Let us do a comparison between the current king of the rocket launch industry, Falcon 9 manufactured by SpaceX and the upcoming contender Neutron being designed by Rocket Lab, a company originating in New Zealand. While you may not have heard about Rocket Lab, it is probably the most successful and innovative rocket launching company besides SpaceX. They were the first to make fully 3D-printed rocket engines and launch a rocket made from carbon fiber composite into orbit.

I could go on about all the cool stuff Rocket Lab has done, but the bottom line is that this is a company with a strong track record. They have been busy developing Neutron for a year now and recently put their new Archimedes engine on display. Ten Archimedes engines will power the Neutron rocket: nine for the first stage and one for the second stage. Rocket Lab has also shown the completion of tooling and moulds to build the rocket. When a company has created moulds and tooling for a rocket, it means they are confident about having finalized all key design work.

This new milestone in development allows us to start making some comparisons with Falcon 9 in terms of design choices, specifications, and performance. More specifically, I want to cover the following topics in this article:

• Specifications – How do they compare in size, weight, power, and payload delivery capacity?

• Engines – How does the Merlin engine of the Falcon 9 compare with the new Archimedes engines?

• Material choice – One rocket is made from aluminum, while the other is made from carbon fiber composite. What are the tradeoffs?

• Rocket shape – Why do Neutron and Falcon 9 have such different geometries?

• Reuse – What makes the Rocket Lab approach to reuse so different from that of SpaceX?

Before jumping in and comparing Neutron to Falcon 9 it is important to keep in mind that Falcon 9 has been refined and optimized over many years. Thus, we can assume that there is plenty of scope for improvement on the first version of Neutron released. For instance, the current Falcon 9 rocket use the Merlin 1D rocket engine. It is a refinement of the previous Merlin 1C engine. The Merlin 1C boasted 420 kilo Newtons (kN) of thrust, while the currently used Merlin 1D doubled that to 854 kN. That radical improved allowed SpaceX to stretch the Falcon 9 rocket from 47.8 meters to its current 70-meter length, which resulted in the mass of the rocket growing from 333 tons to 549 tons.

You need roughly, 5490 kN to lift 549 tons off the ground. The nine Merlin 1C engines on the Falcon 9 v1.0 only gave a combined force of, 4940 kN. Thus, without improving the power of the Merlin engines, it would not have been possible to lift the current Falcon 9 off the ground.

Rockets come in many shapes and sizes. Some send only a few hundred kilograms into orbit, while others send several tons. It is useful to get a sense of where Falcon 9 and Neutron fit in the rocket landscape.

Thus, I have made a diagram below showing relative size of some of the most widely used rockets in the industry. These rockets could be classified as medium lift launch vehicles. It refers to rockets capable of putting 2 – 20 tons of payload into orbit.

The leftmost rocket is Ariane 6 which is being developed by the European Space Agency to replace Ariane 5. The Ariane rockets have been popular rockets for launching commercial satellites into orbit. Next follows Soyuz-2 which is the workhorse of Russian Roscosmos. It is an evolution of the Soyuz, which first got used in the 1950s. No rocket has had more launches than Soyuz and its derivatives. Before Falcon 9 this rocket dominated the commercial space industry. Both American and Russian astronauts rode on this rocket to the International Space Station (ISS).

For height comparison, let us sandwich in Neutron between the first version of Falcon 9 that was launched and the current Block 5 version.

While both versions of Falcon 9 are taller than Neutron, you will see that on other metrics such as mass, Neutron is typically located somewhere between the two Falcon 9s. Another interesting detail is that despite Ariane 6 looking massive, it is actually slightly lighter than Falcon 9. And Soyuz is not quite as light as you might have imagined from looking at its size.

What is the explanation for these apparent contradictions? In relative terms, rockets have thinner metal walls than soda cans. Thus, the fuel and oxidizer is what constitutes most of the mass. Both Falcon 9 and Soyuz-2 use Kerosene fuel (RP-1) at all stages. Kerosene is very dense. Ariane uses Hydrogen fuel, which is very low density. That means much larger fuel tanks are required to hold the same amount of fuel.

The payload each rocket can send to orbit matches their total mass somewhat, although there are some outliers. Most notable is Ariane 62. The key reason is that these rockets use strap-on boosters with solid rocket fuel. For old space companies such as Arianespace and United Launch Alliance (ULA) which make these rockets, that is a very common strategy. Thus, Ariane with two solid fuel rocket boosters can deliver 10.35 tons into low earth orbit (LEO) while adding another pair of boosters doubles that to 21.65 tons. On the other hand, Ariane 6 with four boosters comes in at 860 tons, making it significantly heavier than any of the other rockets.

Yet, the performance of Ariane 6 still looks underwhelming at first glance. You would think it has low thrust, but the bar chart below shows that it has a pretty good kick upon launch. So, what explains the discrepancy? The reason is that Ariane is better optimized towards delivering payloads into higher orbits. For geostationary orbit (GTO) Ariane puts heavier payloads into orbit than Falcon 9.

Falcon 9 and Neutron are substantially different from these other rockets because no strap-on boosters are used. Why not? Because solid fuel rockets are like the blackpowder rockets you use to celebrate New Year: You ignite them and they go. There is no opportunity to control them with any precision, unlike a liquid fuel engine with a turbo-pump. Thus, you cannot land these boosters after use the way SpaceX does with Falcon 9 and Falcon Heavy.

Now that you got some idea of what kind of rockets Neutron and Falcon 9 are most comparable to, let’s focus our attention on their differences. Notice that both in terms of mass and payload, the Neutron rocket sits somewhere between the Falcon 9 v1.0 and the current Falcon 9 Block 5 variant.

It is normal to aim for good margins first so that you get successful rocket launches, and then begin optimizations and improvements later. The design of Neutron makes upgrades harder. Straight cylinder rockets like Falcon 9 can be upgraded easily by stretching them. Neutron, in contrast, cannot as easily be stretched as it is tapered. The base is 7 meters in diameter, while the nose cone is 5 meters in diameter.

The first announced design of Neutron a year ago specified that Neutron would use seven engines in the first stage. Interestingly, the revised design will use the same configuration as Falcon 9: Nine engines in the first stage and one in the second stage.

Incidentally, it is the exact same configuration used by the Electron rocket currently used by Rocket Lab. It is seen as a practical choice because the second stage doesn’t need more than 1/9th the power of the first stage. It is also practical for reusability because need to throttle back considerably to land a booster with propulsion. Too much thrust and your booster stage will shoot upwards instead of merely slowing the descent towards the ground.

It is quite hard to throttle rocket engines down to an arbitrary low level. It is easier to shut engines off completely. When the Falcon 9 booster lands, it shuts down all engines except one and throttles down to 57 percent of max thrust. Yet even that isn’t low enough to get a controlled landing. Instead, the Falcon 9 booster famously lands with a hover slam. Basically, it fires an engine right before it slams into the ground.

It is interesting to see how common this rocket engine configuration is becoming because the previous generation of Western rockets such as Atlas, Ariane, the Space Shuttle, Delta and Saturn V all used relatively few and large engines. They also used entirely different engines and fuel for each stage, while the thinking of current generation rocket companies is to be able to reuse as many components and technology as possible for each stage to simplify and keep costs down.

Enough about engine configuration. Let us zoom in on a single engine and compare them. In terms of raw power, the Archimedes sits between the old Merlin 1C and the current Merlin 1D. This gives Falcon 9 an advantage in the first stage. Rockets want to get off the ground as quickly as possible to avoid wasting fuel fighting gravity.

But when looking at propellant efficient (Isp) the tables turn. Here, Archimedes with a specific impulse of 329 seconds beats Merlin 1D with only 282 seconds of specific impulse. That means the same amount of fuel can provide thrust for a longer time on a Neutron rocket than a Falcon 9. It is 17 percent more propellant efficient. That advantage grows in vacuum to 18 percent.

What are the practical consequences of this difference? It will matter most for the second stage, allowing it to go higher. That matters, especially if you want to get payloads into higher orbits. That advantage is very clear with the Ariane rocket. The stats I showed in this article for Ariane makes it look like a crappy rocket because it can deliver much less payload to low Earth orbit than a Falcon 9 despite being a rocket of very similar size.

But Ariane has a Hydrogen engine as its second stage, which has a specific impulse of 318 seconds at sea level. However, once in vacuum, it has an Isp of a crazy 432 seconds. Here, Falcon can only reach 311 seconds. In other words, Ariane has a 39 percent advantage at higher orbits. That helps explain why Ariane beats Falcon 9 at delivering cargo to high orbits.

Engines burning methane are much better suited for reuse than engines burning kerosene because methane burns cleanly, while kerosene cause soot to buildup in the engines. We refer to this as coking. Founder of Rocket Lab, Peter Beck, humorously said you can eat your lunch out of the engine bells of the Archimedes engine after they have flown. That means methane engines require significantly less maintenance. There is no need for extensive cleaning of the engines after they have flown.

When describing engines it is normal to categorize them by what we call power cycle which describe a thermodynamic cycle for an engine. That may sound like a bunch of gobbledygook, but basically refers to the steps involved when an engine operates. It says something about how heath is transferred and used to do work. The gasoline engine in your car for instance typically follows the Otto cycle, while a diesel engine use the Diesel cycle. Steam turbines use the Rankine cycle.

We can describe rocket engines with power cycles as well. Two common ones are the gas generator cycle and the staged combustion cycle. I have an article discussing this in more detail:

Gas generator engines are simple and cheap to make. Staged combustion engines come in several variants, but I will only talk about oxygen-rich staged combustion cycle here. These engines are much harder and expensive to make. However, it all depends on the performance you are aiming for. The Merlin 1D engine is a gas generator, which makes it a relatively simple engine. The Archimedes engine in contrast use a staged combustion cycle running oxygen-rich in what we call the pre-burnerwhich drives the turbo pump of the engine.

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A key difference in design is that Archimedes is a low-performance version of a high-performance cycle, while Merlin 1D is a high-performance version of a low-performance cycle.

The benefit of the Rocket Lab approach is that they get much larger tolerances on their engines. If you want to maximize reuse, you don’t want engines to run close to their breaking point but with fat margins.

Falcon 9 is built using aluminum, which is the most popular and common material in the space industry to build rockets. Rocket Lab is going a radically different route by building Neutron using carbon fiber composite. That is an unusual choice, but a very sensible one for Rocket Lab. They already have extensive experience building rockets out of carbon fiber; that is what their existing Electron rocket is made from. New Zealand, where Rocket Lab originates, has a strong expertise in working with composite materials thanks to the yacht industry. Peter Beck himself worked extensively with composites before founding Rocket Lab.

If anyone can pull off building a large rocket out of carbon fiber composites, it is Rocket Lab. Another question is whether one should use carbon fiber. Let us look at the pros and cons.

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Metal is relatively easy to work with. You can bend, roll, melt, hammer, stretch, stamp and weld it. Aluminum gives you most of these advantages while also giving better strength to weight ration than steel, allowing you to build lighter rockets.

For SpaceX, it is a big advantage because their approach to developing rockets is through a hardware-rich iterative approach. More specifically, almost anything they develop is based on making numerous physical prototypes which they te