September 11, 2003

Rising to the challenge

America has been challenged once again, this time by terrorism. Our response this time, like every time before, is to rise up and meet the challenge. Like Japan after Pearl Harbor and the Soviet Union during the Space Race, the terrorists are being met with a national determination not expected when they threw down the gauntlet.

Since rockets are what gets me going, I’m going to describe in some detail what we as a nation accomplished when we went to the moon. This is going to concentrate on the actual hardware, to give you an idea of the complexity and ingenuity involved. Links to reference sources are located at the end.

The American culmination of the race to the moon was the Saturn V rocket (pronounced ‘Saturn Five’). Standing over 363 feet tall with its Apollo payload, fully loaded she weighed 6.1 million pounds.

The first stage of this rocket was 33 feet in diameter, and powered by five F-1 engines, which were the largest liquid-fueled rocket engine ever designed.

Excerpted from the Saturn V News Reference:

At the end of countdown, the five F-1 engines in the first stage ignite, producing 7.5 million pounds of thrust. The holddown arms release the vehicle, and three astronauts begin their ride to the moon. Turbopumps, working together with the strength of 30 diesel locomotives, force 15 tons of fuel per second into the engines.

Ok, now we’ve got ‘liftoff’. The roar is deafening, and spectators six miles away feel the rumble of the earth beneath their feet.

The first start separates from the second stage at an altitude of about 205,000 feet (38 miles in 2½ minutes). It then ascends to a peak altitude near 366,000 feet before beginning its descent. While falling the stage assumes a semistable engines down position and impacts into the Atlantic Ocean at approximately 350 miles down range of Cape Kennedy.

Think about that last part for a second. The first stage separates from the rest of the rocket, and coasts upwards another 150,000 feet before it slows down enough to start falling. The U.S. Navy ensured that there was no shipping in the ‘impact’ areas.

The F-1 engine was the key. None have come close to the F-1 for its sheer size and volume of thrust. The design originated in the 1950's, with an Air Force study. When NASA was formed, it brought along the design program and the Saturn rocket family was destined to use it. The F-1 burned refined kerosene mixed with liquid oxygen. By contrast, the equivalent Soviet ‘moonshot’ rocket design – the N1 – had a first stage that used 30 separate engines. Balancing the thrust of this many engines proved impossible, and the N1 was launched just four times; each one was a disaster ending in abrupt and catastrophic failure.

On to the second stage, again excerpted from the Saturn V News Reference:

For the lunar mission, the second stage takes over from the Saturn V's first stage at an altitude of approximately 200,000 feet (38 miles) and boosts its payload of the third stage and Apollo spacecraft to approximately 606,000 feet (114.5 miles in 8½ minutes). Speed of the stage increases from 6,000 miles per hour to 15,300 miles per hour. Propulsion is provided by five J-2 engines.

The beginning of second stage boost is a two-step process. When all the F-1 engines of the first stage have cut off, the first stage separates. Eight ullage rocket motors located around the bottom of the second stage then fire for approximately 4 seconds to give positive acceleration to the stage prior to ignition of the five J-2 engines. About 30 seconds after the first stage separation, the part of the second stage structure on which the ullage rockets are located (the aft interstage) is separated by firing explosive charges. This second separation is a precise maneuver: the 18-foot-high interstage must slip past the engines without touching them. With the stage traveling at great speed, the interstage must clear the engines by only a little more than 3 feet.

A precise maneuver performed at more than 6,000 miles per hour. "Ullage" is an old brewers term meaning the air space above beer in a vat. To restart the J-2 engine, the propellants must be at the "bottom" of the tanks, with the gas above them. If they aren't in this position, the fuel pumps would "cavitate" or suck up gas and the start would fail. ‘Gas’ in this case is meant literally, because the J-2 used liquid hydrogen and liquid oxygen as fuel.

The 81-foot 7-inch second stage is basically a container for its 942,000 pounds of propellant with engines attached at the bottom. Special lightweight insulation had to be developed to keep its cryogenic propellants from warming and thus turning to gas and becoming totally useless as propellant. The insulation that helps maintain a difference of about 500 degrees between outside (70 to 80-degree normal Florida temperature) and inside (-423-degree F of liquid hydrogen) is only about 1-1/2 inches thick around the hydrogen tank.

The second stage burns for about 6 minutes, generating 1 million pounds of thrust and pushing its payload into space. At the end of boost, all J-2 engines cut off at once, the stages separate, and the single J-2 engine on the third stage begins firing to take it and the Apollo spacecraft into a parking earth orbit.

The third stage of the Saturn V was the first stage ready for flight testing because it evolved from the second stage of the Saturn Ib. It was enlarged, lengthened, and equipped with the J-2 engine for use on the Saturn V. This single J-2 developed 225,000 pounds of thrust.

The third stage fired for not quite 3 minutes to reach orbit before shutting down. At this point, the spacecraft is going 17,500 miles per hour. The astronauts orbited earth up to three times while checking all systems for the upcoming trip to the moon. When ready, the J-2 was restarted for another 5.2 minutes to send Apollo on its way, reaching a speed of 24,500 miles per hour. The trajectory was one which would carry the astronauts around the moon without further thrust and then return to earth for re-entry.

Once underway, the astronauts turned their spacecraft around and docked with the lunar landing module enclosed within the third stage. After extracting the lunar module the third stage was abandoned, which completed the work of the Saturn V.

There were a total of 13 Saturn V’s launched in various configurations. Three examples still exist and are on display at the Johnson Space Center, Kennedy Space Center and the Alabama Space and Rocket Center. Of these three, only the JSC vehicle is made up entirely of former flight-ready (although mismatched) components.

When President Kennedy challenged America to go to the moon, we did it in less than eight years. We didn’t start from scratch, but the engineering obstacles we overcame were immense, and the achievement magnificent.

The Apollo Saturn Reference Page
Information about the Soviet N1
Apollo Saturn News Reference

Posted by Ted at September 11, 2003 08:32 AM
Category: Space Program
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