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

[Rick Tejera]

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
K7TEJ

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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