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Per glindahl's comment from earlier, if we established a space elevator for the moon[1] it would make the delta-v issues moot. Drop off an pick up supplies at the elevator while swinging by on an continuous Earth/Moon translunar orbit.

[1] https://en.wikipedia.org/wiki/Lunar_space_elevator

There would still be some delta-v. Anyone have a handle on how much? I'm too lazy to look up the data and do the math.

First, I've neither done the requisite orbital mechanics calculations nor have I simulated this. But with that caveat, this is how I imagine it would work.

Assuming your tug is in the translunar orbit (going in a figure 8 around the Moon and Earth with the intersection being the Earth-Moon L1 point. And assuming the tug is significantly more massive than your cargo. As you cross the L1 point you can use a mass driver on the tug to 'shoot' the cargo out behind you which would decelerate it to a low delta-v relative to the elevator base (and slightly boost the tug on its way) and when you came around the other side you hook the cargo through a loop like the old mail planes did [1] and pulling it away (which would impart a small deceleration on the tug as it headed back to earth). Depending on masses and how closely you could match things, the amount of delta-v you would need for the tug to keep its orbit true should be very manageable.

[1] https://postalmuseum.si.edu/collections/object-spotlight/sky...

http://i.imgur.com/SqdzxzF.png

You might save 2km/s if you can use an elevator on the moon. Which is not a lot compared to a space elevator on Earth, but it would definitely be useful. That could be half your delta-v budget if you're going between the moon's surface and an orbit around Earth/Mars/Venus.

Translunar injection needs about 10.8 km/s, Earth's escape velocity is 11.2 km/s.

I mean to hook up with the elevator.

if we established a space elevator for the moon

Wouldn't you get the same benefit from a Mass Driver, since the moon is a vacuum body?

A mass driver doesn't let you land cargo. But as a way to get raw material off the Moon it is an excellent solution.

You could run the mass driver in reverse for landing, but you'd have to aim well.

That would be rather difficult :-) I wonder sometimes however if you've ever dropped a magnet down a conductive pipe you see how the eddy currents prevent it from falling through. If you had series of conductive rings that were of a large diameter and you had electromagnets on the outside of your cargo container could you make a landing pad that primarily used the eddy currents to slow you to landing velocity? That would be a fun problem to give the physics class.

That would be an amazing feat with the risk of destroying an extremely expensive piece of equipment.

Is it any more amazing than doing an orbital rendezvous?

You brake before getting close to whatever you want to rendevous with. You couldn't brake if you wanted to hit the nozzle of your mass driver to use it for braking.

You brake before getting close to whatever you want to rendevous with.

This makes me question whether you actually understand how orbital rendezvous works. How about as an exercise, you explain why I would say this. Otherwise, I think the resulting conversation would be tiresome and not worth my time.

The relative speed of two objects docking in orbit is very small. You brake or accelerate as you get close to matching orbits.

The relative speed of a capsule and the mass driver will be very high IF you're using the mass driver to slow down substantially. That's in the definition, you're using it to slow down so the initial relative speed at contact must be high. The higher the speed the less energy you have to waste lowering your orbit (slowing down beforehand). If you're slowing down beforehand enough that your relative speed of contact is low then you don't need the mass driver to brake.

How about you, just as an exercise, compare the difference in velocities of the mass driver and the incoming space craft with the difference of velocities of a capsule docking at the ISS.

Ok, you clearly don't understand enough to understand what you don't understand. You don't have to make the velocities or the positions of specific points on the mass driver and the orbit coincide. All you have to do is to make an orbital path coincide with a great circle pathway on the moon. We can already guide spacecraft with this degree of precision. Anyone suggesting that you'd need to make specific point on the mass driver coincide in position and velocity -- that you'd need to brake first -- just doesn't understand what the discussed device is for in the first place. Basically you've just "proved" that you can't land airplanes, because they come in too fast to brake the same 1/10th of a second they touch the tarmac. o_O

If you can't explain my analogy, then I'm just going to conclude you're trolling based on my previously stated disdain for DK syndrome.

The space elevator even on Earth in GEO is a hopeless, fool's dream.

On Earth, it's hard to build due to much higher gravity, higher required orbit, and the atmosphere. Getting much shorter lengths of much less strong fiber to Moon may be more realistic than 20.000 km of super strong carbon fiber to GEO.

I'm not too clued up on the topic, but I was under the impression that a lunar elevator would potentially require much longer cables than on Earth (primarily due to the lack of spin of the moon). From the Wikipedia article [1]

> There are two points in space where an elevator's docking port could maintain a stable, lunar-synchronous position: the Earth-Moon Lagrange points L1 and L2. L1 is 56,000 km away from the Earth-facing side of the Moon, (at the lunar equator) and L2 is 67,000 km from the center of the Moon's far side, in the exact opposite direction <snip />

> Both of these positions are substantially farther up than the 36,000 km from Earth to geostationary orbit. Furthermore, the weight of the limb of the cable system extending down to the Moon would have to be balanced by the cable extending further up, and the Moon's slow rotation means the upper limb would have to be much longer than for an Earth-based system, or be topped by a much more massive counterweight. To suspend a kilogram of cable or payload just above the surface of the Moon would require 1,000 kg of counterweight, 26,000 km beyond L1.

So, we're talking an ~80,000 km one?

[1] https://en.wikipedia.org/wiki/Lunar_space_elevator

This is correct, unlike my assumption of a shorter cable above. OTOH due to lower gravity, the tensile strength required is still much lower. To quote the very article you lonked:

> the engineering requirements for constructing a lunar elevator system can be met using currently available materials and technology.

So what you are saying is that a space elevator is theoretically possible. I think that's step 0 in building something. Being theoretically possible is the most basic prerequisite.

Just because something is theoretically possible doesn't mean it's feasible or practical to build in real life. For example, we have known that it's theoretically possible to build vacuume sealed trains, aka the hyperloop, since before the first airplane took flight. And yet whilst you see planes fly everywhere, you don't see any vacuum sealed trains.

A lunar space elevator is feasible with today's materials (just a whole lot of them, way too much for current technology to transport to the moon). Here's a nice talk by Markus Landgraf, a mission planner at ESA, at 33c3: https://media.ccc.de/v/33c3-8407-an_elevator_to_the_moon_and...

It's not just about the tensile strength of the materials. The dynamics of a lunar elevator necessitates and extraordinary amount of weight to be used as a counterweight nevermind the length of the cables themselves.

Just because something ticks the checkbox of being theoretically possible doesn't mean it's practical or feasible to build.

Maybe in a few hundred years we'll have the means to build something on that scale. It still takes $10k to put 1kg in LEO, imagine the cost of transferring to the Lagrange points.

Sure, the amount of materials needed exceeds what current technology can get even into LEO by several orders of magnitude. Nobody's proposing to actually build a lunar space elevator. All I'm saying is that a super-advanced civilisation could theoretically build one from the materials available to us today, which is not true for a space elevator from the Earth's surface.

Mars has the radiation problem while in-transit, which is still unsolved.

We have already gone to the Moon. Given that the travel time is relatively short, radiation is far less of a concern.

This is why the Moon is attractive. We know how to get there already and it's not too difficult, all things considered. The problem of how to get there and back with humans aboard has already been solved. It's never been done to Mars.

Without a magnetosphere isn't solar radiation a problem, long-term on the Moon?

Fun fact incoming: there is a mini-magnetosphere centered around Mare Crisium on the Moon.

http://onlinelibrary.wiley.com/doi/10.1029/2009GL041721/abst...

It's only 360km across though...

TMA-1. Erm, CMA-1.

solved by piling dirt over the habitat

IMHO the radiation problem is solved by telling the astronauts that a journey to Mars increases their risk of dying of cancer in the next ten years by 30% or whatever the right number is and asking for volunteers. I'm pretty sure you'd find plenty. I would go, for example.

I was under the impression that the intricacies of creating a small biosphere on Mars pose a vastly larger obstacle than the simple issue or radiation (which can be eliminated by simply shoving enough regolith on top of your colony).

[To put things in perspective] travel time to the Moon is about 3 days, Mars would be about 150 to 300 days (source: Google).

You don't have to use the cheapest, but slowest Hohmann transfer orbit to get to your destination. You can also spend more fuel and get there faster.

A bit of searching suggests that a mildly increased delta-v budget can get your half-year trip down to a quarter-year, but beyond that you're looking at ridiculous costs.

Are you assuming travel is one-way?

That seems okay for probes and maybe problematic for people.

Although the poor slob plan comes to mind:

https://books.google.com/books?id=7ejGJaFjLccC&lpg=PA154&ots...

For me the real advantage of a moon base first is safety. If something goes horribly wrong while we are learning to live on other planets, a return to earth or a rescue mission takes far less time. I am not a space expert though, so perhaps this is wrong?

As NASA has shown in the 30 years, "safety first" does not work as a prime directive, if you want to advance space travel. Musk stated in a recent talk with NASA,"Going to Mars is not for the faint of heart." and "If safety is your top goal, I wouldn't go to Mars."

[1] https://youtu.be/Jv5LjA62Uw8?t=3480 [2] https://youtu.be/Jv5LjA62Uw8?t=3644

'Apollo: Race to the Moon' by Murray and Cox described how the Apollo engineers drew their priorities from Kennedy's speech: Man. Moon. Decade, in that order. There's a difference between genuine safety and bureaucratic safety, something NASA has admittedly forgotten.

I have head that moon dust is reputed to be a consideration, as it creates a type of environmental problem that would not be as extensive a problem on Mars. My understanding is that, due to a lack of fluid and chemical erosion, it tends to be extremely fine and abrasive, and it clings to everything due to static charge, making it hard to keep out of compartments and machinery.

They're both definitely too far for a rescue mission or evacuation that wasn't planned for in advance. In terms of the old eggs/basket metaphor, it becomes a question of whether you want to use half your straw to weave a second basket, just in case the first one breaks, or if you want to use all of the straw to make your one basket as strong, redundant, and well designed as possible.

melt sand it turns into smooth glass, wondering if you couldnt do something similar with a few focused mirrors

Your assertion about getting to Mars with less propellant is super interesting. Can you provide a source for me?

fewer than twenty five seconds of googling: https://en.wikipedia.org/wiki/Delta-v_budget

From LEO to the surface of the moon is 5.9 km/s

From LEO to the surface of Mars is 4.1 km/s

Unless I'm misreading something, both the text and graphic of that page seem to indicate it's 4.1km/s from low Mars orbit to the surface of Mars. I can't find anywhere saying it's 4.1 from LEO.

I get 2.5+0.7+0.6=3.8 to get into Mars Transfer from LEO. If we were to execute perfect aerobraking, the rest would be free. It for sure wouldn't be perfect so some fuel will be needed but not that much.

Source: https://en.wikipedia.org/wiki/Delta-v_budget#Delta-vs_betwee...