Take Back the Sky

Courtesy Dark Horse Comics

by Jeff Cunningham

Like many engineers, I dream of making the fantastic ships and spacecraft of classic science fiction a reality. Take, for example, the simple concept of leaving Earth itself. Space is only 100 kilometers (about 62 miles) away. Your average automobile, which uses principles of internal combustion largely unchanged for over a century, can cover that distance five times over.

Turn that 100 km on its side, however, pointing straight towards the sky, and suddenly, things get much more complex, almost to the point of getting out of hand. Terms like “expendable launch”, “cryogenic tanks”, “reusable stages”, and “orbital refueling” get thrown in, jumbled around and re-arranged to try to manage energy and cost. Even the most advanced launch vehicles in the world, like the Falcon 9 and the Falcon Heavy, even if both vehicles become completely reusable, are still at the end of the day vehicles the size of skyscrapers used to cover one-fifth the distance that a Ford Focus can before its house-size fuel tanks are emptied.

This is why, unlike most people who are fascinated by exotic hyperspace engines and warp drives, as an aerospace engineer, I’m honestly more attracted and interested in the comparatively more mundane features of these ships. Sure, I’m glad the Millennium Falcon can make the Kessel Run in 12 parsecs and all, but I’m far more impressed that it’s able to lift off of a planet and land on another one without stopping to refuel.

Which brings me to the real star of our favorite cancelled sci-fi western television series. You see, while most popular sci-fi franchises will gloss over their faster-than-light drives with technobabble, their sublight counterparts (i.e. “thrusters” or “impulse engines”) get a wave of the hand, at best– except for Serenity, that is, which has a surprisingly detailed in-universe explanation of how a jet turbine engine can work in the vacuum of space– one that not only actually jives well with known science, and is more plausibly science-based than any other fictional ship, but is also actually close enough to reality to the point where you and I may live to see it.

For this special installment of The Science of Firefly, we’ll be taking a closer look at what literally “keeps her in the air” (aside from love, of course): Serenity’s thruster engines, engine pods or thruster pods.

For the sake of this article, I will use the term engines to refer to the jet turbines on either side of Serenity, so as not to be confused with the aft section that glows when preparing to “rabbit” or “go for hard-burn” for long-distance interstellar travel and no-doubt inspired the Firefly moniker– and also because “thruster” is a term more commonly used in real life to describe elements of what’s called a reaction control system, which would correspond to the tiny jets of gas you may see firing all over the ship when she’s performing precise maneuvers like docking.

A Tale of Two Engines

If you think back to scenes of Serenity in flight, you may recall two different modes of behavior that can be observed in her engines:

• While “in atmo,” the engines appear to be turbine-driven jet engines (convenient for kicking baddies into).
• While out in the vacuum of “the black,” however, the turbines are nowhere to be seen, while what appears to be rocket exhaust trails behind.

“So what?” you ask. “Sci-fi does this sort of hand-waving all the time. What difference does it make?” The answer is, actually, quite a big difference.

To understand why requires us to change the way we see planet Earth. We like to think of it as a wonderful oasis, the only place where anything important is possible. The truth, however (from a technical standpoint), is that it’s an engineer’s nightmare. Why? Because of the air we breathe.

Our planet has a really, really thick atmosphere– which is great for outdoor sports and giving lift to wings, but when you try to power your way through it in a car or airplane, you may as well be trying to jog in waist-deep molasses. As you get up to certain speeds– that speed being Mach 1, which actually depends on environmental conditions– you start to slam into the atmosphere like a supersonic belly flop. This is why many daring pilots actually lost their lives as their planes disintegrated while trying to be the first to break the sound barrier.

One of the ways our atmosphere presents a challenge is elementary physics. We all remember the whole “equal and opposite reaction” business, right? Well, in order to get that “reaction” (called thrust), we have to at least be equal, meaning that the air and gasses that an engine pushes out the back have to be equal to or greater than the surrounding atmosphere behind it (preferably equal to). Given how thick and high-pressure it is here at ground-level, that’s actually a lot harder than you’d think. It’s easier when you get up to altitude where the pressure is lower, but as with many other things in life, the hardest part of flight is getting started from a standstill.

That’s where jet engines come in handy. Those turbines you’ve seen in airliners suck in air and compress it to a useful pressure. Some fuel is sprayed into it and ignited, and the resulting controlled explosion (better known as internal combustion) causes the gas to expand even more. As the exploded gases head out the back, they pass by and push on a second turbine, which steals back some of that kinetic energy to help power the one in front, while the rest goes through a nozzle to compress it and speed it up to what will be required to actually produce thrust.

Jets are excellent for use in non-ideal conditions like ground-level here on Earth. They use the oxygen in the air around them to power the combustion process, and are one of the best, most efficiently powerful ways to get up off the ground from a standstill. The problem, as you’ve no doubt guessed, is that they’re completely useless in the vacuum of space. There’s no “back pressure,” as it’s called, so the amount of exit pressure you need coming out of the engine is much less, but it doesn’t matter when you have no air with which to produce that pressure.

At the other extreme, you have rocket engines. Instead of taking in air and combining it with fuel, they bring everything they need with them (propellant), then ignite and force it through a small opening to accelerate it further. Rockets can conceivably be made to work to fit a variety of different conditions– but not all of them at once. You see, a rocket that works really well at high altitudes or in space has the tradeoff of being terribly inefficient at the high atmospheric pressures of ground level. This is why “liftoff” is such a tense affair when you watch live streams of the launches, because it’s when the vehicle has to work the hardest under the most stress.

Via the Kerbal Space Program Forum — which is funny, because this diagram is better than the one in my actual rocket science textbook from my undergrad studies

In general (and I must stress how much I’m oversimplifying the subject of many a PhD here), jets are your best bet for dealing with the low altitudes of ordinary flight, but have a maximum ceiling. Rockets carry all the oxygen they need with them and are the only practical means of propulsion in a vacuum, but they’re terrible at ground level, where atmospheric pressure really puts a damper on its performance and efficiency.

The conditions surrounding a craft on planet Earth change very quickly over a very short distance or altitude (only 100 km, remember?), which is why it’s been so difficult to invent and build a single engine (as opposed to staging) that can go all the way from the ground into space (a single-stage-to-orbit, or SSTO engine). You’d have to come up with a way for an engine to drastically change its own flight characteristics to adapt to these changing conditions, which is not as simple as letting off the gas pedal.

Ever Sail in a Firefly?

So, how does Serenity pull it off? For that, we turn to the official Serenity Role-Playing Game Player’s Handbook, which refers to the engines in question as a “rocket-based combined cycle engine,” or simply “pods”:

Pods are often mounted on movable hydraulic swivels that allow them to shift direction quickly and easily. An engine pod runs in one of three modes. In
atmosphere, it’s open on both ends like a jet engine. Heated plasma from the fusion power plant is routed to the pod, heats up a mass of air, and blows
it out the back to produce thrust. At low speeds, the heated air also spins a turbine that sucks in still more air from in front to get the cycle started. Once the ship goes supersonic, these turbine blades fold back out of the way and the pod continues in scramjet mode. Running the engine in air-breathing mode is very efficient, since most of the thrust comes from the air itself, and not from the plasma.

When the ship breaks atmo, there isn’t enough air for the engine to work on. The intake irises shut, and the pod switches to pure rocket mode. Extra hydrogen “fuel” is routed through the fusion plant to produce a steady stream of high-energy plasma, which the pod pumps out at extreme speeds to produce thrust.

So, a Firefly-class spaceship combines the functions of both a jet engine and a rocket with an intermediary phase, the “scramjet.” Scramjets and ramjets essentially are jets without the turbines. They simply let air rush in like a dog sticking its head out the window of a moving vehicle and it compresses under its own power (which is what makes for great Instagrams as said dog’s jowls flap in the breeze). The catch is, that only works if you’re already in the air and already going pretty fast.

So, from the top– or rather, from the ground, Serenity’s engines:

1. Start as jet engines to get off the ground, then get it up to a high altitude and speed.
2. At said high altitude and speed (around the speed of sound), the turbine blades retract to let the air in unhindered, transforming into a scramjet.
3. Once they start to exit the atmosphere and run out of air, the front end shuts, and the burning fuel just keeps blowing out the back like a rocket engine.

NOTE: As an added bonus, it uses the same fuel that the onboard nuclear fusion reactor does, which in addition to saving weight, also gives them a carte blanche as far as the laws of physics are concerned. Like the Death Star and Iron Man’s powered suit, when energy isn’t an issue, you can pretty much do whatever you please. In this case, Serenity has more than enough energy to compensate for any inefficiencies in its engines with raw power. More on this in a future installment.

They make it sound so simple, right? So why doesn’t this exist yet?

How You Get There’s the Worthier Part

Well, it does and it doesn’t. We’re not there yet, there’s a long way to go, but it’s barely visible beyond the horizon of the future. For the past couple of decades, a good part of American efforts to create an SSTO was in aerospike development, which is where you essentially take the bell-shaped nozzle at the end of a rocket and turn it inside out to allow it to adapt to a wider range of atmospheric pressures at the cost of power. This approach requires the ship powered by it to have a really, really low mass ratio (meaning that its weight with fuel tanks empty has to be less than 10% of what it is when the tanks are full), which will require further advances in lightweight materials like composites.

The little bit about “retracting turbine blades” is one aspect that’s much easier said than done. Each of these blades is a precisely engineered device with no room for even the slightest imperfection, scratch or ding. Barring more of those advances in materials science– which do continue to produce all manner of new ceramics and composites that might actually make this possible– trying to move these things in and out of a supersonic gas flow within the engine would be akin to jumping out of an airplane with your parachute already opened.

There is hope, however. A British company is building their own “synergistic air-breathing rocket engine”, SABRE, to power a spaceplane of their own.

She’s no Serenity, but Skylon is a beauty nonetheless.

Rather than retracting the turbines, the SABRE engine changes phases through some clever workarounds such as helium pre-cooler loops to try to skirt those issues and lesson the blow to performance and inefficiency as atmospheric conditions change. This is no mere CGI render, mind you– successful propulsion tests have been conducted, and they’ve received over 60 million British Pounds’ worth of funding to build this by British defense contractors and the European Space Agency. We’re still a ways off, so rockets will continue to be the most cost-effective way of flying out into the black for a while, but it’s immensely gratifying to think that we may yet live to see the day when doing so is as natural and casual as pulling your car out of the driveway.

Love Keeps Her in the Air

Those rockets are nothing to sneeze at, mind you– new vehicles like the Falcon 9 and Dragon spacecraft are now capable of doing things people could only dream of during the space race. There are rumblings in the aerospace community (and from my own sources in the world of aerospace) that we can expect the first flight of American astronauts aboard a private ship as soon as the summer of 2018, and it’s only fitting that it be named after another “independent” ship (see what I did there?) that carried both the crew and their hopes and dreams out into the Black.

Write a letter at the next chance you get to Elon Musk, Gwynne Shotwell (a Browncoat!)– or heck, go ahead and write the astronauts who have been chosen to fly the thing while you’re at it; they particularly enjoy getting mail from the public.