Space travel using constant acceleration is the ultimate in slower-than-lightspeed drives (STL drives) for interstellar travel. The propulsion system of whatever kind operates continuously with a constant thrust — for the first half of the journey it constantly pushes the ship towards the destination, and for the last half of the journey it constantly brakes, so that the spaceship arrives at the destination at a standstill. This is the defining feature of a constant acceleration drive.
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Constant acceleration drives are interesting for several reasons:
Humans are currently not launching spaceships to the stars because doing so is too difficult and too expensive with current technology. Constant acceleration drives are not an exception to this fact.
A major limiting factor for constant acceleration drives is having enough fuel. Imagine a horse strong enough to pull a wagon carrying enough hay to feed it on a journey from New York City to Los Angeles. This is why contemporary satellites are sent around the solar system using boost-and-coast technology. Boost-and-coast results in long journey times, but it is practical given the fuel limitations of today's technologies. Constant acceleration won't be feasible until the specific impulse for fuel (in layman's terms, the fuel's economy) has become much higher.
There are two broad categories for ways to solve this problem: one is higher efficiency fuel (the motor ship approach) and the other is drawing propulsion energy from the environment as the ship passes through it (the sailing ship approach). Two possibilities for the motor ship approach are nuclear and matter-antimatter based fuels. One possibility for the sailing ship approach is discovering something equivalent to the parallelogram of forces between wind and water which allows sails to propel a sailing ship.
Picking up fuel along the way—the ramjet approach—will lose efficiency as the space craft's speed increases relative to the planetary reference. This happens because the fuel must be accelerated to the spaceship's velocity before its energy can be extracted and that will cut the fuel efficiency dramatically.
A related issue is drag. If the near light speed space craft is interacting with matter or energy that is moving slowly in the planetary reference frame—solar wind, magnetic fields, cosmic microwave background radiation—this will cause drag which will bleed off a portion of the engine's acceleration.
A second big issue facing ships using constant acceleration for interstellar travel is colliding with matter and radiation while en route. In mid-journey any matter the ship strikes will be impacting at near light speed, so the impact will be dramatic.
These are big issues. They won't be solved quickly or easily. But the benefit of solving them is having interstellar space commerce.[1]
Mankind is close to employing constant acceleration technologies to journeys around the solar system. One example of this is the VASIMR propulsion system currently being developed by NASA and former astronaut Franklin Chang-Diaz. The current implementations have high fuel efficiencies but feeble thrust. But when drives can deliver constant accelerations in the .1G to .5G range, journeys between planets will take days not years.
If a space ship is using constant acceleration over interstellar distances, it will approach the speed of light for the middle part of its journey when viewed from the planetary frame of reference. This means that the interesting effects of relativity will become important. The most important effect is that time will appear to pass at different rates in the ship frame and the planetary frame, and this means that the ship's speed and journey time will appear different in the two frames.
From the planetary frame of reference, the ship's speed will appear to be limited by the speed of light—it can approach the speed of light, but never reach it. If a ship is using .5G constant acceleration or greater, it will appear to get near the speed of light in about a year, and have traveled about half a light year in distance. For the middle of the journey the ship's speed will be roughly the speed of light, and it will slow down again to zero over a year at the end of the journey.
As a rule of thumb, a constant acceleration ship journey time will be the distance in light years to the destination, plus one year. This rule of thumb will give answers that are shorter than the correct answer, but reasonably accurate no matter what the G force is as long as it is above, say, a half G.
From the frame of reference of those on the ship the acceleration will not change as the journey goes on. Instead the planetary reference frame will look more and more relativistic. This means that for voyagers on the ship the journey will appear to be much shorter than what planetary observers see. This is important. It means that a journey that appears to take decades or centuries to planetary observers will take years or decades to the journeyers. This difference makes space commerce feasible for the ship crew even though it looks unfeasible for those on the planet they leave behind. A journey from the sun to the galactic core at 1G constant acceleration takes 340 years as experienced by the ship crew and 30,000 years as experienced by Earth observers.[1]
This is something many readers don't understand well, so it bears repeating: The journey times as experienced by those on the ship are not limited by the speed of light. Instead what they experience is the planetary reference frame getting relativistic.
In the ship frame of reference the amount of acceleration applied will change the journey time: Larger accelerations will produce faster journeys. That same journey to the galactic core will take 244 years at 2G and just 110 years at 10G.[1]
This is a half myth because it depends on the frame of reference. This is true for those watching from the planetary reference frame. For those experiencing the journey-those in the ship reference frame-this is not true. In the ship reference frame the ship will change speed in a Newtonian way—push it a little and it speeds up a little, push it a lot and it speeds up a lot. The difference is that from the ship point of view during the middle of the journey the galaxy is rushing by at near light speed, so the world around the ship looks very strange.
Tau Zero, a hard science fiction novel by Poul Anderson, has a spaceship using a constant acceleration drive.