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Orbital Insertion and Space Combat Tactics – Part 1

Published April 18, 2016 in Science , Science Fiction , Space - 0 Comments
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One issue that is almost universally ignored in science fiction these days is the role of orbital mechanics in space combat tactics. Science fiction space combat tends to be written or visualized as if the spacecraft involved have nearly infinite energy and thrust available. Occasionally this is due to the writers or visual effects technicians consciously simplifying things for viewers. Unfortunately, however, most of the time it’s simply due to the writers being woefully ignorant of the subject.

There are a few cases where orbital mechanics can be safely ignored. Obviously, they can be ignored for atmospheric combat. Modern science fiction fans are intuitively familiar with atmospheric combat thanks to watching hours worth of dogfights in film and television. Although many of the details would make a hardened fighter pilot squirm, our intuitions of the basic physics of how these fights occur more or less conforms to reality. They can also be safely ignored when we’re discussing ships traveling in interstellar space (although there are other issues there, mainly the massive velocities of the spacecraft themselves). And in some cases of two “mother ships” occupying very near orbits, we can handwave away orbital mechanics if the fight is focused on “fighter ships” surrounding them. The mechanics still don’t go away, but for the purposes of entertainment we can safely pretend that they do.

Otherwise, orbital mechanics are crucial to space combat tactics. The definitive primer on orbital mechanics for science fiction writers is Ken Burnside’s magnificent essay “The Hot Equations.” Anybody looking to study the matter seriously should start there. Rather than retread ground he has already covered, my intention is to break new ground and discuss some of the implications of the physics discussed by Mr. Burnside.

My endeavor here will be far more modest in scope than Mr. Burnside’s. I wish to discuss merely one element of an entire tactical encounter: orbital insertion. Mr. Burnside has already lain much of the groundwork that we’ll need. Rather than walk through all of his logic, I wish to begin merely by recapping some of his relevant conclusions.

  1. Change in velocity (delta-V) is a finite resource, and it’s of huge importance militarily.
  2. Hiding a spacecraft (stealth technology in space) is essentially impossible because of the heat generated by the thing (even under minimal power) compared to the unrelenting background cold of space.
  3. Because that heat is detectable as infrared light, range of detection is limited really only by the strength of the sensors. In other words, sufficient sensors can detect a spacecraft at a range close enough to infinity as to not matter, militarily speaking.
  4. Thrust is necessary to effect delta-V. In other words, if a spacecraft wishes to alter course, it must emit thrust of some kind.
  5. Thrust is detectable. If that spacecraft changes course, it can’t hide the fact from its adversaries.
  6. You can’t make a battleship look like a rowboat in space. The heat signatures will give away the game.

Let’s begin by synthesizing these ideas together into their logical conclusions. Orbital mechanics require that course corrections (requiring detectable thrust) be made, very often at distances that are detectable in time for adversaries to effectively counter-maneuver. Let’s look at why this is.

Figure 1

Figure 1 represents a lunar transfer orbit of the type used by the Apollo missions. Keep in mind that this is, in space terms, a small distance to travel. Nevertheless, it can illustrate many of our points quite nicely.

For those unfamiliar with the basics of orbital mechanics, here’s how it basically worked for Apollo. The circles represent the Earth (the larger circle in the lower left) and the moon (the smaller circle in the upper right). The Apollo spacecraft launched on board a giant Saturn V rocket from Point 1 and first entered into a Low Earth Orbit (LEO), represented by the circle around the Earth.

At the appropriate time and place – Point 2 – the Apollo spacecraft executed another engine burn. Here on Earth, that would have been equivalent to simply pointing the car in a given direction and then hitting the gas. The car goes straight until you hit something or run out of gas. Space is different. The spacecraft goes straight until either you hit something or some gravity source operates on it. In this case, the gravity source is the moon – and rather than going straight, the Apollo craft was now on a “figure eight” orbit that orbited both the Earth and the moon. In Figure 1, that orbit would go from Point 2 to Point 4, around the moon to Point 8, then back to the Earth at Point 9 and around to Point 2 again. Assuming no orbital decay (which is a big assumption, and very likely incorrect) it would perform this orbit again and again and again. Apollo 13 actually did almost exactly that, performing only mild course correction burns, in order to get the astronauts home as quickly as possible after the spacecraft was damaged.

But Apollo 13 wasn’t designed to do that. The intention was to do what Apollos 8, 10, and 11 had done, and what the later Apollo missions would do. In the successful missions, another “braking” burn was performed at Point 4 on the chart to slow the spacecraft down. Doing so altered the orbit, and transferred it into a pure lunar orbit – the circle you see around the moon. But without the burn, you get the figure eight orbit.

Had the Apollo craft been traveling at different speeds, the results would have been radically different. Just a bit faster or slower, and the craft still would’ve traveled past the moon and swung around back toward Earth – but its aim would have been off. It would have missed the Earth, reached escape velocity again, and been slung off into interplanetary space.

If it had gone a lot faster, it wouldn’t have even swung back toward Earth. Its path would have arced a bit, thanks to the moon’s gravity, and then it would have just kept going – once more headed for interplanetary space. Had it been going fast enough, the moon’s gravity would barely have even warped its trajectory.

The speed of the spacecraft during the transit stage (roughly point 3 on the chart) is of critical importance. Let us consider six categories of speed:

  1. Very Low Speed – the spacecraft is unable to escape Earth orbit and never transits to the moon at all.
  2. Low Speed – the spacecraft does not have enough velocity to enter a full figure eight orbit, and will likely crash into the moon.
  3. Transit Speed – the spacecraft is paced exactly right for a figure-eight orbit.
  4. High Speed – the spacecraft will swing around the moon and return, but at too high speed to re-enter Earth orbit
  5. Very High Speed – the spacecraft will have its trajectory warped by the moon, but will continue beyond it rather than orbiting it at all.
  6. Ludicrous Speed – the moon’s gravity has an imperceptible effect on the spacecraft’s trajectory.

For our purposes, we can ignore Very Low Speed, as it won’t even allow us to maneuver to fight an enemy spacecraft near the moon. We can also safely ignore Ludicrous Speed for all “hard” science fiction scenarios. Any technology that could be built on currently understood physics or engineering can neither accelerate to Ludicrous Speed nor decelerate from it in anything we would consider a reasonable, militarily significant time.

From a military standpoint in a reasonable hard sci-fi scenario, we must therefore assume a spacecraft operating at Low Speed, Transit Speed, High Speed or Very High Speed. We must also assume that the craft will ultimately need to either a) match orbit with an enemy spacecraft in order to fight it, b) make a flyby close enough to launch a barrage at the enemy, or c) enter an eccentric orbit designed to close with the enemy more than once for repeated barrages.

In nearly all cases, this will require our hero spacecraft to make further burns and expend delta-V. The speed at which the craft makes the transit and its decisions of when, where and how quickly to expend delta-V has massive tactical implications. Writers of hard science fiction – and potential future space combat officers – would do well to keep these in mind.

In part 2 we will begin to examine the ramifications of these decisions.

Orbital Insertion and Space Combat Tactics

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