The major milestone this past week was the deployment of the reflector antenna/boom assembly (RBA), which refers to a 5-meter (~16 foot) long boom that holds a 6-meter (~20 foot) diameter antenna at the end of it. For launch the RBA was folded against the spacecraft to fit within the launch vehicle fairing. The boom and antenna together weigh 58 kilograms (127 pounds). Yes, super light for something that size. The boom is a carbon composite structure, made of multiple layers of carbon fiber cloth that are impregnated with a special resin, then baked to create a structure with the strength of steel but much less mass! The antenna is made of a flexible mesh so it could be folded into a compact stowed volume of 30 by 120 centimeters (1 foot by 4 feet) for launch. The mesh is edged with a ring of lightweight graphite supports called a perimeter truss, which opens like a camp chair when deployed. Once deployed the surface shape of the mesh must be accurate within about 3 millimeters (about an eighth of an inch).
The deployment of the RBA occurred in two steps: first the boom and days later the antenna, which made for two heart stopping moments.
The boom was deployed on Wed. Feb. 18 and it was entirely successful with results being very consistent with predictions and expectations (just the way we like it). The whole process took about 20 minutes. First a pyro cable cutter released the boom cradle and then the preloaded tension in the boom moved it away from the satellite bus. A motor then wound a cable that pulled the two-hinged boom to its full extension. If anything would have been anomalous we could have stopped the deployment at any time. Here is a time-lapse video simulation of the whole process:
And yes, the simulation is an accurate prediction of the process. The jerky boom motion is real and occurs when the root and elbow hinges latch. This is a consequence of Newton's 3rd law - equal reaction to latching action! Though I have seen it many times, my heart still skips two beats when I watch that video.
Next up was the reflector antenna deployment, which occurred yesterday (Tues. Feb. 24) and telemetry over the last day confirms it was a complete success. The furled reflector antenna was first released by firing a pyro to open restraints, and stored energy sprung it partially open. A second motor then wound a cable to pull the reflector to its fully opened circular configuration. The whole process took about 33 minutes and here is a video simulation of that event:
This accomplishment was a HUGE deal because it is the first time that such a large microwave-spinning antenna has been deployed in space for a NASA mission. In addition, deploying such a large low-mass structure in a microgravity environment is probably the greatest engineering challenge of this mission and my hat goes off to everyone who made this a success.
One of the biggest difficulties was that the low mass and large deployed size of the antenna made it difficult to test on Earth. Unfortunately we don’t have antigravity chambers (those only exist in Hollywood movies) so testing this huge structure could only be done in pieces and required special equipment for ground testing to compensate for the effects of gravity, which are not felt in space. The ground-testing environment posed more difficulties than space itself because of the unavoidable limitations of this specialized equipment. For this reason, ground testing was supplemented with sophisticated modeling analysis. Here is a great video that shows this testing:
So yes, we got through this major milestone without the need for CPR (on us or the spacecraft). Whew! Next, we will test the antenna by operating the radar and then we will start spinning the antenna slowly until we reach 14.6 revolutions per minute (this spinning action will provide a 1000 km diameter footprint on the ground).
After SMAP launched I was asked how I celebrated such accomplishment and I answered “with ultra-cautious optimism”. Tonight I’ll celebrate with optimism.