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It's always nice to add some extra fuel without adding much to the size of the spacecraft Has 4 sizes of different length Very Fairing friendly due to its shape Has a stock crossfeed switcher Integrated decoupler Aeroynamic nosecap to close the ends or make a boat Stockalike texture will blend in with any craft TweakScale support Download from Spacedock Suggested: - TweakScale (enables bigger sizes) - Any fuel switcher (patch from CryoEngines, Stock Fuel Switch, Modular Fuel Tanks, Configurable Containers) License: CC-BY-NC-SA 4.0
So working on the XenonStorm Mk3's fuel pod design has once again lead me to an old question: "Given a bunch of drop tanks of equal mass and a certain overhead per stage in the form of decouplers (whose presence in upper stages negatively affect all stages below), how many stages before decoupler mass starts really getting in the way, and how many tanks should be in each stage?" Absent decoupler mass, the clear answer is purge dry mass at every opportunity, staging early and often, however, decouplers do have mass, and in the case of my fuel pod, over 1/10 the mass of an empty tank. Also, if later stages are too large, the dry mass can damage the mass ratio more than it would on an earlier stage. To explore the problem further, I built a silly spreadsheet, here dealing with 100 tanks across 10 stages: I put a few important quantities here and there, notably the full and empty numbers for my fuel tanks, decoupler mass (stack decoupler), thrust, Isp, g, and the mass of the ship itself as the ultimate payload. "excess ratio" comes about because when the same engines are used all the way, it's the ln(wet/dry) part of things that makes the delta-v happen, so I took the mass ratio of each stage and subtracted 1 to make them more easy to compare, and allow them to be summed in a way that wouldn't be affected by the number of stages. The fuel pod's actually 7 stacks of tanks stuck on the back of a 1.25-2.5m adapter plate, but for simplicity I've chosen to study the case of 1 long string of xenon tanks with decouplers in experimental locations. A bit of experimentation bore some odd results -- running with the 100 tank concept, I first tried to keep all stages about equal in mass ratio and thus delta-v, winding up with 3/4/5/6/9/11/14/17/23 yielding a sum exccess ratio of 1.1623 and a delta-v of 45.3km/s, as seen above. Reversing this pattern (on the stage early and often concept) gave 1.1612 and 42km/s, supporting the too much dry mass in a late stage argument. Trying something odd, I tested 10/10/10/10/10/10/10/10/10/10, getting 1.1726 and 44.78km/s. I was expecting that to be worse, but very confusingly, the mass ratio sum is better than the even dv split distribution while the delta-v is worse. I'm no longer sure that the sum of the mass ratios really means that much...that or I messed up a formula. I then tried messing around with less stages, trying 12/15/19/23/31 for 1.1784 and 43km/s and 7/11/17/26/39 for 1.1788 and 43.6km/s All of this is of course just half-blind experimentation, though. I get the sense that there's something I picked up in algebra 2 or possibly differential equations that'd lead to a far better solution, as well as answer related questions like whether the fundamental results change noticably with different fuel tanks for decoupler masses. @GoSlash27's stuff on the reverse rocket equation is excellent, but doesn't answer weirder questions like "how much delta-v can I get out of a set number of identical fuel tanks by varying the staging?" What blade made of math might provide a more general solution? Edit: wow, that table looked like a toilet happened. Have a screenshot instead, or a copy of the spreadsheet itself: https://dl.dropboxusercontent.com/u/59091477/Monstrosities/Staging Theory.xlsx