I broke physics and now it's being obnoxious.

[Aminar]

So, for the YA Sci-Fi project I'm working on I have a psuedo-magic system based around the concept of gates(read portals from portal, but with more room to play.)  There's quite a bit of backstory behind this, and more than a little Significantly advanced tech/life is magic.(I'm probably at a 2.5 on the Sci-Fi Hardness scale, but spaceflight itself is leaning towards a 3.5(works with physics at a basic level, but I'm not going to start throwing theoretical physics around much, just account for relativistic time dilation and the like.)
Anyway here's the situation.
Ship A(The Captain doesn't like to name ships.) is running low on fuel.  They have enough to either A-Get to travelling speeds(.9C) or B, slow down from (.9C) but not both.  The ship is being pursued, and is somewhat damaged from a run in with another pirate vessel.  They took the attacking pirate vessel without major harm to it via sabotage and portals.  Now the Pirate ship, Queenswyrd, needs to tow the other ship to travelling velocity.  They'll deal with the fuel trouble of that later, but they don't have the hull integrity to pull this off(because why would you have the hull integrity to tow a ship 6 times your size to .9C)  So what I want to do is put a gate on the back of the ship, fire the magnetic grapple through the gate, and then have it come out at a 180 degree angle to itself, so that the magnet hits a forward facing wall(that is involved in bracing the engines) on the Queenswyrd.)

However, depending on the perspective I take this gets really weird.  If I go from Ship A's perspective it should be getting towed, but if you go from the perspective of the grapple it looks like the cable should be slackening, but then if the cable slackens the two ships will be getting closer together, but if that happens its getting towed.

I'm pretty sure I'm overthinking things, but I can't be certain and it's slowly driving me nuts.

[Wordmaker]

I'm not sure what ".9C" means, but I'm assuming these are starships you're talking about.

Now, most real-world physics kinda goes over my head, even more so when it comes to space and faster-than-light travel. I'm a big fan of warp speed and hyperdrives and anything that makes it easier to just get on with the story.

So this advice comes completely from the perspective of a lay person and what I'd be looking for as a reader.

I would say, just let the ship be towed without the portal. The average reader probably isn't going to worry about, or even notice, details like hull integrity and whether or not one ship can tow another. What's important is that the ship gets towed, so stick to the simplest way of making that happen.

[Haru]

I think if you consider levers and pulleys rather than portals, it might be easier. Basically, what you could do with a pulley is pull something up with less force than you'd need if you lifted it directly, but you will have to pull on a lot longer string. I believe it is antiproportional, so if you want to use half the force to lift, you'll need to pull on the string twice the length you want to lift the object. Portal-engineers probably figured out how to do something like this.
In your case, that would mean you'd need a portal-pulley that reduced the effective mass of the towed ship to a 6th, which in return would mean that for every 6 meters the pirate ship moves, Ship A would move 1 meter. You should be able to adjust this difference by portals as well.

On the other hand, you are not thinking 4th dimensionally, Marty. 

The first thing that came to mind was to create a portal loop for a ship as a runoff. Imagine a slope, and on the bottom you put a portal that leads back to the top. If you roll something down there, it is basically an endless slope, and the object will gather more and more speed as it goes along (in an atmosphere [and by the friction of the slope], that is limited, but you won't have to worry about that in outer space).
Now a slope is probably not the best thing to use for a space ship, but since we are in space, we don't need to use one. We'll just let ourselves be pulled straight towards the nearest sun, portal back a few miles and use that to increase the speed to where we want it. Then you shift the exit portal so that you are facing the direction you want to go and there you have it. You'll have to adjust your trajectory every now and again, but you won't have to use any fuel to get to the speed you want. You'll need quite a large sun to get to the big speeds in a useful amount of time, but you could use smaller ones or even just planets to gather enough speed to get to a bigger system and jump from there.

For example: You'll need 318 days in the gravitational pull of the earth to reach .9C. The gravitational pull of our yellow sun would be able to do it in 46 days. And I think our sun is one of the small ones.

[Wordmaker]

 

That went right over my head, but it sounded smart, so if that works, go with it.

[Haru]

Well, look at it this way. If you are in a car and you accelerate, your speed increases. If the rate of acceleration is constant, and you have a long enough road ahead of you, you could, in theory, accelerate to somewhere near the speed of light. Of course, in your car, you have the problem that friction on the road, friction against the air, friction in the engine itself will only allow for a limited speed.

Now the earth gravity is a source of constant acceleration. If you let a ball fall 2 seconds, it will have double the speed as if you would let it fall only 1 second. Of course the problems remain the same, friction against the air will reduce its speed, and there is a break even point known as "terminal velocity", where the force from the friction cancels out the gravitational force that would accelerate the ball. Also, you can't have it fall endlessly, at some point it is going to simply crash into the ground.

Now if you are in space, there is no air, so there is no (or far far less) friction to stop you. And with the portals, you can create an endless fall, gathering more speed with every second you spend in there. From that point on, it is just a waiting game, and how long you have to wait depends on the size of the planet/sun you use.

[Wordmaker]

Okay, that makes sense.

So are these portals the way that ships reach FTL speeds? Or do they have their own FTL drives as well? Why is there a problem with one ship pulling another along through the portal or with the power of their own FTL drive?

[Haru]

I think the issue there is kind of like pulling an 18 wheeler with a vespa.

The portals would be my idea for them to reach almost the speed of light, since it seems that's what Aminar wanted to go for (.9C means 90% of the speed of light). But you could start there and then add an FTL that requires the ship to be at a very high speed already.

[Wordmaker]

Make the pirate ship bigger/more powerful?

[Haru]

Don't mess with a physicist when he's physicing.

But yeah, that'd work, of course. Or, come to think of it, since Pirates probably want to capture ships on a regular basis, maybe they have a far more powerful engine than they'd need for their ship, so they can tow the big valuable ships to their hideout, once they disabled them.

[Wordmaker]

That would also work. And add some interesting flavour to the pirates. Maybe there's danger to cramming more powerful engines into a smaller ship and that could cause complications on the journey?

[Aminar]

The portals are related to my FTL methods(but ships don't have FTL drives.)  It's all done via use of a very big, very beyond understanding, entity that exists in multiple spaces at once(specifically around black holes it eats, but that's beside the point.

I'm just ignoring the science behind how the ships reach .9C and leaving it at They Can.  I am however counting relatavistic time dilation in as a part of the story(that the faster you get to the speed of light the less time you experience in relation to slower people.  I'm running the math for it, the characters sya they are running the math but don't mention the equations, they just give the conversion.  (IE at .9C 12 Light days of travel feels like 5 something, almost 6 days, but 13 some days have gone by for everyone else.)

(Also, the gates/Portals can only be made on a physical, mostly flat substance.

Now, as to my question, I'm massively confused about his now.  My brother mentioned Pulley's too, but I can't manage a portal pulley given the rules I've designed, so I'm switching what I did seeing I had the bulkhead strength wrong to begin with.  I just need to place the pulling point against the wall the engines push on to rely on their strength to keep the wall from ripping out.

[R4ph]

Actually, by my understanding (and I have a Physics degree, so either I'm right, or about to embarrass the crap out of myself), small ship towing big ship shouldn't be a problem at all.

1) Engine size and structural integrity limits the force you can apply, not the acceleration you can reach.

2) In atmo, that means an RC car can't pull a space shuttle, because the RC can't provide enough force to overcome friction/air resistance.

3) In space, there's no friction or air resistance. So an RC can pull a space shuttle... Really slowly.

4) What this essentially means is that if your F is constant, and your mass goes up, your acceleration just goes down. So it is easily possible to tow a huge ship to .9C, it just takes 6 times longer to get up to speed.

Alternatively, if you wanted to use the slope idea (which is super clever), you could use statites as the solid surface upon which to place the gates, that way they'd stay in place around a sun.

[Aminar]

Wait...  So the structural integrity thing doesn't apply?  Stupid Space not working how I want it to for momentary dramatic tension.  :l 
Oh well, I'll add in a line about wanting to build up speed faster to avoid pursuit, seeing they think they are being pursued.

[R4ph]

There's always running the engine beyond it's capacity (Ah can give ya warp 7 for 38 seconds, cap'n, and nae more!). Leaves the option for something to be burned out/exploded afterwards/during.

Structural integrity is unlikely to play a large factor though, unless the "tow bar" isn't able to withstand Force equivalent to the engine as maximum thrust. But that would seem unlikely, in a vessel designed to withstand the kind of G forces involved in FTL travel. Especially as magnetic grapples are involved, those seem like they'd pop off before the ship damaged.

Maybe you could have multiple grapple lines due to the largeness, to spread the load, and have some of them disconnect during acceleration? Then there's the tension (hehe) of hoping the remaining line(s) hold, with the consequence of losing their ship if the last one disconnects and the ship they're in zooms off.

[Aminar]

No FTL, but quite rapid acceleration to an enforced .9C cap.(enforced by long standing tradition and the dangers of blaa blaa blaa I said so.(I'm sure there are plenty of justifiable reasons made by the ship designers who happen to be much smarter than most of the human race put together.)
I'm honestly not looking to have things explode.  Part of the flavor I'm going for is just how careful people have to be while travelling in space.  I have other causes of tension.  As is, I'm just sticking the grapple to the wall the engine is braced against, so that the ship can accelerate fast enough for her emission trails to disintegrate before the Ships tracing a bounty beacon she set off can catch up.  Because Space Travel is frankly boring, but I, like an idiot, planned on having the whole first book be aboard ship.  Grrrr.