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Mercury-Redstone 3 and 4 were suborbital spaceflights and would have re-entered anyway. Yet, the spacescraft made a retrofire burn after apogee. Was it to adjust the path to land at a specified spot in the Atlantic ocean? Or would the capsule fall too fast otherwise and heat up too much for the heat shield to be bearable? Or a mix of both?

A similar question about the Falcon 9 first stage

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    $\begingroup$ Please, ask only one question per question. Case in point: I know the answer to the F9 question, but I cannot write an answer because my answer wouldn't considered to be an answer to the whole question. $\endgroup$ Feb 23 at 10:21
  • $\begingroup$ @JörgWMittag Alright, I asked the Falcon 9 question seperately and linked it. $\endgroup$ Feb 23 at 10:57
  • $\begingroup$ I believe it was just part of testing the capsules' systems. An orbital flight would need a retro burn, and they wanted to be sure it worked. $\endgroup$
    – John Doty
    Feb 23 at 17:27

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I haven't found a positive statement of why, but the MR-2 postflight report describes the results of not doing it on a suborbital flight (plus an overburn during the boost and additional delta-v due to the escape tower firing):

This greater velocity and the lack of retrofiring (retrorockets are jettisoned in this abort mode) resulted in a range of 363 nautical miles, a maximum altitude of 136.2 nautical miles , a period of weightless flight of about 6-1/2 minutes, with a maximum re-entry acceleration of 14.6g.

This New Ocean (the official NASA history of Project Mercury) also mentions the lack of retrofire as a contributor to the overshoot of the planned landing site.

The retrorockets jettisoned prematurely when the tower aborted, which meant that the spacecraft on reentry would not be artificially slowed down and therefore would gain still more downrange mileage.

The postflight report gives this plot of the planned vs. actual trajectory, but I am not sure how much of the difference is due to the overburn vs. the lack of retrofire.

enter image description here

Also planned vs. actual acceleration:

enter image description here

It is also likely that there was a desire to test out the retrofire system before orbital flight - This New Ocean describes the program management's fear of having an astronaut stuck in orbit and their brainstorming of alternate deorbit methods including a kind of "drag balloon".

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