Robert S. Dean · 23 July 1991 · reply to #2660
> From: Hugh Schoenemann [email removed]
> Subject: (2660) Plastic ships - flights of fancy ?
>
> Greetings everyone,
>
> This may seem a rather stupid question to some, so please indulge
> my ignorance or the possibility it's been discussed here before.
This answer may look a little strange, because I started at the bottom
and worked my way back toward the top.
> I was thinking the other day about the possibility of using plas-
> tics in the construction of starships, that is the external hull.
> Given the weight ratios between "standard" metal structures and
> their plastic counterparts, would it not be more fuel-efficient
> to use plastics and thus benefit from any weight savings that
> might accrue ?
Well, this is the weak link in the whole thing. If you actually have a
structure which _requires_ the strength of a given piece of steel, replacing
it with 'light weight' materials usually gains you little advantage because
the ratio of the tensile strength to the weight is often equal to or inferior
to steel. Replacing a 100 lb load bearing steel member with a 300 lb
aluminum member to get the same strength is not much help. In the real world
savings can often be made because the extra strenght is not really needed.
Besides, the TL7 and TL9 composite structures may well include plastic
components, so it's already taken into account.
> I then thought to myself, "Well, what would happen upon free-fall
> re-entry ? Fried plastic blob hits deck at G ?" Perhaps not.
> Wouldn't it be possible to shield the ship using good ol' ceramic
> tiles a la Space Shuttle ?
Ah! But with grav vehicles we can take the reentry at any speed we want.
We've got power, thrust and duration to burn, and if you want to hover
over one spot for hours while you ease down into the atmosphere, you can
do it. This aspect of Traveller has been somehwat neglected--as for
example in the rule that still says that an unstreamlined hull cannot
land on a planet with an atmosphere...
> Then the problem of aging occurred to me. Plastics age through
> photo-chemical reactions caused by the interaction of natural
> phenomena (sun, wind, rain, local soil etc.) with the plastic's
> own atomic structure (plastics- or organic chemists - please
> shoot me down in flames if this is a misconception on my part).
> This can be useful, for example when designing bio-degradable
> plastics, but would surely be an undesirable trait in plastics-
> for-space-vehicles purposes. Given that UV light (which clearly
> lends a hand in the aging process) is shielded somewhat by our
> atmosphere (unless you live in Antarctica, that is), wouldn't the
> situation be that much worse in vacuum where no such shielding
> takes place ? I suppose solving this problem might help solve the
> next ...
Well, in the real world plastics which are required to stand out in the
sun for prolonged service will frequently have an additive such as
carbon black added as a UV screen. Between internal additives and external
paints and coatings (we could aluminize the hull for example) this could
be ignored.
> I remember hearing years back that all plastics are, to a greater
> or lesser extent, porous. Is this true ? How could this be com-
> pensated for or, preferably, eliminated when considering plastic
> vehicles ?
Quite true. However, the amount of porosity could be considered to be
insignificant in this context. In the course of my job, I've dealt with
helium leak tests performed on containers made out of 1/4 inch thick
high density polyethylene. Helium is used because of its high ability
to permeate any gap. A properly sealed container retains enough helium
for test purposes for many months--which translates into plenty of time
to get to a starport and get another air bottle.
> Note that I didn't consider strength as a factor in my reasoning.
> Why not ? Well, I reckon that a ship-asteroid collision at some
> of the speeds frequently discussed on the TML will turn both ship
> and contents into so much blackberry jelly no matter what the
> ship's made of. Comments ?
While there is little point in considering structural strength vis-a-vis
asteroid collisions, having a ship which is capable of standing up to your
maximum acceleration would be desirable. I'm not sure if inertial comp-
ensators (which I've always viewed as more grav plates mounted to be able
to cancel the drive gravity effect internally) would help the external
structure of the hull or not. Also, I'm not conversant with radiation
shielding effects of plastic. George Herbert has pointed out that the
best reason for the armor 40 hull minimum is to provide long term radiation
shielding for the crew.
Does that help any?
Rob Dean