Orbital mechanics and the challenges of orbital rendezvous isn’t a simple thing to explain, particularly as a non-scientist breaking it down for other non-scientists. But it’s a central part of the Apollo mission profile, so it comes up a lot in my line of work. To illustrate the problem, I typically tell the story of Jim McDivitt trying to rendezvous with the Titan II’s upper stage during the first orbit of Gemini 4 – the story goes that when McDivitt’s pilot instincts kicked in the whole exercise went to hell. I asked McDivitt about that first failed rendezvous when I met him in Florida in November. He promptly and candidly told me that this story, which he’s heard many times, is bull hockey. I learned from the man himself what really happened on Gemini 4. I also learned that Jim McDivitt is, and I say this with the utmost respect, a total firecracker.
Gemini 4 is best known as the mission on which Ed White made America’s first EVA – extravehicular activity or spacewalk. It’s less well known as the first mission to attempt a rendezvous, literally a meeting of two bodies (in this case spacecraft) in orbit. With Apollo committed to Lunar Orbit Rendezvous as the mission mode, figuring out how to fly this difficult manoeuvre was a necessary step on NASA’s path to the Moon.
I first came across the story of Gemini 4’s rendezvous years ago, possibly in high school, reading Gene Kranz’s autobiography Failure is Not an Option. When White’s EVA was added to the Gemini 4’s flight plan, so was the rendezvous goal. A rendezvous but not a docking; Gemini 4 wouldn’t have a proper target or a docking mechanism on the spacecraft. Instead, McDivitt as the mission’s commander would rendezvous with the Titan II’s upper stage and do some kind of stationkeeping exercise in orbit.
The common story is that the Titan’s booster cut off five and a half minutes after launch. The crew let the stage float away for 20 seconds then McDivitt engaged the Gemini’s thrusters to put some distance between the spent stage and the spacecraft. Once there was about a football field’s worth of space between the two vehicles, McDivitt fired his thrusters to propel the spacecraft forwards towards the Titan stage. It was a pilot’s instinct: you move towards your target. But this had the opposite effect; he reported the booster was moving away from him. He tried again, only to find that the distance between the spacecraft and the spent stage increased. After a third try, McDivitt was 2,000 feet from the booster, coming up on night, and had burned through so much fuel that flight director Chris Kraft called off the rendezvous.
It’s a brilliant story that perfectly illustrates the challenges of an orbital rendezvous, which is probably why I tell it so often. Just like Newton described a projectile leaving a cannon, a spacecraft needs a certain amount of speed to achieve orbit. Around 17,500 miles per hour. Too slow and it will just fall back to Earth on a suborbital trajectory (like Al Shepard and Gus Grissom’s Mercury flights). Too fast and it will achieve escape velocity, enough speed to leave the Earth’s orbit entirely and shoot out into the solar system.
Within that sweet spot of orbital speeds, subtle changes to that speed yield subtle changed to the height of the orbit. Add speed and the spacecraft inches towards escape velocity; it goes into a higher orbit. Take away speed and it starts to fall towards the planet; it goes into a lower orbit. This goes against visual cues and everyday experiences. In an airplane (or a car) and increase in speed makes the vehicle move closer to the target. But when these conventional flight instincts kick in in orbit, they produce the opposite results. When McDivitt (according to the myth) used Gemini’s thrusters to increase his speed and close the gap between himself and the booster, the increase in speed took him into a higher orbit, which took him further from his target. The more he increased his speed, the higher his orbit and the further he went. Against intuition, to close the gap he had to slow his speed to go into a lower orbit, get in front of the booster, and speed back up into its orbit and approach it from the other side.
But McDivitt told me that the story I’d heard about Gemini 4’s rendezvous isn’t at all how it happened, though he admitted that it’s a great case study to explain the funny world of orbital mechanics. McDivitt told me that he and White separated from the Titan, about 8 minutes after launch. The booster started rolling slowly and its external lights, an aid in place to help McDivitt keep his rendezvous target in sight, seemed to be on and flashing. But there were only two lights flashing, not the three that should have been on. This made rendezvous a bigger challenge. The booster was a cylinder, so while he could see it on the day side of the planet in the dark he could only see both lights when flying perpendicular to it, and even then he had no depth perception on the vehicle. Flying formation with one bright flashing light isn’t the easiest task.
It was the flashing lights, or lack thereof, that killed Gemini 4’s attempted rendezvous. McDivitt’s pilot instincts had nothing to do with it.
And speaking of Jim McDivitt, I was sufficiently floored just to be talking so freely with such a fascinating figure from the history I’ve devoted my life to that I forgot all of the things I wanted to ask him about. It’s a little after the fact now, but since he was nice enough to agree to speak with me further, I think it’s time I pluck up the courage to send a follow up email.