Saturday, November 8, 2014

Safety problems in the flight procedures for SpaceShipTwo.

Copyright 2014 Robert Clark

 Recent reports are that the co-pilot on the failed SpaceShipTwo flight unlocked the feathering mechanism early:

Two pilots who were close friends, now tied together by one fatal flight.
By Christian Davenport and Jöel Glenn Brenner November 3

 This article says the co-pilot "realized his error" after unlocking the feather and tried to shut down the engine. But it could be he noticed the feather deployed when it shouldn't have even when unlocked, and he then tried to shut down the engine.

 The flight procedures were that the feather should be unlocked at Mach 1.4, not at the Mach 1 it was unlocked on this flight. However, it is not known how much this was explained to be a mission critical element to the pilots. It may have been this was simply treated as something to do to follow the set timeline. Were there training sessions where this was explained that if you do this beforehand it will lead to vehicle disintegration? It is hard to imagine the pilots would make that mistake if it were emphasized the mission critical importance of when the feathering was unlocked.

 This article also quotes a Scaled Composites pilot as stating that normally the co-pilot would announce when Mach 1.4 was reached and the pilot would acknowledge it and command the feather to be unlocked. However, tape of an earlier SpaceShipTwo flight shows this didn't happen on that flight either.

 From the audio you can hear that one pilot state he is unlocking the feather when the motor is still burning. The feather doesn't deploy, correctly, until it is commanded to do so later after the rocket has ceased burning:

SpaceShipTwo's Intense Rocket Ride - Tail View and Cockpit Recording | Video.
Published on Sep 6, 2013

A camera was strapped to the rear of the Virgin Galactic vehicle to capture footage of the rocket engines and feather system at work. The vehicles 2nd powered flight occurred on September 5th, 2013.

 However, it is not announced that Mach 1.4 has been reached when it is unlocked. It is simply stated the feather has been unlocked by one of the pilots and the other acknowledges it.

 A key problem from listening to the video is that the pilots are not calling out the speed and altitude at any time during the burn. The only time they call out the altitude is a few seconds after the engine cutoff when they are close to max altitude. Note that when landing jet airliners when speed and altitude are both critical to a safe landing the pilots are calling these out to ensure they are within the correct range. The pilots should also be calling out both speed and altitude during the engine burn of SS2 to insure this mission critical step of the unlocking is done only at the right time.

 Another problem with the flight procedures also becomes apparent from this video. During that flight in September, 2013, the feathering was unlocked at about 16 seconds into the engine burn, and the feathering deployed correctly only later after engine cutoff. 

 But in the failed flight the catastrophic unlocking occurred only 9 seconds into the engine burn. That leaves a scant less than 7 second window to perform this action of unlocking that will lead to mission success or complete destruction of the vehicle. It's very disconcerting to know this would be the procedure as well for the passenger carrying flights. 

 Since the unlocking at 9 seconds was too early the window is actually shorter than that perhaps only 3 or 4 seconds. Note you can't unlock too late either since you want to ensure the feathering mechanism will be available before engine burnout, when you reach max altitude, when the feather would be needed for landing. Since that safe window for unlocking is so short in just a few seconds, there should be multiple redundant checks to ensure it occurs at the right time. 

 Actually, I'm not really comfortable with it being that short. An advantage of using liquid propulsion is that they have higher performance than hybrids and you can take a longer, more leisurely flight to altitude. This would have the additional advantage that the passengers would not be subjected to as high g-forces as becomes apparent from the pilots voices in the September, 2013 flight.

 In an earlier blog post I noted using liquid propulsion would have allowed Virgin Galactic to reach suborbital flight earlier and more cheaply:

Transitioning SpaceShipTwo to liquid fueled engines: a technology driver to reusable orbital launchers.

 Then in additional to that, there are flight safety advantages to using liquid propulsion.

    Bob Clark


  1. The safety feature on the tail feather is the two-lever control. One is lock/unlock, the other actuates/deactuates feather. It is obviously not very critical when unlock actually occurs during ascent, or they would pay more attention to "exactly Mach 1.4" in these test flights. What failed on the fatal flight was a feather that was uncommanded with the actuate/deactuate lever. The cockpit cameras show that no one touched that lever, according to the NTSB. -- GWJ

    1. If you read the news reports quoting Scaled Composites pilots and officials they are making it seem like unlocking early was *expected* to cause it to deploy early, resulting in loss of vehicle.

      The question I have is whether this expectation was communicated to the pilots.

      Bob Clark

    2. All I know is that public news reports are a little contradictory in the exact details, still. Mid-air breakup is the only thing certain.

      Personally, I think the real question to answer is why the tails feathered, when no one hit the second feather lever, as described by NTSB.

      If there is a "risk of feather" if unlocked too soon, then what is the purpose of the second feather-command lever? This discrepancy makes no sense.