On this day 36 years ago, two astronauts aboard the space shuttle Enterprise took the ship out for its initial test flight. It landed on the back of a 747 before undertaking a series of free flights starting in June that year.
Enterprise was designed as a test ship only, and was never intended to fly in space. Instead, it was used for a series of flying and landing approach tests to see how well the shuttle maneuvered during the landing. The astronauts first flew a series of “captive” flights aboard the 747, then cut the test shuttle loose for five free flights over several weeks.
What lessons were learned and what design changes did NASA implement from the Enterprise test program? And how did Enterprise help shape the future of the space shuttle program? A few clues emerge from the program’s final evaluation report, which was released in February 1978.
– Stopping a hydrazine leak. Hydrazine was used as a fuel for the maneuvering thrusters on the space shuttle, but the chemical is toxic and shouldn’t be exposed to humans. During the first captive flight, an auxiliary power unit was turned on about 18 minutes in. That was part of the plan, but the next part wasn’t: NASA observed fuel was being used much faster than expected in the next 25 minutes. It turned out that a bellows seal in the fuel pump had failed and caused “significant hydrazine leakage” in the shuttle’s aft bay.
– Preventing brake trouble or ‘chattering’. The first indication of trouble came after the second free flight. The astronauts felt a “chattering” (low-frequency vibration) sensation during braking as they were slowing down on the runway. This 16-hertz vibration happened again during “hard” braking on Flight 3. In light of the vibration, the brake control was modified and the astronauts did not feel the vibrations on Flights 4 and 5.
– Minimizing computer vibration. Enterprise’s Computer 2 fell out of sync with its fellow computers as the shuttle separated from the 747 on Flight 1, causing several computer errors. (The other three redundant computers effectively voted the computer off the island, to use Survivor parlance, and the flight carried on.) Ground tests of similar units revealed that the solder keeping the computer attached to the shuttle cracked when subjected to a slight vibration for a long period of time. NASA modified the attachments and the computers were just fine on Flight 2.
– Astronaut training. The astronauts experienced several control problems during Enterprise’s fifth free landing, when they deployed the speed brake to compensate for a landing that was a little faster than planned. As the pilot tried to control the shuttle’s sink rate, the elevons (a control surface for pitch and roll) were elevated more than usual, causing the shuttle to gently head back into the air and roll to the right before landing again. The astronauts could not see any unusual changes in pitch because the nose of the shuttle was not visible from the cockpit. Further, the center of gravity for the pitch changes was so close to the cockpit that the astronauts could not feel the sensation. “The pilot was unaware of any problem other than that he was landing long and trying to get the vehicle on the ground near the desired touchdown spot,” the NASA report stated. Several recommendations came out of this incident, such as more simulations of landings, modifying the flight control system, and stating that speed brakes should not be used just before landing.
Bottom line, though, was NASA said the approach and landing tests accomplished all objectives. The authors of the report called for modifications to these problems and a few others, but said as soon as these situations were addressed the shuttle was performing well enough for further flights. You can read the whole report here.
Enterprise is now on display at the Intrepid Air & Space Museum in New York, but is temporarily closed to the public as the shuttle undergoes repairs from damage incurred during Hurricane Sandy.
