Wild Data-Free Speculation on the SpaceX Landing Attempt

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:

  1. 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.
    1. 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.
    2. 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.
  2. Engine Issues: This category says that the engine for some reason or other didn’t produce the thrust desired at the time desired.
    1. 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.
    2. Engine Underperformance: In this failure mode, the engine was behaving sluggishly, underthrottled, or something else. I think this is somewhat unlikely, but possible.
    3. 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.
    4. 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.
  3. 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.
    1. 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.
    2. 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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Jonathan Goff

Jonathan Goff

President/CEO at Altius Space Machines
Jonathan Goff is a space technologist, inventor, and serial space entrepreneur who created the Selenian Boondocks blog. Jon was a co-founder of Masten Space Systems, and is the founder and CEO of Altius Space Machines, a space robotics startup in Broomfield, CO. His family includes his wife, Tiffany, and five boys: Jarom (deceased), Jonathan, James, Peter, and Andrew. Jon has a BS in Manufacturing Engineering (1999) and an MS in Mechanical Engineering (2007) from Brigham Young University, and served an LDS proselytizing mission in Olongapo, Philippines from 2000-2002.
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16 Responses to Wild Data-Free Speculation on the SpaceX Landing Attempt

  1. Aeolus 14 Umbra says:

    sabotage

  2. Jonathan Goff Jonathan Goff says:

    Aeolus,
    I doubt it. The odds of being caught and getting in serious trouble are way too high. But sure, it’s “possible”.

    ~Jon

  3. Stellvia says:

    Hm, yeah, shades of the CASSIOPE mission if the landing burn failed part way through. If it was an engine failure, they may well still have the engine (albeit slightly bent), which would be cool.

  4. Aeolus 14 Umbra says:

    “Grid fins worked extremely well from hypersonic velocity to subsonic, but ran out of hydraulic fluid right before landing.”

    How do you “run out” of hydraulic fluid? Maybe a pre-pressurized system was expended?

  5. Jonathan Goff Jonathan Goff says:

    Aeolus,
    As Robotbeat pointed out on Twitter, most aerospace hydraulic systems are blow-down systems. You have a tank of hydraulic fluid that is gas pressurized, and you just expend the fluid overboard as you use it. Much, much lighter than a pump and recircularization system, and for a rocket that has to fly for a couple of minutes, it often makes sense. The Merlins used to run off of a hydraulic feed tank, but later they switched it to run off of the RP-1 fuel, since if that runs out your stage is dead anyway. Probably can’t do that though for the grid fins though because they have to run while the engines aren’t running, and probably run much much higher pressure than the ullage pressure of the fuel tank.

    That said, I’m still wondering if it wasn’t a combo of running out of grid fin hydraulic fluid leading to the divert being too far and running out of propellants. That’s totally within the scope of what Elon said, but makes more sense to me. Unless losing grid fin control resulted in them not being able to light the engine, you would assume they should’ve still been able to land. And I imagine they weren’t targeting the grid fins to have the thing crash on the pad if the engine didn’t light. It’d be great to get some clarification, but I still haven’t dismissed the “ran out of propellant before landing” theory.

    ~Jon

  6. born01930 says:

    I thought I read barge was damaged…which means they at least made it to the target. That to mean sounds like a velocity error, running out of fuel seems like would be a total miss. Or maybe the barge needs better thrusters and missed catching the stage?

  7. Peterh says:

    Depending on when the hydraulic fluid ran out, it may work to use RP1 in a pre-presurized system refilled once the engines start. With a gas generator system could the turbopumps be started early, before the combustion chamber?

  8. Pingback: Successful Flyback | Transterrestrial Musings

  9. George Turner says:

    The problem was none of the above. The barge was higher than the GNC program expected because of sea-level rise due to man-made global warming.

  10. Andrew Platzer says:

    A guess. Elon said the grid fin hydraulics ran out during the final landing (short 10% of 4 min = 20 sec) of the 30 sec final burn. The stage probably doesn’t have sensors to detect the failure and reset the fins so they could have been stuck in middle of a pitch maneuver There were high winds at the time of landing so no doubt the rocket was coming down at an angle. The TVC tried to compensate but didn’t have enough range to straighten the rocket in time causing it to crash at an angle and at a higher speed.

  11. Henry Vanderbilt says:

    Another guess: The grid fins were introduced primarily to give more roll authority, IIRC. If they ran out of drive fluid some time before the landing, and weren’t in (or weren’t designed to return to) a neutral position, they may have induced enough roll during the final descent that the stage could guide to the barge OK but couldn’t then land successfully. I can see the engine-gimbal guidance handling, say, a one-half RPS roll rate, but picture the effect of that roll rate on the landing legs if a touchdown is other than completely dead level, or if the landing feet catch on the deck rather than slide.

  12. Robert Clark says:

    Andrew, I didn’t know the last burn lasted for 30 seconds. Anyone know the total length for all the burns for the return portion of the first stage’s flight?
    I think SpaceX should give the F9R hovering capability. Since a single Merlin 1D has too much thrust even when throttled down as far as possible, you would need various methods to reduce the thrust. One would be to put vanes in the exhaust plume that would direct the exhaust partially horizontally.
    Another would be to use a variable mixture ratio. Most rocket engines run fuel rich anyway. Increasing the fuel portion further would reduce the thrust. This might be too expensive a fix though for the Merlins as they have a fixed mixture ratio.
    The exhaust vane approach would be the simpler and cheaper fix but it would have the disadvantage that some of the thrust would be wasted in being directed horizontally, thus burning more propellant than necessary.

    Bob Clark

  13. NA says:

    Judging by screencaps from launch control and barge pictures when it returned to port, it looks like it correctly aligned with the length axis of the barge, but was too far aft (blown by wind?), as video shot seems to show upright touchdown. Judging by the burn marks and the crushed port aft container, it probably came down too far aft, one leg hit the container and collapsed, it landed correctly on the platform with the remaining three legs very close to the collapsed container, then tipped over the container into the sea. Not a lot of recovered debris on deck when the barge returned, notably nothing big like an engine, mostly tank shards and possibly a leg, which suggests a tip over into the sea rather than tipping over into the platform.

    If the loss of grid fin functionality affected it’s ability to adjust for being blown around during terminal guidance, then an inability to compensate for getting blown back by wind at the last moment means they simply got unlucky with the leg striking the container. Allegedly lip reading some of the faces of employees in the video stream had variations of “oh come on!” and Elon supposedly uttered “WTF” as well.

  14. Robert Clark says:

    NA, where was this video?

    Bob Clark

  15. Robert Clark says:

    SpaceX has released video of the attempted landing:

    SCIENCE
    SpaceX releases dramatic pictures and video of failed Falcon 9 landing.
    Video from barge shows how close the rocket came to landing safely
    By James Vincent on January 16, 2015 02:58 am @jjvincent
    https://www.theverge.com/2015/1/16/7555633/falcon-9-barge-landing-images-released

    All of the position, angle and velocity were off. All of these could have been corrected with an ability to hover.

    Bob Clark

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