[AR] crossrange (was Re: SSTO)
- From: Henry Spencer <hspencer@xxxxxxxxxxxxx>
- To: Arocket List <arocket@xxxxxxxxxxxxx>
- Date: Tue, 13 Feb 2018 17:28:44 -0500 (EST)
On Mon, 12 Feb 2018, Henry Vanderbilt wrote:
Within limits imposed by the particular orbit and the vehicle, once in
orbit you can land where you like, yes. The vehicle limit is
crossrange... (Laterally maneuvering while still in orbit, IE changing
your orbital inclination, tends to cost far too large a mass of
propellant to be routinely practical.)
However, that isn't quite the full story, because in many cases there is
an alternative to *lateral* maneuvering.
You're moving along your orbit track, normally at an inclination to the
equator. Your landing site is moving too, because of Earth's rotation,
along a line of latitude. Roughly speaking, your landing would take place
where those two paths intersect, and the trick is to arrange that you
reach the intersection just when the landing site is there too.
If the landing site passes the intersection before or after you're there,
you can do expensive lateral maneuvering to move the intersection east or
west... or you can change the *timing* of your orbit to speed up or delay
your arrival at the intersection. If done well in advance, such timing
changes can be cheap. Given some planning and patience, the added fuel
can easily weigh less than the dry mass needed for greater crossrange.
(Better reentry aerodynamic performance, which is mostly better L/D and
better thermal protection, always seems to involve heavier vehicles with
lower payload fraction and higher development costs.)
The one clear-cut case for crossrange is if you're in a hurry for some
reason: either there's an emergency that won't wait (most likely medical
emergencies *can* wait, by the way, given some on-board interim care --
medevac from isolated areas on Earth often takes days), or your ops tempo
is brisk enough that you're losing serious money by having your vehicles
marking time in orbit waiting for a landing opportunity.
"...first the mission needs must be identified. This does not mean
ceasing to examine vehicle capabilities. It does mean avoiding the
interpretation of vehicle capabilities as mission needs." (Eugene Love,
1966)
Henry
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