Bits and pieces of combat tech (21C setting)...

SPRAY-ON ABLATIVE ARMOR-This is a popular item with tankers, infantry
men, and others who regularly deal with people and things that wish
them to suffer bodily harm. Various formulations and minor variations
exist, with the proponents and critics, but they are mostly similar in
function and use. The American version is typical, manufactured under
various trade names by du Pont, GE, and Consolidated Chemical (among
others).

In use, this item looks like a largish spray-module, connected to a
hose and a pumping machine, which itself is linked to a larger set of
tanks holding the appropriate feedstocks. The machine is not
particularly heavy (except for the feedstocks), but it is bulky and can
be tricky to use without practice.

When operating, the sprayer emits a stream of a thick, high-pressure
white liquid that can be 'sprayed' across almost any flat surface,
where it immediately begins to harden on contact with air (the process
won't work in the absence of free oxygen in the surrounding atmosphere,
it is a required reactant). It begins to harden fast enough that it
will usually adhere even on a vertical surface.

(Note: the spray is _hot_, and materials unable to survive
temperatures of ~200-300 degrees Fahrenheit are not suitable for the
process. Also, the stream emerges under some pressure, so soft
materials and flexible surfaces are not suitable, the moreso since the
material hardens rapidly into a stiff substance requiring a solid
substrate. The main armor of military vehicles such as tanks is a very
good candidate, however.)

Within an hour or so of application, the liquid-foam will harden into a
solid, tough ablative plastic with a DR of about 20 per inch of the
material. Until that first hour passes, the material will be too soft
to be useful as armor. In hot climes, add an extra 15 minutes per ten
degrees above 80 F before it reaches minimum readiness, it happens
faster in cold air, down to as little as 30 min at 0 degrees F (that's
about as fast as it can happen).

The process won't work at all in air temperatures above +185 F or below
-150 F (but this is rarely a consideration on Earth anyway, and if it
did happen the combatants would probably have other worries anyway).

The substance will continue to strengthen for about another 24 hours,
and if left undisturbed it will reach a maximum strength of about DR 60
per inch of thickness. It will be mostly transparent, looking like
someone laid a layer of clear plastic over the underlying substrate
(which is close to the actual fact of the matter). Non-transparent
versions exist that have slightly greater DR and temperature
resistance, but most users prefer the transparent version for its
greater convenience and because it lets camouflage work through it.

With practice, thicker layers can be put down, up to three inches, and
if properly applied they'll 'harden' into a single layer of ablative
armor with as much as DR 180 or so. Applying extra layers over
hardened armor-foam doesn't work well, the new layers tend to refuse to
bond. This makes a skilled armor-sprayer operator a valued member of a
military unit, since maximizing the value of the spray-armor requires
practice and judgement.

The truly wonderful thing about the spray-armor foam from the point of
view of the users is that it's fairly cheap, and thus extra amor can be
applied to almost any suitable surface and then applied anew when the
former layer is chipped/blasted/burned away. It can be sprayed over
tanks and vehicles, or even onto mounds of hard-packed soil if the
operator has some practice at it, giving extra armor.

Extra armor is a very popular concept among combat personnel.

The spray appartus itself weighs only about 20 lbs, the reactant tanks
vary in size according to the amount of the chemicals they contain.
The whole system operates for up to 100 hours on a built-in recharable
power cell.