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

In November of 2020, the rocket company SpaceX was just starting to make some progress in the testing program for their new vehicle, Starship, one of the most ambitious rocket projects in history. One of the prototypes, serial number 8, was on the pad to test-fire the engines for the very first time as a fully stacked vehicle. Almost as soon as the engine lit up, it was clear that something was wrong. A shower of sparks exploded into the dusky sky, and the engine abruptly stopped. The sparks looked innocuous at a distance without a reference for scale, but in reality, they consisted of massive, glowing chunks of the launchpad below the rocket. One of these chunks was blasted into the engine bay, severing an essential cable and severely damaging the rocket. The event brought into the spotlight what is probably the most humble piece of engineering of the entire rocket industry: the pad. How do we build structures that can withstand such insane conditions, what happens when they don’t work, and how might we solve these challenges on other planets? I’m Grady and this is Practical Engineering. Today, we’re talking about launch pads and refractory concrete.

Rocket launch pads are subject to conditions that aren’t very similar to typical infrastructure. There are a lot of creative ways to manage the extremely high-temperature exhaust gases barrelling out of a rocket engine at incredible speeds during a launch. With the Space Shuttle and the in-progress SLS, the launch facilities incorporate a flame trench. This is a structure used to deflect the exhaust gases of a rocket away from the vehicle itself and all the delicate support structures, fuel and power lines, et cetera. But, a launch isn’t the only time that rockets and their fiery engines get close to the ground. SpaceX and other launch providers are now landing rockets propulsively (in other words, with engines). And in most cases, the coming down has a lot less precision than the going up. It isn’t feasible to pinpoint a rocket landing atop a fancy flame diversion structure, at least not yet. Instead, they usually just land on a slab of concrete. But, it’s not just regular concrete. The relationship between heat and that omnipresent gray durable substance is pretty complex, and I have a few demonstrations set up here in my garage so we can learn more.

Concrete is a relatively fire-resistant material. That’s one of the reasons we use so much of it in our buildings and infrastructure: it doesn’t burn. It can provide protection like around the stairwells of buildings. It can also withstand exposure to risky conditions that we wouldn’t allow for other materials, like in warehouses and factories where there’s potential for sparks. Because it is so durable and incombustible, there is a lot of science around the topic of concrete and fire. Engineers have to consider how to design structures that can withstand it. And, if a fire has occurred, we need engineers to inspect structures to figure out whether they’ve been damaged beyond repair or are still safe to use. That can be pretty obvious in some cases, but concrete can be damaged in ways that aren’t immediately clear to the naked eye.

When the damage is obvious, it’s probably because of moisture. Concrete is a porous material, and it can absorb water from the air. But, it’s not super porous. After all, we build dams out of concrete. Moisture can take years to get in after it’s cured. If that water gets too hot, it can turn to steam, expanding in volume within the interstitial spaces of the concrete. And if that steam can’t get out fast enough, it will build up pressure to the point where the concrete breaks. This is known as moisture clog spalling because the water in the pores of the concrete blocks the steam from getting out. Actually, I did try to simulate this effect, but my heat wasn’t enough or my sample was too small and gave the steam too many easy paths to exit. What I really want to show you is how concrete heat damage can be more subtle and insidious.

I’m making a bunch of cylinders of concrete and we’re going to test their strength after exposure to extreme heat. These samples are just made with regular old portland cement concrete from a ready-mix bag purchased from a home center. Just for fun, I’m also making equivalent samples from a specialty concrete that uses materials resistant to deterioration from high heat (also known as refractory concrete). I’m testing three different scenarios: controls left at room temperature with no heat, samples warmed in my oven to 500 degrees F, 260 C., and samples blasted using a gas torch. Two types of concrete times three different temperatures times two samples means I have 12 cylinders in all (but I made a few more just in case something went wrong - they come in handy sometimes). Once they’ve all been heated except the controls, I let them sit in my garage for a week. Now it’s time to break them.

Using a hydraulic press to crush a concrete cylinder isn’t just a lot of fun. It’s the time-tested and industry-approved way of figuring out how strong the concrete is. On almost all construction projects that use concrete, samples of the mix are taken to a laboratory, cured in cylindrical molds, and crushed on a press to verify the concrete was as strong as required. We’re doing the same thing here to see if the heat affected the strength of these samples.

The regular concrete control cylinders broke at 3000 psi or 20 MPa. Unfortunately, the refractory concrete control cylinders maxed out my little press here at 10 tons without breaking. That’s 6,400 psi or 44 MPa. This stuff has small fibers in it to provide some insulation against heat and reduce cracking, and they also help make it much stronger. A fair comparison isn’t going to be possible, but I still think this demo is illuminative - if you’ll pardon the pun. Now I’ll break the heated samples. The ones that went into the oven spent about an hour there to make sure they were fully heated. The portland cement cylinders broke at an average of 2200 psi or 15 MPa. That means they lost about 25% of their compressive strength compared to the unheated samples. We’ll talk about why in a minute. The refractory concrete samples out of the oven still wouldn’t break. They may have lost some strength, but it wasn’t enough to break in my 10-ton press.

The samples that got the blow torch were next, and the effect was dramatic on the portland cement concrete. Both samples broke at around 1300 psi or 9 MPa, losing more than half their original strength. The refractory cylinders did break this time, although it was still at nearly the maximum pressure I could deliver. The lesson here is pretty simple: concrete exposed to high temperatures might look fine even when it has lost a significant amount of strength. But why?

The biggest culprit is microcracking caused by thermal expansion. Concrete is a composite material, after all. It’s made from a mixture of large and small aggregates and cement paste. Most materials change volume according to temperature, expanding when hot and shrinking when cooled. But the materials that make up concrete have slight differences in the way they behave when subjected to changes in temperature. Those differences aren’t so critical when the temperature swings are small. But, when subjected to extremes - like under the heat of a massive rocket engine - microfractures occur at the interfaces between the different components as they expand and shrink at the different rates. I used these waxes that melt at different temperatures to try and estimate the temperature of the blow torch samples. They probably didn’t get much hotter than the oven samples in most places, but directly in line with the flame was scorching, probably over 1000 degrees F, 500 C. That type of uneven heating from a small, incredibly hot source, exacerbates this type of damage. The tiny cracks grow over time, weakening the concrete as they do, and they aren’t usually visible to the naked eye.

Interestingly, once the concrete is broken, it sometimes does carry a sign that it got too hot. Many of the aggregates used in concrete will turn pink after exposure to extreme heat.

Refractory concrete isn’t a single material, but really a general name for concretes designed to withstand high temperatures. Every manufacturer has their special blend of herbs and spices. Usually, they use cement that includes oxides which absorb heat less readily and have reduced thermal expansion. So they’re less prone to deterioration when subjected to extreme temperatures. They also often have embedded fibers that provide insulation and tensile reinforcement similar to the way rebar holds macroscope cracks from growing. These extremely useful properties are taken advantage of in a variety of industrial processes like furnaces, kilns, incinerators, and even nuclear reactors.

Even refractory concrete is subject to damage due to heating. We don’t know what the original strength was, but we do know it dropped below the capacity of the press after being blasted by the blow torch. That potential for damage is especially present in the case of launch pads where concrete is not just exposed to heat but also corrosive gases moving at incredible speeds and sometimes carrying solid airborne particulates capable of eroding even extremely durable materials. Many launch pads use a ceramic epoxy material to repair damaged areas of refractory concrete launch pads or just to provide an extra layer of thermal insulation. It was actually a chunk of this epoxy (called Martyte) that damaged the Starship engine during the static test fire.

This demonstration highlights the difficulties that launch providers face. Landing pads are extremely important. Without them, rocket engines cause extensive erosion, blasting the loose soil atop the planet (called regolith) away at incredible speeds. This is one of the reasons the two recent Mars rovers used a complicated sky crane system for landing. The rovers themselves were lowered onto the planet via cables while the rocket thruster nozzles stayed high above the surface. Once the wheels were safely on the ground, the cables were cut and the crane flew off to crash well away from the rover. It was all to reduce the potential for damage from those rocket engine plumes.

In fact, when you land a rocket on the moon, the exhaust gases are moving faster than the planetary escape velocity. That means, not only can the flying dust threaten the vehicle itself, the engines also send a plume of ejecta flying out like a swarm of microscopic bullets with no atmosphere and not enough gravity to slow them down. If an orbiting spacecraft were to fly through this plume, it would almost certainly be damaged. So, moon landings have to be timed to prevent collisions between orbiting spacecraft and these sheets of ejected regolith.

That’s a lot of complexity that could be solved with a simple square of concrete. But, what seems simple on earth has some interplanetary complications, one more important than others: Concrete is heavy. That’s one of its main features. Concrete structures mostly stay put because their weight pins them to the ground. But that weight is a huge disadvantage if you have to carry the raw materials to another planet. Reducing mass is everything when it comes to launch payloads, and the weight of an entire rocket is often less than that of the pad it takes off from. In other words, we won’t be bringing concrete launch or landing pad assembly kits to the moon, Mars, or elsewhere anytime soon.

There are some creative ideas for building launchpads on other planets that take advantage of local materials, and we’ve even made some lunar concrete using samples brought back to earth. But like almost all tasks that happen outside of earth’s comfort, it’s never as easy as it seems at first glance. The stakes are high, as we saw during the static test of SpaceX’s SN8. When a launch or landing pad fails, it can be worse than if it wasn’t there at all, creating high-speed projectiles that jeopardize the safety of the vehicle and its support equipment, not to mention its crew. It’s a nice reminder that even the humblest provision here on earth - a solid, flat, and durable surface - is an absolute luxury on another world and of the importance of infrastructure in our interplanetary quests.