Good ways to limit space travel?

[thrash]

Originally Posted by Calvin

The other thing to consider is Lagrange points. I had intended to put [interesting things] at those points. Trojan planets in the L4 and L5 of the stars, ...

These don't exist in a binary system with more-or-less equal masses. You need the smaller mass to be less than ~1/25 of the larger mass for the equilateral points (L4-L5) to be stable. The co-linear Lagrange points (L1-L3) do exist, but stable orbits around them (halo or Lissajous) will be extremely difficult to find and maintain.

I would assume that both systems were colonized from somewhere else at nearly the same time, not one from the other. That gives you a space race to drive development, even if it isn't strictly economical. If your default drive is space sails (photon or magnetic), your "breakthrough" could be the fielding of laser/maser or particle beam boosters.

Your target world could have an optically thick atmosphere that limits attempts to map its surface from the original colony systems.

[Shostak]

If you want to limit travel to the third star, stay away from jump drive and stick with constant 1G acceleration--this model is a great way to keep things theoretically plausible and it provides hard-sci artificial gravity. Using this system makes acceleration and fuel storage/consumption huge factors in any travel.

To keep the third star inaccessible, make refueling difficult. This also keeps things believable; after all, why should refueling be an easy option, especially in the emptiness of interstellar space? Since constant acceleration at 1G would eat up a lot of fuel, just declare that it is an in-system-only option--Maybe just a few AU (an already enormous distance). And make refueling take a long time, requiring lengthy layovers--especially long ones if there is not a fuel depot established. This is related to Jason's suggestion that the fuel would only be enough for 1-way, since there would be no industrial infrastructure to support refueling at the destination. Travel to and fro would require an investment of human resources and decades of construction at the far end.

You could also make the orientation of the planes of the systems to each other work to your refueling advantage. Since most baryonic matter will be found roughly within the disk of the the ecliptic, if the stars had roughly parallel ecliptic planes most refueling stations (presumably servicing spacecraft utilizing the raw materials of asteroids and planets) would likely be no closer to the other system than the stars themselves. So there is no convenient half-way point. As a bonus, this coincidental arrangement provides a fertile field for all kinds of "Intelligent Design" theory subplots.

[spacemonkey]

If there was an ancient civilization, maybe one of its defense drones lasted just long enough to destroy the first scout ships, but damaged or destroyed itself in the process. The locals held back until now, with all sorts of theory, fiction, and religious nuttery about the third star/third star people, but someone managed to make the trip and return in one piece with some data. If there's a lowtech society, they could have suffered a tech collapse, so you could place the low tech species on the same planet as the high tech ruins.

[D10]

Maybe when the higher ups saw the oddities reported by scouting robots they concocted a convenient lie layered with half truths about the impossibility of getting there/getting information from there, and began a fnord space program to rush into the potential riches and claim monopoly.

[Daigoro]

If the 3rd star is a captured wanderer, its orbit isn't likely to be circular. Give it an eccentric orbit that has only just brought it close enough to be reachable.

[Fred Brackin]

Originally Posted by Shostak

If you want to limit travel to the third star, stay away from jump drive and stick with constant 1G acceleration

To keep the third star inaccessible, make refueling difficult. .

Just as a note this won't work at all (and the OP seems to understand that). One of the wonders of constant acceleration is that travel time is not linear with distance. It increases as the square root of the linear distance.

As a handy rule of thumb to remember a accel and decal trip of 1 AU takes about 3 days or 72 hours. A trip of 144 AU is of course, 144x as far in linear distance but only 12x as long in trip time at constant acceleration. So 36 days.

A trip 2x as far as that only takes 1.41 times as long rather than 2x as long. even if it absolutely is not possible to fit more fuel onto the ship than that 144 AU trip requires at 1G you can go farther by accelerating slower.

The square factor works with slower speeds as well so accelerating at 0.5G makes the trip last 1.41x as long. Traveling 2x as far still only takes 1.41x as long too. So if you can go 144 AU in 36 days at 1G you can go 288 AU in 72 days at 0.5 G on exactly the same amount of fuel.

If an alien cae and offered us a 0.01G constant acceleration drive I'd jump at it. You'd be accelerating 100x more slowly but trip time would only increase by a factor of 10. Only 3 weeks to Mars at close approach instead of the 6 months current planning is talking about.

As another note, refueling one ship from another ship or a space station requires matched velocities and mid voyage velocities for a constant 1G ship can be quite high. For that 36 day/1G trip they are around 5% of c. So there won't be a lot of refueling halfway through the trip unless you use a Bussard ramjet perhaps.

[Ulzgoroth]

Originally Posted by Fred Brackin

A trip 2x as far as that only takes 1.41 times as long rather than 2x as long. even if it absolutely is not possible to fit more fuel onto the ship than that 144 AU trip requires at 1G you can go farther by accelerating slower.

You probably could, but you wouldn't. Instead, you'd do the acceleration and deceleration at 1g, and spend the time in the middle drifting.

[Fred Brackin]

Originally Posted by Ulzgoroth

You probably could, but you wouldn't. Instead, you'd do the acceleration and deceleration at 1g, and spend the time in the middle drifting.

Back of the envelope calculations say "no" but it's a close thing. Accelerating at half speed adds 15 days to the trip and accelerating at full speed and drifting adds 16.6.

I'd probably find 51 days at half G more comfortable than 17G days of 0G. I believe the plumbing would still work then and it absolutely would not work at 0G.

[Ulzgoroth]

Originally Posted by Fred Brackin

Back of the envelope calculations say "no" but it's a close thing. Accelerating at half speed adds 15 days to the trip and accelerating at full speed and drifting adds 16.6.

I'd probably find 51 days at half G more comfortable than 17G days of 0G. I believe the plumbing would still work then and it absolutely would not work at 0G.

If your calculations are telling you that you can complete your trip faster while moving slower at every point along the way, there's something wrong with your calculations.

If your spaceships are not designed to function in freefall, you're going to have a very hard time parking them.