Ideal Rocket Equation and Delta V Questions.

[errorexe3]

So I've gotten used to eyeing things up in KSP and still having the crafts be successful in getting to a from places, but I want to start using math to get an idea of what my craft is capable of before launching it, the most basic being just how much Delta V I have in a stage but I've run into some issues.

So the rocket equation I found is just (Delta V = Exhaust Velocity * (Initial Mass/Dry Mass)) which gets me an incorrect answer when I use values in KSP.

However on the KSP wiki "Exhaust Velocity" is replaced with "Specific Impulse" when doing Delta V calculations. Is Exhaust Velocity and Specific Impulse the same thing or has something gone over my head?

 

[OhioBob]

1 hour ago, errorexe3 said:

However on the KSP wiki "Exhaust Velocity" is replaced with "Specific Impulse" when doing Delta V calculations. Is Exhaust Velocity and Specific Impulse the same thing or has something gone over my head?

Exhaust velocity = Specific impulse * 9.81

 

[HebaruSan]

2 hours ago, errorexe3 said:

So the rocket equation I found is just (Delta V = Exhaust Velocity * (Initial Mass/Dry Mass))

You also need to take the natural logarithm of that mass ratio:

Delta V = 9.81 * Specific Impulse * ln(Initial Mass / Dry Mass)

[errorexe3]

1 minute ago, HebaruSan said:

You also need to take the natural logarithm of that mass ratio:

Delta V = 9.81 * Specific Impulse * ln(Initial Mass / Dry Mass)

That makes a ton more sense, I haven't seen anything mention it elsewhere, thank you.

[OhioBob]

Sorry, I didn't notice that you left out the ln or I would have mentioned it.  You can read more about the rocket equation here, https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation

[Marschig]

Exact value is 9.80665.