In families with three kids, the middle child always seems to get the short end of the stick. The first child gets all the attention for reaching every milestone first, and the third child will forever be the baby of the family, and the middle child gets lost in-between. Something similar happened with the U.S. manned space program in the 60s. The Mercury program got massive attention when America finally got their efforts safely off the ground, and Apollo naturally seized all the attention by making good on President Kennedy’s promise to land a man on the moon.
In between Mercury and Apollo was NASA’s middle child, Project Gemini. Underappreciated at the time and even still today, Gemini was the necessary link between learning to get into orbit and figuring out how to fly to the Moon. Gemini was the program that taught NASA how to work in space, and where vital questions would be answered before the big dance of Apollo.
Chief among these questions were tackling the problems surrounding rendezvous between spacecraft. There were those who thought that flying two spacecraft whizzing around the Earth at 18,000 miles per hour wouldn’t work, and Gemini sought to prove them wrong. To achieve this, Gemini needed something no other spacecraft before had been equipped with: a space radar.
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NASA had planned extensively for the Gemini rendezvous maneuvers, building a special spacecraft just for the purpose. The Gemini-Agena Target Vehicle (GATV) was an Agena-D rocket equipped with a docking adapter into which the nosecone of a Gemini capsule could fit. The unmanned GATV would be launched separately and inserted into orbit, with a manned Gemini launched shortly afterward. The mission plan was for the Gemini crew to locate the GATV, catch up to it, and perform the docking maneuver.
Even in low Earth orbit, space is a big place, and finding the GATV could prove to be a problem. To fix this, NASA provided the GATV with both optical beacons and a radar transponder. Rather than rely on reflections from a radar set aboard the Gemini, the GATV transponder would transmit a signal a short time after receiving an interrogation signal from the Gemini. This was to avoid the problem of getting accurate measurements of distance and velocity at the close ranges needed for docking maneuvers. The transponder delay was sufficient for the Gemini radar to switch from transmit to receive; the radar would then subtract the delay time to calculate range and relative speed, or range rate.
On the Gemini side, the rendezvous radar was a compact package riding in the very tip of the capsule. The business end was four flat fiberglass discs with interlaced spiral elements wound from center to edge. There was a single transmit antenna and three receive antennas, one each for azimuth and elevation, and one reference antenna. The three antennas were carefully arranged relative to the long axis of the spacecraft, and the azimuth and elevation antennas had servos to rotate them on their axes. The servos would rotate each antenna until the phase of the signal matched the phase of the fixed reference antenna; the angle of the antennas would then be used to calculate the angle to the target in two dimensions.
The range/range-rate meter in the cockpit was a critical instrument for the astronauts. It had to show the distance accurately over a range of 300,000 feet down to zero feet, and it had to indicate that the closing rate was safe for the current distance; the closer the vehicles, the slower the rate should be. The range/range-rate instrument was driven by the analog output from the radar — there was also a digital output that fed into the Gemini computer for navigational calculations. By ramping up a voltage along a linear curve starting at the moment when the transmitter fired and stopping it when the transponder signal was received, a current to drive the range meter was produced.
Despite all the careful planning, the first tests of the rendezvous radar did not go according to plan. Gemini 5, the dress rehearsal for the first rendezvous mission, was supposed to test the radar using a Radar Evaluation Pod, which was essentially the transponder from GATV which the crew could use to practice. The pod was released, but fuel cell problems on the capsule prevented the crew from completing the experiments.
The next mission, Gemini 6, was scrubbed in dramatic fashion. After the Agena rocket with the GATV to be used for rendezvous exercises exploded two minutes into its flight, NASA scrambled to salvage what it could. They decided to launch Gemini 7 with astronauts Jim Lovell and Frank Borman as scheduled, but with a GATV transponder in the nose, letting the spacecraft serve as the target vehicle. The original Gemini 6 was renamed Gemini 6A, and astronauts Tom Stafford and Wally Schirra took off to catch up with Lovel and Borman, already 11 days into a 14-day mission. The rendezvous mission went perfectly, with the two spacecraft approaching as close as a foot apart and keeping station so precisely that no correction burns of their thrusters were needed for a full 20 minutes.
With what was learned from Gemini 7/6a, four of the five remaining missions were able to rendezvous with and physically dock to their GATV — the one exception was Gemini 9a, whose GATV failed to jettison a fairing after achieving orbit. By the end of the program, NASA had reduced rendezvous and docking to practice, and everything that was learned in the process was applied to the Apollo missions, which carried rendezvous radars very similar to those used by Gemini.