How to calculate a ships Delta v

[Absolution]

@Absolution: I get:

Isp1 == (2000*7500) / (62*82.51) ~= 2932.1959 m/s

Isp2 == (373*3000) / (14.17*33) ~= 2393.0198 m/s

?V1 == 2932.1959 * ln((132.79/(132.79-82.15)) ~= 2826.716 m/s

?V2 == 2393.0198 * ln(42.96/(42.96-9.96)) ~= 631.187 m/s

? ~= 4129 m/s

For the 3497 m/s, I\'m guessing you did 2392.34*ln(42.96/9.96)?

~43 tonnes initial mass; ~33 tonnes of fuel burned; ~10 tonnes final mass. So if I understand the equation correctly '2392.34*ln(42.96/9.96)' would be the correct result for the equation, yes? I think you may have swapped final mass with fuel mass burned (yes, it\'s quite a gas guzzler).

[UmbralRaptor]

I\'m confused now:

~43 tonnes initial mass; ~33 tonnes of fuel burned; ~10 tonnes final mass.

Stage 2:

fueled mass = 42.96 tonnes

mass of fuel burned = 9.96 tonnes

Ve of LFE-7E1-A1 engine = 2392.34 m/s

delta-V of the upper stage is 3497 m/s

Did it burn ~10 tonnes, or ~33 tonnes?

[Absolution]

I\'m confused now:

Did it burn ~10 tonnes, or ~33 tonnes?

AHA! My mistake. It burns 33 tonnes worth of fuel. I made a typo and I apologize.

[togfox]

> ISP (m/s) = Total Thrust / Total mass flow rate

For the life of me I had this understood in my head and now it\'s midnight and I\'ve been thinking about a dozen formula\'s and now I\'m muddled.

How does one calculate *TOTAL* mass flow rate for :

- two liquid fuel engines the same type and

- two liquid fuel engines of different type

? Thx.

[UmbralRaptor]

Togfox: calculate the mass flow for each engine, and add them up. eg: for 2 of the same type, multiply what you got for one of the engines by 2.

For 6 Mk 55s around an Aerospike at sealevel connected to standard (200 u/tonne) tankage:

Fuel consumption:

Mk 55: (80*200)/(280*9.81) ~= 5.825 u/s

Aerospike: (250*200)/(390*9.81) ~= 13.069 u/s

Total thrust == 80*6+250 == 790 730 kN

Total mass flow == 5.825*6+13.069 ~= 48.019 u/s

Overall Ve == 790*200/48.019 ~= 3290.393 m/s == 335.4 s 730*200/48.019 ~= 3040.490 m/s == 309.9 s

AHA! My mistake. It burns 33 tonnes worth of fuel. I made a typo and I apologize.

Okay, glad its cleared up.

edit: corrected numbers. *glares at the math error*

[togfox]

Total thrust == 80*6+250 == 730 kN

Tiny correction for those that are following the thread.

Well explained! Thanks! 8)

[Chsuae97]

You can get you Exhaust Velocity (Ve in equation) by multiplying your ISP (if given in seconds, as it is in the KSP VAB hanger) by the standard gravity, which on kerbin is 9.81.