Every spacecraft that achieves orbital velocity must somehow give up that velocity to return to Earth. For Mercury, this meant paying off the energy into an ablative heat shield, a protective covering on the blunt end of the spacecraft designed to “ablate,” or burn off slowly, protecting the spacecraft within. After entry, Mercury detached its heat shield, which would deploy a landing bag designed to cushion splashdown.
Having completed the first orbit on Friendship 7, John Glenn’s spacecraft passed over the Cape Canaveral station to begin orbit #2. During the pass, a flight controller noted that segment 51 was showing that the heat shield and landing bag were no longer locked into place. Such an indication was very serious, if the shield had already deployed before entry, it could mean the spacecraft would burn up on reentry.
Ultimately the segment 51 indication would prove to be in error, and Glenn would reenter safely. Since the indication of the landing bag deployment created uncertainty, Glenn was instructed to enter with the retro rocket pack still attached, which was held by straps beneath the heat shield. Normally the pack would be jettisoned before entry.
Glenn saw chunks of the retro pack burn off as he reentered the atmosphere. At one point he could see on of the straps out the spacecraft window. He remarked “That’s a real fireball outside.” But just how different was Glenn’s reentry from all the other Mercury entries?
We have some evidence to show you. During development of the heat shield, testing had been conducted to determine how a Mercury spacecraft might reenter if the retro pack had failed to detach. Work at Ames on model spacecraft provides us with an excellent look at the difference between a nominal entry vs. the “real fireball.” As you’ll see, there IS a difference.
Today’s footage is testing conducted at Ames Research Center, showing the Mercury spacecraft entry with and without the retropack still attached. Audio is from Aleck Bond, part of the Space Task Group at Langley, VA during the development of Project Mercury.
Final Note: This week’s footage excerpts are from our 6-DVD Mercury set, which is being offered at a special price ($20 off the regular price) through Friday, June 20.