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SERIES 300 STAINLESS-STEEL IN ROCKET DESIGN
Eric Trimbur eat44@pitt.edu, Darini Rajesh dar186@pitt.edu, Emma Lipinski ecl52@pitt.edu
Abstract—Shielding spaceborne payloads from the heat of entirety of a craft’s speed must be slowed to zero within a few
reentry is a difficult challenge with landing payloads on Mars hundred miles. Usually the atmosphere does the brunt of the
and returning vehicles to Earth. Almost all spacecrafts have work decelerating the craft and parachutes finish the job. This
used either an ablative heatshield like that used on the Apollo interaction with the atmosphere has the unfortunate
11 Command Module, or an insulating one like that used on consequence of generating a lot of heat of which the craft
the Space Shuttle. While these technologies are well known, needs to withstand. This is especially true in cases where
they both have limitations that make them unsuitable for some humans are onboard; the craft not only needs not to melt but
missions. SpaceX and their new launch vehicle, Starship, is maintain a non-lethal temperature for the astronauts on board.
meant to be a versatile spacecraft capable of missions The technologies that solve this problem are known as
requiring Earth and Mars reentry. Ablative heatshield heatshields and are often the final stage components of a
technology, while very effective, is an expendable system that spacecraft that take up the most weight [1].
would work poorly on a vehicle like Starship that is meant to While many heatshield designs have been used with
be reusable. Trying to copy the Space Shuttle’s design, nearly perfect track records, new ambitious missions have
another reusable spacecraft, would be a poor idea as the pushed for new technologies. With the announcement of their
insulating tiles are arguably the reason for the program’s new spacecraft, “Starship,” SpaceX is pushing for the vehicle
failure; it took months to inspect the tiles for damage and to have the capability of reentering Earth’s and Mars’s
refurbish them before being able to reuse the craft. This is not atmosphere multiple times without refurbishment. Starship’s
to mention the several times the tiles almost or completely projected entries will cause more damage than that from cargo
failed as in the case of Space Shuttle Colombia. To avoid these resupply missions to the ISS because the orbits they are
woes, SpaceX is opting to use series 300 stainless-steel on returning from will be of a much higher velocity. SpaceX has
Starship. While the steel naturally has a high temperature pushed for a never before used stainless-steel heatshield that
resistance, it is not enough to withstand atmospheric reentry. will enable the craft to carry out its missions [2,3].
SpaceX will employ an active cooling system, running Currently, a prototype of the vehicle dubbed
cryogenic fuel under the steel surface to carry heat away from “Starhopper” has already been constructed and will begin
the surface. While cooling techniques like this have been used testing between March and April of this year. The design
in the past to cool rocket engine nozzles, never have they been choice has brought forth waves of criticism. When looking at
used on the surface of a spacecraft. SpaceX will take the previous technologies and the mission of Starship, it
advantage of stainless-steel’s unique property of gaining becomes clear why the choice was made.
strength at cryogenic temperature in order to reduce the
overall weight of the vehicle into an acceptable margin.
Key Words—Active cooling, Atmospheric reentry, Cryogenic
temperatures, Heatshield, SpaceX.
INTRODUCTION: DIFFICULTIES OF
ATMOSPHERIC REETRY
Whether it be cargo from the ISS or astronauts from the
Moon, returning spacecraft from orbit to the surface of Earth
is one of the most critical aspects of any mission. Even at the
slowest speed for stable orbits, 17000 miles per hour, the
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at high heat fluxes [7]. An important component of PICA was
its ceramic carbon substrate that replaced a more traditional
polymer glue. The ceramic is a lot harder which allowed it to
withstand much higher temperatures without becoming
damaged. This change in material permits the heat shield to
survive in extreme temperatures up to 5,000-degrees
Fahrenheit and high velocities of up to 28,600 mph.
More recently, Space-X worked with NASA to
create a different version of the original PICA heat shield
known as PICA-X. It has been tested on Space-X’s Dragon-1
space craft [7]. The biggest difference between PICA and
PICA-X was that PICA covered an area that was about one
meter in diameter while PICA-X needed a larger area covered
on Dragon-1. The Dragon-1 designers fixed this problem by
creating small tiles of PICA and forming them into a larger
heat shield of about four meters in diameter (PICA-X) [7]. In
FIGURE 1 [4] 2010, Dragon-1 successfully re-entered the Earth’s
Starship compared to Starhopper shuttle. atmosphere during a demo flight, and it was followed by more
International Space Station re-supply missions [7]. Stated in
HISTORIC HEATSHIELDS an article by SpaceX, it is the most advanced heat shield, and
it is expected to last through about one hundred re-entries with
SpaceX’s choice to use stainless-steel implies that little degrading. Also, it can endure the high velocity reentries
current technologies were unsuitable. There are specific from the Moon or Mars that produce harsh temperatures [8].
technologies with relationships to Starship that demonstrate PICA and PICA-X were useful in older spacecraft
alternative technologies. One is the Dragon capsule for which
like Stardust and Dragon-1, but SpaceX’s is looking for
SpaceX invested in developing an ablative heatshield for.
something even more efficient at cooling the surface of its
Another, the Space Shuttle, was a human-bearing reusable
vehicle like Starship which used insulating tiles to protect the new craft, Starship. Seeing that us humans haven’t taken
crew inside. While both technologies worked successfully, a people to Mars yet with the technology we have, we
more in depth look at them is required to understand why they undoubtedly need a new type of heat shield that can guarantee
are not applicable for Starship. entry to Mars and then another reentry to Earth without
refurbishment in-between.
PICA: Ablative Heatshields
Space Shuttle Tiles: Insulative Heatshields
NASA planned to send a spacecraft billions of miles
into space to retrieve materials from comets, so they needed a In 1981, the Space Shuttle Columbia commenced
new way of cooling the craft upon reentry [5]. They turned to NASA’s flight program as the first reusable Space Shuttle [9].
ablative heat shield technology. Ablation is “energy The reusability of the Space Shuttle program revolves around
management through material consumption” [6]. Ablative the insulative tiles surrounding the shuttle that protect it
matter is usually made from two composite materials that during reentry into Earth’s atmosphere. According to the
have contrasting physical properties. When joined, the new Kennedy Space Center, such tiles are composed of “a low-
material has traits that are different from the original two density, high-purity silica 99.8-percent amorphous
components. When exposed to heat, the ablative matter fiber…insulation that is made rigid by ceramic bonding” [10].
begins to decompose which releases gasses at cool This material can withstand the shock of recurrent extreme
temperatures. These gases flow through the surface of the heating and cooling of temperatures ranging from -250 to
ablator to keep it from getting too hot [6]. This technology almost 3,000 degrees Fahrenheit. In fact, heat on the surface
was necessary for NASA to have any chance at bringing back of each tile dissipates so quickly that seconds after being taken
their Stardust spacecraft in one piece. out of a 2,300-degree Fahrenheit oven, the tile can be touched
PICA, a material that NASA discovered in the early by a hand on its edges [10].
1990s, is a type of ablative heatshield technology; it stands for While the technology of the insulating tiles sounds like
“phenolic impregnated carbon ablator.” According to NASA, a satisfactory system of protecting shuttles, their technology
PICA was originally used as a heat shield for the Stardust is not perfect. On many flights, tiles are lost or damaged upon
mission due to its low density and efficient ablative capacity launching or reentry. Written in an article by Air & Space
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Smithsonian, to be able to perform for its next flight, each The specific stainless-steel that will be used on Starship
shuttle must undergo refurbishment that includes the is 300 series stainless-steel which is visually comparable to
replacement of up to one hundred tiles. A task like this may other steels despite its vastly different properties. Its high
not seem very hard, but each tile is unique in area and nickel content changes the crystal structure to be “face-
thickness depending on where it is on the orbiter and how centered cubic.” This crystal structure gives the steel
much heat that area is expected to endure [11]. The increased strength. Even at cryogenic temperatures, the steel
not only maintains its strength, but can become up to 50%
requirement of tile replacement after each mission is not
stronger [14]. This is in contrast to most other materials, such
feasible for a craft like Starship, as it needs to be fully reusable
as cast iron, which become extremely brittle at cryogenic
for multiple reentries. temperatures to the point that they shatter when impacted
[15]. The structure of a spacecraft must maintain its strength
and ductility through a wide range of temperatures as the fuels
can be cooled to only a few degrees kelvin where the surface
could experience thousands of degrees upon reentry.
It might not seem worth mentioning, but stainless-steel
is shiny. In this way it can act like a superior version of the
reflective white paint used on the majority of spacecrafts.
While the heatshield will protect the windward side from the
heat of impacting the atmosphere, the leeward side still
FIGURE 2 [12] experiences radiant heating from the light produced from the
friction between the craft and the atmosphere. The white
Columbia’s heat shield tiles.
coating helps reflect most of that heat away and the shiny
The loss of tiles surrounding Space Shuttle’s orbiters
property of steel would work just the same, if not better [16].
created the biggest problem during the disintegration of Stainless-steel also has interesting interactions from a
Columbia on January 16th, 2003 during reentry into orbit. thermal standpoint. it is of the least conductive metals there
According to the “Columbia Crew Survival Investigation are. By no means does it insulate the inside of the craft, but it
Report” by NASA, a piece of insulating foam had detached won’t allow the heat to seep inside with ease. The steel is also
during launch which then left a hole in the protective tiles. able to withstand extreme temperatures up to 2500 degrees
Upon reentry, the shuttle’s inner skin most likely wrinkled Fahrenheit before melting, although in order to maintain
due to compression which loosened and detached more tiles structural integrity it would be unwise to push it that far. In
ultimately causing catastrophic thermal damage to Columbia comparison the Space Shuttle tiles could withstand 2300
[13]. degrees Fahrenheit [17]. This qualifies steel to work on both
The Space Shuttle program did successfully reuse many ends of extreme temperatures. Furthermore, after cooling
orbiters; however, the insulating heatshield technology of back down the steel maintains all of its properties of strength
and temperature resistance.
such shuttles is not viable for a shuttle like Starship. SpaceX’s
Starship needs a cooling system that works 100% of the time
unlike the tiles used on Columbia. It also has the goal of being
STAINLESS-STEEL IN ROCKETRY
completely reusable and efficient which insulating tiles have
Starship will be the first spacecraft to use steel as a part
proven not to be as replacement between uses is essential. A of its heatshield, but there are other rockets that used steel for
technology with little to no refurbishment requirement would other reasons. Early Atlas rockets used a balloon like steel
be a better option for the cooling of Starship during reentry structure, where a thin steel shell would be pressurized to give
into Earth’s atmosphere. rigidity to the vehicle. In one infamous failed launch the
depressurization of the vehicle caused the whole structure to
300 SERIES STAINLESS-STEEL fold over itself under its own weight [18]. Later iterations of
PROPERTIES the Atlas rocket switched over to an aluminum structure with
an isogrid pattern [19]. This is just a pattern of triangular ribs
Stainless-steel on Starship was first teased by Elon on the inside of what would otherwise be a hollow shell.
Musk on Twitter when he described the change as While stronger than simple sheets, this isogrid pattern is
“delightfully counter-intuitive.” Stainless-steel is well known difficult to manufacture, something SpaceX knows well as
as a heavy material and isn’t a traditional rocket material. they have used it in their Dragon capsule [20]. If steel sheets
While that is true, there are more properties of stainless-steel could be made to have the same rigidity without the isogrid
than that, some that are obvious with non-obvious pattern, it would be a powerful technology.
applications and some that are lesser-known.
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would stand to reason that Starship will at least use some of
the fuel will cycle back into the engine this way. The best part
is that this effectively kills three birds with one stone, by
running the liquid methane under the surface it cools the
surface, gives the steel strength, and forces fuel into the
engine.
This process is actually similar to that of the ablative
heatshields, but this time rather than a solid material ablating
away it is the liquid methane evaporating off. The advantage
Starship’s technology has over the ablative heatshields is the
ease of replacing what is lost. With ablative heatshields the
surface is lost, and the manufacturing process is too difficult
to refurbish/replace it on Mars. With liquid methane however
the process is almost trivial. Documented back in the earliest
announcement of Starship, known then as the ITS or,
Interplanetary Transport System, the plan was to have one-
way unmanned missions sendoff machines that would
FIGURE 3 [20] produce methane through the Sabatier process. The process
Isogrid pattern in the Dragon capsule. takes abundant CO2 in the Martian atmosphere and reacts it
A shadow of such a technology does exist, carbon composites. with hydrogen to produce the methane. The hydrogen would
The original plan for Starship was to use carbon composites, have been produced by the electrolysis of water which would
a prototype tank was even built and shown off to prove the have been mined from the Martian soil and powered through
technology [2]. In terms of cost however, steel is far cheaper. solar panels [2]. This ensures that missions would be able to
Not only is the carbon composite and expensive material, it is go to Mars, enter the atmosphere, refuel, return to Earth, and
even more expensive and difficult to manufacture, and a lot still be able to reenter the atmosphere here as well.
of the material ends up wasted anyways. Steel on the other The first mission to put humans on Mars could start in
hand is an incredibly mailable material that an be formed into the mid-2020s. Once Starship and its complementary booster,
almost any shape for extremely cheap. This is what has “Superheavy,” have completed construction, there is nothing
allowed for the Starhopper prototype to be constructed so holding SpaceX back from going to Mars; this would be their
quickly [21]. In order to compete in strength to weight ratio own, internal mission. A Starship going to Mars would be
however, Starship will have to use innovative techniques. launched with less than full fuel to help get it into orbit. The
Superheavy booster would then return to launch site, much
STAINLESS-STEEL ON THE STARSHIP like Falcon 9 boosters do already. Then, a fuel bearing version
of Starship would be loaded onto the same booster where it
The design for Starship takes all of steel’s useful would be launched into the orbit as the first Starship. The fuel
properties and marries them into one vehicle. Starship will use would transfer over into the first Starship before the second
liquid methane and liquid oxygen at cryogenic temperatures one would de-orbit and land much like the booster did. At this
as fuel for the Raptor engines. By flowing the fuel underneath point the primary Starship would head off to Mars. When it
the stainless-steel skin, it will add strength without the need arrives, its superior heatshield means it would not have to get
to add any new cooling fluids This also opens up the into orbit first, rather, go straight in for a landing. Going at
opportunity for a never before used cooling technique. There tens of thousands of miles per hour, Starship would crash
will be tiny slits all over the surface of Starship that allow for through the atmosphere, keeping cool the whole way down as
the gaseous form of the fuel to leak out and provide a heat some of the last of the methane fuel is used to both slow down
shielding effect similar to human sweat. While we have pores the booster and leak out through the pores in the stainless-
in our skin that let out liquid water to evaporate, Starship has steel surface, keeping the internals cool. At this speed the craft
tiny slits that let out liquid methane at cryogenic temperatures hits the atmosphere causing the emission of large quantities
that then violently boils off after being exposed to of light. The shiny stainless-steel backside of the craft reflects
temperatures at potentially thousands of degrees Fahrenheit. this into space creating a streak of light at Starship slows
This is the technology that defines Starship’s heatshield [21]. down to just a few hundred miles per hour. At that point the
A similar technology has been in place for cooling rocket Raptor engines would fire slowing the vehicle down to zero
nozzles since the Apollo era, but it has never been used to cool as it lightly touched down on the surface. While the vehicle
an entire vehicle [22]. Systems like this would run pipes along would then be nearly out of fuel, machines capable of
the outside of the rocket nozzle through which the liquid fuel producing the methane gas would be deployed and begin
would flow. The fuel would pick up the heat and begin to making the fuel that would eventually bring the passengers
vaporize pushing the fuel through the system and ultimately back to Earth.
into the engine itself where it would react with the oxidizer. It
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CONCLUSION: THE SHINY STAINLESS- 2019 and the world will hold its breath to see if this project
STEEL FUTURE (OR LACK THEREOF) gets off on the right foot.
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Accessed 1.27.2019. We would first like to acknowledge our parents for
https://www.popularmechanics.com/space/rockets/a2595366 sending us to the University of Pittsburgh and for paying our
3/elon-musk-spacex-bfr-stainless-steel/ tuition. Without them, we would not be in the Swanson
[22] D. Ellis, L. Pagel, D. Scheffer. “Design and Fabrication School of Engineering doing this conference. We would like
of a Radiative Actively Cooled Honeycomb Sandwich to acknowledge Dr. Sanchez for motivating us to keep doing
Structural Panel for a Hypersonic Aircraft.” Langley our best and for always providing us with the support we need.
Research Center. March 1978. Accessed 01.27.2019. In addition, we would like to thank our writing instructor,
https://apps.dtic.mil/dtic/tr/fulltext/u2/a302033.pdf. pp.14-22 Daniel McMillan, for giving us advice and critiques on how
[23] “Dear Moon.” SpaceToday Inc. Accessed 03.08.2019. to improve our paper. A huge shout-out to our co-chair,
https://dearmoon.earth/ Nolan, who gave us great advice throughout the writing
process. Without these people, our team would not have been
able to accomplish a successful, well-written paper to present
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