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Comment by zaroth | Hacker News Reader

It's apparently a fully 3D printed engine with an electric turbopump;

  "Use of this kind of electric motor system produces a 95%
  efficiency as compared to the 50% efficiency achieved
  through standard gas-generator cycle engines."

I can't imagine that statement means what I think it means...?

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My understanding is that it's just talking about the pumps. 95% of the energy from the battery is turned into rotational energy by a motor in the pumps. This pushes the fuel into the reaction chamber of the rocket engine. Contrast this to a gas-generator cycle that uses some of the fuel/oxidizer to run the pumps. It's not perfect combustion (you don't want it to get too hot) turbines aren't perfectly efficient, etc. so you only get 50% of the energy out of the fuel that you use in the turbo pumps.

However, batteries are heavy. I'm hesitant to believe that electric pumps will scale well to larger rockets, but I would love to be proven wrong.

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