From: Larrison <larrison@ix.netcom.com>
Newsgroups: sci.space.policy
Subject: Micrograv comparision -- was: Re: Merits of STS (was Re: Shuttle & govt. / private
Date: Thu, 05 Feb 1998 06:40:06 GMT
Organization: Netcom
Message-ID: <6bbmq8$meq@dfw-ixnews12.ix.netcom.com>
Larrison wrote:
>>
>> Hmm... actually, the ISS does have better microgravity that an
>>un- manned free flyer like the ISF, or Photon or the Chinese
>>reentry system.
>>{much deleted}
>> Similar problems have also hindered the Russian Photon capsules as
>>well (they *dropped* the capsule from a height of
>>several hundred meters while flying it out of the recovery zone).
David Anderman <davida@cwo.com> replies
>I am rather surprised that the normally astute Mr. Larrison would
>actually attempt to compare the poor microgravity environment aboard
>ISS with the Russian Photon capsule.
>In many ways, ISS will prove to be *unusable* for micro-gravity
>experimentation, whereas Photon has flown several commercial
>micro-gravity experiments, and has flown in its various incarnations
>over 1,000 times.
>Some of the more useful microgravity experiments - and more
>importantly, industrial processes - require durations of hours, if
>not days. ISS cannot provide an acceleration free environment of
>this duration, whereas Photon can fly in gravity gradient mode for
>up to 2 weeks.
>And yes, the Russians dropped one of their 1,000+ Photon capsules
>during transport back to the shop. This has no bearing on the on-
>orbit microgravity environment provided by Photon.
>BTW, one of the lessons of the Russian experience in space is that
>having people around microgravity experiments is a bad idea. That's
>why the Photon free-fliers are required despite the existence of
>several generations of Russian space station.
Hmmm .. . my files show that only 11 Photon capsules have flown.
The most recent launches being:
1994 Photon 6, launched 14 Jun 94 (15 day mission)
1995 Photon 7, launched 16 Feb 95 (16 day mission)
1997 Photon 8(N11), launched 9 Oct 97 (16 day mission)
(Note, there were 3 prototype flights from 1985-1987, which are not
included in the usual numbering systems, but Photon 8 was the eleventh
launch of a Photon capsule).
Now, the Photon capsule is a direct descendent of the Vostok
pressurized sphere, which has been used on hundreds of Soviet
reconnaissance satellites, so David is correct in that this type of
capsule has flown many times -- but the microgravity configuration has
only been flown a very limited number of times, versus other
configurations such as the military, Resurs, Bion, and other mission
configurations. In particular, the attitude control maneuvers
required for pointing instrument systems such as on the Resurs
spacecraft, make maintaining a good microgravity environment
questionable. (The Resurs series has flown small 15-30 kg
experimental microgravity experiments, and measured degradation in the
microgravity environment. For some experiments, the Resurs
microgravity level and disturbances are perfectly acceptable.)
For comparative microgravity environments, the Photon capsule is
stated to have microgravity conditions "as low as 1e-5 G" (10 uG),
with measured environments typically greater than this, dependent upon
the specific experimental apparatus inside the capsule. I've had the
actual environment on an operational mission estimated as 100+uG by
folks who have flown on Photon. The Photon capsule is capable of
supporting up to 700 kg of payload, with up to 400 watts (average)
power, over missions of up to 16 days. (Ref: "Principal Technical
Vehicle Characteristics of the Space Vehicle 'Photon'", Glavkosmos
1987, with actual results estimated in unpublished correspondence with
an investigator on Photon 6).
Other free flyers can do substantially better for on-orbit power
and duration. From http://cobi.gsfc.nasa.gov/ISS_SW/tables.html, I
note EURECA can maintain an 1000 kg payload in orbit for 6-9 months,
and maintain a microgravity environment of <100 uG (< 1 Hz) or < 1000
uG (> 100 Hz) while delivering an average of 1000 W.
For pure microgravity environment, the Wake Shield satellite can
provide 300 kg, < 1uG, for several days, with an average of 950 Watts
of power. But both of these systems must be tended on orbit and are
not capable of surviving reentry at the end of their planned missions.
In comparison, data on the Shuttle and Mir has been gathered
through the use of NASA and CNES accelerometers during flights.
The Mir typically has environments of about 500 uG, in the range of 1-
100 Hz, which appears to be primarily driven by the BKV-3 compressor,
part of the humidification equipment in the Mir Core module. This
results in disturbances at harmonics around 24 Hz, with disturbances
in the range of "hundreds" of uGs. From the data measured on several
Mir missions, other equipment contributions (neglecting thruster
firings) are much less. Crew exercise runs around 5000 uG, when the
crew uses the onboard exercise equipment. (Ref: "SAMS Acceleration
Measurements on Mir from Sep 1996 to Jan 1997 by Moskowitz,
Finkelstein, Hrovat, and Reckart (21 Nov 97), and "SAMS Acceleration
Measurements on Mir from Jun to Nov 1995", by DeLombard, Moskowitz,
Hrovat, and McPherson (9 Aug 96) -- both of these reports are
available on the NASA LeRC PIMS web site)
The Shuttle microgravity environment has also been measured on
orbit. For experiments placed in the Shuttle payload bay in a
Spacelab rack, typical shuttle microgravity environments during
periods of crew activity varied substantially depending upon crew
activity levels. When the crew was using the exercise equipment,
accelerations of 50-1000 uG were recorded dependent upon if active
damping was used to reduce the coupled vibrations from the exercise
equipment. (Active damping equipment is baselined for microgravity
laboratory flights.) During sleep periods, the shuttle provided an
environment of < 3uG, in gravity gradient orientation, with the
remaining disturbances primarily from the vehicle structure dynamics,
the Ku-band antenna operations, and the refrigerator compressor in the
Spacelab module. This environment appeared to maintained for some
hours, until the next crew work cycle. (Ref: Microgravity Environment
Description Handbook, NASA LeRC, with a draft available on their PIMS
web site. See also "Summary Report of Mission Acceleration
Measurement for STS -78 (launched June 20 1996)", by Hakimzadeh,
McPherson, Hrovat, Moskowitz, and Rogers -- undated, but also
available through the PIMS web site.)
It should be noted the SpaceLab racks allow the mounting of
several thousands of kilograms of experiments with total Spacelab
power available of up to 6 kW, for up to 16 days (more if the 30 days
EDO kit is used).
For both Shuttle and Mir, thruster firing resulted in
substantially higher disturbances, with as low as 300 uG (shuttle
vernier ACS jets), with other disturbances higher, up to almost 1000
uG depending upon the jets and maneuvers measured. (400 uG for
Progress reboost of the Mir complex.) In both vehicles days would
pass between ACS firings to maintain orbit against orbital decay.
So in comparison we can summarize for existing systems:
Steady State Quiet Crew Exercise Duration Mass Power
------------------ ------------- -------- ---- ----
Photon 10-100+ uG N/A <16 d 700 kg 400 W
EURECA <1000 uG N/A <270 d 1000 kg 1000 W
Wake Shield < 1 uG N/A < 5 d 150 kg 950 W
Mir <500 uG Hours ??? ???
< 5000 uG 30-60d+ ??? ???
Shuttle < 3uG Hours >1400kg* <6000 W
< 50 uG** <16 d >1400kg* <6000 W
[Note: * = experiment mass in 2 racks, up to 8 possible
**= use of active motion dampers on crew exercise
equipment]
As a final point in comparison, I would note the COMCAP
(Commercial Capsule Provider) joint German/ Italian/ Russian venture
using an upscaled version of the Express capsule is offering a payload
of 350 Kg, mission duration's of up to 15 days, 250 W average power,
and a microgravity level of 50 uG. Price is estimated at $40,000 per
Kg of payload (which puts the total system cost at around $15 M). I
have no record of any sales of this system. (Ref: Aerospace Daily 23
Oct 97).
ISS has microgravity as a design requirement, as compared to
Shuttle and Mir, which did not have microgravity level and quality as
design-to requirements. I can't quote specific values for ISS as
measured, since the system hasn't flown. I can't find the specific
design requirement in my files (anyone on the forum know?) but I
recall the design-to requirement is supposed to be <5uG over a large
volume of the Lab module, and maintained for a period of weeks. The
primary disturbance that would violate this requirement is the
required reboost to a higher orbit. The current ISS operations
planning is to perform a resupply/ crew exchange mission and then
reboost to a higher altitude, from which the system will decay over a
period of weeks or months. During this period of time, the ISS design
requirement is to provide high quality microgravity environment over a
region encompassing several racks in the US Lab module and partially
over a region encompassing another lab module. If the system works as
planned, this would provide a fairly high quality environment for a
period of weeks.
Based upon this, I made the observation the Shuttle and ISS have a
better microgravity environment than the Photon, or other free-flying
systems, although I did not call out the exception of the very limited
duration Wake Shield Facility. ISS values are unverifiable at this
time, since the hardware has not flown, but the design requirement is
to be substantially better than Photon.
However, let me repeat a statement from my original posting that
David Anderman didn't repeat. I said:
"Having said that, there has been some questions raised about how
much the microgravity requirement and the constraints it has
imposed on ISS design are worth. The cost of meeting the ISS high
quality microgravity requirement has been substantial."
I agree with David that in the long run, it would be better to
separate microgravity research facilities from permanently manned
space facilities. As WSF has shown, freeflying facilities can offer
substantially improved microgravity environments.
-----------------------------------------------------------------
Wales Larrison Space Technology Investor