Learn about where our rocket's power source come from

In the past our capability in aviation and spaceflight has generaly been defined by the power available: engine horsepower in the early airplanes and later pounds of thrust for jet and rocket engines. New engines usually lead to advanced flying machines, not vice versa. The matter of power, or propulsion as it is more accurately called, is a complex one when considering Mars.

Early pioneers such as Von Braun thought that no breakthroughs in propulsion would be nessesary for a Mars trip, just bigger and more efficient liquid rockets. The first to use rockets were the Chinesse who used solid fuel for propulsion. Still today, solid fuels are used when simplicity is favored over higher performances. Since all the fuel of a solid rocket is stored inside its combustion chamber, no fancy tanks, pumps or valves are required as with a liquid. But unlike liquid rockets, solids can't be turned on and off as willingly. For manned spaceflight their only application in a primary role has been in the two behemoths that lift the space shuttle off the ground. It was a leak in one of these that caused the death of the Challenger crew in 1986.

Von Braun's liquid rckets burned kerosene and oxygen, and that is what was used to put John Glenn into orbit. Later more sophisticated fuels and oxidizers, such as unsymmetrical dimethyl hydrazine and nitrogen tetoxide, were introduced. The Saturn V went back to kerosene/oxygen for the first stage but used hydrogen instead of kerosene for the two upper stages. Hydrogen is more efficient and it is used today as fuel for the space shuttle's main thrust engines.

The efficiency of the rocket motor is measured by a unit called Isp in engineering shorthand. The higher the ISP the better. The old kerosene/oxygen combination produces an Isp of 300 seconds. Hydrogen yields 390 seconds. An improvement. Other exotic fuels have been rejected because they are too toxic, temperamental, or expensive even though they would produce up to 400 Isp. Research contines in the fields of "exotic" fuels, including concepts for "exciting" molecules bombarding them with beams of highenerday materials. In this way Isp's of 1000 seconds may eventually be produced.

To achieve a really high Isp we must abandon chemical rockets and move into other realms. Nuclear engines have been discussed for years but have not yet progressed beyond ground tests. Nuclear propulsion has so far failed to overcome a series of technical and political hurdles. People just don't want another Chernobyls in the sky. However the attraction of nuclear power is simple: a pound of nuclear fuel contains more than 10 million times the potential energy of a pound of liquid hydrogen and oxygen! Extracting that energy safely is the problem.

A nuclear reactor produces heat, but that is just the beginning. Heat alone would not produce thrust. To generate thrust, matter must be expelled from a rocket's nozzle. The faster the particles are ejected the better. In one popular nuclear design, hydrogen is used as the propellant. It is not burned as chemical rockets, but is heated inensely by passing it through a uranium reactor, resulting in very high exhaust velocities and an Isp roughly twice that of a chemical rocket.

Another kind of variation rockets is so-called nuclear electric engine. In this rocket the nuclear reactor produces electricity, whch is used to propel charged particles at high velocities out the back of the rocket. Isp's then times that of chemical rockets can be produced using this technique. In general these rockets have the advantage not only of high Isp but of being able to operate for long peridos of time. On the other hand, they tend to be low-thrust engines. Getting off the ground requires a huge engine, but one that need to operate for only minutes. Nuclear rockets are ill suited for this. Once in space, they come into their own inter-planetary trajectories, where they might produce only a few pounds of thrust but can sustain that level for months. Another problem is that humans must be protected from this radiation. This can require heavy sheidling or inconvenient designs that put the crew as far away from the engin as possible. In the literature the nuclear-electric idea is sometimes called "electric propulsion".

30 years ago the Defense Industry funded something called project Orion, one of the most powerful, bizarre space propulsion plan ever thought of so far. It was to be propelled by a nuclear explosion at its rear end and with that blast, Orion was going to accelerate. To prevent it from being destroyed it was equipped with an alunimum cushion on the tail end of Orion. The pusher plate would have a hole in it through which a series of bombs would be expelled and then exploded some fifty feet away. When the shock wave from each burst hit the pusher plate, it would compres a gigantic shock absorber. When the absorber expanded on the rebound, the Orion would go foward. Its designers calculated a Mars tup might require a couple of thousands of bombs, not unlike the ones dropped on Hiroshima and Nagasaki. The crew would have a rocky ride, but supposedly well within tolerable limits. Like a nuclear rocket, Orion wouldn't have been suitable for lauchpad use but only after a spaceshuip was well on its way. Nuclear contamination from Orion would have been intense. Not so with the normal operation of nuclear engines, but they cause almost as much apprehension as a series of planned explosion. The hysteria centers on what might happen if a spacecraft carrying nuclear material blew up, as Challenger did. The radioactive debris rained back to Earth. There are probally adequate technical solutions to this problem, but the political problems still remains.

Beyond nuclear propulsion, there are other facinating ideas that can produce even higher Isp's (in theory). For more than half a century physicistst have known about anitmatter, particles that exist as counterparts, or mirror images, of normal particles.When a proton and an anitproton collide, the result is a bried but intense burnst of energy. The heat produced can be trapped in a metallic core, through which a propellant gas is circumlated. More complex designs migh use magnets to hold anitparticles, and the heat from their collisions would accelerate plasma through a nozzle. The Isp of these plasma would vary from 1000 to 20000 seconds. Thrust levels of several hundred thousand pounds could result from just a minute amont of antimatter.

Then there is the Sun's energy. Solar power satellites have been proposed to beam electrical energy to Earth, and it is just one step beyond that to use solar energy to propel a spacecraft. First the Sun's heat would be sed in solar elctric cells to generate electricity which in turn would drive an accelerator that would propel ions at high speeds, producing thrust. The power of such a device would decrease as a spacecraft approached Mars. However the amount of power would be sufficient for the task.

Another ingenous idea is the solar thermal roket. It uses concentrated sunlight to heat a propellant. In a sense it is nuclear device, but one whose reactor does not have to be carried on board and the fuel is abundant.

One final idea is the solar sail. The Sun spits out an amazing amount of energy, not just light and heat but also particles such as electrons. It has been estimated that the Earth intercepts only one-billionth of the energy coming from the Sun, and that if the solar flux hitting just one square yeards of Earth could be totally converted to energy it would heat and light a small room. Solar energy also exerts pressure on any object in its path. small object, small force, but if a very thin Mylar sail of extraordinarily large area could be attached to a spacecraft, we would have a celestial sailboat and could hitch a free rise through the Solar System. This plan would accelerate the vehicle slowly, however over a long period of time.

All these ideas are being planned out and some are under developement, however the Nuclear Test Ban Treaty of 1963 prohibits nuclear testing in the atmpsphere or in space, it is doubtlful that the space agency could make any advances in nuclear rockets area. Perhaps a breakthrough in anitmatter might really help us in our voyage to Mars.