SpaceX's Starship SN4 prototype fires rocket engine for first time

Starship will spend most of its time being in high heating conditions (like reentry) or very cold because the rocket fuel is loaded into the rocket at very low temperatures. It turns out that while the strength to weight ratio of carbon fiber is better than this alloy of steel at room temperature, it is impacted negatively by both hot and cold. In contrast, the steel alloy they are using actually has higher strength at low temperature and performs much better than carbon fiber at higher temperatures.

So the whole rocket can be lighter and simpler. Carbon fiber rockets that are going to reenter require a lot of heat shielding that this ship won't need.

And rockets and their engines are designed together. So it would be very difficult to make a design change like moving from aluminum to steel on Falcon without essentially designing a new rocket. Instead they will focus all their energy on Starship and eventually replace the Falcon series.

SpaceX has said it is possible that the build cost of a Starship/Superheavy stack could be lower than a Falcon 9. The variable costs of a launch would also be cheaper while delivering more payload.

This is the rocket that could make a lot of science fiction come true. That is what the excitement is about and we should all hope they get it to work.

Elon Musk cares about one over-arching thing: making humanity an interplanetary species. I believe that everything he does is in service to that goal.

In order to make humanity interplanetary, lift costs must be greatly and sustainably reduced.

In order for lift costs to be greatly and sustainably reduced, everything in the lift vehicle except the propellant must be fully reusable. Also, the vehicle needs to be inexpensive, quick to manufacture and reasonably performant.

In order for the orbital stage of a lift vehicle to be re-used, it must re-enter the earth's atmosphere and effectively shed a large fraction of the chemical energy that was used to accelerate it to 7.8km/s.

In short, the 2nd stage must be able to repeatably handle intense re-entry heat.

For performance (and other) reasons, it must also be able to handle the very low temperatures of liquid propellants.

Given all of these constraints, specific types of steel are literally the only choice.

1. Low material cost

2. Low manufacture cost

3. Excellent high temperature performance

4. Excellent low temperature performance

5. Sufficient normal temperature performance

In fact, Musk has said that his initial focus on carbon fiber was a big error.

Re: Falcon: as others have stated, Falcon 9 is effectively a fixed design. It's now a 'cash cow', and there's no reason to substantially change it. In fact, changing it would risk a lot of profitable contracts that are effectively funding Starship/Superheavy development.

With carbon fiber you get a lot of theoretical nice-to-haves, but you can’t iterate quickly with it. You need specialized tooling for manufacturing plus there is a lot of actual R&D that still needs to be done to manufacture with CF at Starship scale.

SpaceX sees all this effort as a diversion from the goal of actually building a new rocket. Compare with steel where you can just hire a bunch of welders and build the damn thing out in the open.

Falcon will never switch to steel. It makes no sense from a performance POV (Merlins are much less powerful than Raptors so your payload capacity hit is too high) and Falcon is a “dead” platform: only minor incremental updates from now on. Any change to it will trigger recertification with NASA, the USAF and a bunch of clients, which is a no-go. Falcon is SpaceX’s workhorse and money making machine, and it will continue on that role until Starship retires it.

Steel is just so much more workable every step of the way. You can order your special steel from close to anywhere, your fancy epoxy might have two sources. Any engineer can design for steel and be really good at it (they probably even took classes on it in college) so you can have more cheaper engineers working parallel. Your engineers that know their way around CF are gonna be much more rare and expensive. The tool and die side of things is well known. The little tricks that are tribal knowledge for CF have long been compiled and published into textbooks for steel. If you fat finger a relief radius the die maker is gonna ask you to clarify whereas with some modern material that doesn't have that body of documentation yet they'll just assume you know what you're doing. The material handling side of things is well known, you'll never have to trash a part of delay a deadline because someone somewhere didn't store one of your constituent products in the right environment. When you've finally built your rocket anyone can test it. The techniques for inspecting steel are well known and widely practiced. For exotic materials you either have to track down experts or build that tribal knowledge internally (expensive and slow).

These little conveniences compound throughout your entire "technology stack" so to speak and a rocket company has a really tall stack.

It's hard to overstate just how much easier and faster you can work in a known-known like steel vs something that's new, hot and sexy that everyone wants to treat like a trade secret (as is the case with CF).

Source: Former defense sector employee, granted on the software side but you still learn a lot of things through proximity.

The proposed space shuttle successor was designed around carbon fiber fuel tanks / hull and engineering problems with them eventually killed the project.

The material struggles with low temperatures of cryogenic fuels (brittle), high temperatures of re-entry (just burns), oxidation in oxygen tanks. Also they apparently tend to sponatiously combust with rocket fuels.

Some reference: https://www.compositesworld.com/articles/an-update-on-compos...

Additionally the ease of manufacture is not just a manufacturing cost saver, it's a development cost and time saver.

Falcon is a frozen design and won't change, beyond a proposed longer fairing.

How is that a dichotomy?

The Falcon family is wildly successful, but it's small potatoes next to SpaceX's vision for the economics of space launch in the near future, so it seems unlikely that they'd go back to fix what isn't broken.

And no, F9 won't change.

I don't know why you think that cost saving measures are different from measures that "make sense".

Metals can be used in supersonic and cryogenic aerospace applications.

Composites and carbon-fiber cannot, regardless of cost.

Lets talk about the most difficult part, the second stage, ie Starship. The big problem you have to solve, is to build a vehicle that can go from incredibly cold structure to a very hot one. Because you start very cold, and when coming back, it gets really hot.

SpaceX loves deep cryo, meaning not just going to the point of making that fuel/oxidiser liquid, but really pushing it down as far as possible. This has a number of advantages but also many disadvantages. SpaceX took a long time to perfect the deep cryo technology with Falcon 9 (that was of course Kerolox).

Because the fuel heating up it forced their operations to be very tight, and in the beginning cause many launch delays. It also cause the famous AMOS 6 failure, as a pretty interesting mostly unknown effect was taking place dring these temparatures.

The upside is that you increase desnisty and thus vehicle performance, and it helps keep your turbo pumps cool as well.

SpaceX very much wanted to continue this for Starship architecture, as they think its a huge win you are leaving on the table if you don't do it.

What this means is that Starship is incredibly cold on the pad, really as cold as they can get it. Any structural material, needs to be able to handle this. This by itself is very challanging with carbon fiber even with the best carbon fiber technology perfomrance degrades the deeper you go. It forces you to really build a carbon fiber structure like we have never seen before.

While some steels are terrible brittle under that condition, 301 stainless (and of course the new alloy SpaceX is developing) are the opposite. Their strengh actually increases, and basically the colder the better. So with incredibly cheap steel that you can make pots out of, you can basically match the performance of super expensive carbon fiber.

On the hot side, you have the reentry. Starship is design not just to come in from LEO like the Shuttle, but actually come from interplantary and then use the air to break. So this is a much hotter profile then Shuttle. And as most people know, Shuttle heat shield caused endless problems.

With both carbon fiber and aluminum you really don't want to let them get them to hot (500C is pushing the issue). Any such design basically requires packaging most of the Starship in a very thick Heatshield. Steel however is very tolerant of heat, meaning that even under fairly high tempratures (800C ish), once it cools down, your structure is still as good as before.

When designing and insulating heat shield (rather then an non-reusable ablative one) the question is really how thick, and thus how heavy does it have to be. The amount of heat that can be handled by the structure behind the heat shield, directly impacts how thick the shield needs to be.

So the combination of being almost as good as peak carbon fiber in deep cryo, and being clearly a much superior choice for reentery makes it a good choice. Once you combine that with the operational apspects, meaning both the per-unit material cost and much cheaper manufacturing its a no-brainer at that point. This is why I believe Musk called it 'The best desision in the whole design', it just made everything easier.

Another bonus is that you have access to a far larger group of engineers with experinace, plus an even larger gorup of workers that can work with steel reliably compared with carbon fiber.

Now for the First Stage, there is a good argument to be made that steal probebly can't quite reach the peak performance level of a carbon fiber design. Maybe not of an aluminum design. In this case I think its really more about material and manufacturing price and of communality with the Starship.