Spacecraft as homes [Spaceships]

[The_Ryujin]

Originally Posted by Fred Brackin

Sorry if you cleared this up somewhere else but I couldn't follow your process. The thing is that "normal" in Spaceships is 1 system of Armor per segment or 3 per ship. To get by with just 1 system ypu'd have to be using a smaller systems rule.

Again sorry if I just ddin't read you right but I really ciouldn't follow what you were doing.

Originally Posted by Rupert

That's why they divide the area by three - to account for an armour system only covering 1/3rd of the ship.

Pretty much this. In my post I pointed out:

Originally Posted by The_Ryujin

Ok, now as you suggested it's a pretty good guess that TL7 Armor, Steel is RHA with a DR of 70 and weight multiplier of 0.56 and one system of Armor, Steel gives DR 10 on a SM +5 ship. IF we divide the ships surface area by 3 we get ~478ft^3 per ship section. Multiply that by 10 × 0.56 and we find that enough steel to give one section of the ship DR 10 weights ~2,676lbs which is right in the neighborhood of the needed 3,000lbs per system (in fact you get even closer if you round the radius of the ship up to 5ft even).

Though to be fair, I had a lot of pointy math flying about all willy-nilly and probably shouldn't of assumed that everyone realized how surface area was handled when it came to armor and cleared it up better.

So when it comes to armor, if you want the whole ship to considered armored across it's full hull it needs to have at lest 15% of it's mass used up by armor systems (one in each section of the ship, the front, center, and read systems).

[AlexanderHowl]

Well, 8mm steel was considered sufficient for the old Sea Dragon design for a reusable spacecraft (it is also used for submarines), so I wonder we are underestimating the protective power of a given thickness of armor. If we assume 140,000 cubic meters for a SM+10 spacecraft, that ends up with a cylinder 90 meters long and 22 meters in radius (a 2:1 ratio of length to diameter). The area is 15,700 square meters which, if we assume 8mm armor, would end up with 126 cubic meters of armor. That translates to 1,000 metric tons of steel. With three sections, we end up with 1500 metric tons of 12mm of steel.

Now, the economics are interesting. Steel costs around $800 per metric ton in 2019, which translates to $330 per metric ton in GURPS. That means that the materials cost for three sections of steel armor at SM+10 would be around $500,000 in GURPS. The cost of three sections of steel armor at SM+10 is actually $6 million in GURPS, meaning that the material cost should be $1.2 million, suggesting that the armor of the spacecraft is actually a much better grade of steel than normal steel (a molybdenum or tungsten alloy perhaps). Any difference in performance can therefore be attributed to better qualities of steel rather than a smaller spacecraft that could not hold the required reaction mass or the required people.

Another thing to consider is passenger seats. If a component at SM+10 is 7,000 cubic meters and if 80% of the passenger seats is taken up by life support, seats, structure, and overhead compartments, that would leave 2.33 cubic meters per passenger in personal and common space. If we assume that 50% of the volume is common space, that leaves 1.17 cubic meters per passenger. That would be a typical space for an economy seat (luxury seats should probably be considered two normal seats).

[Rupert]

Originally Posted by AlexanderHowl

Well, 8mm steel was considered sufficient for the old Sea Dragon design for a reusable spacecraft (it is also used for submarines), so I wonder we are underestimating the protective power of a given thickness of armor.

We know that 8mm of RHA or equivalent steel gives DR22. For one things, it's the set point for measuring DR. For another, it matches real-world performance. Now, whether the scaling (both when you move away from bullets and bullet-like attacks, and when you move away from sub-inch thicknesses) is correct is another matter, though it's not far wrong for heavy naval shells vs heavy naval armour.

Note that 8mm thickness steel is rather thin for civilian cargo vessels, with 0.5" to 1" being common.

Another thing to consider is passenger seats. If a component at SM+10 is 7,000 cubic meters and if 80% of the passenger seats is taken up by life support, seats, structure, and overhead compartments, that would leave 2.33 cubic meters per passenger in personal and common space. If we assume that 50% of the volume is common space, that leaves 1.17 cubic meters per passenger. That would be a typical space for an economy seat (luxury seats should probably be considered two normal seats).

We know from Spoaceships 7 that removing the life support machinery allows double the cabins or seats, and thus that life support is 50% of the tonnage. As the spaces that the humans have to live in are likely to be rather less dense than the life support machinery and water storage I think you are granting way too much volume to habitats and seating, or at least you're putting it in the wrong place.

The thing is, whatever the average volume of a system might be, it's not constant across systems.

[Agemegos]

Originally Posted by Rupert

Note that 8mm thickness steel is rather thin for civilian cargo vessels, with 0.5" to 1" being common.

Note, too, that the hull of a ship is built to support the weight of the ship against the pressure of water, Spaceships don't need a flotation structure.

The thing is, whatever the average volume of a system might be, it's not constant across systems.

Very much. GURPS Spaceships is by design intent a system that rounds things off to the nearest half-order of magnitude. 70% larger or heavier? 40% smaller or lighter? I didn't notice.