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Relativistic Spaceship

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Re: Relativistic Spaceship

#51

Post by knodi123 »

MilStdJunkie wrote: post #31
AlexAndScripts wrote: post #13

I had no idea you could achieve relevant amounts of time dilation so "easily" with relatively small (1m/s) of constant thrust - such that interstellar travel becomes somewhat viable provided you can continuously accelerate for the entire trip. Googling it, it would take approximately three years of observed time for a traveller to get to alpha centauri at 1g. Could any somewhat-plausible technologies (fusion etc) get…

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 exhaust port into an unimaginable hell cannon.

Re: Relativistic Spaceship

#52

Post by LoganDark »

jvanderbot wrote: post #48
ryandrake wrote: post #27

Earlier quoted context omitted.

Sure, there are plenty of practical problems that would make the trip impossible. The fuel mass alone that would have to be shot out the back to make that trip would be something on the order of 800 million times the mass of the payload (you). So a 100kg person sitting in a 100kg spaceship would require something like 160 billion kg of fuel, assuming zero energy loss in burning the fuel. Relativistic rocket calculato…

Well that depends on the ejection velocity. If you could shoot it out at close to speed of light, you'd need much less.

If you were to shoot it out at close to the speed of light, you'd knock the Earth out of its orbit.

Re: Relativistic Spaceship

#53

Post by throwbadubadu »

layer8 wrote: post #22
Retric wrote: post #10

Earlier quoted context omitted.

Blue shift, it’s no longer radio waves if you’re traveling fast enough.

Of course it’s not microwaves anymore, but it’s still the CMB, that is, decoupling/recombination, not the big bang. Unless you count the CMB as the big bang.

Its remnant, its echo, or just the big bang, however you want to see it.

Re: Relativistic Spaceship

#55

Post by JumpCrisscross »

ianburrell wrote: post #32
JumpCrisscross wrote: post #21

Earlier quoted context omitted.

> Could any somewhat-plausible technologies (fusion etc) get us to that point? Nuclear thermal or nuclear pulse propulsion. No fusion needed.

Nuclear thermal or pulse, fission or fusion, aren't enough for interstellar relativistic speeds. They have way too low specific impulse. They don't even have enough for constant acceleration inside the solar system unless the acceleration is really low or the fuel ratio is huge.

> unless the acceleration is really low or the fuel ratio is huge

You’d need kilotons of nuclear fuel and a 10^3 fuel ratio. But that’s plausible.

The economically-plausible answer is antimatter, where the ratio stays in the single digits and starts permitting deceleration. But I wouldn’t call that technologically plausible at this time.

Re: Relativistic Spaceship

#56

Post by ben_w »

JumpCrisscross wrote: post #21
AlexAndScripts wrote: post #13

I had no idea you could achieve relevant amounts of time dilation so "easily" with relatively small (1m/s) of constant thrust - such that interstellar travel becomes somewhat viable provided you can continuously accelerate for the entire trip. Googling it, it would take approximately three years of observed time for a traveller to get to alpha centauri at 1g. Could any somewhat-plausible technologies (fusion etc) get…

> Could any somewhat-plausible technologies (fusion etc) get us to that point? Nuclear thermal or nuclear pulse propulsion. No fusion needed.

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-mass of fuel", 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.

Re: Relativistic Spaceship

#57

Post by knodi123 »

LoganDark wrote: post #52
jvanderbot wrote: post #48

Earlier quoted context omitted.

Well that depends on the ejection velocity. If you could shoot it out at close to speed of light, you'd need much less.

If you were to shoot it out at close to the speed of light, you'd knock the Earth out of its orbit.

aim a bit to the left.

Re: Relativistic Spaceship

#58

Post by pc86 »

Teever wrote: post #40
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…

What if you didn't carry the fuel? Use stationary lasers from your starting point to power your journey half way until you lasers from your destination to slow your craft down.

Of all the dangers associated with spaceflight it's a pretty interesting concept to add "lasers at the origin shut off for whatever reason" and "lasers at the destination never turn on / are misaligned / turn on too soon / turn on too late" to the list. Not sure I'd want to be in a spacecraft careening through the cosmos with no realistic way to slow it down.

Re: Relativistic Spaceship

#60

Post by robbomacrae »

zonkerdonker wrote: post #36
robbomacrae wrote: post #20

This is something I've wanted to try in VR one day. Does anyone know if there is an app/game available for one of the popular headsets? Is there a technical limit to how realistic the effect could be?

Not exactly what youre looking for, but this type of game (using relavistic physics) has been attempted. Heres one built in unity that could be easily ported to a Quest line device. https://en.m.wikipedia.org/wiki/A_Slower_Speed_of_Light I also found this render (and game!) that show off some aspects of relativistic spaceflight. https://youtu.be/Ix1XlxF66Zk?si=8HzDH3U-CRM-erRB I think the technical limit might be moo…

Thats great! I wonder if the stomach issues would be eased if you were sat in a chair like captain kirk looking through a big panoramic window.
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