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 | | From: | Craig Fink | | Subject: | SpaceShipOne roll problem | | Date: | Thu, 16 Dec 2004 08:43:13 -0800 |
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>> > Adding dihedral really doesn't seem to be the solution to me.
>>
>> Space ship One has no dihedral, and a dihedral effect has gravity as a
>> component. Is it dihedral? No.
>
> According to the flight log as given at the Scaled Composites web site:
>
> (http://www.scaled.com/projects/tierone/logs-WK-SS1.htm)
>
> "SpaceShipOne Flight 16P Anomaly:
>
> Highly publicized in the media, the rolls near the end of the motor burn
> certainly got our attention. Detailed analysis determined that the rolls
> resulted from a mild thrust asymmetry, which was unable to be offset by
> pilot inputs at a flight condition of low directional stability. ... The
> characteristics of excessive dihedral effect and high-Mach low
> directional stability will be corrected on future spaceship designs."
>
There was an interesting article I read about the two rocket designs that
were developed for SpaceShipOne. One of the hybrid rockets had one hole
through the fuel, and the other has four holes through the fuel. They
picked the rocket design with four holes because it had higher thrust
early. More surface area to burn. But this design has a problem when the
walls between the holes gets thin. The thin walls break unpredictably and
chunks of fuel are spit out the back end of the nozzle. Causing large
banging noises as the unburned fuel partially blocks the rocket throat.
Maybe this is part of the thrust asymmetry problem, as the fuel inside the
motor would no longer be symmetric if the thin walls between the holes
break nonsymmetrically.
Also, this could make the four hole design more dangerous than the single
hole design. If a large enough piece of unburned fuel were to block the
nozzle long enough, it could possibly cause a pressure spike within the
engine large enough to rupture the engine.
But, then again, maybe the hybrid design minimizes this danger as compared
to a traditional solid fuel motor. If a pressure spike occurs in a hybrid,
the flow of oxidizer is reduced because of the increased pressure in the
engine. So the hybrid tends to limit transitory pressure spike.
In a traditional solid fuel motor the oxidizer and fuel are already mixed.
So as the pressure starts to build due to the obstruction passing thru the
nozzle, the fuel and oxidizer will continue to burn, further increasing
the pressure. Also, as the pressure increases within the motor, the burn
rate of the solid oxidizer/fuel goes up. Depending on the oxidizer and
fuel that are being used, it is possible for the solid motor to turn
itself into a large firecracker and explode. Which has occured from time
to time on expendable rockets.
The Space Shuttle has a problem with it's solids, slag builds up in a
pocket just above the throat of the engine. Under certain flight
conditions, the slag can slosh out and exit the throat, causing a small
pressure spike in the engine late in first stage.
Craig Fink
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