[amsat-bb] Re: ARISSat-1 - Dumb Question

Rocky Jones orbitjet at hotmail.com
Thu Feb 10 20:52:42 PST 2011

Hello All.. A basic rule of the "internets" is in the discussion of orbits when Hohmann transfers are brought up the easy discussion is over (grin)...but everyone below has the essential points.  When a velocity vector modification is made in an orbit the "area " rule means that the changes to the orbit are increasingly felt as the antipodal or 180 degree point is approached.  IN addition of course other factors act on the orbit and as Bob described it in the cares of ARISSAT the main one would be "drag" as the probe has a lot less mass then the space station...this will quickly remove the satellite from the orbital altitude of the station...although Bob is correct in a perfect world the satellite and ISS would meet again one orbit later.  There is a great deal of difference in "prox ops" and long term orbit modifications.

This is not initiative.  The closest thing to it is in basic pilot training teaching the notion of a "turn around a point".  Here the "change" vector is wind and it takes a bit of instruction to recognize that any changes made "now" do not really manifest themselves until about 90 degrees later.

There are several "strange" things that celestial mechanics drive.  It took sometime to recognize that when the shuttle and station dock, the center of gravity of both vehicles combine to move the CG well outside the combined stack.  (think of how the shuttle  VRS say how a Progress docks as well as the relative mass of the vehicles).  This eventually drove changes in the docking and latching procedure.

Robert G. Oler WB5MZO Life Member AMSAT/ARRL and NARA and soon to be 5N something.

> CC: orbitjet at hotmail.com; ko6th_greg at hotmail.com; g0mrf at aol.com; clintbradford at mac.com; amsat-bb at amsat.org
> From: saguaroastro at cox.net
> Subject: Re: [amsat-bb] Re: ARISSat-1 - Dumb Question
> Date: Thu, 10 Feb 2011 17:51:12 -0700
> To: bruninga at usna.edu
> This time I'll actually type something before hitting send;)
> The two burn maneuver is essentially a Hohmann transfer maneuver. The point of the first burn becomes the new  apogee or perigee (depending on the direction of the burn). The burn will raise or lower the antipodal point from where the burn occurred. At that point a second burn in the same direction will bring the point of the first burn to the new level. 
> But you all probably already knew that:)
> If you want to play around with this try this:
> http://orbit.medphys.ucl.ac.uk/home.php
> Sent from my iPod
> Rick Tejera
> Editor, SACnews
> Saguaro Astronomy Club
> www.saguaroastro.org
> On Feb 10, 2011, at 17:17, "Bob Bruninga" <bruninga at usna.edu> wrote:
> >> a retrograde maneuver will remove ARISSsat from 
> >> the proximity of ISS very very quickly.  
> > 
> > It is interesting that any such one-thrust (arm throw) maneuver will then
> > intersect the ISS exactly one orbit later. In theory that is.  But the
> > difference in drag at that low altitude will usually be enough to have a
> > safe miss distance on the next and subsequent orbits.
> > 
> > I think that is why most space maneuvers require two burns.  One to start a
> > new orbit (but it will still intersect the original orbit on every orbit.
> > Then a second burn somewhere else in the orbit to get rid of that
> > intersecting point?
> > 
> > Bob, WB4APR
> > 
> >>>>> ... Clint.  When ARISSAT is released it will stay in the "Plane" of
> > ISS
> >>> orbit...they will toss it retrograde meaning in the opposite direction
> > of
> >>> the  velocity vector and with its slightly lower velocity the orbit will
> >>> start to  decrease...this is done so that very quickly the orbits will
> > stop
> >>> being "prox  ops" reasonably fast.
> >>> 
> >>> If so, then I believe as the orbit altitude is reduced, the apparent
> >>> velocity increases.....(??) which will cause ARISsat-1 to 'move ahead'
> > of the
> >>> ISS over a few hours
> >>> 
> >>> But didn't we say the velocity would be less than the ISS due to the
> > method
> >>> of deploying it against the velocity vector ?
> >>> 
> >> 
> >> Interesting puzzler, eh?  From what I have read in the past, I think this
> > their logic.
> >> 
> >> What they are trying to do is to separate the orbits of the ISS and
> > ARISSat as quickly as possible, to avoid the potential for a collision.
> > Consider the options:
> >> 
> >> 1.  Throw it sideways to the ISS orbit.  The result is that twice per
> > orbit the two spacecraft's paths will cross, side to side.  Bad idea.
> >> 
> >> 2.  Throw it ahead of the ISS (faster orbit speed).  This will raise the
> > orbit, slightly, and also make it a bit elliptical (up and down).  The
> > higher orbit makes the satellite go behind the ISS, but the elliptical shape
> > also means that the orbits will cross every orbit (but out of phase, so they
> > won't be at the same place when they do).  But, then as the ARISSat orbit
> > decays, they will get closer and closer, potentially getting back to the
> > same place.  Not good, either.
> >> 
> >> 3.  Throw it behind the ISS (slower orbit).  As you note, this will lower
> > the orbit (and make it a bit elliptical), and initially the apogee of the
> > orbit will intersect that of the ISS.  Being in a lower orbit, ARISSat will
> > move ahead of the ISS, and over time, as the ARISSat orbit decays, the two
> > will diverge even farther.  So, this is the safest.
> >> 
> >> At least, I think that's the logic.  If not, pass me some of that tuna...
> >> 
> >> Greg  KO6TH
> > 
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