Sunday, January 15, 2017

SpaceX Nails Return to Flight, Booster Return

SpaceX had a successful return to flight launch from Vandenberg Air Force Base yesterday, putting 10 Iridium satellites into orbit.  In addition, they had a successful recovery of the 1st stage on their drone barge "Just Read the Instructions", hitting the center of the target with the live video feed working completely through the landing, for the first time.  This is the first time they've successfully landed a booster launched from Vandy.

SpaceX is quick to point out the most important part of the mission is that they delivered for their customer, and from a business sense that's absolutely right. It's just us space geeks who follow the ability to recover the boosters as a way to cut costs, and cutting costs is the key to bringing spaceflight to full commercial use.  How common would air travel be if we had to throw away the planes after one use?

Readers will recall that SpaceX had a highly unusual accident during a "routine" test on the pad on September 1st and has been grounded since then.  It was a tough problem to troubleshoot, but armed with just 93 milliseconds worth of data before the explosion, engineers isolated the problem to the rupture of a high pressure helium tank by late September, and then spent another couple of months verifying that with some creative testing.
The Sept. 1, 2016 explosion, which occurred during a routine pre-launch test at Cape Canaveral, initially puzzled SpaceX; about a week later, Musk described the incident as the "most difficult and complex failure" in the company's history. But technicians and engineers eventually traced the cause to the failure of a high-pressure helium vessel inside the Falcon 9's second-stage liquid-oxygen tank.

SpaceX announced earlier this month that it had wrapped up its investigation of the Sept. 1 accident. The Federal Aviation Administration accepted the results of the inquiry and granted SpaceX a license that covers all seven launches required to orbit the 70 Iridium-NEXT satellites.
The Falcon 9 launch system has been changing over to a colder liquid oxygen than other boosters use, called densified liquid oxygen (pdf warning - Master's Thesis).  Liquid oxygen boils at 90.2 degrees Kelvin (-297.3 F).  SpaceX is switching to a colder, denser liquid oxygen at about 65 K to improve engine efficiency.  Like other launch vehicles in the industry, SpaceX uses Composite-Overwrapped Pressure Vessels to contain the helium, and submerges them in the liquid oxygen.  The difference is that SpaceX is the first to use the COPV helium tanks in a liquid oxygen that's substantially colder than the 90 degree Kelvin boiling point.
The previous iteration of the Falcon 9 used Liquid Oxygen at boiling point temperature and began loading its tanks over three hours ahead of launch – permitting the COPVs to be fully chilled prior to applying high pressures. Falcon 9 FT enters LOX load on the second stage with just 19.5 minutes on the countdown clock followed by Helium load just over 13 minutes prior to launch – an aggressive tanking sequence unprecedented in the space launch business.
In a way, this sort of process/handling issue is the best thing that could have happened to SpaceX - ignoring the loss of vehicle, customer's payload, and all the costs.  It's easy to change the rates and times at which propellants are loaded.  It's even relatively easy to redesign a helium tank.  I believe the "aggressive tanking sequence" is to minimize the amount of time the densified liquid oxygen is sitting in the launch vehicle warming up.  That tells me they probably have alternatives to tweak that sequence so that the thermal stresses on the tanks are not as high. 

While there's much to read about, it's also appropriate to acknowledge the team for a successful return to flight and overcoming the recent failure. 



    I saw a better explanation elsewhere a couple of weeks ago, but can't find it now. The bottom line is that they were pushing their margins without looking at the consequences of doing so. Hopefully they have looked closely enough at their new procedures to make sure they didn't introduce any new unexpected failure modes, but that's not the way I would bet. Unfortunately...

    1. For sure, SpaceX has developed some degree of institutional arrogance that keeps them from automatically assuming someone else might be right and they might be wrong. On the other hand, they have succeeded at things that other folks haven't done, and some arrogance kind of comes with the territory. I've seen it in a couple of places I worked.

      I read a piece a while back that they thought they could avoid the problems by changing the protocols for loading the helium. I don't know if that was the final recipe for avoiding disaster, or if they're keeping that as secret sauce.

  2. Changing something about how they load the helium, And supposedly going from 3 to 4 copv's in S2. Probably keeping it a bit warmer to avoid frozen oxygen in the overwrap.

    First-and-a half landing from Vandenberg. There was one that landed just fine - And then one leg folded up (Iced-up locking collet) and the whole thing tipped over.