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> This is far from ideal because multiple small engines gives poorer power to weight than fewer big engines Are you sure about that? The Merlin engine Falcon 9

> This is far from ideal because multiple small engines gives poorer power to weight than fewer big engines Are you sure about that? The Merlin engine Falcon 9 uses has the highest thrust-to-weight ratio of any rocket engine currently being used, more than double that of the much more powerful RD-180 used on the Atlas V. You'll lose some advantage because structural support and plumbing for more engines is heavier, but I'd expect they'll still come out ahead. The Raptor has been downsized from the original plans, and Musk has said that was to improve thrust-to-weight ratio.

simonh 5y ago
It’s a fair point but how many engines have been through such an aggressive optimisation and upgrade program? Arguably a bigger brother of Merlin going through the same process should be more efficient.
- Denvercoder9 5y ago
  > Arguably a bigger brother of Merlin going through the same process should be more efficient Why? I can't think of any reason why more powerful engines would fundamentally be more weight-efficient.
  - simonh 5y ago
    The volume of an object goes up with the cube of it’s diameter, while its surface area increases relative to the square of its diameter. That means if you quadruple the volume of a combustion chamber and nozzle, the area of the chamber and nozzle only doubles. A similar effect applies to the plumbing. That doesn’t mean the mass only doubles, it’s a bit more than that, but less than 4X.
    - Veedrac 5y ago
      Most parts of a rocket engine are structurally pressure vessels, where this rule doesn't apply, as wall thickness also scales with size. As I understand it, the main driver of scale is that the combustion chamber has an optimal size. Too small and you get incomplete mixing, too large and you get instabilities.
    - Denvercoder9 5y ago
      Rocket engines are far too complex to simply apply the cube-square law. In particular, the thrust of a rocket engine does not scale with the volume of the combustion chamber, but with the nozzle exit area (since exhaust velocity is independent of engine size). Thus, following your reasoning, thrust goes with the square of engine diameter, and weight as well, so thrust-to-weight is independent of the engine size.

simonh 5y ago

It’s a fair point but how many engines have been through such an aggressive optimisation and upgrade program? Arguably a bigger brother of Merlin going through the same process should be more efficient.

Denvercoder9 5y ago
> Arguably a bigger brother of Merlin going through the same process should be more efficient Why? I can't think of any reason why more powerful engines would fundamentally be more weight-efficient.
- simonh 5y ago
  The volume of an object goes up with the cube of it’s diameter, while its surface area increases relative to the square of its diameter. That means if you quadruple the volume of a combustion chamber and nozzle, the area of the chamber and nozzle only doubles. A similar effect applies to the plumbing. That doesn’t mean the mass only doubles, it’s a bit more than that, but less than 4X.
  - Veedrac 5y ago
    Most parts of a rocket engine are structurally pressure vessels, where this rule doesn't apply, as wall thickness also scales with size. As I understand it, the main driver of scale is that the combustion chamber has an optimal size. Too small and you get incomplete mixing, too large and you get instabilities.
  - Denvercoder9 5y ago
    Rocket engines are far too complex to simply apply the cube-square law. In particular, the thrust of a rocket engine does not scale with the volume of the combustion chamber, but with the nozzle exit area (since exhaust velocity is independent of engine size). Thus, following your reasoning, thrust goes with the square of engine diameter, and weight as well, so thrust-to-weight is independent of the engine size.