Starships - Mass vs Volume

[atpollard]

Moved to avoid excessive topic drift ...

Actually, it says (in MT) that it requires Striker/MT AV40 for micrometoroid and radiation protection. Even in Aluminum, that's still going to be a significant chunk of mass - 33cm basic steel equivalent - 66cm of aluminum at .8x the mass of steel is about 1.6x the mass of steel, and twice the volume. (BSD is listed as 14x the strength, but 15x the density, so 15/14 the overall mass.

You want lighter spacecraft? You reduce the armor ratings required. (Disposables like current launchers only need AV8... 2cm equivalent..

And there's the error in the design system. It is, however, consistent in all editions...

The masses of the equipment are reasonable in MT and TNE, and match closely with those of other design systems and with density calcs I've done back of the envelope on autmobiles, busses, and trains (done back in the 90's).

So the only thing which is unrealistic is the armor required. A scout courier, for example, has a base armor mass for 1cm of 40Mg. Aluminum hull for the same strength would be 64Mg. Times thickness. BSD would be 15/14 (about x1.07).

Are you sure about the figures for Aluminum?

I read that for (water) ship hulls, the aluminum plating needed to replace a steel hull would be 1.5 times as thick (so a 1 cm steel plate would need to be 1.5 cm of aluminum plate to achieve the same strength) but that the total weight of the aluminum hull would be half the weight of the steel hull (if the 1 cm steel hull weighed 1000 kg, then the 1.5 cm aluminum hull would weigh 500 kg).

In contrast, your values indicate that an aluminum craft would be heavier than an equivalent steel craft (64 vs 40), which seems contrary to the automobile industry, modern warships, APCs, and the Apollo rockets/space shuttle which all selected aluminum over steel to save weight.

 

[aramis]

No, I'm not.. checking the sg, it's 2.5-2.8, vs steel's 7-8 (by alloy), so i've misread a table. 2x volume and 1/3 weight, that puts it at about .75 equivalent mass, but still at a huge volume penalty. (MT, sadly, ignores volume). (Yeah, I read the table wrong; It was from my crib notes of WWFS... and so I forgot I'd already integrated volume and SG into the columns for use.)

I AM sure the armor hardness of aluminum is half that of steel. It's structual value is better than that, but resistance to impact isn't. And that is what the minimum is set for: armor values. (And FF&S and MT make no distinctions between structural values and armor values, as a simplification.)

Still, the major component mass wise breaks down to:
Drives
Hull Armoring
Cargo.

Giving the drives a mass of 2Mg/m² is pretty consistent with naval drives, where the drive itself is typically 1/3 to 2/3 the actual bay volume, and has specific gravities in the 3-8 range.

Drives comprise a major factor in mass; in Bk2/MGT, a type S has 16Td of drives. That's 224 cubic meters; that's about 450 tons. That's about 1/2 the unladen mass, right there.

a 1cm thick slab of steel masses 70 to 80 kg/m²; a 1400m² has a cubic surface area of about 751m². that's 7.51m³. Given aluminum doubling, that would be 14 cubic meters at SG 2.2 means 28 Tons. Now, keep in mind the 33x for AV40. 928 tons. Instead of the 600 or so of the canon design.

It's the armor value that is the problem. Traveller presumes THICK armoring. (Hulls and bulkheads are described as 10cm thick, for example.) It's not even the materials being bad; it's just the requisite armor thicknesses. (The armor on the shuttle is 3cm of fabric, or 4cm of ceramic, depending on where, plus a millimeter or two of aluminum.)

But, given the design paradigms, yes, nominal weights are 5-10E6 g/m², with outliers of 2 and 20, unladen.

 

[Andrew Boulton]

Loaded mass

.              MT     TNE
Scout/Courier   918    698
Free Trader     2280  1918
Sub Merch       4840  3906
SDB            11150  6075

 

[BillDowns]

Aluminum armor. That's, hmmm, an interesting thought.

I do feel compelled to point out that the use of aluminum in naval warships was curtailed back around the time of the Falklands war. Not because of supposed deficiencies with regard to attacks, but because the 15 or so years of experience pointed the lack of strength to resist hull warping in high seas, and reduced resistance to fires. The Belknap's aluminum superstructure was blamed for her major fire after colliding with the John F. Kennedy.

OTOH, a small cruiser colliding with a carrier is going to lose anyway.

 

[atpollard]

Yeah, the fact that Aluminum is the main ingredient in many (solid) rocket fuels tells you something about aluminum and fire control.

 

[BlackBat242]

Powdered aluminum is, but we are talking great hunks of solid aluminum.

The main problem with aluminum and fires is not burning or exploding, but melting.

Of course, before the aluminum melts, all personnel, equipment, and stores in the area have been destroyed by the heat, so its kind of a moot point.

Fatigue cracking, corrosion, and the like are the main reasons aluminum has fallen out of favor in warship construction.


Or has it?

One of the two competing designs for the USN's new LCS (Littoral Combat Ship) is a trimaran... with an aluminum hull.

One (USS Independence LCS-2) has been delivered to the USN, and is scheduled for commissioning this saturday (16 Jan 2010), and the second (USS Coronado LCS 4) recently began construction. They are being built by the USA division of Austal, an Australian ship-builder.

 

[far-trader]

...Fatigue cracking, corrosion, and the like are the main reasons aluminum has fallen out of favor in warship construction.

I thought I'd read it was cost or availability? At one time anyway. Or I could be mistaken.

One of the two competing designs for the USN's new LCS (Littoral Combat Ship) is a trimaran... with an aluminum hull.

Part of that is a stealth factor isn't it? Aluminum being more radar absorbant (or transparent?) than steel. Or is that another bit of foggy memory?

 

[BillDowns]

Fatigue cracking, corrosion, and the like are the main reasons aluminum has fallen out of favor in warship construction.

This is what I have read, but I also haven't made a thorough study, either.

One of the two competing designs for the USN's new LCS (Littoral Combat Ship) is a trimaran... with an aluminum hull.

This makes me wonder if the reason for the fatigue factor might not be galvanic corrosion. THe Belknap had a steel hull and aluminum superstructure. Perhaps an all aluminum hull and superstructure will provide different long-term results.

I do know that aluminum welding costs more than the usual steel welding, although this cost is continuing to decrease. And, of course, maritime aluminum costs around 6 times the cost of steel.

IDK

 

[Shonner]

The main problem with aluminum and fires is not burning or exploding, but melting.

And evaporating. Only concrete foundations remain when such structures go up in smoke.

 

[Cryton]

Could be it has something to do with the supposes aluminum/Alzheimers connection...

But I forget...