ONE LARGE STEP CLOSER TO MARS

For the last few years, space enthusiasts, otherwise known as full-blown nerds such as myself, have waited patiently for the latest generation of SpaceX rockets. And with the most recent Starship update, we have seen some big changes made to the SpaceX family of rockets.

If you have spent a lot of time on the internet lately you are aware of the change: SpaceX switched the body of its rocket, or fuselage, from a carbon-fiber composite to an all-stainless steel build.

If you have seen photos or videos, the rocket is very striking. The Starship Mark 1 stands 164 feet tall, dwarfs most surrounding buildings, and its look appears to be straight out of  a sci-fi film. It just catches the eye with the stainless steel, which makes it, for lack of a better term, just so shiny.

After admiring the stunning appearance of the rocket and watching Elon Musk’s update speech delivered in front of it, while a video drone tries its best to fit Musk and Starship into the same frame, I began to ask myself a big question: Why switch to stainless steel?

For those who don’t know rockets, the name of the game is saving weight. To gloss over a wealth of intense rocket engineering science, the more the fuselage weighs, the more fuel a rocket needs to get to its destination. But fuel adds to the weight, so you have a heavier rocket, which then needs more fuel to lift the fuel and now, a larger fuselage as well, which adds more weight…

This is what  astronaut Don Pettit describes as “The tyranny of the rocket equation.”

So, back to the question: Why would anyone in their right mind switch the fuselage from extremely lightweight, durable, carbon-fiber composite, to bulky, not-used-on-rockets-since-the-1960s stainless steel?

On the surface, it almost seems like the material was switched for looks alone, but as you delve in, the move starts to make sense.

BLAST FROM THE PAST 

Stainless steel fuel tanks were used in the Atlas rockets in the ’60s when rocketry was barely out of its infancy. They essentially acted like big metal balloons and could collapse under their own weight, which – fun fact – led to the creation of WD-40, which actually stands for water displacement-40, originally produced to prevent rusting when the rockets were used as underwater, submarine-launched ballistic missiles. 

The only reason these rockets did not collapse under their own weight was fuel pressure. As the rocket and fuel tanks want to buckle and fail, the pressure of the fuel keeps them stable. Still, one fueling mistake and the rocket will come cascading down and you can kiss roughly $200 million goodbye; that’s not a good look for stainless, at all.

It’s true that stainless steel adds a significant amount of weight while not adding a proportional amount of strength to the rocket.

However, there are far more factors at play, such as thermal conductivity, which describes how a material conducts heat. Aluminum, the standard for most SpaceX rockets, and rockets in general, conducts a lot of heat very quickly, which requires extensive add-ons to protect the fuel. For instance, on NASA’s space shuttle, the distinctive orange paint on the external fuel tanks wasn’t actually paint at all: It was sprayed-on insulating foam to prevent the fuel from boiling off.

While NASA eventually switched aluminum alloys, the process of getting those booster rockets flight-ready resulted in wasting most of the material. And although SpaceX and NASA are similar, they are not by any means the same: SpaceX is Musk’s private business, so profit is important; NASA is a government agency, so profit is not of any concern. SpaceX has to make decisions on material with costs and profit in mind, which is why it originally opted for carbon fiber fuel tanks and fuselages. 

TOLERATES EXTREMES

But carbon fiber has a few major flaws. First, it costs roughly $135 dollars per kilogram (2.2 pounds) for a 164-foot tall rocket and its fuel tanks. That is a pretty penny. Second, carbon fiber needs to be woven very specifically, and for a large fuel tank and rocket, this requires a massive facility, which SpaceX couldn’t find.

Lastly, and by far most important, is the matter of strength vs. maximum operating temperature. Meaning, sure, you might have a material that is strong and light, but can it withstand huge changes in heat?

While aluminum works very well for current SpaceX models, which are designed for Earth orbit and re-entry into Earth’s atmosphere, Starship is designed to be an interplanetary vehicle, more particularly, a Mars mission. 

Why is this important? Because fuel for rockets is cryogenic, which means it is sub-zero temperature when loaded. Then the rocket has to light the engines, which generates extreme heat, and after the flight is done, has to re-enter the atmosphere, which generates even more heat as SpaceX rockets light their engines again to make a soft landing. 

This new rocket, as designed, would have to take off from Earth, which means sub-zero to extreme heat, enter Mars’s atmosphere, which means extreme re-entry heat, and land, which means even more heat. On Mars, it has to refuel, which means back to sub-zero, light engines again, fly back to Earth, re-enter Earth’s much thicker atmosphere, which means much more heat, and burn its engines again for landing.

And then – as a reusable starship – be able to be able to do it all over again. And again.

Carbon fiber, aluminum alloys, or basically any other material simply cannot handle these huge differences in temperature, but steels can. Stainless steel out performs almost any conventional rocket material in withstanding these huge swings in temperature. It, more than any  other proposed material, can meet the requirements for manned spaceflight to Mars.

To clarify, this current rocket is referred to as Starship Mk1. It is a prototype of the much larger BFR (Big Falcon Rocket) which would go to Mars. Starship Mk1 isn’t flight-ready yet, but once it is, it will serve as a stepping stone for the future of manned spaceflight.

SpaceX from the beginning has been about creating innovative rocket designs, and disruptive space technology. In the words of Musk, “Is it in the future, or not? If it’s not, then who cares.”