Math going wrong. Can't calculate Delta-V using Newtons Law

[GoSlash27]

paul23,
 It does. You have to right click on the engine. It's in "units" (fuel plus oxidizer), so you have to divide by 200 to get the mass flow rate in tonnes/sec.

Example:

 

If T=1389.1kN and "m"= 98.682 units/sec, then m-dot = 0.49341 tonnes/sec. This yields an exhaust velocity of  T/mdot = 2,815.3 m/sec

Verifying this the normal way, 287.1s x 9.807 = 2,815 m/sec.

Kinda silly to do this, since, as I said before, the Isp is actually sitting right there...

Best,
-Slashy

 

 

[Kerbart]

Let's get at least the terminology right!

First, I_sp is Specific Impulse and thus written as I_sp (Impulse, specific), not as ISP. “It's only semantics.” Maybe, but the quickest way to foul up your calculations is by being sloppy.

Second, given the “you forgot the oxidizer” and “units” comments, it’s propellant (“that what comes out of the exhaust”) and not “fuel.” This is why water bottle rockets work, despite the fact that they don't use any fuel at all (just water, but that's the propellant, not the fuel). Don't forget that the nuclear engine emits zero fuel (that's the nuclear core inside it, after all) and uses purely propellant (it doesn't get burnt, even though it's LF in the game, in absence of pure Hydrogen).

[Loren Pechtel]

20 hours ago, Thormack said:

Thanks very much for comments guys.

Anyway, can anyone point more exactly what is so wrong with my spreadsheet?

As I'm getting around half the actual delta-v that I should as result.

I might have to do some mass adjusts, nevertheless, there must be something else wrong besides it.

 

 

The basic problem is your averaging the mass approach is horribly wrong.

When the rocket is fuller you get less delta-v per unit of fuel burned than when the rocket is emptier.  This is a curve, not a straight line and thus can't be reasonably approximated with an average.

Your choices are:

Simulate it second by second.

Use the rocket equation.

Apply calculus--in effect, derive the rocket equation.

6 hours ago, Kerbart said:

Let's get at least the terminology right!First, I_sp is Specific Impulse and thus written as I_sp (Impulse, specific), not as ISP. “It's only semantics.” Maybe, but the quickest way to foul up your calculations is by being sloppy.

I_sp is routinely written ISP when you don't have easy access to subscripts.

[pincushionman]

11 hours ago, paul23 said:

Does KSP give the instantaneous/constant mass flow rate anywhere?

Yes, the maximum fuel and oxidizer flow rates are shown in the engine descriptions, stated in units per second. The unit mass of each propellant can be worked out by comparing tank capacities and masses.