Gimbals & Gimballing
Ships’ compasses are mounted on gimbals, fixtures that allow the needle to stay steady against the pitching and rolling of surging seas. Sailors know that a steady compass helps to steer a true course.
In rocketry, the nozzles are “gimbaled,” providing positive control of the thrust from the business end of a rocket engine and assuring that the rocket stays on-course. Rocketeers know that an unsteady nozzle can ruin your whole day.
In many designs, the hydraulically-operated gimbal system takes control inputs from the on-board flight computer to steer the vehicle along its proper trajectory. The hydraulic system is charged with enough juice to get the vehicle to altitude. If the gimbaling gets out of whack, it’s mission over.
Getting a rocket to space takes a whole lot of things going right. But unknown factors can grab hold…and things go wrong.
Speaking of rockets, as a hired hand on a launch vehicle project some years back, my task was to examine, specify, calculate, test, analyze and predict all issues that had something to do with RF in general and EMC in particular. The project was a privately run, but partially supported by the same organization who put an Apollo LEM down gently in the soft dust of the Sea of Tranquility. As it would turn out, our ending did not go so well, our vehicle landing—not-so-softly—in the chilly waters of the Atlantic.
Frank called one day and said there was a space-task and would I be interested? I’ve always been a fan of space projects. Frank said that there wasn’t any kind of SOW or RFP for this project, so he advised to “Bid a hundred hours and see what happens.” With baby number two needing diapers and shoes, I bit and I bid it. Soon enough, I was on-board.
The launch vehicle was of relatively simple design (if anything that breaks through the atmosphere can be “simple”) with a modest mission: launch a couple of science projects into low Earth orbit (LEO). The experiments were designed to collect some data in the stratosphere and drop back to Earth by parachute for retrieval and analysis.
We hired hands out-numbered the employees by a good margin, much like a myriad of other operations scattered about in non-descript office buildings around the various fountains of money around the Capital Beltway. My office mate Steve, also a hired hand, was in charge of the telemetry and had been on similar programs before. He took me under his wing, so to speak, and guided me to the right meetings and interpreted the corporate culture for me.
Steve and I both contracted with Jack, a distant voice that grumbled in my phone (I still have yet to meet him). Jack had a dozen or so guys on this project and he took a piece of each of us, extolling the virtues of working lots of hours “charge as many hours as you can, Mike. Fifty, sixty a week is no problem.” Sure Jack, no problem.
As the telemetry guy, Steve’s job was to collect data on the rocket during ascent. The vehicle was wired with dozens of accelerometers, strain gauges, temperature sensors and the like. Data from these sensors flowed back to ground control to monitor the progress of the rocket (and to inform design modifications on the next vehicle—if there was to be a next). If something were truly to go awry during flight—like if it was out of control—a big red button in the control room would be pressed, fast.
Settling back in his chair, hands behind his head, Steve swiveled in my direction and said “One of the first things you have to do is to get to know George.” George was the project lead, which meant that he was in charge of herding the cats.
I wandered over to George’s office, knocked and entered slowly and looked around. The four walls of his office were lined with large sheets of paper covered with project diagrams, milestones, sticky notes, red stars and yellow highlighting. George was head-down in a pile of papers, scratching away furiously. “Have a seat.” He motioned to a chair that was stacked with thick spiral-bound design review documents. I sat those on the floor and took the chair.
George started reeling off a list of priorities. I jotted them down as quickly as I could. “Mike, we’ve got to get a ground diagram worked up! We absolutely must! And the antennas, take a look at the antennas! Are they a problem? Also, get a hold of Vivek and ask him about the ordnance; he’ll know. Burt can help you with the GSE—just get to him before noon. And shielding! Is the wire shielding good enough? Hank can give you the wiring diagrams. Here, take a look at this.” He handed me a legal pad-sized paper with a hundred lines, interconnecting numbered boxes: IMU, FC, BATT1, BATT2, COMM, DWNLNK, TLM. “This is my shot at the ground diagram. What do you think?”
I took the paper. “Yeah, good” was all I could muster. George returned to scribbling something on a sticky note, stood up and put it up onto the web of lines on the wall.
“OK. See you at the meeting tomorrow morning. I want a report on everything.” He stood, turned his attention to the diagram on the wall, tapping his chin with his pen.
Time to go and get started.
One of the first tasks was to analyze the potential cross-coupling between the various antennas on the craft. There was the telemetry downlink, a communications connection to the TDRSS constellation (a collection of transmitters orbiting about the planet for space-ground-communications) and the all-important “self-destruct” receive antenna for the worst-case scenario that the lift-off needed to be aborted by the range safety officer. The analysis involved calculations of margins, based on output powers and antenna patterns. Some of the antennas were moved, just to be on the safe side.
All the Write Specs
Another task was the generation of EMC testing and performance specifications. We favored the NASA General Environmental Verification Specification (GEVS) which was developed for the Space Shuttle program. The only non-cookie cutter activity was to determine tailored requirements for RS03 (radiated susceptibility) levels. I needed information on the launch site and for this I went to Burt, who was managing the ground operations and Ground Support Equipment (GSE). Burt was helpful, although he had a funny habit of pounding three mai-tais at lunch at the local Chinese restaurant. I’m not sure how he was able to function in the afternoon, so, as George suggested, I always caught him first thing in the morning.
“There’s a big radar down there at the launch site.” Burt told me one morning, sipping coffee (I suppose) from his mug, which may as well have been attached to his right hand. “It’s operating at around 2 GHz, if I recall correctly. It’s kind of close to our launch pad, so heads-up!”
Got it. I marked 2 GHz on a spreadsheet, FREQ: 2GHz. LEVEL: TBD.
Another task was to analyze safety margins for the dozen pieces of ordnance on the craft to make sure they wouldn’t go off inadvertently—an ill-timed explosion being a bad thing. There were two kinds of these devices: Explosive bolts were designed to blow at a certain altitude to cleave the aerodynamic structure (the fairing), which would separate out and away from the craft. It would be bad form for these bolts to blow early, say, on the launch pad because of a nearby RF source (like a pesky radar). The second kind there were the self-destruct charges that were designed to destroy the vehicle if it went off-course.
This kind of analysis is often referred to as a HERO analysis which is ironic, because if you “blow it,” you won’t be a hero. The basic analyses was not heavy on Maxwell, but a general wag at how much coupled energy would find its way into the wiring and (potentially) induce a triggering voltage across the pins. End game: use a triple-shielded cable and good connector and bond the heck out of the thing. Should be OK, but everybody likes calculations, so I put together something with a bunch of dBs to shore up the argument.
How to Ground a Rocket
The other part of the job involved “grounding,” George’s favorite topic. I know how some perfectly sane engineers get worked up about ground (Run! Hide! Ground Loops!), but developing ground concept on a flying thing is not all that difficult, once you realize you can’t trail a long wire out the back of the thing and that’s not how “ground” works anyway. The avionics packages on the rocket were a collection of metal boxes places around the core of the rocket’s fuselage, which was non-conductive composite. The solution for “ground” was really to create an “equipotential structure.” This would also serve as the mechanical mounting for the avionics and other electronics.
All the boxes were mounted to a six-sided panel arrangement: flight computer, radios, power distribution, batteries and telemetry subsystems were arranged according to function and the interconnection hierarchy. To minimize the generation of errant voltages across the structure, we selected a thick, heavy printed circuit board material: double-sided un-etched fiberglass + copper. To bond the boxes, some sort of non-oxidizing goo was slathered on the footprint of each of the boxes. This “ground plane” structure was beneficial to overall system performance because all the interconnect cables were staked tightly to the plane, minimizing coupling/radiating loops. Each of the six panels was bonded along the edge to its neighbor.
Five Out of Five Engineers Recommend Testing
Tests of the avionics were performed during development to see if there were any real issues. A lot of these tests were informal, bench-level and incremental. The only hairy part of that task (one of those: ‘Why did I take this job?’ moments) was when I flipped on my power amplifier and blew a main breaker, crashing the avionics exercise mid-test. After a searing glare from the Chief Scientist and an angry “If you broke it, Violette, you’re going to pay for it!” we reset the breaker, the test resumed and the whole business re-booted without hiccup (I moved my amp to another circuit, of course).
Eventually, functionality was beat into the works, we ran some system-level EMC tests and the payload was moved to the launch site to be bolted onto the launch vehicle.
Fat Boy at 2 Gigs
The radar was indeed close, about 600 meters from our launch pad. Hoo-boy. I took some measurements and found peak levels around 300 V/m. Ouch. I rechecked the HERO analysis and decided there were sufficient margins, but what about the other systems? Navigation? Communication? We never tested at levels this high. I reported back the measurements and a note of caution ‘See if they can at least turn the radar off during launch.’ It was the last time I touched the project.
A Few Hertz Hurts
Steve called me about eighteen months later.
“Hi Mike. Did you hear about the rocket?” Sometimes a question is posed in a certain way, with the answer embedded in the question.
I said ‘no’. (I think I whispered my reply, actually.)
“She splashed.” He let that sink in a moment.
Wow. I thought about the radar, testing, all the design reviews, the guesses. The entire sweep of the project rushed by. What happened? EMI?
Steve laughed “No no. You’re off the hook.”
Phew. What happened then?
Steve explained. “We run a ‘tap test’ on the rocket once it’s on the pad.”
Tap test? The last tap test I ran was in college.
“Yeah, a tap test checks for vehicle resonances. Technicians whack the thing all over. The accelerometers record the structure response and any resonances, or ringing.”
“Turns out the vehicle was resonant at a few hertz, but it was overlooked. As the vehicle lifted off, the thing rang like a bell. The gimbaling system tried to compensate, commanding the nozzles like crazy.” He paused. “The hydraulics were exhausted at less than a minute and with no control, the thing heeled over, split apart and the rocket tumbled into the ocean.”
Steve continued. “It’s hindsight, but all we needed was a simple filter in the software to reject the structure resonance. “ He let that soak in and added, more quietly. “The range destruct system didn’t even work because all of the antennas were ripped away when the fairing blew off.” He repeated. “We needed a filter in the software.” He sighed. “Live and learn, I guess.”
Live and learn indeed. The reality is that to put something above the atmosphere, safely and in the right spot, so many things need to go right. That’s why it’s called ‘Rocket Science.’
remembers the first Moon walk (the same night the cat had a litter under the living room couch), Space Food Sticks, drinking Tang and the awesome Saturn V, The King of All Launch Vehicles. He can be reached at firstname.lastname@example.org.