Let's Talk About Near-Lightspeed Travel

[quasarrgames]

Wishing to make a little novella set a few thousand years in the future where interstellar travel is comon. In the hopes of using time dilation as a plot device, I want to make some ships capable of travelling at close to c with a constant 1g acceleration, while staying within the realm of possibility. I was hoping you guys could help to make sure that the propulsion concepts i was planning to use were still scientifically accurate, for the most part.

To solve the "where did you get all that antimatter?" hole, I know that the collision of two specific kinds of white dwarves can generate lots of antimatter. Since fusion reactors somewhat replicate the conditions inside a star, it seems feasible that a very powerful, unstable fusion reactor could produce antimatter. So what if the reactor destroys itself in the process! You have antimatter! Let's just assume it's a very expensie fuel source.

The only somewhat cheaty thing i plan to do is give humanity thermocouples that are close to 100% efficient. I'd prefer not to have to deal with hundreds of kilometres of radiators. 

Now on to the ideas:

Antimatter- So i know that typical matter-antimatter collisions produce exhaust velocities of about 0.33c. Too low for me. I also know that pure electron-positron annihilation produces velocities closer to 0.56c, which is what i need. Is there any possible way to magnetically store electrons on their own, or to supercharge normal matter with hundreds of electrons apiece and store that? (don't mind if the magnetic storage takes crazy amounts of energy (thermocouples!) or if some of the fuel decays over the two-year transfer time due to quantum tunneling).

Also, what are the actual equations that are used to determine the destructive capabilities of antimatter? 

 

Bussard Ramjet- I know that these supposedly have a "terminal velocity" of 0.2c, but this assumption seems to imply that the hydrogen is sped up to roughly the same speed as the ship before being fused. I was planning to use a series of magnetic wireframe nozzles a couple hundred kilometres across placed a few light minutes at maximum in front of the ship to ionize the incoming hydrogen (using beamed power from the ship) and nudge it into the ship's reactor. These nozzles could also use the ionized hydrogen to propel themselves at the same pace as the rest of the ship.

 

Light Sails- Probably incapable of near-lightspeed travel, but maybe a cheap way to transport cargo at reasonable speeds. A ship could use an engine to get close to a star, and then unfurl a solar sail at perihelion. A quick calculation based on the amount of power the earth gets tells me that a circular 100km sail placed 6.4 million km from the sun (the closest we can get a spacecraft with our current technology i believe) would be hit with about 3.0*10^16 watts of energy (give or take an order of magnitude). Is this reasonable?

 

Black Holes- Could a black hole be used as a gravitational slingshot to get ships close to the speed of light?

 

Thanks for reading this.

[p1t1o]

Well you have a lot of rules here like “I don’t want that really difficult thing that physics needs, how can I not need it?”

If you can manipulate “crazy amounts of energy” without qualms about detail, then its difficult to know how much restriction you are happy to accept from real physics.

You may have to make some sci-fi hand-wavy concessions to tailor your universe to your desires.

This is ok, it is very difficult to write a “pure science” sci-fi novel and not have it be quite tough reading.

Obviously, visit project rho for plenty of relevant detail.

Here are some pertinent points though:

Antimatter:

Storing “pure electrons” or other similar-charge carriers is extremely problematic, the repulsion is immense.

Relevant: https://what-if.xkcd.com/140/  (basically, if you put a colossal amount of antimatter [and matter] in one place, you get a titanic explosion, put a colossal amount of electrons [and nothing else] in one place and you might wipe the universe. Maybe. Most of physics breaks down if you use enough of them.)

But if you hand-wave away the storage, you can squeeze a ludicrous amount of energy into it, and just by releasing the containment, the carriers will be ejected with extreme force.

Equations for destructive force: well of course E=Mc2. To properly deal with the physical effects of such a weapon, of course the science get s very complex very quickly. Mainly because of the different types of energy released and the different effects they will have on the target. I think it is generally accepted that using E=Mc² to get an energy figure, and converting that into a TNT-equivalent-yield (you know, megatons, kilotons etc.), gives you an illustrative, if not precise, idea of the forces released.

But it gets a little more complex (a lot more complex) because the energy is released in many different forms. Depending on the reaction, much of the energy can be carried away by uncharged particles, EM radiation or neutrinos, which are very difficult to utilize for thrust, because you cannot direct them or because they do not carry very much momentum.

 

Bussrd Ramjet: You get nothing for free with a ramjet, that “nudging” will consume energy which will equate to drag. The maintenance of a huge wirseframe nozzle whole light-minutes ahead of your ship via beamed power will consume energy which will equate to drag. Compressing hydrogen that is whizzing through your engine at relativistic velocity will be problematic.

All in, you can hand-wave away some of the drag and justify (within your own universe) an improvement on the 0.2c, but getting it up to high relativistic speeds where time dilation is significant (lets say 0.7c and higher, as a minimum) is stretching the improvements quite far.

 

Solar sails – IF you combine these with large, high power laser installations at departure and arrival, probably your best bet for realistic high-c travel. The Advantages of not carrying around your energy source and reaction mass are huge.

  

Black Holes – Yes, but only if the black hole itself is moving at high relativistic speed. The speed increase you get is not proportional to the mass of the body, but to its speed relative to you. You cant, for example arrive in a black hole system and use it to stop. But you can use it to change direction. Or if the hole was passing through another system at high speed, you could use it to help brake into that system.

HOWEVER, if the hole is spinning rapidly (VERY rapidly, this can happen apparently, accordin to theory) then it may be possible to extract momentum from the ergosphere, where space is “spun” or “twisted” by the hole. Less like a slingshot, more like jumping onto a record player. Depending on conditions, you could get a significant boost, yes.

 

TL;DR – what you want, a constant 1g acceleration essentially for unlimited time, is a very, very tall ask. Even using antimatter as an energy storage medium will only get you so far with real physics.

Bonus FunFactTM - if you could have such a drive, with unlimited fuel, you could explore the entire observable universe in just a few centuries of “ship-time” due to extreme time dilation.

 ProTip – write the story anyway, but do not go into detail about the “impossible/implausible” parts. Alastair Reynolds has some books out at the moment with almost your exact premise – a group of individuals who spend their time travelling the galaxy at high relativistic speeds, outliving multiple civilisations as a matter of course due to time dilation. He deosnt go into detail about the propulsion, but they are great stories none the less.

 

**edit**

Thought of something else - If there was a planet/body/mass orbiting a black hole, close enough to have a relativistically-relevant orbital velocity, you could slingshot around that, and get a goodly speed boost. However, I am not sure if any single body of mass can exist that close to a BH, maybe easier if it was a very, very large BH (which still [naturally] have very strong gravity near the event horizon, but the gradient can be very low, so it wont rip you apart. As long as it doesnt have an active accretion disc that is.)

Edited September 5, 2017 by p1t1o
I really wanted to type "wipe the universe"

[Nuke]

for your electron containment issue, look to bussard's other invention, the polywell. a fusion concept that only tries to contain the electrons so that ions have a well to fall into, since all ions are crossing the center it really does wonders for your fusion rate. i suppose you can also use it for an electron storage system, but wouldnt it be easier just to generate them on the fly either chemically or with an electron gun for your propulsion needs? polywell should also work for containing positrons provided there isnt a better trapping scheme available.

[Bill Phil]

Thermocouples can't help you here. They operate off a temperature gradient. A hot side and a cold side. The heat doesn't just go away. It's stuck, and the only way you can get rid of it is by radiating it away. Even if thermocouples could help, there's so much energy that even extremely high efficiencies will still leave enormous amounts of heat in the ship. 

But I think you could get away with hot radiators, something in the many thousands of kelvin. This is super technology, but you have that anyways. In any case, I think they could gain access to the required super materials. 

http://www.projectrho.com/public_html/rocket/thermodynamics.php

[wumpus]

Anti matter: your best bet.  If you are that worried about the exhaust velocity, I suspect you haven't worked through the rocket equation and how it applies to relativity (i.e. outrunning your exhaust velocity isn't all that hard, New Horizons did it).  If you don't mind your characters making anti-matter, an Orion-type spacecraft is an obvious near-c device.

Bussard Ramjets: I think they've been proven not to work, although that likely assumes using the material for fusion.  You might get such a beast faster with using antimatter to heat up the hydrogen, but that is likely complication for the sake of complication.  Stick to antimatter orions.

Light sails: lots of practical problems with this one, but probably easier than making antimatter.  The catch is that you likely have a brief window to supply all the power, and your sails have to be virtually *perfectly* reflective.  Note that the only way any other engine doesn't have this problem is by doing most of your acceleration well beyond the star system you are leaving (simply because it takes longer to get to speed: check how far away from a star you would be to get to .1c while accelerating at 1g.  That's how far your lasers have to focus.

Black holes:  "Slingshot" maneuvers should really be thought of as "bouncing off".  While a black hole might have virtually infinite momentum, I doubt you can use a slingshot to accelerate any faster than it is going.  The mass ratio of an existing probe to Jupiter might as well be infinite, but you don't see them accelerating by high multiples, likewise I don't expect that black holes would give any more advantage than Jupiter (unless you happen to find one rotating around another at relativistic speeds (if this is possible the tidal forces would be a major issue), this might make a great "base" and launch facility, but you still need to slow down somehow (assuming you survived the tidal forces).

I'd simply stick with the anti-matter.  Orion or classic "rocket" design, either will work (presumably a Bussard Ramjet works to supply the matter for antimatter, this does wonders for the rocket equation).  Remember, hard SF is called hard for a reason, and part of the reason "the golden age of SF is ten" is because you don't see the errors.

[Spaceception]

Black hole starships could work too, using miniature black holes could propel a starship to .1c in 20 days. I don't know if they could get to relativistic velocities, but you could always tweak the rules, it is sci-fi

Antimatter may be your only way to reach relativistic velocities without having to bend anything too much, but it would require a lot of it.

Below are 2 pictures, tell me if they don't show, but they should.

https://drive.google.com/file/d/0B2oaCxNoSFNQVG5HUmRXMk1fYzQ/view?usp=drivesdk

https://drive.google.com/file/d/0B2oaCxNoSFNQNzVqT19MX1U1eVk/view?usp=drivesdk

 

The mass:ratio one shows you how much antimatter/matter you would need for every kilogram of payload. For example, if you had a mass ratio of 1.2, you would need .6 kilos of antimatter, and .6 kilos of matter to get 1 kilo of payload to .18c.

The gamma factor one, shows how much time would pass depending on your velocity, so traveling at .6c means time will pass 1.25x faster relative to the universe.

Pics are from the book Wizards, aliens, and starships.

(Did I get anything wrong?)

Edited September 5, 2017 by Spaceception

[NSEP]

You can't have a black hole slingshot unless you get to one! And if you are making one yourself, here is a tip: make sure you don't suck up and destroy your spacecraft or homeplanet, or make it unnoticeable and puny.

Maybe you could use a negative mass 'slope' to go forwards and accelerate. (If negative mass exist and can be produced)

Edited September 5, 2017 by NSEP

[sevenperforce]

Off-topic, but I just had the thought of two co-orbiting black holes rotating perpendicular to their orbital plane, creating an omnidirectional relativistic slingshot for anyone brave enough to plummet into their ergospheres.

Never mind the frame-dragging yuck that would happen.

I second the idea of antimatter orions, especially if you do antimatter-catalyzed fusion bombs. Those are super lightweight and require far less antimatter than the various alternatives.

For fine adjustments you could consider something like a fission fragment drive.

[HebaruSan]

17 hours ago, quasarrgames said:

in the future where interstellar travel is comon.

Everyone wants to jump directly from the current "near-impossible, totally impractical" to "common."

I'd expect a long (possibly indefinite) period when interstellar travel was achievable but rare, with departures and arrivals as historically memorable as a world war. Narrative-wise, I'd consider that kind of world an under-exploited domain of sci fi. The universe is intimidatingly big; let it be big! It's OK if we personally can't reach the end of every distant frontier ourselves. That just means there will still be adventures for our great great grandchildren to have.

Quote

In the hopes of using time dilation as a plot device, I want to make some ships capable of travelling at close to c with a constant 1g acceleration, while staying within the realm of possibility.

That's going to be very expensive no matter how it's done. Even for a Type II civ, you're talking about enough energy to complete many other megaprojects. Do the characters and their civilizations have important reasons to expend the necessary money (energy) on such travel?

Quote

Bussard Ramjet- I know that these supposedly have a "terminal velocity" of 0.2c

I believe the limit is 0.12c; double check that if it becomes relevant to your story. And the workarounds don't work; however you're accelerating the inputs, you ultimately need to expend energy doing it.

Quote

transport cargo at reasonable speeds

Call me a killjoy again, but cargo? On interstellar routes? What mere substance or product would be cheaper to transport interstellar-ly than to harvest or manufacture in a local solar system? Won't we find iron, gold, uranium, etc., in the same general relative abundance wherever we go?

Edited September 6, 2017 by HebaruSan

[Nuke]

8 hours ago, NSEP said:

You can't have a black hole slingshot unless you get to one! And if you are making one yourself, here is a tip: make sure you don't suck up and destroy your spacecraft or homeplanet, or make it unnoticeable and puny.

Maybe you could use a negative mass 'slope' to go forwards and accelerate. (If negative mass exist and can be produced)

curious if mach effect could create negative mass for at least a fraction of time (think duty cycle). mach effect thrusters are the thing that em drive fanatics should be paying attention to but arent for some reason. the idea is that mass fluctuates (specifically the mass of energy storage devices) and that you can get free thrust from oscilating that mass at the same frequency as its natural mass fluctuation. i have the hunch that if you use a very low density material that you can make the mass appear negative for a small amount of time provided that the amplitude of the fluctuation is bigger than the mass. i considered this as a potential negative mass source for alcubierre drives.  because what good is limitless delta-v if you cant break the light barrier. wouldnt technically break conservation of momentum because a massive object needs other massive objects for that value to mean anything, essentially its stealing momentum from the universe.

Edited September 6, 2017 by Nuke

[p1t1o]

8 hours ago, HebaruSan said:

Call me a killjoy again, but cargo? On interstellar routes? What mere substance or product would be cheaper to transport interstellar-ly than to harvest or manufacture in a local solar system? Won't we find iron, gold, uranium, etc., in the same general relative abundance wherever we go?

Thats easy.

Art. Information (news, communications etc.) People (slaves, passengers, clones). Foodstuffs. Animals and other organisms (especially those non-native to the destination).

[kerbiloid]

On 05.09.2017 at 4:46 PM, quasarrgames said:

a few thousand years in the future

This makes things much easier.
Post-human hivemind(s) will enjoy fast, 500-years lasting, interstellar journeys between the closest stars.
(Optionally - keeping their "crew" human bodies in vats, taking them out from time to time to use. Like a KSP player with Kerbals and DeepFreeze mod).

500 years * 0.01 c = 5 ly

Every 500 years they will have a short, 50-years, stop to refuel, have a rest and grow up a new crew instead of depleted.

15 hours ago, HebaruSan said:

What mere substance or product would be cheaper to transport interstellar-ly than to harvest or manufacture in a local solar system?

Spoiler

Water. ET invaders usually occupy the Earth for its water.

1. Antimatter and exotic matter from a neutron star or a white dwarf magnetosphere.
Like the antimatter magnetic scoop in KSPI, but huge and inside much stronger fields.

As the nearest industrial neutron stars are 400+ l.y. from here, they can be used only by an adult, experienced civilization.
So, by the post-human hiveminds without any doubt. For them, 400 ly is a several thousand years trip. Like in age of sails.

But we can take a look at Van Maanen's star.
Just 14 l.y. from here, cold (4000 K), but still a white dwarf, probably with enough strong magnetic field to make the mining meaningful.

2.  Hypernuclear materials (if some of them can be enough stable to use) - chemical elements expanding the periodical table. (Say, dilamda-3-antixi-5-carbon nanotubes to build paper-thin UFO hulls).
Obviously to be produced near a neutron star or a white dwarf.

3. Special products requiring myriads of nanobots to produce, to avoid the greygoocalypse.
In case of leak, let them eat a planet near another star, instead of Solar System.

4. Pure rare metals being mined with antimatter blasts.
Mining antimatter (see above), blasting into dust asteroids and planets, gathering and filtering the dust to extract myriatonnes of pure cesium, sending it to the Solar System.
(Instead of those puny beltalowdas with their pickaxes and shovels).

P.S.
When I finished writing this, now I no longer understand, who needs those dull useless alphacentauri and epsiloneridani.
Screw the proxima, go Van Maanen!

P.P.S.
Bitcoins. Just imagine how many bitcoins can be mined in a white dwarf (say, Van Maanen's) magnetosphere and transmitted to the Solar System with a light ray.
Word "money" will be replaced with "maaney".

Edited September 6, 2017 by kerbiloid

[quasarrgames]

On 05/09/2017 at 9:57 PM, HebaruSan said:

Everyone wants to jump directly from the current "near-impossible, totally impractical" to "common."

I'd expect a long (possibly indefinite) period when interstellar travel was achievable but rare, with departures and arrivals as historically memorable as a world war. Narrative-wise, I'd consider that kind of world an under-exploited domain of sci fi. The universe is intimidatingly big; let it be big! It's OK if we personally can't reach the end of every distant frontier ourselves. That just means there will still be adventures for our great great grandchildren to have.

Very good point. Perhaps i should have clarified what i meant be "common".

This would be a future where most humans had uploaded their consciousnesses into machines. People could engage in "interstellar travel" uploading their consciousness across the vast black to a new world, all they'd need is a powerful radio broadcaster/reciever.

However, you'd still need interstellar fleets for creating new colonies and for physical enforcement. How else would you fight back against alien agressors, or reclaim mutinous colonies that stopped communicating with other worlds? And what of the few flesh-and-blood human colonies scattered throughout your domain? How do you keep them in check?

That's what the story would be about. The physical humans trying to maintain and control the tangled interstellar web they call society.

Obviously having near-lightspeed travel would prove an invaluable advantage. If one empire sends their battleships to a star 10 lightyears away at 0.05c, and another empire sends them at 0.1c, then the second empire has 100 years to weaponize their destination and repel others. Thus, empires would be in constant competition to create the fastest ships possible.

On 05/09/2017 at 9:57 PM, HebaruSan said:

That's going to be very expensive no matter how it's done. Even for a Type II civ, you're talking about enough energy to complete many other megaprojects. Do the characters and their civilizations have important reasons to expend the necessary money (energy) on such travel?

I believe the limit is 0.12c; double check that if it becomes relevant to your story. And the workarounds don't work; however you're accelerating the inputs, you ultimately need to expend energy doing it.

You weren't kidding. I didn't realise just how much energy it would take until actually doing the calculations. Accelerating a small aircraft carrier to 99% of the speed of light would require as much energy as the ENTIRE EARTH gets from the sun in A DECADE!

Holy hannah that's a lot of energy. I now see why everyone's so skpetical about near-lightspeed travel.

However, i'm still keeping hope alive that we'll find a way.

And about the Bussard Ramjet, that estimate seems to require a LOT of assumptions. In all the designs i've seen the ramjet deflects incoming hydrogen into its exhaust nozzle at angles of 45 degrees or more. That seems like a lot of energy wasted. If you simply put the collector far in front of the ship and deflected the incoming hydrogen into your engine by only a fraction of a degree, you'd save a ton of energy and drag. Combine that with a low-drag scoop like the one designed by Cassenti, and cause the fusion to happen by inertial confinement as it passes through the bulk of the ship instead of being caused by the shear compressive forces of the hydrogen striking the ship, and the design seems a lot more probable to me.

Then again, i'm not an astrophysicist, just saying what seems logical to me. Sorry if it comes off as ignorance.

 

On 05/09/2017 at 10:22 AM, p1t1o said:

Thought of something else - If there was a planet/body/mass orbiting a black hole, close enough to have a relativistically-relevant orbital velocity, you could slingshot around that, and get a goodly speed boost. However, I am not sure if any single body of mass can exist that close to a BH, maybe easier if it was a very, very large BH (which still [naturally] have very strong gravity near the event horizon, but the gradient can be very low, so it wont rip you apart. As long as it doesnt have an active accretion disc that is.)

oooh now i like THAT idea!

 

20 hours ago, kerbiloid said:

This makes things much easier.
Post-human hivemind(s) will enjoy fast, 500-years lasting, interstellar journeys between the closest stars.
(Optionally - keeping their "crew" human bodies in vats, taking them out from time to time to use. Like a KSP player with Kerbals and DeepFreeze mod).

500 years * 0.01 c = 5 ly

Every 500 years they will have a short, 50-years, stop to refuel, have a rest and grow up a new crew instead of depleted.

  Reveal hidden contents

Water. ET invaders usually occupy the Earth for its water.

1. Antimatter and exotic matter from a neutron star or a white dwarf magnetosphere.
Like the antimatter magnetic scoop in KSPI, but huge and inside much stronger fields.

As the nearest industrial neutron stars are 400+ l.y. from here, they can be used only by an adult, experienced civilization.
So, by the post-human hiveminds without any doubt. For them, 400 ly is a several thousand years trip. Like in age of sails.

But we can take a look at Van Maanen's star.
Just 14 l.y. from here, cold (4000 K), but still a white dwarf, probably with enough strong magnetic field to make the mining meaningful.

2.  Hypernuclear materials (if some of them can be enough stable to use) - chemical elements expanding the periodical table. (Say, dilamda-3-antixi-5-carbon nanotubes to build paper-thin UFO hulls).
Obviously to be produced near a neutron star or a white dwarf.

3. Special products requiring myriads of nanobots to produce, to avoid the greygoocalypse.
In case of leak, let them eat a planet near another star, instead of Solar System.

That sounds interesting as well. Ha, what about blasting a neutron star with antimatter and harvesting pure neutronium that flies off it? (silly idea, but i wonder if it would actually work...)

 

On 05/09/2017 at 2:08 PM, Bill Phil said:

Thermocouples can't help you here. They operate off a temperature gradient. A hot side and a cold side. The heat doesn't just go away. It's stuck, and the only way you can get rid of it is by radiating it away. Even if thermocouples could help, there's so much energy that even extremely high efficiencies will still leave enormous amounts of heat in the ship. 

But I think you could get away with hot radiators, something in the many thousands of kelvin. This is super technology, but you have that anyways. In any case, I think they could gain access to the required super materials. 

http://www.projectrho.com/public_html/rocket/thermodynamics.php

What about channeling the extra heat into a small amount of secondary reaction mass?

 

On 05/09/2017 at 2:54 PM, Spaceception said:

Black hole starships could work too, using miniature black holes could propel a starship to .1c in 20 days. I don't know if they could get to relativistic velocities, but you could always tweak the rules, it is sci-fi

Antimatter may be your only way to reach relativistic velocities without having to bend anything too much, but it would require a lot of it.

The gamma factor one, shows how much time would pass depending on your velocity, so traveling at .6c means time will pass 1.25x faster relative to the universe.

Pics are from the book Wizards, aliens, and starships.

(Did I get anything wrong?)

That's an interesting idea too. A kind of black hole battery. And i was expecting time dilation to be much more drastic. Oh well. Using multi-decade sleeper ships works too.

 

P.S. It's strange to me how new estimates for the energy requirements of alcubierre drive travel are SMALLER than the energy estimates for near lightspeed travel.

[Spaceception]

@quasarrgames Yeah, but if you had a very theoretical drive (Maybe one that sapped energy from another universe, though, haha, that probably wouldn't work), and were able to constantly accelerate at 1g forever, you could get to the Andromeda galaxy in 25 years, and the other side of the galaxy in a human lifetime. Although billions upon billions of years would pass outside the ship.

Edited September 7, 2017 by Spaceception

[ChrisSpace]

On 6 September 2017 at 11:57 AM, HebaruSan said:

What mere substance or product would be cheaper to transport interstellar-ly than to harvest or manufacture in a local solar system?

Alien technology/materials that can't be reproduced by humans? Small portable wormholes connecting different points in the galaxy?

[Cunjo Carl]

@quasarrgames

You've got a novella's worth of reading in this thread already, but I figured I'd add my two cents. First, it's awesome you're taking on the challenge of writing and taking the time to do some good solid world building for it. Using our present science as a tool to help build your world is a great idea, and I like that you're aiming to use it constructively without trying to get mired in the engineering concerns too heavily.  Funnily, just like in today's life, in scifi typically the only people who understand how things work are a handfull of salty engineers. Other folks have other priorities, and if the story follows them instead it may not need to dive too deeply into the specifics of how things do what they do.

Regardless, we got you covered for engines. There's a few options that should meet your story requirements, sound rad and be realistic enough to be interesting. Before diving into it though, let's talk briefly about what a rocket does:

1. Store or collect energy
2. Convert that energy into a useable form
3. Use energy to convert mass into momentum

Using the same 1.2.3. to refer to these aspects we can look at some modern systems:

Spoiler

 

In a traditional rocket, that could be:
1. Stored chemical fuel
2. Burn it to create insane pressures
3. Use the choke and nozzle to direct an exhaust stream straight back at high speeds.

With ion thrusters, that could be
1. Collect sunlight
2. Convert sunlight into electricity
3. Use electricity to launch a Xenon plasma straight back at high speed.

 

Each of these three aspects has a value that determines how awesome your rocket can be.

1. Stored energy -> Specific energy J/kg
    Gathered energy -> Specific power W/kg
2. Energy conversion -> efficiency (unitless, and normally relative to Carnot efficiency)
3. Mass to momentum -> Isp (s)

Something very wondrous happens when you have enough energy available (as we might in the far future). Every kind of rocket becomes functionally equivalent! Because of relativity, if you store enough energy it'll always have a mass following e=mc2 and so always has a specific energy of c² at maximum. Assuming good conversion, no matter how we convert this energy to momentum it'll always make an Isp of c/g₀. As a fun example, if we're ejecting a proton at near the speed of light, and we push all the energy from a 1gram matter/antimatter annihilation into it, that proton will become 1gram heavier! The net result is we've taken 1gram off the ship and ejected it out the back at light speed. Alternatively, if you use the same energy to heat something up to white hot and launch photons out the back (photon rocket) you'll get an identical amount of momentum. This is called the Energy-Momentum relationship. That's kinda cool, so let's look at a handful of options which are at or near this hypothetical limit. *

* Naturally there may be stretches and gaps in technical abilities. If I had all the answers for building an interstellar corvette, I'd totally go do it instead of write about it! That said, armchair physics is fun.

Stored momentum rocket:

Spoiler

 

1. Spin up charged particles in a pair of large rings (LHC style). As you continue to load energy into them, the particles will stop having their normal rest mass, and will start becoming heavier and heavier according to e=mc².  Perhaps we could take the particles up to 0.95c, where they'll have 3x their original mass. This is actually very slow compared to modern day systems, but of course our future ship will have a great many more particles. Because they're going in circles, the particles will also release synchrotron radiation which would need to be reabsorbed as energy and fed back into the system constantly (using your thermoelectric generators perhaps). The exact nature of the synchrotron radiation depends heavily on the particle energy and size of the rings, but most of it would be in the form thermally absorbable photons in any case. As an additional wrinkle, in today's systems, there's also 10x the particles' energy stored in the magnetic fields required to keep the particles spinning, but hey... it's the future.
2. The energy's already in a useable form, being high velocity particles.
3. Simply redirect some of the particles out the back of the ship. Provided you have two rings with counter-spinning particles, you can do this without inducing a torque on yourself.

Because this would be easy to throttle, I think it would be a fun way to have an 'impulse drive' for high TWR maneuvers. Also, the particles don't need to be highly relativistic for it to work. You can have them at .6c to allow for rings much smaller than the LHC! Of course your Isp goes down accordingly, but eh. Look up 'particle accelerators' for more. Funnily, in modern systems, the mass of the particles is probably 20-25 orders of magnitude less than the accelerator's mass so TWR is a bit dismal , but again, in the future this could change radically!

All in all, I think this is the most simple and tenable system to write about. Your characters could 'gas up' using large scale Hydrogen fusion at any convenient icy asteroid with a mass ~10,000x the ship's mass. The system doesn't involve anything too exotic.... Besides miles of superconducting wire and megawatt switching electronics that is

 

Positron rocket:

Spoiler

 

  1. Positrons can be made in a few ways including sufficiently intense fields of light, or intense electron collisions both of which are kinda neat. I think even in the far future they'd be a trick to store, but you could use them to make a stream of antimatter positrons (+ matter electrons) and launch them in the direction you want to travel. Then, your ship can follow the stream while using these particles for energy to accelerate the halfway there, and decelerate the halfway near the destination. It'd require some astronomically precise aiming, focusing and accelerating, but it would provide a convenient excuse for your characters to stop off in star systems every few dozen parsecs to make a new stream. The positrons + electrons in the stream could be brought to speed with the ship and 'caught' with huge frisch grids and  electrostatic optics, and then guided around the outside of the ship to the back.
 2. Conversion: Positrons could be guided  and focused into a cavity in a thin walled chunk of Titanium. They would convert (annihilating with an electron from the metal) into a pair of gammas (high energy photons), which would escape the titanium and fly out in random directions. The Titanium would then recuperate lost the electron by absorbing one that was in the beam through a similarly focused electron beam or a superconducting wire... I doubt real titanium would stand up to this treatment, but it's a good stand in for some futuristic material that has amazing conductivity, low Z, and excellent refractive properties.
 3. The gammas can be caught with a 2m sphere of a billion exquisitely tiny hair-like (60um) fiberoptic light pipes coated with Platinum. These could slowly bend and redirect the photons straight out the back of the ship to work as an intriguing form of photon rocket. The curvature on these light pipes would need to be several meters, or alternatively their diameter much smaller.

To make energy for this positron beam, you'd need to do something along the lines of fusing all the hydrogen in an ice asteroid ~10,000x the mass of your ship. Given its size the fusion reaction would probably need to occur in free space (perhaps using magnetic confinement).

The rate of positron absorption at 1g acceleration would be 3*10^-8 times the mass of your ship per second or 1kA/kg... That's a lot . Electron positron cross section in freespace is pretty small, so we can wave away significant problems with 'premature anhiliation' , but the trail would still sparkle with telltale 511keV light for others to see. Intrigue!

 

 

Blackhole photon rocket:

Spoiler

 

1. Keep a small blackhole (3 million metric tons) perhaps suspended in a magnetic field behind the ship?
2. The black hole will be constantly radiating erroneously high energy gamma rays and exotic particles (often short-lived).
3. If the black hole is held out the back end of the ship, its radiance could be caught and thermalized in an exceptionally refractory material (hopefully ~5000K) The heat would then get transferred to large and white-hot sails with carbon on one side and some polished white-body refractory (4000K) on the other. The carbon side of the sails would emit white light photons and create momentum as a photon rocket. If my sums are right, to achieve 1G acceleration on a black hole, we'd need ~30TW of photon emission or about a square kilometer of carbon film at ~4000K. More likely, the sails would be several square km. That's big! This all assumes the ship is fairly light relative to its black hole.

Interestingly, as the black hole evaporates it gets dramatically hotter! Its radiance goes up with the inverse square of its size, so if you think in terms of the rocket equation, by the time you've gone 2c of deltaV, your black hole will be ~8x more intense!  This could be a cool mechanic where your characters need to push their rocket farther than they intended and the excess power pushes them to uncomfortable accelerations while heating up the ship and punching holes in their sails. Then, before everything goes wahoony shaped they have to eject their warpcore! I mean... black hole. *ahem*.... Anyways, to prevent this people would probably do their best to keep the black hole fed, and how this would be done is pure speculation. Maybe shoot gammas back at it? The little sucker would be smaller than a nucleus and impossible to get normal matter close to due to its radiance. Also, I neglect neutrinos for these calculations. You may need to invoke a technomagical device that suppresses their formation, or alternatively a.... 1.5x smaller black hole with an Isp ~.5 - .8c/g₀ would also do. The numbers are tricky on this.

 

 

AntiHydrogen rocket:

Spoiler

 

1. Store huge quantities of antiHydrogen using magnetic confinement. As a gas the volume would necessarily be quite large, but as an alternative, there's some preliminary papers out there showing that you can make metastable Hydrogen metal, which could allow you to store larger masses of antiHydrogen in a normal space. Once again, you would use magnetic confinement, but for the metal it would be an entirely different and more efficient mechanism! Antihydrogen is a tricky material to create, both because it's hard to make antiprotons and, once made, it's hard to slow them down enough to form an atom with a positron (antielectron). Still, in the future, I'm sure they'll have figured out these difficulties. The process would require an enormous energy source, again. Fusion on an ice asteroid 10,000x the size of your ship is my favorite fallback it seems :).

2. Put the matter and the antimatter together . Here, if the matter we use is anything but hydrogen, you get tons of Neutrons liberated from the reaction, which will cause nearby things to become fissile and transmute a bit. If we go with Hydrogen however, there's some impressive energy losses to neutrinos. Fortunately neutrinos interact by the weak force, and those reactions are often chiral in a way that you can choose the direction the decay products go.... I'm not 100% sure about this application, but it sounds right by the stories I've read of CP violation. We may be able to make the neutrinos mostly go out the back. Maybe. In this  case we could have nearly 100% mass conversion to useable energy or usefull momentum.

3. Make a photon rocket like with the black hole, however the sizes can be much smaller, scaling with your ship mass rather than the mini black hole's mass.

 

 

I've spared details on these systems, as well as others to prevent the post from reaching critical mass. Feel free to ask if anything sparks your interest.

In any case best of luck with your scifi! I hope your characters have some fun toys to play with.

 

[Jim DiGriz]

On 9/5/2017 at 6:46 AM, quasarrgames said:

Wishing to make a little novella set a few thousand years in the future where interstellar travel is comon. In the hopes of using time dilation as a plot device, I want to make some ships capable of travelling at close to c with a constant 1g acceleration, while staying within the realm of possibility. I was hoping you guys could help to make sure that the propulsion concepts i was planning to use were still scientifically accurate, for the most part.

To solve the "where did you get all that antimatter?" hole, I know that the collision of two specific kinds of white dwarves can generate lots of antimatter. Since fusion reactors somewhat replicate the conditions inside a star, it seems feasible that a very powerful, unstable fusion reactor could produce antimatter. So what if the reactor destroys itself in the process! You have antimatter! Let's just assume it's a very expensie fuel source.

 

You could hypothesize that we have GUT-scale knowledge of elementary particle physics and have discovered and understand the mechanism behind proton decay.  Then you could have X-boson catalyzed proton decay mechanisms that would turn protons into positrons which would then annihilate with an electron and produce a total conversion power source.  Unless it could also be made neutrinoless you'd have losses there.