Humans have mostly depended on chemical combustion to power most of their transportation needs. From motorcycles to airplanes and especially rockets, we generally depend on energy from chemical fuels to cause motion. The enormous amounts of fuels we use are of course, causing pollution. The issues aren’t just limited to environmental reasons in the case of rockets – chemical fuels are bulky and not too efficient. We now have cars and motorcycles that run on purely electricity and even planes are hinted to arrive in the coming years. Speaking again of rockets, from recreational rockets to space as well as military rockets, we still don’t have any that solely depends on electricity. What could be the reason behind this? Is it possible to make electric rockets at all?
Related: Should We Be Concerned About Rocket Pollution?
Chemical rockets remain the most used and most popular rocket propulsion system to date. They depend on energy from chemical combustion of fuels, which can be either solid or liquid – like liquid hydrogen, petroleum, liquid methane, hydrazine, and many more. Here, chemical energy from burning fuel is converted to kinetic energy to generate thrust, lifting the rocket upward. One of the major problems with them is the amount of fuel they consume and the requirements of large tanks to contain the fuel and oxidizers.
While talking about types of rocket systems, electric rockets – in fact electric propulsion systems – often cross our mind. However, these are propulsion systems for space travel and not really suitable for flying rockets and putting them into orbit. The reason being that they still generate relatively low thrust and are inconvenient for launch within the atmosphere. Electric propulsion systems are basically of three types – electrostatic, electrothermal and electromagnetic. All these uses electric energy – either as heat, electric charge or electromagnetic field – to accelerate propellant. However, may be in the future, technology will bring us a rocket launcher that also performs the work of a propulsion system. Development is already underway with propulsion engines like VASIMR engine, NASA’s Solar Electric Propulsion (SEP), Arcjet engines and more.
In a TED interview in 2013, Elon Musk, CEO of Tesla Motors, said “…I think all modes of transport will become fully electric, with the ironic exception of rockets. There’s just no way around Newton’s Third Law…” Is the mastermind behind the electrical motor giant right? Is it really impossible to power rocket motions through electricity alone? To attempt this, let’s delve into the principle behind electric vehicles and its applicability to rockets.
In an electric vehicle, chemical energy stored in batteries is used to power the movement of electric motors that in turn causes the wheel of the vehicle to spin. Here the conversion of chemical energy into kinetic energy does not involve combustion as compared to chemical rockets and conventional cars. The friction that comes into play with the spinning of wheels on the rough roads causes forward movement in accordance with Newton’s Third Law of motion – the compulsion of an equal and opposite reaction for any action (motion). Therefore, the big problem with electric rocket is this need for an equal and opposite reaction – motion in the opposite direction of lift. Chemical rockets are able to solve this by shooting fuel or gas at its end causing the rocket to accelerate in the opposite direction.
Herein lies the issue – since there is nothing in space to push against, rockets will always need to eject some sort of mass (or energy) in order to gain velocity in the opposite direction. There’s just no workaround to that sort of limitation set by the laws of physics.
The thrust system alone, for electric rocket, is not the only problem; but even the amount of thrust that needs to be generated is a huge burden. A rocket needs to attain a velocity of at least 40,270 km/h (25,020 mph) to escape the earth’s gravity and move into orbit. Attaining this high a velocity, especially in the atmosphere with motion powered by electricity alone is close to impossible with our current technology. Airplanes generally require a velocity of 150–180 mph (240–285 km/h) or even less for normal take-off.
NASA has recently announced that it is developing a fully electric aircraft named ‘Maxwell’, or ‘X-57’. The aircraft will be powered by 14 electric motors with the first test flight planned for 2017. This is indeed fascinating technology, although the top speed may only be 175 mph (282 km/h) for now. NASA is also very excited to see how this technology will evolve as it has a potential to transform the entire aviation industry and perhaps even rocket systems.
It’d be wise to remind ourselves that there does exist a vast ocean of unexplored science, along with our limitless imagination of course. While there really isnt any violating Newton’s Third Law, there must be a workaround to it, waiting for us to be discovered.