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In Star Wars: The Last Jedi

It's a major plot point that the Resistance must find some way to escape the First Order before their ships run out of fuel.

In space, there is very little resistance to motion, so once something starts moving, it tends to stay moving. This is why real life spacecraft barely need to burn fuel once in orbit of something. When

the rebel ships are being chased by the First Order, they appear to stay the same distance from the ships after initially moving out of effective range.

The only way this makes sense with both distance and fuel burning is if they are accelerating at the same rate. At some point I believe someone says the Resistance ships are faster (I am assuming this refers to acceleration, because neither fleet should have a terminal velocity other than the speed of light), so I suppose they were only accelerating as fast as the First Order to save fuel, and had determined they were not sufficiently fast to totally outrun the First Order.

However, I find it hard to believe that two fleets could accelerate, near the maximum capacity of one of them, for at least 18 hours and not go into hyperspace. These are ships that can traverse systems in minutes if they choose, and I find it equally hard to believe that the First Order would only use a fraction of their available power in pursuing the rebels. How then were the rebels able to run out of fuel without changing their velocity significantly?

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    Star Wars physics? – jeffronicus Dec 20 '17 at 2:54
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    ... one possible explanation, for example, would be that when traveling through real space the ships use something along the lines of the intertialess drive from E. E. 'Doc' Smith's Lensman series. (Come to think of it, that would also explain how the Falcon could get to Bespin with a faulty hyperdrive before the crew died of old age.) – Harry Johnston Dec 20 '17 at 4:25
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    Running shields to protect the Rebel ships from bombardment probably uses fuel, whether the ships were stationary, or using their engines to travel. – mrflash818 Dec 20 '17 at 4:34
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    @KaiQing I wasn't making a joke. If you are exposed to the vacuum of space, bubbles will form in your blood, causing your body to swell in size, and your lungs will bleed, but you'll have about 10 seconds before you lose consciousness and about a minute before you die. You won't freeze instantly, nor will you implode. Leia's survival was actually plausible, assuming she could stay conscious long enough to Force pull herself to safety. – Amy Dec 26 '17 at 19:20
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    @KaiQing My only point is she won't freeze instantly or implode. Not that she will be unharmed. Sorry, but the idea that she would freeze instantly (or at all) or implode is just absurd. That isn't what happens in the real world. Space is an insulator, meaning her body would heat up until she died. Not freeze. – Amy Dec 27 '17 at 22:34
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The captain of The Finalizer (General Hux's flagship) says:

They're faster and lighter, sir. They can't lose us, but they can keep out of range where our cannons are ineffective against their shields.

I think the important part is "They can't lose us", which implies that "faster" doesn't mean the Resistance fleet doesn't actually have a higher top speed. Instead, it probably means that they're able to maneuver faster. So if the First Order started gaining too much, they could turn sharply and re-establish the distance before The Finalizer could react.

These maneuvers that keep them out of range would consume fuel - in addition to the fuel needed to power the shields.

(I realize this isn't exactly supported visually in the movie - it definitely looks like they're travelling in a straight line the whole time. But if they are, it seems like the First Order would have caught up to them - or at least they'd be able to send a ship ahead of the Resistance Fleet and ambush them. The only way they wouldn't be able to do that is if the Resistance Fleet was not actually moving in a straight line.)

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    This seems like a well-reasoned answer. – Edlothiad Dec 20 '17 at 8:09
  • Thank you... I didn't realize how they could run for 18 hours without loosing the bad guys.. Your explanation is helpful, it's also explain why someone has to stay (sacrifice) on the rebel flagship at the end, instead of 'putting a brick on the accelerator' (don't tell me it was not possible). – dna Dec 20 '17 at 8:12
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    I thought "they can't lose us" referred to how the First Order was tracking the Resistance fleet. If the Resistance tried any fancy maneuvers to get the Order off their tail, they would be ineffective because the Order would still know where the Resistance ships went. – David Z Dec 20 '17 at 8:26
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In real world physics, thanks to relativistic mass increase, the faster you go, the more energy to accelerate, so I don't necessarily see why they would hit lightspeed.

However Star Wars physics seems to have many differences from real world physics anyway. In particular space battles in star wars seem like they are largely inspired by a combination of WW2 era air and naval combat, so there seems to be a fair bit of handwaving/physics different to make this work.

Presumably maintaining the shields on the rebel vessels required fuel to power them as well.

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    In real world physics, you never actually reach light speed, but you do approach it asymptotically - much like how an object falling in an atmosphere asymptotically approaches its terminal speed, though for a different reason. – David Z Dec 20 '17 at 8:24

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