Even casual space fans have heard it, “Tonight’s launch window will be from 6:00pm to 8:00pm.” Or, “SpaceX’s launch window this evening will be an instantaneous launch window at 9:12pm.” What is a launch window, and why does SpaceX use instantaneous launch windows?

What is a launch window?

In the shortest or terms, a launch window is a time period for which a specific launch has to happen to achieve a particular orbital trajectory. An easy way to think of this is thinking of being on a highway approaching an exit you intend to take. You do mental calculations, whether you notice it or not, on how fast the traffic is between you and the lane you need to be in to exit, how fast you are going, what speed change is needed to safely exit, what the speed limit of that exit is, and what traffic is on your ramp or next roadway. In this situation these calculations happen quickly, you speed up or slow down accordingly, and you make your exit.

Developing a launch window depends primarily on the orbit to which mission planners intend to launch. Using this information, the launch location, and the capabilities of the spacecraft, launch windows are developed. Most launch windows are a couple of hours long because the spacecraft has enough propellant to achieve the desired orbit over a range of time. More specific orbital trajectories, such as polar orbits, will necessarily have smaller launch windows, because the orbit will only pass over the launch site once per day, whereas a normal (i.e. not SpaceX) launch to the ISS will have a longer launch window because the ISS will pass over the launch site multiple times in a short period, based on its inclination, or angle around the earth.

So what is an instantaneous launch window?

As stated above, launch windows are dictated by quite a few factors. In some instances, if the launch site does not pass underneath the orbital trajectory at all, there can be no launch from that site. In other cases, the orbital trajectory may only pass over the launch site once per day, in which case the launch window would be very small. Spacecraft capability plays a large part in launch window calculations.

With SpaceX in particular, the spacecraft limitations require instantaneous launch windows for many flights. The Falcon 9, when intended to be reused, uses densified fuels. This means that the RP-1 and liquid oxygen (LOX) used by SpaceX are only a few degrees above freezing. This allows them to load more fuel, and thus have enough to launch and land the rocket. By the very nature of these propellants, SpaceX has found that once the rocket is loaded, they will only remain at the desired temperature for a short amount of time. When they first started using these densified fuels, they attempted normal launch windows, but once they passed their first attempt, the process of defueling and refueling the rocket proved to be too long to have more than one attempt in a normal launch window. Thus was born the instantaneous launch window.

Almost all ISS resupply, and future crewed flights, require instantaneous launch windows, as the inclination of the ISS will not allow refueling between crossings of the ISS over the launch site. Flights that require a geosynchronous transfer orbit (GTO), which are all flights sending satellites to geosynchronous orbit, will typically have a long launch window. SpaceX, for a long window flight, will determine a fueling time. Once the rocket is fuelled, the launch window then becomes instantaneous due to the densified propellant. Other orbital trajectories may have long launch windows depending on necessity.

For those cases when they fuel and then fail to launch instantaneously, they will fall back to their secondary launch window, which is typically within a few minutes of the previous window, but on the next day. This phenomena is due to the not quite 24 hour rotational period of the earth.