Can rockets be launched from airplanes? The pioneer of air-launched rockets - Pegasus rocket

The Pegasus program began in the spring of 1987 and was self-funded by the then Orbital Sciences Corporation and Hercules Aerospace Corporation without any government funding. However, during the early development of the flight test and the first few flights, NASA provided a test mother aircraft converted from a B-52 bomber, which also covered part of the cost. In addition, the first two customers, Orbital Communications Constellation and Earth Observation Constellation, paid part of the payment in advance.

On April 5, 1990, NASA test pilot and former astronaut Gordon Fullerton commanded the launch of the carrier aircraft and carried out the first launch of the Pegasus rocket. The Defense Advanced Research Projects Agency (DARPA) was the first customer of the Pegasus rocket. At that time, DARPA signed five launch intentions, but only one was actually implemented, and the other four were handed over to the US Air Force.

By 1994, Orbital Sciences Corporation purchased a retired Lockheed L-1011 airliner and named it "Stargazer" in homage to the USS Stargazer in "Star Trek: The Next Generation".

In 1993, NASA issued a tender for a Small Expendable Launch Vehicle Service (SELVS) that required slightly better performance than the original Pegasus. In 1994, Orbital Sciences Corporation and Hercules developed the XL version to meet NASA's requirements. The Pegasus XL has longer first and second stages, but the third stage remains unchanged. The flight operation procedures remain the same, and the wings of the Stargazer have been strengthened to carry a larger rocket, which is now the main model.

Pegasus received very good reviews in the United States. The development team led by Antonio Elias received the National Medal of Technology from then-US President Bush in 1991.

The original Pegasus launch price was $6 million, without the HAPS upper stage. The Pegasus XL launch price increased significantly, and NASA's Ionospheric Connection Probe (ICON) mission was launched on this rocket. The total contract amount reached $56.3 million, which included "fixed launch service costs, spacecraft processing, payload integration, tracking, data and telemetry and other launch support requirements", and the launch was carried out on October 11, 2019.

In its 44 launches, Pegasus had three mission failures (STEP-1, STEP-2, and HETI/SAC-B), two partial failures (USAFMicrosat and STEP-2), and then 30 consecutive successful flights, with a total success rate of 89%. The first partial failure on July 17, 1991 resulted in seven USAF microsat satellites not entering their intended orbits. The failure on November 4, 1996 resulted in the loss of the Gamma-ray Burst Identification Satellite.

Rocket structure

The biggest difference between Pegasus and ground-launched rockets is that it has wings. It has not only main wings, but also tail fins. The tail assembly consists of a rear skirt, a horizontal tail, a single vertical tail, and a servo. The tail assembly is a traditional all-aluminum structural frame and a stressed skin, and the tail's rotating shaft is made of titanium alloy.

Schematic diagram of the Pegasus rocket structure

As a solid rocket, Pegasus inevitably faces the problem of poor accuracy. Solid engines have thrust eccentricity, that is, there is a lateral offset and directional deviation angle between the thrust and the rocket axis. This situation is much lighter on liquid engines. The upper stage of Pegasus is set up to correct this deviation. If the user does not have high requirements for accuracy and does not set up a HAPS upper stage, the three-stage rocket will complete the mission 10 minutes after separating from the carrier aircraft, and the rocket and satellite will separate. If equipped with a HAPS upper stage, it will send the satellite into the final orbit.

Pegasus XL flight procedures without HAPS

Pegasus XL rocket has HAPS flight program

During the flight, the rocket's attitude is regulated by a cold gas thrust vector control system. The rocket is equipped with a liquid nitrogen tank, which generates control torque by spraying liquid nitrogen.

Flexible choice of launch location

The Pegasus rocket fully demonstrates the advantages of air-launched rockets, which is that it can flexibly choose the launch location and launch direction, and even flexibly choose the location where the aircraft takes off.

The main base for rocket carriers is Vandenberg Air Force Base, which can achieve orbital inclinations between 70° and 130°, including polar orbits and highly inclined orbits. If a larger inclination is required, additional analysis and coordination with the range management department is required. If a lower inclination is to be achieved, it can be achieved by drastically changing the orbit of the rocket in the air, but this will consume a lot of propellant and cause a decrease in carrying capacity. Similarly, when launching at Vandenberg Air Force Base, there may be some special landing area safety requirements, which make certain inclinations impossible to achieve. The good thing is that local measurement, control, tracking and communication facilities can be used when launching here.

Pegasus can also be launched from other ranges, including Wallops Air Force Base (WFF), the US Air Force East Coast Base (ER), and the Reagan Test Site (RTS). If launched from the East Coast, it will use the facilities of NASA or the US Air Force East Coast Range.

Optional launch sites for Pegasus

If you want to launch a satellite to an equatorial orbit, you can take off from the Kwajalein Atoll test range. According to the contract signed by Orbital Sciences Corporation and the test range, during the launch window, the relevant facilities can be used first for mission support for Pegasus launches.

Generally speaking, space launches require satellite manufacturers and rocket manufacturers to send their products to the launch site, where they are processed and assembled before taking off. However, Pegasus uses solid rockets and air launch, which allows the carrier aircraft to carry the rocket to "assemble" the satellite. In 1997, after being integrated at Vandenberg, Pegasus was mounted on the Stargazer and flew to Madrid, Spain, where it combined with the Spanish-made MINISAT-01 satellite and then headed to Gran Canaria for a successful launch.

Flight procedures

After the main components of Pegasus leave the factory, they are transported to Vandenberg Air Force Base for assembly. There is a hazardous cargo handling area specially opened for Pegasus rockets, nicknamed "hot pad".

There is a complete set of rocket and satellite processing facilities in the "hot pad", including ground support equipment, assembly and integration test equipment, orbital transportation equipment and transport vehicles. Some equipment is used for the transportation, loading and unloading of rockets, some for integration and testing, and some for ensuring the environmental control of satellites. The processing facilities provide the necessary equipment to achieve the combination of satellites and rockets, but the processing equipment of the satellite itself needs to be provided by the satellite owner.

After the satellite and rocket are combined, a series of test activities will be carried out, including phase testing, flight simulation, etc. Among them, the rocket's flight computer needs to be specially tested. During the test, the main subsystems of the rocket, including the pneumatic control surface, the air conditioning control system, and the steering gear command system must be ensured to be qualified.

As for the satellite itself, it should be at the launch site 30 days before launch. If it is not launched at Vandenberg Air Force Base, but at an airport selected by the user, it should be launched 10 days in advance. Before the satellite and rocket are combined, the satellite itself should be tested. To this end, some coordination work needs to be carried out with the launch site, and dangerous goods and dangerous processes should be coordinated 120 days in advance.

Pegasus is not something that can be launched just by hanging it on an aircraft. Generally speaking, it is hung on the carrier aircraft three or four days before launch, and then various mechanical and electrical interface tests are carried out. At this time, the aircraft needs to use ground vehicles for power and air supply. Sometimes some pyrotechnics are used on satellites, which are only installed when the launch is approaching. Therefore, there is a hatch on the rocket fairing, which is installed one hour before the aircraft takes off.

When all the tests are completed, the ground vehicle will disconnect all connections with the carrier aircraft, and the power of the aircraft engine will be used to supply power to the rocket and satellite to maintain the environment inside the fairing. Before takeoff, all the fuses on the rocket must be turned on. Within 10 minutes after the aircraft takes off, the power supply must be switched to the battery on the rocket. 45 seconds before launch, the rocket will perform a self-check again. After passing the self-check, a signal will be sent back to the launch control room, and then the pilot will release the rocket according to the control room's instructions.