Starship SN8 High Altitude flight went perfect. Well, except for the big boom at the end. But the boss seems to be happy with it.
If you recall, Starship SN8 was supposed to have launched on wednesday. Unfortunately, the engines decided to auto shutdown right before liftoff. Didn’t leave the pad. Didn’t blow up. That’s ok. Try another day.
As it happens, that day was the next day (yesterday as we post).
So, pre launch, I was watching a few youtubers who normally cover this sort of things.
Starship SN8 Youtube Coverage
Starship SN8 High Altitude flight
The profile was more or less known. Starship was supposed to launch with its three Raptor engines (a first), climb to about 12,5 Km, flip to its belly, freefall down to low altitude, and then flip to its tail and do a propulsive landing. And everything went fine up until the very last stage.
You can get a better sense of Starship SN8 flight profile by this great infographic by Tony Bela.
How it went down – Get It?
As Starship rose to the skies, we noticed the shutdown in sequence of its engines. The last one – serial no. 42 (hummmm…) – did so right before the belly flip to descent. That assist is to be expected, but many (me included) were wondering if the other two shutdowns before were part of the plan. That question would latter have a bigger meaning.
The belly skydive was going fine. Starship does this so that its reentry envelope can accommodate both Earth and Mars atmosphere density and gravity characteristics. We could do a whole article just around this subject alone. Let’s just say that no other spacecraft has reentered the atmosphere with such a dynamic control system.
As it stands, the same flap system has some role to play in the landing flip. The lower ones retract to allow for a faster bottom spin. At this same time, we could notice that the two lower engines fired up at this moment with high gimbal to assist. (Gimbal Musk??)
Finally, as the flip ended (in a very controlled manner for my surprise), one of those two engines quit. The other started making some funny green flame. The third engine – Serial no. 42, did not reignite. It became quite apparent that there was just two much speed arriving at the landing pad.
Finally, what was a very exciting event ended up in a big, beautiful and very SpaceXesque Rapid Unscheduled Disassembly. Or RUD for short.
I have to admit. Before the launch, I had a feeling that everything would go fine except for the last bit. And it did. But not how I expected. I thought that the hardest thing to control would be the landing flip. It involves two pairs of flaps electrically controlled, two not so flight proven orbit grade engines and a freakn big tin can Tintin spacecraft to actually flip. (If you didn’t get that reference, you probably are not from Europe.)
Actually, it boiled down to low fuel pressure. Boiled down. Get it? Ahhh… Forget it.
Fuel header tank pressure was low during landing burn, causing touchdown velocity to be high & RUD, but we got all the data we needed! Congrats SpaceX team hell yeah!!— Elon Musk (@elonmusk) December 9, 2020
Elon Musk explained that the header tanks failed to provide adequate pressure for the landing burn. SpaceX opted to have these smaller tanks installed so that the fuel wouldn’t slosh around during that agressive last flip. The Raptor engines have very high pressure pre burners that run the combustion cycle. Any cavitation could spell disaster. (Cavitation, you know? That thing that is not good for your coffee machine, tug boat or stealthy submarine.) So you want fuel readily available at any point of the reentry profile.
Falcon 9 doesn’t have these tanks. It’s always engines down reentry means that the fuel is basically always at the bottom of the engine feeding lines. For space, they can do this little old trick of moving the spacecraft in the opposite direction of the engines with cold gas thrusters. That way, you avoid having the fuel at zero g during burns.
The green stuff coming out of the engine seems to be the engine itself. As a result of the low header tanks pressure, the unbalanced mixture must have been corroding some copper components. As I understand, that’s how green fireworks… work.
I’m no expert in these matters. But one guy that seems to have a very good grasp is Scott Manley. In this video, he does a great post flight tear down of the event. I don’t want to steal his words, so have a look.
So the rocket blew up. As I stated at the beginning, the boss was happy with it. SpaceX gets basically all the data that they need for future development. The video feed from the craft’s cameras was so good. One can only wonder how much lower bandwidth telemetry was sent.
I’d say that the biggest loss was the engines. If you think about it, SpaceX already has a bunch of those prototypes lying around. What they will need is a whole bunch of Raptor engines for the upcoming boosters (Super Heavy or whatever they name it these days).
Besides, what is SpaceX going to do with all those big soda cans. At Least now, SN8 occupies way less space.
At the end of the day, almost all check marks were crossed and the one that wasn’t doesn’t seem to me to be the hardest bone to chew. Like those still untested tiles that have to cover the bottom half of Starship to actually take the heat from orbital or interplanetary speeds on entry/reentry. Think about that.
I leave you with a SpaceX follow up video left on twitter and Instagram:
Starship landing flip maneuver pic.twitter.com/QuD9HwZ9CX
— SpaceX (@SpaceX) December 10, 2020
Just to be extra cheeky.