Friday, March 2, 2012

Ramjets to the Stars

The fundamental problem encountered with any form of space travel is the large amount of energy required and finding a means to supply it. A spacecraft which carries all of its fuel is very inefficient as a large amount of energy has to be spent transporting this very fuel. This is especially true for spacecraft that need to attain very high velocities such as in the case for interstellar space travel. The distances involved in interstellar space travel are so huge that even getting to the nearest stars, a spacecraft needs to travel at a significant fraction of the speed of light in order to arrive within a human lifespan.

The ship was not small. Yet she was the barest glint of metal in that vast web of forces which surrounded her. She herself no longer generated them. She had initiated the process when she attained minimum ramjet speed; but it became too huge, too swift, until it could only be created and sustained by itself… Starlike burned the hydrogen fusion, aft of the Bussard module that focused the electromagnetism which contained it. A titanic gas-laser effect aimed photons themselves in a beam whose reaction pushed the ship forward - and which would have vaporized any solid body it struck. The process was not 100 per cent efficient. But most of the stray energy went to ionize the hydrogen which escaped nuclear combustion.
- Paul Anderson

Proposed by physicist Robert W. Bussard in 1960, the ramjet is a method of spacecraft propulsion where the basic idea is to collect fuel from the rarefied interstellar medium to propel the spacecraft. Projecting out in front of a ramjet is an enormous ramscoop which collects hydrogen protons from the interstellar medium and funnels them into a central fusion engine which fuses hydrogen into helium to generate thrust. The enormous size of the ramscoop means that it can only be made up from an immaterial net of electromagnetic forces. If the process of hydrogen fusion is 100 percent efficient, the speed of the ramjet can approach arbitrarily close to the speed of light.

In practice, radiation loss is expected to occur and this prevents the hydrogen fusion process from being 100 percent efficient. Hence, the maximum speed of such a ramjet cannot come arbitrarily close to the speed of light. For example, if 20 percent of the fusion energy is lost as thermal radiation, the maximum speed attainable by the ramjet as a percentage of the speed of light will be 89.41 percent. This lowers the performance of the ramjet as an interstellar spacecraft. Interestingly, this also lowers its performance as a ‘time machine’ for the exploration of the future.

A brief mission study is now presented to show the performance of a hypothetical ramjet. The fusion reaction occurring within the ramjet’s fusion reactor is assumed to have a mass-to-energy conversion efficiency of 0.71 percent and of the energy generated, 80 percent gets converted into useful propulsive energy while 20 percent is lost as thermal radiation. The ramjet is assumed to have a total mass of 10,000 metric tons and a ramscoop intake diameter of 7,000 kilometres. Finally, the interstellar medium is assumed to have a density of one proton per cubic centimetre. Starting from an initial velocity of 1 percent the speed of light, the time required for the ramjet to accelerate to 80 percent the speed of light is 454 days and the distance covered during this time is 0.632 light years or 5.98 trillion kilometres. Due to the effect of special relativity, the time elapsed for an observer onboard the ramjet is 369 days.

Alpha Centauri is a binary star system located at a distance of 4.37 light years away. Aimed for a flyby of Alpha Centauri and starting with an initial velocity of 1 percent the speed of light, the same ramjet will take 2006 days to get to Alpha Centauri. Onboard the ramjet, an observer will experience a time elapse of just 1103 days. However, the flight duration will be somewhat longer if the ramjet were to gradually decelerate to a stop as it approaches Alpha Centauri. Things become more interesting if the fusion engine of the ramjet is 100 percent efficient where all of the energy generated from the fusion process gets converted into useful propulsive energy. In this case, the ramjet will take 1818 days to get to Alpha Centauri, while an onboard observer will experience a time elapse of 626 days.

1. Bussard R W (1960), “Galactic matter and interstellar flight”, Astronautica Acta. 6:179-194.
2. C. Semay and B. Silvestre-Brac (2007), “Equation of motion of an interstellar Bussard ramjet with radiation loss”, Acta Astronautica, Volume 61, Issue 10, p. 817-822.