Apollo 13’s Mysterious Space Wind
By Jerry Woodfill
The above picture suggests the mysterious nature of the space wind causing Apollo 13’s reentry angle to shallow as the spacecraft neared Earth. Of course, any elementary school science book describes space as airless. Without air, there can be no breeze blowing. Space is almost a pure vacuum. Sure there are molecules of oxygen and other gases moving through space but they are so few that there is no air pressure in space capable of supporting a breeze.
Nevertheless, for Apollo 13 journey home to Earth, it appeared that was a space wind. A subtle blowing of some kind was affecting the trajectory. The retro officer called for two separate firings of the lander’s engines to null out the angular error imparted by the “space wind.” The first burn employed the lander’s descent engine while the second utilized the lander’s thrusters to correct for the shallowing angle. But there would be a “point of no return” when corrections could no longer be made.
“What was cause the shallowing angle?”, wondered the retro officer. Was something venting from Apollo 13’s command ship? Perhaps, the lander was venting? But a polling of all the flight controllers and engineering team found nothing being expelled from either vehicle. If the source was not venting of a gas, how could the retro decide whether it would continue or cease as the time of reentry approached.
This was yet another problem when God’s answers to prayer took hold. For some unknown reason, the retro felt a calm assurance that the wind would cease…that he should make no added corrections beyond that final firing of the thrusters.
After the service module and lunar lander were jettisoned, the wind stopped. God had known its source despite the ignorance of the flight controllers and engineering team. It was the lander’s cooling system venting into space. Never before had a lander made the trip back to Earth with the crew capsule. The brief time the lander accompanied the command ship for a normal Moon landing was too short to notice venting from its cooling radiators. God had, indeed, answered prayer speaking to the Retro with the still small voice of “inner peace” that “the wind would cease.” In fact, the capsule’s heat shield plowed into the entry corridor within 1/20 of a degree of causing a severe difficulty. The activity below simulates both the space wind and the firing of the lander’s engines to correct the entry angle.
Materials: Several ping pong balls (one painted red, the other blue), a roll of duct tape, card stock, a school cafeteria table at lease 6-8 feet long, a wide roll of white table topping paper. [Optional equipment for alternate version of the activity: several hair blowers.]
Setting up the activity: Select eight players for the game called Apollo 13 Reentry. Have four players kneel shoulder to shoulder facing the table on either side of the table’s length so that their mouths are level with the table top. Explain that the idea is for each player to use his/her ability to blow air from the lungs through her/his mouth and lips to propel Apollo 13 toward Earth into the reentry corridor. However, it will require the same kind of teamwork which the Retro and his fellow flight controllers exercised. Instead of firing thrusters to guide the vehicle into the entry corridor, the students’ breath will serve as eight independent reaction control thrusters rocketing Apollo 13 safely to Earth.
A short explanation about the nature of thrusters and rocket engines might be given here enlightening the students about rocketry. The lander’s engines were of two types: The descent engine could be both gimbaled and throttled. Gimbaling is a term for turning or orienting the rocket in a desired direction. Throttling is the ability to control an engine’s thrust. The lander’s ascent engine was of the second type. It had a fixed thrust and position. Ask the students which type of engine are they. [Of course, because the head can be pitched, yawed, and rolled, their respective breaths can be gimbaled, i.e., directed. Likewise, can their breath be throttled by how hard they blow air from their lungs out of their mouths.] The lander’s ascent engine was produced a constant thrust and fixed direction which, of course, would be unlike the students’ thrusting ability.
At this time, place a ping pong ball at the lunar end of the table. The opposite end of the table (Figure 1.) has an opening in its center much like a hockey goal fashioned by taping the card stock along the table’s edge. [Fold the card stock so that approximately 6 inches extend along the length of the table as shown above in Figure 1.] Assure there is at least a one foot opening between the card walls. Obviously, the card stock will keep the ball on the table. The challenge for the students is to blow the ball through the one foot reentry corridor opening.
An embellishment would be to picture the blackness of space with stars on the card stock to the right of the opening and a burning fiery view of Earth on the card to the left of the one foot opening. In this way, the team would be reminded how difficult and potentially disastrous was the situation for the Retro Officer that April 17th, Friday morning in 1970.
Explain to the students they have but one opportunity to “get it right.” They have failed the test if the ping pong ball ends up in either corner to the left or right of the entry corridor. [Note: Fold the cards at a right angle around the corner of the table. This is to prevent the students on the end nearest the opening from blowing the ball back into space for a second try at reentry.] It is very apparent that the students must perform as a team to prevent the ping pong ball from ending up in a corner.
Other Approaches: Rather than students blowing the ping pong ball, hand-held hair blowers or inexpensive battery driven fans can substitute for students’ breath. Of course blowers/fans must be on opposite sides of the table. [Caution: Make sure hair dryers are on the cool setting. Also, be careful about plugging the driers into a wall electric outlet overloading it.]
Still another approach would feature four red-shirted players competing against four blue shirted opponents. On opposite sides of the table, alternate red and blue players such that a red player faces a blue player. Add a one foot reentry corridor to the table opposite the earlier one constructed of card stock. .
The blue team blows toward the ping pong ball toward one end, while the red team tries to blow the ball through their corridor. The game can use one ping pong ball painted blue, or a second ping pong ball painted red. Be ready, though, for a bit of rowdiness when two teams compete in this fashion. Some instructors might choose to pit the blue team as the boys in the class against the red team of the girls in the class.