Quote from C_Star:
"I think there would be some loss due to increased drag due to higer AoA required to pull g's and to maintain lower speed afterwards. Any idea how that could affect the height gained?"
Welcome! That may be the nub of the issue. OG implies it above:
"A zoom climb being essentially a pull up manoeuvre one has to consider the 'g' available to execute such a pull up. One might expect the lift curve to be nonlinear up near alphamax, so that is one complication. Ignoring any nonlinearity but allowing for the zero lift AOA, we might expect an available 'g' of about 1.1 at 14.5 deg AOA falling to 1.00 at 17.5 deg."
That's just one of the reasons I'm uneasy about my ski-jump analogy (but thought I'd air it anyway, particularly as it's now the wintersports season).
In the Harrier ski-jump, the vertical acceleration results from the reaction between the vehicle and what amounts
to terra-firma.
The A320 has to generate it by an increase in AoA (and therefore drag),
(I'm going to be simplistic and empirical here. The Harrier ski-jump looks like a gentle, continuous curve, rather than the level-change ramps at a multi-storey car park, but I'm going to describe the latter in the hope that the principle would remain roughly the same.)
Once the rotation of the A320 has been completed and the climb established, the extra lift associated with the increasing AoA can be used to enable a gain of altitude. Well, not much, because it has lost airspeed in the rotation... The airspeed decay-rate for a given thrust (still negligible) has increased, due to the climb angle. As r-r-rat points out, the a/c is seriously on the wrong side of its drag curve...
The Harrier is supported by the ramp throughout, and does not need any lift from its wing. That must reduce the drag considerably.
Once the two a/c have left their respective "ski-jumps" (in the case of the A320, when it reaches alpha-max), they become semi-ballistic (please pardon that expression, but I can't think of a better one). The A320 follows a quasi-parabolic (
) trajectory at alpha-max. The Harrier presumably has to rotate to a suitable AoA, perhaps not far short of the stall, and vector its thrust upwards?
The A320 will return rapidly towards its start-altitude, and the resulting VS will have to be arrested before the treetops are reached, The Harrier, on carrier ops, has the luxury of having the extra height of the carrier deck to play with before it hits the water...