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This comment below this question has me somewhat convinced that it would be hard for a Falcon 9 booster to abort a landing at the last minute (say within the last 100 meters or something like that) and either remain nearby to attempt again, or move to an alternate nearby location.

This might be in response to an unexpected event at sea (e.g. a rogue wave or a flock (school?) of flying fish with bad timing) or an unexpected event, vehicle or whatever on land. This may not be likely to happen in the immediate future, but since the plan is a substantial ramp, it might come in handy unexpectedly some day.

(The hypothetical flying fish could interfere with laser or microwave ranging - giving enough false position and/or false doppler velocity information to temporarily confound the Kalman filter.)

I'm not asking if this is a good idea, or really useful or practical or something you could rely 100% on being successful. These are good questions, but here - I'd just like to ask if this is even possible, and if so, what are the major challenges?

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  • $\begingroup$ It should be possible but the cost (fuel penalty on capacity and engineering would probably be prohibitive.) $\endgroup$
    – Antzi
    Jul 25, 2016 at 11:11

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The landing abort burn itself should not be the problem. But getting back from an ascending rocket to a descending rocket fast enough seems impossible to me, for two reasons:

A) Remember, that even with a single engine on at lowest thrust level, the rocket would accelerate away from earth. So after the landing abort, all engines have to be turned off. But with a positive velocity (away from earth), the rocket will experience negative g-forces, accelerating it to earth faster than gravity alone would! This is because the rocket in free fall has zero g-force, but with air drag, its movement away from earth is decellerated, meaning, that the rocket is accelerated towards earth faster than by gravity alone!

Negative g-forces have drastic consequences. Whatever little fuel is left at the bottom of each of the two tanks, now suddenly feels, that "up" and "down" have just been reversed, and starts moving towards the new "down", which is, where the "up" used to be! You can simulate that at home in your kitchen easily: Fill a glass with water (your "fuel"), take the glass in a hand, then push the glas up suddenly (the abort burn), then down again (the phase, where gravity wins). Even though you never turned over the glass, the water spills all over, when you do that fast enough!

Of course, the fuel tanks of the rocket are closed, so the fuel will not spill out during the phase of negative G-forces. And the fuel will start flowing back to the bottom, once the rocket has reached its maximum height and starts falling back to earth, because air drag now causes positive G. But will enough "air"-free (more precisely: helium-free) fuel collect at the bottom during the few seconds of free fall?

B) As already written, the rocket might become instable. While going up with engines off, it gets slower and slower, so that it is harder and harder to control by the grid fins. The fuel sloshing in the tanks add to this instability. The thrusters might be able to keep the rocket upright, but I am not sure, if they can.

C) During the non-powered phase of flight, the rocket will hardly be able to offset wind drag. Of course, the rocket could already stear in the right direction to counter wind during the abort burn. But in the case of unsteady wind (which on a nominally performing rocket is the most likely problem to require a landing abort in the first place!) the exact required direction might be hard to predict - and the final correction of the wind shift will be hard.

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    $\begingroup$ @uhoh: There's also atmospheric drag. When the rocket is moving upwards with the engines off, both gravity and drag are pushing it downwards, and that adds up to more than 1g. $\endgroup$
    – Ilmari Karonen
    Aug 25, 2016 at 3:08
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    $\begingroup$ @uhoh: Nope. The fuel accelerates downwards at 1g. The rocket body accelerates downwards at 1g + whatever extra acceleration drag forces cause. End result is that, as soon as the engine is turned off at any positive vertical velocity, the fuel starts moving upwards relative to the rocket. (Yes, this is also an issue e.g. when starting a liquid-fueled upper stage engine during ascent; ullage motors are one traditional solution.) $\endgroup$
    – Ilmari Karonen
    Aug 25, 2016 at 3:39
  • $\begingroup$ @IlmariKaronen I've cleared my comments - they don't have any educational value, but I think yours are helpful and should stay. $\endgroup$
    – uhoh
    Aug 25, 2016 at 4:32
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    $\begingroup$ @KaiPetzke thanks for your answer! Your answer has given me a lot to think about - I appreciate you taking the time to explain it all so clearly and carefully! $\endgroup$
    – uhoh
    Aug 25, 2016 at 4:37
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The fuel margin is a deciding factor: I get the impression they're very close to running out of fuel during the landing.
And jkavalik is right: the stage would do an 'abort burn' to stop the descent and build some speed going up, and then the engine has to be shut down so the stage can come down again. At that point the stage becomes unstable, and the current configuration may not have enough control authority to keep it upright.

Stability is governed by the center of gravity: this must be forward of the center of pressure. For an almost-empty stage, the CoG is far aft of the CoP. The absence of a nose cone just adds drag.

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  • $\begingroup$ If I understand right, it would be "unstable" because it is moving up with a substantial velocity but without a nose cone, and possibly it simply forgot to lower the gridded fins during the emergency ascent. If this is a hypothetical emergency, what about using only the center engine, and throttling to 60%? In that case things would happen more slowly, and possibly the upward velocity would not be so fast. I think it's "unstable" because of aerodynamic pressure. If it's moving slowly, the torque will be low and it may not actually turn much in this particular case. $\endgroup$
    – uhoh
    Jul 25, 2016 at 13:37
  • $\begingroup$ it's unstable because the weight is concentrated at the aft end. I've amended my answer. $\endgroup$
    – Hobbes
    Jul 25, 2016 at 14:04
  • $\begingroup$ Just invoking the word "unstable" isn't enough. If it is moving quite slowly, rather than supersonic, it may actually not start to rotate very fast at all, and the gridded fins can apply at least some counter torque if the angle is kept below a certain amount. This needs some real numbers. Satellites are put in "unstable" orbits, and can sometimes go months without attention. It's the timescale of the instability that's important, and that's quite velocity dependent here. $\endgroup$
    – uhoh
    Jul 25, 2016 at 14:17
  • $\begingroup$ For an example of time scales - if you read Roberts 2002 that I reference here, SOHO was in an exponentially unstable halo orbit. The instability had a doubling time of 15 days. In this situation, if one engine at 60% thrust can manage to push the rocket back up without too much velocity, with gridded fins operated in a clever way, maybe it can be prevented from flipping. $\endgroup$
    – uhoh
    Jul 25, 2016 at 14:23
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    $\begingroup$ @uhoh just some wild guessing here - single-engine landing burn is ~30s, during abort you want to go some significant portion of that burn height before retrying, and as we assessed, you want to go slowly so you need at least another 30s, possibly more to get the height and you burn too much fuel just fighting gravity. And thats supposing the engine can throttle enough to het to TWR 1 or only a very small percent over that. $\endgroup$
    – jkavalik
    Jul 26, 2016 at 15:49
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The current theory is that in fact, when landing on the ASDS in the ocean, the stage actually targets just off the barge and only diverts there late in the process. Thus it is more about diverting if safe, as opposed to diverting if dangerous. I have no official source, but this appears to be serious speculation in the forums on the topic.

If you watch the landings you can see that the stage cants and seems to fly into landing at an angle, diverting hard to get onto the ASDS.

Converesely on land landings, it seems to come mostly straight in, since by the time it is close enough for the cameras to see the divert from offshore of LC-1/LZ-1 (Darn you Hans Konigsman for confusing us) to a safe landing has been out of sight.

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  • $\begingroup$ That's interesting. Here I've mentioned " ...abort a landing at the last minute (say within the last 100 meters or something like that)" - when you say late in the process, is it within the last 100 meters, or actually earlier? $\endgroup$
    – uhoh
    Jul 25, 2016 at 15:59
  • $\begingroup$ Alas SpaceX is not talking about this, but it is fairly near the landing. The computer decides pretty close to landing. $\endgroup$
    – geoffc
    Jul 25, 2016 at 16:07
  • $\begingroup$ OK thanks! I'll watch some videos and see if I can detect it. If you have seen one where you believe it's evident, please post a link or just put in your answer. As you already know, 100 meters is only 2.5 x the height of the stage itself, and therefore also pretty close and fairly near the landing! $\endgroup$
    – uhoh
    Jul 25, 2016 at 16:11
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    $\begingroup$ I believe, that landings on the barge have those intense last-second horizontal corrections, not because they target off-barge first, but because wind is stronger at sea. As wind is not uniform, there is always an error in the prediction of the effect of the wind on the rockets position, and those errors are higher at sea. $\endgroup$
    – Kai Petzke
    Aug 24, 2016 at 12:11

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