Problem with calculating rocket ISP in KSP

[freQuensy]

I am playing KSP v 1.8. and I want to calculate the time that the rocket stage will operate. So I use definition of ISP:

Spoiler

As we can see, the error is exactly 10 times. The same error persists when flying with other engines (for example 24-77). I cannot understand that something is wrong in my formulas, or does the game somehow determine the specific impulse and thrust of the drive? What's the problem here? Surely someone asked the same question.

[KerikBalm]

You forgot g: 9.81 m/s

[freQuensy]

4 hours ago, KerikBalm said:

You forgot g: 9.81 m/s

Where?

[KawaiiLucy]

https://www.grc.nasa.gov/WWW/k-12/airplane/specimp.html

Isp = F / ((dm/dt) * g0)

=>  dt = dm * g0 * Isp / F

and that you already managed to integrate

[OHara]

17 hours ago, freQuensy said:

Surely someone asked the same question.

Yes, many people do.   

The usual meaning of 'impulse' as force integrated over time,
and the usual meaning of 'specific' as per unit mass,
would give a 'specific impulse' with units m/s as you wrote at the top of your notes.

Conventionally, in rocketry, people state an I_sp with units of seconds,
as if they had measured the force in kg-force rather than Newtons, so you need that extra factor of g=9.8m/s² to make the units work.

In KSP, you might reasonably wonder "g for which planet?"  The answer is 'g' for the planet where the I_sp was measured or specified using local kg-force, so Kerbin, which has the same 'g' as Earth.

Edited July 30, 2021 by OHara
not a whiteboard

[KerikBalm]

2 hours ago, freQuensy said:

Where?

https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation

Quote

where:

 is delta-v – the maximum change of velocity of the vehicle (with no external forces acting).

 is the initial total mass, including propellant, a.k.a. wet mass.

 is the final total mass without propellant, a.k.a. dry mass.

 is the effective exhaust velocity, where:

 is the specific impulse in dimension of time.

 is standard gravity.

Its just a definitional thing. Its good that you could derive a measure of Isp on your own.

[GoSlash27]

On 7/30/2021 at 5:00 AM, freQuensy said:

Where?

freQuency,

 In the form you're using, Isp is the amount of time, in seconds, that an engine would fire while producing the same thrust as the weight of its fuel. In order to translate between mass and weight, you need g.

Best,

-Slashy