Study: 1960 ramjet design for interstellar travel—a sci-fi staple—is unfeasible | Hacker News Reader

Study: 1960 ramjet design for interstellar travel—a sci-fi staple—is unfeasible

arstechnica.com

72 pointsBerislavLopac36 comments

It 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)

Larry Niven has a few e.g. 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?

Use of Bussard ramjets is a distinct historical phase in the numerous stories set in Niven's known space [0] universe - they are how mankind reaches and colonises the nearby stars before being given access to FTL hyperdrive by the Outsiders during the first Man-Kzin War.

[0] https://en.wikipedia.org/wiki/Known_Space

"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.

Yes: https://memory-alpha.fandom.com/wiki/Bussard_collector

I think the problem is more that if you do FTL in normal physics, it's really hard to plot around the time dilation thing. There have been a few stories that have played with this, but generally speaking, having anybody who sits on a planet basically instantly dying from the perspective of anybody on a fast ship is pretty limiting from a narrative perspective.

Andy Weir's Project Hail Mary has interstellar travel at 0.9c with induced coma for the crew.

The incredible Deepness in the Sky and prequels, for example. Perhaps Bobiverse? All top tier scifi.

There’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.

If 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?

You'll have to disassemble the bodies, obviously.

They’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.

Why would that happen only to the antimatter and not to matter?

The physics is quite complicated and I certainly don’t understand it fully.

Let's go Jamie, We've got some planets to mine.

Bussard ramjet

Can I have your autograph?

I just saw this video explaining how ramjet engines work >> https://www.youtube.com/watch?v=kHSJiO_Fvvw . It is under 8 minutes btw. Fascinating stuff.

Not 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?

Magnetic 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.

This 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.

https://www.sciencedirect.com/science/article/pii/S009457652...

Looks like it's back to interstellar sails:

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

And 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.

0.12c would still be incredibly useful. Proxima Centauri in 35 years? Game changing.

for 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

If 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.

What would happen if you entered its orbit at that speed?

edit: according to 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.

Because 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.

The eye-opening bit for me was that any system even to seriously consider would have to be millions of km long.

I'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

That 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.

A 4,000 km diameter is the figure in the article.

A 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.

Ramscoops feature prominently in the Queng Ho series by Vernor Vinge, e.g. Deepness in the Sky.

Is 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?

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