Rocket Staging: Is it Art or Engineering?

Blog Post created by benz on Dec 13, 2016

  It can be both. A look at the engineering behind some beautiful footage

We’ll be back to our regular RF/microwave test focus shortly, but sometimes at the holidays I celebrate by taking a look at other examples of remarkable engineering. I have a strong interest in space technology, so you can imagine where this post is going.

Let’s get right to the fun and the art. Then to the fascinating engineering behind it. Even if you didn’t know what you were watching, you’ve probably seen the footage of staging (i.e., stage separation and ignition) of the Apollo program Saturn V moon rockets. Some have realized the artistic potential and have added appropriate music. Here’s a recent example: I suggest you follow the link and enjoy the 75-second video.

Welcome back. Now let’s discuss a little of the amazing engineering and physics involved.

Staging is an elaborately choreographed event, involving huge masses, high speeds, and a very precise time sequence. By the time you see the beginning of stage separation at 0:15 in the linked video, the first-stage engines have been shut down and eight retrorockets in the conical engine fairings at the base of the first stage have been firing for a couple of seconds. They’ve got a combined thrust of more than 700,000 pounds and are used with pyrotechnics to disconnect and separate the first stage from the second.

At this point the rocket is traveling about 6,000 mph, more horizontally than vertically. The first stage looks like it’s falling back to earth, but its vertical velocity is still about 2,000 mph. It’s almost 40 miles high and in a near-vacuum, and will coast upward about another 30 miles over the next 90 seconds.

In the meantime, the five engines of the second stage will be burning to take the spacecraft much higher and faster, nearly to orbit. Before they can do this, the temporarily weightless fuel in the stage must be settled back to the bottom of the tanks so it can be pumped into the engines. That’s the job of eight solid-rocket ullage motors—it’s an old and interesting term, look it up!—that fire before and during the startup of the second-stage engines.

Those engines start up at around 0:20, an event that’s easy to miss. They burn liquid hydrogen and liquid oxygen, and the combustion product is just superheated steam. For the rest of this segment you’re looking at the Earth through an exhaust plume corresponding to more than one million pounds of thrust, and it’s essentially invisible. That clear rocket exhaust is important in explaining the orange glow in the image below, at about 0:36 in the video.

Still frame from film of Apollo Saturn V staging interstage release

Aft view from Saturn V second stage, just after separation of interstage segment. The five Rocketdyne J2 engines burn liquid hydrogen and oxygen, producing a clear exhaust. Image from NASA.

The first and second stages have two planes of separation, and the structural cylinder between them is called the interstage. It’s a big thing, 33 feet in diameter and 18 feet tall. It’s also heavy, with the now-spent ullage motors being mounted there, and so it’s the kind of thing you want to leave behind when you’re done with it. Once the second stage is firing correctly and its trajectory is stable, pyrotechnics disconnect the interstage.

That’s what’s happening in the image above, and perhaps the most interesting thing about this segment is how the interstage is lit up as it’s impacted not once but twice by the hot exhaust plume. I saw this video many years ago and always wondered what the bursts of fire were; now I know.

The video shows one more stage separation, with its own remarkable engineering and visuals. At 0:43 the view switches to one forward from that same second stage, and the process of disconnection and separation happens again. The pyrotechnics, retrorockets, and ullage motors are generally similar. The fire from the retros is visible as a bright glow from 0:44 and when it fades, at about 0:46, the firing of the three ullage motors is clearly visible.

The next fascinating thing happens at 0:47, when the single J2 engine ignites right before our eyes. We get to see right down the throat of a rocket engine as it lights up and achieves stable combustion. Once again, the exhaust plume is clear, and only the white dot of the flame in its combustion chamber is visible.

The third stage accelerates away, taking the rest of the vehicle to orbit. Remarkably, for an engine using cryogenic fuels, this special J2 can be restarted, and that’s just what will happen after one or two Earth orbits. After vehicle checkout, the second burn of this stage propels the spacecraft towards its rendezvous with the Moon.

I could go on—and on and on—but by now you may have had enough of this 50-year-old technology. If not, you can go elsewhere, online and offline, and find information and images of all kinds. The links and references in my previous Apollo post are a good start.