What we know about Flight Test 13

The short summary seems to be it's an attempt to verify every fix for every problem from Flight Test 12. The current schedule is still this Thursday, July 16th, NET 6:45 PM EDT 5:45PM local (CDT) or (2245 UTC)  The launch will be from the new Pad 2 at Starbase Boca Chica. 

The most notable change is the inclusion of real, functioning Starlink satellites inside Starship’s cargo bay. SpaceX previously tested the ship’s payload deployment mechanism using simulators mimicking the mass and dimensions of the company’s next-generation Starlink Version 3 broadband satellites.

Like FT-12, this will partly be a test of the "Pez dispenser" approach to carrying the V3 satellites into orbit. The majority of the satellites deployed will be Size and Weight mock-ups simply to provide weight and mass properties of the Starlink V3 satellites to test the Pez dispenser. The first couple of satellites will have more electronics than the mock-up V3 satellites will have,  but the source article on Ars Technica doesn't give exact numbers for mock-ups or real V3 satellites. SpaceX's website for the mission says there are six satellites of the 20 modified for tests. 

The satellites will not be part of SpaceX’s operational network, but engineers will attempt to briefly establish laser communication links between the Starlink V3s and other spacecraft flying in low-Earth orbit. If successful, these links will validate Starlink V3’s interoperability with SpaceX’s previous generation of Starlink satellites. 

The overall mission plan is suborbital, just like FT-12 and all the previous Starship flights, and all of the satellites will reenter and burn up in the atmosphere, so there will be a few tests involving the functioning satellites but those are going to be lost.

Six of the satellites have been modified with a suite of cameras to scan Starship’s heat shield and transmit imagery down to operators to continue testing methods of analyzing Starship’s heat shield readiness for return to launch site on future missions. Several tiles on Starship have been painted white to simulate missing tiles and serve as imaging targets in the test.

FT-12 was intended to do a restart test of the Ship's Raptor 3 engines but was unable to attempt that. FT-13 intends to complete restart, which is considered the last major milestone that needs to be verified before going for an orbital insertion. 

Without a doubt, though, the most important part of the mission is how well the heat shield functions. 

“What’s the single biggest remaining problem for Starship? It’s having the heat shield be reusable,” Musk said in February on the Dwarkesh Podcast. “No one has ever made a reusable orbital heat shield. The heat shield’s got to make it through the ascent phase without shucking a bunch of tiles, and then it’s got to come back in and also not lose a bunch of tiles or overheat the main airframe.” [EDIT - change that first use of "No one has ever made a reusable orbital heat shield" to say "rapidly reusable heat shield ..." Musk says that in a different video I've seen - SiG]

“We have brought the ship back and had it do a soft landing in the ocean. We’ve done it a few times, but it lost a lot of tiles, and it … would not have been reusable without a lot of work,” Musk said. “If you want to be able to land it, refill propellant, and fly again, you can’t do this laborious inspection of 40,000 tiles type of thing.”

On this flight, SpaceX will test out modified tiles and attachment mechanisms to gather flight data on different heat shield options. The shield will also have “load sensing tiles” to take measurements as the vehicle experiences higher dynamic pressure during ascent than on previous flights. This higher dynamic pressure will put “added stress on the tile attachments in exchange for increased payload to orbit capability,” SpaceX said.

Artist's conception of the Starship Pez dispenser deploying a Starlink satellite. Image credi: SpaceX