Is the concept of the Epstein Drive based on actual theorized scientific research which is considered plausable?

Question

I've always found it interesting when we see real life taking its cues from fictional arts, especially when it comes to Science Fiction technology. There are plenty of examples of classic books, TV shows, or movies having introduced what was at the time a fantastic futuristic tool or capability. Then, decades later would be eventually developed and used in everyday society.

Some examples are George Orwell's book 1984, published in 1949, which predicted the wide use of PCs and the Internet.

This question relates to a more recent a series of books, and now TV show, The Expanse. Taking place centuries in the future, many of the technologies used in The Expanse are well beyond our current technological capabilities, some being more plausable than others.

My question relates to one of these examples which I hope is plausible, but I just don't know, the Epstein Drive.

The Epstein Drive is a commonly used propulsion system in The Expanse universe, one which makes interplanetary travel for humans both feasible and efficient. However, it seems to defy what I understand as current limitations in material/fuel efficiency capabilities. Unfortunately, I am not knowledgeable enough in this discipline to understand if something like the Epstein Drive is actually feasible, why it would be, or how it could be accomplished.

However, my asking how this could be feasible, which requires one to suspend reality momentarily in order to answer, seems outside the scope of this board. Instead, I will ask the following:

Are the concepts of the Epstein Drive, a fictional propulsion drive technology of the book and TV show The Expanse, in any way based on actual theorized scientific research which could render the fictional capabilities included in the story technologically plausable at some point in the future?

Answer 1

From what I've been able to find, the "fluff" is that the Epstein Drive electromagnetically accelerates the exhaust, improving propellant consumption over a "standard" inertially-confined fusion drive. Electromagnetically accelerating plasma is certainly physically plausible...real world ion and plasma thruster designs do just that. The problem is that electromagnetically accelerating plasma takes power, with the power requirements scaling with the square of the exhaust velocity. These drives are limited to low amounts of thrust not because they can't be built to handle more, but because current power sources don't have enough power density to let them do better.

The whole idea of a fusion rocket is to get at most of that power by fusion. The fusion heats the propellant directly and you don't have to generate and handle the power output in electrical form, you just need enough to keep the fusion reaction going and run the magnetic nozzles/etc. Fusion would have extremely high energy density, and hopefully, not having to convert it all into electrical form would let you achieve higher power densities and more thrust...you just have to fuse your fuel fast enough and not burn yourself up in the process.

An "Epstein Drive" that electromagnetically accelerates fusion rocket exhaust would improve propellant consumption as described, but it would also add terawatts to the electrical power requirements and render the fusion portion largely superfluous. Even just doubling the velocity (and halving the propellant requirements) with perfect efficiency would mean that the "accelerator" would account for 3/4 of the power output (and a much bigger fraction of the power input). Thus, in reality not all that useful.

Answer 2

The properties of the fictional Epstein Drive are:

• Very high ISP with an exhaust velocity at least several percent of the speed of light
• Very high thrust while achieving that ISP or near to it, enough for 5+ g burns.
• Fusion powered.
• Based on the first two parameters, the thrust power is insanely high - this is not a fission sail, NTR, or low-power fusion thruster.
• The ships are not dominated by heat radiators, which means that of all the immense power handled by the engine, not even a very small fraction needs to be dissipated as waste heat by radiators.

The first four properties are within the theoretical capabilities of nuclear fusion. The fifth is very, very difficult to achieve, though it is not fundamentally forbidden by theoretical physics. It is not clear how it can be pulled off.

The basic idea is that nuclear fusion fuel is compressed and heated by some method so that it fuses, and then the resulting ultrahot plasma is allowed to expand outward through a rocket-nozzle-shaped magnetic field. This turns heat and pressure into motion, just like in a chemical rocket.

The slight problem is that while we have achieved breakeven with fusion, nobody has even approached doing it in a useful way, let alone with a compact reactor that can fit in a spaceship.

Most realistic designs that have been considered for this (and which have less performance than the Epstein Drive) use inertial confinement fusion, where discrete pellets of fuel are flash-compressed with converging lasers, particle beams, or pulsed magnetic fields (Z-pinch), causing them to basically explode. Most realistic designs also have the exploding pellets and the magnetic field nozzle outside of the spaceship so that waste heat can be radiated away without heating up a reactor wall.

A good example is the D-T VISTA concept. This doesn't reach anything like the Epstein Drive performance, and it's basically shaped like a cone made of heat radiators. We could do a lot better if we had quasi-magical forcefields able to reflect radiation and channel plasma.

For a lot more information (appropriately for this question, aimed at fiction writers but very much based in factual science), check out Project Rho's page on torchships (a name often used in science fiction for spacecraft with this level of performance).