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Orbital mechanics for Fragon's approach
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JF Mezei
Oct 10, 2012, 5:44:42 AM10/10/12
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When Dragon is 250 metres directly below the station, how come it
remains directly under the station ? SHouldn't it be moving ahead
because at lower orbit, its horizontal speed will be greater ?
Or is the speed difference for that delta altitude so small that during
the hour or two that it spends under the station, it only moves a couple
of metres ?
And when it is given the OK to go from 250 down to 30metres distance,
how is that accomplished ? Does it increase its horizontal speed to
raise its orbit, or does it just push itself upwards ?
When it reaches grappling location, does Dragon have an elliptical
orbit and they have to grapple and berth it before it starts to want to
go back down to perigee ?
Or does it wait a quarter orbit at 10m and fire its thrusters to
circularise its orbit before the arm grapples it ?
If the arm were to grapple it while it has a elliptical orbit, would the
arm be strong enough to "pull" Dragon up to keep it at same distance
from station when Dragon wants to go down towards its perigee ?
remains directly under the station ? SHouldn't it be moving ahead
because at lower orbit, its horizontal speed will be greater ?
Or is the speed difference for that delta altitude so small that during
the hour or two that it spends under the station, it only moves a couple
of metres ?
And when it is given the OK to go from 250 down to 30metres distance,
how is that accomplished ? Does it increase its horizontal speed to
raise its orbit, or does it just push itself upwards ?
When it reaches grappling location, does Dragon have an elliptical
orbit and they have to grapple and berth it before it starts to want to
go back down to perigee ?
Or does it wait a quarter orbit at 10m and fire its thrusters to
circularise its orbit before the arm grapples it ?
If the arm were to grapple it while it has a elliptical orbit, would the
arm be strong enough to "pull" Dragon up to keep it at same distance
from station when Dragon wants to go down towards its perigee ?
Brian Gaff
Oct 10, 2012, 5:52:16 AM10/10/12
to
I thought the whole idea is that it is faster and this tests the station
keeping systems out.
Brian
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"JF Mezei" <jfmezei...@vaxination.ca> wrote in message
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keeping systems out.
Brian
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"JF Mezei" <jfmezei...@vaxination.ca> wrote in message
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Me
Oct 10, 2012, 4:11:29 PM10/10/12
to
On Oct 10, 5:44 am, JF Mezei <jfmezei.spam...@vaxination.ca> wrote:
> When Dragon is 250 metres directly below the station, how come it
> remains directly under the station ? SHouldn't it be moving ahead
> because at lower orbit, its horizontal speed will be greater
> When Dragon is 250 metres directly below the station, how come it
> remains directly under the station ? SHouldn't it be moving ahead
> because at lower orbit, its horizontal speed will be greater
> Or is the speed difference for that delta altitude so small that during
> the hour or two that it spends under the station, it only moves a couple
> of metres ?
Yes.
> the hour or two that it spends under the station, it only moves a couple
> of metres ?
> And when it is given the OK to go from 250 down to 30metres distance,
> how is that accomplished ? Does it increase its horizontal speed to
> raise its orbit, or does it just push itself upwards ?
>
> When it reaches grappling location, does Dragon have an elliptical
> orbit and they have to grapple and berth it before it starts to want to
> go back down to perigee ?
>
> Or does it wait a quarter orbit at 10m and fire its thrusters to
> circularise its orbit before the arm grapples it ?
>
> If the arm were to grapple it while it has a elliptical orbit, would the
> arm be strong enough to "pull" Dragon up to keep it at same distance
> from station when Dragon wants to go down towards its perigee ?
thrusters.
Once the arm is attached, Dragon is part of the ISS. There are brakes
on the arm joints.
JF Mezei
Oct 10, 2012, 8:07:05 PM10/10/12
to
On 12-10-10 16:11, Me wrote:
> fires upward.
Would it be correct to state that at that point in the final approach
from 250 down to 10, the software does not consider orbital mechanics in
a proactive way, and simply reacts in whatever way is necessary to keep
the ship in the centre of the trajectory plan ?
In other words, as it moves closer, it fires in whatever direction is
needed to keep the ship on that centreline and maintain the approach
speed. So if orbital mechanics cause the rate to drop because the Dragon
is in elliptical orbit, the ship will fire some more to keep the rate at
the preset value. Is that correct interpretation ?
At what altitude difference between Dragon and the Station do orbital
mechanics cease to be significant from a guidance/navigation point of
view ?
> Once the arm is attached, Dragon is part of the ISS. There are brakes
> on the arm joints.
Bringing back orbital mechanics here, if Dragon were in a very
elliptical orbit, would it be correct to state that if its apogee were
at same altitude as station, then when Dragon reached apogee its
forwards speed would be much slower than that of the station which is in
a circular orbit ?
So, in order for Dragon to by at same altitude and same horizontal speed
as the station (to allow the arm to grapple it), by definition, must it
not also be in a circular orbit ?
As Dragon takes its time to get closer to station (a couple of orbits),
doesn't its software unknowingly circularise its orbit when it does
whatever it takes to remain prececisely under the destination point ?
> fires upward.
Would it be correct to state that at that point in the final approach
from 250 down to 10, the software does not consider orbital mechanics in
a proactive way, and simply reacts in whatever way is necessary to keep
the ship in the centre of the trajectory plan ?
In other words, as it moves closer, it fires in whatever direction is
needed to keep the ship on that centreline and maintain the approach
speed. So if orbital mechanics cause the rate to drop because the Dragon
is in elliptical orbit, the ship will fire some more to keep the rate at
the preset value. Is that correct interpretation ?
At what altitude difference between Dragon and the Station do orbital
mechanics cease to be significant from a guidance/navigation point of
view ?
> Once the arm is attached, Dragon is part of the ISS. There are brakes
> on the arm joints.
elliptical orbit, would it be correct to state that if its apogee were
at same altitude as station, then when Dragon reached apogee its
forwards speed would be much slower than that of the station which is in
a circular orbit ?
So, in order for Dragon to by at same altitude and same horizontal speed
as the station (to allow the arm to grapple it), by definition, must it
not also be in a circular orbit ?
As Dragon takes its time to get closer to station (a couple of orbits),
doesn't its software unknowingly circularise its orbit when it does
whatever it takes to remain prececisely under the destination point ?
Brian Gaff
Oct 11, 2012, 1:51:21 PM10/11/12
to
In the sub line how on earth do you mispell Dragon?
Brian
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"JF Mezei" <jfmezei...@vaxination.ca> wrote in message
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--
From the Sofa of Brian Gaff Reply address is active
"JF Mezei" <jfmezei...@vaxination.ca> wrote in message
Dr J R Stockton
Oct 11, 2012, 4:03:00 PM10/11/12
to
In sci.space.station message <5075438c$0$47925$c3e8da3$dd96...@news.ast
raweb.com>, Wed, 10 Oct 2012 05:44:42, JF Mezei <jfmezei.spamnot@vaxinat
ion.ca> posted:
>When Dragon is 250 metres directly below the station, how come it
>remains directly under the station ? SHouldn't it be moving ahead
>because at lower orbit, its horizontal speed will be greater ?
>
>Or is the speed difference for that delta altitude so small that during
>the hour or two that it spends under the station, it only moves a couple
>of metres ?
No. The relative distance moved per orbit is three pi times the
difference in altitude, for circular orbits in inverse-square fields.
A proof is at <http://www.merlyn.demon.co.uk/gravity3.htm#Ov>.
--
(c) John Stockton, near London. Mail ?.?.Stoc...@physics.org
Web <http://www.merlyn.demon.co.uk/> - FAQish topics, acronyms, and links.
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raweb.com>, Wed, 10 Oct 2012 05:44:42, JF Mezei <jfmezei.spamnot@vaxinat
ion.ca> posted:
>When Dragon is 250 metres directly below the station, how come it
>remains directly under the station ? SHouldn't it be moving ahead
>because at lower orbit, its horizontal speed will be greater ?
>
>Or is the speed difference for that delta altitude so small that during
>the hour or two that it spends under the station, it only moves a couple
>of metres ?
difference in altitude, for circular orbits in inverse-square fields.
A proof is at <http://www.merlyn.demon.co.uk/gravity3.htm#Ov>.
--
(c) John Stockton, near London. Mail ?.?.Stoc...@physics.org
Web <http://www.merlyn.demon.co.uk/> - FAQish topics, acronyms, and links.
Correct <= 4-line sig. separator as above, a line precisely "-- " (RFC5536/7)
Do not Mail News to me. Before a reply, quote with ">" or "> " (RFC5536/7)
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