The demise of the Bussard ramjet. Long live the Bussard ramjet. Can we save it @ Worldbuilding?

Question

Robert Bussard's ramjet, as featured in Tau Zero and most recently noted by Carl Sagan in Cosmos, may be dead. In 1960, Bussard proposed a space propulsion system that relied on a nuclear electric propulsion engine that generated a powerful electromagnetic field. This field would draw in ionized hydrogen (protons) from the interstellar medium (ether?), pass it through a constricted magnetic field, and ignite a fusion reaction. The field would channel the excess energy aft, and thus generate nearly limitless propulsion.

Bussard's ramjet had a nearly fatal blow a short time after, when it was discovered that the concentration of interstellar hydrogen was significantly lower than he had hoped. A young college student at MIT, John Fishback, came to the rescue in 1969 (a good year for space) and came up with an axial scoop field configuration that seemed to save the ramjet. Sadly, this young man committed suicide a few years later while working on his doctorate at UC Berkeley, no doubt a loss for science.

There is a new paper (the first in nearly 50 years on this topic) that has reanalyzed Bussard's ramjet and Fishback's revisions. The good news is that the ramjet still appears to be physically possible. The bad news is that it now borders on the absurd. In next month's issue of Acta Astronautica, Schattschneider and Jackson (2022) argue that a surprisingly large solenoid radius (2,000 km to 4,000 km) would be needed to scoop up enough hydrogen to cause fusion. Current materials science (including graphene) limits the absolute speed the ramjet could travel at, thus dramatically reducing its effectiveness for interstellar travel (c.f. this Ars article).

From a hard science perspective (well, medium-hard), what are some ideas to save this ramjet design? I'll start first - please feel free to improve on them.

• Create a powerful magnetosphere, and allow the resulting magnetotail to funnel the hydrogen into the reaction chamber.
• Use a carbon nanofiber weave to create a very large surface area, then use implanted magnets to generate the powerful EM field
• Change the model. Instead of direct flights between two stellar points, have the journey go via various stars. Scoop up hydrogen where it is more concentrated to replenish an onboard supply.
• As for the current materials not being hard enough - couldn't we just use stronger magnetic fields to contain the reaction and also to deflect items away from our carbon nanoweave?

Answer 1

There are multiple problems with pure ramjets, none of which are easy to overcome. The two big ones that spring to mind are:

1. Thrust is proportional to the mass flow through the rocket. In space with a nice homogenous distribution of suitable materials, mass flow into the scoop is proportional to the ship's velocity. Unfortunately, drag forces on the scoop will be proportional to the square of the ship's velocity.

   Various efforts have been put towards mitigating drag losses, but they can't eliminate it which means that your ramjets will always have a "top speed" which might be lower than that available from a more conventional rocket. Irritatingly, I've misplaced my references for this particular issue so I can't quantify it. I might update this answer in the future if I remember (most of the interesting stuff is in The Spaceflight Handbook, if you can find a copy).

2. Most of the stuff you find in space (or the universe as a whole) is plain old hydrogen, some molecular, some monatomic. Hydrogen fusion obviously works (exhibit a: the Sun) but only on stupendously long timescales mediated by the decay of diprotons into deuterium which is a very infrequent event. Quoth wikipedia, "The average proton in the core of the Sun waits 9 billion years before it successfully fuses with another proton".

   There are workarounds for this, sorta... Whitemire proposed in Relativistic Spaceflight and the Catalytic Nuclear Ramjet that you could use the C-N-O fusion cycle or Ne-Na fusion cycle (which I don't have a handy link for) which fuse protons with heavier elements, but the heavier elements don't get used up (and losses might be replenished by any heavier particles that get scooped up). The problem there is that such hot reactions are fiendishly difficult with vast bremmstrahlung losses. Obviously giant stars manage it, but scaling it down to spacecraft size remains unobtanium.

The best alternatives that I'm aware of are Ram Augmented Interstellar Rockets, for which I don't have any free and publicly available sources, and I've misplaced my copy of The Spaceflight Handbook but Project Rho has a summary. Basically RAIR spacecraft have a limited supply of fuel on board, and so don't have the infinite delta-V that was imagined for classic ramjets. Unlike pure rockets though, they make use of scooped matter in their engine cycle and so can have a greater delta-V than pure rockets with the same amount of fuel.

There are two kinds of these, more or less.

One involves carrying ⁶Li or ¹¹B fuel, and use the incoming stream of protons to trigger fusion in those fuels. Although sometimes called "catalytic", the boron or lithium isn't a mere catalyst and is actively consumed during operation and is probably impractical to replenish in flight as you won't be able to scoop up enough of it. There's a minimum speed required here so that the incoming proton stream has enough energy to induce fusion, and (for p-11B, at least) the velocity of the fusion products isn't as high as other reactions, if Project Rho is to be believed, which causes losses through propulsive inefficiencies.

So, as is inevitable with all interstellar rocket designs, we end up with antimatter.

You build yourself a suitable antimatter rocket (the classic would be "beam core", though for various complex reasons an antimatter reactor connected to a plasma rocket might be better) but you only need your rocket to carry enough matter to get up to scooping speed, and after that everything is "free".

For an interstellar rocket, halving the amount of fuel/reaction mass you need to carry for a given delta-V is a big deal. The ram-augmented antimatter rocket can fly faster than its unaugmented cousin, travelling further in the same period of time. For an interstellar rocket, this would seem to be a pretty good design.

In a slightly less hard scifi setting there are even better alternatives. If you can some way to either convert mass directly to energy or convert matter to antimatter, you might not even need to carry your own fuel. The Orion's Arm universe project uses monopole-based mass-to-energy conversion (monopoles are theorised to trigger proton decay) which is not quite implausible but is right on the edge of it. Other possibilties include Q-balls (which require supersymmetry to be a thing, and it is increasingly looking like it isn't) which might let you convert matter to antimatter or other nuclearites like strangelets. You are of course in charge of your setting, and you get to decide which kinds of unobtanium can, in fact, be obtained.

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To address your suggestions:

• Create a powerful magnetosphere, and allow the resulting magnetotail to funnel the hydrogen into the reaction chamber.

You kinda need a powerful magnetic field of some kind. Its exact nature is somewhat open to debate. If you had magnetic monopoles, you could create a much stronger field in a much smaller volume (because the external field strength of a monopole follows the inverse-square law, but the far-field strength of a dipole follows an inverse cube law) or have a much larger scoop, but monopoles are unobtanium.

• Use a carbon nanofiber weave to create a very large surface area, then use implanted magnets to generate the powerful EM field

Without wish to sound rude, this sounds like technobabble.

• Change the model. Instead of direct flights between two stellar points, have the journey go via various stars. Scoop up hydrogen where it is more concentrated to replenish an onboard supply.

If you're doing that, why not just park and fill up with more conventional fuels instead? This just sounds like an awkward way to hoover up reaction mass that you then have to carry around with you, which is the complete opposite of the ramscoop promise.

• As for the current materials not being hard enough - couldn't we just use stronger magnetic fields to contain the reaction and also to deflect items away from our carbon nanoweave?

Field strength alone won't save you, though obviously more powerful fields are very desirable. You still have to deal with the drag problems, which is a very difficult problem that remains unsolved.

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Personally, I think interstellar rockets are a bit of a dead end (because of the old rocket equation) and something like Kare's Sailbeam is the way to go. I've waffled on about that in multiple other answers though, so I'll spare you all this time.

Answer 2

They could be used as part of a larger infrastructure. Some writings on this point out that the route could be pre-seeded with material (not necessary hydrogen; I don't recall the details). So, you have slow bulky ships "paving" the routes, and this allows many fast light craft to traverse that route efficiently.

Answer 3

Use light instead.

In the lab, physicists can move around atoms in a vacuum using laser light. It is possible to herd a cluster of atoms into a Bose-Einstein condensate this way; in other words, to "solidify" (or at least concentrate) gas out of the vacuum using a variant of "laser tweezers".

Now when you're using light, you're in the realm of optics, which is one science that has really been advancing lately - the funding beneficiary of too much post-2001 focus on surveillance, I think. Crazy, impossible advances include "free-space data-carrying bendable light communications", where "light bends by itself". (I'm still not totally convinced there isn't a trick about this... but it's what I read). For purposes of sci-fi, to the boundaries of my tiny knowledge, this sounds like a way to argue that you can send out a beam of light with any shape - such as blossoming to a cylinder and then paralleling the spacecraft's trajectory in space. If the beam points approximately forward, then all the energy is going into collecting future fuel, even if it takes light years for all of it to be absorbed. That beam can be tuned to shepherd hydrogen atoms or ions inward at an ever increasing velocity until they meet whatever final stage you have to finish concentrating the material.

I should add that if you look over the Centauri Dreams article, the ramjet is not quite as dead as you'd think, since the distance travelled is proportional to the square of acceleration. If you accept travel times in the hundreds of years, a plain ramjet can be absurdly but perhaps not impossibly large. But yes, it's time for a rethink.