Launch Vehicle Environment and Its Effects on Spacecraft Design

Another attribute of the launch process, which has a major effect on the spacecraft design, is the launch vehicle environment. Usually the structural design of the spacecraft (see Chapter 9) is not governed by its lifetime in orbit, which could be 10 years, but rather by the few minutes it spends climbing to orbit on the launch vehicle. We have already seen how much energy has to be released in a controlled manner during this relatively short period of time to achieve an orbital state for the spacecraft. It is not too surprising, therefore, to realize that the spacecraft is exposed to a lot of noise and vibration, and is subject to significant levels of acceleration to boost its speed from effectively zero to 8 km/sec (5 miles/sec) in a few minutes to reach orbit.

For launch spectators, one overriding impression is the wall of sound that hits them a few seconds after they see the rocket engines ignite, despite the fact that they are kept at a safe distance from the launch complex. Launch is a very noisy affair, and even more so for the satellite payload sitting on top of the rocket. The acoustic field encountered by the satellite is harsh, despite the satellite being contained within the launcher fairing. Large amplitude and damaging vibrations can be excited in flexible structures, such as solar panels or large antennas, by this level of noise.

Similarly, it is not surprising that the energetic processes occurring in the propellant feed pumps, the combustion chambers, and the rocket nozzles at the base of the launch vehicle cause a high level of vibration. Astronauts riding a man-rated launch vehicle to orbit invariably report quite a rough ride!

In addition to noise and vibration, the third main environmental effect to which the spacecraft is exposed during launch is acceleration. Figure 5.9 shows a typical acceleration profile against time for an Ariane 5 launch vehicle, where the acceleration is given in units of g's. To understand the impact this has on the spacecraft design, we need to recall the discussion in Chapter 1 about the force of gravity at Earth's surface. If you drop something, it will accelerate toward Earth's center, increasing its speed by 10 m/sec (32 feet/sec) for every second it falls. This acceleration of 10 m/sec/sec, usually denoted by 10 m/sec2, is the reason we stay stuck to the floor. This environment, in which we experience our normal weight, is sometimes called a 1g environment. However, when we ride to the top of a skyscraper in a high-speed elevator, while the elevator is accelerating upward to gain speed we feel heavier (see Fig. 2.2b). And so it is with launch vehicles. Figure 5.9 shows that the level of acceleration for this particular launch vehicle can be in excess of 4g—four times Earth's surface gravitational acceleration— which means that the spacecraft and its component parts effectively weigh four times their normal Earth surface weight. It is not too difficult to see the effect this has on the spacecraft structure, since its job is to mechanically support all the various parts of the spacecraft—payload instruments and subsystem elements—which are effectively much heavier under severe acceleration.

Clearly, the spacecraft structure design engineer has to take into account the launch vehicle flight environment to ensure that the spacecraft does not fall apart on the ascent to orbit.

Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

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