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> The SpaceX Merlin engine uses the same basic RP-1/LOX cycle as the Saturn V’s first stage engine, which is more than half a century old now. If I build an el

> The SpaceX Merlin engine uses the same basic RP-1/LOX cycle as the Saturn V’s first stage engine, which is more than half a century old now. If I build an electric car with the same rubber technology as cars have used for a century, this would not mean I hadn't pushed the limits of technology. > Rocketdyne’s engine produced about 7-8 times as much thrust (which is why SpaceX needs 27 engines on three boosters for a Falcon 9 Heavy, versus 5 for a Saturn V.) "Thrust per engine" is a bad metric since, for larger payloads, it can be better to add more engines than make them bigger, and that metric immediately disqualifies that strategy. (For instance, more full independent engines means more redundancy. Which strategy is better is going to depend on the details.) We wouldn't say that 18-wheelers are worse technology than 4-wheeled earth movers because the latter have more weight per wheel. Ultimately, the thing we care about is most mass to highest orbit at lowest risk. But if you want to concentrate on raw engine performance, you probably should look at metrics like ISP (which SpaceX is indeed not pushing the limits). > A more accurate summary is SpaceX is retreading well-worm ground that was trod before many of us reading this article were born, bringing modern materials and manufacturing capabilities to bear improving the same basic designs. That’s cool. But that’s not pushing the boundaries of space flight technology. Surely re-useable first stages is the biggest advance by far in spaceflight technology in decades? It sounds like by "space flight technology" you just mean engines, but chemical rockets are within about a factor of 2 of the maximum efficiency allowed by the laws of physics. So within that paradigm, you're just not going to get revolutionary (multiple-orders-of-magnitude) improvements in the things we care about (mass to orbit) by improving engines as opposed to things like re-usability. So I think by your definition, the only thing that would be really be an improvement would be nuclear rockets, or something else even more extreme.

rayiner 8y ago
> If I build an electric car with the same rubber technology as cars have used for a century, this would not mean I hadn't pushed the limits of technology. That's a misleading comparison, because we're not talking about some ancillary aspect of the design (like rubber tires). This is more like designing a car with the same basic engine technology as a 1969 Impala, except you're using an aluminium engine block and modern crumple zones. > "Thrust per engine" is a bad metric since, for larger payloads, it can be better to add more engines than make them bigger. More engines can also mean more chances for failure. (You can make the design resistant to engine-out conditions, but some engine-out conditions are less recoverable than others.) Regardless, the Falcon 9 Heavy also has substantially lower maximum thrust than the Saturn V. > Surely re-useable first stages is the biggest advance is spaceflight technology in decades? It sounds like by "space flight technology" you just mean engines, but chemical rockets are within about a factor of 2 of the maximum efficiency allowed by the laws of physics. So within that paradigm, you're just not going to get revolutionary (multiple-orders-of-magnitude) improvements in the things we care about (mass to orbit) by improving engines as opposed to things like re-usability. So I think by your definition, the only thing that would be really be an improvement would be nuclear rockets, or something else even more extreme. I don't even really disagree with that point. Unless there is some groundbreaking advance in rocket engines, you're not going to be "pushing the boundaries of spaceflight technology." It's just like how we don't say Intel is "pushing the boundaries of CPU technology" every time they widen the vector units on the same basic Core architecture they've been using for almost two decades. It's a mature field where revolutionary improvements aren't possible with foreseeable technology. But that doesn't mean you get to relabel incremental improvements as revolutionary.
- jessriedel 8y ago
  > That's a misleading comparison, because we're not talking about some ancillary aspect of the design (like rubber tires). I think within the context of chemical rockets, engines really are sorta ancillary! Not to the same extent as rubber tires (I used that extreme example just to make my point clear), but engines is just not where the large improvements in things we care about will come from. > More engines can also mean more chances for failure. Agreed. I wasn't arguing for either strategy since I don't know enough to have an opinion. I just wanted to point out that more engines was a plausible best strategy, depending on engineering details, and we shouldn't choose metrics that immediately disqualify it. > Regardless, the Falcon 9 Heavy also has substantially lower maximum thrust than the Saturn V. Agreed, Falcon 9 is not an example of an advance in total lift capacity, and total lift capacity is important. BFR/Starship may exceed Saturn V, but if so it will be a very slight edge which is not impressive after half a century. So I agree that this is not where the advances are happening. But, to pick another extreme example, the even older Spruce Goose was only recently dethroned in total wingspan, but this doesn't mean no dramatic advances in flight technology have occurred. > But that doesn't mean you get to relabel incremental improvements as revolutionary. abledon's original comment said "pushed the limits of spaceflight technology". I was the first person to use the vague word "revolutionary", and I immediately clarified what I meant: orders of magnitude improvement in metrics we care about. And I think the SpaceX tech, both completed and under development, really can lower mass-to-orbit prices by a factor of 100.
  - rayiner 8y ago
    > I think within the context of chemical rockets, engines really are sorta ancillary! Not to the same extent as rubber tires (I used that extreme example just to make my point clear), but engines is just not where the large improvements in things we care about will come from. They're not ancillary because propulsion performance metrics really drive the things you can do with the rocket. (Same thing in airplanes.) Acknowledging that "large improvements" will not happen in that area is a concession that we've hit a pretty hard wall on what we can do with this technology.
    - 8y ago

rayiner 8y ago

> If I build an electric car with the same rubber technology as cars have used for a century, this would not mean I hadn't pushed the limits of technology. That's a misleading comparison, because we're not talking about some ancillary aspect of the design (like rubber tires). This is more like designing a car with the same basic engine technology as a 1969 Impala, except you're using an aluminium engine block and modern crumple zones. > "Thrust per engine" is a bad metric since, for larger payloads, it can be better to add more engines than make them bigger. More engines can also mean more chances for failure. (You can make the design resistant to engine-out conditions, but some engine-out conditions are less recoverable than others.) Regardless, the Falcon 9 Heavy also has substantially lower maximum thrust than the Saturn V. > Surely re-useable first stages is the biggest advance is spaceflight technology in decades? It sounds like by "space flight technology" you just mean engines, but chemical rockets are within about a factor of 2 of the maximum efficiency allowed by the laws of physics. So within that paradigm, you're just not going to get revolutionary (multiple-orders-of-magnitude) improvements in the things we care about (mass to orbit) by improving engines as opposed to things like re-usability. So I think by your definition, the only thing that would be really be an improvement would be nuclear rockets, or something else even more extreme. I don't even really disagree with that point. Unless there is some groundbreaking advance in rocket engines, you're not going to be "pushing the boundaries of spaceflight technology." It's just like how we don't say Intel is "pushing the boundaries of CPU technology" every time they widen the vector units on the same basic Core architecture they've been using for almost two decades. It's a mature field where revolutionary improvements aren't possible with foreseeable technology. But that doesn't mean you get to relabel incremental improvements as revolutionary.

jessriedel 8y ago
> That's a misleading comparison, because we're not talking about some ancillary aspect of the design (like rubber tires). I think within the context of chemical rockets, engines really are sorta ancillary! Not to the same extent as rubber tires (I used that extreme example just to make my point clear), but engines is just not where the large improvements in things we care about will come from. > More engines can also mean more chances for failure. Agreed. I wasn't arguing for either strategy since I don't know enough to have an opinion. I just wanted to point out that more engines was a plausible best strategy, depending on engineering details, and we shouldn't choose metrics that immediately disqualify it. > Regardless, the Falcon 9 Heavy also has substantially lower maximum thrust than the Saturn V. Agreed, Falcon 9 is not an example of an advance in total lift capacity, and total lift capacity is important. BFR/Starship may exceed Saturn V, but if so it will be a very slight edge which is not impressive after half a century. So I agree that this is not where the advances are happening. But, to pick another extreme example, the even older Spruce Goose was only recently dethroned in total wingspan, but this doesn't mean no dramatic advances in flight technology have occurred. > But that doesn't mean you get to relabel incremental improvements as revolutionary. abledon's original comment said "pushed the limits of spaceflight technology". I was the first person to use the vague word "revolutionary", and I immediately clarified what I meant: orders of magnitude improvement in metrics we care about. And I think the SpaceX tech, both completed and under development, really can lower mass-to-orbit prices by a factor of 100.
- rayiner 8y ago
  > I think within the context of chemical rockets, engines really are sorta ancillary! Not to the same extent as rubber tires (I used that extreme example just to make my point clear), but engines is just not where the large improvements in things we care about will come from. They're not ancillary because propulsion performance metrics really drive the things you can do with the rocket. (Same thing in airplanes.) Acknowledging that "large improvements" will not happen in that area is a concession that we've hit a pretty hard wall on what we can do with this technology.
  - 8y ago