[amsat-bb] Re: Since We Are Off Topic Somewhat.... And drifting slightly

Greg D. ko6th_greg at hotmail.com
Sat Feb 16 11:30:03 PST 2008

Hi Tony,

Yes, this all makes perfect sense, and is what I thought should be happening.  But I distinctly remember a website talking about how to get from one orbit to another - Hohman transfers, I think - and how you needed to turn around and fire your engine in the other direction at apogee to slow you down.

Thinking back, and looking at some pictures, perhaps what they were indicating was that the rocket burn would be in the other direction when viewed from above the orbital plane.  But I do remember the "slow down" phrase, and that's what stuck in my brain because it certainly seemed (and is) wrong.

Of course, now I can't find the site...  Or, if I did, it's probably been corrected by now.  At least, let us hope so.

Thanks for indirectly letting me know that I'm not crazy...

Greg  KO6TH

> Date: Sat, 16 Feb 2008 17:32:00 +1100
> To: ko6th_greg at hotmail.com; k3io at verizon.net; amsat-bb at amsat.org
> From: vk3jed at gmail.com
> Subject: Re: [amsat-bb] Re: Since We Are Off Topic Somewhat.... And  drifting slightly
> At 03:55 PM 2/16/2008, Greg D. wrote:
>>To go from a low circular orbit to a higher one, you fire your 
>>rocket behind you to pick up more speed.  The "point" of the burn 
>>becomes the perigee of the new elliptical orbit, and half-way around 
>>is the new apogee.   Ok, so far, so good.  Now, if at the instant of 
>>apogee you
> Yep. :)
>>did nothing, you'd fall back down to perigee, and back again to 
>>apogee on the next orbit.  But since you fired your rocket to speed 
>>you up in the first place, to circularize the orbit you fire your 
>>rocket at the point of apogee to slow you down, and in fact to a 
>>slower speed than you started.  I would think that would make you 
>>drop more steeply down on the next orbit, probably to a lower 
>>perigee than you started.
> Nope.  You fire the rocket to speed you up, which raises the perigee.
>>  Instead, I'd think you should fire in the same direction as the 
>> first burn, to make things round, but that would make you go even 
>> faster, which Mr. Kepler said was wrong.
> You're forgetting that the speed of a satellite in a non circular 
> orbit varies as the satellite moves.  In fact, Kepler's Laws state 
> that in you were able to attach a string (with a very high elasticity 
> and low tension!) between the satellite and the point around which it 
> is orbiting, this string would sweep out an equal area every second.
>>My head is spinning (no pun intended)...  Where did I go wrong?
> Time for a thought experiment.  I'll enlist the aid of Q of Star Trek 
> (Next Generation) fame to shrink the Earth to a point, and the 
> satellite to a very small size (maybe the size of a grain of 
> salt).  We'll put the satellite into a highly elliptical orbit, with 
> a perigee of 1 metre above this "Earth singularity", and an apogee of 
> around 7000km (equal to 800km above the Earth's real surface).  As 
> our satellite orbits this imaginary shrunken Earth, it picks up speed 
> until it's travelling at some incredible speed at perigee (any 
> mathematicians want to work it out, might even be relativistic, but 
> let's assume Newtonian physics apply at all times and velocities - Q 
> can make anything happen ;) ).  What happens at apogee is more 
> interesting.  We're now approximately 7000 km above the singularity, 
> and at the instant of apogee, there is no vertical motion, the 
> velocity is horizontal, and only a few metres per _hour_ (since the 
> orbit is very narrow).  Now, at the moment of apogee, we accelerate 
> our tiny satellite to around 27000 km/h horizontally in the forward 
> direction (Thanks Q for the push ;) ).  Hey presto, it's now in a 
> circular orbit, approximately 7000km above the Earth singularity.  I 
> can now get Q to put the Earth and Relativity back to normal, and we 
> have the satellite in a nice 800km circular orbit. :)
> Hope that extreme, imaginary example helps illustrate what's going on. :)
> Metric was used, because that's the direction NASA and space science 
> is headed.  I can work in either system myself... ;)
> 73 de VK3JED
> http://vkradio.com

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