[Note that this article is a transcript of the video embedded above.]

On April 20, 2023, SpaceX launched it’s first orbital test flight of its Starship spacecraft from Boca Chica on the gulf coast of Texas. You probably saw this, if not live, at least in the stunning videos that followed. Thanks to NASA Space Flight for giving me permission to use their footage in this video. Starship launched aboard the Super Heavy first stage booster, and was the tallest and most powerful rocket ever launched at the time. There was no payload; this was a test flight with the goal of gathering data not just on the rocket, but all the various systems involved. The rocket itself was exciting to watch: some of the engines failed to ignite, and a few more flamed out early during the launch. About 40 kilometers above the ground, the rocket lost steering control and the flight termination system was triggered, eventually blowing up the whole thing.

But a lot of the real excitement was on the ground. Those Raptor engines put out about twice the thrust of the Saturn V rocket used in the Apollo Program. And, all that thrust, for several moments, was directed straight into the concrete base of the launch pad, or as SpaceX calls it, Stage Zero. And that concrete base wasn’t really up for the challenge. Huge chunks of earth and concrete can be seen flying hundreds of meters through the air during the launch, peppering the gulf more than 500 meters away. A fine rain of debris fell over the surrounding area, and the damage seen after the road opened back up was surprising. Tanks were bent up. Debris was strewn across the facility. And the launch pad itself now featured an enormous crater below.

Although the FAA’s mishap report hasn’t been released yet, there’s plenty of information available to discuss. Rocket scientists and aeronautical engineers get a lot of well-deserved attention on youtube and around the nerdy content-sphere, but when it comes to the design and construction of launchpads like stage zero, that’s when civil engineers get to shine! What happened with Stage Zero and how do engineers design structures to withstand some of the most extreme conditions humans have ever created? I’m Grady, and this is Practical Engineering. Today we’re talking about launch pads.

Humans have been launching spacecraft for over 65 years now. And so far, pretty much the only way we have to propel a payload to the incredibly high speeds and altitudes that task requires, is rockets. Rockets produce enormous amounts of thrust by burning fuel and oxidizer in what amounts to a carefully (or not so carefully) controlled explosion. By throwing all that mass out the back, they’re able to accelerate forwards. But what happens to that mass once it’s expelled? When the rocket is flying through the sky, the gasses from its engines eventually slow and dissipate into the atmosphere. But, most rockets (especially the big ones), don’t get to start in the sky. Instead, they’re launched from the earth’s surface, and the small part of the earth’s surface directly below them can take a heck of a beating. Hot and corrosive gasses move at incredible speeds and often carry abrasive particles along with them. To call a rocket launch “thunderous” is often an understatement, because the sound waves generated are louder than a lightning strike and they last longer too.

Dealing with these extreme loading conditions isn’t your typical engineering task. It’s niche work. You’re not going to find a college course or textbook covering the basics of launch pad design. Instead, engineers who design these structures work from multiple directions. They use first principles to try and bracket the physics of a launch. They look at what’s worked and what hasn’t worked in the past. They use computational fluid dynamics, in other words, simulations, that help characterize the velocities and temperatures and sound pressures so that they can design structures to withstand them. But eventually, you have to use tests to see if your intuitions and estimations hold up in the real world.

It’s no surprise that one of the world leaders in successful launchpad engineering is NASA. And their historic Launchpad 39A in Cape Canaveral, Florida is a perfect case study. This pad, and it’s sister 39B, were originally built for the enormous Saturn V rocket, the cornerstone of the Apollo Space Program that first sent astronauts to the moon. Just like the SpaceX facility in Boca Chica, 39A is situated on a coast with the water out to the east. Most rockets launch in that direction to take advantage of earth’s rotation. The earth itself is already rotating to the east, so it makes sense to go ahead and take advantage of that built-in momentum. But some rockets blow up before they make it into space. So it’s best to choose a launch location with a huge stretch of unpopulated area to the east, like an ocean!

Launch Complex 39 was constructed on Merrit Island, a barrier island east of Orlando. NASA decided early on that water was the best way to move the first and second stages of the Saturn V rocket, so several miles of canals were dredged out. Over three quarters of a million cubic yards of sand and shells were produced by this dredging and used as fill for construction. Some of that material was used to build a special road called a crawlerway connecting the Vehicle Assembly Building to the launchpad. But a lot of it was used to construct a flat topped pyramid 80 feet or 24 meters tall. This structure would ultimately become the launchpad. If you’re a fan of the channel, you might already be thinking what I’m thinking. Huge piles of material like this settle over time, and I have a video all about that you can check out after this! NASA engineers let this structure sit before the rest of the launchpad was built. It’s a good thing too, because it settled about 4 feet, well over a meter!

Why did NASA bother building such a massive hill when they could have simply built the pad on the existing ground? It was all about the flame deflector: a curved steel structure that would redirect the tremendous plume of rocket exhaust exiting the Saturn V during launch into a monumental concrete trench. This would keep the plume from damaging the sensitive support structures around the pad or undermining its foundation.

But why not put the trench into the existing ground rather than building a massive artificial hill? The answer is groundwater. Siting a launch pad so near to the coast comes with the challenge of being basically at sea level. If you’ve ever dug a hole at the beach, you know the exact problem the launchpad engineers were facing. Imagine trying to install expensive and delicate technology inside that hole. Of course we build structures below the water table all the time, and I have a video about that topic too. But with the cost and complexity of dewatering the subsurface, especially considering the extreme environment in which pumps and pipes would be required to operate, it just made more sense to build up. On top of that gigantic hill, thousands of tons of concrete and steel were installed to bear the loads of the launch support structures, the weight of the rocket itself while filled with thousands of pounds of fuel and oxidizer, and of course the dynamic forces during a full scale launch.

But that’s not all. Along with the enormous flame trench, and the associated flame diverters, which have gone through various upgrades throughout the years, NASA employed a water deluge system. This is a test of the current system on pad 39B. During a launch, huge volumes of water are released through sprayers to absorb the heat and acoustic energy of the blast, further reducing the damage it causes on the surrounding facility. Check out this incredible historical slow-motion footage of a Space Shuttle Launch. You’ll notice a copious flow of water both under the main engines on the right, and under the enormous solid rocket boosters on the left. In fact, a lot of the billowing white clouds you see during launches are from the deluge system as water’s rapidly boiled off by the extreme temperatures.

39A has seen a lot of launches over the years, more than 150. The first launch was the unmanned Apollo 4 in 1967, the first ever launch of the Saturn V. The bulk of the moon missions and space shuttles launched from 39A, and more recently SpaceX themselves have launched dozens of their Falcon 9 and several Falcon Heavy rockets from the historic pad! But when you compare it to the Stage Zero structure in Boca Chica, at least its configuration during the first orbital test, the differences are obvious: No flame diverter; no water deluge system; just the world’s most powerful rocket pointed square at a concrete slab on the ground. And, I think the results came as a surprise to no one who pays attention to these things. Elon himself tweeted in 2020 that leaving out the flame diverter could turn out to be a mistake.

That concrete, by the way, isn’t just the ready-mix stuff you buy off the shelf at the hardware store. I have a whole video about refractory concrete that’s used to withstand the incredible heat of furnaces, kilns, and rocket launches. This concrete has to be strong, erosion resistant, insulating, resistant to thermal shock, and immune to exposure from saltwater since launchpads are usually near the coast. NASA used a product called Fondu Fyre at 39A and SpaceX uses Fondag. But even that fancy concrete was no match for those raptor engines. Even during the static test fire, there was some damage to the concrete pad, and that was only at about half power. The orbital test and the full force of the rocket completely disintegrated the protective pad and cratered the underlying soil, spraying debris particles for miles.

In a call after the launch, Elon said that, although things looked bad on the surface, the damage to the launch pad could be repaired pretty quickly, noting that the outcome of the test was about what he expected. And even though many might have expected the extensive damage to the pad and surrounding area, it sure wasn’t mentioned in the Environmental Assessment required before SpaceX could get a license to perform the test, whose sole purpose was to document all the environmental impacts that would be associated with building the facility and launching rockets there. Nowhere in the nearly 200-page report is a discussion of the enormous debris field that resulted from the test, and yet there are actually quite a few laws against stuff like this.

For just one example, there are federal rules about filling in wetlands, of which there are many surrounding the launch facility. If you can’t do it with a bulldozer, you probably can’t do it with a rocket, and spraying significant volumes of soil and concrete into the surrounding area likely has the regulator’s attention for that reason alone, not to mention the public safety aspects of the showering debris. The launch also caused a fire in the nearby state park. The FAA has effectively grounded Starship pending their mishap investigation, and several environmental groups have already sued the FAA over the fallout of the launchpad’s destruction.

Even if the FAA comes back with no required changes moving forward, SpaceX themselves aren’t planning to do that again, and they’ve already shared their plans for the future. An enormous, watercooled steel plate design is already well under construction as of this writing. This design is, again, very different than what we see at other launch pads, basically an upside-down shower head directly below the vehicle. That’s the nature of SpaceX and why many find them so exciting. Unlike NASA that spends years in planning and engineering, SpaceX uses rapid development cycles and full-scale tests to work toward their eventual goals. They push their hardware to the limit to learn as much as possible, and we get to follow along. They’re betting it will pay off to develop fast instead of carefully. But this wasn’t just a test of the hardware. It was also a test of federal regulations and the good graces of the people who live, work, play, and care about the Boca Chica area. And, SpaceX definitely pushed those limits as well with their first orbital test. It’s still yet to be seen what they’ll learn from that.