Falcon Heavy for moon shot

[tater]

2 minutes ago, sevenperforce said:

The Saturn V was far more efficient than the Falcon Heavy, based on ISP. It used liquid hydrogen upper stages. 

But the mission profile was not very efficient. 

Good thing they never needed that inefficient mission prof... oh, wait, Apollo 13.

Seems like the suggested FH profile is needlessly dangerous, though it looks to be substantially lower in dv requirement (almost half). I suppose as a private venture they could risk whatever they'd like.

[Motokid600]

I think I'd rather see the F9H fly first before speculating future missions. Wonder why SpaceX is being so hush-hush about it. IIRC it was supposed to fly this year.

 

Question: I'm looking at a picture of the F9H. In its full reusability mode what happens to the bracing structure for the radial boosters? Do they take it with it upon separation then land with this brace hanging off the side? Does the core keep it and land with it. Or is it seperated all together after staging?

Edited March 16, 2016 by Motokid600

[Rakaydos]

5 minutes ago, tater said:

Good thing they never needed that inefficient mission prof... oh, wait, Apollo 13.

Seems like the suggested FH profile is needlessly dangerous, though it looks to be substantially lower in dv requirement (almost half). I suppose as a private venture they could risk whatever they'd like.

Apollo 13 was already off free return (aiming for the landing site) when the accident occurred. they had to burn back onto a free return trajectory (and a faster one) as their first course correction, before shutting down the command module.

Mind, instead of having a LEM Lifeboat, you have the return capsule- there's no reduldant life support if an accident on the scale of Apollo 13 happened.

[tater]

Yeah, so D2 lunar has no redundancy at all. Apollo 13 did need to do a burn, but that accident could just as well have happened during a much longer phase of transit, where the return was indeed free. With no secondary or tertiary engines, most any failures would be fatal. NASA certainly wouldn't go for it, that's for sure.

[sevenperforce]

5 hours ago, Rakaydos said:

Apollo 13 was already off free return (aiming for the landing site) when the accident occurred. they had to burn back onto a free return trajectory (and a faster one) as their first course correction, before shutting down the command module.

Mind, instead of having a LEM Lifeboat, you have the return capsule- there's no reduldant life support if an accident on the scale of Apollo 13 happened.

 

5 hours ago, tater said:

Yeah, so D2 lunar has no redundancy at all. Apollo 13 did need to do a burn, but that accident could just as well have happened during a much longer phase of transit, where the return was indeed free. With no secondary or tertiary engines, most any failures would be fatal. NASA certainly wouldn't go for it, that's for sure.

Yeah, I wasn't thinking NASA per se. The whole concept would be more of a capability demonstration for SpaceX. If Elon really wants to put a colony on Mars, he needs to prove something dramatic. Like this. 

The guys on Apollo 13 were hella lucky. The LM was never planned for use as an emergency lifeboat; it was assumed that any failures significant enough to cripple the CM beyond repair would be immediately fatal. The explosion happened in what was basically the only place that would wreck the CM without explosively depressurizing it or ruining the other two modules. Damage to the re-entry capsule or to the LM would have also been fatal; there would have been no possibility of a rescue mission. 

The biggest concern is delta-V. And with the exception of the crasher stage burn, my mission profile has abort options with dV and secondary engines to spare at every point. 

[fredinno]

37 minutes ago, sevenperforce said:

 

Yeah, I wasn't thinking NASA per se. The whole concept would be more of a capability demonstration for SpaceX. If Elon really wants to put a colony on Mars, he needs to prove something dramatic. Like this. 

The guys on Apollo 13 were hella lucky. The LM was never planned for use as an emergency lifeboat; it was assumed that any failures significant enough to cripple the CM beyond repair would be immediately fatal. The explosion happened in what was basically the only place that would wreck the CM without explosively depressurizing it or ruining the other two modules. Damage to the re-entry capsule or to the LM would have also been fatal; there would have been no possibility of a rescue mission. 

The biggest concern is delta-V. And with the exception of the crasher stage burn, my mission profile has abort options with dV and secondary engines to spare at every point. 

If he wants to put a colony on Mars and needs something dramatic, there is a large list of either less radical missions or missions that are also applicable to Mars missions. For example, Mars Flybys, Moon Flybys, Lunar Orbital missions, Venus Flybys, Venus Orbit, and Mars Orbit (possibly Mars moons if Elon is willing). Those would bring huge amounts of prestige, in the case of the lunar missions, are possible with one FH (two for the planetary flybys, probably MCT or 4 falcon Heavy for the planetary Orbital missions). Lunar Orbital and flybys need minimal modifications in comparison to lunar landings, and the planetary flyby and orbit missions are going to need a HAB built (essential for any Mars Missions).

 

Lunar landings are a dead end if you want to build a colony on Mars.

6 hours ago, tater said:

Good thing they never needed that inefficient mission prof... oh, wait, Apollo 13.

Seems like the suggested FH profile is needlessly dangerous, though it looks to be substantially lower in dv requirement (almost half). I suppose as a private venture they could risk whatever they'd like.

Still, dead astronauts would destroy faith in their Mars program and make SpaceX lose all the prestige gained in the last few years faster than Challenger killed Shuttle-Centaur.
 

6 hours ago, Motokid600 said:

I think I'd rather see the F9H fly first before speculating future missions. Wonder why SpaceX is being so hush-hush about it. IIRC it was supposed to fly this year.

 

Question: I'm looking at a picture of the F9H. In its full reusability mode what happens to the bracing structure for the radial boosters? Do they take it with it upon separation then land with this brace hanging off the side? Does the core keep it and land with it. Or is it seperated all together after staging?

The struts retract back onto the core apparently.

[fredinno]

7 hours ago, sevenperforce said:

FH has crappy BLEO performance, I know. But that's all the more impressive if they can pull off a manned lunar landing. Funding would be needed, sure, but we're talking less than a billion total. Hell, Elon might go himself...if not for the landing, for a preliminary free-return loop. 

Who would run SpaceX without Elon?

6 hours ago, sevenperforce said:

The Saturn V was far more efficient than the Falcon Heavy, based on ISP. It used liquid hydrogen upper stages. 

But the mission profile was not very efficient. 

But it had contingency by going to Lunar Orbit first in case the landing site turned out to be not-so-great after all, could use the orbiter for extra science, and had more capability overall (3 people to Lunar Orbit instead of 2).

Also, the Saturn V was more primitive due to being a 60s rocket.

And any direct ascent profile is automatically less efficient than a comparable LOR mission- that's why all serious mission proposals used LOR- it just offered that much more performance. The Dragon V2 is very mass-efficient due to its interior basically being empty aside from seats and a small control center.

[DerekL1963]

11 hours ago, fredinno said:

Who would run SpaceX without Elon?

What makes you think he's running it now?

[sevenperforce]

11 hours ago, fredinno said:

If he wants to put a colony on Mars and needs something dramatic, there is a large list of either less radical missions or missions that are also applicable to Mars missions. For example, Mars Flybys, Moon Flybys, Lunar Orbital missions, Venus Flybys, Venus Orbit, and Mars Orbit (possibly Mars moons if Elon is willing). Those would bring huge amounts of prestige, in the case of the lunar missions, are possible with one FH (two for the planetary flybys, probably MCT or 4 falcon Heavy for the planetary Orbital missions). Lunar Orbital and flybys need minimal modifications in comparison to lunar landings, and the planetary flyby and orbit missions are going to need a HAB built (essential for any Mars Missions).

Lunar orbit and lunar flybys would be a good step, yes, but Dragon V2 is a landing vehicle, expressly intended to have capabilities for landing on Earth and Mars as well as airless worlds. And a manned landing demonstration on the moon, realizeable with existing platforms, is far cheaper than a manned landing demonstration anywhere else.

I mean, delivering an unmanned payload to Mars would be a good idea too. But a manned landing isn't going to happen anywhere other than the moon, not without decades of development.

7 minutes ago, DerekL1963 said:

11 hours ago, fredinno said:

Who would run SpaceX without Elon?

What makes you think he's running it now?

Yeah, I was going to say "Gwenne Shotwell, just like now".

11 hours ago, fredinno said:

Quote

The Saturn V was far more efficient than the Falcon Heavy, based on ISP. It used liquid hydrogen upper stages. 

But the mission profile was not very efficient.

But it had contingency by going to Lunar Orbit first in case the landing site turned out to be not-so-great after all, could use the orbiter for extra science, and had more capability overall (3 people to Lunar Orbit instead of 2).

And any direct ascent profile is automatically less efficient than a comparable LOR mission- that's why all serious mission proposals used LOR- it just offered that much more performance. The Dragon V2 is very mass-efficient due to its interior basically being empty aside from seats and a small control center.

My profile still has contingency; as long as the Falcon upper stage engine ignites successfully after passing EML-1, you have more than enough dV for abort to orbit/flyby and return at any time. Plus, landing sites aren't as big of a deal now, since we have far better lunar surface mapping than we did during Apollo. Abort to flyby or orbit still has the same "extra science" capability that the CM had (they were mass-restricted too, after all). And the only reason Apollo had three people in lunar orbit was because they needed a warm body flying the CM for the LOR and return; crew payload to the surface would be the same.

If LOR is automatically more efficient than direct ascent, then show me a mission profile which can successfully pull off a two-man landing in a Dragon V2 using an expendable Falcon Heavy FT as your launch vehicle. By my calculations, expandable FH FT can deliver a payload of 19.6 tonnes to LLO, not including the 3.9 tonne Falcon upper stage, so that's your mass budget (and remember, you need 1.31 km/s of dV to get back to an aerobraking trajectory).

[Nibb31]

12 minutes ago, sevenperforce said:

Lunar orbit and lunar flybys would be a good step, yes, but Dragon V2 is a landing vehicle, expressly intended to have capabilities for landing on Earth and Mars as well as airless worlds. And a manned landing demonstration on the moon, realizeable with existing platforms, is far cheaper than a manned landing demonstration anywhere else.

No. It's designed for Earth reentry and landing. It is not designed for landing on other planets. Landing anywhere else than Earth would require significant modifications which would turn it into something else than what it currently is.

[fredinno]

1 hour ago, sevenperforce said:

Lunar orbit and lunar flybys would be a good step, yes, but Dragon V2 is a landing vehicle, expressly intended to have capabilities for landing on Earth and Mars as well as airless worlds. And a manned landing demonstration on the moon, realizeable with existing platforms, is far cheaper than a manned landing demonstration anywhere else.

I mean, delivering an unmanned payload to Mars would be a good idea too. But a manned landing isn't going to happen anywhere other than the moon, not without decades of development.

Yeah, I was going to say "Gwenne Shotwell, just like now".

My profile still has contingency; as long as the Falcon upper stage engine ignites successfully after passing EML-1, you have more than enough dV for abort to orbit/flyby and return at any time. Plus, landing sites aren't as big of a deal now, since we have far better lunar surface mapping than we did during Apollo. Abort to flyby or orbit still has the same "extra science" capability that the CM had (they were mass-restricted too, after all). And the only reason Apollo had three people in lunar orbit was because they needed a warm body flying the CM for the LOR and return; crew payload to the surface would be the same.

If LOR is automatically more efficient than direct ascent, then show me a mission profile which can successfully pull off a two-man landing in a Dragon V2 using an expendable Falcon Heavy FT as your launch vehicle. By my calculations, expandable FH FT can deliver a payload of 19.6 tonnes to LLO, not including the 3.9 tonne Falcon upper stage, so that's your mass budget (and remember, you need 1.31 km/s of dV to get back to an aerobraking trajectory).

You can do landing tests on Earth- Mars has an atmosphere, so it's more reminisent of that.

Dragon V2 alsodoes not have the delta v unless you use drop tanks-  https://www.reddit.com/r/spacex/comments/33f75a/just_how_much_deltav_will_the_dragon_2_capsule/

shows the delta V of the capsule is ~400 m/s. Even with nozzle extensions,that's too low for a direct ascent mission anyways.

[DerekL1963]

2 hours ago, sevenperforce said:

2 hours ago, DerekL1963 said:

14 hours ago, fredinno said:

Who would run SpaceX without Elon?

What makes you think he's running it now?

Yeah, I was going to say "Gwenne Shotwell, just like now".

That's what my sources tell me - though Elon Musk is the money man, and the idea man, and the very public face of SpaceX...  It's Gwynne Shotwell that's actually doing virtually all the heavy lifting.

[sevenperforce]

1 hour ago, Nibb31 said:

No. It's designed for Earth reentry and landing. It is not designed for landing on other planets. Landing anywhere else than Earth would require significant modifications which would turn it into something else than what it currently is.

I know that Elon has a tendency to make exaggerated or overly-optimistic statements, but I don't see anything particularly exaggerated about his claim that Dragon V2 on Falcon Heavy can deliver a 2-4 tonne payload to Mars, ostensibly without modification. Terminal velocity would be a few times higher, I suppose, but this could be decreased by using chutes in combination with propulsive landing.

1 hour ago, fredinno said:

You can do landing tests on Earth- Mars has an atmosphere, so it's more reminisent of that.

Dragon V2 alsodoes not have the delta v unless you use drop tanks-  https://www.reddit.com/r/spacex/comments/33f75a/just_how_much_deltav_will_the_dragon_2_capsule/

shows the delta V of the capsule is ~400 m/s. Even with nozzle extensions,that's too low for a direct ascent mission anyways.

Landing on Earth will be part of operations, yes. 

You would absolutely need an internal auxiliary tank; no question about that. But I outlined how much dV you would have. With a crasher stage, direct ascent to Earth return has lower dV requirements than LOR.

[fredinno]

1 hour ago, sevenperforce said:

I know that Elon has a tendency to make exaggerated or overly-optimistic statements, but I don't see anything particularly exaggerated about his claim that Dragon V2 on Falcon Heavy can deliver a 2-4 tonne payload to Mars, ostensibly without modification. Terminal velocity would be a few times higher, I suppose, but this could be decreased by using chutes in combination with propulsive landing.

Landing on Earth will be part of operations, yes. 

You would absolutely need an internal auxiliary tank; no question about that. But I outlined how much dV you would have. With a crasher stage, direct ascent to Earth return has lower dV requirements than LOR.

You can still do a orbital insertion with the Falcon 2nd stage. And using a drop tank (internal tanks would fill up all the room, and leave none for supplies or humans) +fuel cells would make a Dragon V2 landing on the moon be about as modified as Shuttle>SLS. Prepare for a long development cycle.

 

You just don't seem to realise the problem: the "modifications" you propose are NOT minimal, and would cost a lot of money for something Elon is not even interested in. Real life is not KSP, remember that.

[tater]

Musk has a little of Jobs reality distortion field to play with, I think.

[sevenperforce]

20 hours ago, fredinno said:

You can still do a orbital insertion with the Falcon 2nd stage.

A LOR mission profile is challenging.

The primary issue is that you have to bring along two Dragons rather than just one, which really wrecks your mass fraction. The other thing is that you can't use the Falcon upper stage for your return trip. The crew has to be in the Earth re-entry vehicle for launch due to abort safety requirements, so you have to stack the modified Lunar Dragon on top of the Falcon upper stage, with the unmodified (or loosely modified) Dragon V2 on top of it:

Because they have to be in the unmodified Dragon V2 (1) for launch, but then break away and dock nose-to-nose with the Lunar Dragon (4), which then must also break away for descent, the Falcon upper stage (5) can't be used for the Earth return transfer. So you need drop tanks (2) on the unmodified Dragon V2 inside its service module trunk (3) so it can serve as the Earth return transfer stage. 

As you pointed out, though, the Dragon V2 is very mass-efficient. So I don't know how much mass can be gained by removing the parachute and heat shields from the Lunar Dragon and replacing them with a larger tank. The underside would need to be completely rebuilt...probably using the same landing legs, but with a larger fuel tank in place of the heat shield and a stripped-down aeroshell. It needs its own power supply as well, and it needs an inflatable airlock in place of the egress hatch.

Recall that the dV needed for ascent-to-transfer is more than the dV needed for descent-to-ascent.

I don't think it's possible to expand the fuel tank capacity enough to have both ascent and descent. So you'd still need to use the Falcon upper stage as a crasher stage. If we suppose that the Lunar Dragon can have all those modifications made without increasing dry mass beyond 4,200 kg, and we estimate a crewed payload of 1,103 kg as in my original estimate, then it is going to need 2.07 km/s (allowing 100 m/s of descent plus a little over two minutes of hover time), which corresponds to 4,640 kg of fuel and a total mass of 9,943 kg. To deliver this from LLO to just above the lunar surface, the Falcon upper stage will need roughly 1.87 km/s, or 10 tonnes of fuel.

But if the return Dragon V2 has a dry mass of 4,200 kg, a loaded mass of 5,303 kg, and needs 1.31 km/s of dV to return, it will need 2600 kg of fuel. Even if we ignore the mass of the drop tanks and consumables, that's a total launch mass of 6,800 kg which, combined with the 9,943 kg mass of the Lunar Dragon, means the Falcon upper stage only has 2.87 tonnes of fuel remaining after LLO injection.

21 hours ago, fredinno said:

And using a drop tank (internal tanks would fill up all the room, and leave none for supplies or humans) +fuel cells would make a Dragon V2 landing on the moon be about as modified as Shuttle>SLS. Prepare for a long development cycle.

You just don't seem to realise the problem: the "modifications" you propose are NOT minimal, and would cost a lot of money for something Elon is not even interested in. Real life is not KSP, remember that.

Far more development on the LOR approach. Internal tanks won't fill up the room; I explained how much available space there is in the very first post.

As it turns out I hadn't ever played KSP until after I started this thread.

[tater]

Just put a Clamp-o-tron Sr. on the top of 5, and the bottom of 1 and you're good to go  (as long as we are playing legos with real rockets, lol)

If you were concerned about safety enough to try and bother with the complicated stack you show, you could just do an EOR/LOR hybrid. 2 launches, the 2d docks the lander to to the transfer/return vehicle. FH is 1 and 5, 2d launch (F9) is the 4 (lander with a crasher stage as the trunk?).

This whole exercise is sort of backwards, and reminds me of trying to fit missions to SLS/Orion. You don't build a vehicle, then figure out how to make that work, you pick a mission, then build the best vehicle to do the mission. If you have 40 tons, and your mission would be easy with 50 tons, but incredibly hard with 40, why would you limit yourself to 40?

[DerekL1963]

1 hour ago, sevenperforce said:

As it turns out I hadn't ever played KSP until after I started this thread.

Playing KSP isn't required to not grasp the principles of engineering or to have other bad intellectual habits.   It just reinforces them.

[sevenperforce]

27 minutes ago, tater said:

If you were concerned about safety enough to try and bother with the complicated stack you show, you could just do an EOR/LOR hybrid. 2 launches, the 2d docks the lander to to the transfer/return vehicle. FH is 1 and 5, 2d launch (F9) is the 4 (lander with a crasher stage as the trunk?).

Yeah, if you do EOR and LOR then you have dV to spare.

The "safest" approach is to do a double series LOR. Send the Lunar Dragon unmanned on a Falcon Heavy to LLO, then do a second launch with a manned, unmodified Dragon V2 to LLO only after the Lunar Dragon has been safely circularized. The second launch performs a rendezvous with the first launch and the crew transfers to verify systems. The first Falcon upper stage has more than enough dV to act as a crasher stage, setting the Lunar Dragon down gently. They conduct their mission, then ascend and conduct a second rendezvous to the unmodified Dragon V2 and they transfer, along with samples and so forth. The second Falcon upper stage will have more than enough dV for the transfer back. In fact, they might even be able to bring the Lunar Dragon back for reuse, or at least for ISS docking and investigation.

Definitely a much more flexible mission profile. If the Lunar Dragon's systems won't come online, they can just make it an orbit-and-return instead. The unpleasant bit is being unable to do it all in one go. But the double LOR is much better than an EOR to LOR.

37 minutes ago, tater said:

This whole exercise is sort of backwards, and reminds me of trying to fit missions to SLS/Orion. You don't build a vehicle, then figure out how to make that work, you pick a mission, then build the best vehicle to do the mission. If you have 40 tons, and your mission would be easy with 50 tons, but incredibly hard with 40, why would you limit yourself to 40?

The goal would be for SpaceX to show that their platforms are capable enough to be used for a wide range of mission profiles without requiring ground-up designs.

[fredinno]

2 hours ago, sevenperforce said:

A LOR mission profile is challenging.

The primary issue is that you have to bring along two Dragons rather than just one, which really wrecks your mass fraction. The other thing is that you can't use the Falcon upper stage for your return trip. The crew has to be in the Earth re-entry vehicle for launch due to abort safety requirements, so you have to stack the modified Lunar Dragon on top of the Falcon upper stage, with the unmodified (or loosely modified) Dragon V2 on top of it:

Because they have to be in the unmodified Dragon V2 (1) for launch, but then break away and dock nose-to-nose with the Lunar Dragon (4), which then must also break away for descent, the Falcon upper stage (5) can't be used for the Earth return transfer. So you need drop tanks (2) on the unmodified Dragon V2 inside its service module trunk (3) so it can serve as the Earth return transfer stage. 

As you pointed out, though, the Dragon V2 is very mass-efficient. So I don't know how much mass can be gained by removing the parachute and heat shields from the Lunar Dragon and replacing them with a larger tank. The underside would need to be completely rebuilt...probably using the same landing legs, but with a larger fuel tank in place of the heat shield and a stripped-down aeroshell. It needs its own power supply as well, and it needs an inflatable airlock in place of the egress hatch.

Recall that the dV needed for ascent-to-transfer is more than the dV needed for descent-to-ascent.

I don't think it's possible to expand the fuel tank capacity enough to have both ascent and descent. So you'd still need to use the Falcon upper stage as a crasher stage. If we suppose that the Lunar Dragon can have all those modifications made without increasing dry mass beyond 4,200 kg, and we estimate a crewed payload of 1,103 kg as in my original estimate, then it is going to need 2.07 km/s (allowing 100 m/s of descent plus a little over two minutes of hover time), which corresponds to 4,640 kg of fuel and a total mass of 9,943 kg. To deliver this from LLO to just above the lunar surface, the Falcon upper stage will need roughly 1.87 km/s, or 10 tonnes of fuel.

But if the return Dragon V2 has a dry mass of 4,200 kg, a loaded mass of 5,303 kg, and needs 1.31 km/s of dV to return, it will need 2600 kg of fuel. Even if we ignore the mass of the drop tanks and consumables, that's a total launch mass of 6,800 kg which, combined with the 9,943 kg mass of the Lunar Dragon, means the Falcon upper stage only has 2.87 tonnes of fuel remaining after LLO injection.

Far more development on the LOR approach. Internal tanks won't fill up the room; I explained how much available space there is in the very first post.

As it turns out I hadn't ever played KSP until after I started this thread.

The point of LOR is to use a smaller, less massive lander carryingthe bare minimum it needs, and keepingeverything else in orbit. Thus, using LOR with solely Dragon V2/3s is contradictory.

 

It doesn't change the fact that you'd need to make at least one almost entirely new Dragon Vehicle Version to make this a reality. It's like saying the Apollo Telescope Module was a 'minor' monification of the LEM descent stage, Skylab a 'minor' modifcation of the S-IVB, or Merlin 1D A 'minor' modification of Merlin 1A.

It's not, and it's kind of pointless to make these unless you want to do this in KSP, or SpaceX suddenly wants to do a moon mission.

[tater]

Yeah, sending anything to the Moon (or any other planet) that is not purpose-built is sort of kooky.

I think the initial conditions of many m³ of additional propellant are pretty dubious, as the vehicle would be so redesigned that it would make infinitely more sense to just design a vehicle from scratch. I could probably retrofit my Land Rover to become a small aircraft, but it would be more complicated than designing a plane from scratch (and the resultant vehicle would be better in all ways, than my "variant").

[Rakaydos]

If the plan is double falcon heavy, double LOR, do you need extra propellant?

With a crasher stage, the Dragon 2 can return to lunar orbit with existing propelant. rendevous with a second Dragon2 with booster, fly both capsules back to earth.

[fredinno]

12 minutes ago, Rakaydos said:

If the plan is double falcon heavy, double LOR, do you need extra propellant?

With a crasher stage, the Dragon 2 can return to lunar orbit with existing propelant. rendevous with a second Dragon2 with booster, fly both capsules back to earth.

Yes, it needs extra propellant.

[sevenperforce]

1 hour ago, fredinno said:

The point of LOR is to use a smaller, less massive lander carryingthe bare minimum it needs, and keepingeverything else in orbit. Thus, using LOR with solely Dragon V2/3s is contradictory.

It doesn't change the fact that you'd need to make at least one almost entirely new Dragon Vehicle Version to make this a reality. It's like saying the Apollo Telescope Module was a 'minor' monification of the LEM descent stage, Skylab a 'minor' modifcation of the S-IVB, or Merlin 1D A 'minor' modification of Merlin 1A.

It's not, and it's kind of pointless to make these unless you want to do this in KSP, or SpaceX suddenly wants to do a moon mission.

The LM massed exactly the same as a standard Dragon V2. 

And Elon has expressed specific intentions for retrofitting the V2 platform for other worlds, so why wouldn't they be interested?

39 minutes ago, tater said:

I think the initial conditions of many m³ of additional propellant are pretty dubious, as the vehicle would be so redesigned that it would make infinitely more sense to just design a vehicle from scratch. I could probably retrofit my Land Rover to become a small aircraft, but it would be more complicated than designing a plane from scratch (and the resultant vehicle would be better in all ways, than my "variant").

The Dragon V2 has more than twice the available internal pressurized volume than the lunar module ascent stage crew cabin had. That's why I proposed adding an internal tank if the crew is reduced to only two. Of course, I admit that this is the trickiest part.

23 minutes ago, Rakaydos said:

If the plan is double falcon heavy, double LOR, do you need extra propellant?

With a crasher stage, the Dragon 2 can return to lunar orbit with existing propelant. rendevous with a second Dragon2 with booster, fly both capsules back to earth.

The descent module needs 1.87 km/s minimum...closer to 2.3 km/s for safety's sake. The stock Dragon V2 with nozzle extensions would only have 941 m/s of delta v. So you need to add extra propellant somewhere. But yes, with a double Falcon Heavy launch, the second upper stage ought to have enough remaining fuel to return both capsules to LEO. I'd have to run the math to be sure. 

[fredinno]

19 minutes ago, sevenperforce said:

The LM massed exactly the same as a standard Dragon V2. 

And Elon has expressed specific intentions for retrofitting the V2 platform for other worlds, so why wouldn't they be interested?

The Dragon V2 has more than twice the available internal pressurized volume than the lunar module ascent stage crew cabin had. That's why I proposed adding an internal tank if the crew is reduced to only two. Of course, I admit that this is the trickiest part.

The descent module needs 1.87 km/s minimum...closer to 2.3 km/s for safety's sake. The stock Dragon V2 with nozzle extensions would only have 941 m/s of delta v. So you need to add extra propellant somewhere. But yes, with a double Falcon Heavy launch, the second upper stage ought to have enough remaining fuel to return both capsules to LEO. I'd have to run the math to be sure. 

The V2 lacks the Delta V to get to Lunar landing and orbit, unlike the LM.

And you have no clue how much space tanks take up, do you... Also, wiring up the tank will be a pain.