3 ms·

Why did they go with a hydrolox first stage when they’re not even using the same engines as the second stage? Same reason as the SLS?

perihelions 3y ago
One major reason is that they're a good fit with solid-rocket boosters. The Ariane rocket family SRB's have commonality with the French SLBM. https://en.wikipedia.org/wiki/M51_(missile) https://en.wikipedia.org/wiki/M51_(missile) - "The three-stage engine of the M51 is directly derived from the solid propellant boosters of Ariane 5."
- lmm 3y ago
  > they're a good fit with solid-rocket boosters What? Why/how?
  - dotancohen 3y ago
    Hydrolox engines have very high ISP, but lack thrust at liftoff. Due to the low density of hydrogen, they require huge tanks that are as wide as possible, to reduce surface area and thus boil off. SRBs work well with wide first stages as that affords strong mounting points. And they provide significant thrust for liftoff.
    - raverbashing 3y ago
      Yeah I wonder how much "tying down" the test required, or if the engine alone is insufficient to make it fly (but still, if you make it too light it might wobble too much, etc). The SRBs are certainly fire and forget but I'm not sure how the ratio of SRBs/Main engine thrust are compared with something like the Space Shuttle
Manuel_D 3y ago
Liquid hydrogen has better energy density by mass than alternatives. Density by volume isn't as good, which makes hydrocarbons a better choice usually. But if a lightweight fuel container can be made, it's the more optimal choice.
- perihelions 3y ago
  It's rather that it's more effective at converting energy to impulse, since the molecular weights of the combustion products are much lighter. LH2/LOx doesn't have a particularly large energy advantage over hydrocarbons, when you're also accounting the mass of the oxygen (as you do with non-airbreathing rockets).
panick21_ 3y ago
Basically when Ariane 5 was designed, the Shuttle was all the hotness and the European were mostly copying the design. And for the Ariane 6 they didn't have time to come up with anything new. Most of the Ariane 6 are just things that were already in development for the Ariane 5 ME. It takes Europe 10+ years at least for a new engine. So Ariane 6 couldn't have used anything new. As for why not using it on the second stage, its simple. Its to big. Ariane 5/6 are late staging system. Meaning the boosters provide a lot of the load to get of the ground. The first stage flies was longer then in systems like the Falcon 9. So the hydrogen is bad for takeoff but if you stage late, hydrogen advantage matters more.

perihelions 3y ago

One major reason is that they're a good fit with solid-rocket boosters. The Ariane rocket family SRB's have commonality with the French SLBM. https://en.wikipedia.org/wiki/M51_(missile) https://en.wikipedia.org/wiki/M51_(missile) - "The three-stage engine of the M51 is directly derived from the solid propellant boosters of Ariane 5."

lmm 3y ago
> they're a good fit with solid-rocket boosters What? Why/how?
- dotancohen 3y ago
  Hydrolox engines have very high ISP, but lack thrust at liftoff. Due to the low density of hydrogen, they require huge tanks that are as wide as possible, to reduce surface area and thus boil off. SRBs work well with wide first stages as that affords strong mounting points. And they provide significant thrust for liftoff.
  - raverbashing 3y ago
    Yeah I wonder how much "tying down" the test required, or if the engine alone is insufficient to make it fly (but still, if you make it too light it might wobble too much, etc). The SRBs are certainly fire and forget but I'm not sure how the ratio of SRBs/Main engine thrust are compared with something like the Space Shuttle

Manuel_D 3y ago

Liquid hydrogen has better energy density by mass than alternatives. Density by volume isn't as good, which makes hydrocarbons a better choice usually. But if a lightweight fuel container can be made, it's the more optimal choice.

perihelions 3y ago
It's rather that it's more effective at converting energy to impulse, since the molecular weights of the combustion products are much lighter. LH2/LOx doesn't have a particularly large energy advantage over hydrocarbons, when you're also accounting the mass of the oxygen (as you do with non-airbreathing rockets).

panick21_ 3y ago

Basically when Ariane 5 was designed, the Shuttle was all the hotness and the European were mostly copying the design. And for the Ariane 6 they didn't have time to come up with anything new. Most of the Ariane 6 are just things that were already in development for the Ariane 5 ME. It takes Europe 10+ years at least for a new engine. So Ariane 6 couldn't have used anything new. As for why not using it on the second stage, its simple. Its to big. Ariane 5/6 are late staging system. Meaning the boosters provide a lot of the load to get of the ground. The first stage flies was longer then in systems like the Falcon 9. So the hydrogen is bad for takeoff but if you stage late, hydrogen advantage matters more.