The Fishback ramjet revisited | Hacker News Reader

The Fishback ramjet revisited

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68 pointsprogramd21 comments

I find it very hard to believe.

If you were trying to make the CNO cycle

https://en.wikipedia.org/wiki/CNO_cycle

work with an interstellar ramjet you would have to stop interstellar H (drag!), retain it in a reactor for a timescale of tens of minutes because of the beta decay that is part of the cycle and continue to retain almost all of the heavy (compared to Helium) elements and vent only Helium and maybe some hydrogen (like the space shuttle main engine this is great for Isp.)

Unless you have a triple-alpha line

https://en.wikipedia.org/wiki/Triple-alpha_process

that can synthesize carbon from helium you would be stuck with whatever inventory of C,N,O you started with and you'd slowly lose it.

It seems much more transformative to interstellar travel that you can brake yourself with a magsail at the destination. The drag is very real but any thrust from the Bussard ramjet seems elusive.

> work with an interstellar ramjet you would have to stop interstellar H (drag!)

The drag would be useful in the second half of the trip but can you make a ramjet that rotates the direction of travel of the reaction mass by 180°? You might be able to collect reaction mass for a return trip though, or final deceleration.

Why rotate the ramjet? All you'd really need to do is absorb the mass instead of shooting it out the tail, that would brake you just fine over a long enough time period.

If you can break at twice the acceleration you can reach the destination much faster because you start breaking later.

Sure, but a ramjet with it's scoop pointing in the same direction as its exhaust is going to have a hard time working well if at all. So you'll be spending some time collecting mass which you can then later spew back out again and during both those phases you are braking. But if you don't want to rotate the ramjet (which is technically quite a challenge, unless you rotate the whole spaceship) then it becomes a nasty engineering problem.

I'd like to see some serious work on the other proposal for "living off the land" in interstellar space.

We know comets and other interstellar bodies contain a lot of hydrogen which has a higher deuterium content than hydrogen on Earth.

Designs such as ITER and stellerators should scale up with increasing size and it's plausible that D+D fusion could be developed on the existing path. In fact a very large "inertial confinement" fusion device was fired based on D+D

https://en.wikipedia.org/wiki/Ivy_Mike

and got a massive positive energy return.

D+D fusion produces He3 and T, both which are better-burning fuels than D+D. These can be burnt all the way up to He4 in one big reactor, but one can imagine a system that separates out some He3 and T to be burned in smaller secondary reactors. D + T fusion produces terrible neutron radiation but you can let the T sit and it decays with a 12-year half life to He3 and use the much cleaner D + He3 reaction in secondary reactors with favorable scaling, less shielding, no breeding system, etc.

Those travelers only need to get to the next comet so they don't need to be terribly fast, but the great advantage they have is a sustainable lifestyle even if they never arrive at another star -- those kinds of travelers might not find planetary systems interesting at all.

As a Larry Niven and Poul Anderson fan, this is a disappointing analysis. But as a human, I love that, despite all the craziness going on, people are still writing papers like this.

As the saying goes, "the marvel is not that the bear dances well, but that the bear dances at all." Bussard ramjets were previously thought not to be feasible at all.[0] Feasible, but ridiculous is a big step up from not feasible.

[0]https://en.wikipedia.org/wiki/Bussard_ramjet#Feasibility

> It is very unlikely that even Kardashev Civilizations of type II might build magnetic ramjets with axial solenoids.

Well, this is a bummer. So, likely unfeasible for type I and II, certainly unfeasible for us... and type III, if they exist, would consider it a toy?

Kardashev classification is a form of log scale. There's a huge continuum between II and III.

This is very true and I guess we should be weary of making assumptions about other civilizations' means and motives...

About the same scale of difference between a family of four and the entirety of human civilisation and industry.

I've always found the Kardashev scale to be kind of silly. It starts off with an extremely ambitious goal, and then goes up two more huge steps from there.

To me it always seemed like a classification system that doesn't fully understand the scale it is trying to use. Or one that assumes exponential unbounded growth as a starting point. The cube square law is one that catches a lot of Sci-Fi authors.

I mean when the Alcubierre drive was first theorized it required gas giant planet sized mass energy to work and refinements to the theory have gotten that down to something reasonable so I would be interested to see what problems can be worked around with further research.

As for type III civilizations, my guess is they would think of it as a toy as the speculation about type III has them doing stuff like harvesting stars on mass for use in stellar engines and galactic scale engineering projects.

To me it seems the paper's conclusion is more that to a KII civilization, a Bussard ramjet is already a toy. If you have the energy output of a sun at your disposal, laser propulsion seems like a vastly more attractive solution, except for the problem posed by deceleration which is not insurmountable. Indeed, the ~Mm scale solenoid discussed in the paper seems something that should be easily within the reach of a KI scale civilization if they really want to build one. Or several.

Indeed. This is purely a guess but my hunch a is that something like Breakthrough Starshot may already provide a glimpse of what will turn out to be the most viable and practical mid and long term solution, even for us...

> The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

I know it's boilerplate, but I appreciate the authors' explicit declaration here that they're not in the pocket of Big Dyson Swarm.

That's what those in the pocket of Big Dyson Swarm would say.

Side topic: Star Trek's Federation starships feature a Bussard collector [1] at the front of the engine nacelles. It's frankly fun that the writers managed to keep touch points back to actual science to rationalize their design choices. There's some great writing about the logic behind the first design of the Enterprise from the original series here. [2]

[1] https://memory-alpha.fandom.com/wiki/Bussard_collector

[2] https://forgottentrek.com/designing-the-first-enterprise/

> The cut-off speeds are orders of magnitude lower than thought before.

So what are they? (ideally to the nearest 5% of C)

From the article:

> With graphene, the most suitable material, the cut-off speed for 1 g acceleration would be approached after an on-board flight time of three years at a distance of about 10 light-years. After that, the acceleration would quickly drop to values that shatter the popular dream of reaching the galactic center in a lifetime.

Note that this is already assuming you can build superconducting rings in space with a diameter of 2000 kilometer and with enough graphene to support them. If we assume for convenience that the acceleration will fall to zero after those three years, the relativistic speed calculator at https://www.omnicalculator.com/physics/space-travel (haven't checked it in depth, but I have no reason to doubt it) tells us we will have reached a speed of about 91% of c.

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