Watching the commissioning of the James Webb Space Telescope, the control team took a couple of days to just monitor how the new space vehicle was performing. That the spacecraft was operating a tad different than pre-mission predictions is not really unusual. In fact, that is normal.
I am reminded of the heady early days of the space shuttle. I was in mission control for STS-1, 2, 3, and 4; all of the ‘test’ flights. We certainly learned much that we had not expected.
One of the areas expect differences is in the thermal response of the spacecraft. A senior manager once remarked to me that thermal analysts, making predictions about how the temperatures would respond in space, should all wear ‘tall peaked hats decorated with moons and stars’. Since I studied heat transfer seriously in engineering school, this struck home. There are always many assumptions and complex geometries to model for thermal predictions. For example, liquid propellant swirling inside titanium tanks held by boron struts attached to aluminum structure with mylar insulation swaddling critical components and the entire structure inside the famous heat tiles. How hard could that be to analyze? No wonder the heater duty cycles were considerably different in flight.
Working in the propulsion systems – reaction control thrusters and the like – and one of our main jobs was to project how much propellant would be used. Attitude control is never pristine; thruster plumes impinge on structure and thrust is never precisely aligned with the principal axes of the vehicle. Unusual results from attitude control thrusting should be expected. Almost every maneuver used more or less propellant than predicted. Those of us who were tasked with ensuring that the gas tank never ran dry before the end of the mission were frustrated by the seemingly simple to analyze phenomenon which never worked exactly as predicted.
On the third flight there were a series of extreme thermal attitudes (think bottom to sun for days) to test exactly how the structure would behave. The shuttle got into ‘banana mode’ – just what it sounds like with the bottom expanding with the heat and the top contracting with the cold. Then tried to close the payload bay doors. Not surprisingly the doors would not meet, like the doors in an old rickety barn. Equilibrium had to be reestablished before that critical operation of payload bay door closure was successful. The flight controllers who were charged with monitoring the thermal state of the orbiter built a device to illustrate the temperature state. This highly scientific device was a wooden base with a short wooden pole which they installed on the top of their console. It was to be a quick visual indicator of thermal status. Several of the flight controllers were from Louisiana and on this short wooden staff was a sliding figure of a crocodile (or was it an armadillo? Memory is misty). When the thermal differences were high, they moved croc up; as things returned to equilibrium the croc went down. They called this device the ‘thermo-gator’. A little bit of flight controller humor that the management tolerated.
Preflight much analysis was done on the navigation systems, particularly expected performance of the Inertial Measurement System (IMUs). This is three highly accurate gyroscopes and associated accelerometers. The IMUs were critical to maintain shuttle attitude as well as to generate updates to the all-important state vector used to calculate the shuttle position. The gyros tended to drift which required updating by two built in telescopes called star trackers. Having to do a star alignment to update the gyros required the shuttle to stop whatever activity and turn to at least two positions where bright stars were in the right position to check alignment. Not so very different from using a sextant to navigate a ship at sea. Remember the shuttle flew well before the GPS system had been built. Predicting gyro drift and the time allowed between star alignments was studied very thoroughly pre-mission. IMU performance turned out to be much better than predicted so the star align maneuvers could be scheduled much further apart. Good news there.
Returning to the propulsion area, during the very first firing of the Orbital Maneuvering System engines a pressure measurement on the inlet of one engine showed a small uptick during the burn. Not enough to cause problems but a funny thing nonetheless. A young back room flight controller on the next shift caught that uptick and kept the data. After the flight he made a presentation to the Orbiter Project manager asking that the fuel line into that engine be inspected – a nontrivial task. After some discussion (a story for another day), the inspection was ordered, and the remains of a plastic bag were found on the filter to the inlet of the engine. Not a good thing. I knew that young flight controller well and wound up working for him later in my career. I hear he’s still working actively at SpaceX; his name is Bill. By the way, those OMS engines are stored for use as the main spacecraft engine for the upcoming Orion missions. Hope we cleaned out those lines well.
Many lessons were learned on the first shuttle flight; there are documents full of ‘lessons learned’. But here is the point: operating a new spacecraft for the first time is full of surprises. Hopefully only small surprises that are easily accounted for. Or perhaps even positive results like the shuttle IMUs. But in the real environment of space, with its extreme conditions, mean that systems never ever operate exactly as predicted. A vigilant operations team watches for all of these things and updates plans and procedures accordingly.
Best wishes to that professional team operating the James Webb Space Telescope! I’m impressed.