Propulsion Systems
  
Assembled in Earth orbit, this cycling transfer vehicle with a 10-megawatt power plant could transport 130-metric-ton payloads to Mars in 6½ months and repeat its circuit every 52 months.

There are many technological obstacles that need to be overcome to make the human exploration of Mars and the other planets in our solar system a reality. One of the main obstacles is the development of an efficient and safe propulsion system. Remember in the early days of the space program few believed we would ever develop a rocket capable of propelling humans into Earth orbit, much less of getting them to the Moon! Because of the very large distances involved, the need to reduce the mission duration for medical concerns, and the weight limitations imposed by the existence of a manned crew and their life support system, the propulsion system we choose must be fast, cost-efficient, and as safe as possible.  There are several propulsion options, each with its own advantages and drawbacks. The basic trade-off is between the rocket's thrust and its fuel efficiency. High-thrust systems accelerate faster but generally consume more fuel. Low-thrust systems take longer to speed up but save on fuel.

The Mars spacecraft would either be launched into Earth orbit on a heavy lifting rocket using chemical engines and then be separated from that rocket, or it would be assembled in low-Earth orbit in pieces and then be sent to Mars. A trans-Mars injection (TMI) burn would send the rocket on its way. Rocket engines designs that currently use chemicals, such as the space shuttle main engines, can be used to bring the ship from Earth (or its components for assembly) but cannot release enough energy to make the long trip to Mars in an acceptable time frame. A human crewed spacecraft would be heavy since it would be carrying supplies and equipment for a long mission and would need to get to Mars quickly to avoid exposing the crew to the dangers of radiation and too much time spent in zero g.

The function of a rocket engine is to apply force to the mass of the spacecraft to get it to move through space. Rocket engines all work the same way. The thrust or impulse provided by the engine changes the speed or velocity of the spacecraft. As the rocket fuel or propellant is used up, the spaceship becomes lighter and, thus, less force is necessary from that point forward. Once in the vacuum of space, after you start moving, nothing affects your flight except the gravitational pull of the Earth, the Sun, and nearby planets (Isaac Newton’s laws of motion). The farther you get from a planetary body, the less gravitational pull is acting on the spacecraft. At a certain point between the two planets, the gravitational pull of the planet you have left behind (that is slowing you down) is replaced by the gravitational pull of the planet you are approaching (that will speed you up).

The term specific impulse is used to define the relationship between the thrust of a rocket engine and the weight of propellant flow. This variable is used rather than the exhaust velocity because it relates the thrust of an engine to the mass of propellant and can be directly compared among different propulsion alternatives. Specific impulse (thrust per unit flow rate of propellant) is measured in seconds. It is one of the elements of the rocket equation, that allows engineers to choose between propulsion systems. Specific impulse is represented in equations by the term Isp. More powerful rocket engines will have a higher thrust-to-weight ratio and, thus, a greater specific impulse. Chemical rockets have a comparatively low specific impulse, or an Isp of 150-450, while nuclear thermal rockets engines have an Isp of 825-925. The space shuttle main engines, which are the best currently existing chemical rocket engines, have the highest attained Isp of 455. Different types of electrical rocket engines have even greater specific impulses ranging from electrothermal rocket engines (800-1200) to electromagnetic (2000-5000) to electrostatic or ion engines (with an Isp of 3500-10000!).

For more details on how rocket engines work, click here. 

Nuclear Propulsion

This nuclear thermal rocket fires upon arrival at Mars to insert the transfer vehicle into orbit.

Two designs for rocket engines to Mars are currently under consideration, the nuclear propulsion engine and the electrical propulsion engine. Both of these engines are more advantageous than the traditional chemical engine because of the large increase in available energy. A nuclear thermal propulsion system can carry a larger payload and accomplish its mission in a reduced time frame. Conventional chemical rockets would take too long to get to Mars. The traditional design approach for a nuclear thermal engine is the use of a solid core, heat exchanger reactor.

Liquid hydrogen is pumped through extra-core components for cooling and then through the reactor core to be heated and expanded through the rocket nozzle to produce thrust. The main problem with these engines is that the engine is heated by nuclear radiation emanating from the core. The high temperatures (2,500-3,000K) and huge power production can result in neutron and X-ray leakages. As the primary transfer propulsion system, the spacecraft's reactor would remain inactive until departure from Earth orbit for safety reasons. In short, nuclear propulsion can shorten interplanetary trip times and can reduce the mass launched from Earth, but they can be dangerous and certainly would elicit great concern from environmental groups on the Earth.

For more information on nuclear propulsion rockets, click here.

The basic criteria for a propulsion system to be employed in the future exploration of Mars are

  • the technology employed should be mature and flight-tested.
  • the reliability of the engine must be of a high degree to assure maximum safety of the crew.
  • that its efficiency must allow for a sizable payload, a modest Earth-to-orbit mass, a short travel time, flexibility in selection of the mission parameters, flight plan changes during flight, ample accommodations for the crew, and opportunities for scientific observations.
  • that, in the case of a nuclear system, the operation and testing of the reactor should not present an undue hazard or a cause for concern on Earth and in space.

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