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The Venera landers of the former Soviet Union were quite successful on the very hot surface of Venus.

Which material was used for the parachutes to survive 500 °C?

What about cotton wool? Yes this fiber will burn in air at 500 °C but there is no free oxygen in the atmosphere of Venus.

There is a process for the production of carbon fibers by heating of rayon fibers without oxygen. "Rayon is a synthetic fiber, made from natural sources of regenerated cellulose, such as wood and related agricultural products." Of cause cotton wool is cellulose too.

Thomas Alva Edison held a patent describing several ways of creating the carbon filament including "cotton and linen thread, wood splints, papers coiled in various ways". Carbon filaments used for electric light

See Wikipedia for Carbon Fibers, Kohlenstofffaser and Rayon.

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technically it was fiberglass with acrylic fibers - "стеклонитрон"("glass-nitron").

It was necessary to get down through the hot atmosphere quickly, but not hit the surface too hard. To accomplish this, the main parachute was reefed, a cord wrapped around the parachute cables held the mouth of the chute closed like a goblet, and the capsule descended rapidly. At a temperature of 200° C, the reefing cord was designed to melt, allowing the chute to open fully for a soft landing. Of course the parachute itself had to withstand over 500° C, and it was made of a heat resistant fiber of unknown composition ("glass nitron"). To further prolong mission time, the capsule was precooled to -8° C before separation from the main spacecraft.

http://mentallandscape.com/V_Lavochkin1.htm

Roscosmos: 50th anniversary of the launch of the automatic interplanetary station "Venera-7"

Высокое атмосферное давление позволило заменить двухкаскадную парашютную систему на однокаскадную с рифленым парашютом конусной формы площадью 2,8 м² — это чуть больше, чем площадь купола тормозного парашюта на «Венере-4». Изготовлен парашют был из отечественного материала стеклонитрон. Для обеспечения достаточной прочности купол парашюта изготавливался из 4 слоев ткани. После выгорания нитрона обеспечивалась воздухопроницаемость купола, гарантирующая его надежное функционирование. Соответственно была изменена и автоматика ввода парашютной системы.

The high atmospheric pressure made it possible to replace the two-stage parachute system with a single-stage one with a corrugated parachute of a conical shape with an area of ​​2.8 m², which is slightly more than the area of ​​the braking parachute canopy on Venera-4. The parachute was made from the domestic material "glassonitron". To ensure sufficient strength, the parachute canopy was made of 4 layers of fabric. After the nitron burned out, air permeability of the dome was ensured, guaranteeing its reliable operation. Accordingly, the automation of the parachute system was changed.

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  • $\begingroup$ Acrylic fibers would melt below 200 °C; did the parachute fabric contain acrylic fibers, or does acrylic fibers refer to the reefed cord that melts? $\endgroup$
    – uhoh
    Jun 13 at 11:11
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    $\begingroup$ @uhoh After the parachute is opened, these acrylic fibers are no longer needed. They give "softness" to the fabrics of packed parachute. When theacrylic fibers melts, parashute turns into a hard fiberglass dome. We're not going to use this parachute again, are we? :) $\endgroup$
    – A. Rumlin
    Jun 13 at 11:48
  • $\begingroup$ How cool! Why not mention that in the answer? Or shall I ask it as a separate question? $\endgroup$
    – uhoh
    Jun 13 at 11:51
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    $\begingroup$ @uhoh I'll just add a quote - I remembered how this material is called in Russian. $\endgroup$
    – A. Rumlin
    Jun 13 at 11:53

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