Kerbal Space Program chronicle — part one: achieving orbit for science

Ian Birnbaum


Kerbal Space Program is an open-world universe simulator that specializes in modeling orbits, atmosphere, gravity, and rocket physics. With nothing but your wits and a hangar full of space vehicle parts, your task is to explore. Look up at the night sky. See a planet? See a moon? Go there. There's nothing stopping you except the relentless hostility of physics.

Our home planet is Kerbin, land of the Kerbals. The best and brightest Kerbal pilots from around the world will embark on this program to serve as our rocket test dummies, our astronauts, our lemmings, and our national heroes. They are green and a little top-heavy, like over-saturated Smurfs without the silly hats, but they handle controls reliably and don't eat much.

In this chronicle, I will be recording the first missions of the PCGSA, PC Gamer's ambitious new space program. By being the first PC games magazine to make a home in the stars, we hope to use advanced satellites to gain exclusive news leads and maybe sort out when Half-Life 3 might be showing up.

The plan is simple. First, we'll build and launch a satellite and achieve orbit. Then, we'll put the first Kerbal into space and bring him home again. Then, we'll establish a space station to serve as a fueling depot for longer trips. Finally, we'll plant our flag safely on the Mün, Kerbin's closest celestial neighbor, and come home again.

A note about mods: KSP is still early in its development, and is enjoying a long development cycle with an active modding community . Some mods add features that will eventually be included in the game, while others simplify gameplay by adding autopilots or new spacecraft parts. One of my favorite mods, MechJeb , is one I'll be using for its excellent information readouts. While MechJeb does include autopilot features, I won't be using them. All of these missions will be flown entirely by hand.

On with the first mission...

The sun has risen on the dawn of a new era for Kerbin: the age of the rocket. I make my way to the Vehicle Assembly Bay, the oversize hanger where all rocket design takes place. Along the left side of the room, our engineering teams have thoughtfully laid out all of the fuel tanks and rocket engines and solar panels, everything I need to build rockets, planes, rovers, space stations, and moon bases. Our design teams have been working overtime to prototype designs drawn from other cultures and worlds, so I've got a ton of schematics to choose from. Word is that rocket scientists have been posting their designs online, which will be handy if I need some inspiration.

Every rocket needs a core, a command center. This can either be a cockpit module for a single or multiple astronauts or a computerized remote control receiver. For my first satellite, dubbed PCG 1, the Stayputnik Mk. 2 is perfect: it does the job without weighing very much or sucking up too much electricity.

Every rocket needs a few basic systems to survive, but the PCGSA bureaucracy is still sorting itself out, so these essentials aren't actually written down anywhere and had to be discovered through trial and fiery error. Make no mistake: this information was purchased with the blood of brave Kerbals unfortunate enough to have me as a leader. If I wasn't the head of their space program, I feel confident that they would convict me for war crimes.

Anyway, Reaction Control System thrusters (those little jets of gas that move spaceships around without using the main engine) and a battery are two must-haves, especially for satellites. Assuming the launch goes well, this satellite will be in orbit for years after the main thruster engine is gone, and it will need the RCS thrusters to move around and the batteries to survive orbiting around the dark side of Kerbin.

RCS works best when it's placed over the center of mass, and my scientists helpfully show me where the center of mass, center of lift and center of thrust are through icons in the bottom left of my screen. I've launched a few rockets that just tumbled end over end before exploding in an expensive fireball, and it was because I didn't check out the center of thrust and center of mass readouts. (It also happens to the pros occasionally— I'm lookin' at you, NASA .)

With the RCS thrusters in place, the last thing to add is a pair of solar panel arrays and scientific instruments. This little satellite is a great first step for a couple of reasons. First, it will get up there and do some science: always good. Second, it weighs in at 2.12 tons, according to the MechJeb readout, which is a reliable stand-in for what my first manned craft will weigh. When I design the rocket to get this satellite to space, I can just reuse that same design to take the first Kerbal to orbit.

This is one place where the MechJeb information readouts can come in handy, even though I don't want to use the autopilots. By bringing up the Thrust-to-Weight Ratio reading, I can see that this rocket has a TWR of 3.09 at liftoff. Anything above 1 means that the rocket can successfully reach orbit, so I've got some room to spare.

I'm using a variation on the Titan rocket design made famous by the early NASA missions. It's got a tall, large central engine with a booster rocket pair on each side. It's simple, easy to build, and a pile of dead Kerbal test pilots tells me that, unlike other more complicated designs, it tends to stay balanced without a lot of fuss.

Some egghead from a local university tells me that a key element of rocket design is building in stages. When I first launch, I will want every bit of thrust available firing all at once. When fuel tanks or booster rockets are empty, staging allows me to jettison the dead weight at the press of a button. Looking at the stages plotted by my design team, I do an imaginary dry run through to make sure everything is in order. The last thing to add is a pair of holding clamps that will keep the rocket upright so that the whole thing isn't resting on the delicate machinery of my main engine.

And now, the moment of truth. Sitting on the launchpad, I engage stabilizers to keep the ship upright, throttle up and pull the trigger.

Oh, son of a Kerbal. When I added those clamps, I forgot to address their staging. When the big moment arrives, the clamps release before the engines light. The ship drops its entire weight onto the single engine, which snaps off. Like a revolver dropped down a stairwell, the booster rockets ignite on their own and drag the crippled craft 8,000 meters into the air before the whole mess comes crashing down with a bonus explosion.

Sorry I judged you, NASA. I take it all back.

I sweep all the twisted steel and broken glass off the launchpad, reload the thankfully unmanned ship and try again. This time: sweet success.

My pilots are the best in the world, and they tell me that the rocket should stay perfectly straight up and down for at least the first 10 kilometers. Kerbin's atmosphere is thickest at the ground level and thinner at higher altitudes, so staying upright gets me through the thickest part of the atmosphere with the least amount of drag. After that, I slowly start a gravity turn.

An overeager intern starts to panic, thinking that PCG 1 is about to tip over and explode high above the planet. I calm him down and explain: a gravity turn is where I begin to turn toward a 45 degree tilt to fight less gravity while still gaining altitude. It's counterintuitive, but look at it this way: If you were climbing a really steep hill with a heavy load on your back, would you just point straight up the hill and start walking? No, the easiest way is to zig-zag or spiral around the hill on your way to the top. It takes longer, but your legs will thank you.

Kerbin's gravity well works on the same principle. It's got really steep sides, but nobody's forcing you to climb straight out.

The rush of wind dies down, stars brighten and the quiet isolation of space begins. PCG 1 is in space, but it's not in a safe orbit just yet. I locate my instruments and orient myself with a little orbital geography. Orbits are complex creatures, but for the amateur rocket pilot the important part is the two main landmarks: the apo apsis and the peri apsis. They are the highest and lowest points of an orbit, respectively. A solid orbit is anything above 100 kilometers. Kerbin's atmosphere ends at about 70 kilometers, so 100 kilometers will give me some space (ha!) to breathe (ha ha!).

“Orbits change easiest when you alter them from their opposite sides,” a nearby scientist tells me. Unfortunately I have no idea what that means, so he explains. Turns out, If you want to raise your lowest point, the periapsis, you want to increase your speed from the highest point, the apoapsis. Watching the live tracking on our map screens, I can see that our satellite's projected apoapsis is right at 100 kilometers.

I shut down the engines and let momentum take me the rest of the way. Once I'm on top of the apoapsis, I point directly at the horizon and fire the engines until the projected orbit rises out of the planet and stabilizes at about 100 kilometers.

Almost there. Detaching the fuel tanks and engine, the last thing I do is fire on the scientific instruments and extend my solar panels. Cheers erupt and the control room staff attempts awkward high-fives. Congratulations, Kerbin! We've put a satellite in orbit. It's the planet's first baby step into the great big universe, and it feels good. Historic, even.

Next time, we'll repeat this process with a live Kerbal, then bring him home again.

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