T5.09 starships - air versus water pressure

[dalthor]

I've been wrestling with starship hulls, armor, and pressure. It started out as a simple comparison. How deep could a submergence hull go in water, and how would that equate to depth into a gas giant for refueling purposes?

I would expect that air pressure is easier to resist, but likely more penetrative. A starship hull should be fine against that penetration.

See t5.09 page 253 for a pressure table, along with the damage water pressure causes. I haven't found a water-pressure to air-pressure equivalent table yet.

Page 277 under the submergence hull states that the ability to resist pressure, based on armor, is doubled. This implies that armor itself, including the basic hull, resists pressure.

Page 278 then goes on to discuss the offers of damage on armor - hits in excess of armor means the armor has been penetrated -- or crushed in the instance or pressure.

The interesting thing is that at 1000 meters, water pressure does 500 dice of damage per minute. A frame and plate TL-15 ship has a default of 15 points of armor in general. A submergence hull could handle double that, or 30 dice. This is the equivalent of less than 500 meters of water based on the depths table on page 253.

A modern submarine can do that, so why can't a ship, especially at that tech level, where the armor protection should be pretty dang good?

Something doesn't seem right in the translation and I'm daggoned if I can figure out the disparity.

HELP, please!

 

[Blue Ghost]

Wow. This is a ---> :CoW:

The reason is because in the old CT adventure a ship cold withstand really extraordinary pressures. But I argued against it to something more akin to what you're quoting from the new T5 rule set.

One of the arguments tossed around was that starships / spaeships, are designed like aircraft. The pressure within the vehicle presses on the seal to keep out "bad air' or to seal off the hull from vacuum.

How that design fares in an underwater environment I don't know, but I wouldn't think it would NOT do too well.

I did a search for the old thread, but stopped after reviewing 10 pages of my own archive.

 

[SanDragon]

Page 290 also says hulls built to withstand pressure double the armor's value versus pressure.

But on page 293 on Table C, if I'm reading it correctly, an anti-layer versus pressure has a multiplier of x10!

Figure that one out!

 

[tjoneslo]

I've been wrestling with starship hulls, armor, and pressure. It started out as a simple comparison. How deep could a submergence hull go in water, and how would that equate to depth into a gas giant for refueling purposes?

I would expect that air pressure is easier to resist, but likely more penetrative. A starship hull should be fine against that penetration.

This is wrong. Pressure is pressure. Especially at the level of abstraction given by the T5 rules (indeed all of the traveler rules). I might understand an argument about Hydrogen atmosphere (given the very small size of the hydrogen atoms and their ability to find interstitial space in armor) vs any other kind.

But, in general, I really wouldn't try and take those two apart.

See t5.09 page 253 for a pressure table, along with the damage water pressure causes. I haven't found a water-pressure to air-pressure equivalent table yet.

Page 277 under the submergence hull states that the ability to resist pressure, based on armor, is doubled. This implies that armor itself, including the basic hull, resists pressure.

Page 278 then goes on to discuss the offers of damage on armor - hits in excess of armor means the armor has been penetrated -- or crushed in the instance or pressure.

The interesting thing is that at 1000 meters, water pressure does 500 dice of damage per minute. A frame and plate TL-15 ship has a default of 15 points of armor in general. A submergence hull could handle double that, or 30 dice. This is the equivalent of less than 500 meters of water based on the depths table on page 253.

I have also looked at this as well. And decided this was one of the serious breakdown points of the Vehicle build rules.

While I don't completely understand how the atmospheric pressure translates to Dice of damage, the ratio of increasing damage to depth looks correct.

One thing I got from playtesting GURPS: Atlantis was hull shape has as much or more importance on being able to resist damage from pressure as the amount of armor on the hull. A major reason the modern TL8 submarine can reach the depths it can, it because the pressure hull inside is specifically designed to take the pressure. The armor rating of the hull is much less than a modern tank.

Since the T5 Vehicle design system does not have any hull shape modifiers for armor, the fact you can't build a good submarine isn't surprising. The specific point of breakdown here is hull shape/design for resisting pressure is missing from the T5 rules.

 

[dalthor]

I've come up with the following WAG for now.

Submarines, being specially designed for an aquatic environment, can withstand water pressure at one range band per TL starting at TL-3.

A standard starship can withstand atmospheric pressure at one range band per TL. It is not water resistant.

Note that for starship hulls, they'll leak like a seive LONG before they crush - AV is pretty good for the strength of the hull. I assume that leakage will be around hull openings like hatches. Internal bulkheads, if sealed, will be air- and water-tight until crush depth.

A starship with a flotation hull should not submerge persay, but the hull is water resistant to TL * 10 meters. For example, a TL-15 frame and plate flotation hull could be submersed in 150 meters of water with worrying about leaks. Crush depth is TL*200 meters.

A starship with a submergence hull can be immersed in TL*100 meters of water without worrying about leakage. Crush depth is TL * 200 meters.

Dunno how viable this is so commentary is welcome.

 

[tjoneslo]

I've come up with the following WAG for now.

Submarines, being specially designed for an aquatic environment, can withstand water pressure at one range band per TL starting at TL-3.

A standard starship can withstand atmospheric pressure at one range band per TL. It is not water resistant.

Note that for starship hulls, they'll leak like a sieve LONG before they crush - AV is pretty good for the strength of the hull. I assume that leakage will be around hull openings like hatches. Internal bulkheads, if sealed, will be air- and water-tight until crush depth.

Each 10m of water depth is 1 atmosphere of pressure. It states this at the bottom of the table on p 253.

If it leaks water, it leaks air. Neither is more penetrative than the other. The hull of a starship (or vehicle) will not distinguish between water and air at pressure, and neither should the rules.

A starship with a flotation hull should not submerge per say, but the hull is water resistant to TL * 10 meters. For example, a TL-15 frame and plate flotation hull could be submersed in 150 meters of water with worrying about leaks. Crush depth is TL*200 meters.

A starship with a submergence hull can be immersed in TL*100 meters of water without worrying about leakage. Crush depth is TL * 200 meters.

Dunno how viable this is so commentary is welcome.

The ArmorMaker rules (p225) distinguish between armor (to resist some kinds of damage) and Sealed (to resist pressure or vacuum). These rules are extended to the VehicleMaker, but not the Starship rules. If you think it is important to distinguish between hull crush depth and seals breaking, start with separating these items.

Environmental effect table (p 196) indicates that vacuum inflicts Suffocate-3 damage (3D) , requiring Sealed-10 resist (generally).

I'm going to assume that the Sealed rating of a starship hull is equal to the Armor rating of the hull. This seems to be the implication by omission.

Reading the rules on p277-278, and p293 on armor:

A Starship comes with one layer of armor "for free". Each layer of armor provides AV = TL, and if you have multiple layers of the same armor, the values stack. Each additional layer takes 4% of the ships volume. Presumably this is internal bracing, pressure doors, and the like as well as a thickness of metal or plastic. A submergence hull doubles this value for depth calculations.

Each pressure-1 inflicts 1D damage, meaning you need AV=3 or 4 to resist it, and 6 or more to ignore it. So the latter becomes the "safe maximum depth", and the AV=3.5 becomes the "crush depth" for the vehicle. If you are separating the AV and the Sealed (as with armor or vehicles) use the lower of the two values.

A default armored hull at TL-9 will be sealed to pressure 1.5, (about 15 meters), and a "crush depth" of pressure 2.5 (about 25 meters). A submersible hull gets us to pressure 3 (about 30 meters), and a crush depth of 5 (50 meters).

A default armored hull at TL-15 will be safe to a pressure 2.5 (25 meters) and
a crush pressure of 4 (about 40 meters). A submersible hull doubles these to 5 (50 meters) and 8 (80 meters).

If we decide to add a lot of armor to the hull (12 layers = 48% of the hull), these values become at TL-9 become AV=117 (13 layers * 9) or pressure 19 (190 meters) or submersible hull to pressure 39 (390 meters).

At TL-15 these values are AV=210 for pressure 35 (350 meters) or submersible hull to pressure 70 (700 meters).

to a safe depth of 4 atmospheres (about 40 meters) and a risky depth of 9 atmosphere (about 90 meters).

As SanDragon point's out you can also convert one (or more) layers of armor into "Anti-layers" (p. 278).

If I read this right, you can convert the 12 layers of armor (as above) to "Blast armor anti-layer" which provides x10 protection against pressure. If that's right the TL-9 ship has 12 layers of AV=9 anti-armor, or 12 (layers) * 9 (AV) * 10 (multiplier) = AV=1080 which will resist pressure 180 or 1800 meters depth. With a submersible hull, this is doubled to 360 or 3600 meters.

At TL-15 these values are AV=1800 which is a safe pressure 300, or 3000 meters depth. With a submersible hull this is pressure 600, or 6000 meters depth.

This is Rules as Written, and interpreted by me.

 

[Hemdian]

Each 10m of water depth is 1 atmosphere of pressure.

Isn't that assuming being on a 1G planet with 1 atmosphere of air pressure at sea level? (Actually, I think the depth table gives "pressure in addition to air pressure at sea level" ... but that still leaves the gravity component.)


.

 

[dalthor]

Each pressure-1 inflicts 1D damage, meaning you need AV=3 or 4 to resist it, and 6 or more to ignore it. So the latter becomes the "safe maximum depth", and the AV=3.5 becomes the "crush depth" for the vehicle.

And therein lies the rub, and the reason I am looking for help. All the RAW apply to 1g worlds. How does higher or lower gravity affect the numbers?

Lets take a look at some other factors, based on more reading...

Per page 246, a protected vehicle has minimum AV=12, and Sealed=20. BTW, Sealed-20 is the standard level provided.

Now look at page 259, table W, the entry for a watercraft - a sub. At TL-6 it lists armor-20. This means a submarine can, at TL-6, submerge to 30 meters without risk of crush. Now, to reach armor 20 per the RAW, there are about 3 layers of TL-6 armor on a submarine hull. I'm assuming these are standard layers rather than an anti-layer of some soft.

SO, how does vehicle armor compare to starship armor? Aren't they one and the same in the RAW?

Oh, and FWIW, IMTU the first layer of ANY starship hull is, by default, anti-blast. It would be stupid to have any standard armor layer as the first layer, when for the same price I can have a significant base defense against at least one attack type. Also, by default, IMTU every starship has one coating, too. It is also free, and I usually default that to Reflec. I'll ignore coatings from here out, since they aren't germaine to this discussion.

Not only that, but BY DEFAULT (see armor table C, page 293) when it comes to pressure, ANY type of non-organic anti-armor is AUTOMATICALLY x 10 protection versus pressure. Also, any non-organic or polymer hull is x 100 versus HEAT.

Since most of my TL-xx starships have blast plate or charged plate as the armor type, and that outer layer is anti-blast unless otherwise specified, I have AV=TL*10 versus blast/bullet/frag, AV=TL*10 versus heat, AV=TL*10 versus pressure as well. Charged plate doubles the value, since charged plate is AV=TL*2 protection.

Make it a submergence hull, and the protection is doubled against pressure.

OK, so a TL-15 charged-plate anti-blast submergence hull is TL * 2 * 10 * 2 protection against pressure, or AV=600. This is 100 atmospheres protection with no chance of leakage or crush. (This assumes the maximum roll of 6 on all 100 dice). This equates to more than 1000 meters depth in water - I suppose that isn't too bad for a starship, and this is only one layer of armor.

Still need to figure out how that'll be affected by gravity other than 1g.

Need to do all the calculations for several TL's, and try and equate TL to range band when it comes to starships and submergence, for all hull types, and for both armor and anti-armor.

Sounds like a computer program to write.

Regardless of the case, IMO the TL should have quite a bit more impact on the pressure any hull can withstand.

Ran out of CrImps...

 

[aramis]

Thom, while your statement, "If it leaks water, it leaks air" is a truism, it's not actually true.

It's mostly true for solids...
but many meshes can hold air under water that won't hold water in air. Including US Navy Wool Dress trousers, specifically the ones for CPO/Cadet uniforms in the 80's. They will let water into the mesh, but will hold air when underwater. (This is something we NJROTC cadets were not entirely thrilled with, as it meant jumping into the school pool in Winter Working Blues with temporary authorized swim trunks.)

A number of other fabrics do likewise.
Some even have one-way flow of fluids - these materials are often used in athletic footwear and military rain wear.

A few will let air pass, but not water.

 

[tjoneslo]

Still need to figure out how that'll be affected by gravity other than 1g.

From Wikipedia: https://en.wikipedia.org/wiki/Vertical_pressure_variation

Change in pressure = -1 * density * g * change in height.

So the simple answer is multiply your pressure by the planetary gravity. Seawater is considered an incompressible fluid, so the density does not change with height (or in this case depth).

For gas giant atmospheres, where the atmosphere is more compressible, there is a more complex formula further down the page.

 

[robject]

Page 290 also says hulls built to withstand pressure double the armor's value versus pressure.

But on page 293 on Table C, if I'm reading it correctly, an anti-layer versus pressure has a multiplier of x10!

Figure that one out!

Actually I think p293 has a potential solution to VehicleMaker, which lacks information on how submarines resist pressure. The solution is that "submergence" hulls have an armor multiple against pressure. Using the same multiplier, whether x10 or otherwise, might be nice.

As far as starships go, I would rather stick to the "submergence option" on hulls, than have a specific anti-layer against pressure, unless there's a really really really really really really good reason for it.

...and I suppose there IS a really x 10 good reason for it. Making your ship "submersible" doesn't say how DEEP under water it can go. Hence, armor layers and anti-layers.

 

[Timerover51]

Here is some Real World data.

In 1887, William Hovgaard, who became one of the leading naval architects of the 1900 to 1920 period, designed a steam-powered submarine using 1 inch thick mild steel plating on a 14 foot by 22 foot oblong hull, that would have been capable of going to 300 feet safely. Considering his conservative design standards, and his assumption that mild steel was good for only 10 tons of stress, the sub probably could have gone twice as deep.

US submarines of the Inter-War period used a 16 foot diameter circular hull, made of 5/8 inch mild steel, or about 1.6 centimeters. They had an operational depth of 300 feet, and a probably crush depth of between 450 feet and 600 feet, depending on how you define operational and crush depth. The later WW2 boats, using a 7/8 inch hull of either vanadium alloy steel or high strength steel were good to 400 feet with a probably crush depth of 900 feet. The steel used was HY-45 to HY-50 steel, and still a 16 foot diameter hull. Mild steel would be rated at between HY-30 to HY-35 steel.

On page 640 of the 5.0.9, you have Armor Rating listed by thickness per 1 centimeter. Iron, presumably Wrought Iron is rated at 50, with Steel rated at 70. Mild Steel is rated at about 20% stronger than Wrought Iron plating, so should get a 60 rating. The 70 rating would be for a higher-tensile strength steel, circa HY-45. That would give the pre-war US submarines an armor rating of 1.6 times 60, or 96. The wartime, thick-hull, boats would have a rating of 2.2 times 70, or 154.

Current US submarines have a larger diameter hull made of HY-80 steel, and probably have an operational depth of 1500 feet, with a crush depth of twice that. Note, the pre-war US subs were all of riveted construction, while the wartime boats were of welded construction. Riveted construction was viewed as about 90 per cent as strong as welded construction.

Compared to the thicknesses of tank armor of the WW2 era, the US subs would not have been considered armored to any great degree.

Edit Note: I should add that 1 centimeter of steel is just about 4/10 inch which would equate to 16 pound plate, the standard way of specifying plate thickness in nautical construction. That plate thickness, 16 pounds, was about the same thickness of the plating on the US Navy World War One 4-Pipe Destroyers, or the Royal Navy's World War 2 Flower-class corvettes. In neither case were the ships viewed as armored in any sense of the word. Conversely, the outer hull plating on the Iowa-class Battleships was 60 pound, or 1.5 inches of high-tensile strength steel, which would equate to 3.8 centimeters of at least 80 rating steel, or an Armor rating of 304. That thickness is never counted as part of the side armor protection of the battleships, but should be as it would act as a de-capping plate for Armor-Piercing Capped projectiles. The side armor of the Iowa's was 12 inches of Class A face-hardened plate sloped at 19 degrees to the vertical. Face-hardened plate in World War 1 was rated at 2.6 times the value of Wrought Iron plate, which would give the Iowa-class ships an Armor Rating of 2.6 times 50 times 30.5 centimeters, for an Armor rating of 3,965. That does not take the slope of the Armor into account, but the plate was viewed as equal to 13.5 inches of vertical armor plate. Also, US and British World War 2 face-hardened armor plating was improved over the World War 1 plate.

 

[kilemall]

Just for CT/HG/Striker comparison, Armor-0 hull is rated at 336 mm of hard steel equivalent, and Armor-10 (pretty typical low tech battleship or high tech cruiser stuff) would be 10,800 mm equivalent.

I would expect the SDBs cited as being typical lurk on the ocean floor types to be about in the 5000 mm range.

How sensitive are T5 ships to mass considerations?

I could see ships in general being lighter and below the above old school values to save on all manner of engineering/fuel overhead or gain speed, but the frontier ships would have to be of sterner stuff.

Perhaps part of any 'streamlining' cost should be the assumption of external pressure handling, since it's specifically designed to go into gas giant atmospheres and water as opposed to cheaper more space-oriented hull designs.

 

[aramis]

How sensitive are T5 ships to mass considerations?

Not very. I suspect the drives are a field effect gravity well... not a thruster.