Rockets of Today

TERRAN — USA

Relativity Space was a venture I was ignoring, until they signed a launchpad lease at Cape Canaveral, and let it be known that they had already logged a lot of engine firing time on the test stand. They are taking an approach similar to a number of other New Space rockets, in that they have a bottom stage with multiple “Aeon 1” engines (nine) and a second stage with a single vacuum-bell version of the same engine. Unlike any of the others, they’re using methane as a fuel right from the beginning — something that other companies have considered only after they’d gotten themselves well established.

But that’s not what makes them really stand out. The unique thing about the Terran 1 is that they intend to 3D-print the entire rocket. Lots of other companies use 3D printing to make complex engine parts, but nobody else is considering printing the fuselage, fuel tanks, and so on. At first blush, the idea seems ridiculous. A printed material is never going to be competitive on, for instance, optimizing the strength to weight ratio for tank walls. But they say that by printing everything, they can make an Aeon engine with fewer than 100 parts, and a whole rocket with fewer than 1000. (Most traditional rockets have tens or even hundreds of thousands of parts.) In fact, they have said that some versions of their engine have just three parts.

That’s only the beginning of their ambition. The reason they want to 3D print the entire thing, even if it doesn’t result in optimal properties, is so the process can be automated from beginning to end, so they can make a rocket by pushing one button, with no skilled labor required. It would still take weeks for the build to be completed, but that should certainly achieve some reduction in costs.

We're not done. It can’t actually save money to require no human workers at all — there are diminishing returns there, for sure — so why do they want to do it? Well, so the entire factory can be dropped on Mars, and build rockets there. That’s the end goal of this odd approach. It’s also why they picked methane as the fuel. Maybe when they start building it there, they’ll call it the Martian 1 instead of the Terran 1.

This begs the question of what raw materials the printer would use. They mentioned nickel, so it sounds like they might plan to make a lot of parts out of the same inconel alloy which is widely used for the high temperature parts of engines. Inconel mixes are generally at least half nickel, sometimes three quarters, and nickel should be reasonably easy to find on Mars. The crucial second ingredient of inconel is chromium... and it looks like this may also be fairly abundant. Aluminum and titanium are not rare either, and their tanks and frame are mostly aluminum.

Their giant superduper 3D printer is named Stargate. For the large parts such as tanks, it rotates them on a turntable as it builds up the metal on the top edge. (The results look rather rough and grainy in the video they’ve released so far.) Apparently it not only incorporates some proprietary metallurgy, but also some kind of artificial inteligence features, to help make it autonomous. They claim it will learn to build faster as it gains experience. They are now happy enough with the prototype that they are building several more printers.

Their first rocket, which they claim will in the future be scaleable to different sizes, is fairly small: bigger than an Electron but not bigger than much else. They claim they’ll sell them for $10 million each. I figured that the engine itself would probably have to scale in numbers, rather than size, as it uses an expander cycle — a rare choice for a booster engine, but a good fit for 3D printing, because it uses complex plumbing but does not make severe demands on the materials. From pictures, it looks like it’s a semi-closed design which dumps the unburned turbine propellant into the bell... and does it with a single fat pipe, rather than a ring of little holes such as most people would use for the purpose.

But then they said they plan to make a much larger first stage engine once they get to orbit with the little Aeon 1. This would allow their small rocket to have just one booster engine, thereby reducing the part count further. This engine would be called Aeon R. It would also allow them to build a large rocket, at least as big as a Falcon or Vulcan, called the Terran R. And yes, the R stands for reusable... and in order to compete on cost, they also want to go for second stage reuse. The thing might end up looking like a mini Starship. I’m guessing this large engine would be a gas generator.

So why go for nine little engines on the first rocket, when they say it was “definitely not the optimum choice in hindsight”? Well, they say it’s because this makes it a better pathfinder toward a big rocket, but other companies have tried to make two engines in different sizes and then found that getting just the little one to work was the limit of their capabilities.

This vision has somehow attracted tons of investor capital — a lot more than many of their competitors. It’s also attracted customers to contract for flights on rockets they haven’t even started working on yet — OneWeb, for instance, wants to hire the promised Terran R. But with Virgin Orbit, it’s questionable how it will be possible to make back all that money they’re spending.

Terran 1: mass unknown, diam unknown, thrust 621 kN, imp ~3.5 km/s, type ENm?, payload ~1 t, cost $10M/t.