When your rocket is in an orbit around a celestial body, there are a number of orbital maneuvers you can perform. The maneuvers can change the properties of your orbit. This is called orbital mechanics.
Engine burning[]
- Main article: Burn
A rocket burning prograde.
Prograde[]
Burning in the direction you are flying increases your velocity, enlarging your orbit. It is required to execute a Hohmann transfer or visit other planets.
A rocket burning retrograde.
Retrograde[]
Burning opposite to the direction you are flying in decreases your velocity, shrinking your orbit. It is often used to enter a planet's atmosphere (known as reentry for returning to a planet with an atmosphere and atmospheric entry for entering a planet's atmosphere) to perform a landing.
Hohmann transfer[]
The Hohmann transfer maneuver is simple. It's used to change orbits inside a sphere of influence of a celestial body. This is used to dock two spacecraft together or transfer them to minor celestial bodies (e.g., Phobos and Deimos).
- At periapsis, burn prograde to increase the apoapsis.
- At apoapsis, burn prograde to increase the periapsis.
Gravity assist[]
Main article : Gravity Assists
A gravity assist is a way of using a planet's gravity to increase or decrease the rocket's velocity without wasting fuel.
Changing velocity[]
- To increase velocity, get the periapsis as close to the planet and in the same direction from its center as you're traveling, usually counter-clockwise.
- To decrease velocity, get the periapsis on the opposite side of the planet's direction of travel.
Aerobraking[]
Aerobraking at Venus.
- Main article: Aerobraking
Aerobraking is a maneuver used to slow down a rocket using a celestial body's atmosphere with minimal amounts of fuel, or decreasing the apoapsis of a rocket. To achieve the maneuver, the rocket first needs to slow down until its periapsis crosses the upper layers of the atmosphere.
The maneuver is used to conserve fuel by slowing down the orbit (because you only need to bring the periapsis into the upper atmosphere). Reaching the top of the atmosphere multiple times allows more control. When landing from a high orbit on a planet with an atmosphere, aerobraking multiple times is safer than relying entirely on a landing burn. Aerobraking can also be used to decrease the apoapsis of an orbiter.
Aerocapture[]
Main article: Aerocapture
An example of aerocapture
Aerocapture is used to slow down a rocket to an orbit without landing on it. It is used by many rockets to save fuel on their missions.
Flyby[]
A "flyby" is an encounter with a celestial body without intent to get into orbit around it. A common example occurs when escaping the Earth's sphere of influence and meeting the Moon on the way, or when in a Jupiter transfer the rocket encounters a flyby of Mars or minor celestial bodies, but in this case this is uncommon. Flybys can be used when using gravitational assists.
A flyby of Venus may also be used when trying to get to Mercury. Using Venus to slow your rocket and get into an orbit that is perpendicular to Mercury's can be used for an encounter with the planet in the future.
A recreation of the New Horizons mission in a planet-edited Spaceflight Simulator to use a Jovian gravity assist to boost its trajectory to a Pluto encounter.
Oberth effect[]
Main article: Oberth effect
The Oberth effect is a way to save fuel during an encounter. It's a simple stretch of logic that the closer a rocket is to a planet, the faster it goes, and the faster it goes, the more work is done by changing velocity. It's easy to see that as you increase the apoapsis, your maximum velocity increases at the periapsis as a result. To take advantage of the Oberth effect, it is best to do three things:
- Begin burns before the transfer window indicator, such that the rocket passes the indicator roughly halfway through the burn.
- If the rocket's thrust-to-weight ratio (TWR) is low, it may be necessary to split burns into multiple passes. This is done by burning as much as is practical near the transfer window, then allowing the rocket to pass around and encounter the window again. Repeat until the burn is complete.
- At the periapsis, you can use the Oberth effect to excellent advantage. By burning at periapsis, you can increase or decrease your velocity significantly while expending less delta-v. Conversely, burning at apoapsis is considerably less efficient and may result in expending significant amounts of fuel for little reward.
Fuel mass cascade[]
Main article: Fuel mass cascade
The fuel mass cascade is the effect by which every mass of the mission payload generates a list of required additional fuel masses for each engine burn of the entire mission, related to one another.