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.
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.
Smallest orbital anything so far is 31ft(9.54m) long, 20in(54cm) wide, 2.9t/2.6t(2600kg?), does 9lbs(4kg) to random-ish LEO: https://en.wikipedia.org/wiki/SS-520
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...
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.
> 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?
I'm not talking about fuel that you need to get back, we're still at the "leaving Earth" case. The point is that you need, say, 1000 tons of fuel to leave the Earth. Your rocket then will weigh [weight of empty rocket] + [weight of payload] + 1000 tons. And it is this mass that the engines will have to push while ascending. Of course, the fuel gets spent as you ascend - by the time you reach orbit, your rocket is now 1000 tons lighter.
The refueling idea is so that for example you don't need to carry the fuel needed to get to the moon or Mars all in one rocket. You just need to carry enough to get to the refueling orbit - which is much less.
The toys have to be aggressive. You have less than three feet worth of launch rail--by the time the rocket clears the rail it must be going fast enough that the fins make it stable. Meanwhile, it's light, overengineering the body to take a high g load is trivial.
An orbital class rocket--taking that kind of g load is going to break it (just look at the payload specs for the Falcon Heavy--its maximum permitted payload is well below it's performance to low orbit. You load it up to what the engines can do, it breaks. The only use case is when it's going farther than low orbit.) And an orbital class rocket has active steering rather than fins, it doesn't need to be booking it to be stable.
> Our satellite launched on a SpaceX Falcon 9 rocket from Vandenberg Space Force Base in California (USA) on Jan 14, 2025. The rocket mission is a Transporter, and SAT GUS was dropped off in low-Earth orbit at about 375 miles above the surface of our pale blue dot.
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.
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).
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.
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.
> 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.
>> 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
At this point I'm merely talking about size (which I think is clear from the words I use. I don't think "building a mile tall rocket would be hard" adequately describes the difficulty when we haven't even built a mile tall building.
Sea Dragon[1] was only envisioned as 490 feet tall, and as near as I can tell even the Super Orion[2] would only have been 400-600 meters tall. And of course, neither of those was even close to implementation. Therefore I stand by my statement that a mile tall rocket is, for all practical purposes, impossible, and thus has a payload to orbit of zero. If you disagree then add a zero -- surely you agree we can't build a ten-mile-tall rocket?
As far as complexity, I'm not sure what to say. Toy rockets might fail all the time, but the point was complexity, and a toy rocket can be constructed from under a dozen parts. Even larger model rockets have at most a few dozen to a few hundred parts. The part count of the Falcon 9 has to number in the thousands, if not tens of thousands (9 merlin engines with at least several hundred parts each?).
To be clear, I agree with you that complexity increases with capability.
But also, to push back a bit, I don't think complexity aggregates the way you're saying it does. A box of hammers is not more complex than a nailgun, even if it has more parts in total.
> At this point I'm merely talking about size (which I think is clear from the words I use. I don't think "building a mile tall rocket would be hard" adequately describes the difficulty when we haven't even built a mile tall building.
I was assuming you were using a comical example to illustrate a "nightmare to engineer." The comparison to a building doesn't actually work at all. The practical limitation on how high we can build buildings is how fast we can make elevators. Just making something tall is not a problem.
> Sea Dragon[1] was only envisioned as 490 feet tall, and as near as I can tell even the Super Orion[2] would only have been 400-600 meters tall. And of course, neither of those was even close to implementation. Therefore I stand by my statement that a mile tall rocket is, for all practical purposes, impossible
First, the optimal design for a rocket is not to just keep making it taller, and second, size was not the obstacle to either of these projects not being built. That does not at all prove that it is impossible. What kind of world would we be living in we presumed anything that hadn't already been actively pursued was impossible?
> and thus has a payload to orbit of zero.
My point was that this does not equate to a payload of zero. Surely you wouldn't argue that the weight of this mile high rocket is zero, and therefore that there is some curve for the weight of rockets where making the rockets larger starts to make them lighter. Just as we can calculate the weight for something without actually building it, so too can we calculate the payload, and it can increase far beyond anything we can actually implement.
> If you disagree then add a zero -- surely you agree we can't build a ten-mile-tall rocket?
I agree it would be impractical, but not that it would be so non-physical that we couldn't calculate what its payload capacity would be were it to be built.
> Toy rockets might fail all the time, but the point was complexity, and a toy rocket can be constructed from under a dozen parts. Even larger model rockets have at most a few dozen to a few hundred parts. The part count of the Falcon 9 has to number in the thousands, if not tens of thousands (9 merlin engines with at least several hundred parts each?).
Falcon 9 is a liquid rocket designed to take people into space. That is the source of its part count. You could scale up a solid rocket motor to an arbitrarily large size while keeping the parts count exactly the same. It's probably not the optimal way to make a solid rocket of that size, and you'd be missing out on a lot of capabilities that are important for a real rocket, but if you just wanted a toy no more capable than what you buy in a hobby store it would be no more complicated. Conversely, try to make a fully functional falcon 9 complete with 9 working liquid rocket engines small enough to hoverslam on your desk and you have an immense engineering challenge on your hands.
> But also, to push back a bit, I don't think complexity aggregates the way you're saying it does. A box of hammers is not more complex than a nailgun, even if it has more parts in total.
I concur that part count is not the same as complexity, but that point is in my favor. Making something bigger is like adding hammers to a box of hammers. The quantity goes up, and at some point you're going to need to make some improvements to the box if you want to keep adding more hammers, but conceptually it is simple. Making something more capable, like a nail-gun, is much harder.
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.