Although they use the most powerful means of propulsion ever invented to carry them to orbit, it seems unclear how spacecraft maneuver once in the vacuum of Space. This is where Reaction Control Systems come into play.
A reaction control system is a mechanism utilized by spacecraft to perform a number of small maneuvers such as orientational, rotational, and directional adjustments in space. It typically consists of several small rocket thrusters and/or reaction wheels and gyroscopes.
Once a satellite finished getting a “lift” to space on top of a large two-stage orbital rocket, it is often left without any noticeable means of propulsion. Without any means to maneuver, it can run into serious problems when its orientation or altitude needs to be adjusted.
Similarly, the International Space Station (ISS) needs some kind of mechanism to allow it to maneuver to stay in orbit, adjust its orientation, and make small adjustments to accommodate maneuvers like docking procedures.
Explorational probes like the Cassini Spacecraft and Parker Solar Probe also need some mechanism to maneuver between celestial bodies, while instruments like the Hubble and James Webb Space telescopes must execute precise adjustments to track stars & galaxies.
All these maneuvers, from small directional adjustments to boosting a satellite into a higher orbit, are taken care of by a spacecraft’s reaction control system, also known as an attitude control system. The following section details what this system is and how it functions.
What Is A Reaction Control System?
As mentioned during the introduction, a reaction control system is a mechanism utilized by spacecraft to perform a number of small maneuvers such as orientational, rotational, and directional adjustments in space.
All spacecraft that spend any significant amount of time in space and need to perform even the simplest type of maneuver like staying in orbit require a form reaction control system to have some kind of maneuvering capability.
A reaction control system typically consists of several small rocket thrusters or reaction wheels and gyroscopes. In some cases, a combination of both systems can be used together, depending on the type and magnitude of maneuver required.
(The reaction control system of a spacecraft is not the same mechanism as the main engines of an orbital rocket’s first or upper stage that propels it forwards. Learn how orbital launch vehicles are propelled forward in the atmosphere and space in this article.)
To better understand the role each mechanism plays in maneuvering a spacecraft, one must take a closer look at reaction wheels & gyroscopes, as well as rocket thrusters, individually:
Reaction Wheels And Gyroscopes
Almost all satellites and other spacecraft use either reaction wheels and/or gyroscopes to perform a number of orientational, rotational, and directional adjustments.
The two mechanisms often get confused with each other, or the terms are used interchangeably since both reaction wheels and gyroscopes work on Newton’s Third Law of motion, as well as the principle of conservation of angular momentum.
Newton’s Third Law Of Motion states that “For every action, there is an equal and opposite reaction,” which means that when two forces interact with each other, the one force’s reaction is equal but in the opposite direction of the force applied upon it.
This law enables satellites and even the much larger International Space Station (ISS) to use the torque created by the spinning motion of reactions wheels or gyroscopes to rotate in the opposite direction, which allows them to change their orientation (or attitude).
The spinning reaction wheel also explains the principle of the conservation of angular momentum. The spinning motion does not only create torque in the direction of spin but also creates an angular momentum that is at a 90 angle to the spinning motion.
(A heavy wheel spinning at high velocity demonstrates this momentum as it resists any attempt to rotate the axle around which it is spinning. The resistance experienced when the axle is turned is known as the conservation of angular momentum.)
Although they essentially perform the same type of function, reactions wheels and gyroscopes generally differ in the magnitude and accuracy of the motion required.
A reaction wheel generally performs larger attitude adjustments, while gyroscopes perform smaller, more detailed corrections, like stabilizing a space telescope, allowing it to stay focussed on small targets.
(This is a rather vague distinction but will become more understandable as we dive into the workings of larger spacecraft that utilize both these systems, like the Hubble Space Telescope and, to a lesser extent, the International Space Station.)
To better understand how reaction wheels and gyroscopes differ in the way they work, one can have a look at examples of where each is utilized in a specific spacecraft:
1) Reaction Wheels (Momentum Wheels)
A reaction wheel, also known as a momentum wheel, is a type of flywheel which use the momentum of the spinning motion to provide the tongue that forces the satellite/spacecraft to rotate in the opposite direction.
It is used in almost all spacecraft, from small CubeSats and normal-sized satellites to the highly complex Hubble Space Telescope. It has the benefit of using electricity from the Sun as an energy source and not fuel that can possibly interfere with sensitive equipment.
There are typically three reaction wheels on a vehicle to allow a directional change in any of the three dimensions (or all 3 X, Y, and Z axes). A spacecraft can change its orientation by changing the speed at which any of the three reaction wheels spin.
The Hubble Space Telescope is a perfect example of a spacecraft utilizing reaction wheels to change its orientation to point in a new direction. By spinning any of its four reaction wheels to speeds of 1 000 to 4 000 rpm, it creates torques, forcing the vehicle to change its attitude.
(It is also an excellent example of a vehicle utilizing both larger reaction wheels for orientational or attitude adjustments as described above, as well as gyroscopes for smaller, more accurate maneuvers such as focussing and staying focussed on a specific target.)
Reaction wheels are also used in larger geostationary satellites that stay in a fixed position above a location on the planet and are always pointing towards the planet’s surface. Reaction wheels play a crucial role in allowing these spacecraft to maintain this orientation.
Gyroscopes are sometimes viewed as simply smaller versions or an assembly/combination of reaction wheels, which perform more delicate or precise functions, or are deployed in small spacecraft like CubeSats where a heavy reaction wheel is not needed.
As stated earlier, a spinning reaction wheel also explains the principle of the conservation of angular momentum. The spinning motion does not only create torque in the direction of spin but also creates an angular momentum that is at a 90 angle to the spinning motion.
And it is on this principle of angular momentum that gyroscopes operate. Specifically, by rotating a high-spinning wheel, the resistance created by the angular momentum forces the object executing this rotation to turn as well.
The International Space Station (ISS) is a perfect example to use for explaining how this mechanism works in practice. Unlike many other spacecraft, one specific side of the ISS is always facing the Earth. To achieve this orientation, the station rotates once every orbit.
Theoretically, once a rotational movement is set in motion in space, it will continue without any additional action required, so the ISS should be able to continue rotating as it orbits the planet without any further intervention.
But orbiting at an altitude of 400 km (249 miles), the Space Station still experiences some drag from the little air still present, as well as the pull of the Earth’s gravity. Both forces respectively push and pull on the vehicle unevenly, affecting its rotation over time.
To counteract this interference and keep one side pointed at the Earth’s surface at all times, the Station primarily uses 4 Control Moment Gyroscopes or CMGs (although they are sometimes combined with its reaction control thrusters for larger maneuvers.)
Unlike a typical reaction wheel which generates torque by changing the speed at which the wheel spins, the control motion gyroscopes create torque by changing the direction in which each wheel spins, as illustrated in the diagram above.
Each gyroscope can be rotated independently, and by changing the rotation of one or multiple gyroscopes simultaneously, the system generates angular momentum in a specific direction, forcing the ISS to adjust its attitude/orientation.
As mentioned in the previous section, the Hubble Space Telescope also uses gyroscopes, 6 in total, that are used for finer, more “detailed” movements like keeping it pointed at a specific target like a distant galaxy.
Reaction wheels and gyroscopes are invaluable mechanisms that are deployed in almost all spacecraft that orbit the Earth or explore our Solar System and are essential for performing maneuvers like rotational and attitude adjustments.
In many cases, though, these mechanisms are used in combination with smaller rocket thrusters called reaction control thrusters, for example, when a stronger force or faster reaction times are required.
Reaction Control Thrusters
The majority of reaction control systems used in spacecraft that need to maneuver in orbit or travel to other celestial bodies in our Solar System use a combination of reaction wheels & gyroscopes, as well as smaller rocket thrusters. And with good reason.
Sometimes the force provided by reaction wheels or gyroscopes is not strong enough to provide the torque required to maneuver a spacecraft sufficiently, or faster reaction times are needed that require a source of thrust to make small adjustments in a short timespan.
For example, crewed spacecraft like the (now retired) Space Shuttle, SpaceX’s Crew Dragon, and the Progress spacecraft need to perform fine incremental maneuvers during docking with the International Space Station (ISS).
During these docking procedures, the maneuvers not only need to be performed with precision but with a quick response time since they have a limited time to align themselves with the docking ports of the ISS. Reaction control thrusters are used for these operations.
Reaction control thrusters are small rocket thrusters that are strategically placed around a spacecraft’s surface to enable it to perform maneuvers like rotational and attitude (orientational) adjustments in space.
They operate independently from a spacecraft’s main engines that provide the thrust to propel the vehicle in a specific direction. (A large portion of spacecraft using these thrusters, like satellites, space telescopes, and the ISS, only have reaction control systems onboard.)
The thrusters vary in size, and the fuel they use, depending on the spacecraft’s requirements. Larger thrusters used more frequently & requiring more thrust typically use hyperbolic fuels like hydrazine, while smaller cold gas thrusters are used on smaller craft like CubeSats.
Hypergolic fuel is one of the preferred fuels to use in a spacecraft’s reaction control system. It is highly toxic and deadly to humans and can have a severe environmental impact if it combusts in Earth’s atmosphere. It is therefore not ideal for use in orbital launch vehicles.
In space, though, this type of fuel has several advantages. The fuel & oxidizer spontaneously combust upon contact without the need for an additional ignition device. It can also be stored for long periods in liquid form at ambient temperatures without extra insulation.
Several historical & current spacecraft use this form of propellant. For example, the Cassini spacecraft (that explored Saturn) & the DAWN probe (that studied the asteroids Vesa & Ceres) used hydrazine to perform the complex maneuvers required for these missions.
The International Space Station (ISS) uses two main thrusters as well as 16 smaller thrusters for movements that fall beyond the scope of its 4 Control Moment Gyroscopes (CMG) or when more thrust is needed or to assist in desaturating the gyroscopes.
(The thrusters use a form of unsymmetrical dimethylhydrazine as fuel and dinitrogen tetroxide as oxidizer.)
The Apollo Lunar Module LM also used a hypergolic fuel/oxidizer combination (Aerozine 50 fuel & nitrogen tetroxide (N2O4) oxidizer) in its reaction control system. This fuel combination was also used for the engine of the Service Module SM.
Cold Gas Thrusters
Cold gas thrusters are the simplest, most reliable, and cheapest form of propulsion since it typically makes use of compressed cold gas that is released through a nozzle as required.
As a result, it doesn’t require any of the mechanisms needed by traditional liquid or solid-fueled rocket engines, like combustion chambers, ignition sequences, and the additional hardware and “plumbing” associated with these engines.
(Learn more about the different types of fuel orbital launch vehicles use, as well as their advantages and disadvantages, in this article.)
It has the drawback of being less powerful than other forms of propulsion, but in the vacuum of space, minimal thrust is required to maneuver smaller vehicles. Also, partly due to a smaller fuel tank, the thrusters can only be fired in small bursts for short periods.
A cold gas propulsion system has a simple setup that makes it much cheaper and less complicated to integrate into a spacecraft. It usually consists of a fuel tank, regulating valve, and a convergent-divergent nozzle.
It typically uses nitrogen gas as its propellant. Since it is an inert gas, it eliminates possible unwanted chemical reactions with other substances. When the regulating valve opens, the pressurized gas expands at high speed into space, providing thrust for the spacecraft.
As mentioned earlier, cold gas thrusters are normally used in smaller vehicles like CubeSats and other spacecraft that require relatively small maneuvers for brief periods.
For example, the first stage section of SpaceX’s Falcon 9 rocket uses cold gas thrusters to help “flip” the stage back to the correct orientation to re-enter the atmosphere and fire its thrusters to land back on the planet’s surface to be refurbished for future launches.
With no air to push against, a spacecraft can move in a specific direction using its main thrusters, but in the vacuum of space, it will not be able to turn, rotate, or change its orientation. This is where the reaction control system plays a crucial role.
As this article illustrated, a reaction control system uses a combination of reaction wheels & gyroscopes together with reaction control thrusters to allow a spacecraft to make attitude and rotational adjustments, as well as perform several additional maneuvers.
The vast majority of spacecraft orbiting the Earth, as well as probes exploring the Solar System, rely on some type of reaction control system, without which they will be unable to function correctly.
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