Why did the manned Mercury-Redstone flights carry and use retrorockets?

Question

The Mercury-Redstone programme, a subset of the Mercury human spaceflight programme, used a modified version of the Redstone missile for suborbital flights in space. It culminated in two manned flights in 1961 (Mercury-Redstone 3 [short form MR-3, alias Freedom 7, carrying Alan Shepard] and Mercury-Redstone 4 [short form MR-4, alias Liberty Bell 7, carrying Virgil “Gus” Grissom]), the first by the United States; earlier flights had been unmanned (except for, in one case, a chimpanzee).

Oddly, both MR-3 and MR-4 carried a retrorocket pack which was fired just after apoapsis to slow down the capsule. This would obviously have been needed on the later Mercury-Atlas flights, which did go into orbit, but the Mercury-Redstone missions were all brief suborbital hops that couldn’t have gone into orbit even if they’d tried (the Redstone was a single-stage alcolox rocket without anywhere near the performance to put anything into orbit), making the retrorockets unnecessary. They did slow the capsules down, and, thereby, cause the thermal loads on their heatshields to be lower and their splashdown points to be closer to the Cape, but the Mercury heatshield could easily withstand far higher heating loads than these (given that it would have to protect the Mercury-Atlas capsules during reentry from full orbital speeds - and already had protected three unmanned Mercury-Atlas flights by this point in the programme, two of them from orbit), and the U.S. Navy wouldn’t have had any trouble at all going a bit further east to pick them up (especially when one considers the distances to which they would later go to collect the astronauts and capsules scattered all over the Atlantic and Pacific oceans from the orbital Mercury, Gemini, and Apollo flights).

If anything, their inclusion on these two flights would seem to have been a disadvantage, as it introduced an additional potentially-lethal failure mode (what if a retrorocket blew up instead of firing?). Although Wikipedia claims that

[...] At the top of the curve, the spacecraft's retrorockets were fired for testing purposes; they were not necessary for reentry because orbital speed had not been attained. [...]

I can’t imagine that the retrorockets were included merely to test them for service on the manned Mercury-Atlas flights, since they could easily have been tested on the earlier, unmanned Mercury-Redstone flights (and, for that matter, the early unmanned Mercury-Atlas flights) without potentially endangering a human crewmember.

The only possibility that makes any sense to me would be if, after MR-2, with Ham on board, flew a much higher and steeper trajectory than intended (due to a booster malfunction that triggered the rocket's abort sequence at the end of the Redstone's burn),² resulting in the capsule (and its Pan troglodytes occupant) experiencing G-loads much higher than intended (although still completely survivable, albeit quite uncomfortable), the retrorockets were added to the manned Mercury-Redstone missions in order to limit the height and steepness of the capsules’ trajectories (and, thus, the Gs felt during their reentries) should a similar malfunction occur during a manned flight.

Can someone help clear this up for me?

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²: Other malfunctions to occur on that flight included an unplanned rapid decompression (Ham, wearing a spacesuit, was unaffected).

Answer 1

They did slow the capsules down, and, thereby, cause the thermal loads on their heatshields to be lower and their splashdown points to be closer to the Cape, but the Mercury heatshield could easily withstand far higher heating loads than these (given that it would have to protect the Mercury-Atlas capsules during reentry from full orbital speeds - and already had protected three unmanned Mercury-Atlas flights by this point in the programme, two of them from orbit)

All the Mercury capsules, crewed or uncrewed, launched on Redstones used a beryllium heat-sink shield instead of the ablative shield used by the Mercury-Atlas flights. Both the heat sink and ablative solutions were considered early on, but it was decided that the ablative was better for the orbital flights. (I heard this on The Space Above Us podcast, and have found some supporting claims, but no authoritative NASA reference as yet.) The beryllium shield was said to be "half as safe and twice as expensive" as the ablative.

So it's not impossible that the quicker (less total heating time) reentry was desirable for the heat shields on the Redstone flights.

I can’t imagine that the retrorockets were included merely to test them for service on the manned Mercury-Atlas flights, since they could easily have been tested on the earlier, unmanned Mercury-Redstone flights (and, for that matter, the early unmanned Mercury-Atlas flights) without potentially endangering a human crewmember.

The scarier failure mode for the orbital flights would have been the retrorockets failing to fire at all; it's possible they wanted as many reliability tests of the retros as possible before trusting Glenn's reentry to them.

Answer 2

Would you want your test of your racecar's brakes in to be in the Monaco Grand Prix?

I can’t imagine that the retrorockets were included merely to test them for service on the manned Mercury-Atlas flights, since they could easily have been tested on the earlier, unmanned Mercury-Redstone flights (and, for that matter, the early unmanned Mercury-Atlas flights) without potentially endangering a human crewmember.

There's nothing merely about it. The retro-rocket system was critical to success, without it an astronaut would be unable to de-orbit and would die in space. The Mercury-Redstone missions gave the opportunity to test the system where a failure of the system to fire would not greatly endanger the astronaut.

Also important and perhaps not as well understood was it was an opportunity to test the overall system which included mission controllers, procedures, automated systems, telecommunications, radar tracking, engineering support and other factors than just the spacecraft. The memoirs of Chris Kraft, who built mission control and Gene Kranz, who was tasked with handling procedures in all the mercury missions (and of course was a flight controller later) are both clear that the Mercury-Redstone missions were extremely important in 1) preparing the controllers and 2) showing them what they didn't know. Leaving the retro rockets out of tests would have denied NASA important training opportunities.