SpaceShipTwo

Point out news stories, on the net or in mainstream media, related to polywell fusion.

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Giorgio
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Re: SpaceShipTwo

Postby Giorgio » Thu Nov 06, 2014 5:06 am

A society of dogmas is a dead society.

hanelyp
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Re: SpaceShipTwo

Postby hanelyp » Thu Nov 06, 2014 5:22 pm

I am wondering if, with the feathering system unlocked, transonic flow may have overwhelmed actuators normally not engaged at that speed.
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Re: SpaceShipTwo

Postby Giorgio » Thu Nov 06, 2014 6:13 pm

A society of dogmas is a dead society.

GIThruster
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Re: SpaceShipTwo

Postby GIThruster » Thu Nov 06, 2014 9:33 pm

I'm still amazed anyone could survive that kind of accident. Anyone have any details of how that worked?
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Re: SpaceShipTwo

Postby paperburn1 » Thu Nov 06, 2014 11:40 pm

The anonymous sources within Scaled Composites revealed that Siebold was able to unbuckle from his seat and deploy his chute at 17,000 feet (5,181 m). It is very likely that even Siebold fell unconscious from the initial stresses of the breakup and from decompression at 50,000 feet (15,240 m). He would have fallen into an unconscious state at that height and only have woken up once near 17,000 feet (5,181 m) where the atmosphere is denser and at which a human can survive, such as at mountain altitudes in the Andes and Himalayas. Whether he gave a thumbs up to a nearby chase plane is sensational but it would indicate that he was conscious and aware. With the parachute integrated into his test pilot suit, it was critical for Siebold to regain consciousness and unbuckle from his seat in order to give his parachute any chance of deploying. This is likely where the fate of the pilots differ.

Alsbury quite possibly was struck by debris or was injured by G forces and decompression more severely than Siebold. He either never regained consciousness or was somehow trapped in his seat and surrounding debris of the cabin. The circumstances for Siebold in his descent after the breakup were apparently fortuitous and gave him the chance to re-awaken and unbuckle. Comments in press reports from people around the incident or aware of the technology included that the pilots’ parachutes had automatic deployment mechanisms which activate at 10,000 feet (3048 m). In Alsbury’s or Siebold’s situation, without releasing themselves from their seats, the automatic deployment system would not have worked. If the chutes were to automatically deploy while the pilots were still strapped to their seats, the force from the deploying chute would have caused serious injury to the pilot.
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Tom Ligon
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Re: SpaceShipTwo

Postby Tom Ligon » Fri Nov 07, 2014 3:29 pm


DeltaV
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Re: SpaceShipTwo

Postby DeltaV » Fri Nov 07, 2014 5:18 pm

Another possible contributing factor, not yet mentioned, is Pilot-Induced Oscillation (PIO).

In the transonic regime, where this accident occurred, the susceptibility to PIO (and many of the other factors mentioned above) is higher than on either side of Mach 1.

Capturing all of the nonlinear system dynamics in a simulator (aeroelastic structural modes, uncertain transonic flow as shock waves form, hybrid motor thrust variations, liquid oxidizer slosh, etc.) is very difficult. Approximations and numerical table look-ups typically used in the simulator to model certain effects become less and less valid as dynamic ranges, number of active frequencies, cross-couplings, and various departures from linearity increase, reducing the predictive ability of the sim.

Perhaps an unfortunate combination of circumstances, a tragic synergy of dynamic "tolerance stacking" not seen in the simulator for the particular set of test cases that were run preflight.

EDIT: Hadn't read the interweb reports about insider info on the lever unlock at M1, until now. It seems more certain than I'd thought that it actually happened, so my comments above about PIO and "tolerance stacking" seem moot for this incident (still valid for some others).

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Re: SpaceShipTwo

Postby GIThruster » Mon Nov 10, 2014 4:39 pm

It is true this happened on reentry, is it not? Just asking because there are no control surfaces up there. This is why the tail rotates--to act like a drag chute because there is not enough air to have control surfaces. Sounds to me like the drag chute was opened early and couldn't take the abuse.
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Re: SpaceShipTwo

Postby paperburn1 » Mon Nov 10, 2014 5:28 pm

I believe it happened on climb to alt.
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GIThruster
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Re: SpaceShipTwo

Postby GIThruster » Mon Nov 10, 2014 6:52 pm

So we don't know if the rocket was firing or not?

When they say the tail boom was "unlocked" early, at 1.0 gee rather than 1.4 gee, one supposes a little early, not an entire flight phase early. Could this really be what happened?
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Re: SpaceShipTwo

Postby paperburn1 » Mon Nov 10, 2014 10:12 pm

"I would like to emphasize that what I'm about to say is a statement of fact, and not a statement of cause," Hart said, speaking at a late Nov. 2 press conference. There, he used two models of SpaceShipTwo to show how the vehicle "feathers" during reentry. Under normal operations, SpaceShipTwo is flown to a pre-determined altitude and released by its carrier aircraft, WhiteKnightTwo. For Friday's test, this happened at about 50,000 feet.

Shortly after SpaceShipTwo is released, it ignites its rocket engine and pitches upward, flying into a high arc designed to eventually reach the boundary of space. After the spaceship reaches apogee—the highest point of its flight—the vehicle's twin tails rotate upward, stabilizing and slowing SpaceShipTwo as it reenters the atmosphere.

It is not clear what the intended sequence of events was for Friday's test flight, but Hart said that the feathering system deployed prematurely. In order to feather the vehicle, two levers must be moved inside the cockpit. One unlocks the system, and the second deploys the stabilizers.

"About nine seconds after the engine ignited, the telemetry data showed us that the feather parameters changed from lock to unlock," Hart said. The telemetry data was confirmed by a camera inside the cockpit that showed co-pilot Asbury moving the handle. Normally, the feathers are not unlocked until the vehicle reaches Mach 1.4, Hart said. On Friday, Asbury unlocked the feathers around Mach 1.0.

However, neither pilot commanded the feathers to deploy. Shortly after the system was unlocked, the tail stabilizers began moving into their feather positions. Moments later, telemetry and video data stopped. The engine burn was normal up to the extension of the feathers, Hart said. This information runs contrary to speculation that the accident may have been caused by the engine, which was using a new fuel mixture during the flight. Scaled Composites, SpaceShipTwo's contractor, said the engine had been tested on the ground "many times," according to company president Kevin Mickey.

Hart said pieces of SpaceShipTwo, which are spread over a wide swatch of the Mojave Desert, are being moved into a hangar. "Among other things, [the investigators] found the fuel tanks, the oxidizer tanks, and the engine. All were intact and showed no signs of burn-through, and no signs of being breached," he said.
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Re: SpaceShipTwo

Postby JoeP » Tue Nov 11, 2014 4:41 pm

From that, it sounds as if simply unlocking them early, while a breach of protocol, should not have changed the tail configuration. Perhaps a mechanical switch associated with the second lever was activated due to acceleration forces or perhaps some defect. A very unfortunate sequence of events that lead to the tragedy. But easier to fix than an engine/fuel problem. It is amazing that one pilot survived.

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Re: SpaceShipTwo

Postby DeltaV » Tue Nov 11, 2014 10:17 pm

It is still hard to believe that any pilot familiar with this vehicle would intentionally arm the feathering mechanism prior to reaching a low dynamic pressure region, after motor burnout, near apogee.

If buffeting did not cause his hand to bump the unlock lever, my next suspect cause would be hypoxia. They were at 50,000 ft, after all.

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Re: SpaceShipTwo

Postby hanelyp » Tue Nov 11, 2014 10:48 pm

I don't know how much difference it would have made in this case, but it makes sense to me that if instrument G forces approach safety limits the engine automatically throttles down.
The daylight is uncomfortably bright for eyes so long in the dark.

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Re: SpaceShipTwo

Postby DeltaV » Wed Nov 12, 2014 12:55 am

@JoeP
If the lever is unlocked near Mach 1 the normal aero forces alone are probably sufficient to overpower the feather actuators. The pilots would know this. Most primary flight control surface actuators (electric or electro-hydraulic) have modes such as floating-undamped, floating-damped, locked and active, all modes intrinsic to the actuator's local computer, but also selectable by the primary flight control computers. Since feathering is a secondary flight control function, it probably uses much simpler actuators with fewer modes, perhaps just on/off, with very little locking ability in the off state. I'm guessing that the lock function for SS2 feathering is provided solely by the lock/unlock lever in question, on the assumption that it would never be unlocked until dynamic pressure had dropped to where the actuators could hold until commanded to move.

@hanelyp
It's not clear if there is an autothrottle or not on SS2. If there is, then yes, it would make sense to throttle down when any of several parameters exceed safe bounds. But dynamic pressure killed this bird, and that is just 1/2 rho v**2, so chopping the throttle would probably be too little, too late.


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