Editing Project Echo (section)

== Spacecraft ==
The Echo spacecraft (Echo 1, Echo 1A, and Echo 2) were large thin skinned spheres that were inflated in orbit after leaving the atmosphere. These balloon satellites were approximately {{cvt|30|m}} in diameter with a thin skin made of [[BoPET|Mylar]] (a trade name for stretched polyethylene terephthalate or BoPET), and were built by [[Gilmore Schjeldahl]]'s G.T. Schjeldahl Company in [[Northfield, Minnesota]]. The satellites functioned as a reflector, not a transceiver; after being placed in [[low Earth orbit]], signals could be sent from a ground station, reflected by its surface, and returned to [[Earth]].<ref>{{cite web|author1=NASA/Langley Research Center (NASA-LaRC) |title=Static Inflation Test of 135 ft Satellite In Weeksville, North Carolina|url=https://archive.org/details/NIX-EL-1996-00052|publisher=Internet Archive|access-date=March 15, 2020|date=June 29, 1965}}</ref>

As its shiny surface was also reflective in the range of visible light, Echo was easily visible to the unaided eye over most of the Earth. The spacecraft was nicknamed a "satelloon" by those involved in the project (a [[portmanteau]] combining ''satellite'' and ''balloon''). It was used to redirect transcontinental and intercontinental [[telephone]], [[radio]], and [[television]] signals.<ref name=Jones1961/> During the latter portion of its life, it was used to evaluate the technical feasibility of [[satellite triangulation]].

[[File:T. Keith Glennan Shows LBJ Aluminized Mylar Flim Used to Make Echo I - GPN-2002-000025.jpg|thumb|upright=1.0|left|T. Keith Glennan shows LBJ aluminized Mylar film used to make Echo I]]

=== Echo 1 ===
Echo 1 was {{cvt|30|m}} in diameter, had a non-rigid skin made of {{cvt|12.7|µm}}-thick Mylar, and had a total mass of {{cvt|180|kg}}, weighing {{cvt|71|kg}} at launch. During ground inflation tests, {{cvt|18000|kg}} of air were needed to fill the balloon, but while in orbit, several pounds of gas were all that was required to fill the sphere. To address the problem of [[meteorite]] punctures and keep the sphere inflated, Echo 1 included a {{cvt|15.12|kg}} make-up gas system using two types of [[Sublimation (phase transition)|sublimating]] powders – {{cvt|9.1|kg}} of [[anthraquinone]] and {{cvt|4.6|kg}} of [[benzoic acid]].<ref>{{cite web |last1=Davis and Tanimoto |title=Mechanical Development of Antenna Systems |url=https://descanso.jpl.nasa.gov/monograph/series8/Descanso8_08.pdf |website=NASA JPL |access-date=8 January 2022}}</ref> It also had 107.9 MHz telemetry beacons, powered by five [[Nickel–cadmium battery|nickel-cadmium batteries]] that were charged by 70 [[solar cell]]s mounted on the balloon. The spacecraft was useful to the calculation of atmospheric density and [[radiation pressure#Solar radiation pressure|solar pressure]], due to its large area-to-mass ratio.<ref name=Jones1961/> It was held together with Schjelbond, a proprietary adhesive developed by the [[Gilmore Schjeldahl#Schjeldahl Company|Schjeldahl company]].<ref name="gilmour">{{cite news |last=Gilmour |first=Gerry |date=1999-09-11 |title=A mind for business: Schjeldahl's ingenuity helps keep bread fresh, hearts safely beating |url=http://legacy.inforum.com/specials/century/jan3/week38b.html |url-status=dead |archiveurl=https://web.archive.org/web/20110713032131/http://legacy.inforum.com/specials/century/jan3/week38b.html |archivedate=2011-07-13 |work=The Forum |location=Fargo, North Dakota}}</ref>

=== Echo 2 ===
Echo 2 was a {{cvt|41.1|m}}-diameter balloon satellite, the last launched by Project Echo. A revised inflation system was used for the balloon, to improve its smoothness and [[sphericity]]. Echo 2's skin was rigidizable, unlike that of Echo 1. Therefore, the balloon was capable of maintaining its shape without a constant internal pressure; a long-term supply of inflation gas was not needed, and it could easily survive strikes from [[micrometeoroids]]. The balloon was constructed from a {{cvt|9|µm}}-thick mylar film sandwiched between and bonded with two layers of {{cvt|4.5|µm}}-thick aluminum foil.<ref>Staugaitis, C. & Kobren, L. "''Mechanical And Physical Properties of the Echo II Metal-Polymer Laminate (NASA TN D-3409)''", NASA Goddard Space Flight Center (1966) {{PD-notice}}</ref> It was inflated to a pressure that caused the metal layers of the laminate to plastically deform slightly, while the polymer was still in the elastic range. This resulted in a rigid and very smooth spherical shell. A beacon telemetry system provided a tracking signal, monitored spacecraft skin temperature between {{cvt|-120|and|+16|C|F}}, and measured the internal pressure of the spacecraft between 0.00005 mm of [[Mercury (element)|mercury]] and 0.5 mm of mercury, especially during the initial inflation stages. The system consisted of two beacon assemblies powered by solar cell panels and had a minimum power output of 45 [[Watt|mW]] at 136.02 MHz and 136.17 MHz.<ref>{{cite web|title=Echo 2|url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1964-004A|publisher=NASA|access-date=2019-01-30}} {{PD-notice}}</ref>