Most of the metal rockets have gone into the big hole in the sky

I have a guilty secret that I can now share – me I love Delta 4 heavy missile.

No, I didn't like the price, which was unreasonable, sometimes approaching $400 million. This prevented Delta from having any customers other than the US government. I didn't like the low flying rate, only 16 missions in 20 years. This has prevented the rocket's operator, United Launch Alliance, from getting anywhere remotely close to such effective operations.

But there were two things I liked about the Delta IV Heavy rocket, which had its final launch on Tuesday. I loved watching him fly. And I love that, for all its warts, it proved that private companies could develop a heavy rocket. Although the Delta booster is the product of decades of traditional space development, it provided a glimpse into the future of commercial launch we live in today.

More metal than rockets

The Delta IV Heavy launch in June 2016 produced the most amazing launch image I've ever seen (shown below). Launching the secret mission on a partly cloudy day, near the sea, resulted not so much in an image of the rocket launch as in something resembling an Impressionist painting with the huge dust cloud it raised.

It sure was beautiful to see flying.

The other great thing about watching the Delta IV Heavy launch is that the rocket always looked like it was about to explode as a fireball swept through the vehicle.

This is due to its huge RS-68 rocket engines. The RS-68 expendable engine was developed during the 1990s by Rocketdyne, and was intended to be less expensive and more powerful than the Space Shuttle's reusable RS-25 main engines. Both engines run on a cryogenic fuel mixture of liquid hydrogen and liquid oxygen.

The fireball phenomenon occurs because of design differences between the RS-68 and shuttle main engines, and because the RS-68 thrust valve is open longer before oxidizer begins to flow. Basically, when you start the engine, only liquid hydrogen passes through the engine because it is less chemically active than oxygen.

This hydrogen flows out of the engine, and because hydrogen is so light compared to the surrounding air, it rises to the outside of the rocket. When liquid oxygen begins to flow, the excess hydrogen is ignited in a fireball. This happens during the last five seconds of the countdown. This design process was intentional, and the exterior of the missile was configured to resist fireball.

She looked amazing every time.

A glimpse into the future

After the retirement of NASA's Space Shuttle in 2011, the Delta IV Heavy became the world's most powerful operational rocket, with a lift capacity of about 29 metric tons to low Earth orbit. Only a few government-developed vehicles, including NASA's Saturn V lunar rocket and Russia's Energia rover, have greater lift capacity.

There was a time, about 20 years ago, when Delta 4 Heavy was considered the primary launch vehicle for the Orion spacecraft that NASA was developing. With a more powerful upper stage — later known as the Advanced Cryogenic Advanced Stage, or ACES — it could have played a role in human exploration beyond low-Earth orbit.

However, in 2006, NASA Administrator Mike Griffin quashed such concepts by commissioning an exploration systems engineering study, which led to the space agency eschewing commercial options and developing its own superheavy rocket. Eventually this became known as the Space Launch System. NASA spent more than $20 billion developing the SLS rocket, and its launch costs exceed $2 billion.

However, over the past two decades, the presence of the Delta IV Heavy has offered another way. Specifically, through multiple launches of commercial vehicles, known as distributed launches, NASA can promote a more sustainable exploration architecture using less expensive rockets and refueling in space. In late 2019, some within NASA were considering missions to the Moon using a combination of SpaceX's Delta IV Heavy rocket and Falcon Heavy rocket. The Delta IV booster doesn't sound like a champion of commercial launch, but in all honesty it was. One could point to the Delta and Falcon heavy-lift vehicles and say there is a better way.

Of course a different story unfolded. The launch of the Delta IV Heavy rocket has now been completed. NASA, thanks to persistent pressure from Congress, stuck with the massive and expensive Space Launch System rocket. It will likely fly a few more missions.

However, the future lies in distributed launch. Both SpaceX, with its Starship rocket, and Blue Origin, with its New Glenn spacecraft, are developing plans for multiple launches and fuel depots, with the goal of creating a sustainable plan for space exploration. To unlock this future, they've taken the Delta IV Heavy one step further by incorporating reuse.

He is dead, but the future he promised is no longer there.

Listing image by United Launch Alliance