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How much would Dragon 2 or Red Dragon need to be changed to act as a lunar lander? What I mean is not just landing but also lifting off from the Moon's surface to a specified orbit to dock with another spacecraft.

The current designs would need some modification. For a Moon mission the heat shield and the parachute system are unnecessary, so in place of their mass there could be fuel available for the mission. But would it have enough fuel to land and bring the astronauts back to orbit? If not, how much more fuel is needed? What would the total mass be for this? Also what kind of other modifications would it need, for example life support systems, internal power, and spacecraft components (computers, electronics, batteries), protection from radiation, and structural material changes with lighter materials (since it will not serve to make a re-entry into Earth's atmosphere just as a lunar lander)?

This will be costly, since the design could change, but would it be worth it to modify the current Dragon spacecrafts, or would it be better to create an entirely new design with better capabilities for such a mission and that doesn't resemble the current designs?

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tl;dr: If you use (nearly) all your cargo capacity for additional propellant and get rid of heatshield etc. you might barely be able to land the Dragon II from Lunar orbit (back-of-the-envelope result). In all other cases you need to strap on additional propellant.

From the answer to this question, we know that the delta v is more like 450m/s. According to wikipedia, we'd need around 3.8km/s for LLO -> Moon -> LLO. According to my calculations that means around 27t of propellant for an unmodified Dragon. That amount can be reduced to 16t of propellant when we reduce the dry mass by 2.5t (the intended cargo return capacity).

If we do not want to return from the Moon (e.g. refuel there), the standard Dragon II needs around 7.5t propellant for landing while the reduced version needs around 5t.

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  • $\begingroup$ The Apollo Lunar lander had a mass of about 14 tons including fuel, so I think that the Dragon is too large to be a good ascent vehicle. The astronauts should instead be packed together in a minimal vehicle, as in Soyuz. (On the other hand, SpaceX doesn't talk about returning from Mars...) $\endgroup$
    – LocalFluff
    Commented Aug 6, 2016 at 13:45
  • $\begingroup$ That depends on how costly your fuel is. If you can provide it on the ground in sufficient quantities, it might be better to reuse an oversized craft from an economical perspective. $\endgroup$
    – choeger
    Commented Aug 6, 2016 at 13:59
  • $\begingroup$ @LocalFluff SpaceX doesn't talk about returning with Dragon architecture because it can't - not enough delta v. Come September they intend to announce the architecture that will fly round trips, then we'll hear about returns. $\endgroup$
    – Saiboogu
    Commented Aug 6, 2016 at 16:22
  • $\begingroup$ Landing without enough fuel to get back home? Sounds risky. But it is a trade off and reliability seems to me to improve quickly. I've seen some science fiction where that actually worked had a happy ending after some drama. $\endgroup$
    – LocalFluff
    Commented Aug 6, 2016 at 17:23
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From this question, we know the delta v is about 1700 m/s with the stock configuration of the Dragon 2. The heat shield will be replaced with some extra propellant for a Lunar lander. It's really hard to say how much mass that would be, but let's give it a 10% decrease in mass and the same increase in fuel. The Moon requires about 2.3 km/s It seems reasonable that a light load could land on the Moon. But there would not be enough to return to orbit, let alone back to Earth. Still, I'm sure if they wanted to they could re-design it to make such a trip, mostly by bringing a larger payload to the Moon.

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  • $\begingroup$ The answer to my question points out that real budget is much less. $\endgroup$
    – choeger
    Commented Aug 6, 2016 at 13:17

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