@MichealEmouse: **Raguleader **pretty well nailed it.
Extreme low altitude was an effective tactic in my era, the 1980s. We’d happily drive into enemy territory at 100 feet at 500+ knots. Given any kind of non billiard-flat terrain we were confident nobody could detect us at a distance and even if we flew through a valley that had a SA-4 or -6 in it we’d be over the next ridge before they woke up & got rounds off at us.
To be sure, that exposed us to small arms & un-aimed HMG/autocannon barrage fire. But given the lethality of middle-altitude SAM systems against non-stealth aircraft, low altitude was the least bad option available and was very effective at defeating long range detection, SAMs, and most counter air.
In the intervening years there have been paradoxical developments: Lethality of first-world air defenses has increased dramatically. Absent stealth, if you’re tracked you’re doomed. But meantime the enemy we’re actually fighting or planning to fight is doing good to muster a handful of ratty heavy machineguns. So avoiding barrage small arms fire is job #1; do that and you’re golden.
So against half-assed ragtag IS-style opposition you fly straight and level with impunity in the middle altitudes. But against near-peer adversaries …
Nowadays with effective lookdown-shootdown radars, enemy AWACS, and phased-array and AESA ground-based radars plus faster launch systems it’s a different story. Today, if you drop into a valley that has enemy ADA/SAM in it, you’re gonna get shot at. With real high P[sub]k[/sub]. And if the bad guys have counter air, low altitude is no sanctuary at all.
As to FB-111 & SU-30, it was mostly a matter of aircraft size & design priorities. All else equal, a bigger aircraft will have longer range. It’s a manifestation of the square-cube scaling law. -111s could penetrate against 1990s opposition & carry the fuel to go a very long way once behind enemy lines. Simply because they were 2 or 3x the size/weight of an F-15E, much less 3 or 4x the size/weight of an F-16.
We are still fighting today that stealth is not aerodynamically efficient. And stealth is an absolute requirement to survive against a near-peer opponent. So we can’t both survive and go deep unless we use a very large platform *a la *B-2. A more bizjet or big RJ-sized penetrating stealth bomber is possible. But it’ll have 90% of the pricetag of a B-2 for 1/3rd the range & payload. That’s not a winner at the Pentagon budget office, nor in Congress.