Suggestion: Rebalance proc fairing bases and hollow tanks/structrures

[Clayell]

This issue is the result of a conversation I recently had with Unofficial Aesthetic Inspector and Matthew, both of which seem to be an expert on hollow structures/tanks. I have not used them except for one very specific case so I did not realize their benefits. This is in the context of RP-1.

Hollow structures/tanks (tanks are a bit lighter than structures but cost more in tooling) can be used for a large variety of designs, including some that are perhaps intended to used by procedural interstage and boattail fairings. Hollow structures/tanks are far lighter than fairings, and as far as I can tell they only come with 2 inherent disadvantages.

1. Hollow structures cannot decouple sideways into multiple parts like a payload fairing (referred to as a "fairing decoupler")
2. Hollow structures are fairly weak against heat

Both of these disadvantages only make them unsuitable to be payload fairings, which honestly makes sense. A payload fairing would have to be thicker than something like a hollow cone as they need to hold both the fairing decouplres and be more heat-resistant. So I am not suggesting a change to payload fairings.
I am, however, suggesting the re-balance of interstage and boattail fairing bases. For these roles, the advantages of a fairing as listed above do not mean anything. The only advantages they have is the setting up of the rocket. Interstage fairing bases, in particular, have the technical advantage of being able to attach something to its floating node, which makes setting them up much easier than a hollow structure/tank. They also have settings that make it easier to determine the size of the part that is being covered up. However, these advantages only help the player set up the fairing faster, and do not have any advantage in flight. This is unlike hollow tanks, which take longer to set up but are far lighter than fairings as previously stated.

This leads to a kind of unfair advantage, where new players are encouraged to use the easier to use but heavier fairings, (and potentially not knowing about hollow tanks/structures, like me) while more experienced players take the extra time to set up hollow tanks/structures in order to save weight.

In order to solve this problem, I propose several solutions:

• Make fairings that are put on interstage/boattail bases less heavy by default (when the fairing decouplers are off)
• Allow players to change the minimum width (and also density, as in current RP-1 there is a minimum density based on tech level) of fairings for interstage/boattails bases (when the fairing decouplers are off)
• Remove interstage/boattail bases and add another part (or add a toggle) similar to the hollow tanks/structures but with a floating decoupling node

In my opinion, the last option is the best, especially if some text is added to clarify that it is for interstages/boattails/stuff like that so it will draw more attention from new players.

Here are some examples from the two people listed above using hollow tanks/structures for purposes that are currently intended to be used by fairings: (discord images, so paste in a server so you don't have to download)

Hollow Cone being used as interstage 1
Hollow Cone being used as interstage 2
Hollow Cone being used as boattail

[Capkirk123]

I disagree, and in fact, would very much like to make hollow structures heavier. Fairing mass is set to be relatively accurate for historical spacecraft, and decreasing it would effect balance and realism.

Instead, I would like for hollow parts to be changed so that they have the same mass as a non-hollow part of the same dimensions. This should hopefully at least close the gap between the masses of hollow parts and fairings/interstages.

[ProtonEnjoyer]

I disagree, and in fact, would very much like to make hollow structures heavier. Fairing mass is set to be relatively accurate for historical spacecraft, and decreasing it would effect balance and realism.

Instead, I would like for hollow parts to be changed so that they have the same mass as a non-hollow part of the same dimensions. This should hopefully at least close the gap between the masses of hollow parts and fairings/interstages.

Oh yeah, I would love for my 10m First Stage to have a 10t interstage because "hollow mass" is now the same as "filled mass"

[Clayell]

I disagree, and in fact, would very much like to make hollow structures heavier. Fairing mass is set to be relatively accurate for historical spacecraft, and decreasing it would effect balance and realism.

Instead, I would like for hollow parts to be changed so that they have the same mass as a non-hollow part of the same dimensions. This should hopefully at least close the gap between the masses of hollow parts and fairings/interstages.

Was not aware that fairing mass is actually accurate, although I disagree with your solution. I think that setting a minimum width (maybe like .05m? idk the balancing) between the outer and inner diameter and making sure that this leads to a decent mass could fix this.

[lpgagnon]

For hollow proc structurals and decoupler "rings", "same as non-hollow of same dimensions" makes sense (the non-hollow mass predates hollow parts and is more or less based on the supposition that it's "supposed to be hollow").

For fancier shapes (e.g. a proc structural hollow cone with base outer=1m, base inner=0, top outer=top inner=0.5), it gets a bit less obvious what makes sense.

And for tanks, things get even more nebulous. Arguably a hollow tank should be heavier than non-hollow, since now there's an inner wall (but plausibly that can just be handwaved away in the same way we handwave away bulkheads, cylinder-vs-sphere, etc)

(previous discussion, but it's a bit unstructured https://discord.com/channels/319857228905447436/620690446540341261/1046436749028634784)

[Capkirk123]

Yes, for a simple cylindrical or conical structure, all of the structural mass is already in the skin. Making it hollow or not has no effect on the structure, and therefore a hollow and non-hollow part would be expected to have the same mass (if anything, the hollow part may be heavier due to being forced to use sub-optimal bracing to reduce wall thickness).

As for tanks, a toroidal tank is potentially heavier, but we don't currently enforce any kind of shape-based structural mass penalties, so that's a problem for later.

[Clayell]

Yes, for a simple cylindrical or conical structure, all of the structural mass is already in the skin. Making it hollow or not has no effect on the structure, and therefore a hollow and non-hollow part would be expected to have the same mass (if anything, the hollow part may be heavier due to being forced to use sub-optimal bracing to reduce wall thickness).

Hm, the outer skin area would stay the same, but the bases would lose some area due to the hole in the center, and then you would have to add the inner diameter. Might have to do some math to figure out what the relationship is.

[lpgagnon]

Oh yeah, I would love for my 10m First Stage to have a 10t interstage because "hollow mass" is now the same as "filled mass"

Complaining that misusing a procstructural as an insterstage might become too heavy instead of too light doesn't seem like a convincing argument, but this hints at a real issue:

Now that fairings have a mass progression, procStructural stands out as the stuck-at-1951-mass odd one out.

[Clayell]

Hm, the outer skin area would stay the same, but the bases would lose some area due to the hole in the center, and then you would have to add the inner diameter. Might have to do some math to figure out what the relationship is.

After testing around with some stuff, I believe I've found the relationship:
(d1 and d2 are the diameter of the outer and inner circle respectively, h is height)

When d1 = h and d2 = 0 or d1, the surface area will be the same. (the surface area will increase and then decrease equally as d2 increases)
When d1 > h, the surface area will be higher when d2 = 0 than d2 = d1 (surface area will increase and then decrease dramatically as d2 increases)
When d1 < h, the surface area will be lower when d2 = 0 than d2 = d1 (surface area will constantly increase as d2 increases)

So if we assume a normal hollow structure cylinder is longer than half its diameter, (which makes sense for most tanks, as even a diameter of 2 and length of 1.05 is a bit silly but still applies to this) then the surface area will increase as the inner diameter of the cylinder increases. This would lead to more material needing to cover the tank, which would make it heavier.

The relevant equation is this desmos link if interested.

Now I'm interested about cones...

[lpgagnon]

I think you may be missing one point here. When talking about structural parts (not tanks), the non-hollow mass is, as Kirk said, "already in the skin". The part mass is balanced such that a non-hollow cylinder is pretending to be hollow; there is no base area to consider, and the inner area is already taken into account

Your math (which I haven't checked) would apply to tanks, but tanks already get weird simplified math that basically assumes a sphere

[Capkirk123]

Yes, for all intents and purposes, the inner walls and ends of a hollow, structural component are tinfoil. They contribute effectively nothing to the part mass. All of the mass is in the outer wall, and any structure attached to that outer wall, as a monocoque structure.

Tank mass being a function of surface area, rather than just volume, would be nice, but I'm currently just trying to prevent the absurdly low mass of very thin hollow parts.

[Clayell]

Yes, for all intents and purposes, the inner walls and ends of a hollow, structural component are tinfoil. They contribute effectively nothing to the part mass. All of the mass is in the outer wall, and any structure attached to that outer wall, as a monocoque structure.

Tank mass being a function of surface area, rather than just volume, would be nice, but I'm currently just trying to prevent the absurdly low mass of very thin hollow parts.

Ah, I didn't realize that the mass was a function of volume. For that case, I agree that setting the hollow mass the same as the non-hollow mass would work well enough.