With SpaceX’s Falcon 9 and Falcon Heavy launches all over the news, there is a term called “launch window” regularly associated with it. In our today’s post we will learn in depth what does this term actually mean and its importance.
First let’s discuss a simple analogy: if you were watching the 400 metres race from the centre of the track and wanted to intercept one of the runners taking part. One way would be to simply chase the runner you wish to stop by running along the track. If you were fast enough, you might eventually catch up but only after expending a lot of energy and travelling a long way.
A much better way to intercept the target athlete is simply to walk across the centre to the other side of the circular track. It is a much shorter distance and you use a lot less energy and time getting there.
Space launches work on the same principle, just on a much larger scale and with many more variables. E.g: if the rocket launch is to deliver cargo to ISS then it needs to consider the speed of ISS rotating about earth i.e. 28,800 kmph or 18000 mph and earth is rotating at a speed of 1665 kmph or 1040 mph. So a perfect time has to be calculated considering the speed of the rocket, the orbit it intends to insert the payload, the rotational speed of earth and the ISS. Engineers calculate how much time they have—down to the minute or even second—to launch and reach a target. This determination is called the launch window, and it has become a critical part of spaceflight operations.
There is another term called as launch period, which is a collection of days of which launch window is a subset. Launch periods are often calculated from porkchop plots (Figure 1), which show the delta-v needed to achieve the mission plotted against the launch time.
Thus, launch window can be defined as the time frame on a given day in the launch period that the rocket can launch to reach its intended orbit.
Launch window becomes more important for long duration missions such as Mars missions, which has a launch window of few weeks every 26 months, because of repetitive alignment of the planets. As all planets move in long, curved paths around the Sun that take the shape of circular and elliptical orbits. Therefore, straight-line paths do not exist.
For the case of Earth observation satellites, which are often launched into sun-synchronous orbits, the launch window occurs at the time of day when the launch site location is aligned with the plane of the required orbit. To launch at another time would require an orbital plane change maneuver which would require a large amount of propellant.
In more complex cases, including the use of gravitational slingshots, launch windows are irregular. Sometimes rare opportunities arise, such as when Voyager 2 took advantage of a 175-year planetary alignment (launch window) to visit Jupiter, Saturn, Uranus, and Neptune. When such an opportunity is missed, another target may be selected. For example, the ESA’s Rosetta mission was originally intended for comet 46P/Wirtanen, but a launcher problem delayed it and a new target had to be selected (comet 67P/Churyumov-Gerasimenko).