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Wednesday, June 09, 2004

SpaceShipOne article in the latest Aviation Week
chabot imageVia hobbyspace.com: SpaceShipOne article in the latest Aviation Week is quite interesting but unfortunately not available on line without a subscription. Here are some highlights
- The June 21st flight will carry out a full-duration 80-sec burn of the engine.
- The speed should reach Mach 3.5. Peak equivalent airspeed, however, will not exceed the previous flight since the air pressure will drop so much.
- The engine will use a larger nozzle that has not flown before.
- The engine will undergo higher temperatures, higher speed, and lower dynamic pressure at burnout.
- The previous two flights carried full fuel loads but the liquid nitrous oxide was turned off early to terminate the burn.
- A ground firing flowed oxidizer for 100-sec. This consumed all of the fuel plus some of the phenolic liner and caused even the outer casing made of carbon fiber/epoxy to start smoking but it didn't result in a burn through.
- The new nozzle provides a larger expansion ratio needed for good thrust and efficiency at the higher altitudes. This means, however, that it hasn't been tested on the ground since the higher atmospheiric pressure causes flow to separate from the walls and become turbulent.
- Sensors and temperature-sensitive paint indicated in the last flight that temperatures were a bit lower than expected. None of the thermal protection coating was damaged. (This photo shows the red TPS coatings.)
- In the June 21 flight, however, it is expected that the coating will be damaged and need replacement before the next flight.
- The last flight did a supersonic reentry, reaching Mach 1.9 in feather mode.
- Mike Melvill manually damped oscillations in feather mode. The oscillations may have been due to sloshing in the tank but in the June 21 flight all the oxidizer will be burned.
- Asymmetries in the thrust due to possible erosion in the nozzle near the end of the burn could be difficult to compensate since the very low pressure provides weak control authority. The cold gas attitude control thrusters are "very weak compared to a conventional surface biting into thick air."
- "The computational fluid dynamics says it is OK but the pilot will be earning his salary."
- In the last flight the instrument display went dark about one third of the way through the rocket firing. Nevertheless, Melvill was able to navigate by viewing the horizon through the windows and still reached the target altitude. The problem is believed to have been caused by a potentiometer affected by the acceleration and has been fixed.


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