People from Earth to Mars in 30 days
Using a combination of electric and chemical propulsion it is possible to send people in a rocket from Earth to Mars in 30 days. The electric portion of the system would accelerate nine refueling depots to prepositioning orbits. The chemical rocket with a human crew would rendezvous with each of the nine refueling depots, adding about 3056 m/sec in each of ten burns. The nine refueling depots would be in nine independent orbits. The first would pass Earth at 3,056 m/sec, the second at 6,112 m/sec, the third at 9,168 m/sec and so on. The spaceship to Mars would accelerate to 3,056 and rendezvous with the first tanker just as it passes and likewise with each of the others in turn until it finally achieved the delta v for fast transfer to Mars This would follow an orbit of the same aphelion and perihelion as a Hohmann transfer orbit between Mercury and Saturn, but with the inclination, longitude of ascending node and argument of perihelion needed to intersect Earth's and Mars' orbits. The change in velocity (delta v) necessary to change from Earth's orbit to the transfer orbit is 30,561 m/sec. This is just an example trajectory and there are probably transfer orbits that more efficiently use delta v and can make the 30 day transfer with less than nine refuelings. The fuel transfer technology still needs to be developed and practiced to be sure that the fuel transfers and rocket restarts are accomplished without fail, lest the crew be stranded en route. To address the possibility of difficulties with timely, reliable transfer of fuel and difficulties with engine restart, the fuel could be transferred complete with a fresh set of tanks, fresh engine, plumbing and wiring in a stage swap with the tanker. In using this technique, there would be only a mechanical connection between the rocket stage and the Mars bound spaceship. The rocket stage would have its own electrical power and be controlled by radio signals over a short gap. The prepositioning of the fuel depots would begin long before the human crew is launched, on the order of ten years before.
A spaceship for a 30 day voyage could be smaller than one intended for a 180 day voyage because not as much air, food and water would need to be carried. Provision for refueling would add some mass and there would need to be a substantial heat shield for slowing down at Mars. The 30 day mission to Mars may be limited to a fly-by unless other refuelings are scheduled near Mars to cut down the velocity of the spacecraft some before it begins atmospheric braking at Mars. There is a limit to the peak deceleration loading that it is reasonable for humans to endure. The refuelings near Mars and the atmospheric braking in excess of human tolerance could both be done away with if one is willing to settle for a 38 day transfer when Earth and Mars are closest. This uses a modified transfer trajectory with only seven refuelings.
A concern for the use of multiple refuelings during a journey through the solar system is the possibility of failure of one of the tanker craft to achieve its planned orbit. There have been some failures of ion engines during missions. However NASA is working on a new Magnetoplasmadynamic Thruster which should solve the reliability problems. The new electric rocket uses hydrogen instead of Xenon. Since hydrogen is cheaper and available in much larger quantities than Xenon, it is a better choice for reaction mass for a tanker spacecraft. NASA writes that exhaust velocities approaching 100,000 meters per second can be achieved with noncondensable hydrogen plasmas, so a tanker using a Magnetoplasmadynamic Thruster should be able to achieve the delta v necessary for the planned rendezvous. If some failure did prevent a refueling tanker from being in the proper orbit, telemetry would indicate the failure long before the launch of the craft with a human crew. This failure would not cause loss of life because the launch could be aborted.
Some people commenting on this article indicated that they thought the refuelings would be evenly spaced in time along the 30 day or 38 day trip. However, the refuelings would be bunched together near Earth for acceleration and a refueling near Mars for slowing down if needed. For most of the trip the rocket motor is unused and the spacecraft continues on in its transfer orbit. If two hours is allowed for each rendezvous, docking, stage swap and burn sequence in the 38 day trip, all acceleration would be completed in 16 hours at the start of the trip. Deceleration would be accomplished by atmospheric braking at Mars with preliminary refueling and retro-rocket deceleration if needed.
- W. Joseph Armento, PHYSICAL DATA FOR THE PLANETS, THEIR SATELLITES AND SOME ASTEROIDS in the CRC HANDBOOK of CHEMISTRY and PHYSICS, 64th edition, page F-140