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Solar Powered Helios Plane Destroyed in Test Flight
deglr6328 writes "NASA's solar powered Helios airplane has crashed into the Pacific off the coast of Kauai today during its first test using a regenerative fuel cell power supply. Helios held the record for highest prop propelled plane altitude at 96,863 (set 2 years ago) and was making preparations for a 96 hour continuous flight using its 62,000 solar cells during the day while electrolyzing water into hydrogen and oxygen for use in its fuel cells at night. With the capability to carry 200 lb. to near 100,000 ft. for months on end, Helios was eyed with great anticipation by scientists and RF telecommunications buisnesses alike."
Posted AC because I do not have a /. account.
IAARS. (I Am A Rocket Scientist.)
One question that has plagued me since the destruction of Columbia: If there wouldn't have been extreme heat going into the wing, would the crew still be alive? I'm no aerodynamics expert, but isn't it possible, at the point of entry into the atmosphere, when temperatures start to rise, that the shuttle release some liquid nitrogen or some other super-coolant in some manner as to keep homeostasis of the vehicle?
Upon reentry, the Orbiter (the white and black plane-lookin' portion of the Shuttle), is carrying no cryofuels. They are stored in the large red-orange External Tank, and used up during launch. The Shuttle uses LOX and LH2, both of which are f'nasty to deal with and are economical only to generate the immense thrust necessary to achieve orbit. While in orbit, the Orbiter maneuvers using (relatively) small hydrazine thrusters. N2H4 is also f'nasty, but somewhat less so than either LOX or LH2. NASA's Shuttle Basics website provides a good nontechnical overview of mission stages.
The Orbiter doesn't maintain homeostasis during reentry. The bottom gets really, really, really hot. Because the Orbiter is essentially falling back to Earth, the crew wants to bleed off as much speed as possible. By taking advantage of friction with the air, the Orbiter can slow down, and not be travelling at Mach 20 or so when it lands. It is a tricky balancing act among speed, attitude, and heat--the tiles can only absorb so much thermal energy, the crew has only aerodynamic control of the Orbiter's attitude, and there is a whole lot of kinetic energy that needs somewhere to go.
From my understanding of the physics of reentry, and the information available about the Columbia breakup, I do not think that the only factor was heat. The speeds at which spacecraft travel during reentry are so far beyond the speed of sound that aerodynamicists classify them not as supersonic, but hypersonic. The hypersonic regime (generally > M5) is somewhat counterintuitive. Friction with air generates enough heat at reentry speeds (M20 and up) to vaporize graphite and cause dissociation in N2 and O2 molecules, creating an ion cloud around the spacecraft.
We would not be able to travel at hypersonic speeds if not for a quirk of geometry. If you look at a supersonic vehicle, such as the X-1, you will notice that the leading edges of the wings and fuselage are pointed and form very sharp angles. This causes the shockwave formed by supersonic speed to break cleanly around the vehicle, which is good for aerodynamics. If you look at a hypersonic vehicle, like the Orbiter, you will notice a blunt, rounded leading edge and nosecone, which causes the shockwave to separate from the craft, forming a cushion of air. This insulates the Orbiter somewhat from the heat of reentry.
If that rounded profile is compromised, in Columbia's case by loss of tiles on the leading edge, the shock will break as in a supersonic craft, allowing both heat to transfer to the wing, and also subjecting the Orbiter to the considerable kinetic forces generated by air resistance. Heat did not tear Columbia apart. Her own speed did.
-Carolyn Lachance