Hopefully it’s for real this time…
That’s what I was saying to myself that Wednesday afternoon.
Just the day before, I had waited a solid 2 hours for the highly-anticipated 12.5 km test flight for SpaceX’s Serial Number 8 (SN8) Starship prototype.
The launch was aborted at T minus 1.3 seconds.
The reason was unclear, but SpaceX’s entire launch sequence is automated, so a computer must have seen a sensor reading it didn’t like and called off the entire thing.
But it was the next day, December 9, and that meant another shot at launch.
The test flight was a huge success, albeit with *a bit* of a rough landing.
Amongst the flurry of congratulations and “Mars, here we come”s at the Twitter post-launch party, one user asked, “but didn’t that sh*t blow up?”
Well, yes.
Yes, it did.
But let’s dive into some details and see why this truly was a historic event.
Lovingly called “snate” by her creators, SN8 stood at a towering 50 meters, or 165 feet tall, with an extremely wide 9-meter (29.5 ft) diameter.
This thing was huge.
Starship, unlike most other rockets in the past, uses liquid methane (CH4) as its fuel instead of liquid hydrogen. This is because the ultimate goal for Starship is to enable humans to colonize Mars. Since Mars has a carbon dioxide-rich atmosphere, methane can be pretty easily created to refuel Starship for return missions.
Another defining characteristic of SpaceX’s Starship is its stainless steel body. Since the 60s, materials science has progressed very much and stainless steel has long since fallen out of fashion as a material for propellant tanks. Though strong, it’s also very heavy, making it unsuitable for flight structures.
During the infancy of the Starship program, the vision was to build Starship out of a lightweight carbon fiber composite material. But this quickly proved to be way too expensive and not temperature-resistant enough.
Although stainless steel is much heavier than carbon fiber or aluminum alloys, where it really shines is under extreme temperatures. Since Starship will eventually need to take off from Earth, land on Mars, take off from Mars, and land on Earth, it must withstand the cryogenic temperatures of the propellant and the extreme heat of atmospheric re-entry twice per trip.
For a more detailed explanation of the decision to switch to stainless steel, check out this in-depth YouTube video from Real Engineering.
SN8 lifted off the launch pad around 4:45 pm CST (Boca Chica time).
Propelled by three Raptor rocket engines, the most powerful methane rocket engines ever created, Starship took to the skies.
The ascent was particularly “slow and steady.” A few space enthusiasts who streamed their live reactions of the test flight commented on the ascent velocity of the spacecraft saying, “It’s way slower than I thought.”
A minute and 41 seconds into the test flight, one of the three Raptor engines went out, causing the bottom of the rocket to momentarily catch on fire. No one knew it at the time, but this was the intentional first engine shutoff.
When an engine shuts off and no longer spews propellant, some propellant still leaks out at a much slower velocity, causing the bottom of the rocket to sometimes catch on fire for a couple of seconds.
At just over three minutes into the flight, the second engine shut off. When an engine shuts off, it “gets out of the way” and turns away from the center of the rocket. The three sea-level-optimized Raptor engines used in this test have a very impressive range of motion, or gimbal. Check out this short animation for a great visualization of this.
As the flame shooting out of the final active Raptor engine visibly grows in width due to the decreasing air pressure with increasing altitude, SN8 nears its apogee, or highest point, at approximately 12.5 km (41,000 ft).
White vapor spews out the bottom of the spacecraft, venting the extra liquid oxygen as SN8 prepares to do something never seen before.
Four minutes and 40 seconds after liftoff, the final engine shuts off in MECO (main engine cut off).
Over the next two minutes, SN8 executed a controlled freefall using its aft and tail fins to guide the spacecraft into horizontal “belly flop” position. The 1,400-ton heap of metal fell through the sky at terminal velocity while an army of computers and sensors onboard made micro-adjustments to the fins as well as calling for occasional bursts of air out of the cold air boosters on the side of the spacecraft.
Six and a half minutes after launch, two of SN8’s Raptor engine fired up for the landing burn. Making the bottom of the spacecraft more aerodynamic than the top by tucking in the tail fins allowed SN8 to rapidly reorient itself vertically, “point end up, flamey end down.”
In a mesmerizing, criss-crossing dance of the two plumes of purple flame, SN8 managed to stabilize itself perpendicular to the ground, right above the launch pad.
It was truly amazing to watch.
Then, one of the engines suddenly sputtered out in a flurry of orange flames. The other immediately took on a brilliant hue of green and spit out dark smoke.
SN8 touched down at too high of a velocity and exploded on impact.
SpaceX has been quite private with details and updates of the Starship program. But upon further analysis of the replay and tweets made by Elon Musk, the best explanation that we have for the cause of the explosion is this:
Starship has two fuel tanks and two oxidizer tanks. The main tanks for each are for ascent while the much smaller tanks are for the 15-second-or-so landing burn.
Unlike in cars, where the gas will always flow downwards where it can be combusted, rockets are falling very quickly. That means the fuel in the tanks is also falling very fast, making the motion and flow rate of the fuel to the engines very unpredictable. That’s why pumps are a key component of a functioning spacecraft.
The fuel header tank (tiny CH4 tank in the nose of SN8) did not have a high enough pressure to send fuel down to the engines during the landing burn, causing one engine to go out and the other to be severely weakened. This in turn caused SN8 to have a high touchdown velocity, or a not-so-soft landing.
What about that bright green flame we all saw? Well, the inside of the Raptor engines are coated with copper, and you might remember from chemistry class that copper burns green, so the rocket engine began burning itself up, causing the green flame.
Overall, the test flight was a huge victory. Elon Musk himself had given SN8 a 30% chance of success and we all know how optimistic of a guy he is. Though this test flight wasn't a complete success. It was pretty darn close.
With both SN9 and SN10 already standing on the launch site, 2021 is quickly shaping up to be a monstrous year for not only SpaceX and their Starship program, but for the entirety of space travel and the future of humanity as well.
Elon Musk’s tweet briefly diagnosing the cause of the explosion:
RUD stands for “rapid unscheduled disassembly”, a rather Muskian euphemism for “the rocket went kaboom.”
If you missed it, you can watch SpaceX’s official livestream of the full test flight here: (I linked it to start at T minus 10 seconds, your welcome ;))
Here is SpaceX’s recap video of the test flight: (a highlight reel, so to speak)
And you can find all of the images used below:
