[amsat-bb] (no subject)

Frank H. Bauer ka3hdo at comcast.net
Fri Feb 15 02:13:32 PST 2008


You had two questions in your query and I believe that one of them (the
heating one) was been answered.  Let me answer the second one:

>So in actuality it comes down from space even slower than going up. 
>Yes?  45 min vs 10.

The reason it seems to take longer---45 minutes----to come down (as compared
to 10 minutes to reach orbit altitude), has to do with simple orbital
dynamics.  Or de-orbit dynamics in this case.  As you know, the Shuttle (or
any other vehicle that is purposely deborbited) performs a deorbit burn
about half an orbit before landing.  This burn actually changes the orbit
perigee (lowest point in the orbit) to be well in the Earth's atmosphere.
Or at or below ground altitude if enough propellant is available.  

Remember that the Shuttle orbit period is on the order of 90-100 minutes.
So about 45 minutes after the deorbit burn the vehicle reaches perigee,
enters an appreciable part of the atmosphere, and either burns up (many
small satellites), or has sufficient thermal protection to land on the
ground (Shuttle and large, dense pieces of satellites).

My team used this technique to successfully deorbit the Compton Gamma Ray
Observatory, resulting in a safe splashdown in the Pacific Ocean.  We also
used this technique in no atmosphere to purposefully crash the Lunar
Prospector into the moon at the end of its mission.

Once a vehicle starts to hit atmosphere (the mission control guys call this
"entry interface") the heating and events to landing occur rather
quickly..several minutes.  Because as you start to touch the atmosphere, you
continue to get deeper into the atmosphere because you are dipping further
down to perigee altitude where the drag is much higher than at the entry

I hope this helps.

73,  Frank Bauer, KA3HDO


Message: 8
Date: Thu, 14 Feb 2008 20:18:15 -0600
From: Joe <nss at mwt.net>
Subject: [amsat-bb] Re: Since We Are Off Topic Somewhat....
To: "Ransom, Kenneth G. (JSC-OC)[BAR]" <kenneth.g.ransom at nasa.gov>
Cc: AMSAT-BB <amsat-bb at amsat.org>
Message-ID: <47B4F667.7030304 at mwt.net>
Content-Type: text/plain; charset=us-ascii; format=flowed

Does anyone have the actual speeds at each altitude going up vs coming down?

I tried to find such a graph, chart, table etc, to no avail,


Ransom, Kenneth G. (JSC-OC)[BAR] wrote:

>The amount of time spent in the atmosphere at high velocity (I'd have to do
some digging to give some real time data but basically it is as follows).
>One the way up, the rocket goes through the 100 km of air during the slower
part of the change of velocity and spends very little time in the atmosphere
at the higher velocity.
>On the way down, the object starts to encounter the atmosphere at maximum
velocity and therefore maximum friction. It will be in the atmosphere for
the a much longer period since it comes in a very shallow angle. The object
will have that friction nearly the entire trip to the ground even though the
drag from the atmosphere will slow the object. 
>Kenneth - N5VHO
>From: amsat-bb-bounces at amsat.org on behalf of Joe
>Sent: Thu 2/14/2008 5:55 PM
>Subject: [amsat-bb] Since We Are Off Topic Somewhat....
>Hi All,,
>Since we are somewhat off topic for the moment with the conversation
>about the bird to come down shortly..
>Ok,  the question is,  and I've asked it to several other places, and
>yet to even get a answer,
>The question is,,,
>Space shuttle launches,,  starts off at sea level,  and in less than ten
>minutes goes from  zero to 17K MPh (or so)  And gets to orbital altitude.
>Time to come down,, the de-orbit burn happens to slow it down just a bit
>to cause the orbit to more or less decay.. It's at orbital altitude..
>and moving at about 17K MPh..  45 minutes or so later it's back on the
>ground and moving at zero MPh..
>So in actuality it comes down from space even slower than going up. 
>Yes?  45 min vs 10.
>Ok,  same goes with any satellite,,
>How come,  going up,,  0 to 17K MPh through the atmosphere, all  is
>fine..  BUT
>coming down,,  17K MPh  to 0  unless it has protection  it will  burn up
>in the atmosphere from friction with the air.
>why is it different?

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