PixlBit

Two of the biggest arcade games of the Golden Age of Arcade Games were Scramble and Super Cobra. These side-scrolling tunnel shooters were produced by Konami, and were both released in the same year, with Scramble being released early in 1981 and Super Cobra being released later the same year. Both were notable for being among the earliest forced-scrolling games and for having distinct stages that flowed into one another. Konami considered Scramble and Super Cobra to be precursors to its popular Gradius games, and in some cases even listed Scramble as the first Gradius game.

Scramble

You fly a "jet", which looks more like a rocket ship from a 1950s sci-fi serial, through six different stages, including mountains, caves, tunnels, and futuristic cities, to try to destroy the enemy base at the end. Enemy obstacles include surface-to-air missiles, flying drones, and even a volley of meteors. In the later stages, the environment itself becomes the biggest hazard, as the tunnel narrows and requires you to make sharp turns. Your jet is armed with a forward-firing machine gun and bombs, both unlimited in supply, but its fuel is not unlimited, so you'll need to destroy fuel depots to refuel. 

Scramble was hugely popular, selling 15,000 arcade machines in the US alone, where it was marketed by US pinball manufacturer Stern (the makers of Berzerk). It didn't get any home computer or console ports, but it did get a lot of clones. Penetrator was a clone for the TRS-80 with which I was familiar, which had the unique feature of making you replay the stages in reverse after destroying the enemy base. Bellum was a more direct clone for the Atari 8-bit computer that featured the addition of gravity. 

Super Cobra

Super Cobra was released later in 1981, using the same engine as Scramble. Super Cobra was much harder than Scramble, featuring a greater variety of obstacles including tanks which fired antiaircraft fire, antiaircraft mortars, ram-jets, and more. There were ten stages instead of six. It was a prequel to Scramble, where you flew in an attack helicopter called the Super Cobra, named for an actuall attack helicopter used by the United States military at the time. Your object was to penetrate the enemy base and carry away thousands of dollars in loot. Super Cobra was nearly as successful as Scramble, with 12,000 machines sold in the US by Stern. It was Stern's third most popular arcade game after Berzerk and Scramble. Unlike Scramble, Super Cobra was widely ported to most of the popular consoles and computers of the time by Parker Bros, which also handled home ports of Konami's Frogger.  Super Cobra also featured the ability to continue by inserting more quarters due to its length, a feature which had been introduced earlier that year by SNK's Vanguard. 

As noted above, while Super Cobra was widely ported, Scramble didn't get any official home ports until Konami started releasing arcade collections on PS1 and GBA. Both games are currently available as Arcade Archives titles on Switch and PS4, with Super Cobra being the latest release.