# can someone explain ISP?

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can someone simple explain me specific impulse?

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Think of ISP as the engines efficiency in converting fuel to thrust.  The higher the ISP the more efficiently the engine uses the fuel.

Edited by AVaughan
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It's like your car's gas mileage.

Note that engines work differently in space and on land, which is why an engine's listed ISP for ASL (at sea level) and vac (in a vacuum) are often different. Some engines are better for lifting off, others work better in space.

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Something like this: "how many seconds I get by burning 1KG of this fuel and getting 1KG of thrust on this engine". So, higher is better.

Talking about weight is counterproductive, however, because weight depends of the gravity, and gravity changes on altitude (or distance to the body) and nobody wants to deal with a huge table of a fuel's ISP varying on the current G force. So we use Mass and Thrust, then "technically" we should talk "how many seconds I get by burning 1 kilogram of mass of this fuel while getting 1kN of thrust".

We use seconds on ISP because using Mass and Thrust is cumbersome: some countries use the Imperial Units while others use the Metric System, and we end having to convert Pounds, Kilograms and Newtons all the time. But everybody agrees about what it's a second, so twisting the formulas to get numbers in seconds make life easier for everybody.

The ISP model of KSP is very simplified.  If you want a full explanation, go NASA. Or see this video. :-)

Edited by Lisias
typos, but you already knew it. =P
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1 hour ago, Tewa said:

It's like your car's gas mileage.

Note that engines work differently in space and on land, which is why an eng﻿ine's listed ISP for ASL (at sea level) and vac (in a vacuum) are often different. Some engines are better for lifting off, others work better in space.

1 hour ago, AVaughan said:

Think of ISP as the engines efficiency in ﻿converting fuel to thrust.  The higher the ISP the more efficiently the engine uses the fuel.

i know that isp means efficiency and that some engines work better on vacuum or that high thrust isnt priority in vacuum, i just wanted a simple explanation of it

20 minutes ago, Lisias said:

The ISP model of KSP is very simplified.  If you want a full explanation, go NASA. Or see this video. :-)

i already saw these too and i understand totally nothing xdd

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Thrust is how fast a rocket of unit mass can accelerate per second. [kg (m/s) / s] = [N]

Consumption is how many kilograms of fuel you expend per second. [kg/s]

Isp is how fast a rocket of unit mass can accelerate per kilogram of fuel. [kg (m/s) / kg] = [m/s]

Isp = Thrust / Consumption.

Edited by Teilnehmer
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10 minutes ago, Hunony said:

i know that isp means efficiency and that some engines work better on vacuum or that high thrust isnt priority in vacuum, i just wanted a simple explanation of it

i already saw these too and i understand totally nothing xdd

I'm not sure how much more detailed you can make that explination while remaining simple.  The Isp depends largely on the fuel type and the engine design.

Edited by Capt. Hunt
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Actual rocket science is being discussed, and so the thread has been moved to Science and Spaceflight.

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1 hour ago, Hunony said:

isp means efficiency﻿

Specifically, exhaust efficiency.

Which is why it increases with increased exhaust velocity.

On the other side of the equation is acceleration.

So, high ISP is not necessarily good, if you need to fight off gravity; but high acceleration with low ISP means loads and loads of propellant.

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My understanding of ISP. To move in space you need to throw stuff in one direction so you can go the opposite direction. The faster you can throw the stuff the less of it you need.

Edited by bonyetty
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Simple, barebone explanation, hmm? Let me try.

You have two rockets. 1 ton of payload, 1 ton of fuel on each. One is equipped with a 300 Isp engine. The other with 400 Isp engine. Both are capable of accelerating at 1 meter per second.

Rocket 300 burns for 300 seconds, and achieves the speed of 300 meters per second when it runs dry.

Rocket 400 burns for 400 seconds, and makes 400 meters per second when its fuel runs out.

Do you see the difference higher Isp makes for a rocket now? Thanks to higher efficiency of 400 Isp engine, Rocket 400 managed to reach higher velocity using the same amount of fuel as Rocket 300.

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ISP is thrust per unit of propellant consumed and is the engine's efficiency. It's one of the most important parameters of a rocket engine when comparing different engines. The units can be written as:

N/kg/s or Ns/kg.

But N can be broken down into kg m/s2. That gives:

Ns/kg = m/s

m/s is a speed. What speed? Turns out it's exhaust velocity. The higher your exhaust velocity the higher your specific impulse.

To complicate further, this is often "normalised" to change its units by dividing by Earth's gravity. Why? Because some fools get confused by working in meters but everyone agrees what a second is.

m/s / m/s2 = s

There's also a factor of roughly 10 because earth's gravity has a value, but in terms of units that's the why. ISP in seconds is basically how long an engine can burn for whilst producing 1 unit of thrust from 1 unit of fuel. Longer is better because your end velocity will be higher.

So that's what ISP is. Now why does it change in an atmosphere? It's because back pressure on the engine bell slows down the exhaust. Also the engine bell can't be as large because back pressure causes instability that causes turbulent flow separation which is destructive to engine nozzles. Smaller engine nozzles aren't as good at accelerating the exhaust, so that's a second factor in why it ends up slower.

The higher atmospheric pressure, the more the exhaust is slowed, the lower your efficiency compared to a vacuum.

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i like to think of isp as how long you can run a 1n thruster on 1 unit of fuel. i think the math works out for that and its scalable. meaning if you want a 100n thruster and run it with 100 units of fuel the result would be the same isp. though im not really 100% sure of the units used (i want to say its newtons and kilograms but im not sure). i dont think its what rocket scientists usually mean by it but this approach is somewhat more intuitive.

Edited by Nuke
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12 hours ago, RCgothic said:

ISP is thrust per unit of propellant consumed and is the engine's efficiency. It's one of the most important parameters of a rocket engine when comparing different engines. The units can be written as:

N/kg/s or Ns/kg.

But N can be broken down into kg m/s2. That gives:

Ns/kg = m/s

m/s is a speed. What speed? Turns out it's exhaust velocity. The higher your exhaust velocity the higher your specific impulse.

To complicate further, this is often "normalised" to change its units by dividing by Earth's gravity. Why? Because some fools get confused by working in meters but everyone agrees what a second is.

m/s / m/s2 = s

There's also a factor of roughly 10 because earth's gravity has a value, but in terms of units that's the why. ISP in seconds is basically how long an engine can burn for whilst producing 1 unit of thrust from 1 unit of fuel. Longer is better because your end velocity will be higher.

So that's what ISP is. Now why does it change in an atmosphere? It's because back pressure on the engine bell slows down the exhaust. Also the engine bell can't be as large because back pressure causes instability that causes turbulent flow separation which is destructive to engine nozzles. Smaller engine nozzles aren't as good at accelerating the exhaust, so that's a second factor in why it ends up slower.

The higher atmospheric pressure, the more the exhaust is slowed, the lower your efficiency compared to a vacuum.

This is the bestest explanation. I mean, the 'how long you can run a 1N engine with 1kg of fuel' thing is technically right, but really Isp is exhaust speed divided by 9,8 'cause americans can't learn proper units. Then you think about conservation of momentum ( Mass times velocity equals mass times velocity), realize the two masses are your rocket, and the fuel it carries, and you should see why having twice the exhaust speed will get you going twice as fast, pretty much instantly.

Rune. Rockets are momentum exchange machines.

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Specific impulse. Impulse per unit propellant. Impulse is change in momentum. So change in momentum per unit of propellant. A rocket is effectively a continuous collision, where the propellant and the rocket exert a force on each other. Specific impulse tells us how much the momentum changes per unit of propellant, and in general is dimensionally equivalent to speed. Dividing by the acceleration of gravity gives us the impulse per unit weight, which is in seconds.

With a clever abuse of mathematical principles, it turns out that Delta-V is a function of specific impulse and the natural logarithm of the rocket's mass ratio.

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On 4/29/2018 at 4:58 AM, Scotius said:

Simple, barebone explanation, hmm? Let me try.

You have two rockets. 1 ton of payload, 1 ton of fuel on each. One is equipped with a 30﻿0 Isp engine. The other with 400 Isp engine. Both are capable of accelerating at 1 meter per second.

Rocket 300 burns for 300 seconds, and achieves the speed of 300 meters per second when it runs dry.

Rocket 400 burns for 400 seconds, and makes 400 meters per second when its fuel runs out.

Do you see the difference higher Isp makes for a rocket now? Thanks to higher efficiency of 400 Isp engine, Rocket 400 managed to reach higher velocity using the same amount of fuel as Rocket 300.

On 4/29/2018 at 6:03 AM, RCgothic said:

ISP is thrust per unit of propellant consumed and is the engine's efficiency. It's one of the most important parameters of a rocket engine when comparing different engines. The units can be written as:

N/kg/s or Ns/kg.

But N can be broken down into kg m/s2. That gives:

Ns/kg = m/s

m/s is a speed. What speed? Turns out it's exhaust velocity. The higher your exhaust velocit﻿y the higher your specific impulse.

To complicate further, this is often "normalised" to change its units by dividing by Earth's gravity. Why? Because some fools get confused by working in meters but everyone agrees what a second is.

m/s / m/s2 = s

There's also a factor of roughly 10 because earth's gravity has a value, but in terms of units that's the why. ISP in seconds is basically how long an engine can burn for whilst producing 1 unit of thrust from 1 unit of fuel. Longer is better because your end velocity will be higher.

So that's what ISP is. Now why does it change in an atmosphere? It's because back pressure on the engine bell slows down the exhaust. Also the engine bell can't be as large because back pressure causes instability that causes turbulent flow separation which is destructive to engine nozzles. Smaller engine nozzles aren't as good at accelerating the exhaust, so that's a second factor in why it ends up slower.

The higher atmospheric pressure, the more the exhaust is slowed, the lower your efficiency compared to a vacuum.

15 hours ago, Nuke said:

i like to think of isp as how long you can run a 1n thruste﻿r on 1 unit of fuel. i think the math works out for that and its scalable. meaning if you want a 100n thruster and run it with 100 units of fuel the result would be the same isp. though im not really 100% sure of the units used (i want to say its newtons and kilograms but im not sure). i dont think its what rocket scientists usually mean by it but this approach is somewh﻿at more intuitive.

ok i did understand a bit, but thanks everyone for explaining to me

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High acceleration + low ISP: your rocket moves fast and drains fuel fast

High acceleration + high ISP: your rocket moves fast and drains fuel slowly

Low acceleration + low ISP: your rocket moves slowly and drains fuel fast

Low acceleration + high ISP: your rocket moves slowly and drains fuel slowly

That's the simplest explanation that I can make

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ISP is change in force per second.

A low ISP rocket like the Vega has a 1/100 payload to launch mass ratio. While a high ISP rocket like the Delta-V has 1/20 payload to mass ratio. If you want to make an efficient rocket you want to keep the payload mass to launch mass ratio the lowest, and thus, you also want to have the highest engine ISP.

Efficiency is not alway the main priority in rockets. Vega is very inefficient, but also very cheap.

Edited by NSEP
typo
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5 hours ago, ARS said:

High acceleration + low ISP: your rocket moves fast and drains fuel fast

High acceleration + high ISP: your rocket moves fast and drains fuel slowly

Low acceleration + low ISP: your rocket moves slowly and drains fuel fast

Low acceleration + high ISP: your rocket moves slowly and drains fuel slowly

That's the simplest explanation that I can make

ISP and acceleration are not directly linked. ISP = Thrust / Rate of Propellant Expenditure

1 hour ago, NSEP said:

ISP is the amount of force released per second.

The higher the ISP, the more force is released per second, and the more efficient your engine is.

A low ISP rocket like the Vega has a 1/100 payload to launch mass ratio. While a high ISP rocket like the Delta-V has 1/20 payload to mass ratio. If you want to make an efficient rocket you want to keep the payload mass to launch mass ratio the lowest, and thus, you also want to have the highest engine ISP.

Efficiency is not alway the main priority in rockets. Vega is very inefficient, but also very cheap.

Force released per second? Pretty sure that's not a physical quantity. Closest I can think of is change in force per second, which is a measure of throttle response, not ISP.

Agree that ISP isn't always king.

A stage with a denser propellant and lower ISP may be smaller than an equivalent stage with higher ISP and less dense propellant.

Even between stages with the same propellant density, in a choice between a larger rocket with a Be3 or a smaller rocket (because it doesn't require as much fuel) with an RS25 you might choose the cheaper Be3 because extracting maximum efficiency can be expensive.

Edited by RCgothic
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34 minutes ago, RCgothic said:

Force released per second? Pretty sure that's not a physical quantity. Closest I can think of is change in force per second, which is a measure of throttle response, not ISP.

Yeah, "force released" is like saying "degrees of applesauce"; it's nonsense. I'm sure he was thinking of impulse, not force.

Isp is the amount of impulse produced (change in momentum) per unit of propellant. If you are measuring the propellant in units of mass, then you end up with a "specific impulse" given in distance/time, which happens to be the average exhaust speed of your propellant. If you are measuring the propellant in units of weight, then you'll end up with a "specific impulse" given in units of time, which happens to be the amount of time it would take to burn through a weight of propellant equal to what the engine can lift.

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7 minutes ago, sevenperforce said:

Yeah, "force released" is like saying "degrees of applesauce"; it's nonsense. I'm sure he was thinking of impulse, not force.

Will you believe me if i told it you it was just a little bit of a typo thing ? I don't get to hear 'change in' alot in everyday life.

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The Layman Physicist's Dictionary:

• Force: How hard was the push?
• Impulse: How big was the push?
• Momentum: How much was it pushed?
• Kinetic Energy: How hard was it pushed, and how far?
• Speed: How fast is it moving?
• Velocity: How fast is it moving in that direction?
• Weight: How hard is it to pick up?
• Mass: How hard is it to push?
• Acceleration: How fast are you pushing it faster?

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