A Space & astronomy forum. SpaceBanter.com

Go Back   Home » SpaceBanter.com forum » Space Science » Space Shuttle
Site Map Home Authors List Search Today's Posts Mark Forums Read Web Partners

MSNBC (JimO) - Hubble debate -- a lot of sound and fury



 
 
Thread Tools Display Modes
  #72  
Old April 1st 04, 06:02 AM
Rand Simberg
external usenet poster
 
Posts: n/a
Default MSNBC (JimO) - Hubble debate -- a lot of sound and fury

On 01 Apr 2004 01:38:57 GMT, in a place far, far away, "Jorge R.
Frank" made the phosphor on my monitor glow in
such a way as to indicate that:

The change is 3kmps, which is about a third of the
acceleration that the shuttle needs to get into orbit in the first place.


What's your basis for that number? How much of a plane change were
you assuming?


His number is pretty close to mine (3.05 km/s). I assumed a noncoplanar
Hohmann transfer from HST (310 n.mi., 28.45 deg) to ISS (210 n.mi., 51.6
deg), for a total plane change of 23.15 deg. Delta-V is minimized by
combining most of the plane change (21.76 deg) with the first Hohmann burn.
Of course, both burns must occur at the moment when the RAANs are equal,
otherwise the plane change will be much larger than 23.15 deg.


Jorge, you should be a little embarrassed. Is there any reason to
suppose/assume that they are at the same node?
  #73  
Old April 1st 04, 06:02 AM
Jorge R. Frank
external usenet poster
 
Posts: n/a
Default MSNBC (JimO) - Hubble debate -- a lot of sound and fury

h (Rand Simberg) wrote in
:

On 01 Apr 2004 01:38:57 GMT, in a place far, far away, "Jorge R.
Frank" made the phosphor on my monitor glow in
such a way as to indicate that:

The change is 3kmps, which is about a third of the
acceleration that the shuttle needs to get into orbit in the first
place.

What's your basis for that number? How much of a plane change were
you assuming?


His number is pretty close to mine (3.05 km/s). I assumed a
noncoplanar Hohmann transfer from HST (310 n.mi., 28.45 deg) to ISS
(210 n.mi., 51.6 deg), for a total plane change of 23.15 deg. Delta-V
is minimized by combining most of the plane change (21.76 deg) with
the first Hohmann burn. Of course, both burns must occur at the moment
when the RAANs are equal, otherwise the plane change will be much
larger than 23.15 deg.


Jorge, you should be a little embarrassed. Is there any reason to
suppose/assume that they are at the same node?


Not at this moment, no. But ISS' node regresses at 5 deg/day while HST's
node regresses at 6.5 deg/day, so the nodes will coincide roughly every
eight months. I assumed the smart thing to do would be to wait and perform
the transfer at the next common node. Of course, there's no guarantee that
ISS will actually be at the exact point when the nodes are aligned, but
once the two spacecraft are coplanar at the same altitude, it's a fairly
low delta-V phasing problem.


--
JRF

Reply-to address spam-proofed - to reply by E-mail,
check "Organization" (I am not assimilated) and
think one step ahead of IBM.
  #74  
Old April 1st 04, 07:24 AM
Rand Simberg
external usenet poster
 
Posts: n/a
Default MSNBC (JimO) - Hubble debate -- a lot of sound and fury

On 01 Apr 2004 05:02:59 GMT, in a place far, far away, "Jorge R.
Frank" made the phosphor on my monitor glow in
such a way as to indicate that:

Jorge, you should be a little embarrassed. Is there any reason to
suppose/assume that they are at the same node?


Not at this moment, no. But ISS' node regresses at 5 deg/day while HST's
node regresses at 6.5 deg/day, so the nodes will coincide roughly every
eight months. I assumed the smart thing to do would be to wait and perform
the transfer at the next common node. Of course, there's no guarantee that
ISS will actually be at the exact point when the nodes are aligned, but
once the two spacecraft are coplanar at the same altitude, it's a fairly
low delta-V phasing problem.


You're assuming an impulsive delta V. That's the most expensive
option...
  #75  
Old April 1st 04, 03:01 PM
Jorge R. Frank
external usenet poster
 
Posts: n/a
Default MSNBC (JimO) - Hubble debate -- a lot of sound and fury

h (Rand Simberg) wrote in
:

On 01 Apr 2004 05:02:59 GMT, in a place far, far away, "Jorge R.
Frank" made the phosphor on my monitor glow in
such a way as to indicate that:

Jorge, you should be a little embarrassed. Is there any reason to
suppose/assume that they are at the same node?


Not at this moment, no. But ISS' node regresses at 5 deg/day while
HST's node regresses at 6.5 deg/day, so the nodes will coincide
roughly every eight months. I assumed the smart thing to do would be
to wait and perform the transfer at the next common node. Of course,
there's no guarantee that ISS will actually be at the exact point when
the nodes are aligned, but once the two spacecraft are coplanar at the
same altitude, it's a fairly low delta-V phasing problem.


You're assuming an impulsive delta V. That's the most expensive
option...


Expensive in terms of delta-V or propellant mass? If the former, I'm not so
sure. An impulsive burn concentrates all the delta-V at the common node,
which is where you want it. A long low-thrust burn would, by necessity,
apply much of its thrust at points in the orbit other than the common node,
driving up the delta-V. I will admit I'm not up on the theory of low-thrust
transfers, however.

If you meant propellant mass, I agree. Low-thrust propulsion has much
higher Isp, so the propellant mass can be lower despite needing a larger
delta-V.

--
JRF

Reply-to address spam-proofed - to reply by E-mail,
check "Organization" (I am not assimilated) and
think one step ahead of IBM.
  #76  
Old April 1st 04, 04:16 PM
Rand Simberg
external usenet poster
 
Posts: n/a
Default MSNBC (JimO) - Hubble debate -- a lot of sound and fury

On 01 Apr 2004 14:01:30 GMT, in a place far, far away, "Jorge R.
Frank" made the phosphor on my monitor glow in
such a way as to indicate that:

You're assuming an impulsive delta V. That's the most expensive
option...


Expensive in terms of delta-V or propellant mass?


Propellant mass. It's the most efficient in delta V

If the former, I'm not so
sure. An impulsive burn concentrates all the delta-V at the common node,
which is where you want it. A long low-thrust burn would, by necessity,
apply much of its thrust at points in the orbit other than the common node,
driving up the delta-V. I will admit I'm not up on the theory of low-thrust
transfers, however.


I actually ran some numbers on this a few months ago. While you only
get pure plane change at the node, there are several minutes on each
side of it during which it's still pretty efficient to do it (i.e.,
the ratio of plane change to node change still remains above ninety
percent).
  #79  
Old April 2nd 04, 01:48 AM
Rand Simberg
external usenet poster
 
Posts: n/a
Default MSNBC (JimO) - Hubble debate -- a lot of sound and fury

On 01 Apr 2004 23:54:25 GMT, in a place far, far away, "Jorge R.
Frank" made the phosphor on my monitor glow in
such a way as to indicate that:

(Rand Simberg) wrote in
:

I actually ran some numbers on this a few months ago. While you only
get pure plane change at the node, there are several minutes on each
side of it during which it's still pretty efficient to do it (i.e.,
the ratio of plane change to node change still remains above ninety
percent).


Thanks for the details, Rand!


I mistyped above--I meant the ratio of inclination change to node
change. Your amount of plane change is constant at any point in the
orbit, but it will be a sum of the node change and the inclination
change.

If you really want details, I can send the spreadsheet. It was
actually a pretty interesting problem.

It turns out that if you do a cross-track burn exactly at the top or
bottom, it will always increase inclination--there's no way to
decrease it. In retrospect, this makes sense, the same way that you
can only increase your inclination above your launch latitude,
regardless of whether you launch north or south. But get a little
distance away from that point, and then you start trading off node
change for inclination change (up or down), with minimal inclination
change at the top, and maximum at the nodes.
  #80  
Old April 2nd 04, 05:39 AM
Jorge R. Frank
external usenet poster
 
Posts: n/a
Default MSNBC (JimO) - Hubble debate -- a lot of sound and fury

Chuck wrote in
:

Jorge, what do you think about John's idea in an earlier post about
using an ion propulsion system to push Hubble into an ISS compatible
orbit? Would that be a viable option or would the development/
transition fuel costs, etc. be too prohibitive?


As far as the ion propulsion goes, the basic concepts were proven with DS1,
but that was a smaller beast than HST. To transfer HST to ISS inclination
in a reasonable time (on the order of months rather than years), you'd need
a larger engine than has been successfully developed so far. Some
development cost but no show-stoppers that I can see.

Likewise, using DS1-class ion engines with HST might work but would require
longer transfer times (on the order of years), so you'd need an engine that
can work reliably over a very long period, or multiply-redundant engines.
Again, no showstoppers, just development work and tradeoffs to be made.

In either case, the fuel costs are negligible compared to development
costs. Take all of the above with a grain of salt - I'm a rendezvous guy,
not a low-thrust propulsion guy. I haven't actually run any numbers on how
long these transfers would take.

The biggest development headache may well be automated rendezvous and
capture of HST. All the automated systems developed to date require their
targets to be cooperative to some degree (i.e. equipped with navaids for
the chaser vehicle). The Russian Kurs system requires a rather elaborate
set of RF antennas/avionics on the target vehicle. The systems under
development for ATV and HTV require the target vehicle to have a GPS
receiver and an array of laser retroreflectors.

Trouble is, HST has *none* of those things - it's a completely passive
target, save the (visual) aids on its RMS grapple fixtures and berthing
mechanism. Attaching the navaids to HST becomes a chicken-and-egg problem -
any automated system that could attach the navaids to HST needs the navaids
to be already there in order to get there in the first place.

The *only* vehicle in existence which can rendezvous with and capture such
a completely passive target is the very vehicle which O'Keefe has ground-
ruled out of the solution: the space shuttle. It was designed to be able to
rendezvous with passive targets - hell, it can even rendezvous with debris,
if the situation required it. Its rendezvous radar is capable of skin-
tracking a target with no transponder, and the crew is equipped with
police-style handheld lidars that are likewise capable of skin-tracking an
inert target.

That is not to say that it's impossible to develop an automated system to
rendezvous and capture a non-cooperative, inert, and (by 2007-08) possibly
slowly tumbling target. But the challenge of developing such a system will,
IMO, be greater than that of getting the ion propulsion to work.

--
JRF

Reply-to address spam-proofed - to reply by E-mail,
check "Organization" (I am not assimilated) and
think one step ahead of IBM.
 




Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

vB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump


All times are GMT +1. The time now is 05:58 PM.


Powered by vBulletin® Version 3.6.4
Copyright ©2000 - 2024, Jelsoft Enterprises Ltd.
Copyright ©2004-2024 SpaceBanter.com.
The comments are property of their posters.