Darkstorm406 Posted September 1, 2012 Share Posted September 1, 2012 Hi there, I have previously been playing KSP without getting into any of the science behind it, but various videos and forum posts have inspired me to go and learn some of the fundamental equations for rocket science.I have a good understanding of the 'Rocket Equation' where: ÃŽâ€V = Vexh x In(M1/M0)Where Vexh = ISP(specific impulse) multiplied by G (9.8068 on Kerbin). M1 is the entire rocket's mass, and M0 is the mass of the rocket after the propellant has been burnt.My question is this:Does the specific impulse of a particular engine 'stack' with another? For example, would four engines with an impulse of lets say 320, give me a total impulse of 1280? Which when we times by G, give me a Vexh of 12552.704 for use in the rocket equation? Because when using this to calculate my ÃŽâ€V, I seem to end up with a ridiculously high number (around 10000m/s) which should be more than enough to leave Kerbin on an orbital trajectory. However, I am not seeing these results in game. ...Also on a side note, I would be really grateful if anyone could explain why whenever I am flying a 3-man capsule, using the larger stock parts, about 15,000m up my craft suffers insane amounts of G-force and tears itself apart? The craft literally bends in on itself sometimes, or a structural failure between two connected fuel tanks sends the rocket motor plowing into the rest of the rocket. I have no trouble getting to the Mun and back with the smaller parts and 1-man capsules, but 3-man capsules continue to elude me, even after directly copying other peoples successful designs. Link to comment Share on other sites More sharing options...
sal_vager Posted September 1, 2012 Share Posted September 1, 2012 (edited) Hi Darkstorm406 and welcome to the community Though I can't answer your question, other people here will be able to, though I will say that even with "on paper" excellent performance numbers, you have to take into consideration the increased mass of the engines and fuel needed to feed them, which would reduce your real world performance.On the second issue though, it is now easy to overstress your craft with the new engines especially at high altitudes where the atmospheric resistance drops off, so watch your G meter, plus the new decouplers seem to be too feeble as well Edit, I should have mentioned, when at higher altitudes, to avoid over stressing the craft, reduce the throttle. Edited September 1, 2012 by sal_vager Link to comment Share on other sites More sharing options...
TheCardinal Posted September 1, 2012 Share Posted September 1, 2012 The formula you use doesn't take gravity and resistance (drag) into account (which counteract the acceleration by the rocketengine(s)).If you launch a craft with MechJeb (the ascend pilot) and press the stats button, a screen with all kinds of information will enlighten you completely. You'll see what percentage is used to overcome gravity, how much for drag, how much for steering etc.. Link to comment Share on other sites More sharing options...
Apotheosist Posted September 1, 2012 Share Posted September 1, 2012 @Darkstorm406, specific impulse does not 'stack'. If four engines have a specific impulse of 320 each, then all of them together will still have a specific impulse of 320. As you know, specific impulse is related to the exhaust velocity. If you think about it, each rocket engine has it's own exhaust velocity, and putting more of the same rocket engines next to each other will not increase their exhaust velocity.It gets more complicated if you're using more than one engine with different specific impulse values. This thread explains how to calculate the average specific impulse: http://kerbalspaceprogram.com/forum/showthread.php/15796-A-Guide-to-Basic-Kerbal-Rocket-Design-Through-Rocket-Science Link to comment Share on other sites More sharing options...
Darkstorm406 Posted September 1, 2012 Author Share Posted September 1, 2012 Thanks to everyone for their warm welcomes and input, all of your advice was really helpful. Thank you for pointing me in the right direction Apotheosist and explaining about the specific impulse and clearing up my confusion! Link to comment Share on other sites More sharing options...
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