Manufacturing in space

[Sarevok]

I see the phrase "in situ resource utilization" brought up a lot when discussing space colonization. Some people think for instance a moon base has great economic potential. It can make rocket fuel, solar panels, building materials for space stations. How much of this is true and though ? Can you really make complex machines at a small lunar outpost or asteroid mining facility ?
Are things like small self contained replicating factories space enthusiasts dream of feasible at all ?

[PeZook]

I see the phrase "in situ resource utilization" brought up a lot when discussing space colonization. Some people think for instance a moon base has great economic potential. It can make rocket fuel, solar panels, building materials for space stations. How much of this is true and though ? Can you really make complex machines at a small lunar outpost or asteroid mining facility ?
Are things like small self contained replicating factories space enthusiasts dream of feasible at all ?

Of course you can. It will cost trillions of dollars, and the economic benefits will only concern other space projects, but it can be done.

Think about it: a Space Shuttle costs about 2 billion per launch. It's a vehicle in the same payload class as the Saturn V (though obviously designed for other purposes, so most of its payload is the spacecraft).

That translates into about 12-15 tonnes of stuff on lunar surface. So, to deliver one bulldozer, you need two or three launches, depending on how much mass you can shave from the vehicle.

That's 4 billion per dozer. For that money, you could build a really, REALLY awesome factory on Earth.

Of course, the same thing means that every tonne of resources you can extract or recycle on the Moon saves you millions of dollars.

For example, supporting a man for three months requires about 180 kilograms of drinking water. If you can extract it locally, you save 24 million $ in transport costs for every three months.

To sum it up, all savings from manufacturing stuff in space is long-term. A private corporation is almost guaranteed not to get any return on investment for decades, especially without a space-based industry to supply.

[Sarevok]

I should clarify. The idea was manufacturing in space would be cheaper than lifting everything from Earth. Now while I can buy somehow making rocket fuel from lunar water some of the more ambitious plans struck me as an exaggaration. For instance the whole idea of a self sustaining base on moon or mars. How are they going to make the complex parts for the sophisticated equipment the base requires ?

[erik_t]

It's all about timescales and definitions. Such a facility (sans, say, a 45nm fab) could reasonably be considered to be self-sustaining for day-to-day (or month-to-month) purposes; it will not stop functioning if a supply ship or two is running late. This is a pretty big deal; it means that systems do not need to be as critically reliable (and expensive!) as they otherwise would need to be, and much more effort can be expended doing cool stuff rather than keeping the base's head above water.

On the other hand, such a facility would not be self-sufficient in the oh-shit-an-asteroid-just-slagged-Earth sense.

[Darth Wong]

I see the phrase "in situ resource utilization" brought up a lot when discussing space colonization. Some people think for instance a moon base has great economic potential. It can make rocket fuel, solar panels, building materials for space stations. How much of this is true and though ? Can you really make complex machines at a small lunar outpost or asteroid mining facility ?
Are things like small self contained replicating factories space enthusiasts dream of feasible at all ?

You can do it, but everything is so much harder. To take one example, when we mine and refine metal-bearing ore, we tend to use gravity as part of the refining process. Impurities can be skimmed off the top of molten metal, other junk can be drained out the bottom. In zero gravity or weak gravity, this process is less effective or totally ineffective. If you have to use centrifuges for everything, that's a huge pain in the ass, especially when working with molten metal.

The idea of doing certain things in space is fine, but complete self-sufficiency is bound to be extremely difficult. Things we take for granted in Earth-normal gravity with an atmosphere can be so prohibitively difficult and expensive in low or zero gravity with little or no atmosphere, to the point that it's cheaper to have stuff sent from Earth.

[PeZook]

I should clarify. The idea was manufacturing in space would be cheaper than lifting everything from Earth. Now while I can buy somehow making rocket fuel from lunar water some of the more ambitious plans struck me as an exaggaration. For instance the whole idea of a self sustaining base on moon or mars. How are they going to make the complex parts for the sophisticated equipment the base requires ?

The ideas proposed by serious students of the matter acknowledge the problem. Utilizing in-situ resources isn't supposed to make a base or colony self-sufficient per se, it's just that supply ships will be able to ferry the important parts instead of mundane supplies like water or breathing air.

To go back to the water example: 180 kg of water is simply fuel for a single human, and you have to ship it every three months. 180kg of computer spare parts, nuclear fuel or ball bearings will last the base for years. This means you spend the same amount of cash transporting it, but it's much more cost-effective than ferrying mundane consumables.

Another matter is exploitation of raw resources for things like orbital construction, but I can't do a detailed cost analysis for things like titanium processing in 1/6th Earth gravity vs. shipping processed titanium from Earth, etc. - though launch costs from the Moon are certainly orders of magnitude less.

[Simon_Jester]

Yeah. Lunar gravity, for example, is weak enough that space elevators are actually a plausible launch method, rather than being on the very ragged edge of the theoretical limit of the strength of the strongest materials we've ever heard of.

[Zixinus]

Speaking of space manufacturing: what is easier or harder to do in space, due to the effect of low or zero-gravity? Aside saving long-term hauling costs of shipping water that could be made on-site, what other advantages could be there?

I've heard of orbital factories. Why would these be better served in orbit than on Earth?

[Fingolfin_Noldor]

Speaking of space manufacturing: what is easier or harder to do in space, due to the effect of low or zero-gravity? Aside saving long-term hauling costs of shipping water that could be made on-site, what other advantages could be there?

I've heard of orbital factories. Why would these be better served in orbit than on Earth?

Certain types of crystal growing i know can be done much better in space, but however, this is a topic that is still undergoing substantial research and it doesn't help that there's no concerted effort to look into it what with the ISS shutting down in a few years time except the Russian portion.

[TheLostVikings]

Speaking of space manufacturing: what is easier or harder to do in space, due to the effect of low or zero-gravity? Aside saving long-term hauling costs of shipping water that could be made on-site, what other advantages could be there?

I've heard of orbital factories. Why would these be better served in orbit than on Earth?

When making alloys of different metals the one with the heaviest atoms tend to "sink towards the bottom" creating an uneven distribution. In microgravity you don't have that problem, allowing you to create structurally stronger/lighter materials (by a significant margin), and some materials that are completely unworkable at all down on the planet. Though research in this field is extremely limited at this point as experiments are usually limited to a couple of seconds in freefall on a plane or small rocket.

It's completely unrealistic for the foreseeable future, but I guess anyone advanced enough to have a fully featured space based infrastructure in place it would be stupid not to take advantage of it.

[Simon_Jester]

Of course, the drawbacks include not having an atmosphere to work with, not having any place to dump the waste heat from your blast furnace except for radiator panels, and not having any place to run in the event of an industrial accident. It's not all one-sided.

[TheLostVikings]

Of course, the drawbacks include not having an atmosphere to work with, not having any place to dump the waste heat from your blast furnace except for radiator panels, and not having any place to run in the event of an industrial accident. It's not all one-sided.

If an atmosphere is required for what you do you can pressurize your facility. You can build your facility on an asteroid to gain a large heat-sink for little cost (it might even be what you are extracting your raw materials from to begin with). And if you need to run in case of an industrial accident your facility designers clearly fucked up on an epic scale, surgeons today can operate on patients hundred of miles away, and most of the cars today are almost completely assembled by industrial robots, so if you have some major safety hazard in your facility you can try to design it in such a way as to minimize risk to the workers (if there even are any on-site workers there to begin with, which is not a given).

Sure, Murphy's law can screw everyone over on occasion, but you can minimize risks if you think ahead. Just look at modern nuclear reactors, if they go critical the automatically shut down without the need for any human interaction, they are simply built more intelligently than the older reactor designs.

[fnord]

TLV, from what I understand, fission reactors are generally maintained slightly supercritical (neutron multiplication > 1) to allow for neutron losses and still keep the fission reaction going.

The problem isn't avoiding criticality, but avoiding prompt criticality - where the reactor is now critical on the prompt neutrons alone (prompt neutron lifetime 1e-4 sec in a thermal reactor, 1e-7 in a fast one, as opposed to a delayed neutron lifetime in the single second range and are numerous enough normally to raise mean neutron lifetime in a reactor to the 0.1 sec range). During prompt supercriticality, more than one prompt neutron is produced for each prompt neutron involved in a fission - so even for a k_prompt of 1.005 in a thermal reactor, you have, at the end of 1 second, 1000 prompt neutron lifetimes, a relative power of (1.005 ^ 1000) ~ 150x - could prove embuggerising.

The reactivity insertion required to take a system prompt critical is (close enough to) the delayed neutron fraction (~ 0.4% for U233, 0.65% for U235 and ~0.75% for Pu239), called beta, or a dollar - reactivity insertion of more than $1 causes all sorts of silly buggers and people downloading some brownware .

One of the major factors enabling the Chornobyl cockup was the design's reliance on using the light water coolant to absorb neutrons, thus, as the coolant boils and steam voids form, a positive feedback loop forms.

Pre-Chornobyl RBMK designs had a positive void coefficient of approx $4.50 - if the moderator completely voided, it was a reactivity insertion 4.5x that required to take the reactor prompt critical, with damaging results. Post-Chornobyl:
- more control rods added to surviving units' cores (except, of course the destroyed Chornobyl Unit 4)
- U enrichment raised to 2.4%
- 80 more in-core absorbers to inhibit low-power operation
This resulted in the void coefficient dropping to 70c - the moderator completely voiding is now insufficient to take the reactor prompt critical.

In general, to enable self-regulation, reactors are designed with negative coefficients of reactivity.

• Void coefficient - reactors with voidable moderators (such as xWR) have their fission reaction slowed as the moderator starts to void (eg, from boiling). Not signficiant with non voidable moderators (such as gas-cooled designs)
  Temperature coefficient - all components in the core have seperate temperature coeffs of reactivity.
  • The major ones are:
    fuel temperature coeff - as fuel temp increases, resonance absorptions in nonfissile isotopes increase, putting the brakes on. Liquid fuel forms have the resonance absorptions and also significant thermal expansion, further slowing things.
    moderator temperature coeff - as moderator (eg, water) heats, it expands, reducing its volumetric effectiveness in moderating neutrons, and forcing the neutrons that are moderated to travel a bit further during moderation.

These can be combined into the power coefficient of reactivity - the change in reactivity per unit change in power, which is not necessarily constant.

I am not a nuclear engineer nor nuclear device designer, despite occasionally giving that impression.