How to interstellar travel in a very hard SF universe

[CraigDxHypo]

While I enjoy space opera (Star Trek, Star Wars, etc) my favorite fictional genre is the rarer hard science fiction, stories that involve science and engineering which doesn’t yet exist, but, unlike soft SF and fantasy, don’t can be rigorously explained as possible by present-day science, making clear assumptions that themselves are at least not very implausible according to present-day science.

Though I’ve not seriously written fiction (which is to say, actually started, finished, and published a story in any form) since the late 20th century, I’ve kept a journal of “story ideas” since the 1990s, and kept to a personal quest to build a diamond-hard SF universe in which individual Earth people travel interstellar distances.

Ignoring initially the sociological issues, such as why individuals would want to do this, and how to pay for it, we have an high-level, nuts-and-bolts engineering question: how, with unacceptably soft SF staples like warp drives, traversable wormholes, how can you deliver a single, few, or many effectively living (the definition of this is tricky - more later) Homo sapiens sapiens from Earth to somewhere on the order of 10 lightyears distant? That is, how to do interstellar travel?

I start with the assumption that people have perfected the basic problem of living in outer space – that is, how to keep people alive and healthy for their entire lifetimes outside of Earth’s atmosphere given nothing but a reasonably small amount of per person physical power and a mass of expendable supplies not many much greater than the mass of the people, or a total mass greater than on the order of 10^-10 Earth masses.

Next, I assume an upper limit on the “shelf life” for a live human of on the order of 1000 years. Near future medical technology can allow a person to live about 10 times longer than normal, or, more likely I think, some sort of suspended animation like the science fiction staple “cryosleep” is available. I’ll lift this assumption later in this post.

Next, I conclude that an “automobile” - a vehicle that carries all it needs for its trip - isn’t feasible. Even using an ultimately high energy-density fuel such as stored antimatter and super-advanced reaction mass accelerators, no rocket can reach the speed needed to travel on the order of 10 light years.

I’m left, then, with two high-level solutions:

1. The spacecraft obtains rocket reaction mass, and or fuel from the interplanetary and interstellar medium. The best know example of this idea is the Bussard ramjet.
2. The spacecraft is accelerated by a system it doesn’t carry. The most feasible example of a system of this of which I know is light-sail propulsion system described in physicist Robert Forward’s 1982 novel Rocheworld.

As best I’ve been able to glean from the literature, there’s presently a small scientific consensus that Robert Bussard’s original 1960 proposal, in which the ramjet spacecraft was to not only used the ionized hydrogen it “scoops” as reaction mass, but fuse it to produce sufficient power to accelerate the reaction mass to produce sufficient rocket thrust that the spacecraft ’s maximum speed relative to the interplanetary/stellar medium when the drag of due to its scoop equals that thrust is too low (less than .00017 c which is about 3 time the speed of Voyager 1) for the scheme to be useful. So for a ramjet spacecraft - perhaps better termed a scoop craft - to be acceptable for my hard SF world needs, it would require power in addition to that from the fusion of the scooped hydrogen.

I’m left, then, with artificial light (or other EM radiation) pushed “sail” spacecraft like Forward’s design, or possibly craft propelled by other kinds of streams/beams, such as “fountains” of macroscopic particles.

Comments?

 

[Simon Bridge]

Relativistic effects can mean that the time passed onship can be quite small - so very long journeys can be more practical than short ones - from the POV of the crew.

If long journeys are not an issue, then you can get almost everywhere for very little energy by following minimum-energy pathways... so fuel concerns may not be as big as you'd suspect

You certainly have a colony situation at the destination - or everyone just lives on their ships making planetary contact for raw materials and repairs. kind-of space gypsies.

An approach you haven't mentioned of is attaching a habitat to an object falling through the solar system on a hyperbolic trajectory - you maths will need to be good, or you'd need to be very motivated, since there is no guarantee the object will come anywhere close to another solar system before your habitat breaks down.

See "Salt" by Adam Roberts for an example.

Motivation is a key - maybe someone feels they can colonize a nearby star system for profit or ideological reasons (may appeal to wealthy cults for eg.) but the crew are so used to the shiplife when they get there they don't want to stop?

Niven's story "Spirals" has "escape" as a motivation for converting a space-station habitat to a starship.

 

[DHF]

Writing hard Sci Fi is hard work and I respect anyone taking on the challenge. I am currently writing a piece about Earth's first "manned" trip to Alpha Centuri and in an effort to keep the ship as realistic as possible I learned just how depressing the numbers can get with even modest speeds(by sci fi standards) My ship design comes in at a scant 1000 Metric tons and will be topping out at 11.5%c and even still, Ryan_m_b calculated that during the ships acceleration it would need to burn as much energy each second as the entire Earth currently does. And the ship would need to accelerate for about a year. Its mind boggling how much energy is needed to go even a small fraction of the speed of light.

Good luck on your tale.

 

[Simon Bridge]

My ship design comes in at a scant 1000 Metric tons and will be topping out at 11.5%c and even still, Ryan_m_b calculated that during the ships acceleration it would need to burn as much energy each second as the entire Earth currently does. And the ship would need to accelerate for about a year. Its mind boggling how much energy is needed to go even a small fraction of the speed of light.

Intreguing... for hard SF ... what proportion of the ship mass was fuel?

Good ion thrusters get you an exhaust velocity around 50kmps = c/6000. (using c=300,000kmps)

Assuming a final speed of 0.1c from a standing start, and without invoking relativity,
Tsiolkovsky tells me that (c/10)=(c/6000)ln(M/m)

where M is the combined mass and m is just the ship part. The numbers are fun.

Then there's the fuel needed to power the thruster.

 

[DHF]

@Simon, The dry mass was 1000 Metric Tons. The fusion reactor imparts 644.93 Terajoules Per kg and the exhaust velocity is a little under 12%c. Ryan crunched the numbers and came up with 870g of fuel per kg of dry mass so my total mass would be 1870 Metric Tons. Afterwards I realized they would need reserve fuel and that would throw the numbers off again... you may think numbers are fun but I think they are secretly out to get me :D

 

[Filip Larsen]

The numbers are depressing indeed and severely reduces how realistic it is to extrapolate our current earth-bound traveling culture (e.g. going on vacation, poor people emigrating to start anew, etc) to interstellar scale. It is very difficult to find a realistic reason why humanity would invest such a huge effort only to allow very few people to go roam the stellar neighborhood, and that even includes the premise of a 1000 year life span. And on top of all the hard limits of physics you also have to require that human civilization must have a long term structure stable enough to support such an endeavor (especially important for the light-sail option).

As I see it, the only long term goal that humanity may realistically be able to reach regarding interstellar travel would be the goal of colonization, i.e. seeding nearby solar systems with humans by use of a variant of von Neumann probes [1] which has the additional capabilities to grow humans (and likely various animals) from cloned zygotes and provide them with a safe up-bringing until they can run a stable colony themselves. Since the actual travel time presumably is much less critical for frozen zygotes than for live humans, the feasibility of the probes would be less sensitive to trade-off between weight, travel time and resource consumption. If it becomes feasible to built such a probe on nano-scale it will allow even more freedom in this trade off.[1] http://en.wikipedia.org/wiki/Self-replicating_ spacecraft

 

[Ryan_m_b]

Intreguing... for hard SF ... what proportion of the ship mass was fuel?

Good ion thrusters get you an exhaust velocity around 50kmps = c/6000. (using c=300,000kmps)

Assuming a final speed of 0.1c from a standing start, and without invoking relativity,
Tsiolkovsky tells me that (c/10)=(c/6000)ln(M/m)

where M is the combined mass and m is just the ship part. The numbers are fun.

Then there's the fuel needed to power the thruster.

The key assumption was that exhaust velocity was 0.1c, I worked through the rocket equation from then on (and pointed out the magical nature of that EV). I'm on the go so can't post links easily but you can check out the thread in this forum for the thought process/working out.

To the OP the best way to do interstellar travel in a hard SF setting is SLOWLY. Posit huge advancements in ecosystem engineering and industrial automation then have a mere few thousand colonists travel in a hollowed out asteroid for a few millennia.

 

[Ryan_m_b]

Since the actual travel time presumably is much less critical for frozen zygotes than for live humans

That's not a safe assumption as living humans at least can utilise DNA repair pathways to fix radiation induced mutation (to some degree) whereas frozen cells can't. But then again if you're assuming a robotic system can autonomously create a sustainable ecosystem and technological infrastructure and build machinery for safe ectopic pregnancy and raise a child then you might as well assume it can build a fertilised ovum from scratch

 

[DHF]

Well no matter which way you look at it, as long as we try to stay true to hard sci fi, interstellar travel eventually breaks down. without handwavium the math just isn't on our side. Whether you are talking super fusion drives that can push you at a fraction of c. or whether you are talking a generational colony ship that takes thousands of years to get there...the end result still requires a sickening amount of energy. in order to have a generational crew you would need a self sustaining biosphere and that would take a tremendous amount of power and fuel. I am just not sure which requires more hand waving. Both are fascinating concepts for a story but at the end of the day in order to pitch your story into another star you need to soft boil your hard sci fi.

 

[Simon Bridge]

It is very tricky - with the trickiness depending on how much of a stickler you are for hard physics.
Usually a Hard-SF story won't be 100% real physics - instead you allow yourself a single one-time exception (or two...).

In DHF's case it was the magic reaction drives. Some sort of magic-quality in the drive is the usual exception ... the hard-science art is related to how rigorously you explore the consequences. In Hard SF the exception is central to the story while in soft SF it is an incidental plot device.

With the usual variations in between.

 

[DHF]

Good point. If it was 100% hard it wouldn't be Science fiction it would be a fiction novel. In my case I needed the crew to get to Alpha Centuri within a few decades and the only way I could make that happen was by taking liberties with the propulsion system. I played fast an loose with the ship's mass but since the story was set 250 years from now I didn't feel it was a complete leap to assume stronger lighter materials would be in use. after 250 years ago plastics an advance ceramics would have been total hanwavium to even the most open of minds. Aside from the Fusion drive, the real soft in my science fiction comes from the budget, the assumption that mankind will make the investment to explore outside our neighborhood. given the current state of Space exploration I think that took more liberties then assuming we would master Fusion within the next 2 centuries.

 

[Filip Larsen]

That's not a safe assumption as living humans at least can utilise DNA repair pathways to fix radiation induced mutation (to some degree) whereas frozen cells can't. But then again if you're assuming a robotic system can autonomously create a sustainable ecosystem and technological infrastructure and build machinery for safe ectopic pregnancy and raise a child then you might as well assume it can build a fertilised ovum from scratch

Good point. The "frozen" word was a hand waving on my part to imply some kind of technology that would keep the zygotes unchanged. Since interstellar space is around 3K anyway I was thinking some kind of cryogenics, but nano-(re)construction of zygotes, as you hint at, may be better for the reasons you mention. Even at 3K there will be changes on a molecular level to the DNA so travel time and statistics alone would dictate that at some point you would have to few viable zygotes to form a sustainable colony.

The intention of my post was mostly to point to a possible technology trade-off in colonization scenarios where you can "trade off" unobtainable propulsion technology with more plausible (nano-scale) bio-engineering (but then again, I am not a bio-engineer and I may of course be unaware of any "hard limits" that already has been established in that field).

 

[DHF]

The intention of my post was mostly to point to a possible technology trade-off in colonization scenarios where you can "trade off" unobtainable propulsion technology with more plausible (nano-scale) bio-engineering (but then again, I am not a bio-engineer and I may of course be unaware of any "hard limits" that already has been established in that field).

I have read stories with this sort of plot device before. It is a very interesting one. as far as feasibility goes however you hit a wall no matter which route you take. Each road leads to the conclusion that you need an unlimited power source. My colleges an I have been discussing long term colonization of Mars and we came to the same conclusion, you can't create something from nothing. We take power for granted because our planet is bathed in resources but on Mars you only have limited amounts of solar power and without a solid and very abundant power source you can't build or grow anything. The same obstacle stands for a crew of robots standing by to rebuild civilization. in order to grow the humans you need power. It seems reality is determine to keep us rooted here in the Sol system :)

 

[Filip Larsen]

The same obstacle stands for a crew of robots standing by to rebuild civilization. in order to grow the humans you need power.

You need power indeed, but not necessarily an infinite amount. A seeder probe does not need much energy during travel, perhaps it is even possible to built one that is capable of traveling in complete hibernation (every part of the probe is at a temperature around 3K) and then have it "wake up" as the power levels from the destination star rises. If colonization means colonization on planets (which is more or less the only option here), then it is given that the planet in question must already be in the green belt around its star, so power levels comparable to Earth levels should be realistic.

 

[Plan]

Why do you need additional power for your drives if they're ramscoops?

Yeah, depending on the math, it could take years to accelerate to top speed, and years to decelerate to the destination, on top of the years it takes to reach a particular system. But the closer your ramscoops are to light speed, the greater the relativistic effects on subjective ship time. Your characters can survive multiple system hops with a combination of time dilation and cryopreservation -- "running cold" during those long stretches between the stars.

Those things can actually work FOR the plot with a deft hand. I'd say SF is so saturated with different types of FTL -- from BSG's jump drives to Star Trek's warp to the magic unnamed drives of Star Wars --that it would actually be more exotic to write a story where FTL isn't possible.

Personally, I think way too much SF is happy to make the universe small, which really doesn't respect the vastness of space, the amazing-ness of achieving interstellar travel, and the loneliness in those massive distances between the stars. Most people really have no concept of the distances involved and thus don't understand the enormity of becoming a spacefaring culture.

With our current world so small, our instant-FTL-jumping SF ships are a reflection of our societal attitudes, where we want things almost instantaneously. But colonizing space SHOULD be difficult and inconvenient, the same way it was difficult and inconvenient for the first intercontinental settlers risking months-long, perilous journeys across the ocean so they could claim the untouched lands on the other side.

 

[Plan]

Good point. If it was 100% hard it wouldn't be Science fiction it would be a fiction novel. In my case I needed the crew to get to Alpha Centuri within a few decades and the only way I could make that happen was by taking liberties with the propulsion system. I played fast an loose with the ship's mass but since the story was set 250 years from now I didn't feel it was a complete leap to assume stronger lighter materials would be in use. after 250 years ago plastics an advance ceramics would have been total hanwavium to even the most open of minds. Aside from the Fusion drive, the real soft in my science fiction comes from the budget, the assumption that mankind will make the investment to explore outside our neighborhood. given the current state of Space exploration I think that took more liberties then assuming we would master Fusion within the next 2 centuries.

I don't think anyone's going to find fault with you if you don't explain the global economics necessary to turn humanity into a starfaring culture. SF fans are pretty forgiving with that sort of thing, since it helps get the story to the good stuff.

Also, if you've got ramscoop drives that bring you to 90% light, and you tack on a year each for accel/decel, you can get your characters to Alpha Centauri in less than a decade. That's perfectly reasonable and a plausible tech level for a few centuries from now.

 

[DHF]

The one thing that always bothered me about the Ram scoop was how does the ship slow down? its basically a funnel, grabbing fuel and shooting the exhaust out the back. but to slow down you need to flip the ship and face the engine in the direction you are moving to decelerate. However if the scoop is facing in the opposite direction, how does it collect and compress the fuel?

 

[Simon Bridge]

Why do you need additional power for your drives if they're ramscoops?

... you need to turn the scoops on to start the collection process ... they won't start at break-even.
You'll need to use some of the energy collected to power the scoops.
The incoming fuel needs to be slowed to be used in the reactor.

The one thing that always bothered me about the Ram scoop was how does the ship slow down?

You use stored fuel to slow down and/or have at least one other thruster.

 

[the_wolfman]

The one thing that always bothered me about the Ram scoop was how does the ship slow down? its basically a funnel, grabbing fuel and shooting the exhaust out the back. but to slow down you need to flip the ship and face the engine in the direction you are moving to decelerate. However if the scoop is facing in the opposite direction, how does it collect and compress the fuel?

Modern air planes employ thrust reversal to slow down after landing.
I imagine you could design a ram scoop that has similar capabilities.

 

[Plan]

... you need to turn the scoops on to start the collection process ... they won't start at break-even.
You'll need to use some of the energy collected to power the scoops.
The incoming fuel needs to be slowed to be used in the reactor.

You use stored fuel to slow down and/or have at least one other thruster.

Indeed, but I think readers wouldn't blink at a story that passes over those details to get the plot going. What I'm trying to say is that it isn't necessary to explain that sort of thing unless it's directly related to the plot.

It's like the recent ubiquity of screen-less holographic monitors in pretty much every science fiction movie of the past 5-10 years -- we accept that such tech could exist, we acknowledge it looks pretty cool, but we don't need a 10-minute detour so the characters can explain how those monitors are able to project images into air.

Ramscoops have been discussed quite a bit, and they're used enough in SF to accept that it's a viable drive tech for a plausible hard SF story. I don't think new SF writers have to reinvent the wheel when it comes to drive tech, and I think it's always better to err on the side of relevance -- are readers really going to want to know how the drive works, or are they more interested in where the drive is going to take the characters and what awaits them when they arrive at their destination? It's pretty much always going to be the latter, unless some drive failure is crucial to the plot.

I'm sure you already know this, I'm just stating it for writers who think they need to describe all the underlying tech. I understand the temptation, or the feeling of obligation to explain that stuff, but most times it's easier for everyone to skip it.

 

[DHF]

True enough. The average reader has little interest in the numbers and more in the crew. Speaking personally, I don't put most of the figures on the page. I do however like to work out the mechanics so the tech makes sense to me. Even though the reader won't be treated to how the engines work or how much the ship weighs, I like to know that the ship has a realistic feel on paper so I try to work out things like how fast the ship can go with the given engines, how much fuel they need ect. If it is solid behind the curtains I think it makes more sense on the stage as it were.

 

[Plan]

True enough. The average reader has little interest in the numbers and more in the crew. Speaking personally, I don't put most of the figures on the page. I do however like to work out the mechanics so the tech makes sense to me. Even though the reader won't be treated to how the engines work or how much the ship weighs, I like to know that the ship has a realistic feel on paper so I try to work out things like how fast the ship can go with the given engines, how much fuel they need ect. If it is solid behind the curtains I think it makes more sense on the stage as it were.

That's good, solid world-building, and as a writer it saves you a lot of headaches later on, especially if you become really successful. There are always going to be those fans who are obsessive and somehow come to know more about a writer's invented universe than the writer does his or her self.

 

[DHF]

That's good, solid world-building, and as a writer it saves you a lot of headaches later on, especially if you become really successful. There are always going to be those fans who are obsessive and somehow come to know more about a writer's invented universe than the writer does his or her self.

Very true. I remember reading a book with my son and in said tale the aliens are in the sol system heading for Earth, and the plot needed the journey to take days. In order to justify why they were creeping up on the planet the main character thought out loud about how putting the sub light engines at full burn would get them there in seconds but then years will have passed on Earth due to relativity. I remember how frustrated I was because it made no sense, and wishing that the author had done basic research on relativity an astronomy if she was going to write a book involving space travel. Since then I try to understand how the world in my stories work, so even if I don't publish a tech manual, basic statements by the characters all add up and don't cause reader injury due to face palming.

 

[CraigDxHypo]

The one thing that always bothered me about the Ram scoop was how does the ship slow down? its basically a funnel, grabbing fuel and shooting the exhaust out the back. but to slow down you need to flip the ship and face the engine in the direction you are moving to decelerate. However if the scoop is facing in the opposite direction, how does it collect and compress the fuel?

Bearing in mind that Bussard Ramjets have been designed only on a high, conceptual level, not in detail, even simulated, I think there’s a conventional answer to “how do they slow down”.

Because their electromagnetic scoops produce drag (which is equal to their maximum thrust when they are traveling at their maximum speed relative to the interstellar medium), BRs can slow down “passively” by just turning off their fusion rocket. They can’t slow to zero velocity relative to a typical star or planet, because this drag falls off as the square of their IM speed, so just as they need a “startup rocket” to reach sufficient speed to collect hydrogen fast enough to use their fusion rocket, they need it to match speed to their destination.

In all this conversation about BRs, I think it’s important to consider that, according to some credible recent analysis, they are not practical possible at all – that is, assuming the daunting engineering task of an (optional) long-distance hydrogen ionizer, an electromagnetic scoop, and a fusion rocket can be achieved, the speed at which drag = thrust may be lower than what can be achieved by a conventional rocket.

My intuition is that a workable ramscoop spacecraft would use the scooped hydrogen only as reaction mass, and would need an internal, or better, an external power supply to accelerate that (conveniently already ionized, meaning consisting mostly or protons only) reaction mass to an exhaust velocity high enough to appreciably mass dilate it (for example, to about 0.995 c for a factor of about 10). (Note that the most optimistic expected exhaust speed for fusion rockets is around 700000 m/s =~ .002 c)

This would increase the maximum speed by a factor of the square root of the mass dilation. Taking the 0.00017 c maximum speed I gleaned from a few sources in post #1, and the speed to which the LHC can accelerate protons, about 0.999999991 c, an xtra-powered ramjet spaceship – let’s call it an XRJ – would have a maximum speed of 0.015 c =~ 1 lightyear/68 year.

While not as fast as I’d like, this meets the requirements I put in my OP of on the order of 10 light years within an extended human lifespan on the order of 1000 years.

Indulging in a bit of only semi-bridled engineering optimism, and barely checking my arithmetic, let’s assume a particle-accelerator rocket with 10 times the per-proton energy of LHC, scaled up in exhaust mass by a factor of on the order of 10^15, that can be fit into a reasonable small (with these numbers, on the order of 10^8 kg) spaceship. The per-proton energy is proportional to its mass dilation, so the max speed of this XRJ would be 0.15 c =~ 1 ly/7 y, making for a 40 year trip to Alpha Centauri.

 

[DHF]

I like the idea of using drag to slow down. but the issues I come up to is that the whole point of a Ram Scoop is to avoid excess mass by doing away with bulky power systems and fuel tanks and just use the universe as a fuel tank. I however you need to put in extra generators or back up fuel cells to slow down. then you start to negate the advantage the Scoop had. The idea of using drag to slow down, while an excellent one comes into the issue of needing a tremendous amount of power to generate enough drag to slow down from a fraction of c. Since you are burning X amount of energy to accelerate, it would seem that regardless of what method you were using, you would need to generate the same amount of energy to decelerate.

 

[Decimator]

I like the idea of using drag to slow down. but the issues I come up to is that the whole point of a Ram Scoop is to avoid excess mass by doing away with bulky power systems and fuel tanks and just use the universe as a fuel tank. I however you need to put in extra generators or back up fuel cells to slow down. then you start to negate the advantage the Scoop had. The idea of using drag to slow down, while an excellent one comes into the issue of needing a tremendous amount of power to generate enough drag to slow down from a fraction of c. Since you are burning X amount of energy to accelerate, it would seem that regardless of what method you were using, you would need to generate the same amount of energy to decelerate.

You are unlikely to negate the scoop. The key to the bussard ramjet is that they are not subject to the rocket equation. Bussard ramjets don't need to use propellant to accelerate their propellant, as they use ambient matter.

 

[CraigDxHypo]

Scooped reaction mass + beamed power

I started this thread a month ago with a favorite idea for a transportation technology for a hard SF universe where individual humans not too different from our present-day kind could, in the course of their lifetimes, live around more than one star, that was pretty close to the artificially-pushed (by a gigantic laser closely orbiting the Sun) light sail in Robert Forward’s 1982 novel Rocheworld. Forward’s novel involved a single scientific expedition to Barnard’s star (distance ~6 ly). Since I wanted such travel in my universe to be much more common, I imagined a system of many such lasers defining paths between many stellar systems, the traveling ships being dependent on these lasers serving them reliably for many decades. Since origin and destination have a pushing laser, the need for the Forward’s elaborate multi-part light-sail is eliminated. Call this imagining “a railroad of light”.

A month or ruminating and following this thread on the subject has shifted my favorite to the following.

The ships scoop ionized hydrogen (protons) using a magnetic field like a classic Bussard ramjet, but rather than fusing the hydrogen to generate power, uses it just for rocket reaction mass, accelerating it to very high speed using an “inside out” linear accelerator.

The power to run all of this comes from a beam of … something … sent from big beam-projectors, roughly similar to the laser in the Forward light-sail scheme.

The obvious missing part of this scheme is the vague something nature of the beam, and the shipboard engines needed to convert it into usable power – in this scheme, electricity.

One possibility (I’m very much brainstorming here, not even 1st-order approximating feasibility) is an electron beam. The scoop collects lots of protons, so putting an electron collector on one end and the proton stream on the other should make for a usable electric current, and by exhausting the electrons along with the protons, avoid accumulating a net charge.

Another is a high-speed beam of neutral matter (or nearby streams of net neutral charge particles). This strikes the ship, heating the struck part (“the back”), creating a heat difference between the un-struck part (“the front”), which can be used to run a heat engine. A basic challenge is discarding the heat, a major challenge in all spacecraft . Perhaps the collected matter could be evaporated, to cool at a much higher rate than radiation.

Perhaps the ultimate option is a beam of antimatter. Like a matter beam, it could power a heat engine, but with much higher energy density ((m c^2)/(1-(v/c)^2) vs. (m c^2)(1 -1/(1-(v/c)^2)) ~= m v^2 ). The major challenge with any antimatter power scheme is the likely huge energy cost of creating antimatter, so unless some sort of exotic natural source of it – say some sort of exotic stellar object jet – is plausible, this is likely a non-starter, as artificially created antimatter would be too valuable to waste by beaming into space, most of it being lost (at interstellar distances, any non-magical beam will be many times wider than any practical ship-born target).

I think it’s time for some less-brainstormy estimation and modeling, and trying to work out, to hard SF standards, the engineering of one of the above beam options (or some other). But which one?

 

[Simon Bridge]

The ships scoop ionized hydrogen (protons) using a magnetic field like a classic Bussard ramjet, but rather than fusing the hydrogen to generate power, uses it just for rocket reaction mass, accelerating it to very high speed using an “inside out” linear accelerator.

Are you not discarding a lot of neutral hydrogen fuel there though...
Why not have your energy source ionize the hydrogen and then accelerate both charges in different engines?

The power to run all of this comes from a beam of … something … sent from big beam-projectors, roughly similar to the laser in the Forward light-sail scheme.

The obvious missing part of this scheme is the vague something nature of the beam, and the shipboard engines needed to convert it into usable power – in this scheme, electricity.

Some reason to not use light for this beam?

One possibility (I’m very much brainstorming here, not even 1st-order approximating feasibility) is an electron beam. The scoop collects lots of protons, so putting an electron collector on one end and the proton stream on the other should make for a usable electric current, and by exhausting the electrons along with the protons, avoid accumulating a net charge.

You realize there are electrons in space right? Not just protons by themselves?
Electron beams spread out over long distances though so you'd need one amazing electron gun.

Another is a high-speed beam of neutral matter (or nearby streams of net neutral charge particles). This strikes the ship, heating the struck part (“the back”), creating a heat difference between the un-struck part (“the front”), which can be used to run a heat engine. A basic challenge is discarding the heat, a major challenge in all spacecraft . Perhaps the collected matter could be evaporated, to cool at a much higher rate than radiation.

But how do you make the beam to begin with?

You could use beams of neutrons from nuclear explosions maybe - using a decent sized chunk of rock as a collimator. But then why not just use the gamma and x rays?

Perhaps the ultimate option is a beam of antimatter. Like a matter beam, it could power a heat engine, but with much higher energy density ((m c^2)/(1-(v/c)^2) vs. (m c^2)(1 -1/(1-(v/c)^2)) ~= m v^2 ). The major challenge with any antimatter power scheme is the likely huge energy cost of creating antimatter, so unless some sort of exotic natural source of it – say some sort of exotic stellar object jet – is plausible, this is likely a non-starter, as artificially created antimatter would be too valuable to waste by beaming into space, most of it being lost (at interstellar distances, any non-magical beam will be many times wider than any practical ship-born target).

I was thinking more that each gram of antimatter used at the ship also consumes a gram of the ship. You are basically transporting half you fuel with you.

You could use the reaction with the collected hydrogen though - tricky bit of aiming to get the antimatter beam on the right spot though.

I think it’s time for some less-brainstormy estimation and modeling, and trying to work out, to hard SF standards, the engineering of one of the above beam options (or some other). But which one?

Why not use a beam of microwave frequency photons - these are massless and chargeless and make an electric current all by themselves when they hit metal.

All these versions will involve a momentum transfer at the collector when the particles hit the ship ... you are basically using the light-sail idea, a much narrower beam and using the delivered energy as well as the momentum.

That the idea?

 

[CraigDxHypo]

The ships scoop ionized hydrogen (protons) using a magnetic field like a classic Bussard ramjet, but rather than fusing the hydrogen to generate power, uses it just for rocket reaction mass, accelerating it to very high speed using an “inside out” linear accelerator.

Are you not discarding a lot of neutral hydrogen fuel there though...
Why not have your energy source ionize the hydrogen and then accelerate both charges in different engines?

Having the ship ionize neutral hydrogen in the interstellar medium is certainly an option – the early Bussard ramjet studies included this possibility, using powerful shipboard lasers.

The early studies, and my introduction to the ramjet idea via Poul Anderson’s wonderful 1970 novel Tau Zero, were based on the 2-phase model of the interstellar medium, in which the ISM consists of a small fraction (~1%) of dense, cold (<50 K) molecular (2H) gas and the rest a warm neutral atomic (cold and warm neutral media). In the late 1970s, after the most active period of ramjet studies, a 3-phase model gained the consensus, in which most (more than half) of the ISM is warm (~8000 K) or hot (~ 1000000 K) ionized hydrogen (warm and hot ionized media).

My SF assumption is that a 3-phase model is correct, so most of the ISM doesn’t need power and equipment-expensive artificial ionization for a scoopship to scoop enough of it. There’s still not a detailed, high-resolution scientific model of how ionized and neutral H is distributed in the ISM, which allows for interesting SF plot devices where the ship must find and follow regions rich in WIM and HIM, avoiding CNM

The power to run all of this comes from a beam of … something … sent from big beam-projectors, roughly similar to the laser in the Forward light-sail scheme.

The obvious missing part of this scheme is the vague something nature of the beam, and the shipboard engines needed to convert it into usable power – in this scheme, electricity.

Some reason to not use light for this beam?

At least these – though like my whole “beamed power interstellar railroad” vision, I’ve not given ‘em much rigor yet:

• Higher power. Light consisting of bosons, there’s almost (ignoring a kugelblitz) no theoretical limit on the power of a beam of it, but practically, light must be produced aimed by fermionic stuff, so making beams powerful enough for my beamed power scheme may be practically difficult to impossible, especially if beam-making machine must survive more than briefly.
  Matter beams – which can be described as “missile streams” – avoid these problems
• Diffraction. There are hard optical limits on how concentrated a light beam can be at increasing distance.
  Not so with missile streams. Notice I’ve used the very generic term “missile” rather than “projectile” or “particle”. A projectile can in principle be thrown at a point in spacetime very accurately. If, rather than being an inert subatomic particle or molecule-size projectile, it’s a microscopic machine – a guide missile – it can in principle be very accurate.
  This begins to more resemble “shooting tiny spaceships at big ones”, but doesn’t seem to me outside of the realm of possibilities.

One possibility (I’m very much brainstorming here, not even 1st-order approximating feasibility) is an electron beam. The scoop collects lots of protons, so putting an electron collector on one end and the proton stream on the other should make for a usable electric current, and by exhausting the electrons along with the protons, avoid accumulating a net charge.

You realize there are electrons in space right? Not just protons by themselves?
Electron beams spread out over long distances though so you'd need one amazing electron gun.

I admit that the electron beam option seems far-fetched, especially considering that the best current ISM models suggest that it’s far from neutral and magnetic field-free. Hitting an on the order of 1000 m target at a distance of several lightyears with even a few electrons would, I think, be amazing to the extent of being magical, having it happen in a story a sin against hard SF.

The main attraction of the electron beam option is the simplicity of creating one (electron guns are very old technology) and using it on the ship/target end of the system. I let that blind me to the infeasibility in the middle of the system.

Another is a high-speed beam of neutral matter (or nearby streams of net neutral charge particles). This strikes the ship, heating the struck part (“the back”), creating a heat difference between the un-struck part (“the front”), which can be used to run a heat engine. A basic challenge is discarding the heat, a major challenge in all spacecraft . Perhaps the collected matter could be evaporated, to cool at a much higher rate than radiation.

But how do you make the beam to begin with?

As best I can seen, solutions fall into 4 main kinds:

1. Fully ionize the matter (get all the electrons of the nuclei). Accelerate electrons in one linac, nculei in another, aiming the two beam so they collide at low enough relative speed that they recombine into neutral matter
2. Partially ionize it, and do the same. This would allow the many-atom “microscopic missiles” I describe above to be given enough net positive charge to be accelerated magnetically, then sprayed with electrons enough to be neutral
3. Fully ionize the matter, and just assure that the number of protons hitting the distant shipborn target/collector are nearly equal. However, this runs afoul the “in the middle” dispersion problem described above for an electron beam.
4. Accelerate the matter some way other than magnetically. For example, push very low mass “Starwisps” using microwave lasers, accelerating them to their maximum speed over a distance on the order of an AU or two.

Perhaps the ultimate option is a beam of antimatter. Like a matter beam, it could power a heat engine, but with much higher energy density ((m c^2)/(1-(v/c)^2) vs. (m c^2)(1 -1/(1-(v/c)^2)) ~= m v^2 ). The major challenge with any antimatter power scheme is the likely huge energy cost of creating antimatter, so unless some sort of exotic natural source of it – say some sort of exotic stellar object jet – is plausible, this is likely a non-starter, as artificially created antimatter would be too valuable to waste by beaming into space, most of it being lost (at interstellar distances, any non-magical beam will be many times wider than any practical ship-born target).

I was thinking more that each gram of antimatter used at the ship also consumes a gram of the ship. You are basically transporting half you fuel with you.

You could use the reaction with the collected hydrogen though - tricky bit of aiming to get the antimatter beam on the right spot though.

Correct, you’d need to use the collected hydrogen.

Aiming any kind of beam well enough to hit a astronomically tiny spaceship at interstellar distances is very tricky. I don’t imagine the engineering of getting the collected hydrogen together with the incoming antimatter to be very complicated compared to other parts of the scheme. Consider this “thought prototype” example: decelerate a small amount of the scooped protons, combine them with electrons to make cool hydrogen, gather and pump the gas of liquid hydrogen to the “back” of the ship, release it to intercept the incoming antimatter, then (handwaving) convert the resulting radiation into useful engineering energy. As with present-day ion rockets, such a scheme would need to prevent the ship from accumulating a large net charge, so in addition to the proton scoop, the ship would need an electron scoop, smaller than the proton scoop, because it would only scoop the same flux of electrons from the ISM as positrons from the antimatter beam.

An IMHO excellent primer on the subject of antimatter power is Robert Forward’s 1995 Indistinguishable from Magic. To avoid my needing to recapitulating it here, I highly recommend reading this slim, IMHO underappreciated popular science book (kindly ignore the lurid cover if it offends you, and accept that I’m not schilling for Baen).

I think it’s time for some less-brainstormy estimation and modeling, and trying to work out, to hard SF standards, the engineering of one of the above beam options (or some other). But which one?

Why not use a beam of microwave frequency photons - these are massless and chargeless and make an electric current all by themselves when they hit metal.

Microwave and other non-visible EM beams are certainly worth considering. They’re key in the Starwisp scheme I linked to above, in which very low-mass robotic probes have their instruments and radios powered by microwaves sent across many light years. However, microwaves or radio-frequency EM beams suffer from the same difficulties I described above for visible light beams.

All these versions will involve a momentum transfer at the collector when the particles hit the ship ... you are basically using the light-sail idea, a much narrower beam and using the delivered energy as well as the momentum.

That the idea?

Pretty much.

My main thrust is to take the strength of the Bussard ramjet idea – getting reaction mass from the ISM – and the powered lightsail idea – allowing power to be generated by gigantic machines near stars, rather than on the necessarily much lower mass ships – while discarding their weaknesses – in both cases, too little power to produce enough rocket thrust vs. the drag of the ramscoop for the ships maximum speed to be high enough.