First, before saying anything else–congrats SpaceX! Both on the successful launch, and on coming so close to a successful recovery as well! If SpaceX’s competitors aren’t feeling the heat yet, I don’t know when they will.
As most of you have probably seen, Elon tweeted that while the rocket made it back to the drone ship, it hit at too high of a speed for successful recovery. But as of the time I started this blog post, they haven’t provided any more details.
So, in the grandest tradition of the intertubes, I would like to wildly speculate with almost no data on why they hit too fast. Being a former VTVLer, I have a few theories:
- GN&C Failure Modes: This category of failure modes relate to either the rocket not knowing precisely where it was/how fast it was going, or making the wrong decision on how to bring it in for a successful landing.
- Navigation Error: In this failure mode, the rocket either thought it was in the wrong place or going at the wrong speed. Ie it thought it was higher up and still had time to decelerate, or thought it was going slower than it really was when it hit. I think these are relatively unlikely–in order to stick a landing with a minimum throttle setting near 2G’s, you need to know very precisely where you are and how fast you were going. SpaceX knew this, so they probably put tons of resources into making sure this was done right. I wouldn’t be surprised if they had some sort of differential GPS “ground station” on the drone ship, combined with accelerometers to back out a very good estimate of GPS errors that could be sent to the rocket. Unless they made some implementation error (I doubt it based on the fact that they got the rocket all the way back to the drone ship), the rocket probably knew where it was to within a couple of cm, and probably knew how fast it was going to within a few cm/s.
- Guidance or Control Error: In this failure mode, the rocket knew where it was and how fast it was going, but made some poor decision about how to command the engines in order to bring it to a stop on the pad. For instance, not turning the engine on when it should’ve, or going to gentle on the throttle at first. Once again, I think this one is pretty unlikely, especially with the experience they have with regular F9 flights, previous ocean landings, and F9R dev1 flights.
- Engine Issues: This category says that the engine for some reason or other didn’t produce the thrust desired at the time desired.
- Relight Failure: In this failure mode, the engine either didn’t relight at all for the final burn, or relit too late (maybe after an unsuccessful first relight attempt). While SpaceX knew this was important, making a complex rocket engine that can relight correctly every single time, on-time, is really hard. That said, knowing this, they probably had some sort of backup plan in case the engine didn’t relight (maybe light two outer engines, and do a faster hover-slam?). This sort of failure mode is why I’ve started gravitating back toward helicopter landing in my rocket philosophizing.
- Engine Underperformance: In this failure mode, the engine was behaving sluggishly, underthrottled, or something else. I think this is somewhat unlikely, but possible.
- Engine Shut Down Inadvertently by Computer: In this failure mode, the computer saw a sensor reading it didn’t like, and shut the engine down. Knowing that this would doom the vehicle, and that the odds of false-positives is high, if I were SpaceX, I would’ve either disabled this capability entirely, or made it really hard for the vehicle to decide to shut the engine down in this situation. The engine should give its life for the vehicle, not the other way around.
- Engine Ate Itself: In this failure mode, the engine had a failure. Either a hard start, or a failure after ignition. Totally possible, and hopefully this is something they’ll have data to easily determine whether or not this happened.
- Premature Propellant Depletion: In this category (the most likely one IMO), the rocket ran out of propellant or the engines were starved of propellant before successfully nulling out all of the velocity. I.e. They ran out of gas and hit fast–seeing as how Elon didn’t give a lot of details about how fast the impact was, this is my guess.
- Less Landing Reserves Than Planned: In this case, for some reason the F9R first stage used more propellant either during the flight itself (due to off-nominal engine performance or something else–I didn’t get to watch the flight yet), or during the two burns prior to the final landing burn. Basically they got to the burn, but just didn’t have enough gas left.
- Too Big of a Divert Burn: This may be a variant on 3A, but it’s possible the grid fins got them only so close to the drone ship, and they had to do a big divert burn in order to get back to the drone ship, resulting in having insufficient fuel to finish the maneuver. This one seems the most likely to me, since getting back to the drone ship always seemed like the hardest part of this mission. The good news is if this is the case they can solve this by refining the grid-fin controls, adding more propellant margin, or some other combination of solutions (maybe an extra divert burn a little higher up where it does more good?). As I said, I think this was the most likely failure mode.
There are probably tons of other possible explanations, but those were the ones that popped out to me. Once again, I was doing this as a total fanboy, wanting to speculate about what happened, not in any way a diss on SpaceX or their team. Hopefully once SpaceX has reviewed the data, they can share the conclusions with the rest of us. It’ll be fun to see if any of my guesses were right.
In the other grand tradition of the internet, feel free to speculate in the comments as well!
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