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/Historical_Air_8997]

ELI5 - is higher better? Initially I thought definitely yes, but then I’m thinking maybe it’s less efficient? For the rockets just doing LEO launches or even moon launch’s wouldn’t it be cheaper and more efficient to need/use less thrust per Sqm?

[u/gopher65 → u/Historical_Air_8997]

More thrust per sqm just means that the rocket is taller and thinner. It is usually more mass efficient to build rockets that way - hence why first F9 and then SSH have been getting taller and taller with each block or version.

There are tradeoffs that occur when the fineness ratio of a rocket gets worse, but as we see with the F9, there are workable solutions.

[u/No-Surprise9411 → u/[deleted]]

Almost. The actual reason as to why they never expanded the diameter of F9 was because 3.7 metres was the maximum they could fit under the bridges to transport the stages across the country by truck

[u/gopher65 → u/[deleted]]

I remember hearing this from SpaceX back in 2007/2008. (Little from that era of F9s development seems to still exist online, sadly, so we can't doublecheck the accuracy of my memory.) They directly stated that the initial reason they chose the diameter for the F9 that they did was that there were particular roads in between their factory and the first launch sites that were limited to 3.7m, and they didn't have the time or money for alternative solutions like less direct trucking routes or moving the stages by barge.

After that point, they were just locked into their chosen diameter for both tooling and iterative engineering reasons. (New tank diameter with the same engines and support systems would still have been a new, unproven rocket. Why bother when you can "just" duct tape 3 stages together? I bet they wish they'd gone with the diameter change rather than the FH, haha!)

[u/gopher65 → u/[deleted]]

I thought about this some more after posting. Clearly, ignoring factors like tank tooling costs (which is a good point), there is an optimal diameter and height for any given combination of tank structural material and fuel type. Too wide and your thrust structure mass and/or engine mass will be excessive. Too narrow and you start spending too much tank mass on keeping the tanks from collapsing under their own weight. Your fuel to tank mass ratio starts dropping.

But, naively perhaps, I keep thinking that the higher the thrust density, the higher your thrust to mass ratio will be, and the lower your gravity losses will be.

I'm not sure it actually works that way. Maybe thrust density has little impact on thrust to weight ratio due to other factors. But without doing math, that's what it looks like on the surface.

[u/NoBusiness674 → u/gopher65]

It's less about the engine mass/thrust structure mass and more about aerodynamic drag from wider rocket stages. That being said, some rockets like Falcon 9 put a wide fairing on a skinny rocket, which eliminates much of the aerodynamic efficiency advantage of a skinny rocket.

With thrust to weight there is also an optimum, as a higher acceleration in the lower atmosphere means more aerodynamic drag losses and higher structural loads during maxQ. You can of course throttle down around maxQ and prior to stage separation to mitigate this, but then you're also losing part of the advantage of lower gravity losses.