NYT: Death Sentence for the Hubble?

Neil Gerace wrote:

"Pat Flannery" wrote in message
...


Yes, I thought the moon and sun would be a problem.



If the sun disappeared, skin cancer would too. Bring it on!



Spoken like a true vampire.
If we got rid of the Moon also, there'd be none of those pesky
werewolves to contend with.

Pat

Henry Spencer wrote:
In article ,
Christopher M. Jones wrote:

...However, a telescope in an orbit with such a
low lifetime would also need frequent reboosts, which would
require a lot of onboard propellant and frequent usage of some
sort of thruster. Things which are not terribly compatable
with high precision optical astronomy equipment...

No, it just means you have to choose propulsion systems carefully,
avoiding orthodox hypergolics and other systems which spew out condensible
garbage that can easily contaminate optics. Xenon Hall-effect thrusters
or an ammonia arcjet would be good choices.

True, which is why I did not *completely* rule out the
option. Of especial note is that such efforts come with
rather sizeable down sides (and price tags). Low thrust
propulsion requires nearly continual operation to maintain
a low orbit. This comes at a cost in either
sophisticated scan platforms, inconvenient pointing
constraints, degraded duty cycles or all of the above
(plus additional mass and complexity in power generation
and processing, e.g. solar arrays and PPUs et al).
Somehow, NRO manages to keep Hubble class optical
surveillance satellites in pretty low orbits for a
substantial period of time, so there might be a few
tricks to getting away with that. Though the constraints
for spysats are much different than those for space
science observatories, so the experience may not be
applicable (spysats never create long exposures like the
Hubble deep field, for example).

Of course, none of this comes for free, no matter how
well it works. If manned spaceflight were significantly
more routine and robust than it is today, it might be a
worthwhile tradeoff. However, if it were then it would
likely have the capabilities needed to service more
remote installations, so the concept has something of a
catch-22 built into it.

Pat Flannery wrote:

:Neil Gerace wrote:
:
:"Pat Flannery" wrote in message
...
:
:Yes, I thought the moon and sun would be a problem.
:
:
:If the sun disappeared, skin cancer would too. Bring it on!
:
:Spoken like a true vampire.
:If we got rid of the Moon also, there'd be none of those pesky
:werewolves to contend with.

Actually, getting rid of the sun takes care of that, too. No sun, no
sunlight to reflect at earth and hence no full moons. Voila. Problem
solved.

"Fred J. McCall" wrote in message
...

Actually, getting rid of the sun takes care of that, too. No sun, no
sunlight to reflect at earth and hence no full moons. Voila. Problem
solved.


Ah, but is it really the moonlight or the orbital position of the full Moon?

I mean why not a partial werewolf on a waxing or waning Moon?

I guess the question is, do werewolves come out during lunar eclipses?

Werewolf? There wolf, there castle.

In article ,
Christopher M. Jones wrote:
...just means you have to choose propulsion systems carefully,
avoiding orthodox hypergolics and other systems which spew out condensible
garbage that can easily contaminate optics. Xenon Hall-effect thrusters
or an ammonia arcjet would be good choices.

True, which is why I did not *completely* rule out the
option. Of especial note is that such efforts come with
rather sizeable down sides (and price tags). Low thrust
propulsion requires nearly continual operation...

That's why I mentioned arcjets, which are still low-thrust by chemical
standards but have *much* higher thrust than Hall-effect thrusters.
With them, it should be possible to do occasional corrections rather
than continuous thrusting.

Even resistojets would be worth considering. You don't actually *need*
terribly high Isp for this; the point of the electric thrusters is more
that they can run on storable non-contaminating fluids.

For that matter, you ought to be able to do non-contaminating storable
chemical fuels, if you picked carefully. I would guess that ClF5/NH3
would be non-contaminating. (Don't know if anyone's tried that particular
combination; ammonia is notorious for being difficult to burn well because
it's so stable, but ClF5 is notorious for being so ferociously active that
it gives smooth combustion with anything it gets its hands on.)
--
"Think outside the box -- the box isn't our friend." | Henry Spencer
-- George Herbert |

On Sat, 19 Feb 2005 05:55:15 +0000, Henry Spencer wrote:

For that matter, you ought to be able to do non-contaminating storable
chemical fuels, if you picked carefully. I would guess that ClF5/NH3
would be non-contaminating. (Don't know if anyone's tried that particular
combination; ammonia is notorious for being difficult to burn well because
it's so stable, but ClF5 is notorious for being so ferociously active that
it gives smooth combustion with anything it gets its hands on.)

Including the telescope?

--
Chuck Stewart
"Anime-style catgirls: Threat? Menace? Or just studying algebra?"

Henry Spencer wrote:
That's why I mentioned arcjets, which are still low-thrust by chemical
standards but have *much* higher thrust than Hall-effect thrusters.
With them, it should be possible to do occasional corrections rather
than continuous thrusting.

Are there arcjets large enough for this task? I was
under the impression that there were none large enough
for main propulsion tasks. Although for simple orbital
maintenance, something that large might not be needed.


Even resistojets would be worth considering. You don't actually *need*
terribly high Isp for this; the point of the electric thrusters is more
that they can run on storable non-contaminating fluids.

Agreed (or solids, of course). And perhaps solar sails or
electrodynamic tethers, though those are much less easily
compatible with telescope operation than even ion engines.

In article ,
Chuck Stewart wrote:
chemical fuels, if you picked carefully. I would guess that ClF5/NH3
would be non-contaminating. (Don't know if anyone's tried that particular
combination; ammonia is notorious for being difficult to burn well because
it's so stable, but ClF5 is notorious for being so ferociously active that
it gives smooth combustion with anything it gets its hands on.)

Including the telescope?

That's definitely a combination you want to run fuel-rich. :-)

Handling ClF5 is, shall we say, a "zero defects" kind of operation. Never
mind fuels; the stuff is hypergolic with all normal fire-extinguishing
agents...

(The recommended firefighting procedure for a ClF5 fire is to take cover
at a safe distance. Quickly.)
--
"Think outside the box -- the box isn't our friend." | Henry Spencer
-- George Herbert |

In article ,
Christopher M. Jones wrote:
That's why I mentioned arcjets...

Are there arcjets large enough for this task? I was
under the impression that there were none large enough
for main propulsion tasks. Although for simple orbital
maintenance, something that large might not be needed.

Some quite large ones have been tested, although not recently. The
off-the-shelf ones *are* small, but you could use a cluster of them.

Even resistojets would be worth considering. You don't actually *need*
terribly high Isp for this; the point of the electric thrusters is more
that they can run on storable non-contaminating fluids.

Agreed (or solids, of course). And perhaps solar sails or
electrodynamic tethers, though those are much less easily
compatible with telescope operation than even ion engines.

Solar sails almost certainly can't be made to work for this because of
air drag. Except in very favorable circumstances, it's difficult to get
an excess of thrust over drag with a solar sail below 1000km or so.

Electrodynamic tethers are a possibility, although as you say, making
them compatible with telescope operations would be tricky.
--
"Think outside the box -- the box isn't our friend." | Henry Spencer
-- George Herbert |

"Peter Stickney" wrote in message
...

(The recommended firefighting procedure for a ClF5 fire is to take cover
at a safe distance. Quickly.)

AS it said on the back of my Duracell Abuse Testing T-Shirt:
"If you see me running, try to keep up"

In the outdoors people often ask me how fast they have to run to be safe
from a bear attack. I simply point out they have to only outrun me.



--
Pete Stickney

Without data, all you have are opinions