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A simplified version of the landing choices might be - if there's going to be sufficient propellant, the plan is for the F9 first stage to u-turn and go all the way back to the launch area to land.

If not, a drone ship is conveniently placed downrange so much less propellant is needed to land.

In either situation, are all the tanks topped-off to absolutely 100% of capacity "just in case", or are they filled only with the anticipated amount needed plus a safety margin?

The reason I'm asking what is actually done is that I seem to recall that "leftover" propellant might actually be helpful, since the thrust of the (three?) engines on landing is way more than the weight of an empty 1st stage, even when throttled.

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    $\begingroup$ As discussed in related questions, a high TWR in the final landing approach is actually desirable (up to a point), because it minimizes burn time and therefore gravity losses. $\endgroup$ Commented Jul 18, 2016 at 6:04
  • $\begingroup$ @RussellBorogove Thanks, which is why I'm asking what is actually done, except that in the last question I drifted off into a hypothetical tone. I'll fix that now, and try to round up some previous answers to link. $\endgroup$
    – uhoh
    Commented Jul 18, 2016 at 6:14
  • $\begingroup$ Grasshopper early flights seem to do more vertical movement as does the Blue Origin But of course these are not coming back from space. Is it wrong of me to think that SPX gets controlled landings to this point one day. $\endgroup$
    – user16417
    Commented Jul 26, 2016 at 20:40

4 Answers 4

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All tanks of a launch vehicle are always topped to the 100%. It simplifies the things a lot because the rocket weights the same and behaves the same. With tanks "half-empty" for example the TWR on launch would be different. The weight difference between payloads is something like ~1% of the liftoff mass iirc so that does not change the things much.

The just in case reserve is important too. Recent case of Cygnus OA-6 would have been a failure if the Atlas V and Centaur were not topped to the max. The same for SpaceX CRS-1. The engine-out capability is exactly "if anything bad happens with some engine(s), use the fuel reserves and burn longer, possibly forbidding a landing attempt".

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    $\begingroup$ I would not say too hard, but not needed. $\endgroup$
    – jkavalik
    Commented Jul 18, 2016 at 6:18
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    $\begingroup$ @uhoh: TWR, trajectory, landing place, throttling, maneuver torques and durations - pretty much everything. The operation of the first stage is complex enough. Any "customizations" can be applied to second stage, which is not under rigors of reentry and must adapt to payload demands (target orbit, mass, geometry etc) anyway. $\endgroup$
    – SF.
    Commented Jul 18, 2016 at 9:28
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    $\begingroup$ Max Q depends on the flight path anyways, so I assume it's re-calculated anytime anyways. $\endgroup$
    – PearsonArtPhoto
    Commented Jul 18, 2016 at 10:27
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    $\begingroup$ Here is what motivated my question in the first place - it seems to me it would be much more helpful to keep the weight within a limited range during reentry, where for (most of the time) you don't have control from thrust vectoring and are relying on gridded fins to steer, and where the rocket is "flying backwards" so to speak, and the target is a few meter circle where you have to nail all six degrees of freedom at once, rather than getting to an orbit where a kilometer or a half-dozen meters per second can be easily accommodated. $\endgroup$
    – uhoh
    Commented Jul 18, 2016 at 11:49
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    $\begingroup$ @uhoh no idea if there is any official word on this specifically. I remember reading some discussions about that few months ago (when the troubles with densified propellant were actual) but did not found any specific one now. I believe it is so because the advertised engine-out capability depends on it and there are really not any actual downsides (I know some were mentioned around here and I do not find these important enough) - but without traceable sources thats just my opinion and I should probably specify that in the answer. $\endgroup$
    – jkavalik
    Commented Jul 19, 2016 at 5:51
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There's really no point to not filling the tanks all the way, and every reason to do so.

Reasons to fill all the way:

  • Simplifies procedures
  • Initial liftoff the same for all launches
  • Keeps the maximum acceleration more constant
  • Empty tanks can cause sloshing, which can be bad.
  • Fuel reserve in case of an emergency.

Reasons not to:

  • Saves a very small amount of money (Less than 1% of the launch cost is in fuel)
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    $\begingroup$ If you want, you can add to the "not to" list what I mentioned the comment above. If I were trying to land rockets on boats, I'd like to have a more repeatable re-entry weight while I was learning to do it, and possibly always, if I could. OK I know I know, it's a ship, not a boat, I just liked the sound of it there. $\endgroup$
    – uhoh
    Commented Jul 18, 2016 at 11:55
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    $\begingroup$ @Emrakul I'm sure you are more familiar with explosions than I am, so if you say they're all pretty much the same, independent of size, I'll take your word for it. When a rocket is returning to earth in free fall, doesn't the terminal velocity, Reynolds number, etc... depend directly on the weight? It's the actual aerodynamics of a rocket going backwards with mostly just gridded fins for control trying to hit a few meter circle with all six degrees of freedom that's worrying me. Control theory is just the math - the aerodynamics here is hard. $\endgroup$
    – uhoh
    Commented Jul 18, 2016 at 16:55
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    $\begingroup$ If you have extra fuel, you do a slower more controlled landing to burn off the extra fuel, it's not really that much of a concern. $\endgroup$
    – PearsonArtPhoto
    Commented Jul 19, 2016 at 0:59
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    $\begingroup$ @uhoh you don't pogo :) you just start the burn higher with minimal thrust and let the gravity loses take their toll. $\endgroup$
    – jkavalik
    Commented Jul 19, 2016 at 5:14
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    $\begingroup$ @jkavalik I'm using humor. I am suggesting that due to an unexpected series of events (rogue wave, solar flare and flying fish) that you might want to wait a bit. However, by the time that flying fish would be a problem, you'd be very close. Because the thrust is too strong you can not hover in place, your only choice (if you have any at all) for a re-try is to go back up again, then shut off the engines, then start falling again, then when you get close, start them again and on, and on... Boing! Boing! Boing! $\endgroup$
    – uhoh
    Commented Jul 19, 2016 at 7:42
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No, tanks are not always filled to 100%.

For Falcon 9:

Tank fill fraction, especially on second stage, is mission-dependent (optimizing the rocket equation - adding performance margin makes what you learn in class very idealized).

Cryogenic tanks are usually filled to capacity, though.

a. it limits the amount of analysis that has to be done
b. There are only empty and full sensors on the tanks
c. hard to determine load by head pressure accurately due to prop boil off.

Just easier to launch full and deal the excess on orbit

On the Ariane 1-4 it was routine to fill the second stage only to the level required for the mission. This had to do with the guidance system, they decided to set second-stage engine cutoff at a specified speed.

On the Ariane 40 and 42P the first stage was filled to a lower level, because a full tank would result in the thrust/weight ratio being too low. (source: Europäische Trägerraketen 1: Von der Diamant zur Ariane 4 – Europas steiniger Weg in den Orbit, B. Leitenberger)

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  • $\begingroup$ This is very helpful - thanks! Indeed it makes sense to think twice (or 2n times where n = 1, 2, 3...) when choosing a fill fraction for 2nd stage tanks. $\endgroup$
    – uhoh
    Commented Aug 7, 2016 at 0:54
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For landing, one downside to completely full tanks, on low performance missions is the possible damage a falling stage would cause.

We saw on the several failed landing attempts of the ASDS JRTI and OCISLY the damage that a landing stage (JRTI for CRS-5 (Jan 2015), CRS-6 (Apr 2015), and Jason-3 (Jan 2016) then on OCISLY with SES-9 (Mar 2016)) can do.

When Jason-3 landed, but a leg did not lock and it slowly toppled the explosion of the fuel and oxidizer as the tanks were punctured was impressive. The more fuel remaining, the bigger the potential bang. However since the goal is successful landing, not crashing that may be less of a concern.

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    $\begingroup$ Another possible downside would be if there were so much fuel left that the legs would not be able to handle the weight. $\endgroup$
    – jkavalik
    Commented Jul 18, 2016 at 18:05
  • $\begingroup$ @jkavalik that's a good point! Unless the rocket lands perfectly vertical, the first leg to hit must actually be able to sustain enough force so it can impart a moment to tilt it back to vertical. It's not a trivially simple problem, actually it's a great engineering problem to think about. Nice question and answers here :) $\endgroup$
    – uhoh
    Commented Jul 19, 2016 at 0:11
  • $\begingroup$ @jkavalik that's a good point! Unless the rocket lands perfectly vertical, the first leg to hit must actually be able to sustain enough force so it can impart a moment to tilt it back to vertical. The complete dynamics of landing is not a trivially simple problem, actually it's a great engineering problem to think about. Nice question and answers here, although the answers (including mine) all seem to address the mechanical tipping point assuming unbreakable legs, and not the fact that the legs have mechanical limits of their own. $\endgroup$
    – uhoh
    Commented Jul 19, 2016 at 0:32

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