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

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

so they would need to return to the Earth directly from the surface of the moon

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

Direct ascent requires a much larger initial launch mass, i.e., a larger rocket than has ever been built. Since it took the massive Saturn V to complete the Apollo missions using lunar rendezvous, and SpaceX's Falcon Heavy is smaller, I don't think they are even close to having the ability to land on the moon and return.

(And that's assuming there's not other insurmountable factors preventing Dragon from landing on the moon and taking off, which there probably are.)

Hopefully someone with more knowledge can chime in.

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.

Between flying to the moon and back (no landing) the only difference is fuel and consumables (air/food). The key capabilities are launch, landing, spacecraft communication and operations, and navigational support for computing and executing maneuvers to get into (and out of) a trans-lunar orbit. The more fuel you have the easier it is on the equipment.

Or to put it differently, the difference is that when you get to the Moon you make an orbital injection burn in order to slow yourself down to the point that you can stay in lunar orbit, and then when your done you make another burn to put you back into the trans-lunar orbit.

When you get back you can either do a high speed re-entry or you can do an Earth orbit injection burn.

You still need a vehicle that can carry people for the duration of the trip. You need to get it into a translunar orbit so that you can get out to the Moon. You need to be able to precisely locate yourself in space relative to the Earth-Moon system and course correct to achieve your orbit goals. All of that stuff has to happen either way.

There is an excellent discussion of the various orbit options for getting to the moon in this paper: http://www.esa.int/esapub/bulletin/bullet103/biesbroek103.pd... definitely worth looking over to get a better appreciation on what is involved.

It's not a greater achievement than the Apollo moon landings (it's basically the mission flown on Apollo 13, swinging around the moon without going into orbit and then coming back), but it's still something that hasn't been done since the Apollo 17 mission in 1972.

Hopefully more like Apollo 8 than 13!

I understand that SpaceX is not proposing to orbit the moon. Therefore it is more like Apollo 13. Apollo 8 orbited the moon.

Good point cswilly, I didn't think of that part! And yeah, I meant let's try and not explode stuff too much...

Somehow I think the details of flyby vs orbit were not the intended subject I suspect he wants a generally smooth mission like what Apollo 8 and to avoid any unplanned rapid disassembly like what Apollo 13 had.

Hurray for progress! We're able to repeat what was done in 1972!

Except today it will be done at less than 1/10th the cost. So yeah, progress.

If it will be done. Musk isn't great at the "deliver" part, not to mention how dangerous this mission is.

Musk sends partially re-usable rockets into space for a few years now. A decade ago, all he had were some fancy powerpoint slides.

We sure have different definitions of "delivering".

We're still not 100% that they're reusable in a meaningful way that actually reduces costs. The landings are awesome but if it costs too much to reuse them it's more a stunt than anything because as good as it is to not just chunk that hunk of metal in the ocean every launch if it's cheaper that's how it'll be done.

It's a lot easier to go from a stage that lands to one that has to be ditched than the opposite. Worst case, Musk has the same tech that the rest of the world does, although to me it's clear he has quite an advantage.

Oh yeah and personally I think that it's going to work but it hasn't been actually shown that it's cheaper and that the rockets can safely be reused. And that really won't be well shown until multiple reused rockets have been launched.

An interesting outcome is that SpaceX has been able to analyze the rockets post-landing so that they can learn more about the stresses on the rocket during launch. Of course they have to in order to re-launch the rockets, but this is already a capability that no other space agency has had. This ability could end up paying for itself if it means they can feed the knowledge of the rocket back into the design and launch of a new rocket.

Actually, we're pretty sure (based on what SpaceX has announced) that the engines can be reused, and they're a reasonable % of the booster cost.

He's pretty good at the "deliver" part, where he has trouble is "on time."

Space is dangerous. That comes with the territory. I would be very surprised if this ended up being more dangerous than Apollo 8 just given the advances in materials and electronics.

Hurray for reproducability - we can still do what was last done in 1972 - this is something which eludes most scientists!

Look at F1 in 1972 and F1 in 2017 for an example of what relentless, continuous improvement looks like. Can you imagine what would be like if they'd given up on the series? If all that we had was NASCAR and Indy? The entire automotive industry would be over a decade behind, if not more. Most thigns people take for granted like ABS and traction control might not even exist.

Look at SpaceX as an example of what happens when someone fumbles the ball so hard it takes over four decades to recover it. Virtually nothing happened in that time other than the shuttle program, and that was never intended for travel outside of Earth orbit.

Now we're finally back to innovating and engineering for actual space travel.

We did send plenty of robot missions that wouldn't have been possible in the seventies and produced very valuable science. Rosetta and Curiosity for example are very cool missions.

Consider these very cool missions are about 1/10th of the number of missions America was in a position to do.

Consider an America with the Texas Collider operational where major discoveries were made a decade before the CERN one even existed.

Consider an America where the ARPA internet project could have been one of the least notable breakthroughs in the last thirty years.

It's actually a good analogy. The F1 Cars (equals Apollo CSM vs. Crew Dragon 2) have much improved since the 70s, but the Tracks haven't changed and Silverstone is still Silverstone (and the Moon is still where we expect it should be) so the mission parameters are much the same too; you go round and round seventy-odd times (or, once) and come home.

For a fraction of the cost.

Being the first to land on the moon was never intended to be cheap. So many things had to be invented simply to get there.

Just imagine how economical space flight would be if all of that 1960s technology had been incrementally improved on for efficiency, manufacturing ease, and scale instead of a lot of it being almost literallly mothballed.

There was never any reason to spend that kind of money. It was worth going to the moon once to see what was there, but I have yet to see anybody articulate a reason to go back that doesn't rely on soaring rhetoric and implausible money-making schemes.

Rare-earth mineral security?

Rare-earth minerals aren't actually rare. The name comes from the relatively low concentrations in which you find them naturally. But we won't run out; they're just more expensive to refine as we tap lower concentration sources. Even poor sources are going to yeild minerals at a cost that's a few orders of magnitude less than a what you'd get on the moon.

Rare-earth means low concentrations. There is still a lot of this stuff in the darts crust an it's not destroyed with use.

Yeah, why did they build computers so big and expensive and slow in the WWII? They could have waited for a Skylake Intel, and it would be not only better, but also cheaper...

Which is also the record for Farthest Humans From Earth. Any chance they can pass up the chance to break that record?

Yep, they plan to go as far as 400.000 miles from earth, that's quite a bit farther out than Apollo astronauts ever went (less than 300.000 miles).

Well, it says in the article:

"Once operational Crew Dragon missions are underway for NASA, SpaceX will launch the private mission on a journey to circumnavigate the moon and return to Earth."

I don't know why people are arguing over whether they are landing or not...

because no one read the article :)

Yes, that is exactly right. They are not going to land, for exactly the reasons you say.