Disclaimer: My knowledge about orbital mechanics is basic wiki-self-teaching. I haven’t played Kerbal Space Program or anything similar, so my trajectories probably leave a lot of room for improvement.
In this mission we have to destroy an enemy space station mining Methane in a low orbit around Neptune. Its orbit is extremely low, fast and highly inclined, so it is very hard to reach. High command has acquired some ships with crews from a neutral faction that has a base on the Neptune moon of Polyphemus. The challenges with this mission are mostly about reaching the target in the first place. Killing the undefended station should then be no problem.
Since no enemy defence is expected, our fleet for this mission is built around a Privateer vessel. Lightly armored and armed, but with an impressive deltaV of 11.8 km/s (ability to change this amount of speed with the given propellant). Most other warships have dV in between 4-6 km/s, which would be insufficient to reach our target.
The Privateer is accompanied by a Hydrogen Tanker with an impressive dV of 22.3 km/s, which of course can share some of its propellant. Both ships have a relatively low thrust, with the lower of the two, the Hydrogen Tanker, limiting the acceleration of the fleet to 0.07g.
This is the starting position. In red is the enemy Methane Depot in a extremely low and inclined orbit.
Our own fleet starts in orbit around the Neptune moon Polyphemus.
Our first task is to get out of orbit of the moon. Since it’s gravity is so tiny, a short burn of 5 m/s is enough to escape.
While such a small burn gets us out of the moon’s orbit, it is insignificant for our orbit around Neptune. It stays basically the same. We have to spend a lot of dV more to get somewhere in relation to the gas giant.
Since the time point and orientation of our first burn was already selected to be aligned with our orbit around Neptune, we can simply make this burn larger. I select to do a large burn of 2 km/s of dV, which will take well over an hour to complete.
Wait a second, we are supposed to get down to a lower orbit. Why are we boosting our fleet to an even higher orbit? The answer is a bi-elliptic transfer:
I let that explanation stand for its own…
As our trajectory reaches the orbital plane of our target, we initiate a huge retrograde burn of 6.4 km/s delta-V. This basically kills all our orbital speed and leads to a free fall, crashing dead center on Neptune.
Now lets add a “downward” component to the last burn. Basically we are offsetting our free fall, passing next to Neptune and entering a highly elliptical orbit.
This is the the side view, the horizontal line is our staring orbital plane. While the lowest point (perigee) of the elliptical orbit is touching the target’s orbit (and almost skimming Neptune), the highest point (apogee) will rise again to the position of our last burn. In order to flatten the ellipse and get a circular orbit around Neptune, we will have to spend a lot of delta-V.
Retrograde burns at perigee (lowest point) lower the apogee (highest point), so lets do that and spend 1 km/s dV. A lot more would be required, but we can’t do more at this point. Since we are going so fast close to Neptune, notice the insane amount of distance traveled while doing this burn (orange trajectory). If the burn was made longer, it would eventually continue to a position where it would even enlarge the orbit. Because of this, we will have to make subsequent shorter burns at perigee through multiple orbit. Also notice that we have at this point already spent 10 km/s of our 12 km/s delta-V budget, so some refueling is in order.
So far this has only been a plan for a trajectory. So let’s run time for a couple of hours until the last planned burn is complete.
The privateer is low on propellant, so lets distribute some fuel from the Hydrogen Tanker.
With the hydrogen transfer complete, both sips have about 10 km/s delta-V available.
Our position after 26 hours with the refueling just complete. The next retrograde burn at perigee to further flatten our orbit is set up.
After another orbit, the next burn is prepared.
Another orbit, another burn at perigee.
One more burn. This one is adjusted in length to phase our own orbit with the target. Just being in the same orbit as the target serves little purpose if the target is on the other side of the planet. The intersection needs to be timed to be at the same point at the same time.
After the next orbit we will be in a close enough position to plot an intercept. Maneuvering so deep in the gravity well of such a giant planet is brutal on delta-V. The intercept will cost us another 1.7 km/s and sends us closing in at 394 m/s.
Finally the intercept is achieved. Our privateer rotates side wards to open up with its 60mm cannons while screaming in at 400 m/s.
The unarmed enemy Methane Depot stands no chance and is immediately disabled.
Mission successful.
