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.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.
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 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.