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Old 11th Nov 2014, 13:54
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How it is supposed to work

SpaceShipTwo: How it's Supposed to Work : Discovery News

Granted that this source is credible, it gets more confusing.


Initial analysis ruled out an engine problem. Instead, onboard video and data relayed during the flight quickly led investigators to conclude that for some unknown reason co-pilot Mike Alsbury, whose body was found in the wreckage, moved a lever that unlocked the spaceplane’s pivoting tail section early, before conditions were right for aerodynamic forces to hold it in place.
Once unlocked the tail is not held in place by the pneumatic system, but by aerodynamic pressure on the tail?? Or does it mean that the pneumatic system is not powerfull enough to hold it in place when arrodynamic loads exceed a given limit?
I tried to follow the logic in the patent, while the unlocking is explained in great detail, there is not that much info on the working of the tilting system itself, or I was not able to grasp it.


After SpaceShipTwo is dropped by its carrier jet some 50,000 feet above ground, its tail section, unlike the tail of a regular aircraft, is generating up-lift on the vehicle, rather than a downward force (which on airplanes counters the wings’ lift to generate stable, horizontal flight.)

Once its rocket motor burns out and its fuel is spent, SpaceShipTwo’s center of gravity shifts forward, so that the tail generates downward force, like an airplane. In between boost and re-entry, aerodynamics forces dictate when structural loads shift for safe reconfiguration of the vehicle.
Aerodynamic loads on the tail are not only influenced by speed (subsonic, transonic, supersonic) and altitude (less loads higher up), but also by shift of CG due to burnt fuel. According to the source that leads to a reversal of lifting force on the tail. after burn out. If I follow that logic, then the unlocking of the system would not only depend on the airspeed, but also on the remaining rocket fuel.

That must be a hell of a ride for the pilots to keep that ship pointed on the intended flightpath, now i understand these flight path oscilations some more.

Does that also mean, that unlocking the feathering system at 1.4M gives the tail some freedom to align with aerodynamic loads and thus helps in dampening the flightpath?

Any thouhts on that?
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Old 11th Nov 2014, 17:33
  #42 (permalink)  
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Once unlocked the tail is not held in place by the pneumatic system, but by aerodynamic pressure on the tail??
- I am totally confused by what we know so far of the tail 'feathering' system, but I think we must assume the tail movement is 'powered' somehow, and I can see significant aerodynamic forces moving the tail to 'feather', but I cannot see how the reverse will happen - unless the 're-entry' is inverted.
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Old 11th Nov 2014, 19:52
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- I am totally confused by what we know so far of the tail 'feathering' system, but I think we must assume the tail
movement is 'powered' somehow, and I can see significant aerodynamic forces moving the tail to 'feather', but I
cannot see how the reverse will happen - unless the 're-entry' is inverted.
I too am confused, but some items in the patent seem to clarify some points and raise others.

From para [0018] Pilot-controlled pressurization of actuators 33 drive the aft wing sections and tail booms upwardly and downwardly.

From para [0019] When the wing sections are retracted after reentry, actuators 41 are again pressurized to extend pistons 43 to reengage the locking system.
So it seems that:
- Intentional feathering and un-feathering is by pneumatic upward and downward actuation.
- The feathering lock is re-applied as soon as the downward actuation is completed.

Leaving the following questions open:

- What keeps the booms in the feathered position, as there is no locking mechanism for this.
--- Pneumatic pressure?
--- Aerodynamic forces?

- What keeps the booms in the unfeathered position after unlocking, but prior to feathering?
--- Aerodynamic forces? But only in appropriate flight regimes?
--- Pneumatic pressure? But have the downward actuators been pressurized yet?
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Old 11th Nov 2014, 21:30
  #44 (permalink)  
 
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Actuators

It looks to me like Cylinder 33 controls the tilt. In the diagram it appears much larger the Cyl 41. From the description, I infer that 33 and 41 have the ability the ability to both push and pull rather than rely on some outside force (aerodynamic in the case of 33 and maybe a spring like force - like a leaf sping - in the locking hook in the case of 41) for one direction. If I'm right, then the pneumatic systems have the ability to apply differencial or equal pressure to both sides of the piston.

Applying equal pressure to both sides of the piston would "lock" the piston in place, but only up to a certain force because the air inside is compressible.

Under this scenario, 33 might provide stability above Mach 1.4, but be overcome (depending upon altitude) by aerodynamic forces in the transonic zone.

Another possibility is that a pneumatic failure in 41 could have caused a premature unlock and the pilot was moving the unlock lever to try to fix it.

I'll be reading the NTSB final report with interest.

Last edited by thcrozier; 11th Nov 2014 at 22:22.
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Old 12th Nov 2014, 12:27
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With the additional insight from the Discovery News ref. (RetiredF4 post)

Once its rocket motor burns out and its fuel is spent, SpaceShipTwo’s center of gravity shifts forward, so that the tail generates downward force, like an airplane. In between boost and re-entry, aerodynamics forces dictate when structural loads shift for safe reconfiguration of the vehicle.
So not only must a sufficient Mach number have been reached (to be clear of the complex dynamics of the transsonic regime), also a sufficient CoG forward shift must have been made.

The first transition occurs around Mach 1.4, or about 1.4 times the speed of sound. It’s not an exact time, but dictated by a combination of several technical factors including vehicle speed, altitude and motor nozzle angle. Around Mach 1.4, the lock (which actually is a chunk of machined aluminum) is moved so that the feather is now mechanically free. It won’t move though because at Mach 1.4, aerodynamic forces keep it nailed back.
The Mach number 1.4 in the applicable flight regimes appears to imply both proper "clean" supersonic conditions and the required CoG for tail surfaces down-lift in that regime.

“In addition to the lock, the feather itself has a big actuator that drives it up and down. So just because it’s unlocked doesn’t mean it’s just flopping in the wind and it can go where it wants. In the (initial) boost phase, the aerodynamic forces can overcome that, which is why we lock it in place,” Moses said.
Which suggests that after "the (initial) boost phase" the aerodynamic forces cannot overcome the pneumatic downforces on the feather. Which would imply that positive lift on the tail surfaces up to a certain magnitude can be tolerated. Such positive lift occurances (assumed limited in time and magnitude) may result from the trim of the horizontal tail surfaces, elevon deflections and thrust vector variations (e.g. due to uneven burn and outflow).

Two main pneumatic 625-psi actuators with a 9.5-in bore and 31-in stroke, change the position of the feather ...
Some numbers related to force and stroke had been found by PeterH, with an estimate of tail boom dimensions (someone may have these?) these numbers may then be related to maximum adverse (positive) lift on the tails that can be tolerated.

Which may serve to answer part of the recent questions from PeterH.

And leaves the following questions:
- What keeps the booms in the feathered position, as there is no locking mechanism for this.
--- Pneumatic pressure?
--- Aerodynamic forces?
In line with the previous I would expect Yes and Yes. In addition the proper re-entry attitude relative to flight path - when not controllable aerodynamically - may be set up by the reaction control gas jets.

@thcrozier
Applying equal pressure to both sides of the piston would "lock" the piston in place,
A hard locking in any halfway position is not possible with pneumatics, and I would not expect the feathers to be in any other position than their extremes except when transitioning. Maintaining some pressure on the retreating side of the piston may serve though to smoothen the transition. In the feathered position I can imagine - rather than bi-lateral pressurisation - a uni-lateral pressurisation pressing against a hard mechanical stop.

@BOAC
- unless the 're-entry' is inverted. ,
I understand the the unfeathering motion to be driven by the pneumatics, possibly assisted with forward trimming of the horizontal stabilisers and up-deflection of the elevons to reduce required forces.

Upon inadvertant failure of the pneumatics during the unfeathering phase perhaps a roll to (partial) inverted attitude may be an emergency option (inside or outside of designed procedures). I recall this trick had been pulled by an aeroplane with structural wing failure (distant past, must search deep ...).

Other than that: bail out at safe altitude and speed.

It would be interesting to know if parachutes are forseeen for space flight passengers (?)

Last edited by janrein; 15th Nov 2014 at 07:44. Reason: typo
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Old 12th Nov 2014, 16:13
  #46 (permalink)  
 
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I recall this trick had been pulled by an aeroplane with structural wing failure (distant past, must search deep ...).
At the risk of thread drift I guess you mean this:

Zlin wing Structural Failure Report - Neil Williams
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Old 12th Nov 2014, 17:19
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unplanned test pilot flights

Oh yeah, the aerobatic pilot's famous inverted approach and crash.

As with me on my Viper leading edge flap failure ride, you have to remain as calm as possible and see what works and what doesn't. But you don't have a minute, or even a few seconds in some cases. As with the aerobatic dude, I was flying fast enough to have a small degree of roll control and plenty of yaw, even with the right LEF folded up at 50 or 60 degrees. So don't do something stupid, just sit there!!! Don't slow down, don't go faster, don't pull up, don't roll, just stay there while the gal is happy.




The SS2 guys didn't have a chance. And I seem to recall Mel unlocking the feather mechanism on his severe rolling ride for first leg of the X-prize. Alleged reason was for "dampening" or such.

I do not like allowing or depending on my control surfaces to move on their own. Just my preference, but I have reasons. Even at M 2, the SS2 should be able to shut down the motor and come back safely. Problem is the sucker goes ballistic at "x" altitude/"x" speed, and you are along for the ride to apogee. I would guess that the RCS might be used at or slightly above 100K with motor still running at a low setting, but once that motor shuts down for good you're screwed. Not sure about SS2 nozzle gimbals such as shuttle had. Dick Covey and John Blaha told me that the "return to launch site" procedure was a pipe dream, but they practiced it anyway because the main motors could be gimballed and throttled way back. Imagine rotating a zillion pounds of vehicle using the remaining fuel, main motors and OMS system and then gliding back home, heh heh.

Lastly, I do not understand the procedure to unlock the feathers early except it appears that the mechanism is pneumatic, not pure mechanical. Why not a simple cable and long handle to rotate those big hooks? Leverage. Ditto for deploying the feathers coming back. But maybe loads on the feather suckers require a huge mechanical advantage as they are exerting pressure against the locking hook.

I am sure we will see some design and procedure changes, ya think?
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Old 12th Nov 2014, 17:24
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@wiggy
Quick find!
And I would say relevant (admitted remotely) to the SS2 mission.
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Old 10th Mar 2015, 16:21
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Virgin Galactic to drop Scaled Composites from SpaceShipTwo rocket testing - Aerospace Technology

Scaled Composites is reportedly being dropped from testing Virgin Galactic's new SpaceShipTwo (SS2) rocket.

Scaled Composites president Kevin Mickey told Los Angeles Times that the company will no longer be engaged in SS2 testing and would work as a consultant to Virgin Galactic.



Following the SS2 crash, Virgin modified the spacecraft based on the investigation findings.

Virgin Galactic has appointed Mark 'Forger' Stucky as pilot to its commercial flight team responsible for flying WhiteKnightTwo and SpaceShipTwo.
Consequences of the crash and / or disagreement how or if to proceed further?

http://www.virgingalactic.com/introd...ond-spaceship/
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