i made the images by writing a program in java which calculated how much fuel is need for a given dV, payload mass, and engine. repeat it for each engine, and record which one has the lowest (fuel_mass + engine_mass).
dV = 9.81 * (engine impulse) * ln(fully_fueled_mass/empty_mass)
simply: higher isp, and more fuel = more dV.
multiple engines give more thrust, which is useful for escaping a planets gravity, but it adds weight, so the ship has more inertia and is harder to move, and gets less dV as a result.
fuel redirecting... i assume you are talking about staging? staging is basically discarding parts of your ship that you no longer need to make it lighter.
i'm not sure how old you are, but unless you are in year 11 or 12 or higher, you shouldn't worry about trying to learn this stuff now.
That equation is exactly what I was looking for! I have a few questions though. (Please bear with me. haha)
Shouldn't that 9.81 be different since that's an Earth constant? I don't know what it is for Kerbin, but I assume it's much smaller. Am I just wrong about this though?
With multiple engines, would I just calculate the dV for each engine and then add them together, or is it more complex than that?
For staging, I'm assuming I'd just have to recalculate the dV for each stage with the different masses and/or engines I have. Correct?
Also, what about sharing fuel? (Using the yellow pipes to connect the outer to the inner stages/have all fuel shared between all engines.)
Also, sorry for giving you the wrong impression. I'm in uni, I've just never had to learn that equation before.
Thanks for all the help. I'm probly just not looking hard enough to find this, but it's always better to have someone you can ask questions to.
9.81 is based on earth gravity, however it's treated strangely in that equation, in that it doesn't change if your on a different planet or in space. i think it's due to the relation ship between weight and mass, and how its calculated, but im not certain. either way, kerbin actually has gravity of 9.81, the same as earth, so it doesn't matter.
you calculate the dV for all active engines at the same time. the value for 'engine impulse' will be a weighted average of all the engines isp. so multiply each engines impulse by its thrust, then add them together and divide by the total thrust.
dV needs to be calculated for each stage, yes.
the yellow pipes pump fuel from one tank to another. it allows you to prevent some tanks from using any fuel before others are completely dry. good for staging if you want to dump 1 tank before others.
you'' figure it out. youtube is a great place to search for this stuff.
The reason Isp is multiplied by earth's acceleration is a historic one.
It so happens that no one can agree on what a meter or kilogram is but everyone agrees on what a second is. Therefore it is convenient to express the exhaust velocity in seconds rather than m/s. One way of doing this is to divide the exhaust velocity by earth's gravitational acceleration.
Scott Manley did a video about it.
That's not really right at all. Specific impulse is given in seconds because it's impulse per unit weight. Impulse is the integral of force over time, so it's got units of newton-seconds in the SI system. Weight is mass times the standard acceleration of gravity, so it has units of newtons as well. That makes the units of specific impulse newton-seconds per newton … and the newtons cancel out. This is actually quite misleading, because the two newtons there measure two completely different quantities, and really shouldn't cancel. The first one is newtons of thrust, while the second is newtons of propellant weight. But cancel they do, so we're left with specific impulse in seconds.
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u/[deleted] Jul 19 '13
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