. However, the mission scope of Starship and Super Heavy far exceeds Falcon 9 and an entirely new design has been worked out to meet the additional requirements for interplanetary manned missions.
Strength versus density – the everlasting compromise
Compared to aluminium alloys and carbon fibre reinforced polymers (CFRP), the conventional structural materials used in the aerospace industry in recent decades, stainless steel represents a generally unfavourable option due to its high density. The importance of the material density, or more precisely, the specific strength, for the application in launch vehicles has been outlined in a previous article.
In essence, a low mass of the rocket structure is crucial to maximise the mass of the payload that can be delivered into orbit for a given amount of fuel.
If the specific strength is the only criterion taken into account, stainless steel appears indeed an inferior option against CFRP and aluminium alloys. However, other physical quantities play a major role for launch vehicles due to the extreme environmental conditions like, for example, the vastly different temperatures during ascent and reentry.
The fuel combination for the Raptor engines, used in both the Starship and Super Heavy, is sub-cooled liquid methane and liquid oxygen. The fuel is sub-cooled, meaning cooled to a temperature significantly below its boiling point, to increase its density and to have a safety margin before the fuel starts to boil off and increase the tank pressure.
In order to avoid the tank pressure from reaching a critical level that can cause the tank to rupture, the evaporated fuel must be removed via valves and is no longer available for combustion.
Within the tanks, the fuel is stored at a pressure of about three times atmospheric pressure. A higher pressurisation would offer a marginally higher fuel density but would, in turn, require a significantly more massive tank structure, which renders this option unattractive.
For the Falcon 9, which is equipped with aluminium-lithium alloy tanks fabricated using friction stir welding, the boiling issue is addressed by filling the tanks as close to the launch of the vehicle as possible. Insulation of the tank with foam was dismissed due to the significant additional mass the foam would add.
Only a few millimetres of tank wall separate the fuel at about 66 K  from the atmospheric temperature of about 293 K. The thermal conductivity of the aluminium-lithium alloy used for the previous tank designs is about four times greater than the thermal conductivity of 301 stainless steel. In this case, a low thermal conductivity is favourable to prevent fast conduction of heat from the environment to the cryogenic fuel.
Therefore, stainless steel removes the need for additional insulation and provides a wider time window between the fuelling of the rocket and the actual launch.
Service temperature as a determining factor
The considerations for the material selection up until this point focused on the conditions before and during the launch of the vehicle. However, a fully reusable spacecraft faces even harsher conditions than dealing with the cryogenic fuel and immense mechanical stresses. The Spaceship is designed to be able to make an interplanetary round trip to Mars. The implications of this design goal are far-reaching.
Earlier missions to Mars have shown that the entry velocity into the Martian atmosphere is on the order of 21,000 km/h . To minimise the required fuel for the mission, atmospheric drag is used to reduce the velocity of the vehicle before a final landing burn is conducted. However, the Martian atmosphere has a density of only about one-hundredth of the Earth’s atmosphere, providing very little drag in comparison. Therefore, a very high angle of attack of the spacecraft is needed to use the largest possible surface area for the hypersonic braking manoeuvre.
Entering the atmosphere at seventeen times the speed of sound can lead to temperatures of up to 2000 K on the wind-facing side of the vehicle. Even the best CFPR materials can only sustain up to 480 K before the resin disintegrates and the strength of the composite rapidly decays, leading to structural failure. A significant layer of heat shielding would be required to keep the temperature of the CFPR within its operational limits.
This is where the greatest advantage of stainless steel gets to shine. With stainless steel, the operational window in terms of temperature is expanded to about 1100 K. This lowers the requirements for heat shielding substantially. Moreover, Elon Musk introduced the concept of integrating several components like the tank structure and the heat shield into one, making stainless steel the ideal material for this particular application.
As it can be seen in figure above, only technical ceramics and tungsten alloys offer even higher service temperatures than stainless steels. While tungsten alloys are simply too dense, technical ceramics often come with a severe brittleness, making their application for the repetitive mechanical loads too dangerous because of easy cracking/rupture. These ceramics offer only a minuscule strain before breaking.
Stainless steels, especially at elevated temperatures, can dissipate mechanical stresses via plastic deformation and can survive many stress cycles before breaking.
All energy to the heat shield
Not even the stainless steel 301 that Elon Musk proposed can sustain the extreme temperatures during atmospheric entry. Therefore, the concept of active cooling via the bleeding of either water or liquid methane is being considered to keep the temperature of the steel within its operational constraints.
In detail, the liquid will be pumped between two steel panels on the windward facing side of the vehicle and reach the surface through small pores. Thereby, the liquid will gain heat and evaporate, resulting in a significant cooling effect and dissipation of the heat away from the vehicle. The evaporated coolant is lost with the hot gas flow from the atmosphere.
This concept for heat shielding is superior to abrasive heat shielding in this case, as it requires no maintenance or refurbishment before a second launch and entry. Only the water/methane has to be replenished, which is possible on Mars.
The heat shield tiles are designed with a hexagonal alignment to avoid a straight path for the hot gas to accelerate through the gaps during the entry phase. In a recent tweet, Musk showed the successful testing of the hexagonal tiles up to 1650 K, which is very close to the melting point of 301 stainless. However, the active cooling technique was not applied in the test and is foreseen to cool down the critical areas of the tiles.
The video below is “testing Starship heat-shield hex tiles” taken from the official Elon Musk Twitter account (@ElonMusk).
The (in)significance of cost in space engineering
Historically, the requirements in aerospace engineering were so demanding that cost was often amongst the lowest priorities in the design criteria. No matter how rare a material was and how difficult and costly it was to manufacture, if it was capable of getting the job done, it was likely to be chosen. However, with the progressing commercialisation of space and the strongly rising number of launches per year, this is no longer true.
The raw material cost of the CFRP used in earlier designs is about $135 per kilogram. Furthermore, the machining to manufacture components from the raw material imposes a scrap rate of about 35%, leading to an actual material cost of about $200 per kilogram for the final component. In contrast, stainless steel can be acquired in the range of a mere $3 per kilogram, offering almost two orders of magnitude in cost reduction.
Typically, the cost to launch a kilogram of payload to space far outweighs the cost of the structural materials of the launch vehicle. Nevertheless, in the case of CFRP, the extreme cost reduction that can be achieved with stainless steel was another important factor for the decision. After all, SpaceX intends to construct a fleet of Starship and Super Heavy combinations in order to serve multiple markets, not only to pursue the colonisation of Mars.
Advanced structures: Isogrid and Octaweb
In the course of pushing space engineering to its limits, SpaceX has picked up some well-proven concepts and modified them to be simpler to refurbish or to be cheaper, while maintaining a comparable functionality. One example of this approach is the isogrid structure , which is commonly encountered in many launch vehicle and satellite designs.
The unique isogrid pattern is used to enhance the strength-to-weight ratio of a material. Thereby, the resistance against buckling is increased. Buckling was a major problem for early rocket designs like the Atlas rocket, which once collapsed under its own weight (video below) due to its extremely thin tank walls. The isogrid is an interwoven pattern of I-beams, which acts to increase the stiffness of the overall structure and minimise the amount of material needed.
Apart from its enticing features, the isogrid structure is extraordinarily expensive to manufacture. The commonly pursued route to manufacture such a structure is to start from a thick piece of metal and machine it down with a computerised numerical control. This technique leads to a waste fraction of 95%. For this reason, SpaceX did not use the typical isogrid structure, but obtained a similar effect with stir welded strengthening stringers.
Another impressive design from SpaceX is the Octaweb. The Octaweb is the primary structure bearing the mechanical loads, located at the bottom of the Falcon 9 rocket. This structure carries an array of nine engines and protects each engine from the others by providing a protective bay. The Octaweb is manufactured from a high strength aluminium 7000 series material and bolted, which allows for a much easier and faster refurbishment in contrast to the previous welded designs.
Following the design and testing of the Starship and Super Heavy is an adventure on its own. This article presents the state of the art of some design features and the reasoning behind them. Nevertheless, history has proven that SpaceX follows a philosophy of continuously looking into alternative solutions and does not hesitate to consider and implement radical design changes if they offer superior performance/reliability or a reduction of cost.
The first tests of the Starship design with a prototype “hopper” vehicle are scheduled for 2019 and with them, stainless steel will make its shiny comeback.
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