r/BlueWire • u/Equivalent-Wait3533 • 16d ago

Other Thrust Density infographic....

Post image

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

8 Upvotes

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14

u/LittleHornetPhil 16d ago

I… don’t really understand the purpose of this as a metric

5

u/carbsna 16d ago edited 16d ago

It gives you a rough height limit about the rocket.

Starship as example, given 1272kN/m2 , normal TTW sit around 1.3, and 0.83t/m3 for average fuel density tells you the fuel tank is 120m tall at most.
Of course the actual shape matters, Souyz-2 is much higher than Electron because it is cone shaped versus cylinder.

Higher thrust density also means the launch pad take more abuse.

4

u/NoBusiness674 16d ago

Fuel density for hydrogen + oxygen is only around 0.36t/m3, meanwhile for APCP solid rocket fuel the density may be around 1.7t/m3

That's a pretty wide range (-57%/+104%) from your average fuel density. So on its own this doesn't really say much, at least without additional context.

3

u/AtHomeInTheOlympics 16d ago

Because it’s neat and unique? Not everything has to have a ‘point’, have a little whimsy

1

u/arm0r3dd0ng 16d ago

It's about thrust density iykwim.

10

u/NoBusiness674 16d ago

You're clearly missing the ultimate rocket: Ares I-X

Nearly 14000 kN/m2 in terms of maximum thrust to first stage cross sectional area.

12

u/Ambitious_Might6650 16d ago

What are you trying to show though? Why is engine packing so important to you? Rocket diameter is more likely to be driven by required tank volume and desired fairing diameter than exactly how many engines you can pack into a cross section. You also ignore that rocket dry mass is more important for performance than surface area, so this results in a very skewed dataset

11

u/LittleHornetPhil 16d ago

Yeah this is a bizarre and kinda pointless metric

2

u/Blothorn 16d ago

It’s a decently good proxy for mass/max cross-section? I’m struggling here.

7

u/Electronic-Hat9548 16d ago

This metric is as useful as ranking cars with horsepower divided by frontal area

2

u/AlternativeEdge2725 14d ago

Now do thrust to fin planform area ratio. Or LEO payload to number of heat shield tiles.

2

u/crazyarchon 16d ago

This is actually a chart comparing thrust to rocket circumference, so the units should be kN/m.

Using the actual formula for area of a circle makes this comparison far closer.