Low reference orbit

Low reference orbit ( DOE , low Earth orbit) - the orbit of a spacecraft near the Earth. It is legitimate to call the orbit “reference” if it is supposed to significantly change it - an increase in altitude or a change in inclination . If maneuvers are not provided, or the spacecraft does not have its own propulsion system at all, the use of the name " low Earth orbit " is preferable. In the general case, it is believed that a spacecraft is in a reference orbit if it moves with the first space velocity and is located at a height where the corresponding density of the upper layers of the atmosphere , in a first approximation, allows circular or elliptical motion ^[1] . Such a combination is described in professional language as "the duration of a ballistic existence exceeds the time of one revolution." At the same time, in orbit of this type the device can be located, including, and less than one revolution, in accordance with the flight plan. For example, all payloads launched by the 8K78 Lightning rocket were carried out in a reference orbit from about 2/3 to 3/4 of a turn. With the expansion of the capabilities of the control system of the upper stages of rockets and booster blocks, the time spent in the reference orbit began to vary widely. So, the Indian Mars probe Mangalyan spent about 2 days in a reference orbit.

Typical parameters of the reference orbit, using the Soyuz-TMA spacecraft as an example, can be:

• The minimum height above sea level (at perigee ) is 193 km,
• The maximum height above sea level (at the peak ) is 220 km,
• Inclination - 51.6 degrees
• The circulation period is about 88.3 minutes.

When determining the height of the DOE, it is important to indicate from which model of the Earth it is being counted. Russian ballistics traditionally indicate the height above the ellipsoid, and American balloons indicate the height above the sphere; as a result, the difference can reach 20 km (approximately corresponds to the difference between the equatorial and polar radii of the Earth), and the positions of apogee and perigee can shift.

Since the Earth's daily rotation is involved in putting the payload into orbit, the carrying capacity of the launch vehicle depends on the inclination of the orbit to the equatorial plane. The best conditions are achieved if the DOE has an inclination towards the equator, which coincides with the latitude of the launch pad with which the launch was made. Other inclinations of the orbit lead to a decrease in the parameters of the launch vehicle in terms of the ability to put cargo into orbit. However, launching in the most energetically advantageous direction is not possible for all spaceports, for example, for Baikonur with a latitude of about 46 degrees, it is not possible to launch to inclinations of less than 48.5 degrees due to restrictions on the location of the falling territories of the separated parts of the missiles (exclusion zones). The most commonly used incline when starting from Baikonur is 51.6 degrees, smaller inclinations are rarely used.

The lifetime (the time spent by the SC on the LEO) depends on the ballistic parameters of the artificial celestial body and on the activity of the Sun during this period, which affects the height of the upper layers of the Earth’s atmosphere .

The lower the orbit, the greater the mass of cargo that a booster can bring to it, all other things being equal. Therefore, the supporting orbit is beneficial to do as low as possible. In practice, the time of orbital flight (before entering the dense layers of the atmosphere) for less than one day can cause problems in case of failures on board the spacecraft, therefore, such low orbits are practically not used. In addition, the magnitude of the orbit error affects the minimum altitude of the reference orbit, since with an unfavorable combination of errors of measuring instruments, controls, and external factors, the orbit may turn out to be too low, and the spacecraft will return to the Earth’s atmosphere and burn before it maneuvers. Nevertheless, there are known cases of launching vehicles into orbits with a circulation period of less than 88 minutes and a perigee height of 121-150 km. For example, the Luna-7 automatic station was launched into a reference orbit with a perigee of 129 km. The satellites of the KH-7 Gambit family were launched into similar orbits.

The concept of “reference orbit” came into use with the launch of the four-stage Lightning rocket, the fourth stage of which was launched in zero gravity after about 3/4 of the revolution around the Earth, as was required for interplanetary and lunar AMS.