[AR] Re: crossrange (was Re: SSTO)
- From: Henry Spencer <hspencer@xxxxxxxxxxxxx>
- To: Arocket List <arocket@xxxxxxxxxxxxx>
- Date: Wed, 14 Feb 2018 12:22:17 -0500 (EST)
On Wed, 14 Feb 2018, Uwe Klein wrote:
However, that isn't quite the full story, because in many cases there is
an alternative to *lateral* maneuvering.
do the reentry burn in two steps.
1. brake a tiny bit for an eliptical orbit with a lower perihelion
faster orbit will give you access to the stripe between
your regular 15deg "overpath".
2. final reentry maneuver.
Yes, this is one example of what I was describing. This particular method
is appealing, since if you put the initial braking burn in the right
place, you're essentially doing the reentry deorbit burn in two parts, and
it costs essentially nothing extra.
The limitation is that if you start from a typical LEO, the faster orbit
isn't very *much* faster, because you weren't very far above the
atmosphere in the first place and you can't safely lower your perigee very
much. For example, if you started at 400km (typical space-station
altitude), lowering perigee to 200km (about as low as is safe) only gains
you about two minutes per orbit. Since a 400km orbit has a period of
about 93 minutes, drifting over to nearly the other side of the stripe can
take a couple of days, even with a bit of help from air drag.
Being willing to spend some extra fuel can speed that up, somewhat. If
you lower apogee as well as perigee, that about doubles the drift rate.
And if you're willing to go *up* instead of down, slowing your orbit, you
can drift in the other direction, so you can choose the direction that
gives you a shorter drift. (Here the limit is that you don't want to go
too high, because radiation dose rate rises as you start encountering the
lower fringes of the inner Van Allen belt.) Even so, we're talking about
maybe 12 hours of drift in the worst case, so patience is needed.
(Or else advance planning. For example, it's possible to put your space
station in a "resonant" orbit that regularly passes over your landing
site, if other orbit constraints don't get in the way and you're willing
to do small orbit-maintenance burns fairly frequently.)
And of course, if you're doing something *big* like powersat construction,
something that justifies building dedicated infrastructure, then you can
build an equatorial launch site and put your assembly base in equatorial
orbit, and the problem goes away.
Henry
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