From: Larrison 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 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