Propulsion Systems

propulsionTo put the vastness of space into human perspective, let us assume a present-day space rocket can maintain an average speed of 40,000 miles per hour. Even so, the most direct route to Mars would still be in excess of 50 million miles and would therefore take approximately 2-4 months. Pluto at the edge of the solar system would take over 10 years to reach. However, it would take the same rocket, 72,203 years to reach the nearest star, assuming it could carry or access enough fuel, the descendants of the original occupants were still alive and the rocket had not disintegrated with age.

So what developments might act as a game changer?

It would appear that without the development of radically new systems of propulsion, space exploration will be limited, either in distance or will have to be planned to last for decades or even centuries. In distance terms, the asteroid belt may well fulfil the need for mineral resources for the foreseeable future, while Mars could possibly be terra-formed in some limited way to provide an alternative place of human habitation. If so, then even a modest improvement to existing propulsion technology may satisfy these shorter-term goals. However, longer journeys to the stars will present an altogether different problem even assuming that near light speed propulsion is possible. Without addressing the complexity of Einstein's theory of relativity at this point, we can introduce a few key implications of this theory:

  1. Nothing can travel faster than light
  2. Time dilates or runs slower onboard faster moving spaceships
  3. Space contracts in the direction of motion

In isolation bullet-1 above appears to impose a major limitation to the exploration of even our local galaxy, the Milky Way, and implies an almost total barrier to any practical exploration of the larger universe.

But do bullets 2 & 3 offer a potential escape clause?

The problem with time dilation is that the effects would only become appreciable at speeds approaching light speed, e.g. +95%, which is far beyond any propulsion technology that exists today. However, as shown in the following diagram, the relativity index will fall towards zero as an exponential function of velocity as it approaches the speed of light. So, at 95% of lightspeed, the relativity index would fall to 31%, which would conceptually reduce the onboard time taken to the nearest star, at some 4.3 lightyears, to 1.4 years, although the elapsed time on Earth would be 4.5 years. Of course, a journey to a far more distant star, even within our local galaxy, might be linked to distances of 100's or 1000's of lightyears, such that the onboard and elapsed time would still remain problematic.

Therefore, other approaches may have to be considered. While it does not really resolve the problem of either the onboard or the elapsed time, suspended animation might 'fool' the human aging process such that the journey might be survivable within what we currently understand to be an 'average human life-time'. Another, and possibly more pragmatic, approach might encompass the idea of a deep space vessel becoming an 'Ark' in which the spaceship exists as a moving, space-borne, self-sustaining colony. The Ark would be a place where people lived, and died, over multiple generations in order to reach their final destination in what would amount to a one-way migration from Earth to some distant world. Of course, it might be naive to assume that it will be  'homo sapiens', as understood today, that will journey to the stars, as the challenge of space exploration might favour some other form of intelligent life.

Does this imply that 'humanity' will never go to the stars?

While pure speculation, the idea of space contraction, as tabled in bullet-3 above, may offer a better possibility of 'real-time' travel to more distant stars, while avoiding the problem of both excessive onboard and elapsed time. For example, the idea of a 'worm-hole' is often described as a way of reducing the normal distant between 2 points in space, such that any travel time would be reduced proportionally. However, whether this idea is only realised within science fiction remains to be seen, but it is clear that without some further major developments within the field of spaceship propulsion, the distances between Earth and other planets will remain literally astronomical and the distance to stars, all but impossible. Ultimately, if space exploration is to move beyond the backyard of our own solar system, propulsion systems will require a new paradigm that extends well beyond current science.

Ion Drives

The principle behind the ion drive is based on known physics in which a gas is first ionised to give it an electrical charge, which can then be accelerated by means of an electrical field. The ions are then expelled via the engine nozzle, producing a thrust in the opposite direction. Operating in the near vacuum of space, ions are expelled much faster than the jet of a chemical rocket.

Although ion engines could deliver about ten times as much thrust per kilogram of fuel as a conventional rocket engine, they are very low-thrust devices. As such, ion engines could not be used to take-off from the surface of a planet. For similar reasons, ion engines would not be able to accelerate very fast, but could sustain thrust for much longer periods of time and achieve 'specific impulses' from 10,000 seconds up to a theoretical maximum of 400,000 seconds. Unfortunately, these figures are still a good way short of a specific impulse of a million seconds considered necessary to approach any reasonable percentage of light speed.

Nuclear Pulse

The idea behind this approach has been developed from nuclear-weapons research and works on a surprisingly simple, albeit disconcerting premise. Every few seconds, a small nuclear bomb would be released from the spacecraft and detonated a short distance behind it. The vessel would be equipped with an enormous shielding plate attached to the ship into which vaporised debris from the explosions would slam, thereby propelling the ship forward.

It has been estimated that a refined version of this basic idea could produce an engine with a specific impulse approaching one million seconds that would convert to an eventual velocity of well over 10% of light speed, i.e. 18,600 miles per second. This is well over 3000 times faster than today's rocket propulsion systems. In fact, if acceleration and deceleration time is ignored, such an engine could reduce the one-way time to Alpha Centauri from 72,203 years to 43 years. However, unfortunately even at this speed, the effects of time dilations would only reduce the on-board time to 42.7 years.

Fusion Ram Scoops

One of the key requirements to travelling the vast distances between the stars is fuel-efficiency. Otherwise, the vessel would simply be a huge fuel supply with a tiny ship bolted onto it. Equally, if space exploration is to aspire to travel to the stars, then speeds nearing the speed of light will be necessary. The design parameters of propulsion systems such as fusion ram scoops could potentially meet both of these requirements.

A ram scoop vessel would collect charged particles from interstellar space using a large magnetic scoop and funnel them into an onboard fusion reactor, where they would be converted to fuel. Calculations suggest that a 1000-ton ship with a high-efficiency reactor could accelerate almost indefinitely at 1G towards the speed of light. At 1G acceleration, it would take approximately 1 year to approach near light speed. Assuming that 99% of the speed of light could be attained, then time dilation would reduce to 14%. For every hour passed on Earth, only 9 minutes would pass in the ship. As an example, ignoring the acceleration and deceleration time just for now, the 127 light years to the star Antares would take 18 years according to the clocks on board the ship at 99% of the speed of light. Of course, time dilation only affects the moving vessel and over 127 years will still have passed on Earth. The implication being that it would be unlikely that anybody who saw the ship leave Earth would be alive when it reached Antares, even though the crew were only 18 years older.

Before getting too carried away with the idea that light speed travel is now just a matter of working out the details, it is probably worth remembering that there are a few major problems with this proposal. First, while Einstein's Theory of Relativity seems beneficial in terms of time dilation, the theory also predicts that mass will increase as the craft approaches the speed of light. Increased mass needs increased energy to move it and, at light speed, infinite energy would be required. Other major factors are the complexity of the fusion process in deep space, the density of interstellar matter, the drag of the magnetic field and the braking of the incoming protons, to mention just a few. We have also completely ignored the possibility and effects of colliding with interstellar debris at these speeds.

Warp Drives

Of course, we cannot conclude this discussion without, at least mentioning warp-drives. In principle, a warp drive is a mechanism that would use exotic matter with a negative energy density to warp space-time, such that an object may appear to move faster than light. However, the 'wormhole' may better be described as creating a short cut between two points in normal space, such that a spaceship could, in theory, enter at one point and effectively appear almost immediately at the other end. Unlike time dilation, the journey would take the same time as measured within the spacecraft and by a stationary observer.

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It is realised that some people might believe we are entering the realm of science fiction rather than science future, when talking about warp drives. However, it is worth remembering that Sir Isaac Newton could have calculated the Earth's escape velocity of 25,000 miles per hour back in 1687, even though the fastest transport available to him was still the horse and cart. The possibility that the escape velocity would be achieved in less than 300 years would have been considered as nothing other than science fiction, even to a visionary like Newton. Of course, this said, the warping of the fabric of space-time to create an advanced propulsion technology is still science fiction, as the energy required would be astronomical in terms of our current 'horse and cart' technology.