Problems understanding difference between ISP and trust

[Luc1fer]

Hello fellow kerbonauts!

I just installed interstellar mod yesterday and i have two big questions:

A:What’s the difference between ISP and trust and what uses each one has?

B:How i make the engines work?i mean the mod comes with reactors and other things,but i could not found how to make the engines work,a lot of them just don’t start idk why...

[MaxwellsDemon]

A:   Isp (specific impulse) is the efficiency of the engine -- how long it will run on a given amount of fuel.   Thrust is the total work done at any one time by an engine.   You can have a very efficient, low-thrust engine (like an ion engine) or a high-thrust, inefficient engine (like most solid boosters).   The gold standard, of course, would be to have both high Isp and high thrust, but in practice that's extremely difficult and engines are therefore optimized to emphasize one or the other.  (That's in real life.  KSP models that.)

B:  Can't help you there...don't have that mod!

Edited February 5, 2018 by MaxwellsDemon
addition

[Starman4308]

On B: I would consult in the Interstellar mod's thread.

On A, they are very different parameters. As @MaxwellsDemon said, loosely speaking, you can consider specific impulse as the "efficiency" of an engine.

 

One of the most fundamental concepts in rocket science: to go further, you need more fuel, then you need more fuel to carry that more fuel. The mathematical expression of this comes out in Tsiolkovsky's rocket equation:

dV = Ve * ln(mw/md) = Gm * Isp * ln(mw/md).

 

dV is delta-V, or your craft's capacity to change its speed.

Ve is exhaust velocity. This is equal to Gm * Isp.

Gm is Earth's surface gravitational acceleration, about 9.81 m/sec^2.

Isp is specific impulse. Why we list specific impulse instead of exhaust velocity largely comes down to historical reasons.

ln is just the natural logarithm operator.

mw is the stage's "wet" mass. This includes not just the stage, but also any payload.

md is stage's "dry" mass, once it has been depleted of fuel.

 

In short, to increase a stage's delta-V, you can either improve specific impulse, or add more fuel. Specific impulse is dictated by the engine. Adding more fuel helps... up to a limit.

First off, fuel tanks are not massless. In KSP, fuel tank wet/dry ratios are about 9:1 IIRC. That means the cap on your ln(mw/md) term per stage would be ln(9/1) = 2.197, and that's if your ship is nothing but fuel tanks.

Second, thrust. A ship that is all fuel tanks has no engine to propel itself. To get a rocket off Kerbin, you need a TWR (Thrust to Weight Ratio) of > 1.0, preferably around 1.4. Even in space, if your TWR is 0.01, you will get awfully bored maneuvering.

 

This brings us to Newton's Second Law of Motion: an object at rest tends to stay at rest unless acted upon by an external force. The mathematical expression becomes F = ma; force (in Newtons) is equal to mass (in kilograms) times acceleration (in m/sec^2). For rocketry, it is convenient to rearrange this to a = F/m; acceleration is force divided by mass.

Gravitational acceleration at Kerbin's/Earth's surface is about 9.81 m/sec^2. Thus, to reach a target of 1.4 TWR, your rocket must accelerate at 1.4 * 9.81 m/sec^2 = 13.734 m/sec^2. This means that at launch, for every kilogram of rocket, you need 13.734 N of thrust.

Since masses in KSP are measured in tons (1000 kg), and thrusts in kilonewtons (1000 N), you would thus need 13.734 kN of thrust per ton of rocket. Convenient how SI units work, eh?

 

This, generally speaking, leads to the following design choices in rockets.

For launch vehicles, you often choose engines which produce a lot of thrust per kilogram, and which don't lose too much specific impulse at sea level. Having a very high sea-level Isp isn't too important: the bigger factor is just reducing the number of expensive first-stage engines onboard while still reaching ~1.4 TWR at launch.

For orbital maneuvers, you often choose engines with a high specific impulse, so as to efficiently use your fuel, and reduce the overall mass, so the rest of your rocket can be lighter. Thrust is often not hugely important unless landing on a moon/planet; I've worked with as low as 0.07 Kerbin TWR.

[MaxwellsDemon]

Funny how often Tsiolkovsky's name comes up.      (Amazing man.)

In a similar vein, I'm reminded of the problem of how much water to carry into a desert.  With horses, which need more water, you need more horses to carry more water for more horses... with camels, it's the same thing, only not quite so many as the horses.   So, a camel has a higher Isp, so to speak.       Being a bit goofy, but it does somewhat illustrate the point.

[NSEP]

ISP is fuel efficiency.

Thrust is acceleration power.

[Snark]

7 hours ago, Juanlu16 said:

A:What’s the difference between ISP and trust and what uses each one has?

As folks have observed:  Isp is fuel efficiency:  higher Isp means you get more dV for the same amount of fuel.  Thrust is how much power it has, i.e. how fast it can accelerate.  By analogy with automobiles:  an engine with really high thrust is like a sports car; an engine with high Isp is like a fuel-efficient hybrid that can go a long distance on a small amount of fuel.

Both Isp and thrust are important, but they're important for different things.

Generally, having high thrust is only important for takeoff (and for landing, if you're on a planet with no atmosphere).  You need the high thrust there because the engine has to be powerful enough to fight the force of gravity.  For example, on Kerbin, a 10-ton rocket needs 98 kN of thrust just to lift its own weight-- anything less than that won't even get off the launchpad.

Having high Isp is much more important after you're in orbit.  That's because, when you want to go from one planet to another, or do other orbital maneuvers, what you really care about the most is dV:  "How many meters per second of speed do I get if I burn all of my fuel?"  For a rocket with a given mass and amount of fuel, the higher your Isp, the more dV you have.  In that situation (i.e. when you're already in orbit), thrust isn't very important-- even a very low-thrust engine is fine.  That's because you're not trying to lift the weight of the ship, and generally you have plenty of time to do your burns so you don't need super-strong acceleration.

That's important to remember:  thrust doesn't give you more dV.  If I have two identical rockets in orbit, both the same mass, both the same amount of fuel, both engines have the same Isp ... but one of the engines has 10 times the thrust of the other.  Both rockets will have the same dV.  The rocket with the higher thrust will accelerate faster, sure... but it will also guzzle its fuel a lot faster, too.  Maybe the low-thrust rocket takes 100 seconds to burn through its fuel supply, whereas the high-thrust rocket does so in only 10 seconds.  But both of them end up going the same speed after they've burned all their fuel, and that's what you really care about.

In general, in KSP, engines are designed to be "balanced" so that no one engine is better than all the others.  So for the most part, engines with high Isp tend to be low thrust (example:  LV-N and ion engines), and engines with high thrust tend to be low Isp (example: SRBs).  So when you're designing a ship, you need to pick the right engine for the job it's doing:  typically, high-thrust engines for takeoff and landing, high-Isp engines for orbital maneuvering.

Here's a simple way to think of the practical implications:

• Isp:  The higher, the better.  More is always better.  Double the Isp is twice as good.
• Thrust:  You either have enough, or you don't.  Ideally you want to have just enough.  Not enough = mission failure.  Too much = bad, because that means you're wasting too much dead weight on engines you don't need.  "How much is enough" depends on your ship mission.  For takeoff/landing, it's determined by the gravity of the planet you're on.  For orbital maneuvers, "enough" is pretty close to zero-- the main lower limit is "how patient are you to do long burns".

 

[KerikBalm]

Isp is essentially a measure of the ratio of thrust vs fuel consumption.

Suppose you have 2 engines, A and B. Suppose they both produce 100 kN of thrust, but B has twice the Isp of A.

A would consume x units of fuel per second while producing 100 kN of thrust. Since B has twice the Isp of A, it would produce x/2 units of fuel per second while producing 100 kN of thrust.

Another scenario would be that B has twice the Isp of A, and also twice the thrust. In that case both engines A and B would consume x units of fuel per second, but B would produce twice as much thrust.

Another scenario would be that B has twice the Isp and half the thrust of A. In this case B would consume 1/4 the fuel per second of A, while producing half the thrust.

Generally speaking, you care about Isp when concerned with dV, and you care about thrust (and the related thrust:weight ratio of the engine) when you care about the ability to generate a certain dV within a given amount of time. Ion engines are great for high dV designs, but their thrust is so low you may not want to use them if all your burns are going to take >20 minutes. They are also pretty much useless for landing on bodies with gravity stronger than Mun.

[wumpus]

While more Isp is better, don't overdo it.  Ion engines don't exactly play nicely with the time acceleration system.  Nuclear (LV-N) engines *are* that great, but fueling them can be a pain (insufficient fuel-only tanks) and you might want more than one for large vessels (or learn all about the Mangalyaan maneuver [sometimes called pe-kicking here]).

I'd learn to use the terrier and poodle engines first (as second and/or third stages*).  They still have reasonable thrust, low cost and the higher Isp of traditional rockets.  Remember, you still need sufficient thrust when circularizing [you start coming down sooner than you might think] and I've found that the LV-N often can't get that job done.  Once in orbit, while you don't have that issue, you lose efficiency as you have to burn long before and after your burn point (in near orbit of some body, out in deep space it doesn't matter).

And consider the camel analogy.  That is pretty much it.

* while it isn't obvious from the equations that Isp matters more and more as you drop stages, you'll find that Isp doesn't matter as much for the first stage (I love big SRBs like kickers) and it becomes critical in your final stages (terriers work wonders thanks to how light they are).  This is mostly because of all the stages above the first stage count as "dry mass" to that first stage: you are in the more linear regions of the exponential curve.  As you have less and less dry mass above you, you go into the upper ranges of the exponential curve and Isp matters more and more.

 

[drhay53]

If you have problems with ISP and trust, you should switch away from Comcast.

[invision]

to use a lot of those engines you need to hook up a nuclear reactor, a power generator, a fuel tank with the correct fuel, and heat shields.

honestly most of the engines need so much crap to get them running and add a lot of weight its becomes a hassle and not worth it

however theres 2 engines that are probably the best engines and its the nuclear solid core and the candle. they dont need anything fancy just straight liquid fuel.

[MaxwellsDemon]

16 hours ago, wumpus said:

(terriers work wonders thanks to how light they are)

 

I unabashedly love the Terrier for upper stages (and for getting largish landers down to and up off the Mun, too).   If it's 1.25m and it's not at the launch site, it's rare I'm not using the Terrier...