The Falcon 9′s second-stage mystery liquid

A couple of astute commenters on my recent Planetary Society blog entry, “SpaceX successfully launches Dragon into orbit,” brought something interesting to my attention regarding the SpaceX Falcon 9 COTS 2 launch video. Shortly after stage separation and ignition of the second-stage Merlin Vacuum engine, a stream of fluid can be seen trickling from the second stage, just above the glowing engine bell. It can be seen in the following video at about forty seconds:

I was equally perplexed, and wanted to find an explanation for the mystery liquid. For starters, let’s check out a video of the second Falcon 9 launch from December 2010, and see if we observe the same phenomenon. Skip ahead to four minutes:

So, not only do we observe the same thing happening, we can actually see a lot more liquid here because the rocket was launched in daylight. Let’s identify some key components here: the large nozzle is the Merlin Vacuum engine bell. The small nozzle darting back and forth is the turbine exhaust, which helps provide roll control during the flight. The gold plate is the interior portion of the second-stage’s quick disconnect panel. This is where the liquid seems to originate. The ...