dietz@SLB-DOLL.CSNET (Paul Dietz) (12/18/85)
The minimum velocity change needed to get from Mars orbit to an ellipse with perihelion at earth orbit is about 2.7 km/sec. Launched from Mars, however, this orbit will reach Earth only once every year or so (if launched at other times it will miss). My question is: what is the minimum velocity change needed to get from Mars orbit to an orbit that intersects Earth, independent of the planets' relative positions? Ignore velocity needed to escape from Mars, and assume the vehicle must intersect the Earth within one orbit. I don't know the answer to this one; I would really appreciate a solution or a pointer to a solution. Paul Dietz dietz%slb-doll@csnet-relay
dsmith@HPLABSC (David Smith) (12/20/85)
> The minimum velocity change needed to get from Mars orbit to an ellipse > with perihelion at earth orbit is about 2.7 km/sec. This could probably be reduced if you're willing to spend more time in transit. I believe the Galileo project had a history like this: funding was slow in coming, slipping the projected launch date past the favorable conditions necessary to launch Galileo directly to Jupiter with an IUS.* So, it was decided to launch the craft into an Earth-crossing orbit, so that ~12-18 months later (I'm fuzzy on the figure), it would get a gravitational slingshot to Jupiter. Unfortunately, that meant an additional 2 years in space between launch and arrival for something to fail. Luckily, DOD decided it needed a higher energy upper stage, and set about to build the wide-body Centaur. So Galileo will ride a Centaur. *IUS = Interim Upper Stage, originally. But since the space tug did not get funded, the "I" now stands for Inertial (whatever that means). <Basenote drift on: switch over to discussion on cheaper launchers> We may get another such help from the military. In AW&ST, Dec.16,1985, p.16 (quoted without permission, and omitting large chunks): Cost to transport materials to Earth orbit using the shuttle is about $1,500/lb., and this cost must be reduced to $150-200/lb. if a ballistic missile defense systems [sic] employing space-based assets is to become affordable, Lt. Gen. James A. Abrahamson, SDIO director, has said. If this cost reduction can be achieved, commercial activities that require access to space will realize long-term benefits from the program even though the primary purpose of near-term vehicles remains military. Recent Air Force studies have concluded that the nation requires short notice, on-call access to space to service and replenish the increasing number of military space systems. Research in hypersonic propulsion, advanced materials and computational fluid dynamics has created a consensus in the scientific community that the vehicle may be be feasible even though there is a high degree of risk in the program. Recent breakthroughs in ramjet/scramjet technology, materials and structural cooling techniques, the availability of supercomputers for design and simulation, and solid oxide fuel cells with unprecedented power-to-weight ratios have contributed to this consensus. The present concept envisions a hydrogen-powered aircraft capable of horizontal takeoff and landing from conventional runways, thus reducing the large support crews now required for shuttle launches. The aerospace plane would operate at Mach 12-25 and altitudes of 100,000-350,000 ft. David Smith