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A larger rocket mitigates the effects of the rocket equation. The wet (loaded with propellant) to dry (empty of propellant) mass ratio is determined via the ro

A larger rocket mitigates the effects of the rocket equation. The wet (loaded with propellant) to dry (empty of propellant) mass ratio is determined via the rocket equation to be the exponential of delta V divided by exhaust velocity. Certain parts of the rocket, such as the external tank structure, scale sub-cubically with the rocket's dimension, as do aerodynamic forces; whereas payload and propellant mass scale cubically. Hence if the rocket is smaller than a critical threshold size, the requisite vehicle structures are too large relative to its propellant capacity to permit the required wet:dry mass ratio to achieve the delta V for orbit. At exactly this size, the rocket can reach orbit with zero payload. As the rocket increases in size beyond this threshold, it is able to carry a payload which is increasingly large relative to the rocket's total mass.

trklausss 1y ago
Added to that, Full-flow stage combustion engines are bigger, heavier, and more expensive, but are way more efficient. So a bigger rocket is the only option to get one of those onboard, and helps with taking more mass to orbit because they are more efficient than other options.
- perihelions 1y ago
  I don't believe there's any performance advantage for full-flow, which SpaceX alone is attempting. The only point is to lower the combustion temperature inside the turbines, at the expense of (much) higher flow rates through those turbines, in order to increase their lifespan. (There's a large difference between staged combustion generally and gas-generator engines, which throw away performance by dumping fuel out of the turbine exhaust).
  - schiffern 1y ago
    Since the temperature limit of available materials is the fundamental limitation (even after making custom high-temp alloys), this allows them to maximize mechanical power from the turbopumps, which raises performance. We might imagine a conservative FFSC design which accepts very low temperatures in exchange for making it easy (low R&D cost) to reach high longevity. Raptor is not a conservative design, so it requires more R&D to achieve that longevity. https://www.youtube.com/watch?v=twnZYPdFgbU https://www.youtube.com/watch?v=twnZYPdFgbU
londons_explore 1y ago
This is also why no hobby rockets get to orbit. Even a 1 gram payload to low earth orbit is beyond what a human-sized rocket can manage due to the way rockets don't scale downwards well.
- LeifCarrotson 1y ago
  How does this compare to the cube-square law scaling effects applied to propeller- and wing-lifted vehicles like quadcopters/helicopters and RC aircraft/jumbo jets? Or even the squat shape of a housefly that zigs and zags through the air like an acrobat compared to the ponderous lift-off of a large goose? I understand vaguely that those operate and scale based on the area (a square function of their length) of their lifting surfaces, and are pulled down by their mass (a cube function of their length). A little Estes toy rocket lifts off the pad much more aggressively (in the blink of an eye!) than a full size rocket...
  - simiones 1y ago
    They are almost entirely unrelated. When trying to leave the gravity well of a planet, the atmosphere is only a dragging force acting to reduce your thrust. It might be proportional to the surface area of the vehicle, but likely not - I think it's only proportional to the surface area of the "nose" of the rocket. But what's certain is that it's strictly a force that hinders you - in a rocket, all of your thrust comes from the engines, you don't get any boost from the air. However, even if you're taking off of a planet with no atmosphere, you still have a huge force to deal with - you need to maintain an acceleration to exit the gravity well of the planet, and you need to burn fuel for that. But you also have to carry the fuel you'll burn with you, so the more fuel you have, the more fuel you'll need - this is what the rocket equation codifies.
    - dylan604 1y ago
      > But you also have to carry the fuel you'll burn with you, so the more fuel you have, the more fuel you'll need Isn't this the entire point of using methane as fuel so that they can build a gas station once they get there so that return fuel is not required to be considered in this equation?
gcanyon 1y ago
But you also have a limit on the other side: going extreme to make the point, we haven't managed to build a mile-tall building yet, and a rocket that size would be a nightmare to engineer (while perhaps technically possible -- you might have to scale up another 10x or 20x to make it physically impossible). So there's some sort of curve, zero at both ends, between overall rocket size and the payload to orbit. The question is where Starship sits on that curve, and to your point it seems likely that it's looking good on that metric alone. But then you have another curve that I think starts small and increases near-monotonically, which is the complexity/likelihood-to-fail factor to the size of the rocket. It's (relatively) easy to launch a toy rocket, (fairly) simple to build a missile-sized sub-orbital rocket, difficult to build a small-to-medium orbital rocket, and apparently very difficult to build a Saturn/N-1/Starship-sized rocket. More props to the crazy '60s team that pulled it off.
- jjk166 1y ago
  > So there's some sort of curve, zero at both ends, between overall rocket size and the payload to orbit. This doesn't follow. Engineering complexity is not a limit on payload to orbit, it is a fundamentally different parameter. Yeah building a mile tall rocket would be hard, but it would get a shit ton of payload to orbit. There is no maximum beyond which making a bigger rocket starts to reduce your payload to orbit. > But then you have another curve that I think starts small and increases near-monotonically, which is the complexity/likelihood-to-fail factor to the size of the rocket. It's (relatively) easy to launch a toy rocket, (fairly) simple to build a missile-sized sub-orbital rocket, difficult to build a small-to-medium orbital rocket, and apparently very difficult to build a Saturn/N-1/Starship-sized rocket. Complexity does not increase with size, people just become more risk averse with size. Toy rockets fail all the time, just nobody really cares. No one would bet the lives of multiple people and hundreds of millions of dollars on a successful toy rocket launch. If complexity increases, it is with capability. If you want to land on the moon, you need something a bit more advanced than a hobby rocket. There is no reason to believe a floatilla of physically smaller rockets capable of achieving any given mission will be less complex in aggregate than a single physically larger rocket.
  - 1y ago

trklausss 1y ago

Added to that, Full-flow stage combustion engines are bigger, heavier, and more expensive, but are way more efficient. So a bigger rocket is the only option to get one of those onboard, and helps with taking more mass to orbit because they are more efficient than other options.

perihelions 1y ago
I don't believe there's any performance advantage for full-flow, which SpaceX alone is attempting. The only point is to lower the combustion temperature inside the turbines, at the expense of (much) higher flow rates through those turbines, in order to increase their lifespan. (There's a large difference between staged combustion generally and gas-generator engines, which throw away performance by dumping fuel out of the turbine exhaust).
- schiffern 1y ago
  Since the temperature limit of available materials is the fundamental limitation (even after making custom high-temp alloys), this allows them to maximize mechanical power from the turbopumps, which raises performance. We might imagine a conservative FFSC design which accepts very low temperatures in exchange for making it easy (low R&D cost) to reach high longevity. Raptor is not a conservative design, so it requires more R&D to achieve that longevity. https://www.youtube.com/watch?v=twnZYPdFgbU https://www.youtube.com/watch?v=twnZYPdFgbU

londons_explore 1y ago

This is also why no hobby rockets get to orbit. Even a 1 gram payload to low earth orbit is beyond what a human-sized rocket can manage due to the way rockets don't scale downwards well.

LeifCarrotson 1y ago
How does this compare to the cube-square law scaling effects applied to propeller- and wing-lifted vehicles like quadcopters/helicopters and RC aircraft/jumbo jets? Or even the squat shape of a housefly that zigs and zags through the air like an acrobat compared to the ponderous lift-off of a large goose? I understand vaguely that those operate and scale based on the area (a square function of their length) of their lifting surfaces, and are pulled down by their mass (a cube function of their length). A little Estes toy rocket lifts off the pad much more aggressively (in the blink of an eye!) than a full size rocket...
- simiones 1y ago
  They are almost entirely unrelated. When trying to leave the gravity well of a planet, the atmosphere is only a dragging force acting to reduce your thrust. It might be proportional to the surface area of the vehicle, but likely not - I think it's only proportional to the surface area of the "nose" of the rocket. But what's certain is that it's strictly a force that hinders you - in a rocket, all of your thrust comes from the engines, you don't get any boost from the air. However, even if you're taking off of a planet with no atmosphere, you still have a huge force to deal with - you need to maintain an acceleration to exit the gravity well of the planet, and you need to burn fuel for that. But you also have to carry the fuel you'll burn with you, so the more fuel you have, the more fuel you'll need - this is what the rocket equation codifies.
  - dylan604 1y ago
    > But you also have to carry the fuel you'll burn with you, so the more fuel you have, the more fuel you'll need Isn't this the entire point of using methane as fuel so that they can build a gas station once they get there so that return fuel is not required to be considered in this equation?

gcanyon 1y ago

But you also have a limit on the other side: going extreme to make the point, we haven't managed to build a mile-tall building yet, and a rocket that size would be a nightmare to engineer (while perhaps technically possible -- you might have to scale up another 10x or 20x to make it physically impossible). So there's some sort of curve, zero at both ends, between overall rocket size and the payload to orbit. The question is where Starship sits on that curve, and to your point it seems likely that it's looking good on that metric alone. But then you have another curve that I think starts small and increases near-monotonically, which is the complexity/likelihood-to-fail factor to the size of the rocket. It's (relatively) easy to launch a toy rocket, (fairly) simple to build a missile-sized sub-orbital rocket, difficult to build a small-to-medium orbital rocket, and apparently very difficult to build a Saturn/N-1/Starship-sized rocket. More props to the crazy '60s team that pulled it off.

jjk166 1y ago
> So there's some sort of curve, zero at both ends, between overall rocket size and the payload to orbit. This doesn't follow. Engineering complexity is not a limit on payload to orbit, it is a fundamentally different parameter. Yeah building a mile tall rocket would be hard, but it would get a shit ton of payload to orbit. There is no maximum beyond which making a bigger rocket starts to reduce your payload to orbit. > But then you have another curve that I think starts small and increases near-monotonically, which is the complexity/likelihood-to-fail factor to the size of the rocket. It's (relatively) easy to launch a toy rocket, (fairly) simple to build a missile-sized sub-orbital rocket, difficult to build a small-to-medium orbital rocket, and apparently very difficult to build a Saturn/N-1/Starship-sized rocket. Complexity does not increase with size, people just become more risk averse with size. Toy rockets fail all the time, just nobody really cares. No one would bet the lives of multiple people and hundreds of millions of dollars on a successful toy rocket launch. If complexity increases, it is with capability. If you want to land on the moon, you need something a bit more advanced than a hobby rocket. There is no reason to believe a floatilla of physically smaller rockets capable of achieving any given mission will be less complex in aggregate than a single physically larger rocket.
- 1y ago