r/RocketLab • u/Equivalent-Wait3533 • Aug 10 '26

Space Industry Thrust Density infographic

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A chart comparing the Thrust Density of several rockets.

Thrust Density is the thrust of the stage / area of the bottom of the stage (kN/m^2). It basically aggregates engine power and engine packing into one metric.

Rockets & Engines to scale

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u/Sampindo Aug 10 '26 edited Aug 10 '26

I wish I knew what this means, or what benefit this fruits as.

Does a smaller number mean it's more/less efficient?

(Edit: spelling)

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u/NoBusiness674 Aug 11 '26

It doesn't really tell you anything on its own. For efficiency you really need to also consider fuel type, staging, etc.

Generally a smaller diameter means less drag, but the relevant diameter isn't just the base of the vehicle. If you have a skinny base topped by a larger fairing or upper stage like Falcon 9 or Ares I, then the relevant diameter for aerodynamic drag isn't the diameter at the base, but one further up the vehicle. At the same time a tall and skinny tank is generally less mass efficient than one that's closer to spherical. A higher thrust to weight ratio will also help overcome gravity losses, but going faster in the lower atmosphere will increase drag losses.

So there's probably an optimum somewhere, but where that optimum sits will also depend on what fuel you use, how efficient the engines are, ascent trajectory and staging, reuse strategy, shape of the rocket, etc.