New to the game, and to rocket science. Could someone explain the engine stats to me?

[taukarrie]

Trying to read up all over the forums taking my first steps into KSP. Seems like one of the most important initial things to know would be how I can use the engine stats to gauge fuel efficiency, power, etc but there really isnt much int he way of tooltips and tutorials in-game yet. could someone help me understand the following engine stats and how I would use them to determine what engines i should use for what jobs?

Engine Max Power:

Engine Min Power:

IsP at Sea Level:

IsP in Vaccum:

clearly performance stats. im guessing the engines perform differently at different altitudes, but i dont know what IsP means. I also assume that i should be comparing my rocket's weight with these numbers to determine which engines are best suited and how many. i understand that different stages will make things vary. but i dont really have a baseline to work with yet.

Propellants

Liquid Fuel: (0.9)

Oxidizer: (1.1)

I know these are the fuel components for the rocket engines. but every engine seems to have the same number values. i assume this is a consumption rate, but what is the unit measure?

[Amoun]

Welcome to the forum.

FIRST OF ALL, I am new my self, and most of the stuff I looked up. Secondly (I do not mean this in a bad way) Google is your friend....and wikipedia too.

ISP-Specific Impulse (Isp) (I copied this) http://wiki.kerbalspaceprogram.com/wiki/Terminology#isp

The Isp defines how effective a propulsion system is. The higher the Isp the more powerful is the thrust applied to the rocket with the same fuel mass. The Isp is usually given in seconds but actually the physically correct unit is distance per time which is usually given in meters per second or feet per second. To avoid confusion which unit of speed is used, the physical correct Isp (in distance/time) is divided by the surface gravity of Earth (9.81 m/s²). This results in a value given in seconds. To use this Isp in formulas it must to be converted back into distance per time which requires multiplying with the surface gravity of Earth again. As this value is only used to convert between those two units, the specific impulse doesn't change when the gravity changes. It appears that KSP use a value like 9.82 m/s² and thus using a little less fuel.

When I select engines, I mainly look at max power. When you play around with it you will get familiar with which ones are for which application. Intuitively, you will use engines with more power at lower altitudes and less power in higher/vacuum.

That's the most I can give you right now...I am learning my self still. But I hope this at least somewhat helps.

[capi3101]

The information you seek is on the wiki - specifically at http://wiki.kerbalspaceprogram.com/wiki/Parts.

When you're looking for max engine power, look at the Thrust column - that's how much thrust the engine will output at full throttle. You're going to want that to be greater than the total mass of your rocket * 9.81 (Kerbin's gravity), or your ship is going nowhere.

Minimum engine power is, of course, zero in all cases.

Amoun has already covered what Isp is - specific impulse. It's basically a fuel effiicency value; the higher this value, the further you can travel with that engine. How far? You'll can calculate that with the Tsiokolvsky rocket equation - natural logarithm (total mass / dry mass) * 9.81 * specific impulse = delta-v, in units of meters per second.

"Dry mass" = the mass of your rocket once all the fuel's been spent.

I believe that the flow measurements for fuel are in units of tonnes per second, unless I'm mistaken. You can largely ignore that data in practice, though.

[arq]

Max power is the max thrust the rocket engine can provide. If your thrust-to-weight ratio (Thrust/(Mass*9.81)) is <1, your rocket will not leave the pad. Ideally (for most-efficient launch), you want this number to float in the 1.8-2.5 range for getting off Kerbin, but it doesn't matter much once you're in orbit.

Min power is 0 for all engines except solid rockets. Once you activate an SRB, it burns at full power until it runs out of fuel.

ISP is the 'efficiency' of the rocket. Rockets are least efficient in thick atmospheres (at sea level) and most efficient in vacuums. Don't worry about what the number means (I never do), just know that a higher ISP will get you further for a given amount of fuel. Also, the atmosphere thins out very quickly, so by the time you hit 5-10km the vacuum ISP is all that really matters.

Propellants are what the rocket burns. All rockets use the 0.9/1.1 mix at the moment, same with fuel tanks. The only exceptions are the airplane parts, which use liquid fuel/intake air, and ion engines, which use xenon/electricity.

Mass was not listed by you, but it is actually very important. There is a 'correct' rocket for every job. Even though the NRV has double the ISP of any other rocket, it weighs a lot and provides very little thrust. This makes it poorly suited for a launch stage and too big for small orbital stages (less than a few tons). An engine needs to push both your rocket and itself, and the more massive they are the harder this is.

Ultimately, the two most important stats for your rocket are TWR and delta-V. TWR is really important for getting into orbit or landing with rockets, though it matters little once you're actually in orbit. Delta-V is very important, it says how 'far' your rocket can go, in terms of how many m/s it can accelerate with the fuel it has. You can look up the equations for delta-V, but I won't go into that here. But know that it takes roughly 4600dV to get off Kerbin.

If you are really interested in these stats, try the mods Kerbal Engineer Redux or Mechjeb, as they will calculate these and lots of other parameters of your rocket (doing this by hand is very tedious).

[sojourner]

The quick answer on ISP: fuel economy. The higher the number the more fuel efficient it is.

[rpayne88]

As far as getting into orbit, you want a TWR of approximately two. You will get off the ground with anything higher than one, but the higher your TWR, the faster you will get into orbit. dv is more important once you are in orbit. You can have a TWR of less than .000001 and still change to another orbit. If your TWR is this low, though, it will take quite some time to make a noticeable change. A good plan for space flight is to use an engine with a very high TWR as the first stage of your rocket to push you off the ground. Then ditch it and light off a second stage with a TWR greater than one until you are in a stable orbit. On Kerbin, a stable orbit is any orbit whose perigee is higher than 70km (above Kerbin's atmosphere.) Once you are in orbit, efficiency is the name of the game. Use high Isp engines such as the LV-N, LV-909, Poddle, or, if you have the patience, the PB-ion.

TWR:

Engine mass does not change, unless the engine breaks off of your rocket. Thrust is how much force, measured in kilo-newtons (kN) that each engine can provide at full power. 1kN equals 1000 Newtons (N.) Newtons are used to measure force (if you have not taken, of remember from, physics.) You can calculate how much force your rocket is applying in a downwards direction by multiplying its mass by Kerbin's gravity. As with every where in physics, the metric system is used. You calculate a rocket's "force" on the ground using the equation F=ma, or force equals mass times acceleration. In this case, acceleration is Kerbin's gravity, which incidentally is the same as Earth's, 9.8m/s^2 (32ft/s^2.) Lets say your rocket has a mass of 1000kg and is siting on the pad at KSC. You multiply the 1000kg by 9.8m/s^2 to get 9800N, or 9.8kN, of force. That means you would need 9.8kN of thrust to make your rocket "float" over the pad or at least 9.81kN to make your rocket ascend. You can find your TWR by taking the ratio of your thrust and comparing it to your weight (which is not the same as mass.) Lets say you were cheating and using infinite fuel and you had a hypothetical engine that provided exactly 9.8kN of thrust. Your TWR would be 1.00. In KSP, as in real life, however, your mass is constantly decreasing as you burn fuel. This means your thrust (assuming no throttle input from you) stays constant while your mass (and your weight) decrease. This causes your TWR to increase as you burn the engines.

Delta V (dv):

Dv is a measure of the extent of how much your rocket can change its velocity (not the same as speed.) Dv is officialy notated as a triangle (Greek symbol for delta, used in physics to show change in a variable) then a v. First off, velocity has a vector to it. This means it has a direction to the speed. In space your velocity is critical to your orbits. In order to raise your orbits, you thrust pro-grade, thus increasing your velocity. To lower your orbit, you thrust retrograde, thus decreasing your velocity. Dv is a measure of how much you can change your velocity, usually measured in m/s or km/s (1km/s=1000m/s if you live in the U.S. like I do and don't know.) For example, if you wanted to raise your velocity (assuming no outside influences such as drag or gravity) from your current velocity of, lets say, 1000m/s to 2000m/s then back to 1000m/s you would use 2000m/s or 2km/s. Why? You use 1000m/s dv to increase your velocity to 2000m/s per second. You then use an additional 1000m/s to slow yourself back down. Dv can be calculated using only simple algebra and the Tsiolokovsky (I would butch his name trying to pronounce it) Rocket Equation. The equation is as follows: Dv=ve*ln*(m0/m1). In plain English, delta v equals the product of the effective exhaust velocity times the natural logarithm of the quotient of the initial mass divided by the ending mass. Ve needs to be computed on its own. When it is factored into the the equation, the equation looks like this:

Dv=(g*Isp)*ln*(m0/m1)

Where g= the gravity of a body

Isp= the specific impulse seen on the engine statistics.

So, the final equation, in plain English, is delta v equals the product of gravity times specific impulse times the natural logarithm times the quotient of the initial mass divided by the ending mass. If you have a calculator, all you have to do is plug the numbers into the equation to get the correct answer.

Orbits:

Orbit is defined as free fall around and object. For example, if a baseball is thrown fast enough (ignoring aerodynamic drag,) by the time it should have hit the ground if the object was flat, the surface has curved away from the ball. As long as the ball continues traveling at this velocity, it will never hit the ground.

Orbital Information:

Just some useful terms to know:

Apoapsis (sp?, AKA: Apogee): The highest point in your orbit or ballistic trajectory. You will never be farther from the surface than this without input.

Periapsis: (sp?, AKA: Perigee): The lowest point in your orbit. You will never be closer to the surface than this point without input. On ballistic trajectories, your periapsis will be zero and the object will impact the ground at some point.

Eccentricity: A measure of how circular your orbit is. An eccentricity of zero means you are in a perfectly circular orbit.

Orbital Period: The time it takes to complete one orbit.

Types of Orbital Transfers:

These are used to place your craft on a trajectory to intercept another object. The most common type of transfer is the Hohmann transfer. In this orbit, you place your periapsis above your target if you are in front of it, and below it if you are behind it. Raising your periapsis will cause you to move slower, allowing your target to "catch up" to you. Lowering your periapsis will cause you to accelerate, allowing you to "catch up" to your target.

Rocket Design:

An efficient rocket can place a high payload fraction into orbit. A payload fraction is the fraction made by dividing the weight of the rocket and payload (a capsule or spacecraft, for example) by the weight of an otherwise empty rocket (fuel is included in an empty rocket, just not the payload.) An easy way to achieve a high payload fraction is to "stage" a rocket. Staging involves dropping empty fuel tanks and other parts when they are no longer needed, leaving another stage to continue pushing a payload into orbit. A design commonly used in KSP is called "Asparagus Staging." Using Asparagus Staging, one pair of boosters on opposing sides of a central booster core feed into another pair of boosters on opposing sides and so on until fuel reaches the central core. The first pair of boosters feed all the engines on the stage, including their own, then fall off after they have depleted their fuel. Then, the next pair burns and so on. Other things to take into account when designing a rocket are structural stability and control-ability. Struts are often used to increase the structural stability of a rocket or craft to prevent it from falling apart during flight. RCS, reaction wheels, and fins/ control surfaces help to control a vehicle.

A Note on Space Planes:

Space planes do not need a TWR greater than one to take off (although it is still a good idea to have a TWR greater than one.) The lift generated by their wings counteracts the force of gravity when they are designed correctly. Remember, though, that they spend much more time in the lower atmosphere and are subjected to the pressures affiliated with low altitude flight. Spaceplanes are difficult to design. If you do decide to design them, make sure the center of lift is behind the center of mass or the aircraft will do a low altitude loop and crash shortly after takeoff. For beginning players, it is a good idea to wait to design successful rockets before attempting to design a spaceplane.

Hope this has answered your questions and provided you with enough understanding of the game for you to enjoy it. Please remember, I do not know who you are or what your educational level is. I apologize if it seems like I was treating you like an idiot. I saw that you were also new to rocket science and wanted to clarify some core principles of KSP to you.

Edit: There are mods that do most of this work for you if you do not like the idea of crunching numbers for each rocket. Or you could resort to trial, error, and fireworks displays over KSC.

Edited August 2, 2013 by rpayne88
Fixed gramatical errors