Oolite Bulletins

ffutures wrote:

How many years do you want to spend travelling? I went with constant thrust because without it you're talking say a year to Mars, several years to the outer planets.

Smivs wrote:

Constant thrust is desirable certainly. As well as vastly reducing journey time, a constant thrust of say 0.5G would overcome most of the gravity issues.

Desirable, yes.. practical, no. As impractical as aerodynamic spaceships, in fact.

ffutures wrote:

Try not to think about how much energy you need to do this...

Precisely. My point exactly.

Diziet Sma wrote:

Desirable, yes.. practical, no.

Depends on your drive type. An ion drive could use less reaction mass under constant thrust than a conventional combustion drive would use in bursts. For journeys which aren't time-critical (especially on uncrewed ships) they're potentially very useful (even on faster ships using conventional drives for main acceleration and deceleration, you might fit ion drives to give a bit of extra speed without too much extra fuel)

Another possibility for propulsion of at least some ships - again, perhaps only uncrewed ones, or perhaps as an alternative to freighters for less fragile cargo - would be to stick giant mass accelerators in high orbit and use them to do the acceleration and deceleration with less need to deal with "where do you stick the fuel"

SteveKing wrote:

One thing... holds the thing together... materials science... structural weakness... inversely proportional to size... inefficient to construct... joints in a frame or hull... integrity... bigger... inertial stresses (torsional/centrifugal/impact)... manoeuvrable... upper size... inefficient to construct... tend to be more exo-skeletal... contain the atmosphere... geometrically simple.

Geodesic sphere. This was a solved problem before the first rockets ever went into space. Why use ground-launched rockets at all? Balloons are the smart way to get to the upper atmosphere. Such a pity NASA have always valued theatricality over practicality.

It seems this thread has taken on a varied subset of thoughts related to space ship design. Getting back to the original question: It seems to me that it would be depend on the technological advancement made by spacefaring beings. Simple, practical, feasible and economical are the appropriate adjectives for human-kinds first spacecraft designs. But, if we are allowed to learn - and that is a big IF - we may find spacecraft design taking off in totally different tangential directions.

I'm with Day on the 'everything form space' question. Getting resources from the asteroid belt might itself be a limiting factor in producing ships in orbit. Getting a 'plant' to the belt might prove the same. I've always imagined space craft and stations to be built of concrete (ever since seeing the station at Lave all those years ago). A peristaltic pipe from the surface (attached to an elevator) would allow a concrete (possibly using moon dust or 'waste' as the aggregate) to be 'sprayed' onto a mold. Concrete has some very special properties and I'm sure it's not beyond the whit of man to devise a cement that could cure at low temperatures (given that you'd need some kind of environment for the workers even this may be unnecessary).

Propulsion has always fascinated me and I love the idea of a solar sail (and even have some 'Clipper' and 'yacht' like designs in my head for Oolite), strictly in system though.

NigelJK wrote:

Getting resources from the asteroid belt might itself be a limiting factor in producing ships in orbit. Getting a 'plant' to the belt might prove the same.

No, it would work the other way round I suspect. Most of the plant would be in orbit, and selected asteroids would be steered towards Earth for processing. The asteroid belt is enormous, so it doesn't make sense to put stuff out there - where would you choose, because one area would have no advantage over any other. Much more sensible to send 'harvester' ships out to the belt which could identify worthwhile targets and return them to Earth orbit for processing.
Concrete? Sounds daft to me but then a lot of good ideas do to start with, so why not. At least there would be lots of raw materials around from all the used-up asteroids.

Smivs wrote:

Much more sensible to send 'harvester' ships out to the belt which could identify worthwhile targets and return them to Earth orbit for processing.

Disaster scenario generator, isn't it?

I already imagine the movie with the tracted asteroid crashing into earth ; and later the united nations unanimous bill forbidding to put asteroid plants in earth orbit.

Day wrote:

I already imagine the movie with the tracted asteroid crashing into earth ; and later the united nations unanimous bill forbidding to put asteroid plants in earth orbit.

Unlikely, I think. Any asteroid sent earthwards would be very closely watched, on a course that everyone would know about and be able to plot, and would be many years in transit. It would only be possible in any case if we had relatively easy access to good asteroid-deflecting technology (which would be something worth developing in any case).

I suppose theoretically you could have some super-villain make a late, violent course change to an inbound asteroid which couldn't for some reason be corrected, but if a super-villain has access to that level of technology - not to mention that amount of disposable energy - then we'd have more immediate problems closer to home anyway ...

@Disembodied: agreed in part.

What about this, a hack at the last moment?

The speed of the asteroid would be slowing when approaching earth.
The asteroid designed orbit depends on its mass and its speed. If you lose the speed/acceleration vector control when being almost on-orbit to a terrorist/villain/rogue state/playful hacker, who would push just a little bit in earth direction, then the asteroid would crash without possibility of being countered.
Each second passing would mean the energy necessary to counter it would increase.
The necessary energy would increase proportionnaly with the altitude decrease (potential energy = 1/2*m*g*h), and the altitude would decrease along the square of passing time (acceleration towards earth being g).

So, the time that the problem is detected, it may already be too late to be corrected.

PS: in my scenario, I imagine the asteroid being steered by solar wind, through sails, with target paths sent via hackable telecommunications. Seems a cheap way to convey them.

What a complicated way of giving the terrorist/super-villain the opportunity!
Once selected, the target asteroid would be gently nudged onto a 'near-miss' trajectory towards Earth. It wouldn't need steering or drive or anything, it just slowly heads roughly towards Earth. At the appropriate moment, it is captured by a Tug which safely tows it to its final destination. Safe and simple.

Day wrote:

@Disembodied: agreed in part.

What about this, a hack at the last moment?

The speed of the asteroid would be slowing when approaching earth.
The asteroid designed orbit depends on its mass and its speed. If you lose the speed/acceleration vector control when being almost on-orbit to a terrorist/villain/rogue state/playful hacker, who would push just a little bit in earth direction, then the asteroid would crash without possibility of being countered.
Each second passing would mean the energy necessary to counter it would increase.
The necessary energy would increase proportionnaly with the altitude decrease (potential energy = 1/2*m*g*h), and the altitude would decrease along the square of passing time (acceleration towards earth being g).

So, the time that the problem is detected, it may already be too late to be corrected.

PS: in my scenario, I imagine the asteroid being steered by solar wind, through sails, with target paths sent via hackable telecommunications. Seems a cheap way to convey them.

The last-minute hack looks like the only viable option, but it would be very tricky to do (and would require people to be daft enough to allow their multi-gigatonne floating rock to be open to external hackers, although we shouldn't rule that sort of stupidity out ... or the possibility that an automated system couldn't be suborned in some other way). I'm also not sure if it would ever be a wise idea to push asteroids into an earth orbit that required the asteroid to be slowed down on approach (or, worse, aerobraked): better to send it on a path where it would be gravitationally captured by the earth-moon system. This seems (following a quick google for "gravitational capture") to be fairly mind-bogglingly complicated, involving Lagrange points and suchlike, but for economy as well as safety it looks like the way to go.

Smivs wrote:

At the appropriate moment, it is captured by a Tug which safely tows it to its final destination. Safe and simple.

That does rather depend on having tugs that can capture and easily and quickly manoeuvre mountain- or island-sized asteroids! With anything approaching what is currently technically conceivable, "the appropriate moment" would probably be 10 or 20 years before the rock arrives ...

L1 would suit nicely - but I still think the Moon has possibilities as a shipyard.

Cody could be on to something, you know.
All the plant and yards on 'our' side of the Moon, and aim the asteroids at the other side - they would even be conveniently broken up, and nobody would notice all the extra craters

The Earth-Moon L4/5 points might be decent for heavy industry - gravitationally stable, and a long way from anything else. You'd need a lot of radiation shielding, though.

Subverting the asteroid on the way in could be made relatively difficult - more straightforward would be to capture the processing plant (or more practically, simply own it) and drop the asteroid into a collision orbit from there.

Obviously came in a day late, but I totally agree with Cody about the moon

Cody wrote:

...but I still think the Moon has possibilities as a shipyard.

It's far enough away to be safe from 'accidents' that are earth bound, but close enough to be easily accessible from earth. An asteroid can probably be manouvred into moon orbit reasonably safely and just as easily as into earth orbit, and before/after mineral extraction, could be just 'dumped' onto the moon as Smivs quips.

If NigelJK is right about concrete (or some sort of aggregate based hardened slurry) as a hull material, then perhaps a moon-orbit or L4/5 point (as cim suggests) for a manufacturing station/space dock. Use some sort of railgun technology to boost the abundent bulk materials from the moon into orbit if they weren't available as part of an asteroid.

I don't imagine that the sourcing of raw materials for ship construction will ultimately be difficult, after all its just a 'trucking'/logistics problem. The bigger problem will be the refinement of raw materials to useable ones in space - how much serious research has gone into metal extraction/refining in a vacuum?