Author Topic: Study: 1960 ramjet design for interstellar travel—a sci-fi staple—is unfeasible  (Read 15020 times)

Offline su27k

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https://arstechnica.com/science/2022/01/study-1960-ramjet-design-for-interstellar-travel-a-sci-fi-staple-is-unfeasible/

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In Poul Anderson's 1970 novel Tau Zero, a starship crew seeks to travel to the star Beta Virginis in hopes of colonizing a new planet. The ship's mode of propulsion was a so-called "Bussard ramjet," an actual (though hypothetical) means of propulsion which had been proposed by physicist Robert W. Bussard just a decade earlier. Now, physicists have revisited this unusual mechanism for interstellar travel in a new paper published in the journal Acta Astronautica, and alas, they have found the ramjet wanting. It's feasible from a pure physics standpoint, but the associated engineering challenges are currently insurmountable, the authors concluded.

Paper: The Fishback ramjet revisited

Offline Vultur

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"Currently insurmountable " seems like an odd (obvious) statement- we can't even make a net power fusion reactor, and a Bussard ramjet presumably needs proton-proton fusion which is way harder, even beyond the other challenges like the scoop itself, surviving collisions at ridiculously high speeds, etc etc.

Online ccdengr

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Hasn't the Bussard ramjet been questionable since about 1978?  See Heppenheimer, T.A. (1978). "On the Infeasibility of Interstellar Ramjets". Journal of the British Interplanetary Society. 31: 222

Not referenced in this recent paper, so perhaps something is different.

Offline Surfdaddy

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Peripherally related - not sure where to ask this.

Nowadays we know more about the interstellar environment. "Lots" of dust, rogue planets, black holes, etc.
Even if we managed to get up to significant sub-light speeds (say, 0.1c or higher) - does the environment make interstellar travel impractical as we likely couldn't travel fast enough and not hit too much debris to destroy/damage a ship at those speeds?

Online ccdengr

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Even if we managed to get up to significant sub-light speeds (say, 0.1c or higher) - does the environment make interstellar travel impractical as we likely couldn't travel fast enough and not hit too much debris to destroy/damage a ship at those speeds?
Presumably if we figured out how to get to those speeds in the first place, dodging, deflecting, or destroying debris would be feasible.

The wikipedia page https://en.wikipedia.org/wiki/Bussard_ramjet is a pretty good place to start with the literature, most of which is focused on basic questions of energy production.  See Anderson's TAU ZERO or the recent movie PASSENGERS for some discussion of the effects of debris. :)

Online ccdengr

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Hasn't the Bussard ramjet been questionable since about 1978?  See Heppenheimer, T.A. (1978).
I haven't read and can't find the Heppenheimer paper online, but http://www.projectrho.com/public_html/rocket/slowerlight3.php says
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T. A. Heppenheimer published a paper... noting the problems with the p-p chain for fusion without citing Dan Whitmire’s solution. Heppenheimer notes radiation losses but does not cite Whitmire and Fishback, who addressed the problems of bremsstrahlung and synchrotron radiation in the reactor and the scoop field.
Worth reading the projectrho page for background.
« Last Edit: 01/11/2022 08:15 pm by ccdengr »

Offline FattyLumpkin

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Apparently there are only two ways to salvage (sort of) the Bussard RJ concept. One is bringing "an easier type" of fuel along to fuse and then mixing it with what the scoop collects as reaction mass. The Ram Augmented Interstellar Ramjet. (RAIR),
or most feasible, laying down fuel "packets" infront of the stove pipe like vessel prior to vessel launch. Fuel and Vessel would be lauched in a way to where the vessel intercepts the fuel in a "head on" fashion vis-a-vis opposition Olberth launching. Opposite directional Olberth launching of both fuel and vessel is to increase velocity of both to ensure high velocity impacts.
Fusion ignition occurs when the space "ship" impacts the fuel at high velocity. There is no scoop on the second version to create "drag" in the IP or ISM.

Offline endlesslimitation

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Anyone have a good resource on the "scoops" proposed in the standard bussard ramjet design? I've always been curious what specific geometry of magnetic fields would be useful to collect and concentrate ISM fuel.

Offline FattyLumpkin

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I believe the Cassenti toroidal field scoop concept was suggested to try to help mitigate the drag effects of what Bussard originally envisaged as/for his ram scoop.
I image you are aware Andrews and Zubrin found the original Bussard scoop concept to create enough drag in the ISM to limit the vehicle to +/- 12% of C....that's if P-P fusion were possible and the ISM were dense enough to collect that amount of hydrogen. Note the the drag + lack of adequate fuel collection and our inability to even consider P-P fusion make the the BRJ untenable. Looking for my reference pertaining to the original Bussard collector concept.   See if I can find it.
OK , looked aroud for a little while...if my source is correct the Bussard  Ram scoop consisted of a series of loop coils with largest diameter at the "mouth" of the scoop, and loop size would decrease in diameter as the scoop approached the "throat"...main "ingestion" point of all particle-matter prior to being collected and fed into the fusion reactor.
Of note: the magnetic field of a coil 10 meters in diameter could strech out to as far as 100 km into ISS depending upon power and frequency of the ciculating current.
 K Hambsch
« Last Edit: 01/14/2022 06:08 pm by FattyLumpkin »

Offline endlesslimitation

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Thanks I'll have to Google that Cassenti toroidal field!

Yeah it's always seemed like one of those "cool if by some miracle the engineering is actually possible" concepts, but not anywhere near the top of the most promising interstellar concepts list.

I had heard vaguely about the drag issues, but never heard a number placed on it. Honestly 12% of c sounds pretty good... Well ahead of engineering-constrained nuclear pulse propulsion, and fast enough to send a colony ship to alpha Centauri even without fancy cryogenics, digital DNA, or artificial wombs. But yeah, won't be holding my breath for advanced fusion that can be done efficiently in a feather weight space bound reactor.

Offline edzieba

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Apparently there are only two ways to salvage (sort of) the Bussard RJ concept. One is bringing "an easier type" of fuel along to fuse and then mixing it with what the scoop collects as reaction mass. The Ram Augmented Interstellar Ramjet. (RAIR),
or most feasible, laying down fuel "packets" infront of the stove pipe like vessel prior to vessel launch. Fuel and Vessel would be lauched in a way to where the vessel intercepts the fuel in a "head on" fashion vis-a-vis opposition Olberth launching. Opposite directional Olberth launching of both fuel and vessel is to increase velocity of both to ensure high velocity impacts.
Fusion ignition occurs when the space "ship" impacts the fuel at high velocity. There is no scoop on the second version to create "drag" in the IP or ISM.
If you're going for an impulsive Oberth burn, you may as well skip the interstellar medium and the enplaced fuel packets altogether, and built a Bussard scoop to ingest the stellar atmosphere as propellant (nice and dense, and pre-heated!). That'd be a REAL sundiver of a probe!

Offline Greg Hullender

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I see the article sort of sloughs off the question of whether you can usefully fuse protium (ordinary hydrogen) in such an engine. The sun does it, but it does it very slowly. All man-made fusion uses deuterium and tritium, but those are quite rare between the stars.

If you can't solve the protium fusion problem, the other engineering challenges of the ramjet are irrelevant.

Offline octavo

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Maybe this is a dumb question, but how does a Bussard ramjet slow down? Surely the scoop must always face the direction of travel? So once you're at 12% C, how do you stop?

Offline edzieba

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Maybe this is a dumb question, but how does a Bussard ramjet slow down? Surely the scoop must always face the direction of travel? So once you're at 12% C, how do you stop?
Either keep the scoop on but not fusing, and use it as a drag brake. Or keep your scoop facing forwards and point your engine exhaust backwards for retro-thrust.

Offline qraal

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I believe the Cassenti toroidal field scoop concept was suggested to try to help mitigate the drag effects of what Bussard originally envisaged as/for his ram scoop.
I image you are aware Andrews and Zubrin found the original Bussard scoop concept to create enough drag in the ISM to limit the vehicle to +/- 12% of C....

That's not what they found. What they did find was that a simple magnetic loop to scoop interplanetary matter would produce too much drag.

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that's if P-P fusion were possible and the ISM were dense enough to collect that amount of hydrogen. Note the the drag + lack of adequate fuel collection and our inability to even consider P-P fusion make the the BRJ untenable.
 K Hambsch

Part of the problem is that the p-p reaction is awfully slow. In Solar core conditions it happens due to tunneling and the effective half-life of the protons is billions of years even so. A few million degrees hotter and the CNO Cycle is much faster and the dominant fusion cycle in all Main Sequence stars heavier than the Sun. But it has some slow steps too, so it might not be practical either - something Dan Whitmire handwaved in his paper on using it in ramjets.

Offline Vahe231991

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Hasn't the Bussard ramjet been questionable since about 1978?  See Heppenheimer, T.A. (1978). "On the Infeasibility of Interstellar Ramjets". Journal of the British Interplanetary Society. 31: 222

Not referenced in this recent paper, so perhaps something is different.
The paper titled "The Fishback ramjet revisited" focuses largely on John Fishback's variant of the Bussard ramjet envisaged in 1969.

Offline InterestedEngineer

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It's feasible from a pure physics standpoint, but the associated engineering challenges are currently insurmountable, the authors concluded.

Funny argument, there's a whole slew of people who assert that if it's physically possible, then "it's just engineering".

Offline daveklingler

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Wait...they're saying that it's theoretically possible, but we don't know how to build one yet?

I'm shocked that we can't build a Bussard ramjet, in this day and age.  Next they'll be telling us they can't build an interstellar spacecraft.  Or a General Products hull.

Offline Lampyridae

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Wait...they're saying that it's theoretically possible, but we don't know how to build one yet?

I'm shocked that we can't build a Bussard ramjet, in this day and age.  Next they'll be telling us they can't build an interstellar spacecraft.  Or a General Products hull.

Clearly we just need to find some Slaver stasis boxes.

The lighthuggers from Revelation Space were supposed to be some sort of ramjet but they had insanely advanced technology for p-p fusion (not even the crews seemed to really understand it). It would require fiddling with the weak force somehow and maybe something else like a muon. p-p molecules are possible with muon catalysts but they fly apart almost immediately, unless one of the ps undergoes beta+ decay.

Otherwise, maybe a ram augmented fusion engine, with interstellar p and stored d, giving you He3 exhaust and ~5MeV. Perhaps some kind of breeder cycle might work using the interstellar p.

Who knows, maybe Fermilab is onto something with their possible fifth force discovery and that could eventually be applied.

*I am not a physicist
« Last Edit: 08/16/2023 06:25 pm by Lampyridae »

Offline briantipton

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Wait...they're saying that it's theoretically possible, but we don't know how to build one yet?

I'm shocked that we can't build a Bussard ramjet, in this day and age.  Next they'll be telling us they can't build an interstellar spacecraft.  Or a General Products hull.

Clearly we just need to find some Slaver stasis boxes.

The lighthuggers from Revelation Space were supposed to be some sort of ramjet but they had insanely advanced technology for p-p fusion (not even the crews seemed to really understand it). It would require fiddling with the weak force somehow and maybe something else like a muon. p-p molecules are possible with muon catalysts but they fly apart almost immediately, unless one of the ps undergoes beta+ decay.

Otherwise, maybe a ram augmented fusion engine, with interstellar p and stored d, giving you He3 exhaust and ~5MeV. Perhaps some kind of breeder cycle might work using the interstellar p.

Who knows, maybe Fermilab is onto something with their possible fifth force discovery and that could eventually be applied.

*I am not a physicist
What surprises me about this paper is the attitude that "we can't reach the galactic core in our lifetime, so the ramjet is a bust" * That is setting the bar rather high! I call the results of the paper quite encouraging.

Granted the paper also says that the assumptions are wildly optimistic in a number of areas so the results represent an upper limit, not the expected outcome. And, I'll also grant that the very premise of using the p-p reaction in a ramjet is a stretch. But the actual conclusion is that, given those optimistic assumptions the limits of a graphene structure (which is NOT unobtainium) will limit the ramjet to 3 years of 1g acceleration. That would mean a velocity of 0.997c and a tau factor of 0.08. So, they're right that the galactic core is out of reach, but hundreds of stars would be within a few decades travel. I'd settle for that! Unfortunately, I suspect p-p fusion and the other practical problems are not going to be wished away and we will be lucky if we can even make RAIR work.

*I think this is a nod to a very specific sci-fi story that encounters exactly this problem.

 

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