Guiding to orbit

The Saturn V took care of its own guidance and, assuming everything went
smoothly with the ascent, the crew had little to do except to keep a careful watch over it by running Program 11 (PI 1) on their computer, which displayed their speed, height and how rapidly that height was changing. Pll also drove their displays to show what their attitude should be through­out the ascent, so that any deviation could be seen. Should the commander have to take over control of the Saturn, he would fly it by following the cues given by Pll.

Eugene Cernan, who commanded the only mission to launch at night, later spoke about having trained to fly his Saturn V to orbit manually, a task no commander wished upon their mission, yet one which appealed to their test-pilot credo. "The launches – both from the Earth and from the Moon – were the only truly automatic phases of the mission, but we could take over and fly it manually to orbit. Aborting during Earth launch was the last thing I wanted to do, so I trained and planned. It was a lot more difficult at night than in the daytime because you didn’t have horizons and things to look at; you had to look at the stars. We had several modes of failure that could have degraded systems. The worst would have been for all the guidance to fail so that you literally had to fly it by the stars.

Now, I can never prove that I could have done it. But I did it a lot of times in simulators and really did – and still do – believe that I could have flown that Saturn V to orbit. It’s one of those things where you say T hope it never happens; but I dare you. I’ll show you. If you do fail, you just watch.’ You had to have that attitude; and I think that attitude is reflected across the cockpit. You develop confidence in each other, and, from that, the teamwork evolves.”

Making certain that a rocket gets to where it needs to go is a significant part of what is commonly referred to as ’rocket science’, although it would be better described as rocket engineering as, like all engineering, it is merely underpinned by science. Though the bedrock of rocket guidance is mathematics and physics, the basic concepts behind it are not so difficult to understand. What a space rocket is usually trying to achieve is to reach a point above most of Earth’s atmosphere at a defined time, and to be travelling at a certain speed and in a particular direction

when it gets there. Fulfilling these criteria should result in the rocket and its payload travelling in the desired orbit around Barth.[2]

The Saturn V. and many other launch vehicles after it, handled its guidance in two distinct and separate ways: one dumb, the other smart. It started off dumb, switching to smart once it was beyond the majority of the atmosphere. The dumb technique went something like this. "I don’t care where I am.’- says the rocket’s computer. ‘Tin just going to manoeuvre myself upwards through the air. tilling over in a fashion that’s been programmed into me. and I’ll see where I get to at the end.’’ This is termed open-loop control because information about the effect of a steering command was not fed back to influence subsequent commands. Engineers began the flight with this guidance philosophy because it was considered unwise to have the Saturn potentially make large steering turns while it was travelling at high speed through the denser regions of the atmosphere. For the first three minutes or so. the rocket flew according to a pre-programmed tilt sequence, a series of manoeuvres designed to ensure that its structure endured minimal sideways aerodynamic forces. This tilt sequence consisted of four major manoeuvres.

The first such manoeuvre was the 1.25-degree yaw that tended to scare onlookers during the first few seconds of ascent as it steered the Saturn V away from the launch umbilical tower. Once clear of the tower and upright again, it then made its second manoeuvre, rolling around its long axis to align the minus-г axis, the cast-facing axis, with the flight azimuth. Remember that, when sitting on the pad. the launch umbilical tower was to the north and the spacecraft’s hatch faced cast; the minus-z axis also pointed directly east and in that position the vehicle’s azimuth was 90 degrees. The roll manoeuvre’s job was to aim this axis in the direction they wanted to go so that thereafter the whole space vehicle would only need to make a simple tilting manoeuvre around its у axis, and start picking up horizontal speed. For most Apollo missions, the flight azimuth was around 72 degrees, a direction around cast- northeast which allowed for the most efficient path to a highly desirable free-return lunar trajectory that the early Moon missions would take. Apollo 15 and Apollo 17 had flight azimuths very near due east which helped them to access the northerly lunar sites that they had to reach.

Once the rocket had aligned its own coordinate system with its flight azimuth, the third and largest manoeuvre of the tilt sequence began; a very slow pitch-over to take them from a vertical attitude towards the horizontal as they began to accelerate not just upwards, but along the flight azimuth. The whole of the S-IC’s flight was carried out in dumb mode. The smart mode of rocket guidance came later.