Orbital Insertion and Space Combat Tactics – Part 2
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.
We left Part 1 with a discussion of the lunar transit orbit shown in Figure 1. Specifically, we showed that we must account for orbital mechanics. Here in Part 2 we will examine specific orbital decisions and how they effect our space combat tactics.
Transit Speed is our baseline. A spacecraft traveling through this orbit at Transit Speed will, left alone, continue into a “figure eight” orbit. A civilian spacecraft traveling an energy efficient path might adopt something like this approach. Depending on the goals of the civilian craft, it would most likely follow the standard Apollo model described in Part 1 and execute a burn somewhere around Point 4 to transition into a solid lunar orbit.
Keep in mind that these positions are relative and in constant flux. An enemy spacecraft in lunar orbit could be at any point along that orbit. However, that position is eminently predictable for a non-maneuvering spacecraft. If we know its position at time t, we can calculate its position at time t + x. But predictable doesn’t mean convenient. It may well be that the timing of our own orbital insertion coincides with the enemy craft being positions near Points 5 and 7. In this case, our military craft would also have to execute a burn to insert itself into a true lunar orbit as well or else it would never actually encounter the enemy vessel.
The timing of this burn carries a great deal of significance, however, and it’s greatly dependent upon our spacecraft’s velocity along the transit path through Point 3. At Transit Speed or above, we’ll have to execute a breaking burn to slow the craft down. At Low Speed, we’ll have to execute an acceleration burn to speed the craft up.
Beyond that, however, is another consideration. Merely inserting ourselves into the same orbit as our adversary isn’t enough. This point is key: any two spacecraft traveling in the same orbit are necessarily traveling at the same velocity. If their velocities are not the same, their orbits will change. This is counter-intuitive to many, but it is a necessary mathematical consequence of orbital mechanics. The military consequence of this is that once two spacecraft are in the same orbit, their relative positions on that orbit will never change.
Suppose for example that I insert myself into lunar orbit at Point 4. At the time I insert into orbit, my opponent is at Point 8. Unless one of us executes another maneuver, we will continue to orbit in that same relative position to each other – separated by more than a third of the circumference of the orbit – until our orbits decay. We will never encounter each other.
There are two solutions to this problem. The first is to adjust our velocity during the transit through Point 3. Speeding up or slowing down here will adjust the timing of our arrival, and by timing it appropriately, we can then ensure that we enter orbit close enough to our adversary to engage. The downside to this approach is that our opponent will see us coming and has a chance to adjust his own orbit to throw our timing off.
The other option is to enter a higher or lower orbit than our adversary. Here, we face another trade off that has tactical significance. Just like a modern air battle, we will find that altitude and speed are our friends. All else being equal, a higher altitude orbit is better. Likewise, all else being equal a faster orbit is better.
Unfortunately, all else is not equal and we can’t have both. The mathematics of orbital mechanics is a brutal dictator that we cannot escape. The catch is that absolute velocity and relative speed of motion are not the same things in orbit. If I move into a lower orbit, my absolute velocity will be increased. This is necessary to offset the higher force of gravity at the lower altitude. I will actually complete an orbit in less time than my higher orbiting adversary. The reason is apparent: even though I am actually traveling more slowly, the distance I must cover to complete an orbit is far less than that of a higher orbit.
As an example, the International Space Station orbits in Low Earth Orbit (LEO) at roughly two hundred and fifty miles and completes an orbit in 92 minutes. By comparison, a geostationary satellite orbits at an altitude of roughly twenty-two thousand miles and completes one orbit in 24 hours.
If I want to catch up to my adversary, then, one solution is to insert myself into a significantly lower orbit than him, come around behind him, and then raise back up into his orbit. This requires inserting myself at the correct orbit point, decelerating (perhaps substantially, depending upon my transit speed) to enter the low orbit, and then accelerating again at the correct time to enter his orbit.
This maneuver has distinct advantages over simply re-timing my insertion into his own orbit. First, it’s less predictable. That means that our adversary has a harder time responding to it. Second, it leaves us with more leeway to correct for any evasive maneuvers our opponent does do. Third, assuming that we’re orbiting in the same direction, it allows me to come up behind him. Yes, in space he can easily turn to face me without sacrificing his orbit.
But that maneuver does actually sacrifice some maneuverability. This is one reason, among many, that the battleships I described in my story “The Fourth Fleet” had main drive engines on both the fore and the aft of the ship. In addition to being able to accelerate or decelerate without turning, the ships maintained an ability to change to either a higher or a lower orbit without turning. Designing a craft this way would be hugely expensive, much like today’s supercarriers, but it would have huge military advantages.
The final maneuver that we could attempt would be to insert myself into a higher orbit than my adversary and let him catch up to us. Then we could descend into his orbit to engage. With a typical ship configuration, however, this could mean letting him approach our rear or sacrificing orbital maneuverability. However, the configuration I just described above could easily handle this without such a sacrifice. In a situation where, say, I was inserting myself into orbit at Point 4 while my opponent was at Point 5, this might allow me to engage him much faster than otherwise.
This puts my opponent is a position of having a higher apparent relative speed, but it leaves me in a higher altitude orbit and with a higher actual velocity, both of which might be advantageous if used properly. One way that a higher orbit is tactically advantageous is that projectile or missile weapons don’t have to accelerate to reach our opponent – instead they would decelerate, and correct timing might mean that they don’t even have to do that. Kinetic weapons (bullets, shells, railguns, etc) might especially benefit from this, as the higher absolute velocity of a higher orbit would impart a higher raw kinetic energy into our projectile. Meanwhile, these same factors would work against our opponent, reducing the kinetic energy of his projectile weapons and requiring extra acceleration for them to even reach our orbital altitude.
As noted above, the exact way in which these factors trade off tactically is dependent upon the design of both our own spacecraft and our adversary’s. In some ways, we can design the craft to neutralize disadvantages of one position or another (energy weapons, such as lasers, would not suffer from the kinetic energy or acceleration problems imposed by a lower orbit). We can also design our spacecraft to amplify the benefits of one position or another (heavy use of railguns, for instance, could capitalize the benefits of a ship designed to operate at a higher orbit), although we must keep in mind that this would also have the effect of amplifying the drawbacks of poor positioning.
In Part 1, we spelled out why we must account for orbital mechanics in space combat tactics. Here in Part 2, we saw how decisions of particular orbits have a large effect on those tactics. In Part 3 we’ll examine some alternative orbits and see how those can drastically alter the tactical situation.
Orbital Insertion and Space Combat Tactics
- Orbital Insertion and Space Combat Tactics – Part 1
- Orbital Insertion and Space Combat Tactics – Part 2
- Orbital Insertion and Space Combat Tactics – Part 3
- Orbital Insertion and Space Combat Tactics – Part 4