When the last Titan 4 rocket lifted off from Vandenberg Air Force Base Oct. 19, it was carrying what one senior U.S. Air Force official characterized as perhaps the most important U.S. national security payload launched in 30 years.

The successful mission was a fitting end to a program whose contributions to the nation’s defense and intelligence capabilities could easily be overlooked. After all, the Titan 4 was referred to by builder Lockheed Martin as the “America’s silent hero” — although anyone who attended a launch of one of the 20-story-tall vehicles might attest otherwise — and the majority of its payloads were classified. Even those that were not, Defense Support Program missile warning satellites, for example, provide a vital national security service that many take for granted.

But one need not have a security clearance to recognize the Titan rocket family’s place in the pantheon of U.S. military machinery. The Titan line started back in the mid 1950s as the United States’ first two-stage ICBM, back before the nation had mastered solid-rocket propulsion technology. The Titan 2 vehicle launched Gemini astronauts in the 1960s, and up until recently, polar-orbiting weather satellites.

The Titan 2, of course, was a mere bottle rocket compared to the heavy-lift Titan 4, which during its heyday was the largest and most powerful expendable launcher in the U.S. fleet.

Payloads launched by Titan rockets, most recently the Titan 4, have almost certainly saved lives. Again the most obvious example is the Defense Support Program, which provided advance warning of Iraqi Scud missile launches during the first Gulf War. And who knows what disasters have been averted by the United States’ ability to keep tabs on adversaries and potential adversaries using the imaging and signals-intelligence satellites that were launched on Titan rockets?

Also notable — and easy to overlook — are the Titan family’s contributions to science. The Viking Mars landers and Voyager deep space probes were launched aboard Titan 3 vehicles, and it was a Titan 4 that launched what has so far been a spectacularly successful Cassini mission to Saturn.

But the Titan program, the Titan 4 specifically, also was a magnet for criticism, and not without good reason. During its time, the Titan 4 was by far the most expensive expendable rocket in the U.S. fleet and, in all likelihood, the world. Even taking size differences into account, the Titan 4’s costs were way out of proportion compared to those of other rockets.

Then of course, there were the failures: four out of 39 launches; two of them visually spectacular, all of them extremely costly. During one disastrous stretch in 1998 and 1999, three consecutive Titan 4 missions ended in failure.

But there’s no such thing as a perfect rocket: launch failures are an inevitable cost of space activity. During the 1980s, then-Air Force Secretary Edward “Pete” Aldridge ordered the first 10 Titan 4s out of concern that exclusive reliance on NASA’s space shuttle for launching national security payloads was a mistake. The 1986 Space Shuttle Challenger accident tragically proved him right.

On the cost issue, the sad fact is that half a century into the space age, there is still no such thing as routine, low-cost access to orbit — at least not in the Western world — in spite of billions of dollars spent in various efforts to field such a capability.

The Air Force’s answer to the Titan 4’s cost was the Evolved Expendable Launch Vehicle (EELV), which was conceived as a single family of rockets based on common hardware that would launch virtually all operational U.S. military satellites. It was supposed to reduce the government’s launch costs by 25 to 50 percent, with most of the savings coming on the heavy-lift end.

But the EELV program is proving far more expensive than anyone envisioned 10 years ago. And while this is partly due to the collapse of a commercial launch market whose promise led the Air Force to switch gears and support two EELV providers rather than just one, it is probably safe to assume that the rockets themselves — Boeing’s Delta 4 and Lockheed Martin’s Atlas 5 — cost far more to develop than originally envisioned.

Now that the Air Force will be supporting the two providers at least through 2010, and with a proposed EELV merger that will effectively eliminate the prospect of savings achieved through competition, there is no telling how the EELV ultimately will stack up against Titan on cost.

Notably, the EELV still has to prove itself on the heavy-lift end of the launch spectrum. The only such mission thus far, a test launch of the Delta 4 Heavy rocket last December, failed to deploy its dummy payload into a sustainable orbit.

It is still far too early in the life of the EELV program to judge its merits against those of the Titan 4. But if what is known now about both programs was known seven to 10 years ago, even the Titan 4’s harshest critics might not have so eagerly looked forward to its retirement.