Rocket Science–Real and Imagined

Every science fiction show that has its protagonists zipping around the solar system and traveling from one planet to another in a few days or weeks instead of months or years requires some fictional propulsion system. Star Trek of course has its warp drive.

The scientific details are a little fuzzy—somehow dilithium crystals are involved—but that isn’t really important anyway. Warp drive is just a device to get us from one interesting place in the galaxy to another. Or in the case of the original 1960s series, Captain Kirk from one attractive green-skinned female alien to the next.

The Expanse, a series currently in its fifth season on Amazon Prime, is perhaps the most scientifically realistic show about crewed interplanetary space travel ever aired. And even here, for dramatic purposes, we have a fictional propulsion system, the “Epstein Drive.” Let’s look at both the realistic and the fictional aspects of the rockets in The Expanse.

THE (ALMOST) REAL THING

A rocket works by throwing stuff in one direction to make the rocket go in the other direction. The more stuff you throw, and the faster you throw it, the faster the rocket goes. There a limit to how much propellant you can carry on a practical spacecraft with humans. Life requires a lot of infrastructure that robot spacecraft can do without: food, water, oxygen to breathe, livable temperatures. The mass of more fuel requires fuel to move it, and there comes a point of diminishing returns.

Therefore, most effort in making rocket engines more efficient focuses on ejecting the propellant exhaust faster. Generally this means heating the propellant to higher temperatures. The most efficient chemical propulsion systems use the combustion reaction of hydrogen and oxygen. This burns quite hot, and the exhaust from this sort of rocket engine leaves at a high velocity.

How do you make a hotter—and faster—exhaust?

Nuclear reactions produce far more energy pound for pound than chemical reactions. In the 1960s, American scientists developed and ground-tested rocket engines that heated hydrogen propellant in a nuclear fission reactor, the same kind of reactor, albeit considerably smaller, that is at the heart of a nuclear power plant. The result is a rocket engine that is twice as efficient as the best chemically powered rocket.

Such a system would likely only operate in space, where safety and contamination worries would be minimal.

Nuclear fission reactors, where heavy atomic nuclei are split into smaller ones, are a proven technology—they have existed for nearly 80 years. Nuclear fusion, where light nuclei are fused into heavier ones, is an even more powerful source of energy. An uncontrolled fusion reaction is the source of the destructive energy of a hydrogen bomb; taming and controlling nuclear fusion remains an unmet challenge in 2021.

But if a fusion reactor could be built small enough to be carried on a spacecraft, it could heat propellant to even higher temperatures, and make for an even more efficient engine. A design under development at the Princeton Plasma Physics Laboratory would provide both propulsion and electrical power for a spacecraft. The engine would be more than twenty times as efficient as the best chemical rocket.

THE (OH, IF ONLY!) THING

On The Expanse Wiki page (https://expanse.fandom.com/wiki), the Epstein Drive is described thusly:

The drive utilizes magnetic coil exhaust acceleration to increase drive efficiency, which enables spaceships to sustain thrust throughout the entire voyage. A ship fitted with the efficient Epstein drive is able to run the drive continuously for acceleration to its goal and then after flipping at about the halfway point is able to run the drive continuously during deceleration. Previous engine designs used propellant less efficiently and could not be run long enough to achieve the high velocities that the Epstein drive permitted.

The same page tells us the Epstein Drive would—somehow—be almost 2000 times as efficient as the still theoretical Direct Fusion Drive!

There are two advantages this would give. A trip between Earth and Mars would only take about three days! There is another advantage here that may be less obvious. By running the drive continuously, the crew would experience zero gravity only at the mid-course flip-over, negating all the bone and muscle-wasting effects of this hazard of long duration space travel.

In the show’s timeline, Solomon Epstein invents his eponymous drive in 2205. We can only hope!

Here is about ten minutes of flying spaceships from the show. It is definitely one of my favorites.