Forces of acceleration

The job of the S-IC and its five F-l engines was to lift the stack to an altitude of 70 kilometres and accelerate it to a speed of about 8,500 kilometres per hour. As it did so, the acceleration felt by the crew gradually increased. In common parlance, acceleration is stated in terms of g-forces, because the force we feel on Earth due to gravity is directly comparable to the force imparted by the acceleration of a vehicle. Therefore, it is useful to relate acceleration forces to something of which everyone has a lifetime’s experience. In this manner, when the Saturn was sitting on the pad, the crew felt an acceleration of 1 g, due entirely to Earth’s gravity.

Because the Saturn V weighed down on the launch pad with almost as much force as the engines were pushing up, the stack initially rose quite gently with g-forces barely above 1. But the consumption of over 13 tonnes of propellant per second lightened the vehicle considerably as it flew. This decreased the mass the engines had to push against and, since they did not throttle, increased the acceleration forces imposed upon the crew.

A small additional source of rising acceleration came from an improvement in the efficiency of the F-l engines as they rose through the atmosphere. A rocket engine works by burning propellants in a combustion chamber. The heat of combustion causes the gases to expand very rapidly and this exerts massive pressure on the walls of the chamber. If one of those walls is missing (because someone has placed a nozzle there), the pressure within the chamber becomes unbalanced, resulting in a force.

Graph of g-forces during first stage flight. (Redrawn from NASA source.)

However, at sea-level, the pressure of Earth’s atmosphere has the effect of slightly capping the open end of the nozzle, somewhat inhibiting the high-speed flow of exhaust gases and reducing the thrust that the engine can generate. By the time the virtually empty S-fC gave its final push, the atmosphere had become essentially a vacuum, which reduced the back-pressure against which the exhaust gases had to contend as they left the nozzle and this had the effect of improving the thrust by almost 20 per cent. Each engine, which had started out with a thrust equivalent to 690 tonnes, was pushing with 815 tonnes force just prior to the exhaustion of the first stage.

In response to these two effects – an increasingly light S-fC and five increasingly efficient engines – the acceleration continued to ramp up ever faster until about 2 ‘A minutes into the flight when, having reached nearly 4 g, it was reduced by the shut­down of the S-fC’s centre engine to a little over 3 g. Under the power of the remaining engines, the g-force resumed its rise towards 4 g as the vehicle lightened further. The early shut-down of one engine not only curtailed the rising acceleration, it also lessened the jolt felt by the stack and the crew when the remaining engines cut out. Additionally, it flagged the Saturn’s computer to think about starting Timebase 2, which eventually began once a specified speed had been gained. The computer then sensed propellant levels in the nearly empty tanks and prepared to shut down the outboard engines. By the time the stage was expected to shut down, about 25 seconds after the centre engine cut-off, the acceleration would again be approaching

4 g.