Overview
Welcome to the #1 rated guide of all time for KSP!!Some math, technical terms & basic mechanics necessary for your success in Kerbal Space Program. This guide is suitable for any human being of average mathematical and technical background.
Fundamental Mechanics
This is a beginners guide to rocket science, made simple!
*refresh if words overlap with images*
In case you don’t know what these are, I’ll explain them!
Velocity SI UNIT : meters/second or m/s
Is the rate at which an object moves, relative to another object. We are all moving at 30 kilometers per SECOND around the sun, which means that our velocity is 30km/s relative to the sun. But, when we talk about velocities here on Earth, we usually reference them to ground. If I say a car moves 10m/s, it’s moving 10m/s relative to ground. Hence, velocity is relative.
Acceleration SI UNIT : meters/second^2 or m/s^2
Is the rate at which an object’s velocity changes. Hence, meters per second PER SECOND! An object accelerating at a rate of 2 m/s^2 means that every second, it will increase its velocity by 2m/s.
Time SI UNIT : seconds or s
Time. A hard one to explain ain’t it?
Distance SI UNIT : meters or m
Just a measure of length!
Motion & Orientation
Things you just NEED to get used to
In everyday life, we can say left or right or up or down to specify direction. In rocket science, we have the same, except in different terms.
-Prograde: the direction in which the spacecraft is traveling
-Retrograde: the opposite direction in which the spacecraft is traveling
-Orbit Normal: Perpendicular to the orbit and upward facing
-Orbit Anti-Normal: Opposite direction of Orbit Normal
-Radial In: facing towards orbiting body (such as a planet or star)
-Radial Out: facing away from orbiting body
Principal Axes
Spacecraft rotates to adjust its flight path, this is much like an airplane. There are 3 axes:
Pitch, Yaw and Roll
Thrust
Thrust is measured in Newtons, which is the unit for force!
From Newton’s laws, we know that objects accelerate due to applied force, the mathematical illustration of this is Force=mass*acceleration or F=ma. In a rocket, exhaust particles are constantly being driven out of a rocket nozzle, that is, particles are being accelerated by the chemical reactions within the rocket. With action comes reaction, since a force is being exerted on the particles coming out of the rocket, a force of equal magnitude is exerted in the opposite direction! This is why rocket propulsion works!
How particles are accelerated by chemical reactions:
When chemical reactions occur within the combustion chambers of a rocket, heat is generated. Thus, pressure within the chamber increases due to the increase in average kinetic energy[1] of the particles. Heated particles want to expand outwards, which is how a steam engine works. The pressure difference between the outside of the rocket nozzle and inside of the nozzle is what causes the particles to move out!(more on that later)
[1] Temperature is proportional to average kinetic energy of an object, kinetic energy is the energy of motion! So particles within onjects are constantly in motion so long as they have temperatures above absolute zero. The higher the temperature, the greater the motion.
Mass and Weight!(THEY ARE DIFFERENT)
Mass and weight ARE NOT the same!
Mass SI UNIT : kilograms or kg
Weight SI UNIT : Newtons or N
Mass and weight ARE NOT the same!!!!!!!!!!!!!
Mass and weight CAN HAVE similar values but they are FUNDAMENTALLY DIFFERENT!
Mass is a measure of how much stuff is in something, a metal ball for instance. Weight, in contrast, is the measure of how much force gravity is exerting on an object! If a metal ball on Earth weighs 981 Newtons, the metal ball has a mass of 100kg, but the metal ball on Mars weighs 371 Newtons! Weight is dependent on the gravitational influence of the mass dominant object(a.k.a planets). Mars has a weaker gravity on its surface compared to Earth.
*Weight is calculated based on F=ma, acceleration on earth is 9.81 m/s^2, the picture below simplifies it to 10 m/s^2*
Isp, Specific Impulse
In rocketry, or any device or vehicle that generates thrust, we use specific impulse(Isp) to measure efficiency. As you may know, a rocket loses mass as long as it is burning fuel. This is due to the fact that the mass of the particles coming out of the rocket nozzle is precisely the mass which the rocket loses. In order to maximize the efficiency of a rocket, the exhaust velocities of each particle must be maximized as well. This is because of F=ma, as discussed previously. Exhaust particles ejected at high speeds HAVE to be accelerated to that speed, which means higher exhaust velocities equal more acceleration, which means more Force. Having said all this, Isp is the measure of thrust produced per unit of fuel, or fuel efficiency.
You can see a rocket engine’s Isp by hovering your mouse cursor over it!
Note:
The unit for Isp is in seconds, this may seem weird, but it is the result of canceling units. For more info check out this NASA page. [link]
Atmosphere
Not all planets within the Kerbal solar system have atmospheres! But it is an EXTREMELY important factor to consider in Kerbal Space Program! Landing and taking off from a planet with and without an atmosphere is very different!
Pressure
What is pressure? Pressure is the measure of force applied to a surface. The SI unit for pressure is Pascals(Pa) or N/m^2(force over area). A familiar non-SI unit is PSI.
A pressure difference causes particles to flow from a high pressured location to a low pressured location, the higher the difference, the faster the flow rate.
This is why compressed gas flows out fast and aerosol cans work!
Due to the Earth’s gravitational effects, air gets thinner as altitude increases, that is, air density decreases and so does atmospheric pressure.
Atmospheric pressure is a bit of an odd thing, the atmospheric pressure on Earth at sea level is over 101,300 N/m^2. This, in literal terms, means that there are more than 10 tons of mass stacked on top of any table that has a surface area of 1m^2(1 meter by 1 meter). The only thing preventing the table from collapsing is because the atmospheric pressure is everywhere, it exists on top, and on the bottom, the forces cancel out! However, this does not mean you can’t destroy the table, simply make the space below it a vacuum! By doing so, you disrupt the balance of force and effectively allow pressure to exert the force(10 tons)!
Drag
Basically, drag is just air friction, it slows your craft down and generates heat! This is vital for liftoff and reentry. The amount of drag depends on the atmospheric pressure, shape and velocity of a craft. More on this later!
Specific Impulse(continued)
Now that we know about pressure, we now can dive into the deeper aspects of specific impulse. From the last section, we learned that atmospheric pressure varies throughout different altitudes. Another concept we learned is that the difference in pressure determines the speed at which particles flow from point to point. In rocketry, this is where particles flow from the rocket nozzle(high pressure) to the atmosphere(low pressure). Since the rocket flies upwards and increases in altitude, atmospheric pressure decreases and the pressure difference between the rocket nozzle and atmosphere outside changes. Therefore, since specific impulse is proportional to exhaust velocity, specific impulse changes too!
Mass Distribution of a Vehicle and Control Systems
Why is mass distribution so important?
During flight, we want the rocket to be as stable as possible. If the mass is not well distributed, gravity acts heavier on the heavier sides, this creates torque, which displaces the rotation of the rocket.
To achieve stability, we have several options. One is to use aerodynamic fins(like flaps on an airplane), the other is to use RCS thrusters, which stands for reaction control system. These thrusters are usually mounted perpendicular to the direction of the rocket(see e in picture below). These thrusters are alot less potent compared to your rocket engine, therefore, when traveling through an atmosphere where air drag dominates, RCS is not very effective. Another method is to alter the direction a rocket nozzle is facing during flight(this is called GIMBAL, see d). In atmospheric flight, fins and gimbal are probably the best.
You should use all of these methods to keep your rocket safe!
Gravity
Why is Newton considered one of the most important scientists in history?
To understand how a rocket works, we must understand the environments in which they operate in. On the surface of the Earth, we are held to the ground by gravity. In other words, a force is being exerted on US downwards, otherwise, we’d be floating all over the place. This force is what we call gravitational force. Recall from earlier sections that F=ma, on the surface of the Earth the gravitational acceleration is 9.81m/s^2, this is true for all objects. But think about it, why is that? Shouldn’t a heavier object fall to the ground faster than a light one? This is a common misconception in which alot of people believe, the truth is that objects that fall slower are simply influenced by atmospheric friction(like a feather). This means that on a planet with no atmosphere(no friction), a hammer and a feather will fall at the same rate. Thus, gravitational acceleration is independent of mass!
Gravity at high altitudes
Why in space do objects feel little or no gravitational forces? At lower altitudes, gravitational effects decrease dramatically because gravitational force(F) and acceleration are inversely proportional to the square of the distance(r)(depicted by the equation below). For those who are bad at math, this simply means that as the distance increases, gravitational effects decrease at a higher rate. To clear things up a bit, what does the word distance mean in this context? It is the distance between the center of mass of the Earth, and the satellite.
However, if you were on a spacecraft in orbit, you don’t feel a force for a different reason. When you are in orbit, you are moving with the spacecraft in a state of free fall. Think of it this way, when you are on the surface of the earth, gravity is crushing you towards the ground, that’s how you stay down and that’s how you feel gravity! However, when you’re in free fall, the spacecraft is moving with you in the same direction at the same velocity, so you’re not crushing towards anything, and hence don’t feel gravity!
*for those who can math
G is the Universal Gravitational Constant
M is the mass of the big object(e.g Earth)
m is the mass of the small object(e.g The Moon or International Space Station)
r is the radius, or the distance between two centers of mass
*Notice that if you move m from the right to the left, F/m=a, therefore a=GM/d^2. Gravitational acceleration is independent of mass!
Basic Orbital Mechanics
Terms
Apoapsis: the highest point in an orbit, where the distance between the satellite and the body it is orbiting is the highest.
Periapsis: the lowest point in an orbit, where the distance between the satellite and the body it is orbiting is the lowest.
Tangential Velocity
To avoid confusion, anything that moves has a velocity MOVES in a STRAIGHT line! A force(such as gravity) can be applied to a moving object, causing it to change direction. An easy way to think about this is to imagine an object orbiting a planet. If the planet suddenly disappears, the object will no longer move in a circle because the gravitational force that makes the orbit possible will cease to exist. The object will therefore move in a straight line, which is representative of tangential velocity.
Gravity in Play
Have you ever attached an object to a rope and swung it around like a degenerate? Guess what! That’s pretty much how objects orbit the Earth! The reason why objects don’t fly off is because of the rope. The rope is exerting a force on the object directed towards the center of rotation. This force is analogous to the force of gravity(depicted by a in figure). For an orbiting satellite, gravity constantly pulls it inwards but the satellite is constantly moving in the perpendicular direction of the pull. The pull of gravity keeps the satellite from flying out, and keeps it moving in a circular trajectory! To achieve orbit, a satellite must have sufficient velocity(depicted by v in figure), or else it will fall back into Earth(see picture below)
Orbits
Here comes the tricky part, across different altitudes, different magnitudes of gravitational forces are experienced. Hence, we don’t exactly need the same amount of velocity to orbit a planet at different altitudes.
So how and when do we fire up the engines and gain some velocity? To achieve orbit, we first gain enough velocity to reach our desired altitude. After we reach that altitude, which is around the apoapsis, we fire the engines once again to gain the tangential velocity we want! Be sure that your rocket is facing the prograde direction when you do this!
Eccentric Orbits
Beyond Circular Orbits
So far we have learned about the simple type of orbit, which is circular. But what happens when you apply prograde thrust in a circular orbit? Your orbit begins to go elliptical! The math of elliptical orbits are quite complex, but we’ll get more into that later.
Elliptical Orbits are useful for traveling to the moon or other planets in the solar system. Here is a graphic demonstrating the trajectory of the apollo lunar missions.
First, the rocket reaches Earth orbit
Second, the rocket achieves an elliptical orbit that aligns with the moon
Third, as the rocket reaches the moon, it begins to burn in the retrograde direction to achieve orbit with the moon, this is called capture.
Lastly, the rocket burns even more in the retrograde direction to perform touchdown
Are you getting the connection? You should realize that all of these processes have to do with altering the trajectory of an orbit. In previous sections we talked about getting to different orbits. To get from a circular orbit to an elliptical orbit you gain velocity in the prograde direction. To get from an elliptical orbit to a circular orbit you decrease your velocity, hence, fire the engines in the retrograde direction.
Gravitational Potential Energy, Kinetic Energy and Delta-V
I’m sure some point in our lives we’ve heard about conservation of energy. Rocketry is no exception! In this case we are talking about the conversion of energy between gravitational potential energy and kinetic energy.
Kinetic Energy
When an object has velocity, it has kinetic energy. Kinetic energy is proportional to the square of the velocity. Its equation is (1/2)mv^2. Where m is the mass, and v is the velocity.
Gravitational Potential Energy
When an object is within the influence of a gravitational force, it has the “potential” to gain kinetic energy while discarding some of its potential energy. Think of a bowling ball for example. A bowling ball suspended 10 meters above the surface of Earth has potential energy because upon release, its gravitational potential energy will be converted into kinetic energy. It gains speed as gravity accelerates it! For this bowling bowl example, the math is quite easy because the gravitational acceleration at the surface of Earth and 10 meters above has very little difference. For objects in orbit, the concept is still the same, but the math is a different story because remember: gravitational force varies with the distance between two centers of mass, and at high altitudes the difference is notable.
Apoapsis and Periapsis
Having said all this, when a satellite is at its apoapsis, it is farther away from the object it is orbiting and thus, has more gravitational potential energy. On the contrary, when the satellite is at its periapsis, it is closer to the object it is orbiting and has less gravitational energy. Since energy is conserved, where there is less gravitational potential energy should be more kinetic energy and where there is more gravitational potential energy should be less kinetic energy. This is why satellites move faster at their periapsis than at their apoapsis!
*This phenomenon can also be explained through Kepler’s laws, if you’re interested, look it up!
Delta-V
Delta means change, V means velocity. If a rocket with a Delta-V of 6000m/s is situated in space, far away from any gravitational influence, it can gain 6000m/s of velocity. Delta-V is a good measurement of what a rocket is capable of as it shows how much it can travel. However, the value of Delta-V does not necessarily mean how fast a spacecraft will be going when it arrives at its destination. If we travel to low earth orbit, we need a Delta-V of 9400m/s, but when we are in Earth Orbit, our velocity is not 9400m/s, it is less. What happened? Some of the kinetic energy is stored as gravitational potential energy(The vehicle loses speed as it flies upwards due to gravity, but it can also regain that speed by falling back into Earth!). Thus, energy is conserved! The Delta-V of a rocket is calculated as the sum of how much velocity each stage can gain.
Here is a map of how much Delta-V is required to travel from one place to the other in the solar system.
Additionally, here is a Delta-V map for the Kerbol system:
Kerbol Delta-V Map[forum.kerbalspaceprogram.com]
Interplanetary Travel
The Hohmann Transfer
The best way to get to another planet is to perform an orbital maneuver called the Hohmann Transfer. The mathematics of this are quite complex but luckily there is a handy calculator available online with step by step instructions to ensure success.
As you may know, the Kerbol system, which is a small version of our solar system, has many planets orbiting the sun at different radii. At different distances from the sun, the orbital velocities and orbit circumferences of each planet(in relation to the sun, cuz planets orbit the sun) are different. Thus, timing is important as you need to know that your destination planet is actually there when you arrive! This is why we need the Hohmann Transfer!
*additionally, not all planets have circular orbits!
The link to the online calculator is here:
ksp.olex.biz
Staging and Different Rocket Engines
Recall in previous sections that we talked about how a rocket engine can have different specific impulse at different altitudes. Now, it’s time to talk about different rocket engines with different specific impulses.
Solid Rocket Boosters(SRBs)
Solid Rocket Boosters have considerably low ISPs, however, they are cheap and powerful. The reason they are powerful is due to their ability to produce thrust. SRBs are not very efficient, but they do output a tremendous amount of thrust necessary for liftoff. Hence, they are not designed to last very long, but they do give rockets the kick it needs to get it going. Its low ISP is one of the reasons why they are not usually used beyond liftoff. Another reason is because you can’t stop an SRB from burning, once its burning, it stays burning until it burns out. You also cannot control thrust output in real time, which limits its uses.
Liquid Fuel Rockets
Liquid fuel rockets have higher ISPs than SRBs, they can be used for liftoff in conjunction with SRBs. Liquid rockets are a little more costly compared to SRBs. You can adjust the thrust of a liquid fuel engine during flight, which is very convenient when you are conducting complex orbital maneuvers. You can also shutdown the engine at any time!
Staging
As you know, transitioning from atmospheric to space flight is better with different sets of engines. To get the most out of your fuel, you don’t want your vehicle to be heavy!(recall F=ma) Thus, you will want to abandon the engines that we no longer need. This is what we call staging. As you saw in the last section, the Apollo lunar mission’s Saturn V rocket has many stages. Each stage has a different purpose. Mess around with the different rocket engines in KSP, you will find that each is suitable for certain maneuvers while others are not!
There can be as many stages as you want! Be creative!
There are two types of staging, the parts that allow for separation are called decouplers.
Airplane Engines vs Rocket Engines
As you may know, airplanes suck air into their engines, rockets DON’T. This means that airplanes can eject exhaust particles that it didn’t carry! This is a tremendous advantage in terms of efficiency for an airplane engine. However, there are limiting factors.
Airplanes
There are a variety of engines that go on airplanes. Commercial airplanes that don’t go beyond the speed of sound use turbojet engines. This is the engine that you should be most familiar with. For transonic flight, the aerodynamics get tricky as we have to deal with shock waves, which is beyond the scope of this guide.
Space
Since the air gets thinner and thinner as we rise in altitude, airplanes no longer have the oxygen necessary for combustion. This is why airplanes don’t work in space, there is no oxygen! This is why rockets have to carry their own oxygen, which are called oxidizers.
Re-Entry
FIRE! FIRE EVERYWHERE!
F**K F**K F**K F**K F**K!!!!!
Re-entry is scary, parts may disintegrate, burn out, especially when you have sensitive equipment on board! If you don’t want them to be damaged, consider mounting a heat shield of appropriate size. Re-entry does not require as much Delta-V as establishing orbit. This is partly because the Earth(Kerbin too) has an atmosphere(the other reason is because you don’t have to climb to space). If you change your orbit just enough for the spacecraft to touch the atmosphere. The craft will enter the atomosphere several times, and it will eventually slow the craft down enough to perform final re-entry. Of course, you can also change the orbit so that the trajectory touches the surface of Kerbin, by doing so you can easier pinpoint your landing to a specific spot. However, there will be more thermal stress on the vehicle due to the increased speed of entry.
Direction of Re-Entry
Always have your heat shield facing the prograde direction upon reentry! So that the heat does not burn the poor kerbals! Unless you like torturing them for fun!
Parachutes, don’t forget them
It’s always a good idea to have spare parachutes. If you have alot of payload to bring home, you need alot of chutes. Unless you are in career mode where part count is limited, strap as many as you want! Do make sure that the parachutes are stored in a service bay or something so that they can survive re-entry! Parachutes only work in bodies that have an atmosphere. The moon is not one of them!
EVERYTHING YOU READ IN THIS SECTION SO FAR IS ONLY FOR LANDING ON PLANETS WITH AN ATMOSPHERE!
Think about it, do you need a parachute to get to the moon? do you slow down when you are close to it? Of course not, the moon has no atmosphere!
So how DO you land on the moon?
Well there are many ways and many good tutorials on youtube. Watch one and you will know!
KSP tutorials
Some things are better explained through interactions
KSP’s starter tutorials are a MUST if you are new! It will teach you how to use the interface to perform what we talked about. You should be familiar with plotting courses for your ship! Without it you will have no clue what to do, even if you know how it works!
Play the tutorials!
Modding
If you look in the construction menu in KSP, you can see that there aren’t very many choices. This can be good or bad depending on your preference. If you’d like the complicate things more, consider installing mods that put a bit of realistic variety into the game. With the realism overhaul modpack, we have loads of different fuels and rocket engines that are real. There is also much more that enhances the realism of KSP. However, KSP is still significantly less complicated than actually building a rocket in countless ways!
Explore the mods!
A handy tool to install KSP mods is ckan, which is a software that automates modding for you. For some mods you may have to downgrade your KSP version, as not all mods support the newest version of KSP.
Link to ckan:
github.com/KSP-CKAN/CKAN/releases/latest
Closing Remarks
Mess around in KSP and make sense of what I just talked about! There is still much to figure out for yourself! If you have any suggestions or find any information erroneous, please feel free to add me on steam! I am always open to new ideas. If this guide has helped you understand the fundamentals of space flight, please give it a good rating.
Consider reading some books on astronomy and physics!
Here are some books that I recommend:
- For the Love of Physics by Walter Lewin
- Astronomy: A Self-Teaching Guide, Eighth Edition by Dinah Moche
- The Grand Design by Stephen Hawking
- Cosmos by Carl Sagan
Please rate!
