## Adventures with Kerbal Space Program part 2

-## Boldly going where no Kerbals have gone before

Continuing from my last post I am still pretty addicted to the space simulator Kerbal Space Program. Over the weekend i tried to make a point of understanding some of the terminology used and some of the basic concepts behind it. A lot of these terms come up right from the start but aren’t explained inside the game itself.

## Basic Physics Terms

Some of these may be basic but it was good for me to re-cap them as it has been a long time since I studied physics. Even then all we covered in secondary school was all related to electricity (our physics teacher was incredibly dull), calculating electrical resistances etc.

It is fair to say if we got to play with rockets etc at school I probably would have paid a lot more attention!

Anyway, back to the topic at hand;

**Mass**

Not to be confused with weight although they are similar they are often confused with one another. Mass loosely refers to the amount of matter that makes up the object (measured in units such as Kg’s). Weight however is the outcome of the mass influenced by gravity (measured in newtons) etc. In KSP (kerbal space programme) the larger the mass of your rocket the more thrust required to lift it. I have fallen victim to this a few times as eventually it reaches a point where you are adding more fuel to cover the fuel required to lift your monstrosity which means again, adding yet more fuel.

**Thrust**

This is a nice easy one to explain. Thrust in it’s simplest form is a force strong enough to move mass. It is a bit more complex in real terms but in Einsteins 2nd and 3rd laws it says when a force is expelled from an object it creates a force of equal magnitude in the opposite direction or more famousely, “every action causes an equal and opposite reaction”. In Kerbal Space Programme (KSP) a rocket for example exerts a downward force which in turn creates an upward thrust! If you don’t have enough thrust it simply wont move!

**Velocity**

Much like mass and weight, velocity and speed are closely linked though shouldn’t be confused with one another. Speed describes how fast an object is moving whereas velocity also describes it’s direction (so 60mph is a speed whereas 60mph east would be it’s velocity). It is quite a complicated topic so I will only brush over what’s important in Kerbal. Taken from Wikipedia here, this is a good description of what it means in relation to an orbit:

“The big difference can be noticed when we consider movement around a circle. When something moves in a circle and returns to its starting point its average velocity is zero but its average speed is found by dividing the circumference of the circle by the time taken to move around the circle. This is because the average velocity is calculated by only considering the displacement between the starting and the end points while the average speed considers only the total distance traveled.” The article also goes on to describe a few important points such as “escape velocity”, which is used to calculate the minimum speed an object (spaceship etc) requires to break free from the gravitational energy or the orbited object (planet, solar system etc).

**Prograde/ Retrograde**

In Kerbal terms I found this was a strange concept to get my head around as I personally thought it isn’t explained very clearly so I made the quick diagram above. The blue object in the middle is the object you are orbiting and the white triangle with the tip of the triangle being the direction it is travelling. Based on that, the green arrow indicates prograde and the yellow indicates retrograde. If you increase your acceleration in a prograde direction you orbit speed increases and gets further from the planetary body, naturally if you accelerate in a prograde direction you actually slow down your orbits relative speed reducing distance to the planetary body.

In planetary terms however it means something slightly different. Prograde is where an objects orbit (spaceship etc) is in the same direction as the object it is rotating on its axis (planet etc), so if the planet rotates clockwise a prograde orbit would be clockwise around it. Retrograde in that case would be the opposite.

**Apoapsis/ Periapsis**

Again like prograde and retrograde, the apoapsis and periapsis is relative to your ship’s orbit. In short if your ship has an eliptical orbit it will have an apoapsis and a periapsis in the orbit. The apoapsis is the highest point of the orbit (or the furthest point away from the planet you are orbiting, if that is easier to visualise), and from that the periapsis is the lowest point in the orbit.

That should cover the most important points to understanding a lot of the main terms in Kerbal!

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