Virgin Galactic Tech
 

Despite extensive gargling I have not found a decent explanation of the ship's systems - if anyone has such I would appreciate a link. In particular, from looking at early test videos, it almost seems as if the wing/tail 'feathering' is drive by air loads at high alpha. If so, how is it unfeathered, and could an inadvertent higher alpha after feather 'unlock' cause movement or is movement regulated?

BOAC: The 2nd column of the 3rd page (numbered 56) of
http://www.boulder.swri.edu/suborbit...Spacecraft.pdf
states that:

Two main pneumatic 625-psi actuators with a 9.5-in bore and 31-in stroke, change the position of the feather ...

http://en.wikipedia.org/wiki/SpaceShipTwo
has a tiny image of the craft, but it's pretty unreadable even if you click to enlarge.
http://en.wikipedia.org/wiki/File:Sp...al_diagram.jpg

Peter - thanks! I had found the mini-jpegs, but that pdf is excellent. I assume therefore they just use high-alpha to 'ease' the feathering load. I see from the 'sim ride' the tail was unfeathered well after apogee.

BOAC. Need to get away from the touchy-feely and other stuff on the other forum.

Apparently the NTSB has decoded several "tapes" early on.

- The mechanism for the feather mode seems straightforward and robust.

- I don't unnerstan why the mechanism is supposed to be unlocked before the vehicle is "coasting" up on its flight path and well above high "q". Passing 1.4M the vehicle is at a decent gee, so doesn't take much to move something too far or not far enuf.

gums: I'm a s/w guy trying to understand the pros and cons of when to unlock.

Not too early
At first sight unlocking just after rocket ignition seems premature and a needless hole in the cheese.

Not too late
I've seen something to the effect that the plane cannot re-enter without functioning feathers. Which gives a
reason for checking that it can unlock before passing the point of no return (wherever that is on the flight path).

To allow the feathers to be rapidly deployed for non-reentry reasons
To quote from another blog: http://www.orbiter-forum.com/showthread.php?p=486049
...because I wonder if SS2 lost attitude control (pitch/yaw/roll) after engine ignition? That might be one reason
why Alsbury pushed the feathering system to the unlock position. Remember the SS1 1st X-prize flight (16P)
in September 2004? Mike Melvill had to deploy the feathers early after it entered a violent roll late in the engine
burn. Maybe something was wrong early in the engine burn this time and Alsbury decided to arm the system
early in order to get it deploy ASAP after it gets above the atmosphere and engine shuts down?
... or course you might proactively unlock in anticipation of such an eventuality, especially on test flights?

It is not unknown for the Airspeed/Mach systems to fail,or under/overread in any aircraft,so it may be that it was an inadvertent early `unlocking`.However,I would have thought that a simple backup with `time vs expected acceleration` may have been useful as a cross-check,both on the ground and in the cockpit.

gums;

https://www.youtube.com/watch?v=0mCFxAsmnk0

This video below looks rearward at the moment of launch, through engine-start and the pitch-up/climb then the apogee and pitch-over. You can see the feathers working.

More interestingly however, the CVR is heard. I'm sure you'll be familiar with some of the sounds as the gee builds during the pitch-up.

Regarding the "unlock" question, from the video/CVR:

Release occurs @ 00:18 into the video. Engine light-off occurs at 00:22"

From crew comments heard on the CVR, unlocking the "feathers" is done @ 00:39, 17" shortly after initial engine light-off.

Actual release of the feathers, ("feather up" comment), near/at apogee for the descent occurs @ 01:18, or 49" after initial unlocking.

We don't have Mach information so don't know at what Mach the unlocking occurred.

The point is, unlocking shortly after engine light-off does not seem unusual according to this video.

In other words, the feathers aren't unlocked just before they're commanded to change position.

Perhaps something in the interlocks, (unlocked, not released) permitted the "feathers" to move; their relatively small structure seems (intuitively...not an engineer!), that they would not tolerate lateral loads well.

As we would know well, once such a failure mode started it would swiftly progress in the same manner as any airplane that had lost its rudder.

Feather moment considerations, abort scenario
 

Thanks for that AWST article, a wealth of information!

By deduction and combination from the various sources:
Feather unlock (and actual feathering?) nominally at Mach 1.4 (says NTSB Hart)
Engine burn of 3rd powered flight had been 20 sec to Mach 1.4 71kft
Engine burn planned 4th (this) powered flight approx 20 sec to approx Mach 1.4
(my assumption, not to change fuel and durn-time/Mach in one step)
Operational goal 2 minute burn Mach 3.4, 370 kft

Feather unlock lever, I wonder, is this:
- A safety lever securing the actual movement of the feather lever?
Or
- A lever disengaging physical locking pins allowing subsequent feathering by the pneumatic actuators
(or inadvertantly allowing uncommanded feathering by aerodynamic forces overpowering pneumatics-only)

Feather unlock c.q. actual feathering at Mach 1.4
- After motor burn-out around Mach 1.4 in third test
- Aerodynamic forces at that altitude/speed acceptable for coasting flight
- Aerodynamic forces at that altitude/speed also acceptable if still powered?
If yes on the latter, then it makes sence to unlock and/or feather after Mach 1.4 in the procedure for longer-burn flights, in order to discover anomaly in the essential feathering function for safe re-entry, and allow early motor-stop, in stead of continuing to push energy into the vehicle.

Hypothesis: Mach 1.4 and associated altitude and weight may be the limit for safe unfeathered re-entry?

Of course the unburned fuel mass after premature motor-stop is an additional weight/energy burden for reentry, the loaded SS2 is 30kLB (AWST), how much of that is the fuel mass? And can the oxidiser be dumped (through the extinguished engine) without re-lighting it?

A jettisonable rocket motor in case of feather failure comes to mind. Although that would also add complexity.

Any thoughts, additional information, corrections on that?

Is it only I who thinks, that the lateral stability feathered and unfeathered does not look great in those videos?

Do the pilots on the controls oppose this wing rocking tendency or is some kind of stabilization system doing the job?

Here's my dumb question - why is 'feathering' required (?after apogee?)? Is it because at the high alpha of 're-entry' the nose-down moment of the unfeathered wings would be too great and require too much reaction control?

RetF4 - I expect with the high sweep there will be ample LE vortices which will almost certainly not be forming symmetrically and 'natural' roll damping looks minimal. I would propose 'leaving it alone' or otherwise the likelihood of 'out-of-phase' inputs would be high. I'm not sure (for many other reasons as well!) I would want to be in that cabin. You'd have terrible trouble with the champagne:)

"Is it only I who thinks, that the lateral stability feathered and unfeathered does not look great in those videos? "

Lateral, and longitudinal; I wondered too. The first thing that came to mind was "PIO".

It occurs on all flights I've viewed the videos for so it's not unique to one flight.

I would think you'd still want to dampen it before serving the bubbly...

In the aft-view video above, I looked for small movements of controls but couldn't discern any although they'd probably be tiny. And oscillations don't stop except briefly at two points: approach to the apogee, and later in the descent when the feathers retract to normal position.

BOAC, gums, Retired4 - if you can look past the soft stuff in this early commercial for Virgin Galactic, there's something of an explanation regarding the feathers @ 2:40 in this video, (which also shows the wing-rocking at the beginning of the video for a flight in October, 2004).
https://www.youtube.com/watch?v=MN-GE4D61Zk


Beginning at 00:45", another video, (first flight with "feathering"), shows what I would call a "significant" pitching & rolling pre-, during and post-feathering.
https://www.youtube.com/watch?v=Lqhzlq7UReA

janrein;

Re, "Of course the unburned fuel mass after premature motor-stop is an additional weight/energy burden for reentry, the loaded SS2 is 30kLB (AWST), how much of that is the fuel mass? And can the oxidiser be dumped (through the extinguished engine) without re-lighting it? "

At 1:20 in the video with the aft-looking camera linked in the post above there's a crew remark, "auto-dump" and a cloud of gas is seen leaving the ship; it occurs again at 02:30 into the video and I suspect that's unburnt fuel and/or oxidizer. It doesn't appear to be dumped from the engine from from behind the tail-camera.

Lateral stability etc.
 

RetiredF4

From an AWST article posted earlier (Sept 7, 2009)
The SS2 reaction control system is remotely similar to the Space Shuttle Orbiter RCS. For the SSO re-entry was considered to start down from 400kft and a concept of "blended control" was applied, handing over roll control first to aileraons, then pitch control to the body flap, and rudder coming in last reaching the transsonic regime. This staged transition from RCS to aerodynamic control is said to have been the main requirement driving to Fly by Wire.
(see e.g. Free Online Courses From Top Universities | Academic Earth , then search ...)

The SS2 has to make the transition both ways and a learning curve has to be made again. One of the many challenges in the development program.

@Janrein
My question was in regard to that rocking and rolling flightphase, where gas thrusters would not jet or later on during reentry no more be used for steering.

@PJ2
Indeed, it is present not only in roll, but looking at more scenes in yaw and pitch as well. It increases when feathering, winds a bit down when feathered, and increasing again when defeathering.
Would some dehidral configuration of the main wing have a better outcome?

Various venting events
 

PJ2
Thanks, triggered me to looking further into that.
( video https://www.youtube.com/watch?v=0mCFxAsmnk0 )

This is what I see and what I think I hear.
0:44 End of motor burn observed
0:53 "There comes ullage"
Fumes observed on centre line, appear to be coming from rotor nozzle
(flushing motor fuel grain clean? dumping of remaining oxidiser?)
1:15 "Feather up"
1:18 Feathers observed coming up
1:20 "Feather dump"
A cloud observed escaping on port side for next 3 seconds
(venting of air from pneumatic feather actuator?)
(with water vapour condensation?)
!:47 "OK feathers down"
2:11 Feathers observed coming into view
2:17 Feathers observed down
2:26 "Autodump are on"
Cloud observed escaping on port side until end of video

Fumes appearing to come from nozzle may actually be excess oxidiser being dumped through the motor (without burning the fuel, flow started well after end of motor burn).

The oxidiser is nitrous oxide, liquid or gaseous.

The fuel is a solid.


BTW the plume from the motor burn can be observed around 0:50 just after end of motor burn, with a slight cork-screw twist due to roll oscillation in last stage of the burn.

At 1:26 the plume can be seen again, twist seems to have further expanded.

Around 2:00 the plume top is seen rising through the horizon, that 1:15 of coasting parabolic flight at that time.


Amazing footage.

Rocking and rolling
 

@ RetiredF4
I find it hard to estimate at what stage the jets will be applied in ascent and until when in descent, for the SS Orbiter the "blended use" was over a wide regime of altitudes and airspeeds.

Equally hard to say if the aerodynamic controls will be frozen during any portion of the trajectory, if they are, the regimes may again be different for each of the axes.

The feathers - going by Rutan´s own statements - are indeed meant to provide most of the stabilisation function when the air becomes progressively denser, I am not sure if the RCS system would be totally inactive in feathered configuration.

Other than that the aerodynamic active or passive stabilisation functions see transitions from subsonic through transsonic, to supersonic (low supersonic so far for SS2). There is hardly a constant regime, it´s all transients, and maybe that´s also a factor in the observed oscillations. I can imagine that the oscillations increase with decreasing q while the jets - if applied to complement the aerodynamic controls - are known to behave less predictably when there is still an appreciable effect of the thinning atmosphere (shockwave patterns).

That said, I have only touched on these complex matters and in a more distant past, so I welcome any other views and corrections.

@Janrain
I have no knowledge concerning SS2 and how it is designed to behave.
I've done a few handful of flights up to M2.3 in the phantom and up to 50.000 feet. This jet was getting the more stable the faster it flew. If i would have encountered such a rock'n roll motion in any flightphase I most probably would have aborted the flight. The problem with such non linear oscilations is, you never know how big the next one will be. And they must create some loads on those feather fins too.

I'm curious to see some footage of the fllight prior to the breakup, wether those oscilations had been present too.

RetiredF4;

Re, "Would some dehidral configuration of the main wing have a better outcome? "

Would that come with slightly increased drag?

And given that, I wonder how "critical" or unstable the design is in order to perform its layered tasks? I haven't flown anything but commercial transports and would never (ordinarily) see such motion but I certainly agree with you regarding the rock 'n roll and aborting the flight.

janrein, thanks for the interpretations - makes sense that "venting" oxidizer from anywhere but the rear wouldnt' be done.

If the controls aren't frozen, the RCS would be very busy and perhaps that's why it remains within certain limits? It's a lot of gyrating and given the air density, without knowing but only guessing, would be difficult for a pilot to control using conventional controls.

Bank angle and roll rate, what limits?
 

@RetF4 and @PJ2

The SS2 has only started to open the lower part of its intended flight envelope and has not even come close to the achieved performance of SS1. It´s a major scale-up, and clearly it needs time.

Here is footage of one of the SS1´s above 100 km flights, have a look at the rolling motions.
I appreciate that in increasingly straight-up motions the notion of bank angle vanishes, however there must be limits to roll rate both for crew and vehicle. Not sure what those would be though.

On the footage that has appeared of the accidented SS2 flight in those 9 seconds or so I have seen no indication of any significant oscillation, maybe more close-up and detailed footage will come out lateron. The NTSB has so far been quite open sharing factual information on the accident.

Not sure if that video is actually first hop for the prize. I watched both on live TV ten years ago. Guess I'll look at the long videos tonight. The clip is the first hop, sorry.

Seemed to me that flight two was the hairy one with the uncontrolled roll. An interview with Mike revealed he just let her roll as he was so high that aero forces were not a player and the RCS had some limits like time and authority. So he just let her roll and took control with the RCS at apogee or maybe lower.

The best part of that video is how effective the feather system worked.

Burt has favored the twin tail design forever. I would have liked a large vertical fin way back on the fuselage. Keep the outboard stuff, but increase roll and yaw stability.

I'm also fascinated by the way the guy survived - being forcibly ejected from the wreckage at 45k+ and to be able to deploy a chute.............