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 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. 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.
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. Still a bit confusing Do I need to know what it is to fly? 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. I'll just pretend I know what that means. Nobody even bother explaining to me anymore, i'm just not gonna get it.
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.
That gravity is just a constant conversion factor due to historical convention. It's about a rocket engine's efficiency. The higher the engine's Isp, the more delta-v you get per unit of fuel. 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.
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. 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.
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. That's a fantastic way to explain it. 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. 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 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 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. 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.
The choice of units is important when you consider where the definition came from in the first place. Why is that conversion coefficient there? 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. 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. Store Page. Kerbal Space Program Store Page. Showing 1 - 15 of 26 comments. Sleepyjoe View Profile View Posts.
It depends on what you want to do. Isp is like your miles per gallon for a car. Your isp gets better in a vacume because the engine doesn't have to deal with atmospheric gases.
High isps are good for boosters or first stages. Low isps are better for for probe engines and manuvering. Rhedd View Profile View Posts. Originally posted by The 'B' Mann :. Last edited by Rhedd ; 13 Jun, pm. So Isp is directly proportional to a vessel's delta-vee, and more delta-vee is always good.
As B Mann said: depends what you're doing. If you're launching from Kerbin, you need to get away from Kerbin and into orbit as fast as possible. That means high thrust, and with one exception, trading thrust for ISP at launch always means you'll end up wasting more fuel than you save because the lower thrust will cause your ship to spend more time in Kerbin's grip.
What's that one exception? Originally posted by GeneralVeers :. Azunai View Profile View Posts. Originally posted by Rhedd :. Last edited by Rhedd ; 15 Jun, am. To test any idea all you have to do is check and test the min and max cases. The min case is a rocket with 1g in thrust.
It stands there unable to take off and the fuel burn out. It did not go anywhere. Now , was that efficient? Hell no, so you want to get away from that then.
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