Re: Mars expeditions (was Re: Proposed sample return mission to Phobos)

Joe Strout wrote:
In article <45DB5950.603@xxxxxxxxx>,
Ian Woollard <ian.woollard@xxxxxxxxx> wrote:
That's not as true on Earth though. We get quite a lot of our goods and services via shipping, and they take relatively long periods to reach us.

True, but our Earth transportation infrastructure is significantly more advanced than our space infrastructure.

But that only makes my point more strongly, the steam rocket isn't an *advanced* delivery system, the Isp is low; but the *costs* are low.

I'm sure a more advanced system could improve on steam, but that would require much R&D and much higher capital costs, and the market wouldn't support it. You don't *want* an advanced transportation system initially. Slow and cheap is good.

But even so, I note that the theoretical energy costs are not the primary driver of transportation costs on Earth, either.

Yeah they are. We don't put things on banana boats because energy costs are irrelevant, we do it because airfreight fuel (i.e. energy) as well as other costs are many, many times more.

The point about Phobos is that the return delta-v is lower so material goods that can be made there are going to be cheaper than from the Moon.

Only if delta-V is the key determinant of cost, and I don't believe that's the case.

No, delta-v is important; but it's the total system costs that are the determinant, and steam rockets steam whoop ass in that regard.

Goods from the Moon can be sent to LEO using mainly an electric mass driver; once that's built,

But you're talking about soft landing many tens or even hundreds of tonnes of equipment (including expensive PVs) on the Moon right there. Or building it with ISRU and, frankly, we don't know how to do that well right now. It's a non starter at this point.

Then there are all the other factors that go into total transportation cost, including warehousing your stuff on Phobos for many months at a time until a good launch window comes along, and so on.

Warehousing costs? You pile it up in orbit and wait for the next regular launch window! What costs?

Electrolysis? I don't think so -- it's more a process of heating the lunar regolith until useful amounts of O2 bake out.

You'd have to liquify it afterwards down to -190C. How you going to do that? A LOX plant is an impressive piece of kit and takes lots of electrical energy. And you have to soft land it all on the moon. And LOX has poor Isp as a monopropellant, so it's not self-ferrying. And once the LOX is in orbit, you still have to have fuel on hand to send it off to where you want it; which would need to be expensively lifted from the Earth and then lifted further to get to the moon. With Phobos, the propellant rolls *down* hill towards the Earth and costs nothing to do that. With fuel from the Earth it *costs* fuel to push the fuel up hill.

This would most likely be accomplished during the lunar day using a big mylar mirror.

Yeah, so you'd have no production for weeks on end. You could probably store enough thermal energy to deal with night passes on Phobos because they're short and you're in a vacuum.

Granted, extracting water from Phobos is likely to be a similar process, but your mirror will have to be quite a bit larger there.

It's only a factor of about two, and the mirror is much easier to do in zero-g; the Sun doesn't move in the sky, unlike the moon. Anyway, it's just aluminium foil.

True, I'm a big fan of water as well... but O2 alone will go a LONG way to helping reduce the mass we need to launch from orbit.

The thing is, with water the entire usage cycle closes; whereas you would still need fuel *as* *well* for O2. And the equipment for O2 is much more massive, and needs to be soft landed on the moon, and O2 is hard to store in space, it tends to boil off, whereas water is space storable.

Nobody is disagreeing with that, but at the moment Phobos looks to be a better bet for initial ISRU, the equipment costs would be much, much lower than that necessary to create lunar oxygen for example

You're making an assumption there based on, what, exactly? I certainly don't agree with it. Lunar O2 production has been demonstrated in a prototype unit, though not with the performance NASA would like -- hence the LunaRox challenge they're currently getting underway. I bet that within a few years we'll have designs that work much better, and I see no reason to suppose they'll be particularly expensive.

I'll take that bet. 5 years, one hundred bucks; *total* estimated system costs to be *below* *total* estimated costs of a steam cycle. Deal? :-)

(Hint: steam cycles are about 1-2 orders of magnitude lower costs, you really think the research will give that?)

there's no high delta-v landing or takeoff maneuvers so you can use ion drives which are slow, but cheap and reliable.

True, but landing on the Moon isn't all that expensive either, and takeoff is essentially free once a mass driver is built.

That's hundreds of tonnes of equipment and lots of R&D and all of it needs soft landing at enormous expense! And mass drivers aren't portable and aren't any use for carrying humans around!

Steam rockets can potentially do all that, and Earth launch masses are a tiny fraction, and are lower tech.

We need to assay places to find the best places to go. Maybe the Moon is it in spite of current appearances, but the current samples we have of the moon aren't particularly encouraging.

How can you say that? Lunar regolith is nearly half oxygen by mass. Oxygen is 8/9ths of the mass consumed by a LH2/LOX rocket. That's a huge boon, and it's only three days' travel away, with a launch window at least twice a day.

Such a thing needs to be looked at as a complete system. The system costs are too high for O2. Also what it means is that the launch and transport costs for the moon are limited by the hydrogen/fuel launch costs from the Earth. With steamers you can repeatedly deliver materials to LEO from Phobos/Deimos without *any* launches from the Earth.

Best,
- Joe
.

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