Artist concept of a nuclear electric-propelled vehicle firing banks of ion thrusters in order to circularize its orbit around Mars.

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.  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 engine designs that currently use chemicals, such as the Space Shuttle main engines, can be used to bring the ship from Earth but cannot release enough energy to make the long trip to Mars in an acceptable time frame. A manned spacecraft would need to make the long trip to Mars as quickly as possible to avoid exposing the crew to the dangers of radiation and to too much time spent in zero g. However, this would require the craft to carry more supplies and equipment which would increase the weight of the space craft, requiring even more energy.

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 currently the best 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!). 

Nuclear Propulsion

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

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, for safety reasons, remain inactive until departure from Earth orbit. 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.

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

• 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
• 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
• and 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

Electrical Propulsion

Ion engine in testing at NASA's Jet Propulsion Laboratory

Gas enters the ionization chamber to increase its temperature. This increase in temperature is done via the ionization process. The gas is bombarded with positive ions from the power source; and, before the gas reaches the nozzle, the accelerated mass of ionized gas is injected with electrons and thrust is obtained.

In an electric engine, the energy needed to accelerate the particles of propellant is provided by an onboard electric power supply connected to both the ionization chamber and the neutralizer at the rocket nozzle. As with solar sails, solar ion engines are mainly practical in the inner solar system, where ample sunlight is available. For more distant missions, it is, in principle , possible to drive an ion engine by a small onboard nuclear reactor.

VASIMR

Called VASIMR (variable specific impulse magneto-plasma rocket), the engine features a modulated ("tunable") exhaust permitting high-thrust operation during departure from and arrival in planetary orbit and gradual shift to continuous low-thrust operation between planets. This results in a highly efficient, potentially very fast propulsion system. This engine could permit both slow, high-payload cargo missions and fast, low-payload crew missions using the same rocket engine. The engine consists of three magnetic cells. 

The propellant, hydrogen, is first ionized by radio waves and is then guided into a central chamber threaded with magnetic fields. There the particles spiral around the magnetic-field lines with a certain natural frequency. By bombarding the particles with radio waves of the same frequency, the system heats them to 10 million degrees Kelvin .  A magnetic nozzle converts the spiraling motion into axial motion, producing thrust. By regulating the manner of heating and adjusting a magnetic choke, the spacecraft pilot can control the exhaust rate. The mechanism is analogous to a car gearshift. Closing down the choke puts the rocket into high gear: it reduces the number of particles exiting but keeps their temperature high. Opening up corresponds to low gear: high thrust but low efficiency. A spacecraft would use low gear and an afterburner to climb out of Earth orbit and then shift up for the interplanetary cruise. A Mars mission would need a 10-megawatt device. 

One feature of this spacecraft is, if difficulties occur within 15 days of Earth departure, it can return to Earth in 75 days (assuming a fully functional propulsion system). A second abort scenario, in which the accidental loss of all propellant is assumed, can return the crew to Earth in 180 days. In addition, during faster propulsion periods, there could be a slight amount of artificial gravity created. Finally, the plasma engine would generate a magnetic field around the ship, which could help to protect the crew from radiation exposure similar to the way the Earth's magnetic field protects us from harmful solar flares and the resulting radiation. 

Questions to think about:

• Which of the three types of engines would you choose for your rocket design? Why?

• What are some of the benefits and drawbacks of each type of rocket?

Next... Living off the Land (pg. 15 of 17)