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Thread: Specific impulse?

  1. #1

    Specific impulse?

    Hello. I'd like to know how, exactly, the specific impulse system works. I'm not fammiliar with rocket science (I just mash a bunch of SRBs toguether, put a parachute and tie it all toguether with struts) so I'd like an explanation in layman's terms. Anyways yeah, how does the specific impulse system work?

    That's pretty much it. I used google, the wiki and the forum search feature to no avail. If anyone could help me I'd appreciate it!

    Thank you.

  2. #2
    Minimalist Rocket Expert
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    Well I could try, But I'm not promising anything.

    The specific impulse for the rockets shows how efficient they are. The higher the specific impulse, the greater the efficiency. So basically a lower specific impulse means the rocket uses more fuel, but does not make more thrust.

    Most rockets have different specific impulse values at one atmosphere and in a vacuum. For example, an engine might have a specific impulse in one atmosphere of 320, and in vacuum of 370. Specific impulse depends on air pressure outside the engine. The air pressure outside the engine at sea level is called "one atmosphere" because you have one whole atmosphere above you pushing down and creating that pressure. As you go higher in the atmosphere, the air pressure gradually drops, until eventually there is no air at all, so it is called a vacuum. Your engines will be more efficient (use less fuel to produce the same thrust) in the vacuum of space than they are at sea level, where there is one atmosphere.

    However, the "Toroidal Aerospike Rocket" has a constant specific impulse, no matter how much atmosphere it's in.

    If this isn't detailed enough for you I could go further into the physics of it.

  3. #3
    Specific impulse is basically the thrust / fuel burn rate. Therefore, if you had two craft that both had 1000 thrust, but one used 50L/s of fuel and the other 100L/s of fuel, the first would have twice the specific impulse.

    This represents the efficiency of the engine, and gives you the relative amount of delta V you can get out of the engine for a given amount of fuel, as compared to other engines.

    For example, if you take one engine with an ISP of 390, and one with an ISP of 350, you will get ~11% more delta V out of the first engine for a given amount of fuel. This is typically most important for orbital maneurvers, where you don't need a lot of thrust, and can do long period burns instead (thick of the probes with ion engines). Therefore, for orbital transfer maneurvers such as trans munar injection burns, or later on burns between planetary orbits, you want to use a high ISP engine, not a high thrust engine.

    In atmosphere, or when taking/off and landing this is slightly more complicated, as you need a certain amount of thrust to take off, so the low thrust high ISP engines cannot help you.

    Hope this helped.


    PS: ISP = specific impulse.

  4. #4
    Another way of looking at it, specific impulse is the amount of time, in seconds, that an engine producing 1 kgf (kilogram of force) would take to burn 1 kg of fuel.
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  5. #5
    Rocketry Enthusiast Nori's Avatar
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    Quote Originally Posted by silent_prtoagonist View Post
    Another way of looking at it, specific impulse is the amount of time, in seconds, that an engine producing 1 kgf (kilogram of force) would take to burn 1 kg of fuel.
    Is one fuel unit in game equal to 1kg?

    How would you calculate actual usage then? Say for instance you were using a engine with a 390 ISP and 250 thrust. What would the fuel usage per second be?

  6. #6
    Quote Originally Posted by silent_prtoagonist View Post
    Another way of looking at it, specific impulse is the amount of time, in seconds, that an engine producing 1 kgf (kilogram of force) would take to burn 1 kg of fuel.
    Thank you. This is pretty much all I wanted Still, the other explanations helped a lot. Thanks everybody.

  7. #7
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    Quote Originally Posted by Nori View Post
    Is one fuel unit in game equal to 1kg?

    How would you calculate actual usage then? Say for instance you were using a engine with a 390 ISP and 250 thrust. What would the fuel usage per second be?
    This is a bit tricky because in KSP fuel units do not correlate to mass in nice numbers. We want to know how many units of fuel we use per second. We can't do that directly, but we can work out the mass flow rate per second. There is a formula for this:

    m=F/Ve

    Where m is the mass flow rate of propellant, F is the thrust force produced by the engine, and Ve is the exhaust velocity of the rocket engine. Exhaust velocity can actually be found by multiplying specific impulse by the acceleration due to earth's (or kerbin's) gravity), which is about 9.81. So our exhaust velocity is

    390*9.81=3825.9m/s

    Now we can plug in all the numbers to find out the mass flow rate.

    m=250/3825.9=0.0653 (to four decimal places)

    So there is 0.0653 kerbal units of mass of propellant being used per second. This however still does not tell us how many units of fuel we use per second. Before we can do that we need to know the mass of one unit of fuel.

    So let's work out the mass of one unit of fuel. Choose any tank. I'll take the smallest one, the FLT-200. Take away the dry mass from the total mass to get the mass of the fuel.

    1.125-0.125=1

    So all the fuel in the tank weighs 1 kerbal unit of mass. The tank has a capacity of 200 units of fuel. We can find the mass of one unit by dividing the mass of the total fuel by the number of units.

    1/200=0.005

    Ok, now we know the mass of one unit of fuel and the mass flow per second, we can work out the fuel consumption per second. We can do this by dividing the mass flow per second by the mass of one unit of fuel.

    0.0653/0.005=13.068

    So there you have it, your engine (Toroidal Aerospike Rocket, I see) uses 13.068 units of fuel per second. That is to a certain degree of accuracy; I suppose this calculation could be made more precise. I hope this helped.

  8. #8
    Senior Rocket Scientist Serratus's Avatar
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    Also, there's a display on right-click menu, of the amount in liters. We don't really know the density of that fuel, but if we assume that it's similar to ours then we just have to calculate the weight from that remembering to first scale it down by a factor of 10 (or was it 11?) and You'll get in Earth Kg how much that fuel weights.
    First Kerbonaut to die by slamming into Minimus surface... from the bottom.

  9. #9
    I'd prefer it if the tooltip would just display the liter per second usage in vacuum (full thrust), like it does when you right click it while flying the rocket *.* I get that the impulse shows which engine is better but to me it's too hard to still make some use of that...

  10. #10
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    There is also the wikipedia article on the subject.

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