Launch pad design
[edit] Launch azimuth
Both of the launch pads at Complex 39, Kennedy Space Center, and the crawlerways that approach them, are aligned with true north. The main hatch of the spacecraft is facing due east and if the whole vehicle were to lift-off, pitch over and fly to the east, its flight azimuth would be 90°.[1]
However, the desired azimuth for each flight depends on the desired orbit. After lift-off the vehicle will roll so it's at right angles to the flight path, then pitch over and fly along the desired bearing.
Apollo launches could target a range of azimuths from 72° to 108° (see diagram). If the desired azimuth is 72° (as for Apollo 8), immediately after launch the vehicle rolls 18° so the hatch faces slightly north of east, then pitches over to fly along a bearing of 72°.
For moon missions, the initial Earth orbit depends on the position of the moon at the time of arrival, so the azimuth depends on the date and time of launch. For Apollo 17, a 2-hour 40 minute launch delay caused an update in the launch azimuth from 72° E of N to 91.504°E of N.
[edit] Launch pad orientation
The shape of launch pad A, approximately 2,297 square feet (0.7 square kilometers), was roughly octagonal. The elevated launch pad, which would rise 39.4 feet (12 meters) above ground level, lay in a north-south direction. This orientation required the crawlerway to make a near right-angle turn before approaching the ramp sloping 5 degrees upward to the top of the pad.[2]
This north-south orientation simplifies launch control calculations - the vehicle were to lift-off, pitch over and fly to the east, its flight azimuth would be exactly 90°.
"Two requirements governed the location of the umbilical tower and the service structure: the need for clear lines of sight from the erected launch vehicle to radar and telemetry stations in the industrial area 3 kilometers to the southwest, and an anticipated launch azimuth of 75 to 90 degrees."[3]
[edit] Reasons to roll
Since the spacecraft doesn't launch into the same orbit on every mission, they'd either have to built a rotating launch pad, or rotate the vehicle after launch. And since the vehicle has to maneuver anyway, it was logical to just roll the vehicle to the desired heading.
Most launch vehicles have "roll programs" to align them with the flight azimuth. Just as a crew likes to be heads-up with shoulders perpendicular to the flight path so do most guidance systems. In fact, many launch vehicle's major axes are aligned like an aircraft with the X-axis running the length of the vehicle. This orientation may be a little confusing on the launch pad but works perfectly for flight.
The reasons to roll are slightly more complex than just steering to a specific launch azimuth, though:[4]
- The roll program was initiated very early, during vertical flight, often to test engine gimballing almost immediately after clearing launch structures. If something was going to go south with the engine gimbals, they figured a crisp roll of the nearly fully-fueled vehicle would precipitate it, leaving them time to get their abort options in line (and giving them a good idea whether or not the thing was actually going to fly downrange and not fall back down on top of their heads).
- While a rocket could conceivably place itself on its azimuth from any roll position, the guidance is designed to align the rocket's body axis co-ordinates to a specified trajectory (specifically, to set angles between the body axis co-ordinates and a stable reference, like local vertical at the launch site). So, no matter where the rocket starts out in terms of pitch orientation, it's going to place itself based on its assigned body axis definition.
- Since your launch azimuth is likely going to vary from flight to flight (or from one launch window to the next), it makes no sense to try and set your pad up so that the desired roll attitude would automatically result from a straight pitchover onto your desired azimuth. The best solution is to not worry about the pad's roll orientation, just roll the vehicle to the desired position after lift-off.
- And, of course, for manned launches, you want to provide specific out-the-window visibility for manual flight maneuvers (like aborts and such). So, your desired attitude factors in, for example, any need for the pilots to see a horizon during various portions of their flight. While Apollo crews rode into orbit in a "heads-down" position (feet away from Earth, heads toward Earth), Gemini crews rode into orbit on their sides, which supposedly gave the CDR the best vantage for eyeballing Titan II trajectory dispersions.
References
- ↑ W. David Woods and Frank O'Brien, Day 1: Launch and Ascent to Earth Orbit, Apollo 8 Flight Journal, updated 2005-08-21
- ↑ NASA Kennedy Space Center Public Affairs, Building KSC’s Launch Complex 39 (3.4M pdf), FS-2002-08-010-KSC Rev. 2006
- ↑ Charles D. Benson, William Barnaby Faherty, Moonport: A History of Apollo Launch Facilities and Operations, NASA Special Publication-4204 in the NASA History Series, 1978, Section 2.4 - Writing the Criteria Book
- ↑ Doug Van Dorn, Why the Roll program, NASA SpaceFlight.com Historical Spaceflight forum, #119686 in reply to #11134, March 10, 2007