Saturday, November 1, 2014

Two Commercial Spaceflight Disasters in One Week

Two commercial launch failures within days is a lot for a fledgling industry to take.  Especially with political critics that are quite undeserved.  One of these failures relates to commercial manned flights,  that being Virgin Galactic's SpaceShip Two.  The other relates to unmanned cargo and satellite launch,  that being Orbital Sciences' Antares rocket and Cygnus cargo spacecraft.  

Update 11-3-14:  with a rocket-powered vehicle,  the first suspect is always the engine when there is a problem.  That appears to be the case with the Orbital Sciences launch failure.  

But with the Virgin Atlantic spaceplane,  preliminary NTSB comments reported this morning indicate there was a problem with the re-entry feathering system,  not the engine.  This one is going to be very interesting when the report is done.  

In any event,  enjoy the rocket technology discussions that follow.

Update 11-5-14:  Published news releases as of today indicate that Orbital thinks their launch disaster was caused by a turbopump failure.  They also no longer intend to use those engines.  The best new information about the Spaceship Two disaster suggests that the two pilots were literally ejected into the air as the cabin broke up around them,  at about Mach 1,  near 50,000 feet.  These reports indicate they were so ejected without pressure suits or supplemental oxygen.  One managed to deploy his parachute and lived,  the other didn't and died.  

Update 4-15-15:  There is no news yet from the NTSB regarding Spaceship Two that has made public release.  However,  regarding the Antares explosion,  it does appear that the turbopump bearings were essentially destroyed in one of the refurbished Russian engines,  probably from foreign object damage.  

Orbital (builder of the vehicle) and Aerojet Rocketdyne (refurbisher of the engines) are at odds over this.  similar to Ford and Firestone Tires a few years ago.  At issue is where the foreign objects that got into the turbopump came from.  Whose fault was it that such debris got in there?  

If it were in the engine to begin with,  how did it escape quality control at Aerojet Rocketdyne?  If it came from the vehicle tanks or piping,  was there not an inlet screen in Orbital's design to catch such stuff?  Both possibilities are rather hard to believe.  

We may or may not ever hear in public which is true.  But there's not a lot of other possibilities for that kind of failure.  

Hybrid vs Solid Rocket Technology

Hybrid rockets “done right” have safety advantages over solid propellant rockets,  if the fuel is just that:  fuel,  without any oxidizer in its formulation.  That can be a rubber,  a plastic,  or even a wax,  and it can have solid powders dispersed in it,  just not oxidizer powders. 

That kind of hybrid ceases combustion when you stop the liquid oxidizer flow.  That makes the vehicle abortable,  since you can shut off the engine,  as with liquid rockets.  Solids burn like dynamite sticks:  once lit,  they burn to completion,  no choice.

Solid propellants are typically a rubber or rubber-like binder loaded with particulate solids,  solids that include the solid oxidizer as powder.  Solid oxidizers are typically ammonium perchlorate or ammonium nitrate these days.  There are some others. 

Solid propellants,  even those that are seriously fuel-rich,  all burn with two physical effects controlling their burn rate.  The prime factor is an inherent burn rate of exposed surface that depends upon chamber pressure per a power function with an exponent under 1,  usually well under 1.  The secondary factor is often called “erosive burning”,  and shows up as a burn rate enhancement (added term) in the presence of hot gas scrubbing along the exposed surface at high speed. 

All exposed solid surface quickly produces massflow,  because there is very rapid surface flame spread from lit areas to unlit areas.  The most common problem encountered with solid rockets is the presence of cracks or voids in the propellant charge,  because these very quickly (in milliseconds) add large quantities of burning surface,  producing massive increases in motor massflow.

This very quickly drives the chamber pressure too high,  overpressuring the case,  and causing a violent explosion.  It typically happens without warning symptoms,  and very quickly (on a few milliseconds time scale).  This is due to the very nonlinear mathematics of what determines the chamber pressure,  which is a balance between pressure-driven surface massflow generation,  and pressure-driven nozzle massflow capability.

The second most common failure in solid motors is a case insulation failure,  leading to a burn-through,  followed quickly (usually just under a second) by the case exploding.  The third most common failure is a type of combustion instability wherein the oscillations cause higher average pressures from an enhanced burn rate,  due to the erosive-burning effect.  These can lead to over-pressurization explosions on a time scale of a few seconds,  but not always.

That kind of combustion instability has an unstable positive feedback of combustion energy released from tiny combustion eddies directly into the oscillations.  It occurs when some natural gas vibration mode in the geometry is close to the small-scale combustion eddy frequencies.

A very distant fourth most common failure is a piece of propellant or other debris coming loose inside the motor,  and getting blown into the nozzle,  where it obstructs the throat,  causing the motor to virtually-instantaneously explode like a pipe bomb. 

The most common problem in the hybrids is a kind of combustion instability that causes very rough operation and vibrations,  but usually without the unstable feedback of the solid.  This is inherent due to the gross unmixedness resulting from generating the fuel from only the erosive-burning effect of the hot port flow (oxidizer plus fuel from upstream plus still-combusting fuel and oxidizer). 

There are localized fuel-rich and fuel-lean pockets of gas.  As fuel and oxidizer suddenly combine in the very turbulent mixing,  these pockets explode.  The bigger the “exploding vortices”,  the stronger the thrust and pressure oscillations.  But because there is no feedback into a chamber-pressure burn rate effect,  the instability doesn’t grow catastrophically out-of-control within seconds,  the way it does in solids. 

The other two fatal hybrid failure modes are the same as solids:  case insulation failure leading to burn-through,  and debris blocking the throat.  Update 11-3-14:  hybrids with no oxidizer in their fuel grains are pretty much immune to the effects of grain cracks or voids.  There is no pressure effect burn rate in these freshly-exposed spaces,  and the massflow scrubbing effect just doesn't reach there hardly at all.  Fuel burnout "tailoff" characteristics can be affected somewhat,  though.  

Historically,  hybrids have suffered design impracticality from very low effective burn rates (more properly,  “regression rates”).  The historical “fixes” for this are (1) multiport grain designs that reduce the effective thickness to be burned,  and (2) adding low amounts of oxidizer powder to the fuel grain formulation. 

In recent years,  a third option has become available:  fuel formulations that liquefy (literally melt) before they pyrolyze into combustion.  That third option is the best.  The multiport option leads to low volumetric efficiency.  Adding solid oxidizer is absolutely the worst,  by far.

Adding oxidizer,  even in amounts too low to sustain combustion with the liquid oxidizer shut off,  converts the hybrid “fuel” into a fuel-rich conventional solid propellant.  That negates the very most important safety advantage of being a hybrid.  Now there is fast surface flamespread,  drive by the inherent burn rate vs pressure effect of a solid,  into cracks and voids.  This restores the same vulnerability to motor explosion as any other solid.  

If you add too much solid oxidizer,  you cannot stop the motor by cutting off the liquid oxidizer.  This negates the other advantage of a hybrid,  abortability. 

A Word About Liquid Rockets

Liquids have three of the very same failure modes that afflict solids and hybrids:  burn-through,  debris blocking nozzle throats,  and combustion instability.  The failure of regenerative cooling (not insulation) is what causes the burn-through.   Combustion instability in liquids takes many forms,  all being related to ignition of mixing eddies that are rich or lean in fuel. 

The unique problem with liquids is turbopump failures,  since nearly all liquid systems have them. (Update 11-3-14:  some hybrids can also have pumped liquid oxidizers,  others are pressure-fed.)  These components are very highly stressed,  and exposed to very extreme conditions of heat and cold simultaneously.  It is not surprising that they fail.  When they do,  there is almost always some kind of explosion. 

Another failure mode known since the V-2 missile of World War 2 is starting at too high a propellant flow rate.  This will literally blow the engine apart,  virtually instantaneously.  It does take a very finite time to spool turbomachinery up from low thrust settings to high thrust settings.  Further,  there are serious limitations on just what the minimum thrust of any particular engine design can be. 

SpaceShip Two’s Loss 10-31-14:

The available data-to-date are spotty at best.  We will have to wait for the NTSB investigation to complete,  before the truth and all the facts come out.  That can take over a year. 

The news photos I have seen indicates that SpaceShip Two dropped from White Knight Two successfully,  and then successfully ignited its rocket engine.  Then there was an explosion. Update 11-3-14:  not an explosion,  a mid-air breakup,  according to very preliminary comments made by the NTSB.  

Photos of the wreckage tell me that the engine compartment was violently destroyed,  also blowing away the tail booms of Spaceship Two.  Without the tails,  the fuselage and its stub wings just become a tumbling ballistic projectile to the desert floor. 

The tail booms,  and that which is recognizable as fuselage wreckage,  appear to have crashed in different places on the desert.  I did recognize the fuselage aft pressure bulkhead in some photos of the wreckage.  There was nothing left aft of it.  Update 11-3-14:  more published photos from more view angles show that there was nothing left on either side of this bulkhead.  

Some accounts I have seen claim CNN as a source for the assertion that the engine shut down,  and then exploded upon a restart attempt.  I do not know anything about that,  myself. 

Nothing I have found among open sources on the internet would indicate that solid oxidizer was a part of the original HTPB fuel grain formulation,  or the new plastic grain formulation on the lost flight.  That’s not to say there wasn’t any,  but nothing released publicly indicates it. 

Everything I saw says fuel-only grain plus liquid nitrous oxide (N2O).  The only change seemed to be the switch from HTPB rubber to a plastic for the binder.  That occurred when Virgin took the motor design in-house,  away from Sierra Nevada,  who had done the HTPB version.

That leaves motor case burn-throughs or debris plugging the nozzle,  from the failure modes listed above.  There is also the possibility that the restart attempt was transiently just too violent an event,  and simply split the motor case open. 

Whatever happened,  the event was violent enough to suddenly “disperse” the entire engine compartment from the aft fuselage,  since the fuselage debris in the desert terminates at the aft pressure bulkhead.  Update 11-3-14:  with a midair breakup,  it was broadside air pressures that broke up and dispersed everything.  It doesn't take an internal explosion to do that,  just a loss of attitude control.  

This same violent event was enough to separate both articulated tail booms from the aircraft.  They can be quite clearly seen about a vehicle length away on each side in the news still photo that was taken a split second after the explosion. 

Pieces of the motor case are going to be hard to find in all of that desert.  It may be impossible to find enough to reassemble it,  to determine if there was a burn-through. 

If there was some kind of mixing baffle inside the motor to help with the low-grade instability of hybrids due to poor mixing,  then it might have come loose and blocked the nozzle.  It might be vulnerable during an ignition transient.  If it can be found,  extreme distortion of the part might suggest this possibility.

Finding out whether restart attempts might be too violent will require motor restart testing on the ground.  Virgin and Scaled Composites certainly have a lot to investigate.  I would suggest doing any such restart tests in a revetment of some kind,  as a ground crew safety measure. 

Update 11-3-14:  With the NTSB focusing up the the feathering system causing a mid-air breakup,  the engine issue revises to confirming that it was operating correctly.  

Orbital Sciences Antares/Cygnus Loss

I have seen news video footage of this loss enough times to know that something was wrong from liftoff.   The engine plumes of kerosene-liquid oxygen engines are very brilliant,  enough to cause camera “image bloom” all the time.  Yet in this launch,  that “apparent fireball” was just too bright. 

A few vehicle lengths off the launch pad I saw the vehicle slowing to a stop,  followed by the first explosion.  That first explosion would have been the range safety self-destruct charge ripping apart the first stage.  The rest of it blew up as multiple explosions upon impact very near the launch pad.

I do not know,  but I suspect either a turbopump or chamber failure leading to a loss of incandescent gas around the engine at vehicle rear.  After a few seconds overheating,  the engine blew apart,  killing thrust,  and precipitating self-destruct.  These are refurbished 40+ year-old Russian-made engines! 

Orbital Sciences is using these Russian-made engines as the only thing available to them in the right size range.  These were originally built in Russia over 4 decades ago,  and were stored most of those years in a barn of some kind in Siberia.  Aerojet Rocketdyne refurbished these for Orbital Sciences.

This says more about corporate (and government) policies regarding offshoring overseas critical US capabilities,  than it does anything about the engines themselves.  If “we” had not frittered-away this country’s rocket engine manufacturing capabilities,  Orbital would not have been forced to use refurbished 40+ year old foreign made units.  Period.  Somebody needs their butt thoroughly kicked over this issue!

From what I can find on the internet,  one of these same engines blew up in a ground test a while back.  I could not locate anything explaining that test failure,  but then,  I am no internet expert.  However,  that’s enough to cast some doubt on this engine.


Orbital has to get to the bottom of this.  They will need Aerojet’s help.  The rest of us need to give them the “wiggle room” to get that job done.  Critics stirring up trouble in the halls of congress are not needed,  and can go stuff their unhelpful “suggestions” where the sun doesn’t shine!

Same goes for Virgin’s fatal problem with SpaceShip Two:  they need “wiggle room” and time to get to the bottom of what happened. 

I can only wish both outfits well,  and may God speed their investigations.  I would gladly help either or both of them.


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