Realistic fast Interstellar Propulsion Methods

[bob012345]

For the purposes of your story you could define realistic not as something you can build today but something that is actually proposed in the peer reviewed literature today as a potentially viable future technology. Here is a paper Breaking the warp barrier: hyper-fast solitons in Einstein–Maxwell-plasma theory by Erik W Lentz in the journal Classical and Quantum Gravity. Yes, the concept is not (yet) practical but SF can extrapolate to a possible future where it is.


https://iopscience.iop.org/article/10.1088/1361-6382/abe692

Here is a slightly earlier version on arXiv:
https://arxiv.org/abs/2006.07125

 

[DaveC426913]

At the risk of being a later-comer me-too who hasn't absorbed all the responses I have a concern about realism:Going on an interstellar journey to collect a fuel is an incredibly expensive way of acquiring the fuel.
You must consider the cost of the interstellar spaceship and its trip into the cost per tonne of fuel you bring back. (How much can you bring back at once? How big a tanker? How much technology to keep it metallic? You're taking all that technology on a goose chase through interstellar space?)

We have hydrogen right here. While it is true that metallic hydrogen is not everywhere, it's not going to stay metallic hydrogen the moment you take it out of its natural habitat. So you need a container to keep it at temp and at pressure. And that surely takes a lot of power.

The question is: how can it use less power to make an interstellar trip than simply recreating the temperatures and pressures here, in a "metallic hydrogen factory"? I mean, if you have the technology to mine and store high pressure high temp hydrogen, you surely have the technology to produce it from free hydrogen.

So, if the premise is "we need to go to that distant star just to mine fuel", it better be a damned exotic source of resources that cannot be gotten by any cheaper means.

And finally: Just how expensive does space travel need to be before its considered impractical to do at all?

Say you can bring back a billion cubic metres of M-H to power ten thousand ships for ten years - but it costs a billion credits for each and every fill up. Just how economical can it be to own and operate a spaceship at all?

 

[sbrothy]

At the risk of being a later-comer me-too who hasn't absorbed all the responses I have a concern about realism:Going on an interstellar journey to collect a fuel is an incredibly expensive way of acquiring the fuel.
You must consider the cost of the interstellar spaceship and its trip into the cost per tonne of fuel you bring back. (How much can you bring back at once? How big a tanker? How much technology to keep it metallic? You're taking all that technology on a goose chase through interstellar space?)

We have hydrogen right here. While it is true that metallic hydrogen is not everywhere, it's not going to stay metallic hydrogen the moment you take it out of its natural habitat. So you need a container to keep it at temp and at pressure. And that surely takes a lot of power.

The question is: how can it use less power to make an interstellar trip than simply recreating the temperatures and pressures here, in a "metallic hydrogen factory"? I mean, if you have the technology to mine and store high pressure high temp hydrogen, you surely have the technology to produce it from free hydrogen.

So, if the premise is "we need to go to that distant star just to mine fuel", it better be a damned exotic source of resources that cannot be gotten by any cheaper means.

And finally: Just how expensive does space travel need to be before its considered impractical to do at all?

Say you can bring back a billion cubic metres of M-H to power ten thousand ships for ten years - but it costs a billion credits for each and every fill up. Just how economical can it be to own and operate a spaceship at all?

I think the technical tern is "in situ resources". Just FYI. It is - of course - not a new idea. (Not that I think you were under any illusions of that kind, mind!). Sometimes these words are helpful for searching.

It seems that in order for scientists to study a concept or idea they need to appropiate it and give it a "serious" name. Like autogenic massacre (Yeh sorry but that's what popped up.)

[EDIT: Which makes me wonder, btw, how autodafé became synonymous with book-burning, but that's for another thread. I realize. Just an observation.]

[EDIT2: Possibly through Girolamo Savonarola now I come to think about it. Sorry, I'll s... up now.]

 

[BWV]

At the risk of being a later-comer me-too who hasn't absorbed all the responses I have a concern about realism:Going on an interstellar journey to collect a fuel is an incredibly expensive way of acquiring the fuel.

Well I know people who will drive across town to fill up at a station offering 10 cent cheaper gas, so perhaps the same thing applies here

 

[sbrothy]

Isn't some kind of magnetic field also required to push radiation, high energy cosmic rays and micrometeorites out of the way during travel. Luckily the Earth comes with one built in. Another kick in the go.... from the laws of thermodynamics.

If you have these stupendous amounts of energy available already, the "natural" thing to do would be to use an entire planet as a spaceship. (Which, in a sense, it already is; albeit one without a useful plotted course.)

Strapping a useful propulsion unit to spaceship Earth is where my imagination breaks down though. Gravity-assists from giant gas planets, neutron stars or black holes perhaps?

At least that way you aren't shipping monkeys in cans and getting people to go isn't really an issue (once some mad scientists decided it had to be done ("because they could" springs to mind as justification).

:)

 

[DaveC426913]

If you have these stupendous amounts of energy available already, the "natural" thing to do would be to use an entire planet as a spaceship. (Which it in a sense already is, albeit one without a useful plotted course.)

A few problems I see:

• Your energy requirements scale up with the size of your ship. Instead of moving a 10⁸tonne ship, you're now moving a 10²⁴tonne ship (that's a septiillion 10⁸tonne ships). Your fuel tanks will scale up too.
  
  Note that 99.99% of your spaceship is inaccessible to its passengers, but you're bringing it anyway.
• Instead of a well-contained interior, you now have to keep an entire planet warm without its sun.
• The blue-shifted radiation will ionize and strip away the atmo to barren rock.

 

[sbrothy]

A few problems I see:

• Your energy requirements scale up with the size of your ship. Instead of moving a 10⁸tonne ship, you're now moving a 10²⁴tonne ship (that's a septiillion 10⁸tonne ships). Your fuel tanks will scale up too.
  
  Note that 99.99% of your spaceship is inaccessible to its passengers, but you're bringing it anyway.
• Instead of a well-contained interior, you now have to keep an entire planet warm without its sun.
• The blue-shifted radiation will ionize and strip away the atmo to barren rock.

Oh yeah. The sun. I forgot about that. Well we'll "just" move an entire Dyson-sphere. I did said "stupendous amounts" right? :)

 

[sbrothy]

uh-oh.... I think this qualifies as "speculation". Forget I said anything. :P

 

[EventHorizon]

I was thinking that if you somehow "fold" the fabric of spacetime, you would get a lesser distance between two points and would take less time to go from one point to another.
Just a random idea though.

 

[Drakkith]

I was thinking that if you somehow "fold" the fabric of spacetime, you would get a lesser distance between two points and would take less time to go from one point to another.

Sure, but at that point you've gone from realistic to fiction.

 

[stefan r]

... REALLY big fuel tank...

... your five-million metric ton spacecraft ...

This measure of "big" is disappointing IMO. If your puny economy ride was parked by Neptune L5 it would need to be 50 km just to be detectable. Need to up that by 5 to 7 orders of magnitude

 

[sbrothy]

I've probably mentioned this before but the most "realistic" option we have now seems to be Dr. Zubrin's Nuclear Salt Water Rocket.

To quote Atomic Rockets:

"[...]. Zubrin then goes on to speculate about a more advanced version of the NSWR, suitable for insterstellar travel. Say that the 2% uranium bromide solution used uranium enriched to 90% U235 instead of only 20%. Assume that the fission yield was 90% instead of 0.1%. And assume a nozzle efficency of 0.9 instead of 0.8.

That would result in an exhaust velocity of a whopping 4,725,000 m/s (about 1.575% c, a specific impulse of 482,140 seconds). In a ship with a mass ratio of 10, it would have a delta V of 3.63% c. Now you're talkin..."

Also Wiki on the NSWR.