Propulsion Concepts Available For Solar System Exploration

In the previous section it was shown how Isp and mass control space travel and missions. If human exploration of our Solar System is the goal, then there are some time constraints to consider given the current knowledge of shielding from high-energy particles and radiation in space. There is a limit to the mass of shielding that can be incorporated into a spacecraft and yet retain a practical mass to accelerate from LEO. In addition, the ability to warn the space travelers is limited to radiation that encounters Earth. From other sources and directions the spacecraft will have to have a basic protection level plus a short-term safe house for more intense radiation. Since the first warning may be the arrival of the radiation, the danger is that the first encounter may be a lethal one so the entire crew space may be required to be in a safe house. The best insurance against this occurring is to minimize the travel time. Statistically a trip of less than a year is relatively safe and a trip of over two years is not, see also Section 7.6. Exploring the Solar System by manned missions means ideally the total travel time is on the order of one year to minimize the exposure of a human crew to hard space radiation, even with a shielded spacecraft. Russian experience with seven orbital stations, however, shows that even a 2-year mission in microgravity may generate irrecoverable physical damage. One solution is to provide a minimum level of acceleration, perhaps one-fifth of Earth's gravity (approximately 2 m/s2), and a weak magnetic field (at least 0.3 gauss) analogous to Earth's magnetic field. The real limitation is that with current systems a one-way travel time to the Heliopause (100 AU) that appears feasible is 9.5 years. This is too long for a human-carrying spacecraft, and we do not know how to construct spacecraft and supply resources for humans for a total of 19 years. So these missions will of necessity be robotic missions.

The requirements for the propulsion can be determined for a specific distance as a function of spacecraft weight with values selected for just two parameters, the total one-way travel time and the average acceleration of the spacecraft. The equations for the speed increment required over orbital speed (D V) for the spacecraft to achieve its destination in the selected time, the spacecraft mass ratio (MR) in Earth LEO for a one-way or two-way mission, the average specific impulse required to achieve the required DV, the acceleration time from orbital speed to orbital speed plus DV (ta), and the thrust required to provide the selected acceleration follow:

Path length p Radial distance / m DV '

Mission time tm s

DV (seconds)

a Nxgo Nx = axial acceleration ("g"s)

where go is the surface acceleration on Earth.

Newton's Third Law-based propulsion will enable Solar System exploration within the previously discussed travel times only if there is sufficient specific impulse and thrust. In range of distances from 5 to 100 AU the mass ratio for a one-way mission is 4 and a two-way mass ratio is 16. This determines the 7sp for the spacecraft departing from LEO the performance of the propulsion system. The

Table 1.6. Propulsion performance for mission to the Heliopause and nearer.

One-way

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