Why do Aeroplanes Get Fewer Engines and Rockets More?

Thanks, I always love getting feedback. The number of engines questions is an interesting one, and your brought up an interesting counterpoint I had not thought of. As I am am not an aerospace engineer by trade, I would have to make an educated guess about these apperent opposite directions being taken.

I see several possible ways of answering this. I’ve put some different alternatives with different heading:

Elon Musk Engineering Philosophy

The large number of rocket engines on Falcon Heavy as well as future SpaceX rockets, is perhaps not a trend at all per say with respect to rockets but rather a choice matching Elon Musk’s philosophy of engineering. The Tesla Model S has 7000 battery cells, while Nissan Leaf has only 140 cells. Tesla is thus an extreme outlier. Tesla made this choice in large part from economic reasons. They used laptop battery cells which where already produced in high volume and optimized. If you started with a blank slate going with larger cells like Nissan would likely have been better. Elon Musk has a nose for pragmatic solutions. As with rocket engines, he understands the importance of volume production. I’ve failed to grasp this importance many times in the past. I remember how I was convined Apple would surpase PCs since they went for the much more advance PowerPC RISC architecture on their microprocessors. In the end superior technology lost out to the ability of Intel of producing much higher volumes of Intel x86 microprocessors and hence throwing more money at the problem and divide the cost over far more units.

Volume Differences For Aircraft and Rockets

Airplanes are produced in much higher volume than space rockets. Hence even if they go for larger engines, they likely still get enough volume on their production that it becomes economical. Consider the problem with the F-1 rocket engine used on the Saturn V. I think only a few dozen of those were every built and each were basically custom made. Once the Apollo programmed stopped there was no market for them. That is a tremendous waste and make the engines quite expensive as you can’t spread the cost over very many launches. By going for small rocket engines which can be used on rockets of different sizes you avoid that problem. Blue Origin seems to make a similar choice. New Glenn will have seven BE-4 engines despite being a smaller rocket than Saturn V. This allows the BE-4 engines to be small enough to be used in the much smaller ULA Vulcan rocket.

Staging Introduce Complexity Not Found in an Airplane

An airplane basically just has one stage, and can thus simplify down to just one engine. A rocket going into space on the other hand can’t possibly be made with just one engine, ever. Each stage needs at least one engine. The problem is that the requirements of each stage is quite different. The bottom stage needs a lot of thrust, fuel efficiency is less of a problem. The upper stage in contrast needs minimum thrust but high fuel efficiency, because you are no longer fighting earth gravity. I you want to reduce complexity of building a rocket, you want roughly the same engine on all stages. Imagine you had gone for one enormous first stage engine. If you want to stick to a single engine type, you would end up with a totally oversized engine for the upper stage. That would waste considerable weight.

In the past rockets have addressed this issue by creating sort of optimal engines for each stage. Large kersone based high trust, low efficiency engines at the bottom and lower thrust high efficiency hydrogen engines on the upper stage. For sattelites one goes even further and stick ion thrusters on which have very low thrust but 10x the fuel efficiency. These use xenon gas. So with this approach we end up with several completely different engine architectures as well as completely different fuels. This creates a lot of complexity, hence higher costs. Airplanes avoid this problem entirely. An efficient airplane does not require multiple types of fuel and engine types.

Using multiple similar smaller engines is thus a sort of compromise. It doesn’t create the most efficient rocket but reduce complexity and cost while avoiding having completely unsuistable engines at various stages.

Predictions About Rocket Engines

I predict that engine sizes will go up for rockets as we have seen for aeroplanes once the market expands. When the total volume of manufactured rockets go up, I think it will be easier to rationalize a choice of larger engines.

But ironically smaller engines is what is going to get us there through their cost savings which will drop prices of launches and hence expand demand.

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Geek dad, living in Oslo, Norway with passion for UX, Julia programming, science, teaching, reading and writing.

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