SpaceX is already docking with the space station which is similar difficulty as launching from the moon and docking in lunar orbit. The problem is getting a rocket with enough fuel on the surface of the moon without damaging it, not the docking in orbit. Which is even easier than doing the same thing from the surface of Mars. The only advantage of Mars is you don't need to burn fuel slowing down but that's more than offset by a significantly deeper gravity well.

On the upside, they can land an un-manned assent vehicle on the surface of Moon / Mars before sending people there. Which can significantly increase your odds of getting back.

PS: Yes, NASA did get to the moon and back several times, but they also took a massive chunk of US GDP to do so. https://en.wikipedia.org/wiki/Budget_of_NASA#/media/File:NAS...

Actually, they are Berthing[1][2] the Dragon craft with the ISS, which means something slightly different - grabbed by the thing you are connecting to and pulled in, vs. a (controlled) bumping into the thing. I believe that Nautical terminology is where that comes from, and means the same there e.g. berthing ships with ropes and dock hands.

1. https://en.wikipedia.org/wiki/Docking_and_berthing_of_spacec...

2. http://space.stackexchange.com/questions/13665/what-is-the-d...

But crewed Dragon V2 will actually dock.

Need to install the new docking module first (the one that was supposed to be installed blew up with CSR-7)

They already launched and mated the second International Docking Adapter already on CRS-9 back in July 2016 so they ISS side is ready.

I don't think either of us know much about this, but I do know that docking with the ISS to unload supplies requires completely different equipment than doing a rendezvous between a lunar lander and a lunar orbiter for Earth return. (That's why I didn't say merely "docking".)

The only craft SpaceX has that can land on Earth or the Moon is Dragon. I am pretty sure Dragon does not have enough delta-v to do a full propulsive landing on the moon (no aerobraking available) and then return to orbit. And they definitely don't have the tech to re-fuel in lunar orbit for the return to Earth. None of this equipment exists.

Not to mention that this would be a totally new lunar strategy, since the Apollo mission returned to earth in the lunar orbiter (Command Module) rather than re-fueling and returning in the lunar lander.

> I am pretty sure Dragon does not have enough delta-v to do a full propulsive landing on the moon.

The information I'm finding on the internet suggests Dragon 2 should run around 400 m/s of delta-V (it only needs on the order of 200 m/s to go from terminal velocity aerobraking to landed on Earth).

TLI to LLO is 800 m/s, and I'd guess that a Circumlunar return trajectory to LLO transfer would be on that order -- which is already out of the Dragon 2's range and you're only trying to get into orbit on the Moon, not return, and definitely not land or take off again.

Its close enough though that a beefed up Dragon 3 might be able to dock with some kind of space station and refueling dock in lunar orbit, then you just need to have a lunar ferry (Apollo sorta downsized all of this and crammed it all on top of a Saturn V for one-time use -- I'm thinking a bit bigger for the whole system and more cough reusable...)

Yea, I am not a rocket guy.

However, the minimum delta V to land on the moon is ~1.72 km/s, and you need another 1.72 km/s to get back into orbit + some safety factor ~2.2-2.4 km/s. By comparison it takes ~8km/s to get from LEO to low moon orbit and back to earth. Though you can reduce this by how hard to hit the earth's atmosphere. So, having fuel to land and take off from the moon is not that much worse than getting to the moon and back.

> By comparison it takes ~8km/s to get from Earth LEO to Moon LEO

No, from Earth LEO some 3.1 km/s will get you to TLO, and then some ~1 km/s will take you to Moon low orbit, so total about 4(+) km/s. 8 km/s is an approximate budget for a round-trip between low orbits.

Sorry, butchered that. I was trying to say the figures are less exact depending on how hard you want to hit the earths upper atmosphere. ~8.2km/s if you want to end up in earth LEO ~8km/s if you want to get back to the earth. You can also save a little if your sending an unmanned craft and don't care how long it takes or skip low moon orbit.

I wonder if spaceX is going to send a permanent colony of bots on moon to make a base there. The latency is a lot lower than Mars and would make a good start.

Sending a couple of humans around moon for tourism is a waste of resources.

Sending a couple of humans around the moon for tourism is a great R&D opportunity, paid for by someone else's money.

See: Apollo 8 mission.

They have paying customers. When you're in business to make money, you do what you can bill for.

Though I don't know why anyone would want to spend a week locked in a tiny can like that, let alone pay big bucks for it. There's a limit to the discomfort I'd be willing to endure for bragging rights.

They're said they have no particular interest in going to the Moon as it wouldn't teach them anything useful about going to or living on Mars and would just be a distraction. Of course if someone else wants to contract them to do so and they can make a buck off it, that might be different.

The Moon is a terrible "start" for Mars. They are very different places, and require solving very different problems. In many ways, Mars is a lot easier (the atmosphere can do a lot of the work of slowing you down, you have the raw materials to make methane (the rocket fuel SpaceX will be using), etc).

People tend to think of the Moon as a "staging point" on the way to Mars, but in reality, it would be a significant detour.

> Sending a couple of humans around moon for tourism is a waste of resources.

One persons waste of resources is another persons revenue. They're not doing it for charity.

So is it possible for them to launch multiple vehicles into Earth LEO, rendezvous with one another and then transfer to Moon LEO?

Dragon cargo berthes with ISS: it pulls close and gets snagged by a human operating an arm. Crew Dragon will dock with the ISS.

SpaceX for a landing mission wouldn't try and build a larger rocket, they'd definitely argue to just do multiple launches instead. Fuel tankering is within their area of interest for Mars.

I believe this (mating tanks given multiple missions) is what they were considering. Imagine you have your reusable falcon 9 boost into orbit a 'fuel only' second stage. Land the first stage, put another one on it, and send it up again. Once you have your 'boost pack' up in orbit, presumably docking with each other into a single unit, you launch your crew which attaches to the 'tug' that takes them to the moon.

ULA has a great writeup of how you might do this if you have on orbit refueling capabilities.

And on the one hand it seems like total science fiction but on the other, Elon is talking about next year.

This can be applied to much more than just fuel, mind you. Interplanetary manned vehicles might very well end up looking like space stations, with a bunch of modules docked together in order to provide the necessary creature comforts to sustain a crew for months on end.

In fact, a permanently-in-orbit transfer vehicle might make sense here; have this vehicle run the actual Earth<->Mars transfers, then just shuttle in fuel/supplies/crew on each Earth arrival.

Either way, a Falcon-like reusable launch system makes plenty of sense here in terms of making this sort of thing possible.

> a permanently-in-orbit transfer vehicle might make sense here

Buzz Aldrin is a big proponent of this approach: https://buzzaldrin.com/space-vision/rocket_science/aldrin-ma...

Jet fighters routinely use so much fuel taking off that the first thing they do after gaining altitude is refuel. Rockets can use the same idea.

Yes but as the ULA whitepaper pointed out there are some challenges to doing so in orbit. For one, the lack of gravity makes it hard to pump fuel. You can spin the tanker to achieve fuel settling but now you are both spinning bodies. Then there is handling cryogenic propellants in space, you get nominal bleed off from warming and again ULA had a really interesting design of an internal combustion engine using the bleed off of Hydrogen and Oxygen as a chiller pump. But suffice it to say, its not as easy as it is flying a jet behind a slow re-purposed jetliner, and doing that is already difficult.

> the lack of gravity makes it hard to pump fuel

Evidently this problem is solved, because liquid fuel rockets have been working fine in space since the 60s. (Liquid fuel rockets have their propellants pumped into the chamber.)

> nominal bleed off from warming

Launch the tanker just beforehand. There won't be time for the fuel to bleed off.

Liquid fuel rockets use Surface tension against the slosh baffles to hold fuel in partially full fuel tanks in place.

Ullage motors are used to settle the fuel at the bottom of the tanks, so the main motor pumps can get to it.

Once the main motor starts providing acceleration, the ullage motors are no longer required.

For cross tanking, keeping the ullage motors running for long enough to transfer all the fuel, without the main motors running might be prohibitive.

https://en.wikipedia.org/wiki/Ullage_motor

I didn't know about ullage motors, thanks for the link. The link also says that only very tiny accelerations are needed - so this shouldn't be a big problem for a tanker. Like I hypothesized, the problem has been solved.

Bearing in mind that the ullage motors need to be of a type that won't suffer from ullage problems. That implies they cant use the main motor fuel.

The saturn 5 used small solid rockets as ullage motors. Alternatively you could use externally pressurised bladder tanks for a range of non-cryogenic fuels. Cold gas or possibly h2o2 as a monopropellant come to mind. Either option has a comparatively low specific impulse, so even running them at very low overall accelerations for extended time periods is likely to be cost-prohibitive.

I like the suggestion elsewhere of spinning the tank. You could also dock, then spin the whole combination ship. If you abandon the fuel station idea, you could just take the extra fuel tank with you, transferring fuel to your internal tank during a main engine burn, then abandoning the empty tank in a wierd orbit.

it's only been solved if it's demonstrated, which it wasn't.

Do the fuels have to remain in a cryogenic state in space? Can much larger containers with warmer fuel be used once the fuel is in orbit? I have no idea if this makes any sense.

A major problem is pressure. As the fuel heats up and boils off the pressure increases and the tanks have to bleed it off or they'll burst eventually. Also the engine design for cryogenic stages assume a liquid fuel and moving enough gaseous fuel and oxidizer isn't in the current designs.

That's a good question. A big bag of fuel hanging outside is a fine place to store it if you don't care about temperature or pressure. You would need some kind of mechanical means to squish the bag to get the fuel out though, so maybe an accordion with a motorized retractor?

I think you are underestimating expansion ratios (ratio of volume of liquid and gas at reasonable pressure). The ratio for water vapour at sea level is ~1000x. The "large bag" might end up being so large that it's weight is a significant fraction of the weight of the fuel it contains.

Even if you brought up a bag large enough to contain gaseous fuel and oxydizer, before you could actuall use the fuel you'd have to compress it again.

This is partly because they don't always take off with full tanks which allows for shorter runways and/or safer takeoff. Depending on the ordinance that a jet may have to carry, it starts making more sense to stay a bit lighter until a minimum airspeed is achieved.

The idea of docking modular components in space is a good one though. Perhaps all you really need to do is lift a bunch of fuel tanks since there is plenty of time to perform burns with smaller boosters on longer distance trips.

The SR71 literally leaked fuel until it reached operating temperature. It had something crazy like 2 minutes to rendevous with a tanker after takeoff.

Using "milch cows" seems like a cost effective solution to fueling/supplying a moon run. Of course, the green-eyeshade crews will get to have the last say.

It's not as advantageous as you'd expect, because your ships now have to have docking collars extra sensors, and extra maneuvering capability. The rendezvous will take extra time and fuel, and the extra time means you need more life support for the crew.

Worse, the extra steps involved create extra risk, both for mission failure and to the lives of the crew. You could get to your fuel depot and discover the battery sytem has failed and your depot is tumbling, or a leak caused ice buildup that prevents you from docking. Or the chillers failed and enough oxydizer got vented into space that you can't continue the mission.

> Right. SpaceX doesn't have a craft capable of lunar orbit rendezvous

Why not? Their Dragon capsules can dock with the ISS; why wouldn't they be able to dock with each, possibly with some kind of adapter? You can imagine one Dragon and a trunk docking with another upper stage that's all fuel tank, heading to the Moon, undocking in orbit, landing, ascending, and rendezvousing in orbit to return home.

The total dv requirements of landing on the Moon and ascending again starting from low Moon orbit is 4 km/s. I haven't done the math, but I suspect that you can put together a workable mission using two Falcon Heavy launches. Now, granted, they don't have all the exact pieces put together quite yet, but I suspect they've been thinking about it.

A lot of the devil here is in the details. You need long term life support that Dragon hasn't proven yet. Deep space communications on the transmit and receive side. Abort scenarios for most or all phases of flight. All of this is untested and requires a huge amount of engineering and manufacturing work. Since this will likely share the same design as the NASA-contracted capsules, these systems will need to pass muster with them. And they don't cut corners.

Minor quibble: you don't need deep space communications. The Moon is laughably close. I am quite confident that we can easily make contact with the existing unmodified Dragon capsules by using something like one of these bad boys: https://www.nasa.gov/sites/default/files/goldstone.jpg

Of course it's laughably overkill for the purpose -- amateur radio operators using much smaller equipment can already send and receive signals to each other by bouncing them off the Moon, which has four times the signal attenuation owing to the doubled distance plus a good deal more in losses from the poor reflectivity of the Moon to radio waves.

That's not to say that LOR is impossible with a smaller payload. Saturn V enabled a large, complex, redundant set of vehicles. The never successfully launched Soviet N1/L3 with half the TLI payload also used LOR but with much lighter vehicles and a smaller crew.

They'll be able to do LOR in the Dragon 2, which would be ready in time. I question whether they have enough endurance for a lunar landing mission, though, and they would still have to develop a lander.