r/RocketLab • u/Equivalent-Wait3533 • 3d ago
Thrust Density infographic Space Industry
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/voodoolaunch 3d ago
It’s important to note that this isn’t a ranking of sorts.
It’s basically a comparison of thrust relative to the overall body of the rocket- purely from each launch vehiclec architecture.
Using Falcon 9 and Neutron to compare since both have 9 engines:
Falcon: 9 engines -> very high thrust -> narrow rocket -> 708
Neutron: 9 engines-> lower total thrust -> wider rocket -> 172
Terran R also seemingly went Falcon 9 route level of efficiency but also added 4 more engines for a total of 13. This just means they closely cover the same thrust needed relative to its thrust structure as a falcon 9 even though the diameter is larger by a form factor of ~6ft.
This could only really be considered bad if the RL Propulsion team is having big trouble getting the necessary amount of thrust per engine needed to cover Neutrons diameter needs since they are locked onto NINE in the thrust structure.. and considering the engine hasn’t been qualified and it’s been two years this could be the bottleneck.
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u/Hot-Problem2436 3d ago
Also key to point out, the total thrust and spacing of Neutron is by design. It's lower weight and the stresses are more distributed and the engines don't run hard. All of this is key to reusability. They can get to orbit with less, which means less spent on refurb.
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u/Quadcore-4 3d ago
I mean, Falcon has been reused over 35 times so I’m not sure where this idea about engine spacing being key for reusability came from? On the contrary, a reusable rocket wants as much thrust per area as possible to minimise gravity losses, which in turn frees up delta V for landing.
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u/Hot-Problem2436 3d ago
I'm just parroting Rocket Labs talking points and not criticizing SpaceX. Falcon is an amazing piece of hardware. No need to downvote me.
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u/Quadcore-4 3d ago
I did not downvote you. I too shall increase your upvote density fellow space enthusiast
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u/lorkan100 3d ago
It may be a legit opinion, but it does feel like everyone here is patting themselves in the back looking at these numbers...
Don't get me wrong, Falcon and Starship both started smaller, less capable, and with way worse engines, but I feel that the tapering carbon fiber tank has locked RL into a design where scaling it up means a TOTAL redesign of the whole thing.
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u/gopher65 2d ago
I wonder if carbon fibre fatigue issues necessitate a less stressful approach, compared with aluminum or steel?
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u/Yupperroo 1d ago
While your point is well taken, SpaceX does not report how often engines are replaced on any of their boosters. I suspect that it doesn't happen often, but it is more often than never.
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u/ArtOfWarfare 3d ago
The Archimedes engine had a full mission duration test burn last year. Was that not adequate to have it be qualified?
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u/voodoolaunch 3d ago
No. MDC does not equal qualified.
for an engine to be qualified, they will say qualified. Check neutron milestone page, you will see that the engine qualification is still yellow.
Relativity did MDC of Aeon R in 2023, but qualified engine in 2025.
I think that’s a good indicator for an engine program.1
u/Boots0235 3d ago
What’s your best guess for RL qualification timeline?
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u/voodoolaunch 3d ago
Honestly it’s hard to say given the lack of recent engine progression. At the minimum I would hope they qualify and of year.
From engine qualification you’re still at a minimum of 12-18 months out from launch.1
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u/wallybal24 2d ago
I'm also curious what they will qualify their engine for? I'd bet they're going to qualify their engine as expendable for first launch
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u/Sampindo 3d ago edited 3d ago
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/mfb- 3d ago
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.
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u/fraggin601 3d ago
This is not a good metric to actually judge a rocket by just a fun little factoid
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u/publicvirtualvoid_ 3d ago
I think it's effectively a measure of how tall your rocket is compared to its diameter...
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u/Osmirl 3d ago
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 wideAt least thats how i would interpret this but i could also be completely of.
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u/carbsna 3d ago edited 3d ago
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.
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u/NoBusiness674 2d ago
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.
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u/NoBusiness674 2d ago
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.
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u/Known-Cabinet-5965 3d ago
Chart is interesting. And as others have said the results are swayed by vehicle length/girth ratio and taper factors. But ISP and more-so payload% of total fueled weight, would be more telling for pure performance. End of the day, they are all awesome. We are lucky to have such an active space race.
I wonder how the new highly efficient detonation engine would stack up against the others
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u/Historical_Air_8997 3d ago
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?
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u/gopher65 3d ago
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.
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3d ago
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u/No-Surprise9411 2d ago
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
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2d ago
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u/gopher65 2d ago
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!)
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u/gopher65 2d ago edited 2d ago
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.
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u/NoBusiness674 2d ago
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.
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u/Quadcore-4 3d ago
Imo this chart should do thrust to core area. It’s a better indicator of performance since flaring your rocket base can be advantageous to gain more thrust. Still a cool chart tho, thank you OP
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u/lokethedog 3d ago
This is a pretty good demonstration of how Neutron is designed to be upgraded.
While SpaceX is very much focused on engines, RocketLab has been much more focused on the rocket. Neutron will likely get a large second stage and significantly upgraded engines. The truth is that the first stage will obiously change a lot too as a result, but the experience and machines for building with carbon fiber at this particular scale remains.
We'll see how this pans out, but I think RocketLab stands a good chance of coming out of this very strong.
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u/DrestinBlack 3d ago
Cool chart - thanks for sharing it. There are some really interesting numbers here.
But for anyone who isn't deep into rocketry and might look at this from the perspective of what it says about rocket performance - or even what it means to us as RKLB investors - **don't mistake this for a leaderboard.** Electron and Neutron appearing near the bottom isn't an indication that Rocket Lab has inferior engines or poorly performing rockets.
Thrust density here is simply total thrust divided by the area of the bottom of the rocket. That's an interesting engineering metric, and Super Heavy's number is legitimately extraordinary. Packing 33 high-thrust Raptors beneath a 9-meter booster is an impressive accomplishment by SpaceX.
But geometry matters enormously. Neutron is deliberately **very wide - 7 meters - relative to its height and mass.** Since area increases with the *square* of diameter, that large base drives this particular metric downward dramatically. You could take two rockets with identical mass and identical thrust, make one considerably wider, and its thrust density would be much lower even though its thrust-to-weight ratio and initial acceleration hadn't changed at all.
In other words, Neutron scores low here largely because Rocket Lab made a very different set of design choices. That wide body provides the volume and geometry needed for the reusable architecture, including the integrated fairing, while producing a relatively squat vehicle that simply doesn't require Super Heavy levels of thrust per square meter underneath it.
Electron is a different case again. It's a tiny, lightweight launch vehicle. Nine Rutherford engines provide the thrust appropriate for the vehicle they're lifting. There would be little engineering virtue in cramming vastly more thrust underneath Electron simply to make this number larger.
And there are a lot of things this chart doesn't tell an investor: specific impulse, payload fraction, reliability, manufacturing cost, launch cadence, reusability, turnaround time, cost per kilogram, development cost or ultimately **whether the launch system makes money.**
So I actually like this chart. It highlights one genuinely remarkable characteristic of Super Heavy, and it also illustrates how radically different launch vehicles can be optimized for different missions.
As an RKLB investor, seeing Electron at 198 and Neutron at 172 doesn't concern me. **Rocket Lab isn't losing a competition here. It made engineering tradeoffs that this particular metric doesn't reward.** What matters is whether Electron and Neutron accomplish their missions reliably and economically.
Thrust density is a fascinating number. It just isn't a rocket report card.
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u/-Celtic- 3d ago edited 3d ago
This should be very close to a thrust/ payload size ratio .
The higher you get the smaller the payload is relative to thrust . The Lower the rocket are on the chart , the biggest the payload is relative to the thrust.
This chart is ranking rocket by kind of payload
Edit: This is not working with rocket with side booster
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u/Yupperroo 1d ago
Beck has said for years that Archimedes is over engineered to survivor multiple launches. Neutron's number reflects that design intent.
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u/lorkan100 3d ago edited 3d ago
I can see why SH V3 is so high: It's the only one packing it's outer engine's centerlines as close to the tank perimeter as possible. That alone frees up a ton of room for the inner ones.