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Study: 1960 ramjet design for interstellar travel—a sci-fi staple—is unfeasible
- ncmncm 5y agoThe eye-opening bit for me was that any system even to seriously consider would have to be millions of km long.
- deleted 5y ago[deleted]
- R0b0t1 5y agoThat is new to me as well. Previous analysis just based on matching input to output momentum showed that a ramjet wasn't as effective as claimed, but this is even worse.
- nkurz 5y agoI'm not familiar with the design, but where are you getting the "millions of km" from? I didn't see that in the article, and my quick skim of the paper[1] suggested that that Ls (the length of the coil) would need to 200 km (given shortly after equation 15). [1] https://reader.elsevier.com/reader/sd/pii/S0094576521005804 https://reader.elsevier.com/reader/sd/pii/S0094576521005804
- Jeff_Brown 5y agoA 4,000 km diameter is the figure in the article.
- ncmncm 5y agoA 4000 km-across mouth, at the front, but how far in front of the pinch point must that be? At issue is how to hold it out there, with the whole structure under compressive stress, without a huge structural mass.
- cwillu 5y agohttps://www.sciencedirect.com/science/article/pii/S0094576521005804 https://www.sciencedirect.com/science/article/pii/S009457652...
- LorenPechtel 5y agoAnd even if you can build the scoop a Bussard Ramjet has a theoretical efficiency limit of .12c unless you can somehow reclaim the energy of the infalling matter.
- antonvs 5y ago0.12c would still be incredibly useful. Proxima Centauri in 35 years? Game changing.
- dendriti 5y agofor a travel time of 35 years, you'd not get very long at Proxima Centauri, because you'd still be doing 0.12c when you arrived
- nine_k 5y agoIf you could keep an acceleration of 1g, it would take about 40 days to accelerate to 0.12c, and as much to decelerate. The problem, of course, that with a Bussard ramjet, you won't be able to achieve nearly as much acceleration.
- fulafel 5y agoWhat would happen if you entered its orbit at that speed? edit: according to https://rechneronline.de/g-acceleration/orbital-speed.php https://rechneronline.de/g-acceleration/orbital-speed.php, if I got my numbers right (0.12 x solar mass star, .12c = 36000 km/s), the orbit radius would be 1230 km, so you'd be almost at the core of the star since its radius is ~ 100 000 km.
- TheOtherHobbes 5y agoBecause if you don't reclaim the energy of matter infalling at 0.12c your craft gets shredded by dust. At that velocity it's like flying through sandpaper. And if you're flying through a cometary halo, your odds of hitting something bigger are much higher.
- zizee 5y agoNot sure where I read it, but someone was saying that whilst the numbers don't work for the ramjet, the magnetic funnel could still be useful in slowing a craft on its approach to another solar system. Does this study rule that our as well?
- beckingz 5y agoMagnetic breaking is still quite feasible. There are some interesting proposals to build "Q" drives, which would use plasma magnets to generate energy from the solar wind and put that energy into ion thrusters.
- pfdietz 5y agoThis approach could also work with interstellar plasmas, using the motion of the vehicle vs. the plasma to generate power, which then is used to generate thrust by expelling reaction mass. This is not perpetual motion: it is concentrating the initial kinetic energy of the vehicle (in the rest frame of the interstellar medium) in less and less mass as reaction mass is expelled. This might be implemented by using diffuse structures to induce a plasma wave in the ISM, and then accelerating particles in that wave's electric field. No large compression of the plasma (as a putative fusion-based drive would need) is required.
- nynx 5y agoThere’s an interesting theory that antimatter from the early universe is effectively condensed into ‘quark nuggets’, extremely dense colorsuperconducting blobs at the center of massive bodies like planets, stars, and asteroids. They can effectively be turned into antimatter at an efficiency of 10%. If these exist, they could be used for rapid interstellar travel without things like buzzard ramjets.
- causi 5y agoIf these exist, they could be used for rapid interstellar travel without things like buzzard ramjets. In what way, if they're at the center of massive bodies?
- Filligree 5y agoYou'll have to disassemble the bodies, obviously.
- nynx 5y agoThey’d be accessible inside of asteroids. There appear to be some asteroids that are spinning too fast for their size, implying they have an unexpectedly massive core, but there is still no conclusive proof the quark nuggets exist.
- corndoge 5y agoBussard ramjet
- senortumnus 5y agoCan I have your autograph?
- FreeFull 5y agoWhy would that happen only to the antimatter and not to matter?
- nynx 5y agoThe physics is quite complicated and I certainly don’t understand it fully.
- optimalsolver 5y agoLooks like it's back to interstellar sails: https://en.wikipedia.org/wiki/Starwisp https://en.wikipedia.org/wiki/Starwisp
- car 5y agoRamscoops feature prominently in the Queng Ho series by Vernor Vinge, e.g. Deepness in the Sky.
- tialaramex 5y agoIt mentions Tau Zero, but actually just how often are Bussard Ramjets invoked in SF stories? Where there's no travel over great distances (e.g. most Cyberpunk and other SF about what happens here in the near future) this doesn't come up at all. e.g. Diamond Age doesn't need a a way to do interstellar travel since the furthest anybody goes is an airship ride to Europe or America. Today's soft SF generally invokes some means of FTL travel (and so no practical mechanism needed) if the story will involve travel between solar systems, maybe it's wormholes, or there's a "warp drive" or that sort of thing. Banks' Culture novels do periodically try to "explain" how FTL travel works in their world but it's basically just magic. No need for a Bussard ramjet when you have magic. And most hard SF avoids interstellar travel altogether because it's such a problem. Greg Egan has it simply take an incredibly long time in the Amalgam stories, Stross doesn't have it in Saturn's Children, even travel between planets is exhausting and expensive - (and although I guess his short story "Bit Rot" sort of counts, I don't think they were using a Bussard Ramjet for whatever they were attempting before the rot set in)
- fsloth 5y ago"how often are Bussard Ramjets invoked in SF stories?" If I recall correctly in Star Trek the glowing blobs at the front of warp engine nacelles of the Enterprise are ramscoops to collect fuel for the impulse engine.
- k1t 5y agoYes: https://memory-alpha.fandom.com/wiki/Bussard_collector https://memory-alpha.fandom.com/wiki/Bussard_collector
- empedocles 5y agoLarry Niven has a few e.g. https://en.wikipedia.org/wiki/A_World_Out_of_Time https://en.wikipedia.org/wiki/A_World_Out_of_Time but I agree that realistic interstellar travel is more often found in older fiction, as we did not yet know how hard it was. What would be realistic now?
- 5y ago
- Jeff_Brown 5y agoIs it feasible for a ship with a diameter of 4000 km to not be destroyed by the space debris it catches on an interstellar flight?
- ahmed_ds 5y agoI just saw this video explaining how ramjet engines work >> https://www.youtube.com/watch?v=kHSJiO_Fvvw https://www.youtube.com/watch?v=kHSJiO_Fvvw . It is under 8 minutes btw. Fascinating stuff.