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I'm trying to understand the details of the 1972 Viking BLDT test missions (in earth's atmosphere). I found this diagram in a book and the general description that the (four) BLDT (balloon drop) tests were done at altitude of 36 to 43 km in order to simulate the density of Mars' atmosphere.

enter image description here

From the diagram it's clear that the retro rockets of the AV (Viking test vehicle) were fired; actually the text says that only in 3 of the 4 tests this was done. But it's not clear at what altitude the retro rockets were fired. From the diagram it looks like the BLDTs weren't a complete retro-powered landing done as it would later be done in the actual mission e.g. as Wikipedia says about Viking 1

At 1.5 km (0.93 mi) altitude, retrorockets on the lander itself were ignited and, 40 seconds later at about 2.4 m/s (7.9 ft/s), the lander arrived on Mars with a relatively light jolt.

The BLDT tests seem to have fired the retro rockets sending the AV high up again, from which it then descended only with its parachute... and even landed "upside down" if the diagram is correct. What was basically the point of this BLDT test sequence, which seems different from the actual Mars landing? The text doesn't clearly say at what altitudes the retro rockets were fired during these BLDT tests, which might shed light on this.

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  • $\begingroup$ Interesting question. $\endgroup$
    – Organic Marble
    Commented May 4, 2020 at 23:55
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    $\begingroup$ Is that a balloon? $\endgroup$
    – DrSheldon
    Commented May 5, 2020 at 0:10

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Note that you're mixing up the lander's "retro-rockets" and the special motors fitted for the tests. The special motors are different from the retro-rockets used by the real landers to touch down on Mars. They were "boost motors" provided solely to put the test vehicle in the proper altitude and speed conditions for the parachute deployment tests.

The BLDT tests were just for parachute deployment, not landing. The test vehicle didn't include the full landing system such as the actual retro-rockets.

The purpose of these tests was to qualify the Viking parachute system behind a full-scale simulator of the Viking Entry Vehicle over the maximum range of entry conditions anticipated in the Viking '75 soft landing on Mars.

Notice the difference between the location of the retro-rockets

enter image description here

Source - Visual Dictionary Online

and the boost motors.

enter image description here

Source - Google Arts and Culture

That said, to answer the question that was asked,

I found data for the AV-4 test in its Post-Flight Test Report

It states the vehicle was dropped at 120,543 feet

enter image description here

and the boost rockets fired 2.04 seconds later

enter image description here

So, 120,543 feet minus how much it dropped in two seconds.

(all emphasis and highlighting by me)

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    $\begingroup$ Also of interest (to me at least), from page "V-2" (no. 29 in the pdf), the boost motor firing sent the AV-4 up higher to 147,186 feet where the parachute (mortar) was deployed. $\endgroup$
    – got trolled too much this week
    Commented May 5, 2020 at 1:48
  • $\begingroup$ Yeah, I found that report pretty interesting, although I was annoyed that they didn't call out the actual motor ignition altitude :) $\endgroup$
    – Organic Marble
    Commented May 5, 2020 at 1:51

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