How do spacecraft manoeuvre in space? Surely, in a vacuum, reaction force will not work?
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Come on mate, it’s not rocket science
A common misconception
It’s a common misconception among the public that when a rocket lifts off, it somehow pushes against the launch pad, or the air around it, to gain altitude. This is based on common sense and everyday experience. Let’s say you wear skates on an ice rink and you want to move forward; you simply have to push on something solid, such as the side of the rink, in the other direction. But common sense is not a good guide in the case of a rocket; for instance, how would you explain that the rocket is still accelerating toward space when it’s high above the pad and moving through clouds? Indeed, how can it change direction in the vacuum of space?
The simple answer is that a rocket moves by pushing on the gas that flame out from its engines. Even though it seems impossible for a massive rocket to move by only venting gas, it’s the simple scientific truth, based on Newton’s third law of motion: for every action in nature there is an equal and opposite reaction. In other words, when one object exerts a force on a second object, that second object exerts a force on the first object that is equal in magnitude, but opposite in direction. So, when a rocket violently pushes gas out of its nozzles, that same gas, a plasma composed of a myriad of tiny atoms accelerated at very high speed, pushes in unison on the rocket, propelling it forward. In the case of one of the most powerful rockets ever built, Nasa’s Saturn V rocket, which propelled Apollo astronauts toward the moon, the thrust of its engines at lift off was equivalent to 7.6m pounds of gas shooting out from behind the rocket every second.
The same thing happens when you’re sitting in a rowing boat and you throw a massive object astern, such as a log or a large rock; immediately, the boat will move forward. The more massive the object you throw, the more the boat will accelerate in the other direction and the faster it will move. Action-reaction!
What about a rocket manoeuvring in the vacuum of space? Indeed, there’s nothing to push against, but it’s the exact same situation as on the launch pad: to move in one direction, the pilot simply has to activate some jet from a nozzle pointing in the other direction, and voilà! The gas that’s ejected in one direction pushes against the rocket and propels it in the opposite direction. Remember Sandra Bullock in Gravity, using a fire-extinguisher to propel herself from one space station to another? Well, that last part doesn’t make sense, but the action-reaction principle, illustrated by the firing of the fire-extinguisher and Bullock flying in the opposite direction, is spot on. One thing she forgot, though: in the vacuum of space, where there’s no friction to slow you down, the only way to stop is to use the principle of action-reaction in the opposite direction, otherwise you simply continue moving at constant speed for ever. Too bad Bullock didn’t bring a second fire-extinguisher with her!
Prof Pierre Chastenay, Université du Québec à Montréal
Combust for thrust
Reaction force (Newton’s third law) is the exact principle that is used in space for propulsion.
In the case of chemical propulsion, propellant is burned in a combustion chamber that produces very hot, high-pressure combustion products. These combustion products are accelerated through a convergent-divergent nozzle (bell shape), which raises the gas velocity to the speed of sound at the throat (point of minimum cross-sectional area) and then further accelerates the flow beyond the speed of sound in the divergent section of the nozzle. This velocity of the combustion products, combined with their mass, is the momentum which defines the reaction force. This can also be achieved with electric propulsion, which emits charged particles at much higher velocity but with much lower mass.
In addition, there is a pressure force acting on the surface of the divergent nozzle section, which is dependent upon the difference between the ambient pressure and the pressure at the nozzle exit plane.
Phil Gadsby, propulsion engineer, Dawn Aerospace
A spacecraft doesn’t move at all, on average
Reaction force is exactly how they work. A spacecraft doesn’t move at all on average. It throws part of its mass one way (the burning fuel which was fully inside the rocket) and it goes the other way. On average, the original mass is still moving at the same rate (conservation of momentum). On Earth, things can push against other objects using friction (what the tyres of a vehicle do to make it move, or the wings of a plane do against the air to help it turn) which you cannot do in space, hence the pointlessness of X-wings in Star Wars films that appear to make the spacecraft bank and turn, when they would do nothing of the sort. A real spacecraft would have to push fuel fast to the right to start to move to the left, but the way it turns looks completely different to the way an aircraft turns in the atmosphere.
Nic Océan, Switzerland
A large fraction of the rocket engines in use today are chemical rockets; that is, they obtain the energy needed to generate thrust by chemical reactions to create a hot gas that is expanded to produce thrust. A significant limitation of chemical propulsion is that it has a relatively low specific impulse, which is the ratio of the thrust produced to the mass of propellant needed at a certain rate of flow.
A significant improvement (above 30%) in specific impulse can be obtained by using cryogenic propellants, such as liquid oxygen and liquid hydrogen, for example. Historically, these propellants have not been applied beyond upper stages. Furthermore, numerous concepts for advanced propulsion technologies, such as electric propulsion, are commonly used for station keeping on commercial communications satellites and for prime propulsion on some scientific space missions because they have significantly higher values of specific impulse. However, they generally have very small values of thrust and therefore must be operated for long durations to provide the total impulse required by a mission.
Julian Dimitrov, Herts
Reaction control systems
Reaction forces do indeed work in a vacuum. Spacecraft are usually equipped with a reaction control system (RCS). These are typically located in specific locations around the exterior of the spacecraft to allow for its orientation to be changed.
Often, spacecraft also have a larger main rocket engine that is used to raise or lower its orbit or change its orbital plane, known as vectoring. Imagine two astronauts floating next to one another inside the spacecraft. If one astronaut were to push the other, both astronauts would move away from their original positions in opposing directions at the same speed (assuming they were of the same mass).
When an RCS thruster or main engine is ignited, the resultant gas is forced at very high speed out of the engine bell or thruster. The movement of this gas from the spacecraft is what provides the reaction force to propel it in the opposite direction. Once a roll, pitch or yaw manoeuvre has been initiated by burning an RCS thruster, the spacecraft will continue to move along the axis of the thruster even after it stops its burn. An equal burn must be made in the opposite direction to then stabilise the spacecraft.
Orbital manoeuvring system
There being no medium in space, there is no resistance to propulsive motion; throw stuff out the back of the spacecraft and you’ll accelerate forward. The faster you throw stuff out the back, the faster you accelerate, but because of the lack of a medium, manoeuvring fins or wings are useless for manoeuvring, there being nothing to manoeuv
re against, so small rockets situated in all three planes (x,y,z) of the vehicle are used in various combinations to bring about the desired orientation. The system is known as the OMS or orbital manoeuvring system, though the spacecraft doesn’t need to be in orbit about a body to use it. The fact that space is mostly empty and possesses no significant medium means that you have to bring all your propellant (which constitutes the working medium) with you, but the upside is that there’s no resistance to motion unless you are far too close to a planet, so a velocity once attained remains your velocity until you hit something, which is not likely because space is very big, and mostly empty.
Therion Ware, Stevenage
Funny to read this question while writing the software to manoeuvre small spacecraft. I’m the software lead at the Glasgow office of a small-sat provider. We make small spacecraft for a huge range of applications.
I’d like to split the question into two types of manoeuvring:
• Attitude (where you’re pointing) and
• Position (where you are)
For attitude adjustments we have two main forms of manoeuvring:
• Reaction wheel. By spinning up and down wheels mounted in the x, y and z directions and suddenly changing their acceleration we have very fine control over the position of the spacecraft.
• Magnetometers. Each company has a slightly different approach to this, but we have large loops of wire embedded in our solar panels. By changing the electric field around the craft using these loops, and having the fields interact with the earth’s magnetic field, we can have the ability to change our attitude, although not with as much fine control as with reaction wheels.
Adjusting position (orbit):
• Typically in the smaller craft, we have no real direct method of adjusting our orbit. In extreme circumstances, such as collision avoidance, we will point our solar panels in the direction the craft is travelling. This causes a small amount of drag with the limited amount of atmosphere you encounter in low-earth orbit and will adjust the craft’s orbit.
• Propulsion. Again, there are a few approaches to this, but the basic concept is the same. Throw something out your craft in one direction and you’ll adjust the orbit. Larger craft will have a store of gas to perform this. For ourselves, who do much smaller craft, you can use a pulsed-plasma thruster, sort of a small spark plug that erodes a small amount of Teflon by sparking across it and firing the resulting debris out the back of the craft.
• There are more exotic forms of propulsion being played around with, such as solar sails, which use thermal pressure from the sun’s radiated energy to push the craft, much the same way a boat’s sail uses the wind.
Colin Waddell, Glasgow
Other posters far more competent than myself have already explained the physics behind this but, just as an aside, it may be pertinent to mention that as space around our planet becomes more and more crowded it’s increasingly important to forewarn other traffic before you fire your thrusters to manoeuvre your spaceship away from its current course.
In some earlier craft still in operation this involves the navigating astronaut obtruding his or her arm from a dedicated pressurised port, or “window”, and giving one of the system of internationally agreed, but complicated, gloved hand signals in order to communicate their intentions. This is not entirely satisfactory as only some humans and very few robotic craft understand, or even notice, the gestures and consequently there have been a number of near misses.
Luckily, the situation has been eased no end now that modern spacecraft are all equipped with an eye-catching and easily understood array of Gordon flashers.