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A subsection of Second-generation engine in Wikipedia's article on RocketMotorTwo says:

New fuel formulation

Rather than use rubber-based HTPB in the solid portion of the hybrid rocket motor—which had experienced serious engine stability issues on firings longer than approximately 20 seconds with the first-generation engine—the Virgin Galactic-developed SS2 hybrid rocket engine would now use thermoplastic polyamide (i.e., nylon) as the solid fuel component of the propellant. The plastic fuel was projected to have better performance (by several unspecified measures) and was expected to allow SpaceShipTwo to make flights to a higher altitude.

The first test flight with the polyamide formulation ended with an anomaly that was attributed to factors unrelated to this change of propellant, but now Virgin Galactic has switched back to HTPB for the most recent flight per this answer and sources within.

According to the Spaceflight Now article Virgin Galactic completes first rocket-powered test flight since 2014:

Engineers also changed the SpaceShipTwo rocket motor design from one that used plastic-based fuel to one that burns rubber-based fuel. Virgin Galactic previously used rubber fuel in the SpaceShipTwo motor, but switched to the plastic-based compound before the 2014 crash. The propulsion system was not a factor in the accident.

Question: Why did Virgin Galactic switch back to HTPB after one launch using thermoplastic polyamide (i.e. nylon)?

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Virgin Galactic (VG) is about as forthcoming with technical details/rationale as SpaceX, i.e., not very. The closest observer of VG I know of is the blog parabolicarc.com. They have covered the engine switcheroo extensively over the years, but it's largely based on rumors. Here's a decent summary from an August 2015 post:

When they switched to the nylon engine back in May 2014, Virgin officials referred to it as a change in fuel grain. Scaled President Kevin Mickey called the engine change a “minor nuance” after SpaceShipTwo crashed last year. None of that was true. (Virgin Galactic Misled Ticket Holders, Public on Complexity of Engine Change – “Minor Nuance” in SpaceShipTwo’s Propulsion System Was Neither)

The nylon engine actually featured a new fuel injection system that required major changes in SpaceShipTwo. I wrote about it at the time. The full truth was revealed in the 9-month NTSB investigation that resulted from the loss of SpaceShipTwo and the death of Mike Alsbury.

Now that the earlier claims have been officially proven false, Virgin Galactic officials are now embracing the complexity of the nylon engine to spotlight the superior simplicity of the new and improved rubber motor. CEO George Whitesides explained it all to Aviation Week:

"According to Whitesides, the change not only provides adequate power but also results in a lighter and simpler installation. The switch to a polyamide-based grain involved changes to the pressurization system that feeds liquid nitrous oxide into the solid fuel of the rocket motor. Those changes included additional piping to improve initial combustion, as well as adding helium to stabilize the motor toward the end of the burn."

It’s a nice bit of spin. And effective, as long as no one remembers what you said before.

In a comment to the post linked above, the blog author Douglas Messier gives a summary of the engine switcheroo from his perspective. It ends like this:

The nylon engine would involves some significant changes to the ship, including a fuel injection system using the wing tanks that reportedly contained methane. The changes to the ship were quite substantial from the much simpler propulsion system that had flown for the first three powered flights. However, the changes were similar to what would have been done for the enhanced rubber engine.

So, the plan was to use the nylon engine in the interim but to eventually pick up internally where SNC left off on the rubber motor and try to improve the performance even more. And that seems to have been what they have done.

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    $\begingroup$ Hmmm, do I understand correctly that the quote is saying that "...the new and improved rubber motor..." retained some or most of "...the complexity of the nylon engine...", which lies in the "...new fuel injection system..."? In other words, is it saying that the fuel injection became more complex with the nylon design, and now that has been retained with the new rubber engine? $\endgroup$ – uhoh Apr 6 '18 at 13:50
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    $\begingroup$ I think you're right. Editing answer. $\endgroup$ – Organic Marble Apr 6 '18 at 14:21

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