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*To*: Steve VanDevender <stevev@efn.org>*Subject*: Re: MARS HYBRID DESIGN II (First Draft)*From*: David@interworld.com (David Levine)*Date*: Wed, 06 Mar 1996 09:57:25 -0500*CC*: "L. Parker" <lparker@destin.gulfnet.com>, Brian Mansur <bmansur@oc.edu>, hous0042 <hous0042@maroon.tc.umn.edu>, KellySt <KellySt@aol.com>, rddesign <rddesign@wolfenet.com>, "T.L.G.vanderLinden" <T.L.G.vanderLinden@student.utwente.nl>*Organization*: InterWorld, Really Cool Stuff Division*References*: <1.5.4b11.32.19960306033955.006a4d10@destin.gulfnet.com> <199603060322.TAA22500@tzadkiel.efn.org>

Steve VanDevender wrote: > > L. Parker writes: > > I am not going to get into the math, but you CAN tack a solar sail to sail > > towards the sun, so what is the problem with using TC? You might have to > > have the Pathfinder tack around TC several times in a decreasing spiral, but > > it should be possible to decelerate with JUST the energy from TC. > > No, please, get into the math. The only way I know of to get a solar > sail to move towards a star is to let it fall in the star's gravity. > The thrust from a solar sail _always_ has an outwards component, never > inwards. You can direct the sideways component to do things like drop > your orbit to a lower radius, but you can't accelerate towards the star > any faster than you would accelerate by falling. This is from http://caliban.physics.utoronto.ca/neufeld/sailing.txt = It might seem at first that the optimal configuration for a = solar sail is one in which the light hits the sail at normal = incidence (perpendicular to the surface). This doesn't turn = out to be the case, though. A sail oriented this way exerts = all its thrust along the line away from the sun. Because the = intensity of the light from the sun falls off as the square = of the distance, the magnitude of this outward thrust must = fall off also as the square of the distance. In this way it = is exactly like gravity. In fact, putting the sail at normal = incidence to the sun has the same effect as would have = reducing the mass of the sun. It places the sail into an = elliptical orbit which moves farther away from the sun for a = while, but must return to its starting point after one = complete revolution about the sun. This is not a = particularly useful configuration. The only way to avoid = this with a sail at normal incidence is for the solar pressure = to exceed the force of gravity, so that the sail goes into a = hyperbolic escape from the solar system. In order to do this, = for the power output and mass of our sun, the sail would have = to mass no more than one kilogram for every 600 square metres = of sail area, including the mass of payload and electronics. [DELETIA...] = So, putting the sail at normal incidence to the sun is not the = best configuration. It is better to angle the sail in such a = way as to maximize the component of the thrust which is = parallel to the direction of travel. This turns out to be when = the angle between the sun and the perpendicular to the sail is = about 35.3 degrees. In this configuration the spacecraft is = being pushed along the direction of travel, and so it climbs = the gravity well. In the counter-intuitive realm of orbital = mechanics, the spacecraft slows down the whole time it is = climbing the well. = = Well, if the only important thing is the component of the = thrust along the velocity vector, it can clearly be aligned = the other way to oppose the velocity vector. This pushes = against the direction of travel, dropping the sail down the = gravity well, causing it to speed up the whole time. A = solar sail, contrary to popular belief, can travel sunward = just as easily as it can travel anti-sunward. David

**Follow-Ups**:**Re: MARS HYBRID DESIGN II (First Draft)***From:*Steve VanDevender <stevev@efn.org>

**References**:**Re: MARS HYBRID DESIGN II (First Draft)***From:*"L. Parker" <lparker@destin.gulfnet.com>

**Re: MARS HYBRID DESIGN II (First Draft)***From:*Steve VanDevender <stevev@efn.org>

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