[HN Gopher] The Fishback ramjet revisited
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The Fishback ramjet revisited
Author : programd
Score : 59 points
Date : 2021-12-21 15:51 UTC (7 hours ago)
(HTM) web link (www.sciencedirect.com)
(TXT) w3m dump (www.sciencedirect.com)
| PaulHoule wrote:
| 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.
| hinkley wrote:
| > 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 180deg? You might be able to collect reaction
| mass for a return trip though, or final deceleration.
| jacquesm wrote:
| 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.
| shadowgovt wrote:
| > 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.
| jacquesm wrote:
| That's what those in the pocket of Big Dyson Swarm would say.
| SideburnsOfDoom wrote:
| > The cut-off speeds are orders of magnitude lower than thought
| before.
|
| So what are they? (ideally to the nearest 5% of C)
| WJW wrote:
| 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.
| wrs wrote:
| 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.
| gene-h wrote:
| 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
| PaulHoule wrote:
| 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.
| BruceEel wrote:
| > 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?
| bragr wrote:
| 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.
| varjag wrote:
| Kardashev classification is a form of log scale. There's a huge
| continuum between II and III.
| ben_w wrote:
| About the same scale of difference between a family of four
| and the entirety of human civilisation and industry.
| jandrese wrote:
| 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.
| BruceEel wrote:
| This is very true and I guess we should be weary of making
| assumptions about other civilizations' means and motives...
| Sharlin wrote:
| 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.
| BruceEel wrote:
| 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...
| mikehollinger wrote:
| 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/
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