STARSHIP - The revolution has begun... (Part 2)

If you haven't read my previous blog post, i highly recommend you to take a look at it first before diving into the greater technicalities listed in this article. Click here to go to part 1 of this 2 part article.

So the latest update is : SpaceX’s upcoming rocket called Starship Super Heavy (formerly known as BFR), will no longer be made out of lightweight Carbon Composites, instead it's gonna be made of stainless steel.

Let's take a look at all of Elon’s most recent claims about stainless steel actually being the best option and see if we come to the same conclusion and explain how SpaceX’s use of this material is a little different as they’ll be using new manufacturing techniques and doing things that have never been attempted before.

I think most of us are getting really excited to see SpaceX’s newest rocket come to life. We’re already seeing their test hopper that will practice short little propulsive flights being built and then watch as the nose cone blows over in the wind…  and we’ve already got a good handle on the actual dimensions of the vehicle as well.

SpaceX’s “StarHopper” , credits - everyday astronaut

This article took me a while longer to write and research because I had to try and grasp a lot of concepts that I’m very very unfamiliar with, and read many research papers even though i know only little about material science. But after studying this stuff for sometime now, I can't stop myself from sharing them.

So let’s dive right in with why is SpaceX switching to stainless steel… the same thing our pots and pans are made out of?

Well there are four main reasons:

• Strength at cryogenic temperatures
• its characteristics at high temperature
• ease of development
• price.

Let’s start off with what seems to be the most controversial aspect, the weight. Yes, if you have say a cubic millimeter of carbon composite vs stainless steel, carbon composite is much lighter. But weight is only half the equation in a structural material. The other big key is strength. And here's the catch. Everything changes under extreme temperature environments. Don’t forget, a rocket experiences unbelievable differences in temperature.

The super chilled liquid oxygen SpaceX puts inside these paper thin tanks is an unbelievably cold -207°C and at the exact same time, the outside can reach temperatures several hundred °C on ascent, and we’ve not yet begun talking about the brutal reentry temperatures. But it’s the cryogenic temperatures that’s really truly the key to stainless steel being advantageous. Most steels become brittle at cryogenic temperatures.

Perhaps you’ve seen videos of someone taking metal, chilling it with liquid nitrogen and smacking it with a hammer. Many metals become so brittle at cryo temps that it’s useless.

But stainless steel with a high chrome-nickel content like "stainless steel 301" actually gets stronger at cryogenic temperatures! At cryogenic temps the strength is actually increased by 50%! That’s in pretty sharp contrast to Carbon Fiber which becomes weaker at these temperatures. And here’s where the weight difference can close. And let’s throw in the "Aluminum Alloy 2219" which is  similar to what SpaceX currently uses on their Falcon 9 rockets

strength | density comparison at different temperatures

So, that got me thinking, why isn’t the Falcon 9 or other rockets made out of stainless steel? That's because of a recent breakthrough in manufacturing technique known as cold forming. Cold forming is when you chill the material down to cryogenic temperatures as you form and shape them for manufacturing. 

But just last year, a company called Dawson Shanahan, developed a technique developed to cold form stainless steel which offers huge advantages in the strength of the material. And like clockwork, Elon tweeted about cold forming at cryo soon after the announcement was made by Dawson Shanahan.

So, now let's talk about reentry, how about when the rocket gets really hot as it comes back through reentry? Well here’s where things get really interesting. When vehicles come back in at orbital speeds, they get absolutely punished. In order to slow down in the atmosphere, that kinetic energy is exchanged for heat. This is why vehicles that come back from space have heat shields. Whether it be an ablative heat shield that purposefully flakes away material and which takes heat with it.

Or there’s heat shields that are able to soak up the heat like the tiles on the space shuttle which didn’t let too much heat reach the aluminum airframe by basically soaking it all up like a really hot sponge and radiating it away very slowly.

The silica tiles of the Space Shuttle would radiate heat very very slowly

Aluminum and carbon composite can’t withstand much more than about 200 degrees celsius before they start to deform, but stainless steel can handle 800 degrees celsius and beyond

But, reentry heat can go beyond that. As a matter of fact, peak heating can get up to almost 1,500 degrees celsius, well beyond the point of being structurally sound. So there will still need to be a heat shield.

Starship will have a few forms of heat shield protection. First off, since stainless steel is shiny, it actually will reflect a good bit of the radiant heat instead of absorbing it! But radiating heat away isn’t enough, nope, here’s where things get even more interesting. SpaceX is looking to utilize the first regenerative heat shield for a spacecraft.

Basically, on the belly of Starship will be another layer of Stainless Steel, but this time they’ll use "310S Stainless Steel" which can handle a higher peak temperature at the expense of some strength. Then between those layers of stainless steel will be stringers which will house some methane when being actively cooled.

Ok, liquid cooling stainless steel isn’t particularly new, but what is is the next step. The SWEATY rocket. Believe it or not, Starship will actually bleed fuel out tiny micro pores as it reenters. These pores will be so small you probably won’t even see them. The cool liquid methane will take a lot of heat with it as it bleeds out, evaporates into a gas and toots it away into the wake of the vehicle.

And this idea also isn’t new. Some airplanes have tiny holes on the leading edge of the wings too? Some planes use a system to push out an anti-freeze type coolant out the tiny pores which keeps ice from building up on the wings. 

But now let's talk about what probably really made SpaceX spin on their heels and totally ditch carbon composites… and that’s time and money.

With carbon composite, you need to cut the fabric, impregnate it with high-strength resin, which can be difficult and then make 60 to 120 layers! There’s also approximately a 35% scrap rate of material too, which makes it so carbon composites are terribly expensive. As a matter of fact, the advanced carbon composites cost about ₹13000 per KG by the time you factor in the scrap material. So how’s that compare to stainless steel? ₹220 per KG...😮

SIXTY TIMES CHEAPER. When something is 60 times cheaper, readily available today, and outperforms the other material, you’d better hop on it! But best of all, since the material is so easy to work with and well known, they’re actually getting started on it NOW. Like right now! This will certainly help achieve some of those lofty goals and timelines.

You can get live updates of the progress at r/spacex or here.

The revolution has begun

We’re once again seeing SpaceX not fall into the trap of the sunk cost fallacy. It might be easy to think this is a disappointment, a letdown or a compromise, but quite frankly it is a compromise. Engineering is always a compromise. And that’s not a bad thing.

So in summary, SpaceX chose stainless steel over carbon composites because it’s about as light, it can handle higher temperatures which means less heat shield is need, which then makes it lighter, it reflects heat which means even less heat shield which again makes it even lighter, it’ll be cheaper and quicker to build AND it’ll look FREAKING AWESOME.

This 2 part article, especially the second part took me a lot of time to assemble data and present it in the right way, so i urge to share it as much as possible. I will be back with an article in a few days regarding the Nobel prize awarded this year and what it means for astronomy adn astrophysics. But until then

Ciao!

Note - A lot of data was collected to write this article full fledgedly yet keep it precise, if you do copy and post it somewhere, do give me credits, like i give credits to everyday astronaut for this article.