SpaceX Has Lots of Work Ahead on Commercial Crew

SpaceX Crew Dragon Weldment Structure (Credit: SpaceX)

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.

  • patb2009

    SpaceX is doing a few new things. 1) COPV tanks in LOX, 2) Subcooling the LOX.

    I’m wondering if the rapid exhaust out of the He tanks is causing the LOX to freeze,
    or if it’s a chemical reaction with the LOX.

  • JamesG

    That article is sooooo last year.

  • therealdmt

    NASA’s not gonna stop half their commercial crew program over a group of old hands being uncomfortable (of course, given that a third significant He bottle incident doesn’t occur before manned flights start!).

    SpaceX is already going about addressing the issue (as Kathy Lueders said), so NASA at the very least has been in consultation with SpaceX over this issue and in coming up with the design changes. Obviously, everyone’s not on board, but if the rocket’s working, they’ll use it.

    At this very moment, Bill Gerstenmaier is busy working away on how to “redefine risk” in an upper level-led NASA effort to justify allowing commercial crew vehicles to fly even though they apparently won’t be meeting NASA’s Loss Of Crew standards of 1 in 270 (due to basically unaddressable orbital debris strikes). There’s gonna be risk, or there isn’t going to be a commercial crew program. And the commercial crew program is now pretty broadly being counted on as the way forward.

  • Jacob Samorodin

    Agreed! Critics will be critics, because that’s their job, or hobby, or they have fallen into an addictive pattern of being cynical, skeptical & suspicious without justifiable cause.

  • Tom Billings

    Or they know their paycheck depends on spaceflight staying something only governments pay for. Never neglect agency cost as a cause of action in any hierarchy.

  • Tom Billings

    Wrong sequence.

    Helium Tank is cooled by the sub-cooled LOX after Helium comes into the tank, to let you store more Helium, but several other things are happening as well. First, in the test on the first of September, the Helium was put in the tanks at a lower temperature, which meant you could put more Helium in faster, which meant you did not have to re-chill as much sub-cooled LOX while you are waiting for the Helium tanks to fill. However, the Helium gas flowing into those tanks also expands, and that cools it further. The initial lowered temperature of the Helium, combined with the expansion of the Helium gas into the tank to drop its temperature even more. That combination of drops in temperature made the Helium so cold that even though the Helium was still a gas, it was cold enough to freeze the sub-cooled LOX on the outside of the Helium Tank.

    Normally, the exterior carbon fiber wrapping of the Helium Tank would absorb some of the sub-chilled LOX in between the carbon fiber wrappings, and as the Helium Tank filled to maximum pressure that still liquid LOX would be squeezed out of the wrappings as a liquid. However, once the sub-cooled LOX had been frozen in the midst of those wrappings, the frozen LOX could *not* be squeezed out. As the pressure inside the Helium Tank increased, the pressure on those wrappings, now infiltrated by frozen LOX also increased. At some point in time, the pressure of the carbon fiber wrappings on the frozen LOX particles was sufficient to begin combustion, moving rapidly to deflagration, which moved rapidly to explosion of the Helium Tank wrappings, and from there to explosion of the second stage itself.

  • Tom Billings

    “At this very moment, Bill Gerstenmaier is busy working away on how to
    “redefine risk” in an upper level-led NASA effort to justify allowing
    commercial crew vehicles to fly even though they apparently won’t be
    meeting NASA’s Loss Of Crew standards of 1 in 270 (largely due to
    basically unaddressable orbital debris strikes,”

    Oh, …he’s got *far*worse* justifications than that to weedle through, since he is now being asked by the acting Administrator to justify a manned flight of SLS on its first flight!

  • windbourne

    He does not react with anything in this low pressure. It has FINALLY been found to react, but only at pressures/temps similar to earths very inner core.
    In fact, it is one of the main reasons why they use He.

  • windbourne

    huh. I thought that the He temp increased, which allowed the LOX to convert from Solid to liquid, which would expand inside the COPV bands.
    No?

  • Tom Billings

    Nope! The Helium Tank *pressure* kept increasing, even though it was cooled by the LOX, and by its expansion when flowing into the Helium Tank. The difference in boiling points, the high heat capacity of Helium, and the closeness of the sub-cooled LOX to its freezing temperature then allowed the freezing of the LOX to take place. After that, the continually increasing pressure, transferred to those COPV reinforcement bands, was enough to trigger the combustion between the Frozen LOX and the Carbon Fiber.

  • therealdmt

    Yeah, I just saw that – wow!