Earlier this month we could see the first real images of the spacecraft prototype with which SpaceX aims to reach Mars. Soon a detail became evident: The vehicle’s cover is made of stainless steel instead of carbon fiber. Why?
Elon Musk himself has finally decided to explain this change during an interview with Popular Mechanics. In the original
Carbon fiber costs $ 135 per kilogram, but 35% of the material must be discarded. When you cut the carbon fiber there is always a spare part that you can not reuse, so the cost of the material is about $ 200 per kilogram compared to stainless steel, which costs only $ 3 per kilogram.
The second reason in favor of steel is that it has a high melting point much higher than that of fiber. Musk explains the limits of both materials:
Normally, aluminum and carbon fiber are limited to a maximum operating temperature of 150 degrees Celsius for extended periods. It’s not much. You can reach point peaks of about 180 degrees Celsius, but above 200 degrees you’re pushing the limit. The material weakens.
Some carbon fibers can reach 200 degrees, but not without structural problems. Steel, however, can quietly withstand temperatures of 820 to 870 degrees Celsius.
Regardless of what the steel holds, it needs to be cooled, and Musk has explained the first details of an innovative cooling system based on perspiration:
In essence, what we have are two layers of stainless steel joined by stringers. You can pump water or fuel between these two layers of the sandwich, but the outside has a series of very small micro-perforations that filter that water or fuel. As far as I know, it’s a system that had never been proposed before.
The final ship that will carry this system will have capacity for 100 people and will be put into boron orbit of the new BFR but has not yet been built. What has been shown in full construction in Texas is a prototype designed to test the resistance in suborbital launches. It was assumed that these tests would begin in about 8 weeks, but a strong storm of wind that has hit the facilities these days has damaged the prototype and will have to wait a few weeks.