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Relativistic Spaceship

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Re: Relativistic Spaceship

#161

Post by JumpCrisscross »

dylan604 wrote: post #159
AlexAndScripts wrote: post #15

Earlier quoted context omitted.

I suppose you would also have to worry about surviving the trip - every atom in the interstellar medium would damage your ship, likely penetrating the entire way through, and electromagnetic repulsion would only work with charged particles.

Or stopping. It takes the same amount of time to slow down as it did to speed up. Presumably, you'd have to rotate 180° so you are now thrusting in the opposite direction. So at the speeds you've reached to get there in 20 years (ship time), you'd just race on by.

> takes the same amount of time to slow down as it did to speed up

Assuming you're going somewhere, you can use atmospheres and gravity to slow down. Decelerating should take less time than accelerating in practical contexts.

Re: Relativistic Spaceship

#162

Post by zabzonk »

actionfromafar wrote: post #143
zabzonk wrote: post #132

Earlier quoted context omitted.

not so much protons (which would be a problem, but could at least notionally be deflected), but atoms.

An hydrogen atom and a proton are roughtly the same thing, right?

no a proton is a hydrogen nucleus (or at least can be seen as one) and has a charge. travelling near lightspeed you would have to worry most about uncharged atoms/molecules (because of their mass) and neutrons, neither of which can be deflected.

Re: Relativistic Spaceship

#163

Post by LoganDark »

hotpotamus wrote: post #144
LoganDark wrote: post #134

Earlier quoted context omitted.

> Mass matters. Exactly. You try accelerating 200kg up to anywhere close to the speed of light (say 80%). That is a lot of force. Technically even photons can exert force on objects, but they have such a small mass that it's a difficult effect to observe.

From memory, photons are massless, otherwise they could not move at light speed. They do have momentum though.

Oh, thanks for the correction.

Re: Relativistic Spaceship

#164

Post by dylan604 »

JumpCrisscross wrote: post #161
dylan604 wrote: post #159

Earlier quoted context omitted.

Or stopping. It takes the same amount of time to slow down as it did to speed up. Presumably, you'd have to rotate 180° so you are now thrusting in the opposite direction. So at the speeds you've reached to get there in 20 years (ship time), you'd just race on by.

> takes the same amount of time to slow down as it did to speed up Assuming you're going somewhere, you can use atmospheres and gravity to slow down. Decelerating should take less time than accelerating in practical contexts.

Right, because space is known to be full of atmospheres to use to slow down. ???

Re: Relativistic Spaceship

#165

Post by MilStdJunkie »

knodi123 wrote: post #51
MilStdJunkie wrote: post #31

Earlier quoted context omitted.

One gee over the course of months is insane . Even if you converted the entire "fuel" reaction mass directly into momentum, you're still carrying tens or hundreds of thousands of tons of fuel for any remotely habitable spacecraft. And no engine has perfect mass conversion. Matter-Antimatter has been simulated to be maybe sixty percent efficient, fusion is around 17%, nuclear pulse propulsion a surprising 7%, fission…

In the book series The Expanse, that was pretty much the only unrealistic technology that the humans had, which allowed them to colonize the solar system. Sure, there was some might-as-well-be-magic alien stuff later in the book, but by page 1, humans have colonized the solar system, purely with a hand-wavy engine that can generate sustained thrust without needing tons of reaction mass and without turning your exhaus…

I'm going to embarrass myself horribly here, but I've only seen the series, so I'm not a hundred percent clear on how much acceleration they're pulling in cruise[1]. If it's something like .3 g then they're fairly close[2] to the limits of something like a fusion pulse drive (tiny nodules of deuterium or whatever, ejected out the back of the ship, then lasered into fusing, then the reaction pushes on the whatever - magnetic field, pusher planet, etc). That would make the solar system something more like what the Atlantic Ocean was in the days of sail.

They'd need refuels every stop though. Like with early coal fired steamers back in the day.

I got a funny feeling that without much better genetic muckety muck, the hard limit will be the human organism itself. Someone will get to the stars = if we don't screw everything up - but the someone won't be human, or maybe nothing like human.

[1] I know in emergencies they pull mad g, but it doesn't seem like something they keep up for long. Would still burn through their deuterium in no time though.

[2] Like, within an order of magnitude or two.

Re: Relativistic Spaceship

#166

Post by ben_w »

ninkendo wrote: post #148
ben_w wrote: post #56

Earlier quoted context omitted.

For practical values, nuclear pulse propulsion would only get you up to a few percent of the speed of light. The best nuclear thermal, gas core, would give you 7000 seconds[0]. A "dusty plasma" rocket (suspend the nuclear fuel in dust form in a magnetic field) might get 100,000 seconds, or just over a day of accelerating at 1g. [0] for the benefit of non-space nerds: the unit of measure is "lb-force seconds per lb-ma…

> which is kinda like "seconds at 1g" if the fuel is a very small fraction of the total mass, which it really won't be in a practical rocket. I think you mean “a very large fraction of the total mass”… generally the best efficiency comes if your fuel mass fraction is high , as it means there is little overhead of things in your spacecraft of things that are not fuel (the mass of the engine, etc)

No, I mean small, because I'm talking about the quality of the approximation not the way to maximise delta-v.

If you have 1 gram of fuel and a 1 ton payload and that fuel has 1e6 seconds of Isp, you can accelerate the ship for 1 second at 9.8m/s/s.

If you have 1 ton of fuel and a 1 gram payload and the same fuel and burn at 1 gram/second, the first second is mostly spent accelerating the fuel, which means you're no longer able to just approximate the Isp as "seconds at 1g" in a nice linear fashion — it starts off at 1 gee in this example, but ends up at 10^6 gee in the last moment, a million seconds later.

Re: Relativistic Spaceship

#167

Post by ithkuil »

tshaddox wrote: post #130
jcadam wrote: post #125

Earlier quoted context omitted.

Could use such a ship as a time machine (well, that only goes one way).

Do you mean that someone in the space ship could, without extending their own lifespan beyond what is already expected, use the space ship to "travel" to Earth's distant future?

Precisely. That's the famous Twin "paradox" (which is not a paradox)

Re: Relativistic Spaceship

#168

Post by EA-3167 »

slow_typist wrote: post #66
EA-3167 wrote: post #46

Earlier quoted context omitted.

It's worth saying that 1G is heavy acceleration to maintain, nothing "mere" about it. When you run the numbers the amount of energy involved basically adds up to needing a rocket made entirely of antimatter, and a similar mass of 'normal' matter to react with. Keep in mind that when talking about long-distance journeys in space under any sort of constant acceleration, the numbers are generally in thousandths of 1G.

But accelerating by 1 g is very convenient for the crew. No muscle and bone degeneration, no space sickness, etc.

It would be cheaper in every sense to turn something like a giant asteroid into a rotating habitat ship than achieve a constant 1g to even the closest systems. Realistically interstellar travel is not a thing that (biologically modern) humans will ever be suited for, robotic probes don't need thousands of kg of food and water to stay alive, don't need artificial gravity, air, or entertainment. The fact that the trip is inevitably 1-way won't bother probes and robots either.

Re: Relativistic Spaceship

#169

Post by CobrastanJorji »

A_D_E_P_T wrote: post #39

~850 years of ship time at max acceleration and the universe is 10^30 years old. At this point, only white dwarfs, neutron stars, and various stellar remnants remain. Galaxies have dissipated, and stars like our Sun are a distant memory. (If any still exist to remember them. Which is, I'll add, not out of the question. Life can cling to structures built around black holes, white dwarfs, etc., and extract energy from…

You could imagine a technically advanced society using it as a sort of penal death sentence. "You were not safe around our people, so we are sending you to the end of time."

Re: Relativistic Spaceship

#170

Post by qayxc »

throwaway11460 wrote: post #101
qayxc wrote: post #91

Earlier quoted context omitted.

The good old Bussard Ramjet. Problem is, recent calculations showed that it's not viable - even for a Kardashev Type II civilisation [1] [1] https://www.sciencedirect.com/science/article/pii/S009457652...

It's not viable to use it to visit the galactic center during a lifetime, but it still flies, if I understand that correctly?

Theoretically it works, yes, though not nearly as well as initially thought. The engineering and the required materials, however, are still questionable and way beyond our current understanding of physics and material science.

It's a similar story for solar sails, unfortunately, though progress is being made in that area. Personally, I think that's the only realistic way of interstellar travel given our current knowledge of physics and engineering.

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