Chapter 9. Rocket Science

Rocket science has a bad reputation: like brain surgery, it has been held up as something too hard for the average Kerbal in the street to understand. In the last chapter, we took a tour of the basics of the game. In this chapter, we’re going to start learning rocket science.

You don’t have to be a rocket scientist to use rocket science, as there are a few simple numbers you can calculate using relatively simple formulas that are going to help you not blow up your rockets quite as much as you would otherwise.

You don’t have to know everything in this chapter in order to have fun playing Kerbal Space Program. That said, KSP attempts to model space flight as realistically as possible and implements rocket science as well as it can. If you understand why rockets, planets, and moons move in the way they do, you’ll find it easier to do more advanced stuff in the game.

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Tip

The Kerbal Space Program Wiki has a good summary of physics and orbital mechanics terminology that may come in handy.

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The Law of Conservation of Momentum

The basic principle that all rockets work by is the law of conservation of momentum . If we have an object with a mass m moving at velocity v , then the momentum of the object will be p :

However, the law of conservation of momentum states that the total momentum in an isolated system is conserved; it can’t change. That means if the “system” we’re looking at consists of two pieces and you throw one of those away from you, then the remaining part is also going to gain a velocity in the opposite direction. The magnitude of that velocity will depend on the ratio of the amounts of mass involved.

Thinking about a rocket (see Figure 9-1 ) you can see how this is going to work out.

Figure 9-1. The law of conservation of momentum states that the momentum must be conserved

When the fuel is ignited, it rapidly transitions from a liquid — or in the case of solid rocket boosters, a solid — into a hot gas that has only one avenue of escape, the nozzle of the rocket. That means that it will escape with some velocity, imparting a velocity (a force) in the opposite direction to the rocket.

Thrust-to-Weight Ratio

Thrust-to-weight ratio is a dimensionless ratio of thrust to weight of a rocket.

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Tip

A dimensionless quantity is a quantity without any physical units and thus a pure number. Such a number is typically defined as a product or ratio of quantities that do have units, in such a way that all the units cancel out.

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Since the instantaneous thrust-to-weight ratio of a rocket will vary continuously during operation — because the rocket will get lighter as it burns up its heavy fuel — the ratio you will see quoted in the VAB (see Figure 9-2 ) or in utility mods like MechJeb (discussed earlier in “MechJeb” ) is normally the initial ratio. This figure, using the initial gross mass of the rocket at sea level, is often used as a good yardstick comparison of the performance of different vehicles.

The thrust-to-weight ratio of your craft can be calculated using Newton’s Second Law of Motion, which states that the magnitude of acceleration of an object is equal to the force applied divided by the mass. The values for force and mass for your rocket are easily gathered from inside the VAB; see Figure 9-2 .

Figure 9-2. Values for the thrust of your engines in vacuum and at sea level, as well as the mass of the rocket, can be found in the VAB screen

If you mouse over a rocket motor, you’ll be given the thrust in vacuum and at sea level — which will be lower as rockets are less effective in atmosphere — in kN, ¹ while the Engineer’s Report button in the VAB and SPH will provide the mass of your craft in metric tons. ²

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Tip

Mousing over the rocket motor and then right-clicking will pin the information panel and display more information. For engines this will include additional information like the amount of fuel burned per second. For fuel tanks this will include the relative mass of liquid fuel and oxidizer.

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We can work out the thrust-to-weight ratio of the rocket, or more correctly the thrust-to-Kerbin-weight ratio, simply by taking the initial acceleration of the rocket and dividing it by Kerbin’s gravity (which is 9.81 m/s at sea level). So, imagine looking at the LT-V45 “Swivel” engine, powering a rocket which weighs 4.7 metric tons:

If the ratio is less than 1, the craft will not lift off the ground.

Specific Impulse

Specific impulse is a measure of engine efficiency. If we’re looking at two different rocket engines that have different values of specific impulse, a higher value indicates that the engine is more efficient. It will produce more thrust for the same amount of propellant.

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Note

Solid rockets can provide high thrust for relatively low cost. This means that they’ve often been used as initial, or booster, stages. This allows the higher specific impulse engines for later stages. The downside of solid rockets is that they have relatively low specific impulse and aren’t that efficient. Also, once they are ignited, there is no way to shut them off.

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The efficiency of a rocket is always lower in atmosphere than in vacuum because the rocket exhaust interacts with the atmosphere, reducing the efficiency; this means that the specific impulse of an engine will be at its highest point in vacuum. In real life fuel flow to an engine stays the same over time, because engineering-wise this is easier, so thrust increases over time as the rocket climbs toward orbit. In Kerbal Space Program engines can be throttled, but the throttle always controls the rate of flow, not the thrust directly.

You can get the specific impulse (I_sp ) of a rocket engine from the VAB screen by mousing over the rocket motor and then right-clicking to bring up the extended information panel.

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Tip

I_sp is a measure of engine efficiency. If we’re looking at two different rocket engines that have different values of specific impulse (I_sp ), a higher value indicates that the engine is more efficient. It will produce more thrust for the same amount of propellant.

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In Figure 9-3 we can see that the LV-T45 “Swivel” liquid fuel engine has an I_sp at ground level (ASL) of 270s, and in vacuum of 320s. If we compare the LV-T45 to its non-thrust-vectoring cousin, the LV-T30, you’ll see that the I_sp of the LV-T30 is slightly higher (280s) at ground level but actually lower (300s) in vacuum. From this we can conclude that the LV-T45 is much better suited for second stages, which operate at higher altitudes, rather than as a first-stage motor.

Figure 9-3. The LT-V45 engine

Values of I_sp can be radically different when in atmosphere than in vacuum; take, for instance, the LV-909 “Terrier.” This small engine has an I_sp of just 85s at ASL but 345s in vacuum. This is actually higher than bigger LV-T30 and LV-T45. Since it is also much smaller and lighter than either the LV-T30 or LV-T45, and has thrust vectoring capabilities, this makes it perfectly suited for use during orbit correction burns or during the ascent or descent phase of a lander onto an airless body (like Mun).

Delta-v

The fundamental currency of rocketry is called delta-v (Δv ), and it’s a measure of the amount of effort that is needed to go from one place to another — or, more properly, to change from one trajectory to another. It determines the capability of your rocket and whether it’ll obtain Kerbin orbit, or tumble out of the sky a few hundred miles from the Space Port.

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Note

Delta-v is pronounced “delta-vee” and not “delta-five.”

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Effectively it is a measure of how large a change in velocity can possibly be made by the spacecraft. This is measured in meters per second. We can calculate the Δv using this integral between two times, which we call t₀ and t₁ :

Here T is the instantaneous thrust, and m is the instantaneous mass. Why instantaneous, you ask? Well, both the thrust (which is the force acting on the rocket) and the mass of the rocket changes with time. For instance, if we’re powering our rocket by burning fuel, then the overall mass of the rocket is going to decrease as we burn the fuel.

This looks complicated, but don’t panic. In the absence of any external forces acting on the rocket, we can simplify this down to the integral of the acceleration, a , over time:

which is simply the magnitude of the velocity change:

For rockets, absence of external forces means the absence of gravity and atmospheric drag. We therefore generally use the “in vacuum” I_sp to calculate the Δv capability for a rocket using the Rocket Equation.

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Tip

In stock Kerbal Space Program there is no way to see the Δv of your spacecraft. However, there are mods that will do this calculation for you and display the values. Among the most popular are MechJeb (see “MechJeb” ) and Kerbal Engineer Redux.

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The Rocket Equation

The Tsiolkovsky (or “ideal”) rocket equation states that the Δv :

where m₀ is the initial mass of the rocket, m₁ is the final mass of the rocket after the fuel has been burnt (commonly known as “dry mass”), and v_e is the effective exhaust velocity, which can be calculated from the I_sp of the engine:

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Warning

Because I_sp is a term calculated using weights of fuel on Earth , the value of g ₀ here is a conversion factor, and is 9.80665 m/s² precisely. Do not substitute the value of gravity on the local planetary body for g₀ .

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If your rocket is multistage, or has solid rocket booster stages perhaps, you should always use the lower-efficiency fuel sources first. Then, later, when you burn the higher-efficiency fuel, the starting m₀ of the rocket for that stage is lower, changing the ratio between m₀ and m₁ . What this means is that you will actually generate a larger Δv from your second stage with the same amount of fuel.

Derivation of the Rocket Equation

We can derive the Rocket Equation from the first principles of conservation of momentum (see Figure 9-4 ).

In Figure 9-4 the initial momentum of the rocket is:

while the final momentum of the system, now consisting of the rocket and the expelled propellant, is:

Figure 9-4. We can use the law of conservation of momentum to derive the Rocket Equation

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Tip

Remember:

• m = Ship’s mass
• v = Ship’s velocity
• v_e = Velocity of the expelled propellant
• dm, dv = Change in mass and velocity

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Since momentum must be conserved, these two things must be equal:

Rearranging this equation, and dropping second order terms, if we integrate over the duration of the burn we will end up with the Rocket Equation.

Center of Mass, Thrust, and Drag

To get into orbit, your rocket must overcome both gravity and drag. Thus, it’s important to understand how the design of your rocket affects its flight.

The center of mass of your rocket (see Figure 9-5 ) is the point within your rocket where all mass is equally distributed around it. If the center of thrust, and the thrust vector, are not in line with the center of mass, then your rocket will not fly straight; instead, it will pinwheel.

Figure 9-5. The center of mass (yellow) and center of thrust, and thrust vector (purple)

This tendency to pinwheel occurs because you would be applying a sideways force to the rocket body. It is the reason why, in real life, the Space Shuttle orbiter’s main engines had the ability to gimbal to direct their thrust not directly downward, but at an angle. This allowed the thrust vector of the engines to pass through the center of mass of the entire launch stack, consisting of the orbiter, external tank, and solid rocket boosters.

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Note

The center of mass of the rocket is sometimes incorrectly referred to as the center of gravity. The gravitational pull from an object weakens the farther you get from it, so if you have a long thin object (like a rocket), then the gravity at one end of the object will be different than at the other. That means a rocket’s center of gravity would be nearer to the gravity source than its center of mass. However, unlike in the real world, in Kerbal Space Program this difference in gravitational pull is ignored, so both centers are in the same place.

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The center of lift is the point where the sum total of all lift generated by parts — wings, control surfaces, and aerodynamic fuselage parts — balances out (although its effect on your rocket’s flight is perhaps better understood if you view it as a center of drag). As your rocket launches, the center of drag will move in a way to place it directly behind the center of mass.

As the rocket burns fuel, the center of mass will change. As the tanks empty from the top downward, what was initially a stable rocket will become unstable as the center of mass moves backward behind the center of drag.

This problem isn’t so prevalent in the real world because most rockets don’t have the fuel and oxidizer mixed in the same tank, as they are in Kerbal Space Program. Instead, the tanks are located at opposite ends of the rocket — for instance, the Space Shuttle external tank had the liquid oxygen at the front, and the liquid hydrogen at the back — so the mass distribution doesn’t change in the same way.

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Tip

One way to avoid your rockets becoming unstable in this manner is to pump fuel to the upper tanks as it launches. Hold Alt and right-click on the forward and rear tanks of the stage, and manually transfer the fuel forward.

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The placement of both reaction wheels and other attitude control parts also needs to account for the center of mass; both should be placed as close to it as possible. This matters more for parts that provide attitude control through thrust, such as monopropellant jets, which we discussed earlier in “Engines” .

In both the VAB and SPH you can switch on the display of the center of mass, thrust, and lift using the toggles in the bottom-left corner of the display (see Figure 9-5 again).

For more detail on space planes, refer back to Chapter 4 .

Orbits

In the most basic sense, orbit can be defined as “falling but not hitting the ground.” Since planets are — at least more or less — round balls, to avoid hitting the ground you need to go fast enough sideways so that (instead of hitting the ground) the planet curves away underneath you, and you maintain your current altitude above it.

The velocity you need to be in orbit varies depending on your altitude above the planet’s surface, so v — the speed needed to obtain orbit — is:

where r is the current distance from your central body, a is the semimajor axis from your central body, M is the mass of the central body itself, and G is the gravitational constant 6.674×10^–11 m³ kg^–1 s² . For circular orbits this can be simplified to:

There are a few things to note here. First, r is the distance from the center of the body, not the distance above the body’s surface. Also, we assume that the satellite, or spacecraft, is above the atmosphere. Finally, as you can see from the equation, the speed the body needs to obtain orbit is unrelated to the body’s own mass:

Since Kerbin has a radius of 600,000 m (600 km), and a mass of 5.2915793×10²² kg, to orbit at 100 km above the surface (r = 700 km) we would need to travel at a speed of 2,246 ms^–1 .

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Note

The scale of the Kerbol System is roughly at one-tenth scale of the real solar system. Because of this, getting to space is quite a bit easier. If you want to play in the real solar system, you’ll need to install mods. See “Summary” for more info.

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Getting to Orbit

The science fiction author Robert Heinlein once said, “If you can get your ship into orbit, you’re halfway to anywhere.”

To reach orbit, you might naïvely assume you should point your rocket directly upward and then, once out of the atmosphere, blast sideways until you reach orbital velocity. Unfortunately, you’d be wrong.

The Gravity Turn

As we learned in “Orbits” , getting to orbit isn’t about going up. Instead, it’s about going fast enough sideways so that as you fall, you don’t hit the ground.

Launching upward, a rocket will expend Δv without gaining the necessary lateral speed to obtain orbit; see Figure 9-6 . A gravity turn is where you allow the force of gravity to adjust your trajectory, effectively “turning” your ship’s path.

Figure 9-6. The forces acting on a rocket before and after a gravity turn

On bodies with no atmospheres, where you don’t have to worry about atmospheric drag (see “Atmospheric Drag” ) the best thing you can do is launch upward and, as soon as you’re going to clear surface features like mountains, turn nearly horizontal and fire laterally.

However, on planets with an atmosphere, like Kerbin, things are a bit more complicated. If the spacecraft turns too late, it will waste fuel fighting gravity losses (discussed in the following section), but if it turns too early it will spend more time in the atmosphere fighting atmospheric drag. We discussed about how to carry out a gravity turn in “Launching” .

Gravity Losses

Gravity drag (sometimes known as gravity losses) is a measure of performance lost while a rocket is fighting gravity. It is the difference between the actual Δv expended and the theoretically lower Δv needed for a change in speed and altitude while in a gravitational field.

For instance, if you are accelerating at 1.5g and gravity is pulling you down at 1g, you’re losing approximately 66 percent of your fuel due to gravity. Upping the acceleration reduces your proportional losses due to gravity. So while adding more solid rocket boosters for launch does not add much Δv overall, it does add thrust-to-weight ratio. Hence, the additional acceleration saves you time and reduces gravity losses.

Atmospheric Drag

While the Kerbal universe has been scaled down to around a tenth or so of the real-life universe, not everything scales in exactly the same way, and Kerbin’s atmosphere ³ is a case in point. As a result, the atmosphere is much thicker with respect to the planetary radius than Earth’s.

Before the v1.0 release of Kerbal Space Program, it was typical to start your gravity turn at around 10 km altitude. This was because the stock aerodynamic system was a very simplified model where, among other things, drag was related directly to an object’s mass and was the same no matter the object’s orientation. This meant that adding a nose cone to your rocket didn’t reduce drag; it actually increased it. However, since v1.0 the atmosphere has been modeled a bit more realistically.

It is therefore important to minimize drag on your rocket during the ascent to reduce Δv losses while inside the atmosphere.

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Tip

Pressing F12 will bring up the aerodynamic forces on your craft, F11 will show the temperature overlay, and F10 enables and disables temperature gauges.

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The Kármán Line

For Earth, the Kármán line lies at an altitude of 100 km (62 miles) above sea level and commonly represents the boundary between the Earth’s atmosphere and outer space. It is around this altitude where the atmosphere becomes too thin to support aeronautical flight, since a vehicle would have to travel faster than orbital velocity to derive enough aerodynamic lift to support itself against gravity.

Things are slightly different for Kerbin: the atmosphere fades exponentially as altitude increases, and disappears entirely at 70 km. The Kármán line for Kerbin is therefore usually accepted to be 70 km, as this is where the atmosphere ends.

The Kármán line is therefore defined as the “edge of space” for Earth, but for Kerbin, at least as most people talk about it, it is the “edge of atmosphere.”

You could orbit a spacecraft at 75 km above Kerbin, because you would be above the atmosphere, but if you tried to orbit a spacecraft at 105 km above the Earth’s surface it would eventually be de-orbited due to atmospheric drag (discussed in the previous section) because on Earth the atmospheric cutoff isn’t as sharp as it is with Kerbin.

In fact, the International Space Station, which orbits around 400 km above the Earth’s surface, needs periodic reboosts to counter atmospheric drag and to maintain its orbtal altitude.

The most commonly accepted definition for Low Earth Orbit (LEO) is between 160 km (99 miles), with an orbital period of approximately 88 minutes, and 2,000 km (1,200 miles) with an orbital period of about 127 minutes. If we take the definition of “near Kerbin” to be Low Kerbal Orbit (LKO), then LKO would extend from 70 km up to 250 km.

Basic Orbiting

During each orbit, your spacecraft will reach maximum altitude, called apoapsis , and on the opposite side of the planet, it will reach minimum altitude, called periapsis . At both apoapsis and periapsis, your vertical speed will be zero. These points are the easiest points to make orbital corrections. The orbital period is the time it takes the spacecraft to complete one orbit around the planet. Remember that the planet is rotating as well, so when your craft has completed a whole orbit, the geography below it will not necessarily be the same unless your spacecraft is in a synchronous orbit (see “Synchronous and Stationary Orbit” ).

Kerbal Space Program has different labels for different altitudes. Table 9-1 shows the boundaries for each range of altitude for Kerbin.

Table 9-1. Orbital heights

Level	Minimum (km)	Maximum (km)
In flight	0	18
Upper atmosphere	18	70
Near Kerbin orbit	70	250
High Kerbin orbit	250	SPI

For more on how to raise, lower, and circularize your orbits, refer back to “Maneuver Nodes” .

Sphere of Influence

As Kerbal Space Program does not simulate multibody gravitation, only the body you are in orbit (or passing) around influences you gravitationally. This is known as the body’s sphere of influence , the spherical space around a body in which it has sole gravitational influence on the spacecraft. Because of the lack of multibody physics, orbital trajectories in the game are entirely predictable.

The radius of the sphere of influence can be found from:

where a is the semimajor axis of the spacecraft’s orbit around the body, and m and M are the masses of the spacecraft and the body, respectively.

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Note

One major implication of the lack of multibody physics is the lack of Lagrange points , which do not exist in the game.

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Synchronous and Stationary Orbit

A synchronous orbit is an orbit where the orbital period equals the rotational period of the body it is orbiting. Kerbin’s day is six hours long, so an orbit with an orbital period of six hours will be synchronous. A special subcategory of synchronous orbits is the Kerbisynchronous Equatorial Orbit (KEO), or stationary orbit. This orbit has 0 degree inclination and an eccentricity of 0 (circular). The spacecraft has no motion relative to the surface.

A solar day is the time taken for a planet to rotate once relative to its sun. On Kerbin, this is 6 hours long. However, for synchronous orbits we need to use the sidereal day, which is the time taken to rotate relative to the fixed stars. The orbital altitude for KEO is 2,863,334.06 m at an orbital speed of 1,009.81 ms^–1 , equivalent to a surface speed of zero and an orbital period of exactly one Kerbin sidereal day (5h 59m 9.4s).

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Note

Interestingly, since Kerbin’s days are 6 hours long, we might surmise that a Kerbal month could be 13 days long, as there is a Munar eclipse once every 13 days when viewed from the KSC.

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The real-life equivalent is geostationary orbit, mostly used for communications and broadcast satellites.

Transferring Orbits

When transferring between orbits, you should remember that lower orbits move at higher velocities, and higher orbits move at lower velocities. To transfer to a lower, faster orbit, you actually need to slow down (burn retrograde to your orbit), whereas to move to a higher, slower orbit you need to speed up (burn prograde to your orbit). This can initially be somewhat confusing.

A good way to keep it straight is by thinking not in terms of orbital speed, but instead orbital energy. The higher the orbit, the higher its energy. So, to get to a higher orbit, you need to burn prograde to the orbit you are currently in, adding energy. And to get to a lower orbit, you need to burn retrograde to your orbit, subtracting energy.

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Tip

If you are in KEO, a good mnemonic is that “in takes you east, and out takes you west.” If you go inward, you will speed up and will travel eastward over the surface, while if you go outward, you will slow down further and will travel retrograde and westward with respect to the surface.

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If you are transferring orbits to rendezvous with another spacecraft (as discussed in “Docking” ), then — at least in Kerbin orbit — for every 1 km you are below the target you will catch up with it by about 7 km per orbit; and reversing that, for every 1 km you are above the target it will catch up with you by about 7 km per orbit. If you are making a correction burn during a transfer orbit, the farther you are away in time from your target, the less important it is to perform the transfer maneuver at exactly the right time, but the more important it is to get the change in Δv correct.

However, if you are performing your transfer orbit between bodies (e.g., Kerbin and Duna) then there are two things you need to be aware of: the planetary phase angle and the escape velocity.

The planetary phase angle is the angle your destination planet or moon needs to be in front of or behind you, while the escape velocity is how fast your spacecraft will need to go in order to escape your current sphere of influence and go onward to intercept the target body.

While it’s possible to calculate both these things manually, you can mostly get by using rules of thumb and some experience. Leaving Kerbin if you are going outward, say toward Duna, then you should do your burn on the dark side of the planet. If you are going inward, toward Eve, then the burn should be on the daylight side of the planet.

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Tip

We covered this in more detail in “Transferring Between Celestial Objects” .

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Hohmann Transfers

The Hohmann transfer is the most frequently used method of changing orbital altitudes.

To transfer from a lower orbit to higher, you first need to burn prograde (speed up) at orbital periapsis (lowest point) until the apoapsis (highest point) reaches the desired altitude. Then when you reach apoapsis you should again burn prograde, raising the periapsis to circularize the orbit.

Conversely, to transfer from a higher to a lower orbit, you should burn retrograde (slow down) at apoapsis until the periapis is lowered to the desired altitude, and then when you reach the new periapsis you should again burn retrograde to circularize the orbit.

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Note

Rocket science can involve quite a bit of calculation. Fortunately, there are quite a few excellent tools online for figuring out things like when to launch, and when to transfer orbits. A good list of these can be found on the Kerbal Space Program Wiki .

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Slingshots and Gravity Assists

A gravity assist is a technique that involves using a planet or moon to change your trajectory. Using a gravity assist, you can actually gain “free” energy from an encounter with a planet.

Gravity assists work like this: as you approach another planet, its gravity begins to pull you toward its center of mass. As a result, you begin to get dragged along by the orbiting planet, and it begins to change your speed relative to the primary body. However, if your craft has too much velocity to remain in that body’s sphere of influence, you won’t achieve an orbit and instead will be flung out, away from the planet you’re encountering, at a different angle and with a changed speed.

The change in speed depends on whether your craft passes in front of the planet, or behind it. If you pass in front, you’ll gain speed relative to the sun (or, if you’re slingshotting off a moon, relative to the planet that the moon is orbiting). See Figure 9-7 for a more visual explanation.

Figure 9-7. A gravity assist

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Note

The amount of change in velocity is limited to double or half your current speed, relative to the primary. This is one of the reasons why using Mun to do a gravity assist isn’t terribly useful — a minimum energy transfer orbit is 250 m/s, so the most you can get out of a gravity assist of Mun is an additional 500 m/s, which isn’t really enough to get anywhere. In fact, if you’re trying to use a Munar gravity assist to get to another planet, it can actually take more fuel than not doing it.

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There’s one other thing to remember when considering gravity assists: the amount of deflection that a gravity assist can do to your trajectory depends on the gravity of the body you’re using. If the body is light, like Mun, you’ll only get a deflection of a few degrees, whereas if you use a very heavy planet, like Eve or Jool, you can deflect by a much sharper amount. The greater the deflection you can achieve, the more speed you can gain (or lose).

Oberth Effect

The Oberth effect is encountered where, when dropping into a planet’s gravity well, you burn prograde at the point of speed. The effect of this maneuver is that the spacecraft will gain more kinetic energy by applying the same impulse as it would outside the gravitational well. The rocket is producing the same force regardless of the distance being covered. However, the faster the rocket is traveling, the more work is done because more distance is covered. Therefore, the most work is done when you’re doing engine burns at periapsis, because that is the point in the orbit where you have the highest velocity and thus the largest distance traveled over the course of the burn.

So while burning the same amount of fuel always produces the same Δv , at higher speeds that Δv translates to a larger change in the kinetic energy, ΔKE . This will result in a larger amount of gravitational potential energy and thus a higher apoapsis.

In practical terms this means that the most energy-efficient time for a spacecraft to burn its engine is at the lowest point in its orbit, the orbital periapsis.

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Tip

While gravitational slingshots do steal kinetic energy from planets, the Oberth effect does not.

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Aerobraking

When an object moves through an atmosphere, it’s slowed down by drag. When an object in space moves through an atmosphere, it’s slowed down a lot .

Aerobraking is a maneuver that reduces the apoapsis of an orbit by flying the spacecraft through the atmosphere at the periapsis of the orbit. The resulting atmospheric drag (see “Atmospheric Drag” ) slows the spacecraft, reducing and circularizing the orbit.

Aerobraking reduces your velocity by converting it into heat. This means that aerobraking is incredibly useful for when you want to land on a planet, or even simply to achieve an orbit: you don’t have to bring extra fuel for slowing yourself down. When you’re using aerobraking to achieve an orbit, it’s called aerocapture .

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Warning

Remember that aerobraking causes heating. Unless you’re passing through a very thin atmosphere, you’ll probably encounter so much heat that you’ll burn up. Always remember to bring a heat shield if you plan to aerobrake!

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Lithobraking

Lithobreaking is an extreme form of aerobraking that uses the surface to slow the spacecraft down for a landing. While the word is normally used as an alternative to the word crashing , it is actually possible to lithobreak spacecraft in Kerbal Space Program, for instance by connecting with the surface at proper angles so as to detatch spent fuel tanks.

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Note

Due to extremely dense atmosphere on Venus, some of the Venera landers used a hard umbrella-like aerobraker in combination with shock absorbers to lithobreak onto the surface. The Russian Luna landers used a combination of retrorockets and gas-filled bags to reach the lunar surface safely, a technique that later became popular for both Russian and US Mars missions.

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Summary

This has been a rapid introduction to rocket science, and if you’ve gotten a bit lost, that’s OK. You can get a long way in Kerbal Space Program using rules of thumb. You just need to be aware that those rules of thumb are based on the underlying equations of the Kerbal universe, its physics.

You also need to realize that Kerbal physics, while pretty similar to real-world physics, isn’t always exactly the same. Sometimes those scaling factors make a difference, and sometimes the physics engine of the game — especially when it comes to the atmosphere — is somewhat simplified and will give slightly different results from what you might expect.

This is the end of Part II . In Part III , we’ll start looking beyond the base game, and start talking about mods: add-ons from KSP enthusiasts that take the game beyond its developers’ wildest ambitions.

¹ A kN is kilo-Newton, or a thousand Newtons, which is the international standard unit of measurement for force and has units of kg ms^-2 .

² A metric ton (in the US) or tonne (in the UK and internationally) is a unit of mass equivalent to a thousand kilograms, or 1.10 tons (US) or 0.984 tons (Imperial).

³ The atmosphere of Kerbin is modeled similarly to the U.S. Standard Atmosphere , with the vertical height scale reduced by 20 percent.