Start window

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Launch window describes the period of time at which a spacecraft can be launched so that the mission can be completed. If the start cannot be carried out within the start window, it must be postponed until the next start window.

The location and size of the launch window depend on the launch site, the energy reserves and the controllability of the available launch vehicle, as well as the actual mission profile.

If the mission consists only of putting a spacecraft into orbit, the launch can take place at almost any point in time. The start window then results from further restrictions, for example the requirement that the start should take place in daylight.

Overflight zone

The first limitation usually results from the permitted overflight zone. The exact position of the trajectory projected onto the ground depends on the starting time due to the rotation of the earth; the start window therefore opens or closes when the web reaches or leaves the edge of the permitted zone. Restrictions may be necessary for reasons of safety (especially on the ground in the event of a crash), the communication range of ground stations or ships for tracing purposes, and in special cases for political reasons.

Manned space flights

For manned space flights, higher safety standards apply, so that the start window may have to be shortened. Among other things, the launch window is limited by the length of time the crew can spend in the spaceship buckled up before take-off. The overflight zone must then take into account possible emergency landing sites.

Rendezvous in orbit

In some cases, the goal of the mission is to rendezvous with a spacecraft that is already in orbit . This maneuver was first carried out in August 1962 when Vostok 4 was launched in such a way that the spaceship approached the first launched Vostok 3 within a few kilometers. Rendezvous now take place several times a year with the International Space Station , which Soyuz spaceships and space transporters fly to.

The location of the launch window results from the fact that a spacecraft in orbit flies over every point on earth whose latitude is less than the orbit inclination at intervals of about 24 hours . Within the start window there is a point in time with the least energy to be expended. If you do not start at this point, more fuel is required. The size of the launch window then results from the energy reserves and the controllability of the launcher. The start window for the space shuttle on the way to the ISS was open for a few minutes, whereas the start of the rendezvous missions in the Gemini program had to take place within a few seconds. On Gemini 11 , the startup window was only open for 2 seconds.

Satellite groups

Particular precision at launch is necessary if the launched satellite is part of a satellite group and the required orbit must be synchronized with several other satellites. In these cases, the start window can be reduced to a few seconds, for example with the satellites of the A-Train .

Moon flights

Since the lunar orbit is elliptical and inclined in relation to the equatorial plane of the earth, the energy requirements for a moon flight vary during the month and from mission to mission. The Apollo program also had to take into account restrictions regarding the choice of landing site (near the equator and not too close to the edge of the moon), as well as the fact that the sun at the landing site was not yet very high above the lunar horizon. This resulted in only one start window per month, which was open for around three to four hours.

Interplanetary probes

The energy requirement for a space probe to other planets depends extremely on the starting time. The most energy-efficient option is the transfer on the Hohmannbahn . The start window opens only once during the synodic orbital period of the target planet. This is over two years for Mars , less for other planets.

For missions to the outer planets, the position of Jupiter , which is mostly used for a swing-by maneuver, must also be taken into account .

The duration of the launch window for a planetary mission is several days to a few weeks, depending on the power reserves of the launcher. Due to the rotation of the earth, the start window can only be started at a certain time during the day, so that the start window opens once a day for a longer period of time. The first attempt to start is usually scheduled a few days before the optimal time so that delays due to technical problems or the weather can be compensated for.

Organizational restrictions

Restrictions of a non-technical nature can arise in order not to allow a mission to last over holidays or comparable occasions and thus to save personnel costs. This category also includes the flight ban issued by NASA for the turn of the year 1999/2000.

General

The term of the launch window that opens and closes became popular with coverage of the moon flights in the Apollo program . At that time people also spoke of the moon window .

For the landing of a spacecraft there is also a comparable period of time at which the brake rockets must be ignited in order to land in the intended area. However, one does not speak of a landing window .

From a time-critical point of view, the start window is a sub-term of the time window .

Individual evidence

  1. ↑ NASA: STS-120 Mission Information (English)
  2. ↑ NASA: On The Shoulders of Titans, Ch 15-3 (English)
  3. ↑ NASA: Start window of the Apollo missions (English)

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