3 ms·

Even spaceX's starship/BFR with LEO refuel wouldn't work?

QuotedForTruth 7y ago
Maybe answering my own question with some further thought. I guess even if it could get there, it likely couldnt return which would be a big waste of a reusable starship.
- knd775 7y ago
  Why do you think it would be unable to return? I'm curious.
  - caconym_ 7y ago
    Naively, a round trip requires twice the delta-v, which means you need to launch with a lot more than twice the fuel. Maybe they could produce more fuel and oxidizer in situ, and maybe Earth is a more attractive target for aerocapture, and maybe gravity assists line up better one way or the other, but it's not a bad starting point to assume a round trip is far more expensive than a one-way trip. I'm not sure on what basis he thinks Starship might be able to get there in the first place.
    - QuotedForTruth 7y ago
      Yeah that was my thinking. And I didn't have a basis for thinking it could get there in the first place which is why I asked the initial question. I didn't realize just how high the delta v requirements were for Saturn.
      - caconym_ 7y ago
        Yeah, I didn't mean to accuse you of making assumptions; I just wanted to call out one of the obvious well, actually replies that my own comment might have received. It is an interesting question for sure, especially given that one of the stated goals for Starship (IIRC) is to enable exploration of the outer planets.
    - knd775 7y ago
      Sorry for the late reply. Titan has lakes of liquid methane rather than water. Raptor is a methalox engine. I think refueling on the planet would be quite possible.
      - caconym_ 7y ago
        There is no doubt in my mind that with the right equipment on the surface of Titan you can refine methane out of the lakes, electrolyze oxygen from the water ice of the crust, and fuel a Starship. Can you fit that equipment in a single Starship, by mass and volume? Is that what we're talking about here? No idea.
dflock 7y ago
The delta-v required for a trip to Titan, according to this[1], is 28.43 km/s. Optimistically, a fully re-fuelled startship in LEO has ~10km/s of delta-v to spend, afaik. [1](https://www.reddit.com/r/space/comments/1ktjfi/deltav_map_of_the_solar_system/ https://www.reddit.com/r/space/comments/1ktjfi/deltav_map_of...)
- Robotbeat 7y ago
  Starship can do better than that if stripped down to expendable use. About 13km/s: https://twitter.com/elonmusk/status/1111798912141017089 https://twitter.com/elonmusk/status/1111798912141017089 Also, you're reading that delta-v map completely wrong. The red arrows mean you can aerobrake. So it only takes about 4.1km/s to reach Saturn if you start out at a highly elliptical (near escape) Earth orbit and do a large departure burn at perigee. (See here: https://en.wikipedia.org/wiki/Delta-v_budget#Interplanetary https://en.wikipedia.org/wiki/Delta-v_budget#Interplanetary ...the 7.3km/s if the burn is done starting in LEO, minus the 3.2km/s benefit you'd get if starting near c3=0. Or you add up the numbers on your delta-v map that can't be done using aerobraking, and you get the same 4.1km/s delta-v if starting from Earth escape.) ...so a fueled up Starship could send a payload direct to Titan and still have enough propellant to propulsively brake back to Earth orbit immediately after sending the Titan payload on its way. (But that's just one option... Lots of other vehicles could do it if you used refueling or some high performance kick stages or in orbit rendezvous.)
  - QuotedForTruth 7y ago
    So it would work! Thanks for the analysis.

Maybe answering my own question with some further thought. I guess even if it could get there, it likely couldnt return which would be a big waste of a reusable starship.

knd775 7y ago
Why do you think it would be unable to return? I'm curious.
- caconym_ 7y ago
  Naively, a round trip requires twice the delta-v, which means you need to launch with a lot more than twice the fuel. Maybe they could produce more fuel and oxidizer in situ, and maybe Earth is a more attractive target for aerocapture, and maybe gravity assists line up better one way or the other, but it's not a bad starting point to assume a round trip is far more expensive than a one-way trip. I'm not sure on what basis he thinks Starship might be able to get there in the first place.
  - QuotedForTruth 7y ago
    Yeah that was my thinking. And I didn't have a basis for thinking it could get there in the first place which is why I asked the initial question. I didn't realize just how high the delta v requirements were for Saturn.
    - caconym_ 7y ago
      Yeah, I didn't mean to accuse you of making assumptions; I just wanted to call out one of the obvious well, actually replies that my own comment might have received. It is an interesting question for sure, especially given that one of the stated goals for Starship (IIRC) is to enable exploration of the outer planets.
  - knd775 7y ago
    Sorry for the late reply. Titan has lakes of liquid methane rather than water. Raptor is a methalox engine. I think refueling on the planet would be quite possible.
    - caconym_ 7y ago
      There is no doubt in my mind that with the right equipment on the surface of Titan you can refine methane out of the lakes, electrolyze oxygen from the water ice of the crust, and fuel a Starship. Can you fit that equipment in a single Starship, by mass and volume? Is that what we're talking about here? No idea.

dflock 7y ago

The delta-v required for a trip to Titan, according to this[1], is 28.43 km/s. Optimistically, a fully re-fuelled startship in LEO has ~10km/s of delta-v to spend, afaik. [1](https://www.reddit.com/r/space/comments/1ktjfi/deltav_map_of_the_solar_system/ https://www.reddit.com/r/space/comments/1ktjfi/deltav_map_of...)

Robotbeat 7y ago
Starship can do better than that if stripped down to expendable use. About 13km/s: https://twitter.com/elonmusk/status/1111798912141017089 https://twitter.com/elonmusk/status/1111798912141017089 Also, you're reading that delta-v map completely wrong. The red arrows mean you can aerobrake. So it only takes about 4.1km/s to reach Saturn if you start out at a highly elliptical (near escape) Earth orbit and do a large departure burn at perigee. (See here: https://en.wikipedia.org/wiki/Delta-v_budget#Interplanetary https://en.wikipedia.org/wiki/Delta-v_budget#Interplanetary ...the 7.3km/s if the burn is done starting in LEO, minus the 3.2km/s benefit you'd get if starting near c3=0. Or you add up the numbers on your delta-v map that can't be done using aerobraking, and you get the same 4.1km/s delta-v if starting from Earth escape.) ...so a fueled up Starship could send a payload direct to Titan and still have enough propellant to propulsively brake back to Earth orbit immediately after sending the Titan payload on its way. (But that's just one option... Lots of other vehicles could do it if you used refueling or some high performance kick stages or in orbit rendezvous.)
- QuotedForTruth 7y ago
  So it would work! Thanks for the analysis.