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  • Writer's pictureKL Forslund

Slouching toward Apoapsis

I launched a Kerbal into orbit last week. Which required learning a bit about launching rockets and physics. It got me thinking about realistic space physics in fiction. I don’t think I’ve read about it represented as the Kerbal Space Program portrays. Wait, a video game portrays space realistically? Not quite, but the math is sort of accurate.

I watched a video to figure out how to pilot the rocket and get into orbit. It’s not like Star Trek where the ship goes up and then you “get into orbit”. For my own memory, here’s the concepts and how they work.

From the Ground Up

The planet Kerbin works like Earth. Shoot a gun straight up, expect a bullet in the head. Launching a rocket is best done near the equator and after an initial vertical ascent, once the air thins more, tilt toward the east. The earth spins east to west, so it helps by not fighting the Earth’s movement. By shooting up and easing into an angle, the path of your rocket projected into the future forms a parabola. If nothing else changes, you’ll reach the apex and then come crashing down. You’ll land farther east than where you started.

In-Flight Directions

The direction your path is moving is called Progade. If you imagine looking out your window in that prograde direction, the launch starts by going up, then starts tilting toward East. When you reach the peak, the ship will start going down. But for a moment, you are going nearly parallel to the earth (just higher up). That apex point is called apoapsis and when you reach it which aimed at your prograde direction, you will be horizontal to the planet’s plane.

Pushing Towards Apoapsis

In the game, you can switch to map mode (recommended) if you tilt the planet on it’s pole, you’ll see that parabolic arc representing your ship hurtling up, reaching the apoapsis point (apex) before arcing down to strike the planet. Here’s the mind blowin part. That arc is an oval that is overlapped by the planet. If you can make that oval/circle bigger, your ship is moving sideways more than falling toward the planet and it falls forward instead of down, missing the planet in perpetual orbit.

To do this trick, we aim the ship at its prograde direction, and when it approaches the apoapsis point, fires its engines. The oval grows, and if you time it right, pulse the engine, etc, soon it becomes large enough that no part of it touches the planet’s atmosphere. Orbit accomplished.

Points for Orbiting

Odds are good, the orbit is an oval and not a circle. One point will be closer to the planet than others. This lowest point is called periapsis. Its opposite is the aforementioned apoapsis. Applying thrust on the prograde or retrograde course at these points makes the circle grow, shrink or reverse the two points. Retrograde is the opposite point from prograde. Firing your rockets retrograde means putting your tail to be in front of the direction you’re going. In space, the direction you are going is separate from the one you or your thrusters facing.

Orbit to Orbit

You may recall hearing how NASA slingshotted satellites around one planet or moon to get speed to move to the next. The missing detail is that NASA launches nothing in a direct path to anywhere. Once you lift off from a world, you get into orbit. Then, you set facing and apply thrust to warp the shape of your orbit circle to reach the orbit of your goal, for example, the moon. Don’t forget to check when to do so. Your ship has to be at the right spot in its current orbit (and angle) to fire at the right time so the orbit expands and aligns with the moon when it and your ship will be there. Then when you arrive, you’ll apply prograde thrust at the apoapsis point which brings the periapsis point of your orbit in, to align with the moon to establish your new orbit. It’s hula-hoop version of inchworm. Space travel is orbit to orbit to orbit.

Calculated Falling aka Landing

Remember the original launch. It formed a parabolic arc, that I said was really a circle overlapped by the planet. Once prorade thrust was applied at the apoapsis point, the circle grew. Guess how we get down. Flip to retrograde, tail first and fire the engine at the apoapsis point. This causes the periapsis to come closer to the planet. Shrinking it enough until it touches the ground or atmosphere and your ship is coming home.

It’s Almost Too Easy

I’m getting a handle on the terminology. I don’t know if I’ll use the techno-babble in fiction. There’s a reason nobody explains how the Rocinante will reach Europa, it just does.


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