Why use stainless steel to build spacecraft, Musk: Good! Cheap!

 Why use stainless steel to build spacecraft, Musk: Good! Cheap!

Netease Technologies News Jan. 24, according to the Volkswagen Machinery magazine, Elon Musk recently tweeted that the Starship would be made of stainless steel instead of carbon fibers. As far as we know, this marks the first time that stainless steel materials have been used in spacecraft manufacturing since Atlas failed in the late 1950s.

In fact, as early as a few days before Christmas, Musk had revealed that more adjustments would be made to the interstellar spacecraft. At that time, Musk claimed that the most advanced carbon fibers made up the main body of the spacecrafts rocket (formerly known as BFR or the Great Falcon Rocket), and that its SuperHeavy rocket booster would be replaced by 300 series of stainless steel. On Jan. 10, Musk posted a test version of the spacecraft on Twitter, saying it was a prototype that could be used for sub-orbital vertical takeoff and landing test flights, flying at an altitude close to 5,000 meters. He turned these tests into jump tests.

Since uncovering the mystery of interstellar spacecraft, Musk has briefly answered many direct questions from curious space observers via Twitter. But two weeks before the announcement, Musk had an exclusive interview with Ryan Dagostino, editor-in-chief of Volkswagen Machinery, at SpaceX headquarters in Hawthorne, California, to discuss in great detail the ideas behind these changes.

The interview summary is as follows:

Dagostino: Youve been busy redesigning the starship?

Musk: Yes. In the manufacture of spacecraft and super heavy rocket boosters, I replaced them with special stainless steel alloys. I thought for a moment. Its a bit counter-intuitive. I spent a lot of energy persuading the team to move in this direction. But now, I believe they have been persuaded. We have been pursuing an advanced carbon fiber structure, but progress has been very slow, costing $135 per kilogram. Then about 35% of the waste, such as cut parts, is no longer usable. In addition, the carbon fibers are impregnated with high strength resin, which is very troublesome.

Dagostino: By contrast, what about stainless steel?

Musk: The biggest advantage of stainless steel is that its cheap enough and fast enough to produce, although its obviously not the lightest material. But in fact, this is the lightest material we can find. The properties and strength of high quality stainless steel are improved by 50% at not particularly low temperatures. Most steels become brittle at low temperatures, such as typical carbon steels. When you spray liquid nitrogen upwards and hit it with a hammer, it crushes like glass.

This is true for most steels, but not for stainless steels with higher chromium and nickel content. In fact, their strength has increased, and their ductility is still very high. So even at minus 165 degrees Celsius, the ductility is good, very tough, and there is no fracture problem. Fracture toughness is a property. If there is a small crack in an object, does the material tend to prevent or expand the crack? How much will a small defect in the material magnify when you experience multiple vibration stress cycles?

Dagostino: So there are some materials that can prevent their cracks from expanding.

Musk: Yes, ceramics (like coffee cups), for example, are not good at preventing cracks from expanding. Once a crack occurs, it bursts like glass. Then, depending on the type of metal you have (some metals have better fracture toughness), fracture toughness varies with temperature. Technically, toughness is the area under the stress-strain curve. When you apply pressure on something, how much does it deform? This is a very important feature.

Stainless steel was used early in Atlas project. Early Atlas was a steel balloon tank. Its defect was that the material was too thin and would collapse under its own weight. This is a steel balloon, which can hardly stand up, but will collapse like a resilient castle, even unable to bear very small payloads. There were many early cases of Atlas crashing on the launch pad and causing disasters.

Heres a technique that I think is important, and thats when you look at it as an aircraft that needs to re-enter the atmosphere. Look, this is another benefit of steel: it has a high melting point, much higher than aluminium. Although carbon fibers will not melt, the resin will be destroyed at a specific temperature. Typical aluminium or carbon fibers can withstand temperatures of about 149 degrees Celsius under stable working conditions. You can use them for short space trips, occasionally withstanding temperatures of 176 degrees Celsius. But when it reaches 204 degrees Celsius, it may be beyond the limit. Some carbon fibers can withstand high temperatures of 204 degrees Celsius, but their strength decreases. Steel can withstand temperatures ranging from 815 to 871 degrees Celsius.

Dagostino: Do you have a complete metallurgical team?

Musk: We have a very good material team, but at first we only used high quality 301 series stainless steel. Another important thing is that it makes a big difference. In the process of rising, you need materials to maintain strength at low temperatures. When entering the atmosphere, you need it to be able to withstand high temperatures. Therefore, the quality of the heat shield is determined by the interface temperature between the heat shield and the air. Whether it is mechanical or adhesive, whatever the interface point is, it depends on the thickness of the heat shield.

For example, on Dragon, the thickness of the heat shield is actually determined by the thermal infiltration of the heat shield, which causes the heat shield to adhere to the shell. So the insulation tile will not be lost in the process of falling. Basically, you dont want to throw away the tiles.

With stainless steel alloys, you can easily withstand temperatures of more than 800 degrees Celsius, so your interface temperature has risen five times. This means that for steel structures, there is no need for any heat shield on the leeward side of the back shell. On the windward side, what I want to do is to install a renewable heat shield. Double stainless steel shell (similar to stainless steel sandwich) essentially has only two layers, you only need to bond them with special materials.

Flow fuel or water between stainless steel sandwiches and then have tiny holes outside, which in fact release water or fuel through the holes outside. You cant see these tiny holes unless youre near them, but you can use transpiration cooling to cool the rockets windward side. So the whole thing looks chrome, like the cocktail blender in front of us. But on the one hand, it is double-decked, and it has dual purposes, that is, to reinforce the structure of the aircraft, so that it will not repeat the fate of Atlas. You have a dual function insulation structure. As far as I know, this has never been proposed before.

Dagostino: Its a huge change.

Musk: Yes.

Dagostino: Where will steel come from?

Musk: Just ordinary 301 series stainless steel. Let me put it this way: 304 series stainless steel is their material for making cans. Its easy to get.

Dagostino: How did that affect your original plan?

Musk: It helps to speed up the implementation of the original plan.

Dagostino: Because its easier to use?

Musk: Yes, stainless steel is easy to process. Oh, I forgot to say: carbon fibers cost $135 per kilogram, 35% of waste, so the cost is actually close to $200 per kilogram. Stainless steel sells for only $3 per kilogram.

Dagostino: Thats a good idea!

Musk: Thats true! (small)

Source: Responsible Editor of Netease Science and Technology Report: Wang Fengzhi_NT2541