What was the point of the Redstone rocket when there was the X-15 rocketplane?

Question

Just a curiousity question, so don't get me wrong for I very much admire and have high respect towards the entire Mercury project, Alan Shepard (the first American in space, a national hero) and Gus Grissom (the 2nd and last Redstone astronaut) and I'd fly on a Redstone into space myself if I could. My question only deals with the point of building and testing the Redstone rocket during the space race while having the X-15 spaceplane.

Since both the Redstone and the X-15 were suborbital spaceflight vehicles I wonder why did they bother with the crewed Redstone rocket in the first place instead of focusing on bringing a man into space on the X-15 while the Mercury project could have put more effort into the crewed Atlas rocket. The X-15's first powered flight was on 17 Sept 1959, flown by Scott Crossfield. So from there on, about two years before Vostok 1 and Mercury-Redstone 3, the USA were theoretically able to fly a man into outer space. However, the X-15's first (U.S.-recognized) spaceflight was as late as 17 July 1962 (Robert White) and its first FAI-recognized spaceflight on 19 July 1963 (Joseph Walker). On 30 March 1961, a few days before Gagarin's flight, Walker reached an altitude of 169,600 ft becoming the first man to reach the mesosphere. If Walker would have flown 19 miles higher he would have become the first man in outer space according to the U.S. definition or 31 miles higher and it would also be an FAI-recognized spaceflight.

Why wasn't Walker allowed to go higher in the intensifying space race? Why were there so many X-15 flights going comparably low instead of putting more effort into reaching outer space? I don't think building and testing the crewed Redstone rocket was a waste of time, but from the space race's point of view (and perhaps also from the taxpayer's point of view) it might have been one. If more effort would have been put into X-15 spaceflights, the first man in space could have been an American, and also perhaps the Mercury spacecraft would fly sooner into orbit, if more effort into the Atlas rocket would have been put.

Therefore, my question is: Why wasn't put more effort into flying a man into space by the X-15 and built the crewed Redstone rocket instead? Were they concurring programs (like today's Crew Dragon vs CST-100 Starliner)? I don't think so because as said there were many comparably low flights of the X-15. The only advantage of the Redstone rocket I think of is that since apogees around 115 mi (185 km) were reached, noone doubts that Shepard and Grissom flew into outer space, becoming astronauts, while the X-15 spaceflights may be somewhat controversial (even the FAI-recognized ones) and the Soviet Union could have claimed these wouldn't be spaceflights by establishing an own threshold. But that's a bit far-fetched I think.

Note: If the flight Mercury-Redstone BD didn't occur but Alan Shepard was launched instead, he would have become the first man in space. But that's a different story, NASA was just overprotective.

Answer 1

The goal of Project Mercury as stated in 1958 was to put a man into orbit safely.

In order to do that, NASA wanted to send animals, first. The Russians had done exactly that, and NASA preferred using a chimpanzee instead of a dog. The X-15 had to be piloted, you couldn't put an animal into it. Thus, the Mercury capsule was needed to put a chimpanzee into space before they could try it with a real human. This would also have the double benefit of validating the capsule as suitable for at least human-like life in that environment.

This was done with Ham on MR-2, on January 31, 1961. It was done with the Redstone rocket because the Atlas rocket simply wasn't usable at that point in time, and using the Redstone rocket allowed the program to progress with these important tests. One very important point they wanted to verify was if people could perform useful work in space, at all. That is why Ham had to push levers. It might seem obvious in hindsight, but wasn't at the time. Having a pilot unable to control a the X-15 during the hop into space would have been an unacceptable risk.

MA-1 took place July 29, 1960 and failed, and it wasn't until after MR-2 that Atlas made it first successful flight with MA-2 on February 21, 1961. And that still was a flimsy success, because MA-3 again failed spectacularly on April 25th, 1961. MR-3 took place just days later, on May, 5th. Using the Atlas rocket for this kind of mission was simply out of the question, it wasn't usable for crewed flight at that point.

The X-15 was irrelevant in this because after having verified that the Mercury capsule could house human-like life in a space environment, it stands to reason that NASA would want to try to put the first human into space in a capsule that had been tested just for that. The X-15 did not fit the bill, the Mercury capsule did.

And thus, they did just that as soon as Atlas became ready.

They only started to put humans into space with the X-15 after they had made the experiences with Mercury and were reasonably sure this was safe.

Furthermore, only two flights (90 & 91, in 1963) of the X-15 ever crossed the internationally recognized border of space, which lies at 100km. Discussions about the border to space had been a heated point of debate since the mid-50s, and by 1961 it became clear that 100km might have more international support (I'm currently trying to track down when exactly the FAI adopted the 100km border). A flight lower than that would risk not being internationally recognized as spaceflight. It is clear that the Soviets would have seized the opportunity to discredit the US by marking US attempts of achieving spaceflight as fake, and then doubling down on it by doing an actual spaceflight shortly after. I don't have a source for this, but I'm pretty sure US was aware of this danger and thus did not try to claim such a first because of this.

For a timeline of Redstone vs. Atlas, see this Q&A.