Getting Started

DISCLAIMER: I am not a rocket scientist, merely an amateur that has read a lot of books. Any and all of the information on these pages may be incorrect or inaccurate.

But since I have yet to find a website like this written by a real live rocket scientist, I had to write it myself, as unqualified as I am. However, if I do put inaccurate information on this site, occasionally Internet readers will send me corrections. This is the "stone soup" method of website design, with me supplying the stones. Frankly this website is a glorified term paper.

There is some controversy over a few technical details on this site. If you disagree with something you read on this site, first check this page. If you still disagree, your best bet is to go to the Usenet newsgroup and the Yahoo group SFConSim-l and present your case there. If it survives both, I will incorporate your contributions.

Be told: since this site is so huge, and people tend to just look at one or two pages, I found it useful to repeat identical stretches of text on several pages. That way people will not have to jump around so much.

Carol Maxwell: "What makes you think you can discover anything? Who are you?"

Allan Maxwell: "Nobody. Nobody at all. But the secrets of the universe don't mind. They reveal themselves to nobodies who care."

If you do not understand why space exploration is so important, just watch this video. Be sure to make it full screen.

Keep in mind that whether you are designing a rocket for your SF novel, or for a science fictional wargame, the ship can be the center of attention. Think about the proud series of vessels which formed the focus for high adventure. Vessels with names like Enterprise, Serenity, Seaview (yes, that's a submarine but the principle applies), Luna, United Planets Star Cruiser C57D, Space Battleship Yamato, Battlestar Galactica, Millennium Falcon, Fireball XL5, Polaris, Space Beagle, Discovery, Valley Forge, Dark Star, and the Nautilus (another submarine). And ships that never quite made it, like the Strategic Star Command Galactic Cruiser Leif Ericson. In many cases, the ship becomes a character in and of themselves.

The more real you make the ship, the more real the rest of the story will be.

If man survives for as long as the least successful of the dinosaurs—those creatures whom we often deride as nature's failures—then we may be certain of this: for all but a vanishingly brief instant near the dawn of history, the word 'ship' will mean— 'spaceship.'

Arthur C. Clarke, quoted by Hugh Downs, Ad Astra, Fall 2008

(ed note: master spacecraft designer Ray McVay comments on the Space Navy Develpment System)

How can a little chart do so much? I've said it before: Soft Science Fiction tries to make technology fit the imagination, and Hard Science Fiction tries to imagine what fits the technology. This chart provides a great framework of technology, logistics, and practical organization to fit one's imagination into.

Introducing RocketCat

This is RocketCat, the official mascot of the Atomic Rockets website. Do not make him angry, you will regret it.

He has a habit of commenting on my explanations, using less diplomacy and more colorful language.

He maintains that he got his name from Elton John, not from any raccoon. He wants to make clear that any rumors about him actually being General Scarr of the Carnivorian Republic are totally unfounded. But he does get a little tense if you mention Ulthar. Among his hobbies are trying to get 1,000 cats to share a dream (which always makes me nervous for some odd reason). I asked him if his parents were Space Cat and Moofa and he just rolled his eyes at me. Which was not a denial.

His favorite games are Homeworlds, Ogre, and The Game of Rat and Dragon.

Romantically he occasionally dates a lady named Erma Felna.

But if RocketCat ever gets his claws into Alf, that pathetic alien life form is going to suffer as he finds inserted into one of his orifices a live thermonuclear warhead with one kiloton's worth of yield for each cat he's eaten.

You will occasionally see Sneaky the insouciant Cat lurking about the website as well. Don't tell RocketCat, the two do not like each other very much.

Math Is Our Friend

RocketCat sez

Alright you ground-grippers! My name is RocketCat! I'm here to make you into steely-eyed missile people, blasting off and leaving all those trekkies choking on your rocket exhaust.

But we are talkin' Rocket Science here, people! That means ... duntDahDUHHHHH!!! ... Mathematics! If this frightens you, well, go have a seat over there by the trekkies. I'm not talking calculus, I mean math you can do on a dollar store pocket calculator. You can handle that, can't you? Learn where the multiply, divide, square, square-root, natural logarithm (ln), and trig keys are; and you'll be fine.

If ya can't be bothered to do the math, then you can always just look at the pretty pictures. And there are one or two pages here that have hardly any equations at all.

Real rocket scientist know what metric units to use in rocket equations (metre—kilogram—second, centimetre—gram—second, SI). I know that none of them are reading this. So, for the benefit of all you ground-grippers, I'm going to explicitly specify what metric units to use in each equation. Because I know if I don't, it will be just too blasted easy for you to make a mistake in units and get an answer that is a thousand times too big or small.

Or if you prefer, it is because I'm trying to make the equations easier for a non-rocket-scientist to use. You decide.

It is all in the Metric System as well. Because the entire non-USA world in general, and the realm of science in particular uses metric. Get over it.

I'll try to explain things simple, so non-science types can still understand. But there are limits. I assume you at least know what the difference is between a planet and a galaxy. You will have a big head-start if you've had a course in high-school physics (and didn't flunk).

If you can master this website, you will be a steely-eye missile person; and any science fiction you create will have better scientific accuracy than 99% of the crap that is out there now. Plus along the way I'll leave you a few quotes from novels and observations that could spark some fruitful ideas to develop for your work.

Don't panic. All the equations in the following are about high school algebra level. They are all easy enough to manage on an average scientific calculator, a computer spread sheet, or a slide rule for that matter. If needed, the reader should review the rules on Significant Figures.

Having said that, you will find the following pages much easier to understand if you have taken an introductory course in Physics in high school or whatever.

If you are using a pocket calculator, try to find the scientific kind. You will need one that handles natural logarithms (the ln key) and square roots (the key). To do square roots using the calculator program that comes with Microsoft Windows, use the "View" menu to set the view type to "Scientific", click the "Inv" checkbox, then use the "x^2" key. Be sure to un-check the "Inv" checkbox when you are done.

If the equation uses tan(), sin(), or other trigonometric functions, pay attention to what input unit the function wants. If you are using a pocket calculator or the Windows calculator program it defaults to wanting the input to be in decimal degrees. If you are using a spreadsheet or writing your own home brew computer program, the TAN() and SIN() functions want the input to be in radians.

If you are going to do the equations in the Relativity section, you will need a calculator that can handle the hyperbolic functions sinh, cosh, and tanh. On the Windows calculator, click the "Hyp" checkbox, then use the sin, cos, and tan keys. Again, remember to un-check the "Hyp" box when you are done.


When I was a boy, you had to do logarithms by looking them up in a book of tables, or with a slide rule. Now you young whipper-snappers can get a calculator at the drug store for $9.95 that will do it with the press of a key.

As per usage in computer programming languages, the symbol "*" means "multiplication". In the following equations, be sure that you use the same units throughout, to minimize that type of error. I generally use meters - kilograms - seconds. Obviously meters * 1000 = kilometers.

Beware that items like a rocket's exhaust velocity are often given in kilometers per second, instead of meters per second. It doesn't matter which you use, but be sure to use the same units everywhere. (For those readers who actually have some knowledge of rocketry: Yes, I know it is inconvenient to use meters per second with all those annoying ciphers. But I still used them throughout this site so as to give one less source of error for those readers who are new to all this.)

Also useful is AU * 1.49e11 = meters. AU's are "astronomical units", the distance between the Earth and Sol. Planetary distances are generally given in AUs, so you have to know how to convert them into the more useful "meters".

1.49e11 is "scientific notation", a fancy way of writing 149,000,000,000 without all those messy zeros. For those who are familiar with scientific notation but not with computer scientific notation, 1.49e11 = 1.49×1011, the "e" stands for "exponent".

"Kilo" means "thousand", e.g., one kilowatt = 1,000 watts = 1e3 watts. "Mega" means "million. Giga" means "billion". There is a handy table of these prefixes here.

Be told that this website uses the mathematical notations in common use in the United States. A comma is used between each group of three numbers and a period is used as a decimal point, e.g., 23,000.17 = twenty three thousand and seventeen-hundredths. 109 is "one billion" with the prefix "giga-", unlike UK nommenclature where 1012 is "one billion" with the prefix "tera-".

Information about the mass and radius of various planets can be found here:

In the sections on power sources and the section on spacecraft weapons many of the items have outputs measured in watts. This website has a nice table of various watt levels and comparison items so as to get a feel for things.

Back to the Past

RocketCat sez

All you young whipper-snappers who think that science fiction was invented in 1977 with the first Star Wars movie, I have to inform you that you are sadly mistaken. SF was old when your great-great-grandfather was born. Get a load of this:

  • "Blaster" dates back to 1925 in Nictzin Dyalhis' When the Green Star Waned. By the time the term was used in the first Star Wars movie it was fifty-two years old.
  • "Disintegrator ray" dates back to 1898 in Garrett Serviss' Edison's Conquest of Mars.
  • "Needler" dates back to 1934 in E.E."Doc" Smith's The Skylark of Valeron.
  • "Stunner" dates back to 1944 in C. M. Kornbluth's Fire-Power.
  • Isaac Asimov invented "force-field blades" in his 1952 novel David Starr, Space Ranger, which was the father of the light-saber.
  • There was a form of "virtual reality" in Sir Arthur C. Clarke's 1956 novel The City and the Stars, and a more limited form in E.E."Doc" Smith's 1930 story Skylark Three.
  • "Tractor Beam" had its roots in the 1928 novel Crashing Suns by Edmond Hamilton, where it is called the attractive ray. Also in 1928 E.E."Doc" Smith's novel The Skylark of Space had attractors which drew an object closer and repellors which pushed objects farther away. The first use of the term "tractor beam" is in Doc Smith's 1931 novel Spacehounds of IPC. Thirty-five years later in 1966 the term was borrowed by the TV show Star Trek. Eleven years later in 1977 it showed up in the first Star Wars movie.
  • "Force Fields" seem to have their origin in E.E."Doc" Smith's 1930 novel Skylark Three. Ray screens defend against deadly weapon beams, and repellor fields turn back shells, missiles, and other weapons composed of matter (in other words it deflects them). In later science fiction, force fields and deflector shields stop both energy and matter, there is no longer two types of field.
  • Dreadnought is from the real world, refering to the largest most deadly type of seagoing warship, the first one was in 1906. The term was popularized for combat spacecraft in E.E."Doc" Smith's Lensman series, first appearing in 1937 inside Galactic Patrol. Smith did have a odd habit of spelling it "Dreadnaught".
  • Zero population growth is discussed in Walter Kately's 1930 story "The World of a Hundred Men."
  • Power from nuclear fusion appears in Gawain Edwards' 1930 story "A Rescue from Jupiter."
  • Atomic bombs are found in Sewell Wright's 1931 story "The Dark Side of Antri."
  • A "tiny computing machine about as large as the palm of a man's hand" (Palm PDA?) is featured in R. F. Starzl's 1931 story "If the Sun Died."
  • And an unprotected man exposed to the vacuum of space but did not explode appeared in Nathan Schachner and Arthur Zagat's 1932 story "Exiles of the Moon."

Of course you shouldn't try to go travel the space lanes without your official Space Cadet handbook (1952).

Or Ron Turner's Space Ace pop up book (1953).

Scientific Hardness

RocketCat sez

We are going to emphasize scientific accuracy here, is that clear? I'll let you have the accuracy slide once and a while, but don't make a habit of it and wash your hands afterwards. I want to see no Technobabble, a bare minimum of Handwavium, and low amounts of Unobtanium.

Burnside Scale of Science Fiction Hardness


Handwavium: It flat out violates laws of physics. We're waving our hands and saying pay no attention to the man behind the curtain. FTL is handwavium in its many forms. I tend to hold that all these designs that ignore thermodynamics are handwavium, as are force fields and gravitic whosimawatchises.

Ken Burnside


A substance with extraordinary properties, capable for example of withstanding a direct hit by a thermonuclear warhead. By extension, the term [which I stole from Chris Weuve's SFCONSIM-L discussion group] is applied to high TECHLEVEL engineering of any sort, especially if it falls outside the constraints of HARD SF.

A lot of people, me included, try to avoid arbitrary use of Handwavium, but the truth is that you can't travel the KNOWN GALAXY without it, because any FTL is pure Handwavium. So get used to it.

Applied Phlebotinum

"Any sufficiently advanced technology is indistinguishable from a completely ad-hoc plot device"
David Langford, "A Gadget Too Far", as a corollary to Arthur C. Clarke's Third Law

Phlebotinum is the versatile substance that may be rubbed on almost anything to cause an effect needed by a plot. Examples include but are not limited to: nanotechnology, magic crystal emanations, pixie dust, and Green Rocks.

In essence, it is plot fuel. Without it, the story would grind to an abrupt halt. It's the science that powers the FTL drive on the starship so the characters can get somewhere, it's the magic that hatches the Egg MacGuffin so the protagonist can save an endangered species, it's the strange things unknown to science or magic that do basically anything. The reader does not know how Phlebotinum would work and the creators hope nobody cares.

According to Joss Whedon, during the DVD commentary for the pilot episode of Buffy the Vampire Slayer, the term "phlebotinum" originates from Buffy writer (and Angel co-creator) David Greenwalt's sudden outburst: "Don't touch the phlebotinum!" apropos of nothing. Whether or not he had the etymological connection in mind, "phlebotomy" is the drawing of blood, coincidentally appropriate in context.

If the phlebotinum in question is simply a physical substance with unusual/extreme properties you are almost certainly dealing with the element Unobtainium.

A.K.A. Handwavium. Compare Author Powers, Hand Wave, MacGuffin, Deus ex Machina, and A Wizard Did It. Contrast Misapplied Phlebotinum.

(ed note: see TV Trope page for list of examples)

Misapplied Phlebotinum

The case of a writer not quite getting their own head around his invention. An invention which is capable of great and astounding things (and often, of literally anything) is used exclusively for much lesser tasks. If you find that after a trip to the fridge you see that the Phlebotinum in question could be used to obsolete entire industries if not render the entire plot trivial then you're dealing with this trope.

Common victims of Misapplication include:
  • Faster-Than-Light Travel:
    • It's actually harder to conceive an FTL system that can't also double as a Weapon of Mass Destruction than it is to conceive one that can. And that's not even getting into the fact that, because of the way relativity works, FTL travel is logically equivalent to Time Travel...
  • Teleporters and Transporters:
    • The technology that allows your crew to travel from the Cool Starship to the planet and back without using a shuttle is the same technology that can park a live warhead in the enemy captain's lap without using a missile. It also makes a nifty Disintegrator Ray if you skip the "rematerialization" end of the process or, if it doesn't work by dematerializing, send the receiving end into the sun. Or only teleporting part of the target. And unless it's ludicrously expensive/has major side-effects, it can be used to greatly reduce shipping costs and delays, and could remove the need for any other planet-based vehicle (if it's cheap and practical enough, you wouldn't even need to walk). This could also be used to dispose of hazardous waste, removing the need for massive landfills or toxic waste dumps. If it converts matter into energy, and you have a way of storing that energy, you could use it as an alternative source of power: converting otherwise useless garbage into a viable power source for other things. This would change the face of society.
    • If the technology works by destroying and reconstructing, there are a number of possible uses that are rarely used, like bodily restoration after injury or death, copying/mass-production of reconstructible objects, copying/mass-production of people, etc.
  • Artificial Gravity:
    • If your Cool Starship has a device that can generate and manipulate Gravity irrespective of Mass then mounting Tractor Beams, Deflector Shields, Inertial Dampeners and even Engines may be redundant.
    • Note that it takes a really strong and accurately-placed gravity field to significantly change the trajectory of a laser beam or anything else moving at relativistic speeds - a field which, apart from theoretically consuming an extremely large amount of energy to maintain (depending on your flavour of Phlebotinum), might have unintended consequences.
    • However, manipulation of a gravity field probably won't get you to trans-light, unless you're in a "gravity is warp" model like GRT and use it to form an Alcubierre Drive.
  • Nanomachines: While they may have more limits in real life, it'd be easier to list the things you couldn't do with nanomachines capable of the kinds of tasks they do in fiction than the things you can, yet they're frequently introduced as a plot-device for one specific thing and never used for anything else.

It is, of course, possible to create Obvious Rule Patches and Required Secondary Powers for all these Phlebotina that prevent the above forms of misuse (and the really good writers even keep it from looking like a form of Fake Difficulty), but many writers merely take them as-is without thinking about the potential consequences.

(ed note: see TV Trope page for list of examples)


Unobtanium: We can't build a physical example of it, but insofar as we can postulate that it can be built at all, the laws of physics say it would behave like thus and so. Calculating the range and damage drop offs of a laser of a given wavelength, aperture size, input energy and conversion efficiency to make a weapon is pretty much unobtainium right now. While Handwavium and Technobabble tell you what you CAN do, Unobtainium usually tells you what is NOT possible.

Ken Burnside

Unobtainium: Of many kinds. Sophisticated materials science is one of the major areas of advancement in this particular universe. Of particular note: deuterium slush, metastable metallic hydrogen, helium-3 and antimatter (more specifically, antideuterium slush) for power, room-temperature superconductors, sapphiroids (the trade name for the high-grade kind is Adamant™ – not adamantium, because it’s not an element; after all, transparent aluminum has been used, even if accurate), carbon nanotubes, highly refractory cerametals and metallic glasses, muon metals, strangelets, raw tangle – oh, and fun nonbaryonic things like exotic matter (you make stargate frames out of it), gluonic string (held together by the strong force, thus with the best tensile strength available), and so on and so forth. Less elementally, various nanofluids with fascinatingly exotic behavior, nanotech composites, and smart and biomimetic materials (living metal, nanowell-bearing programmable matter, etc.), computronium (okay, that’s not an element either, but…), and again, and so forth.

And medically speaking, of course, immortagens.


Unobtainium is the exotic metal or other material that is needed to make the Applied Phlebotinum of a story work. Without it, all your nifty machines and plot-enabling gadgets quit functioning.

Some forms of unobtainium are based on real physics, but beyond the current scope of human engineering, such as room-temperature superconductors; they would revolutionize just about every form of technology, but they are not in and of themselves dangerous or based on some exotic physics-bending principle.

Others are more fantastic "high-grade" unobtainium, such as antimatter, which would be a revolutionary way of storing huge amounts of energy, if it didn't violently undergo mutual annihilation with any conventional matter it comes into contact with, including air molecules and the walls of whatever you're trying to store the damn stuff in. And if it could be made to exist for more than a few minutes.

The most common varieties of unobtainium in fiction sit somewhere in the middle, like materials so resistant to heat and/or damage as to be Nigh Invulnerable compared to other, similar substances. Materials such as mithril, adamantium, and orichalcum (and all variant spellings thereof) are the fantasy version. Thunderbolt Iron is especially popular in fiction (and has some basis in reality – until furnaces were invented, it was the best source of refined iron).

Much mad science uses unobtanium, such as chemicals with impossible properties, universal solvents that can dissolve anything in the blink of an eye, super-explosives that make nitroglycerin look like a weak cough, and plenty of other funny-colored solutions. Following this would be medical and/or chemical wish-fulfillers; Classical real-world alchemy casually referred to carmot, the base substance of the Philosopher's Stone, and Azoth, either the "universal medicine" or "universal solvent". The ancient Greek writer Plato referred to "orichalcum" (Greek for "mountain bronze") in his description of Atlantis.

In Science Fiction, it will usually take one of three flavors: whatever stuff makes Faster-Than-Light Travel possible, closely followed by the stuff that can mess with gravity (if they're not one and the same), and finally, the stuff they make Humongous Mecha and Alien spacecraft out of, which is why they tend to be effectively immune from earthly weapons or environmental damage.

For Willing Suspension of Disbelief, authors may pick out something actively being researched within the scientific community at the time of writing and run with it. Naturally, this risks dating the work when Science Marches On and today's "super technology" buzzword becomes the next generation's comic-book junk science. The current favorite in hard sci-fi is Helium-3 – believed by many to be the fuel of choice for those nifty fusion reactors that should be perfected any time now. Theoretically, it's a safe large-scale energy source with few environmental side effects. But more importantly, though, there's extremely little of it on Earth; on the Moon, it's Not Rare Over There — which would provide a good reason to go there.

See Also: Minovsky Physics when the Unobtainium has well-thought-out properties that are strictly adhered to, and its opposite, Green Rocks, when it can do anything and everything the plot demands.

(ed note: see TV Trope page for list of examples)


Technobabble: "We've reversed the polarity of the tetryon flow through the main deflector dish, and the Borg's shields have dropped, sir." Or, "His midichlorians are more powerful than Yoda's!" or "Our spaceship is pulled through the aether by the outrage of honest politicians." are all examples of technobabble. Technobabble need not be bad, though in general it's only noticed when it is done poorly.

Ken Burnside

Hand Wave

A Hand Wave (also memetically called "Scotch Tape") is any explanation involving the backstory, a retcon, or a use of phlebotinum, which is noteworthy for its lack of detail or coherence. The name comes from academia and techy-land, where a person explaining a process on a whiteboard gets to a part that is not well defined or important so just waves their hand around to indicate that Stuff Happens, then moves on to the important goodies.

Typically, the use of this trope is an indicator of bad writing; a good author is able to explain plot points with the utmost detail without interrupting the story's momentum.

But sometimes, it's better to gloss over something trivial and get on with the story. Tropes Are Tools. When skillfully done, a handwave can make things plausible enough so that the audience achieves a Willing Suspension of Disbelief. It can also just turn the whole detail and its inexplicability into a joke. Scotch tape may not be strong, it may not be pretty, but it may be much better to have some sort of explanation than to have nothing at all.

The Watson is often a valuable source of Scotch Tape. In Science Fiction shows, a handwave is usually conducted with Techno Babble. In fact, an alternate name for Phlebotinum is Handwavium. In the industry, the vague and generic direction given by management to actors, designers, editors and so on is sometimes known as "hand waving", as it is frequently accompanied by a lot of gesturing.

(ed note: see TV Trope page for list of examples)

Techno Babble

Doctor: Looks like a spatio-temporal hyperlink.
Mickey: What's that?
Doctor: No idea, just made it up. Didn't want to say "Magic Door."
Doctor Who, "The Girl in the Fireplace"

Any impressive- and scientific-sounding, but ultimately nonsensical utterance, full of buzzwords.

Most common in Science Fiction (usually the softer kind), but military, medical and Police Procedural themed shows can also use it when they want the underlying technology to sound impressive.

Dishonest technician characters sometimes resort to vague, senseless "technical" babble to make up "serious problems" in the inner workings of a machine and offer to "fix" them for a high price.

When technobabble is used to justify a plot development, it is Scotch Tape. When it is used to solve a problem, it is a Polarity Reversal. When it is used to add to the genre feel, it is Narrative Filigree. Due to its historical use and abuse by sci-fi writers, Technobabble is nowadays played more and more often for laughs or parodied in some way.

(ed note: see TV Trope page for list of examples)


To this list I'd like to add the term "MacGuffinite". It comes from the term "MacGuffin", popularized by director Alfred Hitchcock. "MacGuffin" means a plot device that motivates the characters and advances the story, but has little other relevance to the story. Examples of a MacGuffin include meaning of "rosebud" in Citizen Kane, the priceless statue in The Maltese Falcon, the Heart of the Ocean necklace in Titanic, and the mineral unobtainium in Avatar.

I define "MacGuffinite" as some valuable ore, substance, or commodity that hopefully introduces no unintended consequences to the SF universe you are creating. You have to be real careful, because unintended consequences everywhere. You have to carefully think things through.

The main MacGuffinite for a hard SF universe is something valueable enough to make an extensive manned presence in space a paying proposition. No MacGuffinite = no reason for people to be living in space = a really dull science fiction story.

Burnside Recommendations

The ratio of Unobtainium to Handwavium to Technobabble defines how "hard" your setting will seem to be to the reader. One of the reasons why I (Ken Burnside) love Mote in God's Eye is that they (authors Larry Niven and Jerry Pournelle) have only two blatant pieces of handwavium (the Drive and Field) and they quite carefully worked out the ripple effects of them before using them. Mote has a very high Unobtainium quotient, as does a lot of Heinlein's space fiction.

The Exordium series has a lot of well reasoned out Handwavium that's applied consistently, but has very little that's directly constrained by Unobtainium (A chief example of Unobtainium is the Tenno glyphs). It also uses very little technobabble, though it uses some (mostly when dealing with aliens).

Most of the Lensman series can be seen as Pure Technobabble with a bit of Handwavium thrown in to anchor it in plausibility.

Star Trek and most television SF is a mixture of pure technobabble and some handwavium. Things work because they make the plot work. Things fail because if they don't the plot fails.

Technobabble can easily lead to a farcical read.

Ken Burnside

Mohs Scale of Science Fiction Hardness

"The fun, and the material for this article, lies in treating the whole thing as a game. I've been playing the game since I was a child, so the rules must be quite simple. They are: for the reader of a science-fiction story, they consist of finding as many as possible of the author's statements or implications which conflict with the facts as science currently understands them. For the author, the rule is to make as few such slips as he possibly can."
Hal Clement, Whirligig World

Speculative Fiction fanatics are always raving about how "hard" the science is in various stories — but it's not like you can rub a story with a piece of quartz and see if it leaves a scratch on the plot. So what is "hardness" in SF? Why do some people want it? And how do we put a number to it?

Beginning with the first question: "Hard" Science Fiction is firmly grounded in reality, with only a few fantastic flights of fancy not justified by science, or with the technology being nonexistent in today's world but probably scientifically possible at some point. "Soft" Sci-Fi is more flexible on the rules. Even the fantastical aspects of the story will show a divide — in hard SF, they operate through strict, preferably physical, laws, where in soft SF they work in whatever way suits the story best. What this leads to for hard SF is a raised bar for the amount of scientific research the writer must put into the story, and usually this is shown quite clearly.

Example: a character is shown a machine for traveling into the past and asks, "How does it work?"

  • In soft SF: "You sit in this seat, set the date you want, and pull that lever."

  • In hard SF: "A good question with an interesting answer. Please have a seat while I bring you up to speed on the latest ideas in quantum theory, after which I will spend a chapter detailing an elaborate, yet plausible-sounding connection between quantum states, the unified field theory, and the means by which the brain stores memory, all tied into theories from both Albert Einstein and Stephen Hawking."

  • In really hard SF: "It doesn't. Time travel to the past is impossible."

Unfortunately for analytical purposes, this pattern is not universal - hard SF stories can skip over the details as long as the basic explanation is correct given what's been established so far. Therefore, regardless of the typical stylistic flourishes of hard SF, the only way to define it is self-consistency and scientific accuracy.

Which leads us to the Scale.

  1. Science In Genre Only: The work is unambiguously set in the literary genre of Science Fiction, but scientific it is not. Applied Phlebotinum is the rule of the day, often of the Nonsensoleum kind, Green Rocks gain New Powers as the Plot Demands, and both Bellisario's Maxim and the MST3K Mantra apply. Works like Futurama, Tengen Toppa Gurren Lagann, The DC and Marvel universes, and The Hitchhiker's Guide to the Galaxy fall in this class.

  2. World Of Phlebotinum: The universe is full of Applied Phlebotinum with more to be found behind every star, but the Phlebotinum is dealt with in a fairly consistent fashion despite its lack of correspondence with reality and, in-world, is considered to lie within the realm of scientific inquiry. Works like E. E. “Doc” Smith's Lensman series, Neon Genesis Evangelion, Star Trek: The Original Series, and StarCraft fall in this category.

    A subclass of this class (arguably 2.5 on the scale) contains stories that are generally sound, except the physics aren't our own. Plot aside, they are often a philosophical exploration of a concept no longer considered true (such as Aristotelian physics), or never true in the first place (e.g. two spatial dimensions instead of three). Some of Arthur C. Clarke's stories fall here. However, given the overlap with fantasy, it can prove tricky to even classify a story as SF.

  3. Physics Plus: Stories in this class once again have multiple forms of Applied Phlebotinum, but in contrast to the prior class, the author aims to justify these creations with real and invented natural laws — and these creations and others from the same laws will turn up again and again in new contexts. Works like Schlock Mercenary, David Weber's Honor Harrington series, David Brin's Uplift series, and the 2003-2009 Battlestar Galactica fall in this class. Most Real Robot shows fall somewhere between Classes 2 and 3.

  4. One Big Lie: Authors of works in this class invent one (or, at most, a very few) counterfactual physical laws and writes a story that explores the implications of these principles. Most works in Alan Dean Foster's Humanx Commonwealth series, the Ad Astra board games and Robert A. Heinlein's Farnham's Freehold fall in this category, as do many of Vernor Vinge's books.

    This class also includes a subclass (4.5 on the scale) we call One Small Fib, containing stories that include only a single counterfactual device (often FTL Travel), but for which the device is not a major element of the plot. Many Hal Clement novels (e.g. Mission Of Gravity, Close to Critical) and Freefall fall within the subclass.

  5. Speculative Science: Stories in which there is no "big lie" — the science of the tale is (or was) genuine speculative science or engineering, and the goal of the author to make as few errors with respect to known fact as possible. The first two books in Robert L. Forward's Rocheworld series and Robert A. Heinlein's The Moon Is a Harsh Mistress fall in this class.

    A subclass of this (5.5 on the scale) is Futurology: stories which function almost like a prediction of the future, extrapolating from current technology rather than inventing major new technologies or discoveries. (Naturally, Zeerust is common in older entries.) Gattaca, Planetes, The Machine Stops by E. M. Forster, and the more Speculative Fiction works of Jules Verne fall in this subclass.

  6. Real Life (aka Fiction in Genre Only): A Shared Universe which spawned its own genre, known as "Non-Fiction". Despite the various problems noted at Reality Is Unrealistic, it is almost universally agreed that there is no other universe known so thoroughly worked out from established scientific principles. The Apollo Program, World War II, and Woodstock fall in this class.

(ed note: see TV Trope page for list of examples)

The Tough Guide to the Known Galaxy: Hard SF


Written SF that adheres, or tries to adhere, to plausible science and technology. Therefore it generally implies a fairly modest TECHLEVEL; the most anal Hard SF may even preclude FTL. For obvious reasons, plausible is pretty much in the eye of the beholder.

It is also a moving target. In fact, you can usually date Hard SF particularly well by its technology, which will lean heavily on whatever technical or scientific speculation was fashionable about five years before a book's publication date. If this did not pan out (and mostly it hasn't), the resulting Hard SF will sound very dated within a decade or so.

Rules Of Thumb

Robinson's First Law of space combat

Something hitting at 3 km/sec (kips) delivers kinetic energy broadly equal to its mass in TNT.

Rick Robinson
Robinson's Second Law of space combat

For every kilogram of handwavium you remove from a setting, you add about 10 cubic meters of impossible to maintain plumbing.

Rick Robinson

Most people instinctively know Burnside's Zeroth Law of space combat:

Burnside's Zeroth Law of space combat

Science fiction fans relate more to human beings than to silicon chips.

Ken Burnside

That is, while it might make more logical sense to have an interplanetary battle waged between groups of computer controlled spacecraft, it would be infinitely more boring than a battle between groups of human crewed spacecraft. For more details go here.

Another annoying fact is that realistic spacecraft propulsion systems are incredibly weak. They will take forever to push the ship to anywhere farther than, say, Luna. So SF authors try to jazz things up by postulating more powerful propulsion systems. Alas, they then run full tilt into Jon's Law for SF authors.

Jon's Law for SF authors is closely related to Niven's Kzinti Lesson. It states:

Jon's Law, part 1

Any interesting space drive is a weapon of mass destruction. It only matters how long you want to wait for maximum damage.

Jon Souza
It goes on to say:
Jon's Law, part 2

Interesting is equal to "whatever keeps the readers from getting bored."

Jon Souza

As an example, a spacecraft with an ion drive capable of doing a meager 0.0001g of acceleration may be scientifically realistic and the exhaust is relatively harmless. However, to most of the audience it will not be interesting. "Nine months just to travel to Mars? How boring!"

The author, not wanting his book sales to go flat, hastily re-fits the hero's spacecraft with a fusion drive. The good news is that the ship can make it to Mars in twelve days flat. The bad news is that the ship's exhaust is putting out enough terawatts of energy to cut another ship in two, or make the spaceport look like it was hit by a tactical nuclear weapon.

The author can still use the drive, but must consider the logical ramifications of the wide-spread civilian availability of the equivalent of thermonuclear weapons. Consider: the more energy the drive contains , the worse the damage if an accident occurs. How would you like to have the captain of the Exxon Valdez skippering a tramp freighter with an antimatter drive? That brilliant mushroom cloud you see marks the former location of Clinton-Sherman spaceport. The more devastation a propulsion system can wreck, the shorter the leash the captains will be on.

So one of the logical ramification is that if drives are too powerful, there won't be any colorful tramp freighters or similar vessels. As a matter of fact, civilian spacecraft will probably by law be required to have a remote control self-destruct device that the orbital patrol can use to eliminate any ship that looks like it is behaving erratically or suspiciously.

Most of the nasty effects of Jon's Law are due to the propulsion system's exhaust. The presence of an exhaust is because rockets use Newton's Third Law (the one about action with equal and opposite reaction). Canny SF authors postulate some kind of hand-waving reactionless drive in an attempt to avoid Jon's Law. Reactionless means no exhaust is required. You feed electricity in, and the ship is magically accelerated. The "gravitic impellers" from David Weber's HONOR HARRINGTON series is an example of a reactionless drive.

Unfortunately such canny SF authors then run smack dab into Burnside's Advice, which is;

Burnside's Advice

Friends Don't Let Friends Use Reactionless Drives In Their Universes

Ken Burnside

The trick is making a reactionless drive that doesn't give you the ability to shatter planets with the Naval equivalent of a rowboat (which would throw a big monkey wrench into the author's carefully crafted arrangement of combat spacecraft). Reactionless drives, with no fuel/propellant constraints, will give you Dirt Cheap Planet Crackers. If you have a reactionless drive, and stellar economics where most of the common tropes exist (privately owned tramp freighters), you also have gravitic drive missiles. And avoiding Planet Crackers Done Real Cheap is almost impossible to justify on logical grounds, the SF author is faced with quite a daunting task.

Unscientific Hollywood

I often bitterly complain about the lack of scientific accuracy in TV and movie SF shows. Todd Boyce of Ninja Magic actually works in Hollywood, and explained to me the facts of life about media SF:

To boil down all the possible reasons, it is because of one or more of the following:

0) It's a business

This is a business venture - you put money in with the expectation that more money will come out. The general audience is historically happier watching space ships woosh by shooting glowing bolts of energy than they are watching a slowly rotating spaceship lazily drift across the screen. If you're putting tens or hundreds of millions of dollars on the line, you go for the shooty-wooshy space ships every time, pure and simple.

1) TPTB (The powers that be) don't care.

If what's on the screen looks good, and the storytelling is sufficient, then scientific accuracy rarely if ever matters. If they don't care that cars don't blow up when shot with bullets, why should they care about the theoretical effects of FTL travel.

2) There isn't time to dissect and fix scientific inaccuracies

Once production on a movie is started, it is an unstoppable steamroller with a tight deadline. If the script says a spaceship wooshes by, the people working on the film don't have time to work out what kind of propulsion it uses — they just make the engine glow, push it across the screen in an interesting way and move on to the next shot.

3) The decisions are made in too many places and it isn't even thought about except by people who aren't in positions to make judgment calls.

A jet fighter shoots missiles at a big space ship hovering above a city. The director tells the visual effects supervisor to make it happen. The visual effects supervisor tells the digital effects supervisor to make a space ship and to make a jet fighter woosh by and shoot some missiles at the space ship while he goes off and directs the on-set pyro effects.

The digital effects supervisor tells the modeling supervisor to have his team make a space ship and jet fighter and tells the FX supervisor to have his team make some missiles shoot, engine effects, vapor trails, smoke trails and whatnot.

The modelers build a jet fighter and give it harpoon missiles. The modeling supervisor says it looks good. The digital effects supervisor says it looks good. The modelers are done with their job and get put on another production.

The FX supervisor hands the model to the FX team who look at the fighter and say "um...that's not really the right kind of missile to do an air-to-air attack..." "Sorry, the modeler is off the show and these have been approved. Can't change it now" is the response. So the FX team launches harpoon missiles at the space ship.

The final shot is shown to the director/visual effects supervisor and it looks cool, but don't pick up on the fact that the wrong missile is being used. It's approved and put into the film.

(You're probably sensing that this is a true story and know what movie I was working on at the time.)

4) The script-reader's gauntlet

Writers use descriptive language to express action in their script. They don't often get into technical details because each page of a script is supposed to represent roughly one minute of screen time. A writer who spends his time describing the intricacies of a space ships propulsion system is a writer who finds his scripts in the script-reader's trash can.

People who write heavily technical novels are almost always terrible script-writers as they have difficulty working within the confines and limitations of that medium. The scripts that pass through the script-reader's gauntlet will likely be of the less technical variety.

5) People in film making have education in film making, they don't usually have PhD's in physics/astrophysics. And people who have PhD's in physics/astrophysics don't usually know how to make a good film.

It's not that they aren't smart enough, it's that their focus of expertise is in other areas. That's why they hire consultants if they're trying to do something with any degree of accuracy, but even then, accuracy is desirable only if it doesn't interfere with the storytelling. Often, things are set in motion that can't be changed after the fact anyway and you just have to shrug your shoulders and say "That's the way it has to be" if you learn too late of some scientific ramification.

6) The power of ego

You know how people fall all over themselves when a famous actor is nearby? Its worse when companies deal with well known directors. Just yesterday we were kicked out of the screening room during our dailies because Michael Bay was parking and MIGHT be needing it. With that sort of hysteria going on, are you going to be the one that walks up to him and say "this is totally unrealistic and you need to change it" knowing that saying so will mean the end of your employment?

What the director says goes, and few people have the will or the power to contradict him. Film making isn't usually done by committee, it is done by imperial decree and if the decree is that cars blow up when shot with bullets, then that is the way it is.

I'm sure there's a few others I've missed but, speaking of unrealism in Hollywood movies, I need to get back to work on a sequence involving bits of LA breaking off and sliding into the ocean because the Earth's magnetic field has collapsed.

I'm not kidding.

Todd Boyce

A Final Warning

But beware the dangers of reading this website {very big grin}. Tom Clark had this to say:

Ruined! It's all ruined! And it's all your fault. Particularly you, Nyrath!

I've been having to travel quite a bit lately, and for entertainment I've been reading David Drake's Lt. Leary series of novels. Pretty harmless space opera mostly, similar in vein to the Horatio Hornblower or (more accurately) Patrick O'Brian's series of books about Capt Jack Aubrey and surgeon/spy Stephen Maturin in the Royal Navy during the fight against Napolean.

The basic premise and plot didn't bother me, not even the "sail" metaphor Drake adopted for his FTL system. This drives the ships to be lightweight - a corvette like the Princess Cecille is about 150 meters long and weighs 1200 tons empty - and the ships are restricted to about 2-3.5 Gs of thrust. And yes, they land on planets. Lift-off must be a fairly leisurely affair. Propulsion in atmosphere is by "plasma thrusters" which use fusion bottles as a power source, and water as reaction mass. Outside the atmosphere, they use anti-matter powered High Drives to drive the ship. (They are also apparently converting normal matter to antimatter on the fly for energy. Nice bit of sorcery there.) So far, so good.

Then they get to the armament. The ships mount turreted plasma guns (which we already know won't work because of the work Nyrath has done on his Atomic Rockets page), but as he says its the meme that won't go away. But these are the secondary weapons....

Primary weapons of these ships are kinetic-kill weapons, anti-matter driven torpedoes launched from tubes in the side of the ship. Thirty-ton kinetic-kill torpedoes. And the Sissy carries twenty of them. (Which means half again her mass is weaponry...) Now, the torpedoes accelerate at 12 Gs, which is OK given that their targets only accelerate at 2 G usually...but then Drake goes on to say that the missiles can reach a maximum velocity of 0.6 C in combat conditions. And that they do so in 8 minutes.

Now, before I started hanging out here, I probably would have bought that statement at face value and driven on. But my innocence has been lost, and it kept eating at me, until I started doing the numbers.

- At 12 G, and assuming zero initial velocity, it would take one of those torpedoes about 420 hours to reach that velocity. That's about 18 days, if you're counting. It would also cover 1.4E11 kilometers while its at it. Even with a 6:1 acceleration advantage, I don't want to think about how large a sphere a ship that can accelerate at 2 G continuous for 18 days would be... Did I mention that the torpedoes don't carry seekers on them? They have some sort of rudimentary command link control, but no terminal guidance seekers.

- At impact, each torpedo would unleash the kinetic equivalent of about 35,000 gigatons on its target. This is considerably more than I would expect to have to use to kill a ship-sized fact, with 20 stowed rounds, the Sissie is a threat to a significant portion of the inhabited galaxy. (Since this is a relativistic weapon, I used the formula's found here.

In one book, two torpedoes miss their target and enter the upper atmosphere of an inhabited planet....and disappear harmlessly in "a flash of plasma". Ummm...sorry, don't think so. Even if 30 tons of missile turn into plasma on contact with the atmosphere (and at 0.6 C I don't think they'll have time to melt), they are still 30 tons of plasma moving at 0.6 C! So much for that inhabited planet..... I'm ruined. If Drake had just done a little math, and maybe set the terminal velocity of the torpedoes to 0.06 C, or even 0.006 C, things would have worked out much better. But thanks to the "education" I've received here, slips like that come back to haunt me in my sleep.

Frankly, it would have been fine if Drake hadn't thrown that "0.6 C" number in there - the missiles have a high enough margin of acceleration versus the ships to make them a credible threat. It's just that, as one of H. Beam Piper's characters once said, "C makes for an awfully long lever." Ships capable of tossing around high-relativistic weapons become planet-killers in relatively short order. You don't need redirected asteroids - which is what brought about the end of galactic civilization the last go-round - to kill a civilization on a planet. One corvette with a load of torps can do it handily.

So a word to authors who like to throw around big numbers....Do the math first! Please!

I need the sleep....

Tom Clark
RocketCat sez

You say that knowing the science ruins your enjoyment of things with sketchy science? Boo-hoo, cry me a river. So I suppose your knowledge of grilled steak ruins your enjoyment of hamburgers that taste like cardboard? Well I guess we'd better forbid everybody from ever tasting a steak. They'll be happier that way.

By that idiot logic your knowledge of knowledge is ruining your enjoyment of life. Wrong. Thomas Gray said "Thought would destroy their paradise. No more; where ignorance is bliss, 'Tis folly to be wise." What a load of hooey. Sticking your head in the sand never works.

No, the solution is to demand higher quality SF like Tom Clark said, over and above the fact that scientific knowledge is worth knowing for its own sake.

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