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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. $\endgroup$ – Russell Borogove Jul 18 '16 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 Jul 18 '16 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 Jul 26 '16 at 20:40
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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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  • $\begingroup$ One reason tanks are filled 100% is because it's too hard to recalculate "Max-Q" for each flight? $\endgroup$ – uhoh Jul 18 '16 at 6:16
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    $\begingroup$ I would not say too hard, but not needed. $\endgroup$ – jkavalik Jul 18 '16 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. Jul 18 '16 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 Jul 18 '16 at 10:27
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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 Jul 19 '16 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 Jul 18 '16 at 11:55
  • $\begingroup$ ...oh, and smaller explosions if it falls over. $\endgroup$ – uhoh Jul 18 '16 at 12:05
  • $\begingroup$ @uhoh At some point, an explosion is an explosion, and the size is immaterial to the damage. "We need to build a new rocket" is going to be true no matter the explosion. Also, system weight is far, far less significant to control theory than you might think - it's mostly about the system's dynamics, which don't change much with the weight of the system (except near critical values). $\endgroup$ – Aza Jul 18 '16 at 16:13
  • $\begingroup$ @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 Jul 18 '16 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 Jul 19 '16 at 0:59
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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 Aug 7 '16 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 Jul 18 '16 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 Jul 19 '16 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 Jul 19 '16 at 0:32

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