Aviation Week’s Frank Morring takes a long look at the “cliff” of work SpaceX has ahead of it in developing, testing and certifying the Crew Dragon spacecraft for NASA’s Commercial Crew Program.
SpaceX, by comparison, has what may prove to be serious design and operational problems in its decisions to submerge high-pressure carbon-wrapped helium tanks in the upper-stage liquid oxygen (LOX) tank and to load LOX into that tank with a crew already strapped in on top. Many experienced propulsion engineers are skeptical of the company’s public explanation of the [Sept. 1 launch pad] mishap, and SpaceX says it will redesign the system without specifying when it will do so.
[NASA Commercial Crew Program Manager Kathy] Lueders says it will happen before the launcher takes off with humans on board. “Right now they are on a Block 3 configuration,” she says, “and the commercial crew program will be flying on a Block 5 configuration. We are obviously monitoring the flight experience that they are going through, but have been working through a design and development process with them as they are upgrading to the vehicles we will be using.”
That includes the helium-tank issue. In an “anomaly update” posted on its website, SpaceX traced the Sept. 1 explosion to buckling in one of three composite overwrapped pressure vessels (COPV) in the upper stage LOX tank, “due to the accumulation of oxygen between the COPV liner and overwrap in a void or a buckle in the liner, leading to ignition and the subsequent failure of the COPV.”
However, some launch vehicle engineers with long experience in government and industry say that does not address the wisdom of submerging a carbon-based composite in highly reactive LOX, a potentially explosive mixture. “The slightest shock can set it off,” says one such engineer.
Preventing the reaction can be achieved by designing the chemistry of the composite layup carefully to avoid it, or by coating the COPV with a protective material. A SpaceX spokesman did not comment on the company’s approach, citing “a reluctance to go into too much detail here, given proprietary concerns.”
The full story is worth a read.