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Naive question: "The easiest part about Starship is the booster because its not doing anything F9 isn't already. Just at a much larger scale." Why is it in ge

Naive question: "The easiest part about Starship is the booster because its not doing anything F9 isn't already. Just at a much larger scale." Why is it in general required, to design a new, more heavy rocket - and not "just scale" up an existing design? I mean, "just scaling up" means a different build process and this alone could probably prevent it when, using off the shelf solutions and not build everything yourself (screws and cables included), but are there other fundamental reasons, why usually a redesign is required?

ceejayoz 5y ago
> Why is it in general required, to design a new, more heavy rocket - and not "just scale" up an existing design? Because defense contractors make a lot more money for the new one.
- mcguire 5y ago
  Do you apply this sort of thinking to your job?
  - ceejayoz 5y ago
    I wish I got cost-plus contracting at my job.
- greedo 5y ago
  One of the criticisms of the SLS program is it's re-use of Shuttle Engine technology, so your point is a bit off the mark.
DisjointedHunt 5y ago
Because the assumptions made for one class of payload don't apply to another class of payload with vastly different mission specifications. A great place to start is this amazing video by The Everyday Astronaut: https://www.youtube.com/watch?v=LbH1ZDImaI8 https://www.youtube.com/watch?v=LbH1ZDImaI8 Notice how the type of fuel used itself is a consideration for the type of rockets we may send to Mars if we're considering In-Situ refueling.
funcDropShadow 5y ago
I am not a mechanical engineer, but my general understanding is even mammals cannot be scaled up arbitrarily. I've read that elephants are more or less the maximal size of land-based mammals because of heat transport. The surface is not large enough to transport all the heat away from the body. I could imagine that similar effects limit mechanical designs.
- emteycz 5y ago
  The height of trees is limited by the physical limits of water transport.
- pyuser583 5y ago
  “The surface is not large enough to transport all the heat away from the body” Can you explain that a bit more? I don’t understand.
  - cmyr 5y ago
    As a body increases in size, the surface area increases as a square but volume increases as a cube. The more volume, the more heat, and after a certain point your available surface area is no longer adequate to vent that heat. Another example of a similar principle applies to insects: they don’t have lungs, and absorb oxygen directly through their skin. In earlier eras of earth’s history, there was much more oxygen in the air, and so insects could be much larger, because the same amount of surface area could absorb enough oxygen to fuel a larger number of cells.
  - ncmncm 5y ago
    Look up "square-cube law". It governs everything about animal size. But not vehicles, so much. The principle is that 2x size, i.e. 2x length, 2x width, 2x height, gives you 8x volume and, for animals, mass. All that mass has to be kept fed and aerated, and produces heat. But the surface area didn't go up by 8x. It only went up 4x unless you changed the shape a lot. You probably can't get skin to shed heat 2x as well, so you have to be careful not to produce 8x as much heat, instead. This is all fine up to, say, the size of a giraffe or elephant. But an elephant that runs for five minutes is in for a bad time. Vehicles are mostly empty space and skin, so they have different sorts of scaling problems.
xoa 5y ago
>Why is it in general required, to design a new, more heavy rocket - and not "just scale" up an existing design? I'm not a rocket designer, just someone who has really enjoyed following it for a long time. Someone in the industry could give more specifics, but the basics are straight forward. Everything about a specific rocket is tightly coupled, from material factors to engines and everything that goes into that. To take an obvious one, how do you drive the bigger rocket? If you choose bigger engines, then you need entire new engines (and there are scaling issues there too). If you choose to use more smaller engines, you need brand new plumbing to feed all those engines (a very complex thing itself, last I checked each Raptor engine consumers nearly 800 kilograms of fuel PER SECOND), you need to deal with harmonics and control of more engines, different fluid mechanics, etc. Even merely construction changes with scale. For example scaling up carbon fiber to huge sizes is incredibly hard and expensive, one of a number of reasons SpaceX dropped it in favor of steel instead. Rocket volume obviously scales with the cube compared to surface area with the square of size, material properties change with thickness of various walls, and on and on. All of this also impacts aerodynamics, center of gravity and a host of other things. So yeah! Knowledge and expertise from existing rocket designs is absolutely very useful in designing a new one, Starship directly comes out of a ton of things SpaceX learned from F9, which itself took important lessons from F1 (and "F9" is a much bigger range much faster then typical rockets, from version 1.0 to Full Thrust Block 5, the current one, they changed more and gained more power then entire new rockets sometimes have). A much bigger Starship v2 may take much more from v1 then typical because they now do have something with more scaling potential (steel construction in particular). And a lot of the software no doubt will be directly evolved, Starship landing of course used F9 landing software and hardware lessons. But still, cannot just "scale up", even without things like completely changing fuel. Edit: In reply, jmyeet points out that I glossed over the manufacturing bit too much as well. There is a huge amount of expertise and institutional knowledge in going from a general design of any big complex machine to actually manufacturing it in a reliable fashion. People and the machines that make the machines and all the processes that go into that is a whole 'nother ball of wax. Can find plenty of stories on HN and elsewhere about merely duplicating "the same thing", like trying to manufacture an aircraft in a different factory. Or the level of work in "simply" copying a fab and the lengths that are gone to in order to try to make sure everything is exactly the same.
mcguire 5y ago
Not necessarily directly related to your question, but a conversation I had with a friend who was an aerospace engineer, and who worked on solid rocket design: Remember the Ares I/V designs? They theoretically were claimed to use what were "essentially" shuttle solid rocket boosters (SRBs), but scaled up. In fact, much of the chatter in general forums was that the Ares boosters were shuttle SRBs with an extra segment on top. (SRBs are built of several independent segments of propellant with the external structure wrapped around them. When they're assembled, they segments are stacked with, among other things, o-rings between them---that is what failed in Challenger.) Ok, so you can just stack another segment on top for more power, right? Well, no. SRBs don't burn from the bottom to the top; there is a hole from the bottom of the propellant to the top, and when ignited it is the propellant on the surface of this hole that burns. This hole up the propellant (remember, it's a solid) is not just a round cylinder. If it were, when you lit it on fire, it would start out at a minimum thrust and the thrust would increase as the hole got larger in diameter. This is not so much what you want a rocket engine to do. So the hole is actually a weird star shape that maintains the same surface area as it burns, which requires a bunch of strange parameters known only to the initiates of aerospace engineering magic. (This is what my friend did in grad school. Now he's an enterprise webby project manager thingy.) If you just stack another segment on a SRB, it will burn for the same amount of time with more thrust, which may not be a good idea---remember, this is a mechanical structure that is mechanically connected to whatever you want to lift and the whole thing may not like higher g-loads. So, to just "add another segment" you have to redesign the propellant entirely, then look at the structures and connectors that may not be designed for the new loads, and so on. Scaling up isn't easy. If I remember correctly (See the book Ignition!), one of the major problems for the Saturn V's F-1 engine was the design of the engine injector plate, the thing that sprays fuel and oxygen into the combustion chamber. It had dramatic effects on the stability of the burn (they can and will go BOOM!), the vibration and other aspects of the engine. And that was basically determined experimentally. Also, keep in mind that the big design problem for a liquid-fuel engine is how to feed as much liquid propellant and oxidizers into the engine at the rate necessary for the thrust. They use burning rocket propellant to drive turbine pumps to feed propellant into the combustion chamber, and those pumps are the cutting edge of high-temperature, high-pressure pump design. "Just scaling it up" can be a completely new research and development project.
Aloha 5y ago
The N-1 Moon Rocket indicates that scaling up by putting more of your existing rocket motor on a vehicle may not always been as easy as it appears. As other parallel posters have commented, mechanical devices do not always squish or stretch with ease.
- soperj 5y ago
  Which engine was the NK33 used on previously? It is currently used on a newer version of Soyuz, but I'm pretty sure that it was originally built for the N-1.
giantrobot 5y ago
The thrust to weight ratio of a cluster of rocket engines is not a linear relationship. Adding one more engine might add N+1 thrust but it also adds at least N+1 mass. So you have more thrust but now more mass to lift and a worse mass ratio (the ratio of fuel to non-fuel). To scale your engine cluster you want to add an engine worth of thrust but not an engine worth of mass. To that end you reduce the mass of your piping or use one pump for multiple engines. All those changes require engineering and testing because the dynamics of the system have changed. For instance a fuel pipe of X thickness and Y material may have some weird failure mode if you make it just a little thinner to save mass.
mr_toad 5y ago
A couple of crucial design elements for Starship are a metholox engine (the idea being to make the fuel on Mars), and a reusable second stage. You couldn’t achieve those goals by scaling up a falcon.

ceejayoz 5y ago

> Why is it in general required, to design a new, more heavy rocket - and not "just scale" up an existing design? Because defense contractors make a lot more money for the new one.

mcguire 5y ago
Do you apply this sort of thinking to your job?
- ceejayoz 5y ago
  I wish I got cost-plus contracting at my job.
greedo 5y ago
One of the criticisms of the SLS program is it's re-use of Shuttle Engine technology, so your point is a bit off the mark.

DisjointedHunt 5y ago

Because the assumptions made for one class of payload don't apply to another class of payload with vastly different mission specifications. A great place to start is this amazing video by The Everyday Astronaut: https://www.youtube.com/watch?v=LbH1ZDImaI8 https://www.youtube.com/watch?v=LbH1ZDImaI8 Notice how the type of fuel used itself is a consideration for the type of rockets we may send to Mars if we're considering In-Situ refueling.

funcDropShadow 5y ago

I am not a mechanical engineer, but my general understanding is even mammals cannot be scaled up arbitrarily. I've read that elephants are more or less the maximal size of land-based mammals because of heat transport. The surface is not large enough to transport all the heat away from the body. I could imagine that similar effects limit mechanical designs.

emteycz 5y ago
The height of trees is limited by the physical limits of water transport.
pyuser583 5y ago
“The surface is not large enough to transport all the heat away from the body” Can you explain that a bit more? I don’t understand.
- cmyr 5y ago
  As a body increases in size, the surface area increases as a square but volume increases as a cube. The more volume, the more heat, and after a certain point your available surface area is no longer adequate to vent that heat. Another example of a similar principle applies to insects: they don’t have lungs, and absorb oxygen directly through their skin. In earlier eras of earth’s history, there was much more oxygen in the air, and so insects could be much larger, because the same amount of surface area could absorb enough oxygen to fuel a larger number of cells.
- ncmncm 5y ago
  Look up "square-cube law". It governs everything about animal size. But not vehicles, so much. The principle is that 2x size, i.e. 2x length, 2x width, 2x height, gives you 8x volume and, for animals, mass. All that mass has to be kept fed and aerated, and produces heat. But the surface area didn't go up by 8x. It only went up 4x unless you changed the shape a lot. You probably can't get skin to shed heat 2x as well, so you have to be careful not to produce 8x as much heat, instead. This is all fine up to, say, the size of a giraffe or elephant. But an elephant that runs for five minutes is in for a bad time. Vehicles are mostly empty space and skin, so they have different sorts of scaling problems.

xoa 5y ago

>Why is it in general required, to design a new, more heavy rocket - and not "just scale" up an existing design? I'm not a rocket designer, just someone who has really enjoyed following it for a long time. Someone in the industry could give more specifics, but the basics are straight forward. Everything about a specific rocket is tightly coupled, from material factors to engines and everything that goes into that. To take an obvious one, how do you drive the bigger rocket? If you choose bigger engines, then you need entire new engines (and there are scaling issues there too). If you choose to use more smaller engines, you need brand new plumbing to feed all those engines (a very complex thing itself, last I checked each Raptor engine consumers nearly 800 kilograms of fuel PER SECOND), you need to deal with harmonics and control of more engines, different fluid mechanics, etc. Even merely construction changes with scale. For example scaling up carbon fiber to huge sizes is incredibly hard and expensive, one of a number of reasons SpaceX dropped it in favor of steel instead. Rocket volume obviously scales with the cube compared to surface area with the square of size, material properties change with thickness of various walls, and on and on. All of this also impacts aerodynamics, center of gravity and a host of other things. So yeah! Knowledge and expertise from existing rocket designs is absolutely very useful in designing a new one, Starship directly comes out of a ton of things SpaceX learned from F9, which itself took important lessons from F1 (and "F9" is a much bigger range much faster then typical rockets, from version 1.0 to Full Thrust Block 5, the current one, they changed more and gained more power then entire new rockets sometimes have). A much bigger Starship v2 may take much more from v1 then typical because they now do have something with more scaling potential (steel construction in particular). And a lot of the software no doubt will be directly evolved, Starship landing of course used F9 landing software and hardware lessons. But still, cannot just "scale up", even without things like completely changing fuel. Edit: In reply, jmyeet points out that I glossed over the manufacturing bit too much as well. There is a huge amount of expertise and institutional knowledge in going from a general design of any big complex machine to actually manufacturing it in a reliable fashion. People and the machines that make the machines and all the processes that go into that is a whole 'nother ball of wax. Can find plenty of stories on HN and elsewhere about merely duplicating "the same thing", like trying to manufacture an aircraft in a different factory. Or the level of work in "simply" copying a fab and the lengths that are gone to in order to try to make sure everything is exactly the same.

mcguire 5y ago

Not necessarily directly related to your question, but a conversation I had with a friend who was an aerospace engineer, and who worked on solid rocket design: Remember the Ares I/V designs? They theoretically were claimed to use what were "essentially" shuttle solid rocket boosters (SRBs), but scaled up. In fact, much of the chatter in general forums was that the Ares boosters were shuttle SRBs with an extra segment on top. (SRBs are built of several independent segments of propellant with the external structure wrapped around them. When they're assembled, they segments are stacked with, among other things, o-rings between them---that is what failed in Challenger.) Ok, so you can just stack another segment on top for more power, right? Well, no. SRBs don't burn from the bottom to the top; there is a hole from the bottom of the propellant to the top, and when ignited it is the propellant on the surface of this hole that burns. This hole up the propellant (remember, it's a solid) is not just a round cylinder. If it were, when you lit it on fire, it would start out at a minimum thrust and the thrust would increase as the hole got larger in diameter. This is not so much what you want a rocket engine to do. So the hole is actually a weird star shape that maintains the same surface area as it burns, which requires a bunch of strange parameters known only to the initiates of aerospace engineering magic. (This is what my friend did in grad school. Now he's an enterprise webby project manager thingy.) If you just stack another segment on a SRB, it will burn for the same amount of time with more thrust, which may not be a good idea---remember, this is a mechanical structure that is mechanically connected to whatever you want to lift and the whole thing may not like higher g-loads. So, to just "add another segment" you have to redesign the propellant entirely, then look at the structures and connectors that may not be designed for the new loads, and so on. Scaling up isn't easy. If I remember correctly (See the book Ignition!), one of the major problems for the Saturn V's F-1 engine was the design of the engine injector plate, the thing that sprays fuel and oxygen into the combustion chamber. It had dramatic effects on the stability of the burn (they can and will go BOOM!), the vibration and other aspects of the engine. And that was basically determined experimentally. Also, keep in mind that the big design problem for a liquid-fuel engine is how to feed as much liquid propellant and oxidizers into the engine at the rate necessary for the thrust. They use burning rocket propellant to drive turbine pumps to feed propellant into the combustion chamber, and those pumps are the cutting edge of high-temperature, high-pressure pump design. "Just scaling it up" can be a completely new research and development project.

Aloha 5y ago

The N-1 Moon Rocket indicates that scaling up by putting more of your existing rocket motor on a vehicle may not always been as easy as it appears. As other parallel posters have commented, mechanical devices do not always squish or stretch with ease.

soperj 5y ago
Which engine was the NK33 used on previously? It is currently used on a newer version of Soyuz, but I'm pretty sure that it was originally built for the N-1.

giantrobot 5y ago

The thrust to weight ratio of a cluster of rocket engines is not a linear relationship. Adding one more engine might add N+1 thrust but it also adds at least N+1 mass. So you have more thrust but now more mass to lift and a worse mass ratio (the ratio of fuel to non-fuel). To scale your engine cluster you want to add an engine worth of thrust but not an engine worth of mass. To that end you reduce the mass of your piping or use one pump for multiple engines. All those changes require engineering and testing because the dynamics of the system have changed. For instance a fuel pipe of X thickness and Y material may have some weird failure mode if you make it just a little thinner to save mass.

mr_toad 5y ago

A couple of crucial design elements for Starship are a metholox engine (the idea being to make the fuel on Mars), and a reusable second stage. You couldn’t achieve those goals by scaling up a falcon.