Dilemma with FTL speed

I have a setting where, at one point, a starship would make a round trip from Sol to the edge of the galactic core, roughly 20,000 lightyears away, in about three months. Doing some calculations, the required velocity for this is approximately 160,000 c^note 160003.5045257664781158793347972, to be exact.

Thing is, the ship is later on required to cross distances in the low two-digit lightyears, which at this speed would take mere hours. However, I want the ship to take days with the smaller distance without affecting the travel time of the larger distance. What can I do?

Would it be an acceptable excuse that, for shorter distances, the ship needs to go slower? That is, the faster it goes, the more inaccurate it becomes, potentially missing its target by hundreds of lightyears. So, either it goes slower and arrives on-target, or it goes faster, arrives off-target and has to spend extra time determining its current position relative to the intended destination, then plot and execute a second (and potentially a third, maybe even a fourth) trip.

The FTL method in question is a wormhole/hyperspace hybrid. There is a fixed-destination version that is slightly below 100 c but is pinpoint-accurate, so the above excuse has basis.

edited 11th May '15 3:59:51 PM by amitakartok

Use hyperspace "lanes", maybe. Like the Honorverse's "grav waves" in a way. Along one of these lanes, FTL is basically super fast, so if there's one between where you are and where you need to go...excellent.

If there is no lane going your way, you'll get there a lot slower. This also has interesting implications for traffic and strategic control...

Something like Mass Effects Mass Relays might also work. Similar to the Honorverse's grav lanes, but rather than natural occurences, artificial structures that allow extremely fast travel between them (or a series of them for longer jumps) but if you're not jumping from relay to relay, FTL is a lot slower.

You could also have a character sabotage whatever mechanism it takes to travel that fast right before the short trip, and that same mechanism is repaired or undamaged for the longer trip.

But the sky freeway for the longer trip sounds less contrived.

Or have the ship accelerate, then decelerate past mid-trip. The average speed will be higher as the distance travelled is longer.

In your case : 20,000 LY in 100 days means an acceleration a = 8 LY/day^2 (because d = 0.5*a*t^2, considering only one half of the symmetrical trip). So a trip 100 LY long will take 7 days.

edited 12th May '15 12:11:21 AM by Aetol

You could have it so that the ship is able to 'accelerate' while in hyperspace, so that for long trips it achieves a higher average speed, whereas for shorter trips it has to begin decelerating again before it can build up that same level of speed.

With simple linear acceleration, this potentially makes the travel time proportional to the square root of the distance covered. Then, if a trip of 20000 light years takes three months, a trip of 10 light years should take about two days, and 100 light years about six and a half days.

The acceleration idea sounds good.

The ship in question is alien-made and one-of-a-kind; no other ships of its class have been built for the past 250 million years. The reason why it needs to go to the galactic core in the first place is to seek out an old battlefield and cannibalize the dead hulk of one of its sister ships for self-repair raw materials. Prior to that, it's so badly decayed from age that it has several large chunks missing and the superstructure is completely exposed, with barely any external hull to speak of.

Role-wise, it is to capital ships what an aircraft carrier is to fighters. It is also the only independently FTL-capable vessel in the entire verse, though the crew eventually figures out how to use the FTL drive to catapult non-FTL ships across interstellar distances like a giant slingshot (though the launcher ship itself has to stay in realspace until the transition is complete). Prior to this ship's discovery, slow and gate-based FTL travel was the only type known, so this ship is a major game-changer, allowing its owners to compete with the local military superpowers despite having only a fraction of their resources and manpower.

Human-inhabited space spreads not even a hundred lightyears away from Sol, so the ship literally has no reason to go any farther after it has the repairs it wanted.

If it's some sort of eldritch tech, you don't even need bother with an in-depth explanation IMO. "This weird-ass alien derelict seems to be faster on longer trips, and we have no goddamned idea why".

edited 12th May '15 4:20:06 AM by Aetol

They could just ask the ship's AI.

On the other hand, she'll most likely refuse to answer. After all, they might have no further use of her if she were to help them make her obsolete. But by withholding data and letting them figure it out themselves, she can postpone that for a couple of decades/centuries more.

Oooh, sharing the ship with a not-necessarily-benevolent AI ? It's been done, of course, but that opens all sorts of fun possibilities.

Maybe the AI is faking the speed difference to disguise how the drive actually works.

Going too fast near inhabited planets results in mutations. Kinda like how breaking the speed of sound causes a sonic boom, but one with a fixed and predictable "danger radius" that dissipates over distance, the effects of which can be avoided by taking advantage of the vastness of space.

But then this brings up the question of how often a vessel can travel along a particular route, as there's a risk they'll collide with the previous vessel's Variation Wave.

More like passive-aggressive. She can operate the entire ship completely by herself but having a crew to handle the small stuff frees up processing cycles. With that said, she makes it a point to let everyone know that she does whatever the hell she wants; they can live onboard her because she lets them, not the other way around.

Yeah, she's kinda an asshole. But age does make people cranky, doesn't it?

Is this about getting the ship to those places or the people on the ship? Because if you're just worried about the people or the situation, you could probably just shift them around. For example, "We went on a brand-new ship last time, but now we don't have that luxury. Gotta take this older ship that only goes a dozen lightyears an hour."

edited 12th May '15 11:46:03 AM by AwSamWeston

Perhaps the FTL drive has a minimum speed it must travel at, some mechanical thing required by whatever physics it functions over.

This would make it ideal for long range travel, since you don't care about the minimum speed, but difficult for the short range, as you are likely to overshoot.

Technically, any FTL drive has a minimum speed (the speed of light)...

No really, guys. This is way overcomplicated. You don't have to introduce a new technobabble element to justify the speed difference. It's way easier to just say "the better option broke."

"Captain! Our uber-fast FTL Drive just went on the fritz!"
*Sigh* "Looks like we'll have to switch to the older-and-slower backup drive. How long til we get there?"
"A few days."
"Crap. Would've been nice to get there in a couple hours. Oh, well."

edited 13th May '15 5:11:14 AM by AwSamWeston

Could be worse. Could be the Event Horizon, imagine dealing with that shit.

I have half a mind to introduce a considerably smaller ship having a reverse-engineered version that gets fired up to escape an ambush. It works but almost kills the entire crew, due to its wormhole being unstable and extremely turbulent (imagine a Space Shuttle tumbling end-over-end during reentry), coupled with their inability to turn it off because it fused all the circuit breakers shut.

One option is that the FTL can't bring you all the way to your destination so you've got to do the last leg traditionally. This adds something of a surcharge onto travel time.

The other is that the system is more efficient over long distances than short ones. Imagine if the FTL can't accelerate to it's full speed over a 10 light-year hop.

Consider that airlines typically use slower propeller planes to fly short distances, and jets to fly the longer ones. A turbo prop typically cruises somewhere around 200 miles an hour, while a commercial jet typically cruises about twice that. But it isnt the flight time that dictates which aircraft gets used- it's fuel efficiency- jets are really only economical over a certain minimum distance (depends on how many passengers they carry). So it's entirely feasible that it takes longer to get to a closer destination. Something like that might be happening in your universe, as well.

Very good. If FTL can't be used near planets, then you'll always spend a few days travelling at sub-light speed to get at a safe distance, and again on arrival. Even if short-range FTL trips take hours, your journey will still take days.

Forgot to mention something in that regard. The gate-based slow version of this verse's FTL travel indeed doesn't work in gravity wells. Gates require either a high orbit or a Lagrange point.

Something like "Antigrav Range" of 6 planetasry diameters (Star wars) so that the distorion of the FTL system doesnt fuck up the world.

the planet-range and accelerating ideas are good, and I think you could easily say that there are strict laws on FTL travel, so there's a 'road' structure - imagine interstates versus parkways. You can go really fast on an interstate but you can only go from city to other city 100 miles away - parkways are slower but you navigate nearly every single place on the way. So, shorter distances the fast lanes will just miss completely, and you have to take the slow road. Instead of a maximum speed, it probably has a maximum acceleration per light year limit.