August 10, 2020
When does Mass = Power?
By Bill Hart
Some thoughts on margins
This probably shouldn’t come as a surprise to anyone, but one of my favorite movies is ‘The Martian’. I am a big fan of sci-fi-thrillers, especially if they are heavily rooted in real science (with a little humor to boot). It also helps if a decent portion of the story is based at your workplace.
One of my favorite scenes is about midway through the film, when (spoiler alert) the JPL team explains to NASA’s Director of Mars Missions that they will have to lighten the mass of the escape vehicle, in order to achieve an ascent trajectory high enough for the hero, Mark Watney, to reach the return vehicle. The director is in shock as the team explains that they will have to remove everything that isn’t bolted down — and some things that are — in order to get the mass down. In the ensuing montage, Mark throws chairs, windows and other equipment out of the rocket to the Martian floor. When I saw that scene for the first time, I leaned over and whispered to my wife, “Do you think the JPL team had some margin baked in there?”
As mentioned in a previous blog post, margin is a valuable commodity for flight projects such as Psyche. High margins indicate that a project maintains sizable robustness, in that it is able to handle changing conditions and scenarios that can be politely described as “off-nominal”. Another reason why margin is desirable relates to how a spacecraft design is verified and validated — a topic worthy of a blog post of its own. We try to verify a spacecraft design through testing as much as practical, but there are some things that are impossible to test, short of running the actual mission. For those cases, we typically rely on analysis. With every analysis, however, there are potential uncertainties for which margin can provide an effective mitigation. And when you’re working on a project like Psyche, a journey to a unique planetary body that no one has explored, there are many opportunities for uncertainty.
For the Psyche project, we keep margin on a wide variety of items, even ones that may not be immediately obvious. We keep track of our downlink rates and onboard memory, in order to ensure that we are able to hold the science data we obtain and are able to transmit it to Earth. We keep track of the expected number of cycles of the various onboard mechanisms, to ensure that we are not exercising our hardware beyond the qualification limits. We even track the number of spare equipment ports on certain hardware, in the event that certain trades result in us adding unexpected equipment, such as thermal heaters in order to maintain — you guessed it — margin to temperature limits of the components.
And while those are all important, there are two key resources that come to mind when spacecraft engineers talk about margin: mass and power. Mass is effectively dictated by the capability of the launch vehicle, and directly influences the design team’s philosophy on whether to utilize methods to preserve margin, sometimes at the expense of other parameters. Power is defined by the scenarios in which the mission is operated. If not kept in check, these resources can limit operation of the payload, which in turn can reduce the overall science return of the mission. In theory, the process seems straightforward: keep the mass and power down, and the mission should be successful. But for a mission like Psyche, that uses electric propulsion, management of these resources is a little bit more interesting than more conventional approaches.
Up until the turn of the 20th century, the majority of deep space missions utilized chemical propulsion. Getting to one’s destination required propulsive maneuvers, but they were fairly short in length, on the order of seconds to minutes. In addition, the propulsion systems did not require a significant amount of power, nothing more than on the order of watts to open or close a couple of valves. As a result, mass and power were relatively independent; there were few connections between how their margins affected one another. About the only thing that connected them was that if there wasn’t enough power, one might add a larger solar array or radioisotope thermal generator (RTG), which would increase the spacecraft mass.
This changed in 1998, with a spacecraft known as Deep Space 1 (DS1). Part of NASA’s New Millennium Program, DS1 was designed to perform flybys of near earth objects, while testing a multitude of new technologies. One of these technologies was solar electric propulsion, in the form of an ion engine. Some of my colleagues have talked about electric propulsion in previous posts.
DS1 successfully visited an asteroid and, through a mission extension, successfully flew by a comet. The use of electric propulsion was an enabling element, and paved the way for successor missions, most notably the Dawn spacecraft launched in 2007 that visited Ceres and Vesta. Both of these spacecraft were built here at JPL, and the Psyche team has benefited significantly from their experience and expertise. In fact, several members of the Psyche team worked on DS1 or Dawn.
The high efficiency of electric propulsion meant that those missions would carry substantially less propellant. In fact, solar electric propulsion is one of the key enabling factors for the Psyche mission. When it comes to margins, however, there are two key nuances about electric propulsion.
First, electric propulsion systems require a significant amount of power in order to operate. When operating, the thrusters on Psyche will draw more power than all other systems onboard combined. The more power one has, the higher the thrust and associated efficiency, which means the more mass one can deliver. As a result, unlike conventional propulsion systems, there is now a strong link between mass and power.
The second nuance is that electric propulsion has significantly lower thrust. The thrust that Psyche uses has a maximum thrust of about 270 mN, roughly equivalent to the weight of three US quarters. As a result, the burn times are significantly longer, sometimes encompassing the vast majority of the mission. “How long?” one may ask. Well, the picture below shows the trajectory that the spacecraft will take. With a couple of exceptions, the curve from Earth to Psyche is covered with a grey outline, which represents when the spacecraft is operating its electric thrusters, over 80% of the time.