3 ms·

Right but they were awful because they were built and tested and qualified top-down, not bottom up. Feynman wrote about this idiocy at length: http://science.k

Right but they were awful because they were built and tested and qualified top-down, not bottom up. Feynman wrote about this idiocy at length: http://science.ksc.nasa.gov/shuttle/missions/51-l/docs/rogers-commission/Appendix-F.txt http://science.ksc.nasa.gov/shuttle/missions/51-l/docs/roger...

api 13y ago
I also wonder about the use of hydrogen and how this impacted reusability. From what I've read hydrogen causes things like metal embrittlement, which is why SpaceX is completely avoiding it despite the higher specific impulse it yields.
- InclinedPlane 13y ago
  LH2 is also super cryogenic. LOX and even liquid methane have boiling points higher than LN2. You can build launchers with them without even bothering to insulate the tanks, because the boiloff is so marginal over the duration of the launch. Whereas LH2 absolutely requires a lot of insulation, because it needs to be much, much colder. Worse yet, because of the super low density of LH2 you can't afford even a small amount of boiloff because that hurts your mass fraction immensely, and if you have a LOX/LH2 vehicle you've probably scraped and scrabbled for what meager mass fraction you've managed to achieve (through advanced alloys and so forth). One particular consequence of the use of LH2 with the Shuttle was that it resulted in an ice covered and foam insulated enormous external tank cheek-by-jowl with the delicate TPS coated underbelly of the orbiter during launch, as the vehicle rammed through the air at transonic then supersonic then hypersonic speeds. Another consequence of the LH2 choice for the Shuttle is that it didn't have enough thrust at sea level to get off the pad, so it needed high-thrust, low-cost boosters as a 0th stage. Solid rockets being the obvious go-to solution, those got used. In a very real way the choice to use LH2 resulted not just in much of the cost and complexity of the Shuttle system but also the death of 2 crews and the loss of 2 orbiters.
  - api 13y ago
    Hydrogen rockets sound like a classic case of narrow over-optimization for one or a few dimensions of performance at the expense of the overall picture. The methane direction SpaceX is taking going forward strikes me as almost a no-brainer. It gives you a higher isp than kerosene, is about as easy to handle as LOX, and is super-abundant and cheap. In addition to being basically 95% of natural gas it can also be made electrochemically/thermochemically from water and CO2 or trivially obtained from biomass digestion. The latter options are all possible on Mars. It's also a renewable fuel if you get it from non-fossil sources on Earth.
    - Crito 13y ago
      Hydrogen can make sense, but not really as the Shuttle used it. The second and third stages of the Saturn V used hydrogen, which was in part responsible for the much greater mass to TLI than the N-1 would have had (despite having a first stage with more thrust).

api 13y ago

I also wonder about the use of hydrogen and how this impacted reusability. From what I've read hydrogen causes things like metal embrittlement, which is why SpaceX is completely avoiding it despite the higher specific impulse it yields.

InclinedPlane 13y ago
LH2 is also super cryogenic. LOX and even liquid methane have boiling points higher than LN2. You can build launchers with them without even bothering to insulate the tanks, because the boiloff is so marginal over the duration of the launch. Whereas LH2 absolutely requires a lot of insulation, because it needs to be much, much colder. Worse yet, because of the super low density of LH2 you can't afford even a small amount of boiloff because that hurts your mass fraction immensely, and if you have a LOX/LH2 vehicle you've probably scraped and scrabbled for what meager mass fraction you've managed to achieve (through advanced alloys and so forth). One particular consequence of the use of LH2 with the Shuttle was that it resulted in an ice covered and foam insulated enormous external tank cheek-by-jowl with the delicate TPS coated underbelly of the orbiter during launch, as the vehicle rammed through the air at transonic then supersonic then hypersonic speeds. Another consequence of the LH2 choice for the Shuttle is that it didn't have enough thrust at sea level to get off the pad, so it needed high-thrust, low-cost boosters as a 0th stage. Solid rockets being the obvious go-to solution, those got used. In a very real way the choice to use LH2 resulted not just in much of the cost and complexity of the Shuttle system but also the death of 2 crews and the loss of 2 orbiters.
- api 13y ago
  Hydrogen rockets sound like a classic case of narrow over-optimization for one or a few dimensions of performance at the expense of the overall picture. The methane direction SpaceX is taking going forward strikes me as almost a no-brainer. It gives you a higher isp than kerosene, is about as easy to handle as LOX, and is super-abundant and cheap. In addition to being basically 95% of natural gas it can also be made electrochemically/thermochemically from water and CO2 or trivially obtained from biomass digestion. The latter options are all possible on Mars. It's also a renewable fuel if you get it from non-fossil sources on Earth.
  - Crito 13y ago
    Hydrogen can make sense, but not really as the Shuttle used it. The second and third stages of the Saturn V used hydrogen, which was in part responsible for the much greater mass to TLI than the N-1 would have had (despite having a first stage with more thrust).