What is the ISP of an engine?

[Bilfr3d]

I'm not to sure what this actually is.......

What is the ISP of an engine, and what does it do?

[WafflesToo]

ISP is the Specific Impulse of the engine. It's sort of like the engine's fuel mileage.

You use it to figure out the total amount of velocity change (termed 'Delta-V') the vehicle is capable of using the Tsiolkovsky rocket equation.

dV = ISP * 9.81 m/s^2 * ln(start mass / dry mass)

[Bilfr3d]

ISP is the Specific Impulse of the engine. It's sort of like the engine's fuel mileage.

You use it to figure out the total amount of velocity change (termed 'Delta-V') the vehicle is capable of using the Tsiolkovsky rocket equation.

dV = ISP * 9.81 m/s^2 * ln(start mass / dry mass)

Still a bit confusing............ Do I need to know what it is to fly? Is lower ISP better, or is higher ISP better?

[stringyquark]

That is a measure of how good the engines internet service provider is. Lol

http://en.m.wikipedia.org/wiki/Specific_impulse

It is a measure of the engines efficiency, higher the number the more push per unit of fuel consumed. Check for the difference between in atmosphere and in vacuum.

[stringyquark]

Wow, I'm slow

[arq]

ISP relates to how 'efficient' an engine is, how much fuel it burns to produce a given amount of thrust. An engine with 800s ISP will produce twice the thrust for the same time, or the same thrust for twice the time, or some other combination of those two compared to a 400s ISP engine, if both use the same amount of fuel.

The main place to see where this is used is in the rocket equation, which states that

<delta v> = 9.81m/s^2 * <ISP> * log(<wet mass>/<dry mass>)

where wet and dry mass refer to the rocket with and without fuel.

[Bilfr3d]

Ahhhhhh...... righhhttt! I'll just pretend I know what that means. Nobody even bother explaining to me anymore, i'm just not gonna get it.

[Giggleplex777]

<delta v> = 9.81m/s^2 * <ISP> * log(<wet mass>/<dry mass>)

*natural logarithm

[tavert]

Impulse is an average force times the length of time it is applied for. Impulse has units of force times time, which is equivalent to mass times distance divided by time.

Specific means "per unit [something]". If the [something] is mass of propellant, then you get a specific impulse in units of velocity (distance divided by time). The convention for specific impulse is to get a result in seconds, so the [something] is taken as propellant mass times the standard acceleration due to gravity, 9.81 m/s^2. That gravity is just a constant conversion factor due to historical convention.

[Lheim]

Don't worry bout the math.

It's about a rocket engine's efficiency. The higher the engine's Isp, the more delta-v you get per unit of fuel.

[Seret]

Ahhhhhh...... righhhttt! I'll just pretend I know what that means. Nobody even bother explaining to me anymore, i'm just not gonna get it.

Higher is better. You'll use fuel more efficiently. Engines can have different ISPs in atmosphere or vacuum, so make sure you pick an engine with a good ISP for where you'll be using it.

Check out the vacuum ISP of the nuclear engines, it rocks. That's why everybody uses them for interplanetary voyages because they don't need to use them in atmosphere, where they suck.

Edited October 17, 2013 by Seret

[lammatt]

@tavert pretty much explained it,

but one thing to add,

impulse is also defined as the change in momentum of a body given by the 2nd law

F=ma => F dt = m dv

so you can understand it as: the higher the impulse, the higher the change in momentum you get.

[Saberus]

Let's try to explain without math.

Let's look at the two extremes in the rockets, ISP-wise: The Mainsail and the LV-N.

The Mainsail has a vacuum ISP of 330. One of the lowest ISP of all the liquid rockets. Only the tiny probe rockets and the Mark 55 are lower. So it burns fuel like mad... but unlike those weak engines, this thing packs a whopping 1500 Nm/s of thrust. It's like an 18-wheeler for rockets, it may drink fuel like a man dying of thirst, but it has enough thrust to get just about anything moving in a very short span of time. Short enough that flimsier designs can be ripped apart.

The LV-N has a vacuum ISP of 800. This is the highest ISP of the liquid rockets. Only jet engines(which can't operate in space) or the ion engine has a better ISP. But it only has 60 thrust. In space, it can slowly thrust and build velocity and use just sips of your limited fuel reserves in deep space. This long burning with low thrust can keep even the most delicate designs from being damaged by the stress of acceleration.

On the ground, the ISP is different, accounting for a myriad of factors that will go unnamed, for there are way too many.

On the launchpad, the Mainsail has an ISP of 280. Fractionally, it's actually pretty good, not as much loss as most engines. It was pretty much designed to be used as a lifting stage, so the shape of the nozzle was made to be used in atmo, with a shorter, wider nozzle. Look at most other engines, the nozzle is narrower and longer than the Mainsail if they were made the same size, as in space the perfect nozzle has an infinite length (At least for a de Laval style nozzle).

The LV-N on the pad is a lousy, inefficient engine with an ISP of 200. It will suck down even more fuel than the Mainsail to make the same amount of thrust, and it will take a long time to make that thrust, as it's making only 4% of the thrust the Mainsail can. If you look at it, you will see it's the opposite of the Mainsail, a long, narrow nozzle optimized for deep space use.

Long and short, the ISP is not the end-all feature to look at, it just gives you a general idea of fuel efficiency. Thrust, weight, TWR (Thrust to weight ratio) are also factors to consider. But the difference between atmo ISP and vacuum ISP will give you a general idea of which engines were meant for lifting off, and which were meant for deep-space maneuvering.

[CattyNebulart]

Think gas millage, but instead of city and highway you have atmosphere and vaccum. And just as with gas milage you want a high one, because fuel is heavy.

[Bilfr3d]

Well, I've been away for a few days. I come back today, log onto the forums, open up this, and I see that some people have actually explained it in plain English. Thank you to everyone to help explain it (especially Saberus), and thank you all.

[Kosmo-not]

Impulse is Force multiplied by time. To get specific impulse, we divide that by mass. We end up with Force divided by the change in mass multiplied by unit time. For rocketry purposes, that is Thrust divided by the mass flow rate of fuel. The units of this come out to be a velocity, which we call effective exhaust velocity. To make this specific impulse value readily understood by any unit system, we divide that specific impulse value by something we can all agree to: standard gravity (surface gravity of Earth). The resulting units is time, and time is the same for all our unit systems.

A little trivia for you about KSP Isp:

• In 0.13.2 (this is where I started KSP), the specific impulse of engines was 579s.
  
• In 0.14, we were introduced to the half fuel tank. Using only half fuel tanks, you would have an Isp of 606s.
  
• 0.16 introduced the new way to calculate fuel burn (using Isp values instead of engine's fuel burn rates). This initially led to an interesting exploit because they left in the old code, so we had two lines of code relating burn rate to throttle position. If you had your throttle at 50%, your fuel burn would be at 25%, getting you exponentially greater Isp depending on how long you wanted to wait. This was also the update they added Isp to engine descriptions.
  

[Dave Kerbin]

It's sort of like the engine's fuel mileage.

That's a fantastic way to explain it. I would have almost just stopped right there and said isp is mpg, IN SPACE! (and yes of course it's not that simple since mass factors into the equation and such, but for people new to KSP that simple analogy is great to get them started)

[Kosmo-not]

That's a fantastic way to explain it. I would have almost just stopped right there and said isp is mpg, IN SPACE! (and yes of course it's not that simple since mass factors into the equation and such, but for people new to KSP that simple analogy is great to get them started)

Your analogy is much closer than you think. The fuel in cars take up a small mass fraction of the entire vehicle. They can get away with using one value since it doesn't change all that much throughout driving. You have greater gas mileage when you're near empty. You can do the same averaging with rockets if you want. Gas mileage among different cars with the same engine will vary just as different rockets with the same engine will.

[Dunbaratu]

One way to look at ISP that's easier to understand (and turns out to arrive at the same number) is to think of it as an amount of time.

ISP is how many seconds will one kilogram of fuel last while the engine uses it to push with a force of one Newton.

Now, a force of one Newton is very small, and even at one "tick" up the throttle most engines will be already pushing a lot more than just one Newton.

An engine with ISP of 800 being used to push with a certain amount of force will make the fuel last 4 times longer than an engine with an ISP of 200 pushing with the same amount of force.

In real life, the same engine will have different ISP depending on what throttle setting it's on, with one particular throttle setting being its most efficient setting. But in KSP, an engine has a constant ISP no matter the throttle setting.

[tavert]

ISP is how many seconds will one kilogram of fuel last while the engine uses it to push with a force of one Newton.

Nope. 9.8 Newtons. Isp as a time is just a bad Earth-centric convention that came from dividing pounds force by pounds mass and pretending they cancel. If the US hadn't been so influential in the space race, we'd likely all be using effective exhaust velocity.

[Dunbaratu]

Nope. 9.8 Newtons. Isp as a time is just a bad Earth-centric convention that came from dividing pounds force by pounds mass and pretending they cancel. If the US hadn't been so influential in the space race, we'd likely all be using effective exhaust velocity.

That doesn't really change anything important in the explanation. Using different units for force doesn't change anything that matters in what ISP means. It just means you need a conversion coefficient to multiply by. When KSP doesn't really give you the exact numbers anyway it just doesn't matter. (You don't have a "how many newtons am I thrusting right now" figure.)

[tavert]

That doesn't really change anything important in the explanation. Using different units for force doesn't change anything that matters in what ISP means. It just means you need a conversion coefficient to multiply by. When KSP doesn't really give you the exact numbers anyway it just doesn't matter. (You don't have a "how many newtons am I thrusting right now" figure.)

Yes you do.

The choice of units is important when you consider where the definition came from in the first place. Why is that conversion coefficient there? Because we use metric, and/or we've been taught to be careful about the difference between force and mass. When you're not using metric and you're not careful about the difference between force and mass, you use imperial units and you have a definition of specific impulse without conversion coefficients. Bad convention. If we just used effective exhaust velocity for everything, there would be no confusion and we wouldn't have to have this debate or clarify to countless new folks that the 9.8 factor doesn't change in different locations.

[Dunbaratu]

Yes you do.

The choice of units is important when you consider where the definition came from in the first place. Why is that conversion coefficient there? Because we use metric, and/or we've been taught to be careful about the difference between force and mass. When you're not using metric and you're not careful about the difference between force and mass, you use imperial units and you have a definition of specific impulse without conversion coefficients. Bad convention. If we just used effective exhaust velocity for everything, there would be no confusion and we wouldn't have to have this debate or clarify to countless new folks that the 9.8 factor doesn't change in different locations.

Exhaust velocity is not an intuitive explanation of why the number matters. It matters because it's a measure of given the same amount of thrust, how long can the fuel last while maintaining that thrust.

And the idea that the people in the US space program were ignorant of the difference between mass and force is hogwash. The fact that they're represented with the same word "pound" does NOT mean the people using that word think they mean the same thing. It was well understood that depending on context "pound" might mean a measure of weight or a measure of mass. If used in the mass context, the word "pound" changes its meaning to mean "the amount of mass that would weigh 1 pound at sea level on Earth". The metric Mks system starts from mass as a first principle and then derives a measure of force from that. The system in the US used weight as a first principle and then derived a measure of mass from that that happened to carry the same name. The fact that it confusingly uses the same name does not mean people treated them as identical.

The problem is that the question "what is ISP" needs to be answered first in terms of what it means and then second in terms of what the units are that different conventions use.

Saying that ISP *IS* by definition an expression using pounds is like saying that by definition velocity is meters per second. Velocity is a distance traveled divided by time it took to get that far. The fact that it's meters and seconds in one system is secondary to the definition.

Just like saying ISP is the time it takes to consume a given amount of fuel at a given amount of force is a universal definition of what ISP means that works regardless of what units you use, and is much easier for people to understand.

Edited October 22, 2013 by Steven Mading

[SaturnV]

If you are trying to get to Mun, Duna or make some space station with this game, you don't have to know what exactly it is, just knowing that the hight Isp the better an engine is is enough. But if you're planning visiting Jool, start a return mission to Eve, Understanding of Isp is critical in your rocket design.

[tavert]

Exhaust velocity is not an intuitive explanation of why the number matters. It matters because it's a measure of given the same amount of thrust, how long can the fuel last while maintaining that thrust.

And the idea that the people in the US space program were ignorant of the difference between mass and force is hogwash. The fact that they're represented with the same word "pound" does NOT mean the people using that word think they mean the same thing. It was well understood that depending on context "pound" might mean a measure of weight or a measure of mass. If used in the mass context, the word "pound" changes its meaning to mean "the amount of mass that would weigh 1 pound at sea level on Earth". The metric Mks system starts from mass as a first principle and then derives a measure of force from that. The system in the US used weight as a first principle and then derived a measure of mass from that that happened to carry the same name. The fact that it confusingly uses the same name does not mean people treated them as identical.

The problem is that the question "what is ISP" needs to be answered first in terms of what it means and then second in terms of what the units are that different conventions use.

Saying that ISP *IS* by definition an expression using pounds is like saying that by definition velocity is meters per second. Velocity is a distance traveled divided by time it took to get that far. The fact that it's meters and seconds in one system is secondary to the definition.

Just like saying ISP is the time it takes to consume a given amount of fuel at a given amount of force is a universal definition of what ISP means that works regardless of what units you use, and is much easier for people to understand.

Except that you've thrown in an arbitrary conversion factor of 9.8 m/s^2 in there. Not very universal, is it. The exhaust velocity makes much more sense if you're doing the actual thermodynamic nozzle calculations and talking about conservation of momentum.

Of course rocket scientists are skilled engineers and perfectly aware of the difference between mass and force. As I said a few pages ago, strictly from the definition, specific impulse is just impulse per unit [something]. The historical convention that we now have is that [something] is in units of propellant mass times g0. This is a pretty nonsensical choice, that came from dividing thrust in pounds by propellant flow rate in pounds per second, and calling the result specific impulse in seconds.

The factor of g0 is hidden when using imperial units, or any other gravitational unit system - we could use kiloponds and fold the conversion factor into the unit choice in the same way.

Edited October 22, 2013 by tavert