On Sat, 3 Aug 2002, Randall Clague wrote:
> You missed the discussion last night. Assume the mission is a flyby
> of the rock coming through on the 18th, and the trajectory from
> hereabouts is nearly straight up: the rock is going to pass near Vega,
> which is almost overhead in the evening this time of year. No circ
> burn, and no remote flyback: the first stage can RTLS.
The fact that the intercept point is overhead in the evening says nothing
about when you launch or what trajectory you choose. In fact, orthodox
escape trajectories are launched from nearly the opposite side of the
Earth from where they're going; Earth's gravity bends the trajectory
around to the desired direction.
You don't want a vertical ascent, because the gravity losses are just
horrible. (I once did a calculation of such a launch, which concluded
that you need at least one extra stage just to handle the extra gravity
losses.) Like an orbital launch, you want to pitch over to horizontal as
quickly as you can. Remember, escape velocity is a scalar, not a vector:
achieving that velocity horizontally gets you out just as definitely as
vertically, provided you're high enough not to hit atmosphere on the way.
So you launch 10hr or so *before* the target comes overhead, and a
horizontal escape trajectory ends up being bent around toward the target.
You may not want to go via parking orbit, because an escape trajectory
doesn't need a perigee clear of the atmosphere (it's not coming back) and
achieving one involves a small penalty. But it may actually be the
simplest approach, especially if you already have a vehicle optimized for
that. (As you point out, the first stage won't be custom-built for this
mission.) And a vehicle designed for an orbital reentry probably won't
be able to handle a reentry falling straight down from 1000km+ up, which
is what it would have to do to RTLS.
Henry Spencer
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