last fuel stop before...

Friday 30 January 2009 at 9:52 pm [Those of you not interested in rocketry or spaceflight or physics can probably skip this blog post and the next one; this includes my wife.] Do you ever wonder why rockets are so complicated? The Space Shuttle has a complicated launch system that consists of two big solid rockets and then it goes higher with the smaller engines in the back. The Saturn V consisted of three full stages before getting to orbit, and then it used the third stage again to push the spacecraft out of earth orbit toward the moon. Why the multiple stages? Why not just build a huge rocket that would do the job all at one shot. The answer is complicated, but the core of the reason is that to get something into space requires a huge amount of ENERGY, to get it into orbit, even more. In current spacecraft technology, that energy is supplied by chemical rocket engines. The strictly most efficient way to send something of a given weight into orbit is to shoot it from an enormous gun. However, the huge acceleration (the firing of the gun) would produce acceleration so powerful that it would squash any payload that you tried to launch in this way. Mechanical components produce maximum acceleration limits to the launch method. If humans are involved, that further restricts the maximum acceleration that can be applied. There's another major factor here; the atmosphere. A lot of the energy carried along in the vehicle gets expended pushing air out of the way. The enormous energy requirements lead to a secondary effect. It turns out that to launch a significant distance, a rocket's mass has to be mostly fuel when it's launched. That means that when it's done firing, the vehicle is MUCH lighter than when it started, and so its ACCELERATION is much higher. (Newton's second law: F=ma. Assuming a generally constant engine thrust, or F, then as the fuel burns away making "m" smaller, the acceleration "a" gets larger.) The higher the percentage of the rocket's mass that starts off as fuel, the larger the final acceleration will be. But from above, acceleration is something that we have to limit for safety and structural reasons. So that's why rockets have stages. They provide a way of changing (during flight) into a smaller vehicle that has less thrust but matches that part of the flight profile better. From this book on the Saturn V, here's the chart of acceleration during the mission of the three stages: <?php csdnimage("SV00.jpg"); ?> At the end of the first stage, the Saturn gets up to almost up to 5 gravities. Jet fighters are sometimes stressed to 9 gs, but that isn't usually sustained for very long. Some comparisons of some widely different vehicles. Saturn V rocket: total weight at launch: 6.1 million pounds total weight of fuel: 5.66 million pounds 93% fuel by weight Cessna 172: maximum gross weight: 2300 pounds weight of full fuel (39 gallons): 234 pounds 10% fuel by weight 1996 Ford Escort: curb weight: 2451 pounds full fuel (12 gal): 72 pounds 3% fuel by weight So the rocket performs radically differently between full and empty fuel states. The Cessna 172 will be slightly more limber almost empty as opposed to full. The car you won't notice the performance difference at all.

Labels: saturn5, space

Sorry about that, chief

Wednesday 21 January 2009 at 8:37 pm I've been watching my way through the first season of the DVD edition of the spy spoof Get Smart. One of the extras in the first season is a reunion of the cast that took place in 2003. They discuss, among other things, the origins of some of the bits that became famous in Get Smart. Don Adam's character first appeared in an earlier sitcom where he plays a bumbling house detective alongside Bill Dana in an early version of his José Jiménez character was a bellhop. Adams said in the reunion that he and Dana together wrote a lot of the character bits for his detective that became Maxwell Smart. The detective character (partiularly the grating voice) started in Don Adam's stand-up act as an annoying attourney. He would begin the sketch with: "Ladies and gentlement of the jury, the prosecution has spent the last twenty minutes making an ass of himself. Now it's my turn." He said the lawyer character was based on Nick Charles from the Thin Man movies. I noticed a couple of interesting things in the show recently. The copyright statement is a little different than any I've seen anywhere else: <?php csdnimage("gs00.jpg"); ?> any "firms"? Were they worried that "Control" would be somehow mistaken for the internal workings of the CIA or something? One other thing: <?php csdnimage("gs01.jpg"); ?> Notice the chair in the lower right-hand corner. I'm almost sure that we had dorm chairs like that when I was in college (1991-1995). In the 1960s, I guess they were classy hotel furniture.

Labels: getsmart

please direct your attention to the flight attendants

Sunday 04 January 2009 at 9:01 pm You know the safety briefing that flight attendants give to you when you fly on an airliner? Well private pilots are required to give a version of it to their passengers too. I ran across a checklist for a Piper Arrow that listed an extremely detailed briefing, which got me thinking as to what parts of it are required. FAA regulation 91.519 covers this topic. Basically, you must make sure your passengers know: - when and where they can smoke - how to use the seat belts and when to have them on - location and operation of normal and emergency exits - location of survival equipment - information on ditching (landing in water) if appropriate - use of (normal and emergency) oxygen equipment Regulation 91.21 also covers the use of portable electronic devices. The current version, as of 2009 states:

(a) Except as provided in paragraph (b) of this section, no person may operate, nor may any operator or pilot in command of an aircraft allow the operation of, any portable electronic device on any of the following U.S.-registered civil aircraft: (1) Aircraft operated by a holder of an air carrier operating certificate or an operating certificate; or (2) Any other aircraft while it is operated under IFR. (b) Paragraph (a) of this section does not apply to— (1) Portable voice recorders; (2) Hearing aids; (3) Heart pacemakers; (4) Electric shavers; or (5) Any other portable electronic device that the operator of the aircraft has determined will not cause interference with the navigation or communication system of the aircraft on which it is to be used. (c) In the case of an aircraft operated by a holder of an air carrier operating certificate or an operating certificate, the determination required by paragraph (b)(5) of this section shall be made by that operator of the aircraft on which the particular device is to be used. In the case of other aircraft, the determination may be made by the pilot in command or other operator of the aircraft.

I assume "an operating certificate" is something that applies to airlines. So...if I read this right, then when operating a general aviation aircraft under VFR flight rules, I can allow people to use Cell phones in flight? And even if I'm going to operate IFR, I can still have them used if I determine that it's safe? I'm stunned. For some reason, I assumed there was a hard "don't use Cell phones in an airplane or you'll surely die!" regulation somewhere. Since airlines prohibit their use, I had always assumed that the same rule applied to me as a generation aviation pilot. My briefing (to flight instructors) has pretty much been "seat belts fastened? Cell phones off?". Guess we learn something new every day.

Labels: faa, flying, regulations

hollowpoint

Sunday 04 January 2009 at 12:32 pm There was a fascinating article in the New Yorker about John Coster-Mullen, a photographer who over many years and using secondary evidence, puzzled out the designs of the first atomic bombs dropped by the United States in 1945. The article is a long but fascinating read, about his process of piecing together clues and photographs and fragments saved by machinists and other people who worked on small pieces of the bombs. Coster-Mullen has also produced a book about his findings, published in 2003: Atom Bombs: The Top Secret Inside Story of Little Boy and Fat Man. His partial diagram of the internals of the Little Boy (the Uranium-fission bomb; the Fat Man was based on Plutonium) is available on Wikipedia under a creative commons license. I remember when these diagrams first surfaced a few years ago, presumably when this the book first became available. One very interesting item about this process is Coster-Mullen's re-discovery of the central Mechanism of Little Boy. The bomb worked by slamming two sub-critical pieces of Uranium-235 (which undergoes self-accelerating fission) together to form a super-critical piece. The assumed setup always has been that a slug of Uranium was fired into a cup Uranium. The fact was the opposite, as illustrated in the diagram. The target was a cylinder held in the bottom of the bomb, and a hollow cylindrical projectile was fired at it, embracing the target slug. My interest in this subject is about the physics involved, rather than the destructive possibility. It's also about the process of secrecy vs. discovery, and figuring out how something was made, even as monsterous a device as that one.