FB, 1.7M limit = aircraft speed limit for the small fin - directional stability; very logical. This may also have some relevance to the rolling ‘g’ limits which were tied to the fin size.
ARXW, thanks. I did not read 300kts as a min speed, but if so, then not too bad, but I don’t recall it.
Many of the early test firings were at the ‘edge’ – to define and validate the firing zone.
Yes the RT had a ‘good’ head-on performance against supersonic targets. The design assumed that the homing head would see a reheat plume and navigate on that, the fusing system would sort out the intercept angle and adjust accordingly; the expected threat was Blinder.
In practice, there was a limited head-on capability against kinetically heated targets – mainly supersonic, and also those with exposed jet pipes or hot fuselage vents. I recall one forward hemisphere ‘acquisition’ against a Buccaneer.
Several RTs were fired at MPC on simulated head on profiles (supersonic launch), but the realism was limited by the subsonic Jindivick target. At one time there was talk of using one of the ‘BQM’ (similar) supersonic targets either at Aberporth or at the range just off the Hebrides.
Much of the surmise on speeds is broadly correct. The lightning was at its best between M1.3 and M1.5, and could intercept all but the very highest targets from this speed range (excluding tail intercept / vis ident of Concorde).
From a legal 56,000ft limit (pressure jerkin not the aircraft), Firestreak could be expected to add a further 15,000ft capability depending on launch speed, Red Top even more.