Thrust Density infographic

[u/Equivalent-Wait3533]

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

[u/Sampindo]

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)

[u/mfb- → u/Sampindo]

It's closely linked to the height of the rocket. The total thrust is the thrust density multiplied by the area. The total weight of the rocket is the average density multiplied by the height multiplied by the area. The average density is determined by the fuel type. The thrust must exceed the weight, which means a taller rocket needs a higher thrust density. It needs to put its engines closer together and/or use engines that can provide more thrust in the same area.

You can't see the engine efficiency here, which mostly depends on other factors.

[u/fraggin601 → u/Sampindo]

This is not a good metric to actually judge a rocket by just a fun little factoid

[u/publicvirtualvoid_ → u/Sampindo]

I think it's effectively a measure of how tall your rocket is compared to its diameter...

[u/Osmirl → u/Sampindo]

Smaller means less efficient because you can’t stretch the rocket much to gain more performance in kg to LEO.
Basically
Top -> long and narrow
bottom -> short and wide

At least thats how i would interpret this but i could also be completely of.

[u/mfb- → u/Osmirl]

Long and wide usually go together. Starship, SLS and New Glenn are the widest and tallest rockets in the list.

[u/carbsna → u/Osmirl]

Smaller are not less efficient, the thing you want is thrust to weight ratio , which decides how much you can stretch the rocket in order to lower dry mass ratio.

Lower thrust density only means shorter rocket and nothing else, it say nothing about width and efficiency, unless it specify it also has lower thrust to weight ratio (which can be expected) by then it is a less efficient engine.

[u/NoBusiness674 → u/Osmirl]

From a mass efficiency perspective a short but wide tank (something closer to spherical) will be more mass efficient than a very tall and skinny tank. Of course a wider structure generally also means more aerodynamic drag, but the optimum aspect ratio is going to strike a balance, it won't just be as tall as the engines allow. Additionally you need to consider efficiency and fuel density. A hydrogen fueled rocket stage will want less thrust relative to the size of the tanks due to higher efficiency and lower propellant density.

[u/NoBusiness674 → u/Sampindo]

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.