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  
http://www.washingtonpost.com/news/business/wp/2014/11/03/two-pilots-who-were-close-friends-now-tied-together-by-one-fatal-flight/

 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.
http://exoscientist.blogspot.com/2014/01/transitioning-spaceshiptwo-to-liquid.html

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

    Bob Clark

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

 Copyright 2014 Robert Clark

 A new book by Tom Bower on Richard Branson, “Branson: Behind the Mask”, claims the hybrid engine on SpaceShipTwo still does not have enough power to get the vehicle to the altitude for suborbital flight. Doug Messier on his blog ParabolicArc.com has been reporting on the technical problems developing the hybrid engine for some time.

 There has been much speculation actually that Virgin Galactic will have to transition to a liquid fueled engine to achieve suborbital flight. In point of fact, independent studies have shown that SS2 by switching to liquid fueled propulsion, can be suborbital on its own without even needing the carrier craft WhiteKnightTwo:

SpaceShipTwo could be single stage to suborbit says ESA firm

by Rob Coppinger on 29 April, 2010 

http://www.flightglobal.com/blogs/hyperbola/2010/04/spaceshiptwo_could_be_single_s/

Reusable Space Plane Idea Intrigues Europeans.
Rob Coppinger, SPACE.com Contributor
Date: 01 May 2012 Time: 04:30 PM ET

The Vinci suborbital space plane's structure and cryogenic fuel and oxidizer tanks are depicted in this illustration.
Credit: ESA

http://www.space.com/15494-vinci-space-plane-suborbital-flight-idea.html

  This would be by using the hydrogen-fueled Vinci engine. The Vinci is soon to be introduced on the Ariane 6. However, the existing HM-7 engine used on the cryogenic version of the upper stage of the Ariane 5 could also be used. The advantage of this is that it has been in use for decades and is well-characterized. You would probably need to place an extra one on the Ariane 5 upper stage to be able to lift the SS2. Still the engine and the stage are already developed and the cost of the addition of an extra engine should be comparatively small. The development cost of the SS2+WK2 combo has reportedly reached into the few hundred million dollars range. In contrast, the addition of an already existing engine to an already existing stage should be simpler, quicker and far cheaper than creating a new engine, hybrid, from scratch.

 The reason for the choice of the hybrid for the SS2 rather than a higher performance liquid-fueled engine was the idea that a hybrid engine could not explode. However, the accident in 2007 at Scaled Composites due to a nitrous oxide explosion has destroyed that misperception. Indeed because of the instability of nitrous oxide one team involved in developing a rocket propelled car suggests nitrous should not be used for passenger flight:

Observations and comments on Cal/OSHA report (Inspection No: 31081103) on fatal accident at Mojave test site of Scaled Composites at the Mojave Air and Space Port, 26th July 2007.

While it is most advisable to apply the established safety protocols relating to liquid oxygen, such protocols, in themselves are not sufficient to ensure the safe handling of Nitrous Oxide. The unique physical properties of N2O require further protocols above and beyond those used for liquid oxygen.
Safety protocols for N2O, in a rocket motor system, should include (in addition to the protocols used for Liquid Oxygen)

   * The detailed study of materials compatibility of all components in the system
   *  Avoidance of high temperatures at all points in the system
   * Stirring of large tanks
   * Avoidance of the gaseous phase both during apparatus filling and in use
   *  Purging of lines and valves immediately prior to ignition
   * Not using any component that may have previously absorbed N2O –   especially fuel grains

We are not confident that, even with these additional precautions, that we yet know enough about N2O to consider it a safe oxidiser for use in passenger flight. In the light of what we do know, safety must remain a major concern.

http://www.knightsarrow.com/rockets/scaled-composites-accident/


  Then the SS2 hybrid engine should no longer be considered to have an advantage over a liquid fueled engine. Then the fast and low cost development possible, especially with using an already existing engine, should push the decision to using liquid fuel. In fact by doing so SS2 probably could already have been flying by the originally announced date of the first suborbital test flights of 2007.

 The importance of their making that decision then and of their making that decision now goes far beyond that of just suborbital rockets however. If you look at the specifications of the cryogenic Ariane 5 upper stage, you see it could be propelled, with the SS2 aeroshell around it, well above the speed needed for suborbital flight. In fact it could be in the high Mach range envisioned for example for the X-33. A stage like that though could be used for a reusable first stage booster for a two-stage to orbit system.

 Now, since the first stage is generally much larger and costlier than the upper stage, a reusable first stage could significantly cut the cost to orbit of a two stage system. This in fact is what DARPA wants with its reusable spaceplane program.

 So Virgin Galactic giving SS2 a liquid fueled propulsion system could have a system to satisfy the requirements for DARPA's reusable spaceplane. In fact, it could already have had such in 2007.


   Bob Clark