Apollo’s Re-entry Window

Apollo Reentry Topic Heats Up

Recently we published a reader’s question about the returning Apollo space craft. Click here to read.

Our good friend John Sullivan commented: Wow that was GREAT stuff! The actual re-entry plane of Apollo was about 5 degrees, I think, as opposed to 0.5 degrees of the Shuttle. The faster, steeper approach of Apollo brought the temperatures (under the protective insulation of compressed air molecules) to about 2,750 C (5,000  F) degrees as opposed to the 1,650 C (3,000 F) degrees the Shuttle is designed handle. This is the primary reason thermal tiles prove so impractical for the Orion, and why it is so challenging to design a reusable outer surface for the Shuttle follow-on. The difference between 1,650 C (3,000 F) and 2,750 C (5,000  F) degrees is not a measure of subtraction but rather of exponential complications and challenges. Again, a great article!

Robert brand (EOA Editor): Thanks John for your support of what we publish here. Coming from someone as knowledgeable as you, it is a great compliment. John, I’m so glad you raised this and the actual figures. For those interested, the angle of attack of course had a lot to do with the speed of reentry of the returning Apollo capsules. It was moving a lot faster than an earth orbiting shuttle. Like a fast stone skimming across a pond, Apollo spacecraft would have simply bounced off the earths surface if the angle of attack was too shallow. If they hit it at too steep an angle it would have heated up too fast and the heat shield would have failed. There is basically a window where the conditions are just right for our technology and we have to hit that window. The shuttle is too slow to bounce off the surface of the atmosphere and off into space like Apollo could have, but it is also too fragile to tolerate a steep angle of attack, thus the 0.5 degree angles that John mentions.

The returning Apollo 13 capsule (celebrates it’s 40th anniversary in April 2010) was at risk of missing the “window”. Because of damage due to an earlier explosion it was unable to use the service module rockets and the mid course corrections on the way home had to be done with the lunar module engine – steering was a problem. As we know, they survived, but there were so many unknowns for them. Was the heat shield damaged, were they at the right angle for the window? Could they disengage the damaged service module? The biggest danger was that there was no heater for the parachute gear and the question of whether it was frozen and would not deploy was in the back of everyone’s mind. Earth was their only safe haven, but the final minutes of the flight could see them die so close to safety.

All went well with Apollo 13 and they did a marvelous job both with the astronauts who were extremely sick and hypothermic and the ground staff who provided the support and information needed for their return.

Yes, these guys were freezing and their core temperatures were low. I once suffered hypothermia  while abseiling down a 700ft (200m) waterfall. The leader advised us to bring wetsuits that were too thin for the conditions (more water flow than expected). A tiny ledge that I was forced to waiting on was getting a refrigeration effect and high winds. My body quickly shut down and I was starting to make bad judgements. The ledge that I was sitting on was the size of a chair seat and I thought that I was clipped on to a tie-off point. I was sitting there for some time shaking uncontrollably waiting for a rope to be untangled by others. When it finally was untangled and I secured myself to the rope for the next descent, I realised that in my weakened state and poor mental condition that I was not actually safe and I had not tied on at all. Once out of the water I was back in the heat of an Australian summer and an hour later was well enough to make a slow retreat out of the valley. My brief touch with hypothermia was tiny compared to the Apollo 13 crew.

The Apollo 13 crew were weakened like this for days. They were extremely sick and every activity took extreme mental will power to complete. I can only imagine what they went through after my brief exposure. Getting that reentry window right and being so close to death health-wise was a super human feat that amazes me every time I watch the Apollo 13 movie. There is a nice part where one of them puts a label over a switch to make sure he does not operate it by accident in his weakened mental state. I can understand that!

The following is from www.nasa.gov (note the Apollo 13 service module had just been jettisoned at this point of the narrative):

“Moment of Truth”

At 9 a.m., Houston time, with just two and a half hours left before reentry, Mission Control gave Swigert and Haise the go-ahead to fully revive Odyssey. Inside the frigid cabin, water clung to the instrument panel, so much that before the men could proceed they had to wipe it off. They could only hope there was no water behind the panel that might cause an electrical short as they activated Odysseys systems. Miraculously, the command module came back to life with no ill effects.

There was only one thing left to do before reentry. Unlike Odyssey, the lunar module Aquarius had no heat shield and was not designed to withstand the fiery plunge into the Earths atmosphere. Before they could come home, Lovell, Swigert and Haise would have to say goodbye to the craft that had saved their lives.

With Lovell safely inside the command module, the men sealed off the hatchway and then, with a mixture of sadness and gratitude, cast off the lander. In Mission Control Joe Kerwin radioed an appreciation: “Farewell, Aquarius, and we thank you.

By this time Mission Control was jammed with controllers and off-duty personnel awaiting the outcome of this life-and-death drama.

“Everybody says you’re looking great,” radioed Kerwin. “Welcome home.”

“Thank you,” came the response, and then there was silence.

Apollo 13 was slamming into the atmosphere, creating an ionized trail that blacked out communications for what seemed an eternity. A full minute after the blackout’s scheduled end, there was no signal. Finally, Jack Swigert’s voice came through clearly: “Okay, Joe.”

On the big screen at the front of Mission Control, and broadcast on televisions around the world, viewers saw a clear, calm morning in the south Pacific. Still no one could breathe easy. They couldn’t be sure the mechanism to deploy the parachutes had survived the frigid conditions of the last four days.

Finally, after long minutes, the TV picture showed Odyssey descending under three beautiful, perfect parachutes. As the command module came gently to rest on the Pacific, cheers and applause filled Mission Control. As much as the astronauts, the heroes of Apollo 13 were these men and women in Houston, throughout NASA and at contractors around the country. Gene Kranz could not hold back tears.

For years afterward, many called Apollo 13 NASA‘s finest hour.

Less than eight months later, in January 1971, Apollo 14 lifted off with three redesigned oxygen tanks and other improvements, to complete the mission that Lovell’s crew had been denied, and begin the scientific exploration of the moon in earnest.

Photos courtesy of NASA

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