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Old 06-01-2016, 09:58 AM   #21
Fred Brackin
 
Join Date: Aug 2007
Default Re: Good ways to limit space travel?

Quote:
Originally Posted by Shostak View Post
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.
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Old 06-01-2016, 10:09 AM   #22
Ulzgoroth
 
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Default Re: Good ways to limit space travel?

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Originally Posted by Fred Brackin View Post
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.
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Old 06-01-2016, 10:35 AM   #23
Fred Brackin
 
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Default Re: Good ways to limit space travel?

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Originally Posted by Ulzgoroth View Post
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.
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Old 06-01-2016, 10:54 AM   #24
Ulzgoroth
 
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Default Re: Good ways to limit space travel?

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Originally Posted by Fred Brackin View Post
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.
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Old 06-01-2016, 11:28 AM   #25
Fred Brackin
 
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Default Re: Good ways to limit space travel?

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Originally Posted by Ulzgoroth View Post
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.
<shrug>Do the math yourself. Don't just yell at people who do about what happened when they did the math. I did nothing more than see how long it would take to cross 144 AU at a constant 5% of c and add that to the 36 days of acceleration.

As to parking, if I had 1 G capability I'd normally be landing and not parking.

This is starting to be a sidetrack.
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Old 06-01-2016, 11:38 AM   #26
Ashtagon
 
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Default Re: Good ways to limit space travel?

Swap out your contant-with-fuel-1g drive for one that also requires the presence of a functional gravity well to operate. That way, the drive simply fails (or slows down unacceptably) past a certain distance from the primary stars.
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Old 06-01-2016, 12:02 PM   #27
Ulzgoroth
 
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Default Re: Good ways to limit space travel?

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Originally Posted by Fred Brackin View Post
<shrug>Do the math yourself. Don't just yell at people who do about what happened when they did the math. I did nothing more than see how long it would take to cross 144 AU at a constant 5% of c and add that to the 36 days of acceleration.
...Seriously, actually doing the math is unnecessary to show why your conclusion cannot be correct.

But fine. 0.5 g accelerating over 72 AU (to the mid-point) takes a little more than 24 days and gets up to a bit over 10 million m/s (3.4% of c). So that's our fairly terrifying half-delta-V. And actually, this paragraph is entirely unnecessary to my point, though I'll use it at the end to deliver actual trip times.

Doubling the acceleration means spending half that time covering half the distance, and then covering the second half of the distance drifting at maximum speed. The maximum speed is twice the average speed of the continuously accelerating ship, so that takes a quarter of the aforementioned time. All told, 3/4ths the time, roughly 18 days, to the mid point, and of course the same from the mid-point to the destination. For a little more precision, it comes to 36.4 days, vs 48.5 at reduced thrust.

Trip time formula if you want to do it out:
Spoiler:  

Quote:
Originally Posted by Fred Brackin View Post
As to parking, if I had 1 G capability I'd normally be landing and not parking.

This is starting to be a sidetrack.
1 G isn't enough to take off or land unless you're doing it aerodynamically.
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Last edited by Ulzgoroth; 06-01-2016 at 12:10 PM.
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Old 06-07-2016, 08:04 AM   #28
Captain Joy
 
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Default Re: Good ways to limit space travel?

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Originally Posted by Calvin View Post
I think I'm probably going to use the jump drive option.
In that case, just make jump drives require jump gates. The first ship(s) there will be the very long expensive mission(s) to build the first jump gate in 3rd-star's system.

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Originally Posted by Calvin View Post
The radiation idea is good, both the lethality and the interference with communications, it was actually what I was imagining. The jump drive option just happens to solve the second issue of in-system travel without the 1g drive. It also has less of a "footprint" than the radiation thing, which might raise other issues.
The radiation was a good idea. The lose-contact-with-any-probe-sent was a good idea. The highly eccentric orbit was a good idea. You have options, which is nice; one or more could be in effect.

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As a total aside, the Space book is really great.
Agreed.
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