Astro2110 – The Solar System

NASA has revived the dreams of many optimists hopeful of smashing the dire predictions of the ideal rocket equation. Insane designs have long been dreamed of by lunatics attempting to create breakthroughs with the power of the atom, but a NASA research program in the 70’s laid encouraging groundwork for a possible fission rocket. NERVA engine designs use supercritical fission reactions to heat and expand liquid hydrogen, that is then expelled from a nozzle to generate thrust. This is different than classic chemical rockets, which use combustion to generate the entropy necessary to expel matter from a nozzle at high speeds. A NERVA engine powered rocket only needs to carry hydrogen with it as fuel, where chemical rockets require both hydrogen and oxygen to generate thrust.

The essential issue with rockets is that the amount of fuel needed to escape Earth orbit must comprise a large majority of the rocket mass. The rocket body must be as light as possible to allow for any payload transport. The Saturn V rockets were able to transport 4% of their mass to orbit as payload, while the theoretical limit of chemical rockets is 5% payload. This means that even substantial investment will not improve transportation capabilities by a significant amount. This is why you see companies attempting to reuse rocket bodies and engines – reusability is really the only way to decrease the immense cost of transporting mass to orbit.

Russia and the US are now attempting to produce fission-powered rocket engines to expand human capabilities in space. Reaching Mars currently takes around 200 days for unmanned spacecraft. A craft with crew and supplies to potentially establish a colony, along with the fuel to reach Mars, would have a much longer journey with current propulsion methods. Nuclear propulsion, with its high potential energy density, could increase the payload mass percentage of a Mars bound spacecraft, loosening the daunting design constraints that exist around any such mission. Atomic reactors will also be necessary to power human colonies on other worlds, as they are easily the most transportable energy generation solution.

This writer argues that humanity’s only hope of large scale space exploration lies in the use of the highest potential energy densities possible: nuclear fission and fusion. Fusion is unlikely to be a reality for generations, especially in spacecraft. It is essential that our private space companies and governments pursue meaningful methods of atomic propulsion, else we may never get to travel beyond the Moon in a permanent way.

In the 1950’s and 60’s, optimism about the new discoveries related to fission of atoms and available energy ran rampant. Ambitious designs of all kinds were created based around the new marvel of nuclear energy. Eager to harness the massive potential of fission and fusion reactions, scientists and engineers at NASA began to ponder the usefulness of nuclear energy for space travel. The high energy density available in nuclear based fuels could be the key to humanity’s escape from “the tyranny of the rocket equation“. The tyranny: even the most theoretically ideal example of a chemical rocket will only be able to transport around 5% of its initial mass to orbit. This makes any space exploration with chemical based propulsion methods difficult to imagine.

One of the most ambitious designs for nuclear based propulsion was Project Orion. Freeman Dyson, the visionary behind this seemingly absurd project, imagined a spacecraft that would be propelled by harnessing kinetic energy from nuclear bombs detonated at the rear of the craft. A large “pusher plate” attached to an impressive shock absorption system, would absorb pressure from nuclear pulse explosions and utilize the energy to accelerate the Orion craft. Slowing down would be simple: all that would be necessary would be the detonation of bombs in front of the spacecraft to decelerate upon arrival.

The advantages of the Project Orion spacecraft would be an incredibly rapid travel speed. Designers claimed that a voyage to Mars could take less than 90 days with the theoretical accelerations offered by their designs. Disadvantages: Passengers on board of an Orion craft would be exposed to lethal doses of radiation upon the detonation of a single pulse. Seeing as plans for a Mars mission involved around 1024 fission bomb pulses, large advances in our ability to create materials capable of radiation shielding would be necessary for this idea to ever leave the drawing board.

However, the challenges presented in the implementation of an Orion inspired mission seem much more accomplishable than the construction of fusion powered ramjets that may be the future of space travel. One thing is for certain: chemical based propulsion will never allow humanity to achieve its goals in space due to the massive separation distances in and outside of our solar system. Anybody interested in hearing about Project Orion from the lead designer’s son should check out the video posted below.