So you've made it to space... Now what? There's a lot more to space exploration than just the launch. While overcoming Earth's gravity and making it to orbit is the hard, Maneuvering your spacecraft once in orbit is even harder. In this article I'll break down the basics of orbital maneuvering terminology and everything you need to know to change your orbit to whatever you need it to be.
Before we do anything else, lets define what an orbit actually is. An orbit is any path where the velocity of an object is so great that it revolves around another object indefinitely, the acceleration due to gravity is cancelled out by the lateral velocity such that the object travels in an elliptical path without ever touching the other object. An object can also be suborbital, meaning that if it stays on its current path it will fall back to the ground. A rocket that fails to develop enough velocity at launch will be on a suborbital trajectory and will return to Earth.
On earth whenever an object "maneuvers" it moves from one place to another and are otherwise stationary, but in orbit objects are always moving at extremely high velocities. Maneuvering in orbit means changing the path that an object is moving along. The way you change your orbital path in space is by firing your engines. Rocket engines burn fuel and oxidizer in a combustion reaction that produces energy, the rocket uses this energy to change it's velocity, which in turn changes the rockets orbit. Any time you fire your engines for a set amount of time is called a burn.
When planning maneuvers while in orbit, there are 6 directions that you can move your craft in. The first two directions are prograde and retrograde. Prograde is the same direction as your current velocity, it can be thought of as pointing forwards. Burning prograde will raise the altitude of your orbit 180 degrees from your current position in your orbit. Retrograde is the opposite direction of your velocity, or backwards from the direction you're travelling in. Burning retrograde will lower the altitude of your orbit 180 degrees from your current position in your orbit. Next is radially in and radially out. Radially in is the direction pointing directly inwards, towards the center of your orbit. 90 degrees from your velocity vector. Burning radially in will lower your orbit 90 degrees in front of your current position, and raise your orbit 90 degrees behind your current position. Radially out is the direction pointing away from the center of your orbit again 90 degrees from your velocity vector. Burning radially out will raise your orbit 90 degrees ahead of your current position and lower your orbit 90 degrees behind your current position. Both radially in and radially out lie on the orbital plane. And lastly we have normal and anti normal. In mathematics the term normal means 90 degrees, or perpendicular to something. The normal direction points 90 degrees up from the orbital plane. Burning in the normal direction increases your inclination. The anti normal direction points 90 degrees down from the orbital plane. Burning anti normal decreases your inclination.
A maneuver node is a point in a spacecrafts orbit where a burn will be performed to change it's expected orbit. Burns can be in any of the 6 above directions, or any combination of them in order to achieve a desired final orbit. Maneuver nodes are used to plan future actions at specified times in order to reach a desired orbit.
An encounter is when two objects in orbit pass by each other in close proximity. This is basically the same as an intercept and the two can be used interchangeably in most situations. Encounters are most commonly seen when going to other planets, where the space craft intercepts the planet along its orbit.
A rendezvous is when two objects remain in close proximity with each other for an extended period of time, with the relative velocity between them being close to 0. Rendezvous are performed when docking to objects together in orbit, like space stations.
When talking about intercepts or rendezvous in orbit, we often want to know how close the two objects will get to each other during the next period. The closest point of approach, or CPA, is the point in an objects orbit during a given period where it will be closest to its target. CPA changes every orbit as the two objects are both constantly moving, so the two objects may pass closer together in future orbital periods than the current one.
Now that we have discussed orbital terminology, let's put it together and talk about what you can do once in orbit. There are a number of different maneuvers you can perform while in orbit, depending on what sort of mission you're trying to do. Let's start with the basics.
A circularization maneuver is when you adjust an elliptical orbit to reduce it's eccentricity. For more information on orbital terminology you should read Apoapsis, Periapsis, and Everything In Between. Circularization burns can be performed at either the apoapsis or periapsis of your orbit. If you perform a circularization maneuver at your orbits periapsis, you must burn retrograde to reduce your apoapsis, making it close in altitude to your periapsis. Likewise, if performing a circularization burn at your apoapsis you must burn prograde to raise your periapsis. Circularization burns generally do not change the semi major axis significantly.
An inclination change is when you change the inclination of your orbital plane relative to a reference plane. You can change your inclination relative to another space craft to match it's orbit, or relative to the object your orbiting to to change it's path around it. To increase the angle of inclination of your orbit to the reference plane, burn in the normal direction at the ascending nodes, or the anti normal direction at the descending node. To decrease the angle of inclination of your orbit to the reference plane, burn anti normal at the ascending nodes and normal at the descending node.
If you want to go from one orbit to another orbit you can perform a Hohmann transfer maneuver. Start by burning prograde in your current orbit till your apoapsis is at the desired altitude of your final orbit then burn prograde again at your apoapsis raising your periapsis until it is at your desired altitude. These two maneuvers together make the Hohmann transfer maneuver and it is the most common way to change a spacecrafts orbit around an object.
Often times during spaceflight, your mission will require you to intercept another object, whether it's a planet or another spacecraft. To put yourself on an intercept course requires two things: your spacecraft needs to be on the same orbital plane as the object you're trying to encounter and your spacecraft's orbit needs to cross the object you're trying to intercept. The first thing you need to do is match your spacecrafts inclination with your targets inclination. Find the point where your orbit crosses the plane of your targets orbit and plan a maneuver there. Burn radially up or down to change the inclination of your orbit to match your targets inclination. Once you've matched inclinations, raise or lower your apoapsis so that its on your targets orbit, this will be the point your intercept the target. Now plan a maneuver at your current apoapsis to raise or lower your periapsis. If you raise your periapsis above your targets orbit, your orbit will take longer to complete and your target will start to catch up to you, likewise if you lower your periapsis your orbit will be shorter and you'll start to catch up with your target. Raise or lower depending on whether the target is closer to being in front of you or behind you. Once you do that and your target is close to you in orbit, adjust your periapsis again to be closer to your targets orbit, you should see your closest point of approach with the target get closer and closer, plan your maneuver burn so that on the next revolution around the planet, you're as close to your target as possible.
A rendezvous fallows the same steps as an encounter maneuver, but with one final burn at the closest point of approach. This final burn is made in the retrograde direction relative to the target object, bringing the relative velocity between the two objects to 0, essentially parking them in orbit next to each other. Once two objects have completed a rendezvous in orbit they can be docked together, if the two objects aren't docked then the slight differences in their orbits will cause them to gradually drift apart.
Once in orbit, if you want to land you need to perform a deorbit maneuver. A deorbit maneuver is when you burn retrograde to cancel out your orbital velocity until your spacecraft is on a suborbital trajectory.
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