engine stuff I'm not sure about...

[Dewm]

This is one area I wish KSP was more clear on.. engine stuff.

So take a engine, any engine. they have stats... here are my questions.

I know isp has to do with fuel economy, but what does isp stand for?

if I have an engine that is 300 isp, or two engines that are 450 isp.. which is more fuel economic? do I have to devide isp by number of engines? so that 450/2 so I am really getting 225 isp?

Thrust. Same thing with engine thrust.. if I have a engine that produces 500 thrust or 2 engines produce 300ea. which is better? do I add the thrust values together? so 300+300 = 600 thrust?

last but not least.. mass. I see people saying something is 100 tons etc.. is 1 mass = 1 ton?

Those are my questions for now. Thanks!

[rryy]

Isp stands for specific impulse. If you have two (or any amount) engines with the same Isp, the total Isp is the same, so the two 450 Isp are better.

Thrust behaves the way you would expect, so you just add up the thrust of each engine for the total thrust.

In the game, the units of mass are measured in tons (metric tons).

[Dewm]

That helps alot! thanks!

[Dewm]

So if I have 12 nuke engines..

60 thrust and 800 isp

60 x 12 = 720

So I'll have 720 thrust but still keep the 800 isp in vacuum?

[Specialist290]

Another fact about Isp: When you have multiple engines with different Isp ratings firing at the same time and drawing from the same fuel source, the average Isp of the total setup depends on the amount of thrust each individual engine provides.

It's hard for me to put into words exactly how it works, so I'll link to the relevant equation on the KSP wiki.

EDIT: Yep, that's exactly right.

[Spica]

Here are my answers to your questions. However, beware, they may not all be right.

1. Isp stands for Specific Impulse. This is how it is measured: You have your engine, let's say the LVT-45: Since its Isp is 370 Seconds in a vacuum, if it were to output 1 Foot-Pound of thrust, and had one pound of fuel to draw from, it would burn for 370 seconds.

2. Since 450 is higher than 300, the two engines with 450 seconds of Isp are by FAR better. They will generate more thrust per pound of fuel than the single engine with 300 seconds of Isp. Even though the single engine may drain fuel more slowly. If you have multiple engines on a stage, I believe you average their Isp's depending on their relative thrusts. (i.e. one engine with 400 thrust and 400 Isp, and two engines with 100 thrust and Isp each will average to 600 thrust and I believe an Isp of either 266 or 300. (I may be wrong, I just did this now)

3. Thrust is literally the thrust output of the engine in question in Kilonewtons. They add just like a normal number. For example, the two engines with 300 KN of thrust have more total thrust than the single engine with 500 KN of thrust. However, more thrust is not always better, you also have to weigh in Isp to make sure that your rocket is fuel-efficient, but still has enough thrust to lift off the ground. In orbit though, you can give efficiency a greater role in your design because you are not trying to fight gravity all the time.

4. Yes, I believe one mass unit is equal to one metric ton in KSP

Spica

Edited August 14, 2013 by Spica

[blizzy78]

So if I have 12 nuke engines..

60 thrust and 800 isp

60 x 12 = 720

So I'll have 720 thrust but still keep the 800 isp in vacuum?

The 800 Isp won't change because it is a measurement of fuel efficiency. No matter how many engines of the same model you add, you can't change fuel efficiency that way.

[stringyquark]

I've just learned to watch out for the nuke engines, sometimes the extra weight just isn't worth it depending p on mission objectives.

[Supernovy]

I have one small correction, the overall Isp is not the thrust-weighted average of the Isps, it's the mass flow rate weighted average.

In some cases it may be easier to calculate the total thrust and mass flow rate and find the overall Isp from that (Isp*9.82 = Thrust/Mass Flow Rate).

[Dispatcher]

I've just learned to watch out for the nuke engines, sometimes the extra weight just isn't worth it depending p on mission objectives.

The atomic engine is either very bad or very good, depending on your needs. Its downside is its mass. Its a poor choice for an aircraft or even a space plane (at least launched from a runway), but it is an excellent choice for an orbit to orbit craft. Its fighting much less gravity, no air drag and the iSP makes it the best stock engine to use in space (for the power, that is).

[Skorpychan]

So if I have 12 nuke engines..

60 thrust and 800 isp

60 x 12 = 720

So I'll have 720 thrust but still keep the 800 isp in vacuum?

Yes, but you'll be using 8 times the fuel with eight engines. It's a choice between time at keyboard and fuel efficiency.

Especially if you mount them outboard to stop them overheating, as extra engines bring extra mass of girder, tank, and struts to attach them with.

[Dewm]

Sorry guys. I think I'm in over my head. XD

SOOO

isp is more like mpg?

And if this is true.... THEN

if I have 4 rockets with a isp of 800 I am still using 4 times the fuel... so I would really have a TRUE total isp for the entire ship, of 200.

Kinda like you can have 2 engines that get 30mpg but if you stick both engines in the same car your car will really be getting 15 mpg.

Am I way off base?

[Zeroignite]

iirc, the breakeven mass where a nuke engine's efficiency is worth it's own mass is ~9 tons of fuel+payload.

[Specialist290]

Sorry guys. I think I'm in over my head. XD

SOOO

isp is more like mpg?

And if this is true.... THEN

if I have 4 rockets with a isp of 800 I am still using 4 times the fuel... so I would really have a TRUE total isp for the entire ship, of 200.

Kinda like you can have 2 engines that get 30mpg but if you stick both engines in the same car your car will really be getting 15 mpg.

Am I way off base?

As far as Isp goes, adding another engine with the same Isp doesn't actually change the Isp for the whole set.

What adding extra engines does do, however, is add extra mass. Unless you balance it by adding extra fuel to compensate (which usually defeats the purpose of adding extra engines to provide more thrust in the first place), this brings your mass ratio down, which in turn brings your delta-v down, because you're effectively trying to lift more total mass with the same amount of fuel.

Incidentally, there's a set of webpages here (EDIT: Missed one) that I always recommend to anyone who takes an interest in getting into the nuts and bolts of rocket design, especially if they want to work out the basics themselves, because they're pretty much the pages that served as my own introduction. They're written mainly from the perspective of real-world rocketry, but much of it applies to KSP as well.

Really, I'd highly recommend taking a gander at that website in general, because even the stuff that isn't directly relevant to the game can serve as awesome sources of inspiration.

Edited August 15, 2013 by Specialist290

[Frederf]

Isp is "effect per fuel" ratio so four engines is four times the effect with four times the fuel rate so they cancel and the efficiency is unaltered, it just happens faster. As specialist says the downside is that more engines mean more mass. More mass means that the same effect (momentum change) from your thrust will be acting on a more massive vessel for a lesser speed change total. If you take a 10 ton space ship and make it 11 tons, you would expect 10/11th as much speed change with the same Isp and fuel.

[ZenithRising]

So if I have 12 nuke engines..

60 thrust and 800 isp

60 x 12 = 720

So I'll have 720 thrust but still keep the 800 isp in vacuum?

You will, but you'll also have added 27 tons to the mass of your craft!

Since you'll be using the nuclear engine for orbit to orbit transfers, the extra thrust won't actually get you anymore delta-v, it will just reduce the time of your burns. A longer burn at a proportionally lower thrust with the same Isp and the same mass will have the same delta-v. So if you get your thrust at a lower mass, you'll be able to get more delta-v out of it.

[TheDarkStar]

If you take a 10 ton space ship and make it 11 tons, you would expect 10/11th as much speed change with the same Isp and fuel.

It doesn't actually work like that, mainly because of the natural log in the equation.

WARNING: MATH

The equation for delta-v (total change in speed) is dV = I_SP * ln (starting stage mass / ending stage mass). All liquid fuel/oxidizer tanks are 8/9 fuel, 1/9 dry mass. If you had two idealized ships (massless engine, 9/1 starting to ending mass, second one has 1 ton dead weight) with 10 and 11 ton masses, you'd get:

First Ship: dV₁ = I_sp * ln (10 / (10/9))

Second Ship: dV₂ = I_sp * ln ((10 + 1) / (10/9 + 1))

Simplify and you get:

dV₁ = ln (9) = 2.1972

dV₂ = ln (99/19) = 1.6506

Divide them and you get 1.3311, which is not 11/10.

Note that you get decreasing returns on actual rockets as your starting to ending mass ratio approaches 1/9 (because of the fuel to dry mass ratio in the engines). In a single stage, the LV-N approaches about 18000 m/s and the ion engine appreaches about 34000 m/s (this might not be correct).

tl;dr

The dV you get is not linear.

[Skorpychan]

Sorry guys. I think I'm in over my head. XD

SOOO

isp is more like mpg?

And if this is true.... THEN

if I have 4 rockets with a isp of 800 I am still using 4 times the fuel... so I would really have a TRUE total isp for the entire ship, of 200.

Kinda like you can have 2 engines that get 30mpg but if you stick both engines in the same car your car will really be getting 15 mpg.

Am I way off base?

Effectively, yes. That's not how ISP actually works, but it's a decent enough approximation for designing rockets in KSP.

Also of note is that nuclear engines overheat when attached straight to the orange jumbo tanks. Either mount them on nacelles, or don't use orange tanks.