Early yesterday morning SpaceX launched Starship again for the vehicle’s fourth integrated flight test. Here we saw the booster return to Earth and complete a landing burn with minor complications. We also saw a hard-fought reentry of the upper stage that eventually ended with a splashdown in the ocean albeit not quite a fully intact Starship.
For a while now SpaceX has been working on the vehicle’s heat shield with the goal of creating a lightweight, easily manufacturable, and heat-resistant tile. That’s much easier said than done, however, and it seems they have a bit more work to do. Here I will go more in-depth into the current state of the heat shield, relevant testing of tiles, Starship’s steel construction, and more.
Starship’s Heat Shield
At its current state, Starship’s heat shield needs some work. What’s important to point out however is the fact that SpaceX is well aware of this and has been for quite a while. Even on yesterday’s launch the company purposely lifted off with two entire tiles missing and one thin tile replacing a standard tile. In these specific spots, they included extra equipment to track heating and how the vehicle would hold up in those different environments. In other words, they are still experimenting and trying to find the best approach to vehicle reentry.
Focusing more on the launch, most have seen the footage of upper stage reentry eventually eating through and nearly destroying Starship’s flap. While we only get a view of the same flap for practically the entire reentry, it can be assumed that the other 3 flaps could have had some similar damage to an extent. In this instance, the specific weak point was the seal that covered the gap between the flap itself and the main body of Starship. This is where the plasma initially broke through and then began basically eating the flap away from the inside. Interestingly, it seems SpaceX was actually a bit worried about this exact spot even before the launch took place. Specifically, in a partially released interview between Tim Dodd and Elon before the launch, Elon was quoted saying, “Sealing that hinge gap and not having hot gas just flowing super fast through where the flap hinge is, cause if you get hot gas flowing through rapidly that will cook anything, including the tiles. We got a hot gas seal at the forward and rear flap hinge and one of the key questions is does that seal work. We think it will work but it may not work” he said.
This is a very promising sign as it showcases SpaceX’s understanding of the vehicle and proves they were a bit unsure about the strength of that seal in particular. Beyond this, in a tweet from Musk referring to the comments he made before the launch, he said, “Not a difficult prediction! We will have this nailed for next flight.” He went on to say, “Note, a newer version of Starship has the forward flaps shifted leeward. This will help improve reliability, ease of manufacturing and payload to orbit.”
The combination of the flaps shifted toward the back of the vehicle relative to reentry along with alterations to the flap seal could very well alter the state of Starship and keep it perfectly intact following reentry. Similar to a lot of other developments and changes SpaceX makes to the vehicle, they usually just test it in an actual flight environment to get the data and then move on from there. It seems like we are watching that exact process in action but related to the heat shield. On the third launch, the vehicle broke up entirely around 65 km high. This time around it made it to the water but definitely had more than ideal damage.
Elon also tweeted about a week ago in late May which gave some good insight into their approach and general plans related to the heat shield. Here he said, “This is a matter of execution, rather than ideas. Unless we make the heat shield relatively heavy, as is the case with our Dragon capsule, where reliability is paramount, we will only discover the weak points by flying. Right now, we are not resilient to loss of a single tile in most places, as the secondary containment material will probably not survive. I will explain the problem in more depth with Everydayastronaut next week. This is a thorny issue indeed, given that vast resources have been applied to solve it, thus far to no avail.” This comment was before the fourth flight test which we know now has changed some of the ideas about what the vehicle can survive and cannot.
A Steel Rocket
When talking about Starship reentry and its heat shield, it’s important to also focus on the vehicle’s primary build material which is steel, an uncommon choice in rocket manufacturing. Steel has a very high melting point, especially when compared to other common rocket manufacturing materials used commonly within the industry. When asked about why they chose steel Musk was quoted saying, “Anything much above 200 degrees Celsius for carbon fiber or aluminum you start falling off a cliff from a strength standpoint. But for steel, you go to 800 degrees Celsius and it’s fine, even 1000 degrees can be fine” he said. Taking some of the tiles off the ship for the last launch is a good example as even in those specific areas it would have posed a much greater risk to the vehicle had it not been steel.
Musk went on to say, “So for the ship, this means that the heat shield mass is significantly reduced because the heat shield mass is determined by the temperature on the back of the tile that is then transmitted to the hull. So if the hull is steel, then you can have thin heat shield tiles, whereas if the hull is carbon fiber or aluminum you need to have thick heat shield tiles. And you also need no heat shielding at all on the leeward side of the ship. So it is actually lighter than the most advanced carbon fiber vehicle” he said. These points are interesting especially as SpaceX tests different tile thicknesses and works to find the best balance of weight and strength.
With all this in mind, the steel body of Starship gives SpaceX quite a bit of leeway in regards to reentry and what the vehicle can handle. That being said, the heat shield definitely still needs work in order to not only keep Starship fully intact, but safe for the eventual transport of payloads and even humans.
Looking back at the reentry footage showcases the importance of a combination of steel and tiles. Once the plasma had found a weak point and gotten through the seal, that flap should have been completely destroyed. Despite all the impressive properties of steel for reentry, alone it’s not quite enough. Quite quickly the plasma began eating through most of the flap’s steel construction. Had reentry been a few minutes longer the rest of the flap easily could have been completely destroyed to the point a landing attempt would not be possible.
As far as the next launch attempt, already Elon has made some bold comments about what to expect. Yesterday he tweeted saying, “I think we should try to catch the booster with the mechazilla arms next flight!” While that would be very entertaining, it seems unlikely that will actually be attempted on the next flight. During IFT-4, there were a few Raptor engine issues during liftoff and the relight of engines for the landing burn. Specifically, just a few seconds in one of the Raptor engines on the outer ring shut off and would remain that way for the rest of the flight. In addition, as Super Heavy was falling toward the water, it attempted to relight all 13 engines but one did not. You could also see some debris from what looked like an explosion likely related to that specific engine.
Even though these two instances during the mission didn’t have any significant negative effect on either the launch or landing thanks to all the additional engines, it means there is still some work to do with the Raptor engines. Super Heavy can operate without all 33 engines but that doesn’t mean SpaceX can afford to have engines going out on each mission. When talking about catching the booster on the next attempt, the company probably wants to be more confident in the system and the Raptor’s ability. Even though SpaceX is in the process of building a second launch tower next to the first, for the time being, that is their only tower. If a failed booster catch attempt were to take place it could cause some major damage to the Stage 0 infrastructure.
In reality, the more likely future result is even more testing to make sure that system truly is reliable before they route the booster back to the launch site. They did however mention plans to test the chopsticks during the last flight attempt while the booster was heading back toward the ground. Whether or not the catch attempt happens on the next launch doesn’t change the fact that SpaceX is working toward this goal and it’s not far away. The other big questions for the next launch have to do with the FAA and general approval. Since the first launch, this has improved a lot with the time between each launch only getting smaller. We can expect the next launch in about 3 to 4 months as SpaceX upgrades the heat shield, upper stage design, and any other tweaks they deem necessary. They definitely got more flight data than ever before that launch which will no doubt be applied to future missions.
This not only includes the upper stage making it through reentry but even the booster got further than before. On the last attempt, it was destroyed before it ever made contact with the water. This time it touched down before slowing flipping on its side while still in one piece. As for the upper stage, it managed to complete its flip maneuver as confirmed by SpaceX. They had even mentioned that they commanded the shutdown of its engines as the flip and landing burn were completed. Considering data is the main goal of these flights that makes these events very valuable for the company.
Conclusion
SpaceX knows they need to work on Starship’s heat shield but they have a good idea of where the weak points are. Even with some of the problems, the vehicle still survived reentry and managed to make contact with the water in the very end. We will have to wait and see how it progresses and the impact it has on the space industry.