5 ms·

Mars launch window is every two years. It is very inefficient to launch at other times. As for moon, I'm surprised with the estimate you have provided. Apollo

by vl 2y ago

Mars launch window is every two years. It is very inefficient to launch at other times. As for moon, I'm surprised with the estimate you have provided. Apollo needed just one launch for each mission. Even if SpaceX will do orbital re-fueling, it's just two-three launches, why would you need more? BTW, the idea of getting heavy Starship to the moon and back is interesting, but at the end flying the vehicle optimized for re-entry far away and back is suboptimal. My prediction that they quickly will go to specialized LEO-LMO vehicles with LEO re-fueling.

avmich 2y ago
> Even if SpaceX will do orbital re-fueling, it's just two-three launches, why would you need more? Wikipedia says Starship weights 120 ton empty and 1320 ton fueled, plus 100 ton payload (approximate numbers). That means fuel weights 1200 ton. So to carry fuel to LEO to fuel up a Starship you need 1200 / 100 = 12 flights. You can change this number maybe 2 times into both directions, but I doubt you'll fuel Starship with just 2 or 3 flights of tankers. Would be glad to err here.
- vl 2y ago
  But you don’t have to fuel it fully to go to the moon and back. It’s a bit hard to compare to Apollo since Apollo dropped stages at every step of the process, but it seems they used 70 tons of fuel in the third state of Saturn V for original trans lunar injection of 45 ton Apollo. Apollo itself was 2/3 fuel. So it’s ratio of 15 tons to 90 tons. I.e. 1/6.
  - avmich 2y ago
    > But you don’t have to fuel it fully to go to the moon and back. Numbers: from LEO (low Earth orbit) to TLI (translunar injection) - approx. 3.1 km/s; from Moon hyperbolic to LMO (low Moon orbit) - approx. 0.8 km/s; from LMO to the Moon surface - 1.6 km/s, back to LMO - 1.6 km/s, total - 7.1 km.s . Numbers are optimistic, no errors. You absolutely, positively have to add delta-V for landing, Apollo LEMs, for example, had ~0.6 km/s, and also for docking - about the same, so total is already 8.3 km/s . This is more than Starship needs to get to LEO after SuperHeavy boosted it from the Earth. So... no, if you're planning to get to the Moon anything near the Starship regular payload - 100 ton - you absolutely have to fuel it up fully.
    - vl 2y ago
      Yes, you are correct, I forgot about payload. With payload my estimate produces full refueling as well.
avmich 2y ago
> My prediction that they quickly will go to specialized LEO-LMO vehicles with LEO re-fueling. Two comments here. First, we assume now SpaceX is going to have Starship HLS - human landing system - which doesn't go back to Earth, doesn't have flaps or heat shield, and is going to be used between low Moon orbit (LMO) and Moon surface - maybe one roundtrip, maybe more. Yes, for each following roundtrip HLS needs to be refueled. Second, Musk mentioned "Moon base Alpha" in his talk. Having a serious Moon base makes it possible to produce some of propellants there. Oxygen is plentiful in the form of oxides on the Moon, and by mass it's 2/3 - 3/4 of the propellant load of the Starship, so it might be useful to produce it on the Moon.
- Paradigma11 2y ago
  Having a serious moon base enables us to build space ship designs not dominated by aerodynamic and gravitational limitations.

by vl 2y ago

avmich 2y ago

> Even if SpaceX will do orbital re-fueling, it's just two-three launches, why would you need more? Wikipedia says Starship weights 120 ton empty and 1320 ton fueled, plus 100 ton payload (approximate numbers). That means fuel weights 1200 ton. So to carry fuel to LEO to fuel up a Starship you need 1200 / 100 = 12 flights. You can change this number maybe 2 times into both directions, but I doubt you'll fuel Starship with just 2 or 3 flights of tankers. Would be glad to err here.

vl 2y ago
But you don’t have to fuel it fully to go to the moon and back. It’s a bit hard to compare to Apollo since Apollo dropped stages at every step of the process, but it seems they used 70 tons of fuel in the third state of Saturn V for original trans lunar injection of 45 ton Apollo. Apollo itself was 2/3 fuel. So it’s ratio of 15 tons to 90 tons. I.e. 1/6.
- avmich 2y ago
  > But you don’t have to fuel it fully to go to the moon and back. Numbers: from LEO (low Earth orbit) to TLI (translunar injection) - approx. 3.1 km/s; from Moon hyperbolic to LMO (low Moon orbit) - approx. 0.8 km/s; from LMO to the Moon surface - 1.6 km/s, back to LMO - 1.6 km/s, total - 7.1 km.s . Numbers are optimistic, no errors. You absolutely, positively have to add delta-V for landing, Apollo LEMs, for example, had ~0.6 km/s, and also for docking - about the same, so total is already 8.3 km/s . This is more than Starship needs to get to LEO after SuperHeavy boosted it from the Earth. So... no, if you're planning to get to the Moon anything near the Starship regular payload - 100 ton - you absolutely have to fuel it up fully.
  - vl 2y ago
    Yes, you are correct, I forgot about payload. With payload my estimate produces full refueling as well.

avmich 2y ago

> My prediction that they quickly will go to specialized LEO-LMO vehicles with LEO re-fueling. Two comments here. First, we assume now SpaceX is going to have Starship HLS - human landing system - which doesn't go back to Earth, doesn't have flaps or heat shield, and is going to be used between low Moon orbit (LMO) and Moon surface - maybe one roundtrip, maybe more. Yes, for each following roundtrip HLS needs to be refueled. Second, Musk mentioned "Moon base Alpha" in his talk. Having a serious Moon base makes it possible to produce some of propellants there. Oxygen is plentiful in the form of oxides on the Moon, and by mass it's 2/3 - 3/4 of the propellant load of the Starship, so it might be useful to produce it on the Moon.

Paradigma11 2y ago
Having a serious moon base enables us to build space ship designs not dominated by aerodynamic and gravitational limitations.