Count the Ansari X-Prize-winning test pilot of SpaceShipOne, Brian Binnie, among the critics of SpaceShipTwo.

Six days before SpaceShipTwo broke-up in-flight and crashed, killing Scaled Composites test pilot Mike Alsbury, Binnie addressed a public meeting of the Explorers Club in New York City.

In remarks recorded by US cable channel C-SPAN and posted online on 3 November, Binnie explained why he decided to leave the Virgin Galactic/Scaled Composites SpaceShipTwo programme earlier this year and join a competitor, XCOR Aerospace.

In nature, Binnie says, the size of the heart organ scales along a precise curve from a rabbit to a lion to an elephant. But the design of the SpaceShipTwo rocket motor was not “on the curve".

“That has been the hold-up for SpaceShipTwo,” Binnie adds.

XCOR announced that Binnie had joined the company last April. One month later, Virgin Galactic announced that it was changing the fuel formulation of SpaceShipTwo’s rocket motor from a rubber-based solid fuel to a plastic-based fuel.

A team of US National Transportation Safety Board (NTSB) investigators has reported that all three tanks aboard SpaceShipTwo were found intact in the wreckage, suggesting the fuel was not the reason that the vehicle crashed. The NTSB discovered that the tail feathers of SpaceShipTwo deployed too soon, even though the pilots had not completed a two-step command sequence. The aircraft broke apart 2s after the uncommanded tail feather deployment at a speed over Mach 1.0.

Although not implicated in the crash, there are still questions whether SpaceShipTwo’s rocket motor was viable. The fourth powered test flight was intended to answer some of those questions.

On 4 October 2004, Binnie piloted SpaceShipOne on the second of two suborbital space flights within 10 days, allowing the vehicle financed by Microsoft co-founder Paul Allen’s and designed by Burt Rutan to claim the $10 million X-Prize.

A decade later, Binnie is critical of the design philosophy that led to the hybrid motor selection and feathered control configuration of SpaceShipTwo, preferring instead the horizontal take-off and liquid-fuel-powered XCOR Lynx design pursued by his new employer.

“If you think about the world of aeronautics [and] airplanes, when you go to build a new airplane you first define the powerplant that’s going to make this thing work and then you build the airplane around it,” Binnie says. “You don’t first build an airplane and then go, ‘Where’s my engine?’ That’s kind of the difference, if you will, between what was going on between SpaceShipOne and SpaceShipTwo and XCOR.”