4 ms·

Wait, what?

credit_guy 5y ago
I think what the GP is trying to say is that reusable rockets had been known and built by NASA 50 years ago. More precisely, the Moon lander was just such a rocket that was able to land and then to lift off, albeit on the Moon, not on Earth. But the concept was there, it was just a matter of scaling it up. By the way, I don't personally buy this argument, but it's not completely meaningless either.
- ceejayoz 5y ago
  The moon lander wasn't really reusable, though. It was a two stage disposable rocket that happened to hang out somewhere during the interstage period. Calling it "reusable" is like calling the Apollo capsules reusable because they go up, then down. SpaceX benefits enormously from fast computers, advances in metallurgy, CAD, etc.
- hwillis 5y ago
  They may also be referring to the McDonnell Douglas DC-X. It flew and landed itself in 1996, but it was heavily based on the 1967 Douglas SASSTO. Neither project was government funded or ordered. There's no particular reason to think it could have even been built in 1967, and even in 1996 it may not have been possible to land a rocket after reentry. Coming in backwards at supersonic speed is incredibly challenging. Flipping around would have been even more so. Transitioning from supersonic to subsonic would have been a total guess. The DC-X certainly could not have done any of those things. The OP may also not understand the incredible gulf in difficulty between landing a suborbital rocket and a real first stage. A suborbital rocket doesn't weigh that much more going up than it does going down. The F9 first stage weighs less than 5% of what it has to lift on the way up. Getting a rocket to output 25% thrust is easy; use four engines and turn off 3. You can't just turn off 19 engines; the F9 first stage is huge and needs huge power swings to remain stable. SpaceX built custom sparse voxel GPU solvers to design around combustion instabilities and supersonic flows around the rocket. Even in the 90s they would have had to make huge concessions for the sake of running 20+ engines at once. There's absolutely zero chance they could have done it in the 60s or 70s.
  - nexuist 5y ago
    The thing is, you use phrases like: > may not have been possible > incredibly challenging > a total guess Which, yeah, rocket science is hard! But the above is not difficult because it's impossible to solve for, it's difficult because we never tried! Nobody, government or otherwise, thought that rapidly iterating on rocket technology would be a good idea. This above all was the ticket to SpaceX's success - when you can throw rockets away for science, you can learn a lot in a short amount of time. Did some things need to be invented before SpaceX could use them? Obviously, although I have doubts that the computations being performed by these "GPU solvers" really require a modern custom-built GPU and couldn't have been done on FPGAs or arrays of microcontrollers from the 80s. If we understand the math being crunched, it's just a matter of throwing more computing power at the problem.
    - hwillis 5y ago
      To put those phrases in context, I think they would have required more effort than the Apollo project. > Obviously, although I have doubts that the computations being performed by these "GPU solvers" really require a modern custom-built GPU and couldn't have been done on FPGAs or arrays of microcontrollers from the 80s. Here's a talk by the guys: https://www.youtube.com/watch?v=vYA0f6R5KAI https://www.youtube.com/watch?v=vYA0f6R5KAI The Cray 2 could do <2 gigaflops. So a single simulation with 10,000 points per side at 1 microsecond timesteps for .1 seconds with ~50 flops per cell-step would have taken 79 years to run, assuming the Cray had 50 cycle fetch/store times. Engineers need to run lots of simulations to come up with a working design. And that's just simulating something around the scale of a single nozzle- you'll need to be doing sims for a 40 meter tall stage as it flips around. Also, the cray had a 256 Mword memory. So you could only store a single timestep of a grid of 400x400x400 cells anyway. The supercomputers of the 80s were really just not very useful for things like this.
      - nexuist 5y ago
        Going to watch the video now. Thank you for the explanation! Graphics isn't my primary field so admittedly I don't know what I'm talking about. One thing I love about SpaceX is how engineers are allowed to do talks like this, I barely hear anything from the engineering division at BO or ULA.

Wait, what?

I think what the GP is trying to say is that reusable rockets had been known and built by NASA 50 years ago. More precisely, the Moon lander was just such a rocket that was able to land and then to lift off, albeit on the Moon, not on Earth. But the concept was there, it was just a matter of scaling it up. By the way, I don't personally buy this argument, but it's not completely meaningless either.

ceejayoz 5y ago
The moon lander wasn't really reusable, though. It was a two stage disposable rocket that happened to hang out somewhere during the interstage period. Calling it "reusable" is like calling the Apollo capsules reusable because they go up, then down. SpaceX benefits enormously from fast computers, advances in metallurgy, CAD, etc.
hwillis 5y ago
They may also be referring to the McDonnell Douglas DC-X. It flew and landed itself in 1996, but it was heavily based on the 1967 Douglas SASSTO. Neither project was government funded or ordered. There's no particular reason to think it could have even been built in 1967, and even in 1996 it may not have been possible to land a rocket after reentry. Coming in backwards at supersonic speed is incredibly challenging. Flipping around would have been even more so. Transitioning from supersonic to subsonic would have been a total guess. The DC-X certainly could not have done any of those things. The OP may also not understand the incredible gulf in difficulty between landing a suborbital rocket and a real first stage. A suborbital rocket doesn't weigh that much more going up than it does going down. The F9 first stage weighs less than 5% of what it has to lift on the way up. Getting a rocket to output 25% thrust is easy; use four engines and turn off 3. You can't just turn off 19 engines; the F9 first stage is huge and needs huge power swings to remain stable. SpaceX built custom sparse voxel GPU solvers to design around combustion instabilities and supersonic flows around the rocket. Even in the 90s they would have had to make huge concessions for the sake of running 20+ engines at once. There's absolutely zero chance they could have done it in the 60s or 70s.
- nexuist 5y ago
  The thing is, you use phrases like: > may not have been possible > incredibly challenging > a total guess Which, yeah, rocket science is hard! But the above is not difficult because it's impossible to solve for, it's difficult because we never tried! Nobody, government or otherwise, thought that rapidly iterating on rocket technology would be a good idea. This above all was the ticket to SpaceX's success - when you can throw rockets away for science, you can learn a lot in a short amount of time. Did some things need to be invented before SpaceX could use them? Obviously, although I have doubts that the computations being performed by these "GPU solvers" really require a modern custom-built GPU and couldn't have been done on FPGAs or arrays of microcontrollers from the 80s. If we understand the math being crunched, it's just a matter of throwing more computing power at the problem.
  - hwillis 5y ago
    To put those phrases in context, I think they would have required more effort than the Apollo project. > Obviously, although I have doubts that the computations being performed by these "GPU solvers" really require a modern custom-built GPU and couldn't have been done on FPGAs or arrays of microcontrollers from the 80s. Here's a talk by the guys: https://www.youtube.com/watch?v=vYA0f6R5KAI https://www.youtube.com/watch?v=vYA0f6R5KAI The Cray 2 could do <2 gigaflops. So a single simulation with 10,000 points per side at 1 microsecond timesteps for .1 seconds with ~50 flops per cell-step would have taken 79 years to run, assuming the Cray had 50 cycle fetch/store times. Engineers need to run lots of simulations to come up with a working design. And that's just simulating something around the scale of a single nozzle- you'll need to be doing sims for a 40 meter tall stage as it flips around. Also, the cray had a 256 Mword memory. So you could only store a single timestep of a grid of 400x400x400 cells anyway. The supercomputers of the 80s were really just not very useful for things like this.
    - nexuist 5y ago
      Going to watch the video now. Thank you for the explanation! Graphics isn't my primary field so admittedly I don't know what I'm talking about. One thing I love about SpaceX is how engineers are allowed to do talks like this, I barely hear anything from the engineering division at BO or ULA.