I'm currently watching the live stream of the CRS-5 launch.

They've been cycling through several different camera angles including one which looks a lot like bubbles in water, or perhaps blobs of water floating without gravity.

I was wondering what this footage is showing?

If you watch that video there's an example at about 34:26, I also took some print screens:

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They look a lot more like bubbles when the footage is in motion. Here's a video of the SpaceX/CRS-5 launch showing this:

  • 1
    Is it my imagination or did something fall off at 28:54? – Lightness Races in Orbit Jan 10 '15 at 16:25
  • Yes. Probably a cover for one of the thrusters, in prelaunch photos you can see all of the thruster nozzles are covered. – Hobbes Jan 11 '15 at 19:04
up vote 26 down vote accepted

That's from within the propellants tanks of the upper stage after Dragon separation and the microgravity environment saying hello. Lack of acceleration (stage engine is shut off during separation) acting on propellants when looking at them from inertial frame of reference (the tank itself) means they're essentially in free-fall together with the stage propellant tank they're in.

With a slight kick of the Dragon separation, propellants have sloshed and that produced droplets of the liquid fuel and oxidizer (the first image shows the RP-1 fuel tank, the other two the liquid cryogenic oxygen, or LOX tank, while the embedded video interchangeably shows both) within the tanks, interacting with each other while no other external force acts on them any different than the stage itself. Once the stage engine is reignited, some acceleration is introduced back to the tank and it essentially catches up with the propellants, forcing them towards the aft of tanks in the opposite direction to the stage's acceleration vector.

As a corollary, this is the reason why some upper stages require separate ullage motors to push propellants back towards the aft of the tank, while others might use different systems to achieve that, like with attitude control system's vernier thrusters for a slight stage kick, a wick within the tank that supplies fuel through their surface tension, or pressurant ullage gas and a membrane (aka a bladder) to provide propellants pressure in the feed line.


For additional explanation on how liquids slosh in microgravity, see this NASA's ScienceCasts: The Strange Way Fluids Slosh on the International Space Station video presentation of the SPHERES-Slosh experiment:

  • Thanks for the reply, so the bubbles are liquid propellant and they're floating in oxygen? Or some other gas? Or is it the other way around, are the things which look like bubbles cavitations? – thexacre Jan 10 '15 at 10:46
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    @thexacre They're not bubbles they're the liquid propellant itself. What you called blobs in your question, as opposed to bubbles. – TildalWave Jan 10 '15 at 10:49
  • @TidalWave What I was trying to get at is what are the blobs suspended in? Is it just some form of gas they take with them and pump in to avoid creating a vacuum? Thanks for the edit too, question looks much better with embedded pictures and video :) – thexacre Jan 10 '15 at 10:52
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    @thexacre Vacuum wouldn't form within the tank if there's still some volatiles in it since they would evaporate and increase the pressure in the tank in their gaseous state. Actually, the upper stage tanks need to be purged for any residual propellants once they're done with their job so they don't explode due to thermal cycling and produce orbital debris. I'm not exactly sure how SpaceX solves that, but it looks like they use ullage pressurant (usually helium gas) and a membrane, which would then also be used to supply propellants pressure to the engine before ignition. – TildalWave Jan 10 '15 at 11:06

While TildalWave's answer is mostly correct, he missed one important thing. This is not the RP-1 fuel, it's Liquid Oxygen, the oxidizer - floating in microgravity after the Merlin 1D Vacuum engine shutoff. There are two very clear giveaways for this.

  1. RP-1 has a yellowish hue, whereas Liquid Oxygen has a more characteristic blue color, which can be seen in these images taken from the top of the LOX tank.

  2. The RP-1 tank has an insulated pipe from the LOX tank above running through it, to facilitate the delivery of LOX to the Merlin engine. No such pipe is seen in this picture.

And yes, as TildalWave also mentioned, small ullage motors are commonly used to "push" the fuel back down to the outlet of the tank, by inducing a tiny forward accelaration on the stage. In Falcon 9's case, pressurized Nitrogen delivered via cold gas thrusters is used to provide this kick. Some rockets, such as the Delta IV, use solid motors.

  • 2
    The first image in the question is RP-1. The remaining two images obviously aren't, so that leaves LOX. – TildalWave Jan 11 '15 at 11:41

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