Advantages of an equatorial launch

I know that a launch from the equator to a zero inclination orbit gets all
of the advantage of the Earth rotation, but I was wondering if this
advantage works the other way round too. In other words, does a reentering
spacecraft from the due east equatorial orbit have significantly less
demanding conditions than a spacecraft entering from a non-zero inclination
orbit? Has this advantage ever been exploited? (The only spacecraft that I
know of that reentered from an almost equatorial orbit was ESA's ARD, have
there been others?)



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Alejandro Zuzek wrote:
I know that a launch from the equator to a zero inclination orbit gets all
of the advantage of the Earth rotation, but I was wondering if this
advantage works the other way round too. In other words, does a reentering
spacecraft from the due east equatorial orbit have significantly less
demanding conditions than a spacecraft entering from a non-zero inclination
orbit? Has this advantage ever been exploited? (The only spacecraft that I

Not much.
However, the atmosphere rotates along with the earth, so there will be
around a kilometer a second in it between retrograde and prograde orbits.

For say a 45 degree inclination prograde orbit, it's only around 300m/s
difference between that and 0 degree.
Not in general a big number, compared to the getting on for 10Km/s that
you need to slow down by.

Alejandro Zuzek wrote:
I know that a launch from the equator to a zero inclination orbit gets all
of the advantage of the Earth rotation, but I was wondering if this
advantage works the other way round too. In other words, does a reentering
spacecraft from the due east equatorial orbit have significantly less
demanding conditions than a spacecraft entering from a non-zero inclination
orbit? Has this advantage ever been exploited? (The only spacecraft that I
know of that reentered from an almost equatorial orbit was ESA's ARD, have
there been others?)

The major advantage is that there is no "window". Since the spacecraft
orbit always crosses the launch point (or landing point), you have to
wait one orbit at most to land.

Contrast this with the Space Shuttle. After launch its orbit is tilted
to the Earth, so the Earth is spinning under it. After one 90 minute
orbit, the launch site has moved hundreds of miles to the west. To land
back where they started they have to wait, sometimes long periods, until
their orbit crosses their landing site. Although the site moves for a
zero inclination orbit as well, it moves toward you, not to the side.

In fact it is exactly this "drift" that caused the Shuttle to look the
way it does. Many early Shuttle designs were much smaller and used
small, straight wings that were only effective in the lower atmosphere
for landing. However when the Air Force agreed to support the project,
they demanded many things, one of them being the ability to abort back
to the launch site after a single orbit. Since their missions were
primarily polar, the "drift" would be up to 1000 miles, and thus the
Shuttle had to use the current delta-wing shape in order to dramatically
increase re-entry manuverability.

Maury

"Alejandro Zuzek" writes:

I know that a launch from the equator to a zero inclination orbit gets all
of the advantage of the Earth rotation, but I was wondering if this
advantage works the other way round too. In other words, does a reentering
spacecraft from the due east equatorial orbit have significantly less
demanding conditions than a spacecraft entering from a non-zero inclination
orbit? Has this advantage ever been exploited?

I don't know about Earth, but this certainly applied to the Jupiter probe
on Galileo. The had to land going with the rotation to reduce an enormous
entry speed to something barely survivable. Of course this effect on
Jupiter is much more signficant since it's bigger and rotates faster
(12.6 km/sec as opposed to 0.46 km/sec at the equator)

(The only spacecraft that I
know of that reentered from an almost equatorial orbit was ESA's ARD, have
there been others?)



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"Alejandro Zuzek" wrote in message ...
I know that a launch from the equator to a zero inclination orbit gets all
of the advantage of the Earth rotation, but I was wondering if this
advantage works the other way round too. In other words, does a reentering
spacecraft from the due east equatorial orbit have significantly less
demanding conditions than a spacecraft entering from a non-zero inclination
orbit? Has this advantage ever been exploited?

Some more info on the entry of the Galileo probe....

From: http://www.agu.org/sci_soc/young.html

The probe entered the jovian atmosphere at 6.54E N latitude, (just
north of the equator). This near equatorial entry was dictated by the
probe having to take advantage of the rotational velocity of the
jovian atmosphere (which is greatest near the equator) in order to
minimize the relative velocity between the atmosphere and probe during
the high-speed entry.

(The only spacecraft that I
know of that reentered from an almost equatorial orbit was ESA's ARD, have
there been others?)