gravity-gradient stabilization

{{Short description|Method for the stabilization and the orientation of various spacecraft}}

Gravity-gradient stabilization or tidal stabilization is a passive method of stabilizing artificial satellites or space tethers in a fixed orientation using only the mass distribution of the orbited body and the gravitational field. The main advantage over using active stabilization with propellants, gyroscopes or reaction wheels is the low use of power and resources. It can also reduce or prevent the risk of propellant contamination of sensitive components.{{cite news | first = Kay | last = Grinter | title = Retrieval of LDEF provided resolution, better data | date = 8 January 2010 | publisher = NASA | url = http://www.nasa.gov/centers/kennedy/pdf/416359main_jan8color.pdf | work = Spaceport News | pages = 7 | access-date = 2014-01-22}}

File:Drawing of the GEOS-A Spacecraft.png

The technique exploits the Earth's gravitational field and tidal forces to keep the spacecraft aligned along the desired orientation. The gravity of the Earth decreases according to the inverse-square law, and by extending the long axis perpendicular to the orbit, the "lower" part of the orbiting structure will be more attracted to the Earth. The effect is that the satellite will tend to align its axis of minimum moment of inertia vertically.

Early satellite usage

GGSE-1, launched in 1964, was a low Earth orbit satellite{{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/displayTrajectory.action?id=1964-001B|title=Trajectory: GGSE 1 1964-001B |publisher=NASA|date=14 May 2020|access-date=3 February 2021}} {{PD-notice}} equipped with a passive oscillatory damping mechanism attached to the spacecraft via an {{cvt|8.5|m}} rod of metal tape. The entire mechanism and rod together weighed less than {{cvt|4.5|kg}}. The damping mechanism, developed by General Electric, was a metal sphere, {{cvt|12.7|cm}} in diameter, containing another metal sphere with a silicone damping fluid between. A small bar magnet attached to the inner sphere aligned that sphere with the Earth's magnetic field. As the satellite oscillated about its local vertical because of gravity gradient forces, the outer sphere of the damper rotated about the inner sphere, dissipating the oscillatory energy in the form of heat from the viscous drag of the fluid. This system was more effective than the damping spring-and-weight system used on a previously launched Transit satellite in that it provided equal damping about all three axes of the satellite while the older damper provided no damping about the yaw axis and less damping of the roll axis than for pitch. The new damper also was effective immediately whereas the older technique required several weeks for the spring-mass to compress into operational position. GGSE-1 worked as hoped. Its stabilization system successfully oriented the satellite to a local vertical within 5° of accuracy and damped out oscillations within three days of orbit.{{cite magazine|url=http://archive.aviationweek.com/issue/19640224#!&pid=56|title=Gravity Gradient Device Orients Satellite|magazine=Aviation Week & Space Technology|publisher=McGraw Hill Publishing Company|page=57|date=February 24, 1964|access-date=3 February 2021}}

From 1966-69, gravity-gradient stabilization was tested in low Earth orbit on several satellites of the United States Air Force's OV-1 series with a system called Vertistat. Consisting of three {{cvt|15.5|m}}-long horizontal booms forming a 'y' and two {{cvt|19|m}}-long vertical booms,{{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1969-025A|title=OV1-17|publisher=NASA|access-date=16 July 2024}} Vertistat was used unsuccessfully on OV1-7, OV1-86, and OV1-17, but successfully on OV1-10 (launched 11 December 1966).{{cite magazine| date = 1987| title = The Orbiting Vehicle Series of Satellites| magazine = Journal of the British Interplanetary Society| location = London| publisher = British Interplanetary Society |last1=Powell|first1=Joel W.|last2=Richards|first2=G.R.|volume=40|pages=420–421}}

Gravity-gradient stabilization for satellites was attempted but unsuccessful on the geosynchronous orbit Applications Technology Satellites ATS-2, ATS-4 and ATS-5 launched from 1966 until 1969.{{cite web|title=Applications Technology Satellite Program |date=22 May 2016 |url=https://science.nasa.gov/missions/ats/|publisher=NASA|access-date=December 31, 2022}} The Department of Defense Gravity Experiment (DODGE) satellite, launched July 1967, was the first successful use of the method in a near-geosynchronous orbit.[http://space.skyrocket.de/doc_sdat/dodge.htm Gunter's Space Page: DODGE]

Later satellite usage

The lunar orbiter Explorer 49 launched in 1973 was gravity gradient oriented (Z axis parallel to local vertical).{{Cite web|url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1973-039A|title=NASA NSSDCA Spacecraft Details - Explorer 49|last=|first=|date=|website=|access-date=}}

The Long Duration Exposure Facility (LDEF) used this method for 3-axis stabilization; yaw about the vertical axis was stabilized.{{Cite web |url=http://apps.dtic.mil/dtic/tr/fulltext/u2/a266026.pdf |title=Lesson learned from the Long Duration Exposure Facility. Stuckey. 1993 |access-date=2017-05-15 |archive-date=2017-04-29 |archive-url=https://web.archive.org/web/20170429051855/http://www.dtic.mil/dtic/tr/fulltext/u2/a266026.pdf |url-status=live }}{{rp|7}}

Gravity-gradient stabilization was attempted during NASA's TSS-1 mission in July 1992, but the project failed due to tether deployment problems.{{Cite journal |doi = 10.1007/BF02506678|title = A technical overview of TSS-1: The first Tethered-Satellite system mission|journal = Il Nuovo Cimento C|volume = 17|pages = 1–12|year = 1994|last1 = Dobrowolny|first1 = M|last2 = Stone|first2 = N. H|issue = 1|bibcode = 1994NCimC..17....1D|s2cid = 120746936}} In 1996, another mission, TSS-1R, was attempted but failed when the tether broke. Just prior to tether separation, the tension in the tether was about 65 N (14.6 lbs).NASA, [http://hdl.handle.net/2060/19970011947 TSS-1R Mission Failure Investigation Board], Final Report, May 31, 1996 (accessed 7 April 2011)

Crewed flight usage

The first attempt to use this technique in human spaceflight occurred on September 13, 1966 during the US Gemini 11 mission. The Gemini spacecraft was attached to the Agena target vehicle by a {{convert|100|ft|m|adj=on}} tether. The attempt was a failure, as insufficient gradient was produced to keep the tether taut.{{Citation

| first = Kenneth| last = Gatland

| author-link =

| title = Manned Spacecraft, Second Revision

| place = New York, NY, USA

| publisher = MacMillan Publishing Co., Inc

| year = 1976

| pages = 180–182

| isbn = 978-0-02-542820-1

}}

On August 11, 1978, during the Soyuz 29 mission to Salyut 6, the station was put into gravity gradient stabilized flight for materials processing experiments with the Kristall and Splav furnaces.{{Cite book |last=Newkirk |first=Dennis |title=Almanac of Soviet manned space flight |date=1990 |publisher=Gulf Pub. Co |isbn=978-0-87201-848-8 |location=Houston}}

External links

[https://web.archive.org/web/20050907232724/http://roland.lerc.nasa.gov/~dglover/sat/ats2.html NASA on ATS-2]
[http://space.skyrocket.de/doc_sdat/ats-4.htm Gunter's Space Page on ATS 2, 4 and 5]
[http://gges.epfl.ch A new kind of Earth Sensor using a proof mass on a MEMS created by EPFL students]

References

Category:Spacecraft attitude control

Category:Spacecraft propulsion

Category:Spacecraft components

Category:Tidal forces