SirusKing

Inspired by one of ferrams posts... BEHOLD; THE MEGA CESSNA; KING OF THE SKIES Suprisingly, for a craft with only 100kN of thrust at the front, it flew pretty well. I suppose the gigantic frame and wings provide it with so much lift, a light breeze will pick it up. Suitably, it flew like a paper airplane.

I'm actually really damn stupid, wow. Disregard everything I have said. Looking into stress applied to the structure, these rockets actually cut it really close to the plastic deformity region, which I didn't expect at all. I did calculations and from what I can see using cylindrical stress, if the saturn V for example had hull walls 0.3mm thinner it would buckle under its own weight; wow. Since the rockets do actually need to increase wall thickness roughly linearly with weight then, the mass would qaudrouple if you doubled the radius. Welp, my bad. TBF, almost none of the mass is in extra stuff like valves and control points, its all in the engines and the fuel tanks.

If I came across as angry, I apologize. I've pointed out the flaws I thought it has and I stick with it, though. One of that actually is significant compared to the main hull, and regardless, you can just scale it up as a partial function of volume. I'm not saying basing it on surface area is perfect, just that its better than basing it purely off volume. Doubling the radius will not at all quadruple the weight. The parts do fit fine with stock actually, so they are also likely basing it off of volume, but there are no parts bigger than the 3.75m diameter ones to compare to.

If I came across as angry, I apologize. I've pointed out the flaws I thought it has and I stick with it, though.

I can't code for excrements, but I did suggest a pretty simple method already; Surface area multiplied by thickness, thickness is a function based on a tiny portion of volume. Hell, just basing it off surface area would still make it better than current.

I only noticed this when making an SSTO for real solar system which obviously needs big fuel tanks, and so hovers near the asymptote-like part of the Delta-V graph, where the initial mass of the rocket actually matters a lot. On these, most of the mass does actually come from the tanks, with crew and engines only adding a small portion of the mass of the rocket. I wondered why my designs didn't match my estimates back when I did use RO on a previous version and I realise now its because the mass of the tanks was still about 50 tonnes out. I could double the size of the tanks, but because it was so close to that pseudo-asymptote of the delta V graph, it would only give it an extra 150m/s. Halving the initial mass of the tanks however gave me an extra 2000m/s.

So, yeah, the problem still stands. What RO must be doing is just giving it a set multiplier that reduces the mass to more realistic levels for some tanks. Instead of basing it off volume and adding a multiplier, it should be based on surface area with a modifier for a far more accurate model, even in stock mode (just the multiplier is bigger than in RO). With the whole "supporting its weight" problem, this only really applies to upwards cylinders, meaning that the length of the cylinder effects it much more than the width. This is because pressure=force x area, and since the area increases with radius, the actual pressure being exerted onto the hull would be significantly less than 2x which it currently is. Overall, this model based on volume just doesn't make sense.

Does this keep happening or does the multiplier change? If not, it still suggests a considerable problem with the way its calculated. Making a tank wider, even with supports being included, would always increase the weight to volume efficiency. This problem exists in stock at least. on an unrelated note, I am assuming you are using a non 1.2 version of RO, so I am unable to test myself.

If you double the radius, does the mass also double or does it quadrouple? That does seem like a much better number.

This is stock. IDK if realism overhaul changes this, that might be why I have never noticed this.

Then the aproximation is horrible. Using a real life example; The external fuel tank of the space shuttle. An incredibly rigid structure easily able to support its own weight, weighs 30 tonnes in total when dry. Remaking this tank using procedural tanks, makes it weigh closer to 270 tonnes, 9x as much as real life. The actual massive tanks walls were less than half a center meter thick (albiet being made of more than just steel). A full steel verson would likely weigh about 60 tonnes, not 260 tonnes. This works for numerous other real life examples. If you actually just base it off size (maybe just times the total result by an extra 20% or something), you get a far better estimate than basing it off volume. The only time this "making up for supporting the weight" would actually hold up is for very large rockets, and when even the SLS tanks are almost entirely hollow, clearly this isn't that significant. Since your surface area requires a depth too, you could have this be your scaling variable, basing the depth of the material on the volume of the overall tank, say, 0.001+0.000001*volume. For the space shuttle tank, this would give... Dimensions: 8.4m diameter by 47 meter length aprox cylindrical. Volume: ~2900m^3 Surface Area: ~1300m^2 Density of steel: 8000kg/m^3 Volume of actual surface area: 5.07m^3 Mass of tank: 40560kg, a much better estimate.

The mass of the propellant would follow a linear relation with volume but the actual base mase of an empty tank wouldn't. Wider/more spherical tanks are **supposed** to hold more fluid for a given mass of tank. In real life, you would have to include the surface area of the internal compartments for seperate oxidiser/fuel tanks inside the outer hull, and also valves and such, but they wouldn't be anywhere near the mass of your current ones. With your tanks, if you double the radius of a cylinder, its mass increases by 4 times, which just should not happen.

There is a serious problem with your tanks; you are basing the base mass of your fuel tanks on the volume of the tank, when in reality this is not actually true. Fuel tanks are almost always entirely hollow and so only the surface area counts, meaning the mass should be calculated from what the actual shape is and not just its volume. This is very significant, since it gives big rockets a much smaller maximum economical delta V for things like single-stage-to-orbit rockets. This should be simple enough for conical and cylindrical tanks but tapered/rounded stuff could be ran as a function based on roundness averaging from the cylindrical to two cylinders, - the flat ends, + a sphere.

Does each launch site disapear when you switch to another? I had the unfortunate incident of trying to land on a runway elsewhere on the globe, only to find no KSC...

Anybody know if this works with real fuels, as in, it cools cryogenic tanks?