Schirra’s Stellar Navigation

John Glenn trains in a Mercury simulator, the Mercury Procedures Trainer. Credit: NASA

John Glenn trains in a Mercury simulator, the Mercury Procedures Trainer. Credit: NASA

Simulators have always been an integral part of spaceflight. In the case of the all important reentry and landing phase, simulators were like analogue versions Google Earth: reproductions of landscapes from specific altitudes taught astronauts to look for when lining up their spacecraft for reentry. But as Wally Schirra learned on his Sigma 7 flight, landing simulators can’t prepare you for everything. 

Launched on October 3, 1962, Sigma 7 was the third manned orbital mission of the Mercury program. In addition to his NASA-set mission objectives, Schirra’s personal goal was to manually control his capsule’s attitude throughout the flight using as little fuel as possible. He was flying on the heels of Scott Carpenter’s Aurora 7 mission, the one Schirra an the other Mercury astronauts (save John Glenn) referred to as “Stigma 7.” This was the infamous flight where the astronaut used so much fuel reorienting his spacecraft to look around in orbit that barely had enough to orient his capsule for a safe reentry. The other astronauts felt Carpenter’s performance put them all in a bad light and they were eager to prove that a pilot could be a help rather than a hinderance in space.  Schirra named his capsule Sigma 7 to reflect the engineering perfection he expected of the flight.

Carpenter during emergency egress training before his flight in 1962. Credit: NASA

Carpenter during emergency egress training before his flight in 1962. Credit: NASA

Schirra achieved his self-determined primary objective. After drifting through space for six orbits (which took about nine hours and 13 minutes), Schirra’s fuel tanks were more than half full when the time came to orient his capsule for reentry. And it was a good thing, too. As it turned out, he needed it.

The Mercury capsule’s onboard computer was smart enough to align it for reentry, but Sigma 7 couldn’t get a good yaw reading. The capsule was in the right place but twisted around its central vertical axis. Firing his retros at that angle, Schirra risked missing his landing point. He risked missing the ocean entirely. The astronaut’s only recourse was the align the capsule manually before manually firing his retros.

All the Mercury astronauts had trained for this, particularly Schirra. He and Gordon Cooper had devised the training method. The astronauts painted a large weather balloon to look like the Earth and put it underneath a simulator so the view out the simulator’s window would was the same one they’d see from orbit. They learned how to line up for reentry using landmarks. But their simulated Earth reflected clear weather around the globe. When Schirra was in space lining up for the real reentry, the landmarks he relied on to guide him were lost under heavy cloud cover.

Wally Schirra poses in front of model Mercury capsule. Credit: NASA

Wally Schirra poses in front of model Mercury capsule. Credit: NASA

But Schirra, a former Naval aviator, didn’t blink. He turned his attention away from the now unhelpful Earth and looked to the star. Stellar navigation, a common practise in the Navy, now became his best course of action. Using the stars as reference points, he got Sigma 7 in the perfect orientation for reentry then promptly engaged the capsule’s automatic attitude control system. He let the computer hold his altitude steady through the descent; it happily gobbled up the ample fuel left in the tanks. Schirra landed less than five miles from his planned impact point.

Sigma 7 was described as textbook flight in the post-mission report, and Schirra was lauded for his role in the success. There was no better verdict for an astronaut who had set out to prove his worth in a spacecraft.


  • Stu Young says:

    I knew about Schirra’s engineering savvy, and his drive for perfection in his test flights. I must confess, however, that I never really studied his Mercury flight, but am more familiar with his Gemini and Apollo 7 flights. I never knew about his spacecraft’s yaw reading problem. Now, I’m even more impressed with Schirra!
    One “oops:” didn’t you mean 3rd Mercury orbital flight (Glenn, Carpenter, then Schirra)?
    Enjoyed this post, as always; and congratulations on becoming increasingly well-known, and being asked more often for your journalistic and video contributions on an ever-increasing number of space-related websites! You’re “living the dream;” and you deserve it!
    -Stu Young

  • pcawdron says:

    Nice improv by Schirra… shows a calm mind under pressure

  • Clive Foden says:

    Just reiterates the importance of the best computing device in the universe!

  • Capcom says:

    there was no computer in the Mercury capsule. You are probably referring to the Automatic Stabilization Control System (ASCS) which was a sophisticated multi-mode control system, which in fact included also a “fly-by-wire” mode, but it was no computer. Even if we want to refer to an “analog” computer (they do exists, like those used in the Concorde), I wouldn’t say that the Mercury spacecraft got one.
    I understand that the term “computer” can be used in a very generic way, but referring to “onboard computer” can be pretty misleading in the context of a spacecraft.
    Otherwise, this as been yet another interesting read. Now, can you provide some reference for it? I checked Schirra’s flight report and there is no clear mention of an attitude problem before the retro-sequence. On the other hand Schirra radioed, as part of the alignment check before that, that he was using both Jupiter and Fomalhaut.

  • EO says:

    Wow, what a great story of staying cool under pressure and the power of training. You would think they would have had a better simulation of the Earth below than a painted weather balloon.

  • […] Amy Shira Teitel, “Schirra’s Stellar Navigation,” Vintage Space, January 26, 2013, […]

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