[Spaceships] Million Merchant Marathon

[Fred Brackin]

Originally Posted by Anthony

You might. It depends how much velocity you want to build your mass driver for. I suspect it's easiest to have a modest size window.

I suspect that for brobdinagian purposes like terraforming you want to keep all of your terraforming machinery working as close to 24/7 as possible.

[scc]

Originally Posted by sir_pudding

I find the combination of worrying about low-dv transfer orbits with large scale planetary engineering to be incongruous. If the former is a concern you really probably don't have the technology or the resources to do the latter. If you can confidently spin up 5x10^24 kg of Venus by 500 m/s, then you should be able to give a infintesimal fraction of that mass a similar delta-v.

I'm not planning on spinning Venus up. At most I'd go for what is necessary to get the atmosphere off, which imparts just over 3 kph at the equator and is like a 50% increase.

Originally Posted by Fred Brackin

You don't uee transfer orbits for mass driver payloads. You're not using limited reaction mass and you have no manned crews whose time in space you're trying to limit. You fire off payloads continuously and they get where they're going when they get there.

Even if you are using mag sails they're not well suited for transfer orbits. Mag sails don't have limited Delta-V to be conserved by using a Hohman orbit. They make thrust (in small amounts) on a continuoius basis without any energy expenditure after you deploy the sail.

This is most of why I originally thought you were using rocket ships. Rocket ships need Hohman orbits.

Originally Posted by Anthony

You might. It depends how much velocity you want to build your mass driver for. I suspect it's easiest to have a modest size window.

Originally Posted by Fred Brackin

I suspect that for brobdinagian purposes like terraforming you want to keep all of your terraforming machinery working as close to 24/7 as possible.

So just firing off tanks full of gas or liquid has some problems. #1 is controlling their entry into Cis-Lunar space. Keep in mind the the original idea of using Mass Drivers in this sort of manner was for getting Luna material on the Moon, the catcher ships where designed to shred the blocks of pressed together material, I'm going to need to put them in boxes with some sort of maintenance equipment, which I'll want to reuse. #2 is stopping them from hitting each other mid flight. Items launched during the two week least time interval don't all take the same time to travel, those launched first arrive last and those launched last arrive first, so there's a nasty crossover problem, and the moment you start saying it's firing all year despite the rate of launch being steady the rate of arrival will not be.

[sir_pudding]

Building megastructures like parasols or solletas is also a thing I wouldn't expect a civilization that can't do direct transfers to be capable of either, nor flinging Kupier belt ice at inner planets (a task you also certainly want to be doing as straight billards), etc. People who are talking casually about liquifying planetary atmospheres, and timelines for Vemus in decades aren't worrying about the rocket equation (especially if you don't even have rockets).

This is a "what do the gods need with space trucks" thing, but if it works for you, do continue to ignore it!

[scc]

It's not a solid sun shade, it's a collection of much smaller shades that serves the same function and that is possible because of a reactionless drive that's worse then Mag-Sails at this sort of thing.

[Varyon]

Originally Posted by scc

So just firing off tanks full of gas or liquid has some problems. #1 is controlling their entry into Cis-Lunar space. Keep in mind the the original idea of using Mass Drivers in this sort of manner was for getting Luna material on the Moon, the catcher ships where designed to shred the blocks of pressed together material, I'm going to need to put them in boxes with some sort of maintenance equipment, which I'll want to reuse.

If your reactionless drives can be used as "space brakes", strapping one of those onto the cargo box and activating it remotely should slow it down enough for orbital assets to scoop it up. Failing that, perhaps use your magsail+particle accelerator idea.
EDIT: Also, if you can't afford to have single-use cargo containers (as implied by the desire to be able to reuse them), you probably can't afford to terraform anything.

Originally Posted by scc

#2 is stopping them from hitting each other mid flight. Items launched during the two week least time interval don't all take the same time to travel, those launched first arrive last and those launched last arrive first, so there's a nasty crossover problem, and the moment you start saying it's firing all year despite the rate of launch being steady the rate of arrival will not be.

I don't see this as being a problem. Venus and Mars move laterally relative to each other, so it's not like you have a string of boxes in a straight line along their path - each leads the target. If you do have a case where two boxes might intercept each other, the solution is simple - alter the velocity of the second box a bit, and it will have to follow a different path. I doubt you'll be able to saturate the target enough that there will literally be no path that lacks the risk of a collision.

[Fred Brackin]

Originally Posted by scc

It's not a solid sun shade, it's a collection of much smaller shades that serves the same function and that is possible because of a reactionless drive that's worse then Mag-Sails at this sort of thing.

The thing is the _scale_ of that collection.

The diameter of Venus is c. 7500 miles. The equation for the area of a circle is pi x radius squared. 3750 squared x 3.14 is c, 44 million square miles.

Even if each mini-sunshade only blocks a mile of sunshine you've built 44 million of the things (all controlled by robots) to get total blackout.

The society that built 44 million robo-sunshades then proceeded to build 44 million robo-freighters to carry off the unwanted atmosphere. Being robot-controlled it doesn't matter if it takes a long time to make the trip on unassisted mag sails.

[SydneyFreedberg]

And they can’t spare a couple million robots to clean up Africa? Grumble grumble I’ll stop now.

[scc]

Originally Posted by Varyon

If your reactionless drives can be used as "space brakes", strapping one of those onto the cargo box and activating it remotely should slow it down enough for orbital assets to scoop it up. Failing that, perhaps use your magsail+particle accelerator idea.
EDIT: Also, if you can't afford to have single-use cargo containers (as implied by the desire to be able to reuse them), you probably can't afford to terraform anything.

The containers could be, and honestly under that sort of model they could likely boost themselves out of Venus orbit and into Earth/Luna orbit. But at that point you don't have people doing stuff and there's a reason most sci-fi has people doing stuff, it actually makes it interesting. And then I'm back to normal cargo ships just using a different drive system.

As for reusing the cargo containers, they're going to need equipment to keep their cargo cool and now drives systems and this is a government program, not throwing away perfectly usable equipment is frowned upon.

Originally Posted by Varyon

I don't see this as being a problem. Venus and Mars move laterally relative to each other, so it's not like you have a string of boxes in a straight line along their path - each leads the target. If you do have a case where two boxes might intercept each other, the solution is simple - alter the velocity of the second box a bit, and it will have to follow a different path. I doubt you'll be able to saturate the target enough that there will literally be no path that lacks the risk of a collision.

Well half the time they're on other sides of the sun. No mass driver is going to have perfect accuracy. And changing the trajectory of one container possibly means changing others down the line.

Originally Posted by Fred Brackin

The thing is the _scale_ of that collection.

The diameter of Venus is c. 7500 miles. The equation for the area of a circle is pi x radius squared. 3750 squared x 3.14 is c, 44 million square miles.

Even if each mini-sunshade only blocks a mile of sunshine you've built 44 million of the things (all controlled by robots) to get total blackout.

The society that built 44 million robo-sunshades then proceeded to build 44 million robo-freighters to carry off the unwanted atmosphere. Being robot-controlled it doesn't matter if it takes a long time to make the trip on unassisted mag sails.

So I've currently got the construction of the shade taking place over 20 years, but I can extend that. Now each shade will have a control/central module about the size of a car sitting in the middle of a square mile frame with a reflective plastic film (think aliminium foil) stretched over that. These things a likely a lot less substantial then you think.

Originally Posted by SydneyFreedberg

And they can’t spare a couple million robots to clean up Africa? Grumble grumble I’ll stop now.

It's not technology that's stopping, it's politics. Europe trying to enforce environmental protection standards in Africa would go down about as well as Russia's current invasion of Ukraine.

[Varyon]

Originally Posted by scc

The containers could be, and honestly under that sort of model they could likely boost themselves out of Venus orbit and into Earth/Luna orbit. But at that point you don't have people doing stuff and there's a reason most sci-fi has people doing stuff, it actually makes it interesting. And then I'm back to normal cargo ships just using a different drive system.

Most sci-fi also makes heavy use of superscience drives for a similar reason - with or without a human crew, transporting things through space with hard-science drives is boring. Even with superscience, transporting things through space is likely to be boring, unless something goes wrong.

I honestly have no issues with inefficiently using manned ships for this kind of stuff for the service of drama, although I suspect you're either going to either have things be fairly boring or will need to have interesting problems occur with strikingly-unrealistic frequency. If that's the campaign you want, go for it.

A possible idea is to have it be that the bulk of the transportation is done using an efficient method like mass drivers, but it is a legal requirement that a certain proportion of the transportation be done by small businesses, which use manned craft. The race would therefore represent only a small fraction of the total amount of nitrogen being transported, making it more believable.

Originally Posted by scc

As for reusing the cargo containers, they're going to need equipment to keep their cargo cool and now drives systems and this is a government program, not throwing away perfectly usable equipment is frowned upon.

Ha ha ha ha ha ha ha ha ha ha ha ha ha ha *deep breath* ha ha ha ha ha ha ha ha ha ha ha ha ha ha!

Originally Posted by scc

Well half the time they're on other sides of the sun. No mass driver is going to have perfect accuracy. And changing the trajectory of one container possibly means changing others down the line.

Accuracy is a potential issue, yeah. Some way to alter the container's course would be useful - one of the drive ideas that allow you to slow it down at the end would work just fine here as well.

As for altering the velocity (and thus trajectory) of later containers, yeah, you'll basically have to maintain a database of the trajectories of every previous container that is still in transit and make certain each new one doesn't come close enough to any of those to risk a collision. The software they're using to plot the courses to start with can probably handle that without much issue.

Originally Posted by scc

So I've currently got the construction of the shade taking place over 20 years, but I can extend that. Now each shade will have a control/central module about the size of a car sitting in the middle of a square mile frame with a reflective plastic film (think aliminium foil) stretched over that. These things a likely a lot less substantial then you think.

Aluminum foil of standard thickness weighs around 4.5 kg per square meter, or 4500 metric tons per square kilometer. 44 million square miles is around 114 million square kilometers, so a sunshade consisting of nothing but that foil would weigh in at 513 Pg (513,000,000,000 metric tons). To put that in perspective, that's roughly 10,000 times the weight of the Great Wall of China. And you're going to have some inefficiencies due to it being made up of separate components, plus you'll need to add in the weight of the attached robots. Even if your reflective plastic is somehow lighter than such an incredibly-thin sheet of aluminum foil, you're talking about an absolutely massive edifice, here.

And, honestly, using some sort of metal may be a better option than plastic. Plastic requires that you either grow plants and turn them into bioplastic, or drill for oil to turn into more traditional plastic. Earth currently generates somewhere north of 340 million metric tons of plastic each year, so you'd need 1,500 years worth of modern plastic production to build your sunshade. Probably better to harvest your needed materials from asteroids - you won't find any oil, but there's plenty of metal to go around.

[Fred Brackin]

Originally Posted by Varyon

And, honestly, using some sort of metal may be a better option than plastic. d.

They need to make the shades out of photovoltaic film so they an absorb that sunshine and use it to power the reactionless drve units that do the stationkeeping.

The thrust units also need to be around the edges rather than in the center. You would need a stiff disc for a center mount. There'll be some extremely strong cables involved too.