Space Force X-37B to use new Aerobraking Maneuver

On October 10th, 2024, the United States Space Force (USSF) revealed that the X-37B Orbital Test Vehicle, currently undertaking its seventh mission in space, will begin performing a new aerobraking maneuver to change its orbital trajectory.

This peculiar maneuver has been used for some time, especially for interplanetary missions. It consists of taking advantage of the atmosphere of a planet, just enough for atmospheric drag to slow down the spacecraft by itself. By doing so, the orbiting spacecraft can be led to a lower altitude without the weight of extra propellant for maneuvering.

The X-37 spacecraft surrounded by hot plasma, performing aerobraking to change its orbital trajectory around Earth, showing the spacecraft over a sun-lit view of the Earth, with clouds covering the ocean. — *The X-37B performs an aerobraking maneuver. Credits: USSF/Boeing*

The USSF X-37B is a spaceplane used as a testbed for various classified experiments in Low Earth Orbit. Resembling the Space Shuttle in its black and white coloring, it is equipped with an advanced Heat Shield necessary for its return to Earth, after year-long missions.

The current Orbital Test Vehicle mission was launched on December 28, 2023, aboard a SpaceX Falcon Heavy needed to get the spacecraft on a highly elliptical orbit. One of the objectives was to test the effects of radiation on materials and perform other activities in this new environment. The aerobraking maneuvers to be attempted will help the spacecraft gradually descend to its usual operation altitudes after its testing objectives for high altitudes are completed.

A capable and secretive spacecraft

The X-37 spacecraft was developed by Boeing for the United States Air Force and has performed 6 successful missions, beginning in 2010, and spending several years in space by now.

The X-37B Orbital Test Vehicle-6 (OTV-6) on the Shuttle Landing Facility. Credit: U.S. Space Force — *The X-37B Orbital Test Vehicle-6 (OTV-6) on the Shuttle Landing Facility. Credits: USSF*

There currently are two existing X-37B spacecraft either deployed, in refurbishment, or stored. The public doesn’t have much information about this secretive spacecraft and its objectives in its various missions. The fact the orbit was confirmed to be highly elliptical in this week’s press release was notable.

X-37B is launched aboard a conventional rocket, with a service module for extra propellant and experiments attached to its aft end. It is also equipped with a payload bay that can host other apparel and mission components. During aerobraking maneuvers, the spacecraft will jettison its service module, according to the press release, having to follow standard Space Debris procedures for safe disposal.

The X-37B Orbital Test Vehicle-6 (OTV-6) being incapsulated inside Atlas V Fairings. Credit: U.S. Space Force — *The X-37B being incapsulated inside Atlas V Fairings. Credits: USSF*

Another mission objective that can only be speculated about is the “Space Domain Awareness” testing cited in the USSF press release. Whether it consists of proof of new technologies or tests related to military applications is not known, however, the Chief of Space Operations General Chance Saltzman stated the mission and the new maneuver is “an incredibly important milestone” as the USSF seeks “to expand our aptitude and ability to perform in this challenging domain”.

Different and novel applications of aerobraking

Aerobraking has been a proven method for some time now. It was derived from knowledge acquired in decades, following the return of manned missions from Earth Orbit or the Moon. It was first used for interplanetary missions, particularly by ESA and NASA for Mars. The limited mass requirements for such missions determined very strict margins for propellant use, being needed both for maneuvers and attitude control.

MRO while entering the mars atmosphere render. Credits: The space collective — *The Mars Reconnaissance Orbiter aerobraking. Credits: NASA/JPL*

To slow down the spacecraft and set it to Martian orbit, engineers devised aerobraking to be a controllable and effective method that would not require extra propellant. Not to use heavy heat-resistant materials, some of the aerobraking maneuvers only take advantage of the higher portions of the atmosphere, requiring hundreds of successive passes to lower the orbit, executed over the span of months.

More recently, the Orion Capsule during the Artemis-I mission performed what NASA called a Skip Entry. Its purpose is to use aerobraking effects on a reentering capsule, to lower the overall effects of the high-G, direct reentry to Earth’s surface, dividing it in successive “passes” to slow down the spacecraft.

An altitude/range to splashdown graph showing the effect of the skip entry aerobraking maneuver during reentry of the Orion Capsule for Artemis 1. altitude lowers, reaches a minimum, then reaches the maximum Skip Altitude and then lowers again, reaching the ground — *The effect of Skip Entry for the Orion Capsule in Artemis 1. Credits: NASA/JSC*

With Skip Entry, the spacecraft experiences less stress and can land more precisely, in smaller specified areas for splashdown, without the need for multiple rescue vehicles set along a long possible reentry path.

An audit led by the NASA Inspector General, however, published on May 1st, 2024, highlighted numerous problems, some with the capsule heat shield, prompting a delay in Artemis-II.

Aerobraking has now the potential to change how satellites destined for Earth orbit are designed, since the possibility of changing orbits without using fuel necessary for station-keeping, could help make missions longer, safer, and less polluting.