Materials Science limit on ship size?

[Quint]

I seem to recall a thread or portion of a thread where someone did the science to figure out the actual size limits on ships based on engineering limits, etc. Am I imagining this thread or does someone have it bookmarked? I tried to search for it but am not having any luck.

D.

 

[atpollard]

I seem to recall a thread or portion of a thread where someone did the science to figure out the actual size limits on ships based on engineering limits, etc. Am I imagining this thread or does someone have it bookmarked? I tried to search for it but am not having any luck.

D.

There are empirical rules of thumb for stability like length = 20 x width as a maximum slenderness to avoid buckling, but think about the size of some of the cast iron structures of the 19th century and any materials limits are going to be beyond ship sized ... millions of dTons even at TL 4.

 

[aramis]

I seem to recall a thread or portion of a thread where someone did the science to figure out the actual size limits on ships based on engineering limits, etc. Am I imagining this thread or does someone have it bookmarked? I tried to search for it but am not having any luck.

D.

Those are based upon monocoque hulls. Chris Thrash did the math; it's about 10KTd for minimum armor.

Note that the implications in Bk2 and Bk5, due to not internal structure tonnage losses, are that the designs are in fact monocoque hulls, or close enough to it.

 

[atpollard]

Those are based upon monocoque hulls. Chris Thrash did the math; it's about 10KTd for minimum armor.

Note that the implications in Bk2 and Bk5, due to not internal structure tonnage losses, are that the designs are in fact monocoque hulls, or close enough to it.

Did he model it as a pressurized container?
I would have thought the figures should be higher.

 

[aramis]

Did he model it as a pressurized container?
I would have thought the figures should be higher.

remember that a monocoque suffers badly from the square cube-law, as strength is a factor of cross section... and goes up with tonnage as a 2/3 power function. Which is why 727 and similar don't use them, instead having frame built...

I don't know if he factored in pressure, but note that the issue is acceleration mass vs cross-section of materials.

 

[AnotherDilbert]

Note that the implications in Bk2 and Bk5, due to not internal structure tonnage losses, are that the designs are in fact monocoque hulls, or close enough to it.

Not necessarily.

Armor: Hulls may be armored with strengthened exterior skins and interior bracing.

The hull size and armour formulae are extremely simplified. They don't say "A 1000 dT ship is exactly 14000 m³", they say "A 1000 dT ship has 14000 m³ usable interior space", however that is realised.

Or so I have always believed.

 

[Timerover51]

There are empirical rules of thumb for stability like length = 20 x width as a maximum slenderness to avoid buckling, but think about the size of some of the cast iron structures of the 19th century and any materials limits are going to be beyond ship sized ... millions of dTons even at TL 4.

Cast iron was not used in construction during the 19th century, wrought iron was. And are you using construction beams for the basis of the 20-1 ratio?

Is the original poster thinking of what can be safely built on an Earth-type planet where the construction has to bear a 1G load? Is the constructed ship fully supported throughout it length and beam during construction, and will always be landing where the hull is fully supported?

The current Lake cargo ships at 1000 feet long, with a length to beam to hull depth ratio of 10 X 1 X 0.5 is close to the limits for a steel-hull framed ship of that type. The very large cruise ships are running around a 7 to 1 hull to beam ratio, with hull depth from the keel to the top continuous deck about hall of the beam. I should also note that I have served on the Marine Forensic Panel analyzing why a ship has sunk when no survivors are present.

If you are operating a streamlined ship on a 1G or higher planet, you would also have to take into account where the ship can be safely landed with sufficient ground surface strength to support the hull. Basically, you will need support for the entire hull, not necessarily continuous support, but what a large ship today has for keel blocks when they are dry-docked.

It should be noted that I am a "small ship" universe person, and view the practical limits on ship size to be circa 10,000 Traveller dTons. I am probably going to use the 1977 Book 2 rules for ship construction for an alternate Traveller sector that I am working on.

 

[atpollard]

Slenderness ratio (20:1) was for loaded columns to prevent failure in buckling. It was a quick and dirty rule for preliminary estimates in Architecture School to keep us out of trouble with a 4" diameter x 100' tall column. It also works for beams to avoid that magic slab that will span 100' unsupported with only 1' of structural thickness. Ultimately, it placed you in the ballpark for wood, steel and concrete structures, so it was easy and handy to use for deckplans and starship sections. The load on a column could be far greater than 6x the dead load of the column.

 

[tjoneslo]

Those are based upon monocoque hulls. Chris Thrash did the math; it's about 10KTd for minimum armor.

Note that the implications in Bk2 and Bk5, due to not internal structure tonnage losses, are that the designs are in fact monocoque hulls, or close enough to it.

http://www.freelancetraveller.com/features/shipyard/hulls1.html

https://www.prismnet.com/~thrash/hulls.html

Here is Chris' article in full if you want the math.

 

[Timerover51]

Slenderness ratio (20:1) was for loaded columns to prevent failure in buckling. It was a quick and dirty rule for preliminary estimates in Architecture School to keep us out of trouble with a 4" diameter x 100' tall column. It also works for beams to avoid that magic slab that will span 100' unsupported with only 1' of structural thickness. Ultimately, it placed you in the ballpark for wood, steel and concrete structures, so it was easy and handy to use for deckplans and starship sections. The load on a column could be far greater than 6x the dead load of the column.

Thank you, I thought that the 20 to 1 ratio came from structural beams.

 

[atpollard]

Thank you, I thought that the 20 to 1 ratio came from structural beams.

It applies to them as well. Beyond 20 to 1 it begins to twist and fails in some uncommon buckling regimen. I remember being told it was related to geometry and was generally material independent. So it should still apply no matter how strong future wonder materials become.

 

[Travellerspud]

here is his conclusion;

"To the extent that my assumptions are valid, very large (1Mdton) vessels are physically possible and practical at high TLs, so long as care is taken with acceleration and density."

 

[Brandon C]

here is his conclusion;

"To the extent that my assumptions are valid, very large (1Mdton) vessels are physically possible and practical at high TLs, so long as care is taken with acceleration and density."

This is why I decided to cut populations as a way to reduce ship size. Less money means less tonnage, and more ships are more useful than large ships (say, ten 5000 ton warships instead of one 50000 ton warship) for a given tonnage budget.

 

[aramis]

here is his conclusion;

"To the extent that my assumptions are valid, very large (1Mdton) vessels are physically possible and practical at high TLs, so long as care is taken with acceleration and density."

That care includes distributed thrust centers...

 

[Quint]

Many thanks!

No wonder I couldn't find it here on COTI - I didn't read it here...

D.

 

[Whipsnade]

No wonder I couldn't find it here on COTI - I didn't read it here...

It might have been here at one time, Chris "tidied up" when he left.

I very much seem to recall reading it here or reading a copy here.

 

[boomslang]

It might have been here at one time, Chris "tidied up" when he left.

I very much seem to recall reading it here or reading a copy here.

I have it saved offline as a text file, but he did put a copyright on it, so uploading the whole thing to CotI without express consent might exceed "fair use" as such.

On the other hand, there is always this online version, which relies upon certain admitted assumptions about the feasibility of bonded superdense armor (e.g., ignoring what happens when the power inevitably fails with catastrophic results) as well as the infeasibility of acceleration compensation when using gravitic drives (relying upon internal structural bracing instead).

The basic math still holds up in terms of such hull structures being potentially unable to support themselves when grounded, however.

It is worth mentioning that the version online at FT uses notably different methodologies and comes to a notably different conclusion than the (later?) version I have offline; namely, it all comes down to assumptions that inform how much displacement is dedicated to structural integrity -- reinforced designs that devote a lot of displacement toward not shattering can be built much larger than ordinary hulls that are intended to carry relatively significant percentages of actual payload around.

Here is the largest fair use except from my archival version that I am comfortable posting:

Well, now we know. To the extent that my assumptions are valid, if all vessels 
(whether armored or not) are restricted by engineering practice and naval 
architectural standards to a size where 4% of the vessel's volume is 
adequate to sustain 6g nominal acceleration, then their maximum displacement 
will be 5,000 dtons (~7,000 dtons for buffered planetoids).  Larger vessels 
and space structures are feasible, but only at the cost of greater fragility 
(real or perceived) and restricted acceleration.

Using the alternate assumption that armor material counts towards structural
requirements, very large (1Mdton) armored vessels are phyically possible and 
practical at high TLs -- though unarmored vessels should realistically still 
be restricted to 5,000 dtons.

FWIW, IMTU I strictly limit vessels to no more than 10Kdtons (using the CT Alien Module 2 K'kree starship rules for "special circumstances" outside normal B2 architectures), and anything larger is inevitably a space station that has to be very, very gently towed if it is going be moved anywhere.

 

[Whipsnade]

Thank you for all that, Boomslang.

I'm sure it will help Quint a great deal.

 

[Timerover51]

I have it saved offline as a text file, but he did put a copyright on it, so uploading the whole thing to CotI without express consent might exceed "fair use" as such.

On the other hand, there is always this online version, which relies upon certain admitted assumptions about the feasibility of bonded superdense armor (e.g., ignoring what happens when the power inevitably fails with catastrophic results) as well as the infeasibility of acceleration compensation when using gravitic drives (relying upon internal structural bracing instead).

The basic math still holds up in terms of such hull structures being potentially unable to support themselves when grounded, however.

It is worth mentioning that the version online at FT uses notably different methodologies and comes to a notably different conclusion than the (later?) version I have offline; namely, it all comes down to assumptions that inform how much displacement is dedicated to structural integrity -- reinforced designs that devote a lot of displacement toward not shattering can be built much larger than ordinary hulls that are intended to carry relatively significant percentages of actual payload around.

Here is the largest fair use except from my archival version that I am comfortable posting:

Well, now we know. To the extent that my assumptions are valid, if all vessels 
(whether armored or not) are restricted by engineering practice and naval 
architectural standards to a size where 4% of the vessel's volume is 
adequate to sustain 6g nominal acceleration, then their maximum displacement 
will be 5,000 dtons (~7,000 dtons for buffered planetoids).  Larger vessels 
and space structures are feasible, but only at the cost of greater fragility 
(real or perceived) and restricted acceleration.

Using the alternate assumption that armor material counts towards structural
requirements, very large (1Mdton) armored vessels are phyically possible and 
practical at high TLs -- though unarmored vessels should realistically still 
be restricted to 5,000 dtons.

FWIW, IMTU I strictly limit vessels to no more than 10Kdtons (using the CT Alien Module 2 K'kree starship rules for "special circumstances" outside normal B2 architectures), and anything larger is inevitably a space station that has to be very, very gently towed if it is going be moved anywhere.

Was the person quoted talking strictly about starships? Five thousand Traveller dTons equates to about 70,000 wet displacement tons, and there are a lot of ships considerably larger than 70,000 wet displacement tons in operation. They have to deal with some fairly severe acceleration loads from water operation.

 

[boomslang]

Was the person quoted talking strictly about starships?

Starships and non-starships, under G-loading.

There is a fair amount of math, and it is significantly different between the two versions in the links tjoneslo gives a few posts back.

Those two versions do have distinctly-different explorations of all the gritty details; the second linked version is effectively the same as the version I saved offline. (Christopher Thrash didn't necessarily date each and every one of the revisions as it evolved over time, so there is some guesswork required as to which version and its assumptions comes after which, and no version is necessarily definitive without his say-so.)

But math is math and the assumptions are explicit, so it's all useful, one way or another. To your point, water exerts plenty of force, but that loading is of a different nature than free acceleration is. The buoyant Empress of the Seas does not blast away from the dock at 6Gs, even though there is massive horsepower pushing against massive resistance. By contrast, in space there is negligible resistance, and the structure's own inertia is the dominant obstacle.