Basic ISP question

[justidutch]

Hello again, another silly basic physics question from the same budding rocket enthusiast who asked about Delta-V in http://forum.kerbalspaceprogram.com/threads/100534-Basic-Delta-V-question

Anyways, here is my physicist-facepalming question:

Does the ISP rating of an engine change if the engine's thrust is reduced via a throttle or setting its limiter below 100%? In other words, if my engine has an ISP rating of 290, and I set its limiter to 80%, does this change the effective ISP rating?

I have looked at the documentation throughout the Wiki, and although I do try and enjoy trying to figure out the math, it is certainly not my background and I struggle with it. I've been playing the game for about a year now and figure its about time I tried learning what the heck is going on behind the scenes.

Thanks in advance for any insights!

Edited November 25, 2014 by justidutch
set to 'answered'

[Laie]

In this game: no.

ISP depends on air pressure and nothing else.

[justidutch]

Ok. I only ask because of the equation stated:

So I was wondering if we change the value of Ft, then Isp would also change.

As found at http://wiki.kerbalspaceprogram.com/wiki/Specific_impulse#Formula

Edited November 19, 2014 by justidutch

[EtherDragon]

Ok. I only ask because of the equation stated:

Isp = Ft / m

So I was wondering if we change the value of Ft, then Isp would also change.

You're forgetting one basic point - both Ft and m are reduced proportionally when you decrease throttle, so, Isp remains the same.

In other words:

at 80% throttle Ft is at 80%, as well as m being at 80%. Net result Isp is unchanged.

[justidutch]

Thinking to myself here:

Maybe if the value of Ft changes, then the value of m also changes, keep the ratio the same? If we change the thrust, does "the fuel consumption in kg/s" also change?

Thanks EtherDragon!

Edited November 19, 2014 by justidutch
ninja'd!

[DrD]

I gave up trying to fully understand Isp, now I just think of it like fuel economy in cars. Higher Isp = better mileage. Works for me.

[Starman4308]

I gave up trying to fully understand Isp, now I just think of it like fuel economy in cars. Higher Isp = better mileage. Works for me.

It isn't really very hard. The faster the propellant is going out of the nozzle, the more force it is applying on the rocket, courtesy of Newton's Third Law (for every action, there is an equal and opposite reaction, thus the rocket pushes the propellant, and the propellant pushes the rocket). Impulse is how much momentum is being transferred between propellant and rocket, and specific impulse is amount of impulse per kilogram of propellant... which happens to be equal to the velocity of the exhaust gases. Thus, specific impulse is both a measure of exhaust gas velocity, and the amount of momentum transferred to the rocket per kilogram of propellant.

In short: the faster your propellant is traveling when it exits the nozzle, the harder it's pushing your rocket (per kilogram of propellant). It says nothing about thrust (ion engines fire very small amounts of propellant at very high velocity, thus have high Isp but miserable thrust), but factors strongly into efficiency.

What makes it slightly complicated is the division by gravity. Way back in the day, rocket scientists got tired of converting between meters/second and feet/second when communicating with each other, so they just divided by Earth surface gravity (~9.81 m/s^2). This converts specific impulse from m/s (or ft/s) to units of seconds; scientists could then multiply by gravitational acceleration to get back to m/s or ft/s. While it is, to some extent, a convention, there is one physical way to think about it: Earth surface gravity would take this long to accelerate something from 0 to the same velocity as the rocket exhaust.

[DrD]

It isn't really very hard. The faster the propellant is going out of the nozzle, the more force it is applying on the rocket, courtesy of Newton's Third Law (for every action, there is an equal and opposite reaction, thus the rocket pushes the propellant, and the propellant pushes the rocket). Impulse is how much momentum is being transferred between propellant and rocket, and specific impulse is amount of impulse per kilogram of propellant... which happens to be equal to the velocity of the exhaust gases. Thus, specific impulse is both a measure of exhaust gas velocity, and the amount of momentum transferred to the rocket per kilogram of propellant.

In short: the faster your propellant is traveling when it exits the nozzle, the harder it's pushing your rocket (per kilogram of propellant). It says nothing about thrust (ion engines fire very small amounts of propellant at very high velocity, thus have high Isp but miserable thrust), but factors strongly into efficiency.

What makes it slightly complicated is the division by gravity. Way back in the day, rocket scientists got tired of converting between meters/second and feet/second when communicating with each other, so they just divided by Earth surface gravity (~9.81 m/s^2). This converts specific impulse from m/s (or ft/s) to units of seconds; scientists could then multiply by gravitational acceleration to get back to m/s or ft/s. While it is, to some extent, a convention, there is one physical way to think about it: Earth surface gravity would take this long to accelerate something from 0 to the same velocity as the rocket exhaust.

Well thanks for trying.

It still for me comes down to fuel efficiency. You get more delta V from the same amount of propellant with a higher Isp. I understand that it says nothing about thrust, you may have to burn far longer for the same delta V with a different engine, but bottom line is how much fuel do I need for a given delta V? Higher Isp = less fuel. All the rest just complicates a simple concept and is only necessary if you're going to actually calculate how much delta V your design will give you. Like most of us I use a mod for this (KER.)

And the gravity thing still escapes my understanding, I don't get why it's used as a conversion. I mean does earth's gravity just happen to have the exact value needed to convert one unit to another? It's like saying to get from pints to liters you should multiply by the airspeed velocity of an unladen swallow.

[Red Iron Crown]

And the gravity thing still escapes my understanding, I don't get why it's used as a conversion. I mean does earth's gravity just happen to have the exact value needed to convert one unit to another?

It's a convenience thing. Seconds are acceptable units in both metric and Imperial measurements, but Isp in seconds needs to be multiplied by an acceleration constant to convert to exhaust velocity. Earth's gravity is an acceleration constant that every engineer or rocket scientist would already have memorized in their measurement system of choice, so it is convenient to use.

The downside is that it confuses just about everyone at first, the first two questions are almost always "Does I need to replace it with local gravity when operating elsewhere?" followed by "Why include gravity at all when it's not important to calculating delta-V?".

As for trying to put Isp in simpler terms for understanding but not necessarily calculation, I find it best to think of it as "amount of acceleration per unit of fuel".

[MarvinKitFox]

In this game: no.

ISP depends on air pressure and nothing else.

In real life, reduced fuel flow into the same engine would tend to lead to reduced chamber pressure, which directly reduces ISP.

But in here... no.

Of course, what KSP DOES do to ISP under air pressure is quite ... wrong.

It just makes you use more fuel, for the same thrust.

What it is supposed to do is reduce thrust, for the same fuel use per second.

shrug.

Close enough.. It is a game, not a sim.

[justidutch]

It's like saying to get from pints to liters you should multiply by the airspeed velocity of an unladen swallow.

By that do you mean an African swallow, or a European one?