Most Interesting Rocket Engines

Rocket Engines which are interesting from a technological and historical perspective.

In previous articles I’ve talked about theoretical aspects of rocket engines, what rocket engine and propellant to use etc. But lets talk about concrete rocket engines actually in use today and what makes them interesting.

Rutherford Engine

One fascinating modern engine is the Rutherford Engine from Rocket Lab. This is a fairly small and simple engine.

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The Rutherford Engine is the worlds first electric rocket engine.

What makes it interesting is that instead of a turbo pump, the centrifugal pump feeding the combustion chamber is driven by a brushless electric motor. The motor gets electricity from a lithium ion battery. This is quite a novelty which has probably only happened due to the big advances which has been made on battery technology these last years.

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Direct current from the battery is turned into alternating current (AC) by the inverter and fed to an electric engine, driving two pumps. One pump for the fuel and one for the liquid oxygen. This causes the fuel and oxygen to enter the combustion chamber at high pressure. The fuel is sent in tubes spiraling around the nozzle to cool it down, otherwise it could get so hot that it would get destroyed. Illustration from Wikipedia by Duk.

Another interesting aspect with this engine is that most of it is 3D printed. It powers the Electron rocket which is only 17 meters long and 1.2 meters in diameter, and which can lift a payload of 150 to 225 kg into orbit. That isn’t a lot, but due to advances in manufacturing, electronics and miniaturization we can now make really tiny satellites. CubSat e.g. only weigh about 1 kg and has dimensions of 10x10x10 cm.

Merlin Engine

The interesting thing about the Merlin engine, is about the realization that making progress in space exploration isn’t always about using more sophisticated technology. In fact one of the main culprits which has retarded progress in the space industry is the focus on exotic and expensive designs. For instance past plans for Mars exploration have been insanely expensive. Partly perhaps a legacy of the space race between the US and the Soviet Union where it was all about being first, dam the costs.

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The Merlin Engine. 9 of these are used on the Falcon 9 rocket’s first stage. Photo by Steve Jurvetson

Merlin is about realizing that you have to balance efficiency against cost and complexity. Getting 20% extra launch capacity by using exotic engines and fuels might not be worth it if it doubles the price of the rocket. The SpaceX Falcon 9 rocket, costs around 60 million dollars per launch while the competition has been 300–400 million dollars. That is 5–7 times the cost. And they can’t launch 5–7 times as much per launch.

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The fuel pumps are driven by a gas turbine which is made to rotate from the hot gasses from the pre-burner going through it. Illustration from Wikipedia by Duk.

Part of the reason for such a significant lower price, is the design decision behind the Merlin engine. It is based a simple open cycle turbo pump design from the 1960s. This is well understood and easy to work with. It is also a fairly small engine. The Falcon 9 e.g. chose to use 9 Merlin engines in the first stage rather than fewer and larger engines. This means SpaceX has the opportunity to do volume production of Merlin engines which makes it possible to cut costs.

Using lots of smaller engines also helps handle landing of the Falcon 9 booster (booster is the first stage of a rocket). When the Falcon 9 booster comes back to earth to land, it will be very light as almost all fuel has been spent. The rocket thus needs to reduce thrust considerably to no shoot up in the air again when starting the rocket engines. Most rocket engines can’t throttle down a lot. And being able to reduce the throttle a lot requires more complicated and expensive engines. By using 9 engines, they can reduce the thrust to one ninth simply by turning off all the other engines.

Of course if building lots of simple small engines is such a good idea, why was it not doing before? E.g. the Saturn V moon rocket used five huge F1 engines. A reason for this is that back in those days any engine failing meant the whole rocket failed. So more engines meant higher chance of failure.

Today it is the opposite. Engine control computers and sensors are a lot more sophisticated, so engine failure can be detected earlier and the engine can be shut down before it blows up other engines or the rocket. And the engine control can calculate adjustments in thrust and orientation of the other engines to compensate for the loss of an engine. Thus with modern engine control, more engines means higher reliability. In fact Falcon 9 has already succeeded with a mission with one lost engine.

So one way of looking at it is that the SpaceX revolution is about replacing simple software and advance hardware with advance software and simple hardware.

Another part of the equation is modern production techniques and quality control. While it is an old fashion engine it has been designed with modern computer aided design to be as good as possible with simple means. Hence e.g. the Merlin engine has very good thrust to weight ration. That is, it give a lot of thrust relative to its weight. It also uses 3D printing extensively in production. This is very useful in rocket engine production because there are so many tubes and shapes. In the old days this was a time consuming process with lots of welding.

Space Shuttle RS-25 Engine

I am not a big fan of the Space Shuttle, and that is partly why I have devoted most of this article to SpaceX and Soviet/Russian engines. But it deserves mention specifically because I am not a fan, in particular because it is such a useful illustrations of the kind of tradeoffs one must make. The RS-25 engine was developed back in the 70s but it is still one of the most powerful and efficient rocket engines in existence. E.g. none of the Russian kerosene engines I talk about here are anywhere near as efficient as the RS-25 engine.

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The Rocketdyne RS-25 is a liquid-fuel cryogenic rocket engine, burning LOX and hydrogen. Photo by Steve Jurvetson

Like a lot of American space technology, they are highly sophisticated and expensive. But they are also very reliable engines. Anyone concerned with the biggest and the best would love these.

However in technology I’ve never been a fan of complexity and that goes for both programming languages and rocket engines.

Using hydrogen as fuel meant superior efficiency (high specific impulse) for the RS-25 engines, but it also meant dealing with a complex fuel. It requires cryogenic cooling, it easily leaks out of tanks as hydrogen atoms are too small to easily contain. Anything requiring that much cooling will require venting to avoid dangerous buildup of pressure, which will happen if you fuel tank is getting heated up by e.g. sunlight in outer space.

It also means really big tanks, which is very noticeable on the Shuttle.

It is not without reason that building hydrogen fuel cell cars have been so difficult. Storing and transporting hydrogen is very difficult. Still people always get tempted to use hydrogen due to its efficiency. Hydrogen fuel cells are more efficient than other fuel cells by a big factor just as hydrogen powered rocket engines. Hydrogen fueled airships also provided more lift than alternative gasses. Yet in every case, hydrogen proved to be a far too impractical gas to deal with.

But most of all I am against the whole idea of the Space Shuttle, because it was an all in one solution. It was supposed to serve every possible need. But all in one solutions are almost never a good idea. The latest example that comes to mind is that F-35 fighter jet which like the Shuttle tries to serve too many needs and ends up being hideously complex and expensive.

Saturn V F-1 Engine

The American moon rocket Saturn V used five of the largest rocket engines every built, the F-1. No engine of this size has ever been built since.

Author David Woods puts the power of the F-1 engines in perspective:


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Picture by NASA of present day engineers dressed up as engineers from the Apollo era. Shows the sheer size of the F-1 nozzle.

Getting appropriate stability for such large engines was difficult to achieve. That the Americans solved this problem is on of the main reasons why they managed to beat the Russians in the space race.

In fact the Russians have built more powerful rocket engines but only by fitting it with multiple nozzles. For instance the RD-170 used on the Energia rocket was more powerful, but used four nozzles, making it look like four separate engines on the outside. The RD-170 is the precursor to the widely used Russian RD-180 rocket engine.


This Russian made rocket engine is called the Ferrari of rocket engines. It is a high performance design with high performance. It is a kerosene engine with impressive specific impulse (efficiency) and thrust, combined with relatively low weight.

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Although it is a single engine, the RD-180 has two nozzles. This is to achive stability, as building large nozzles is difficult. However on top there is just one turbopump.

The reason I chose to write about this, is because the story of the RD-180 tells an interesting story about rocket engines, technology, the US and the Soviet Union all in one.

It is easy to think that Russia and the Soviet Union was outcompeted by the US in space, because the US got to the moon first and has had several impressive missions to Mars. So when it comes to headline grabbing news about space, the US has tended to dominate since the late 60s. However when Russia annexed Crimea and there was talk of sanctions against Russia, it got pointed out in the media that the US was totally dependent on Russian space industry. Not only could American astronauts only get sent to the International Space Station onboard the Soyuz spacecraft, but the US also relied on Russian rocket engines, specifically the RD-180 for their own rockets. More specifically the Atlas V launch vehicle by United Launch Alliance (ULA).

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Explaining rocket terminology. What is the difference between a rocket, space craft, space vehicle and launch vehicle?

Space Rocket Terminology

Just to clarify, terminology can be a bit confusing at times, since we talk about rockets, space ships, space craft, launch vehicles etc. A rocket is a pretty generic term and could mean anything from a Saturn V to a tomahawk missile. So when we say launch vehicle, it is to indicate that we mean a rocket that carries payload into orbit e.g. a satellite. Also to be accurate a typical launch vehicle has multiple stages and thus is made up of multiple rockets stacked on top of each other or connected in parallel. Also important to not mistake is, that a launch vehicle is not a space craft. A launch vehicle is used to get a space craft into space. A space craft is what is actually moving around in space. So e.g. we distinguish between the Soyuz launch vehicle and the Soyuz space craft.

Anyway how could the “winner” of the space race end up in such a situation?

The Soviets was behind the US on hydrogen rocket engines, and thus tried in the 1960s to build their own failed moon rocket the N1 to use kerosene engines for all stages, unlike the Saturn V which used it only for the first stage. They also were not able to build really large engines like the F-1, developed in the US. It is hard to achieve stability with large engines. Instead the Soviets tried to solve the same problem by having lots of smaller but more efficient kerosene engines in the N1 moon rocket. This resulted in the NK-33 rocket engine, which was a oxygen rich staged combustion cycle engine. Such engines were believed by western engineers to be impossible for decades. When Aerojet bought NK-33 engines from Russia in the mid 1990s they discovered that oxygen rich engines which they had deemed impossible had existed in Russia for 30 years.

It was the Russian experience with oxygen rich engines which allowed them to build the RD-170 kerosene engine in the 1980s. RD-180 is simply a scaled down version of this one. RD-170 was made for the Russian Energia which was a Russian response to the American Space Shuttle. The RD-170 was the engine for the Zenith rockets which was used as strap on boosters for the Energia launch vehicle.

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The Energia launch vehicle. On top is the Buran similar to the American space shuttle. Attached to the main body are 4 Zenith booster rockets. Each booster has a single RD-170 engine with four nozzles.

And here we can see how the Russian consistent focus on kerosene engines for everything paid off. You typically need these for the first stage of any launch vehicle. But the American F-1 engines for the moon rocket was completely oversized for usage in any other launch vehicle and the Saturn V was just way too big and expensive to use on a regular basis. The Shuttle was in similar fashion a dead end. The strap on boosters on the shuttle were solid fuel and couldn’t be used as any stand alone rocket like the Russian Zenith rockets. While the RS-25 engines used on the Shuttle itself was useless as first stage on a regular launch vehicle as they were complicated and expensive hydrogen fuel engines. In fact they were not made for cheap production but for easy maintenance and repair as they were designed primarily for reuse.

Thus when ULA needed engines for the first stage of Atlas V, it made sense to go for high performance Russian kerosene engines. It didn’t seem strategically problematic then because they did in fact pay to get the blue prints from the Russians. In fact the plan was for RD-180 engines to be manufactured in the US. So they ended up designing the Atlas V for RD-180 engines. And you can’t just swap out engines to anything afterwards. Rockets are usually designed for specific engines. It turned out later that despite having the blue prints, manufacturing the RD-180 engines in the US was impossible.

The high pressure, oxygen rich propellant proved very difficult to deal with. Oxygen is a powerful oxidizer, which corrodes metal quickly. In a rocket engine having a surplus of oxygen will just easily cause corrosion and ruin the strength of the metal and thus blow the whole engine apart during operation. They Russians however had developed a deep skill in working with Titanium alloys which could deal with this. It was not easily transferable knowledge as it was more like an artisan job, then something you could write down the receipt for.

In fact for some reasons Titanium because a Soviet speciality early on. This is why American airplane manufacturers to this day buy critical airplane parts in titanium from Russia. It was one of the mysteries during the cold-war how Russia had managed to build Titanium submarines. As the material was considered too difficult to work with to make it practical to build large structures like Submarines in Titanium alloys.

So despite loosing the space race, Russia ended up dominating the launch business, as they focus on smaller cheap and efficient kerosene engines suited this business well. In fact until this year (2017) Russia has tended to have the majority of launches. This was evident even back in the cold war. The Space Shuttle never allowed the US to have very many launches. E.g. if we look at launches for various years, lets pick 1980 as an example. Back then the US had 15 launches in a year but the Soviet Union managed a staggering 89 launches.

In short the RD-180 rocket engine, says a lot about the different choices the US and the Soviet Union made in the space industry.

What about BE-3, BE-4 and Raptor Engines?

When this article was originally written neither Blue Origin’s BE-4 or SpaceX’s Raptor engine was close to finished. Both of these engines are perhaps the most interesting engines made in a long time, in particular the Raptor engine. I will write an article about these engines in the future, which I will link to here. Until then I highly recommend this Raptor Engine video by the Everyday Astronaut.


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