'Porkchop' is the First Menu Item on a Trip to Mars
2 Nov 2001
(Source: Jet Propulsion Laboratory)
Jet Propulsion Laboratory
Ancient cultures looked to the patterns of tea leaves or animal entrails to divine the course of the future. At JPL, the course of a future Mars mission can be found in a porkchop.
Porkchop plot, that is. In the sometimes peculiar vocabulary of JPL mission designers, that nickname describes the porkchop-shaped, computer-generated, contour plots that display the launch date and arrival date characteristics of an interplanetary flight path for a given launch opportunity to Mars or any other planet.
Developing a porkchop plot is the first thing on the menu when mission designers are scoping out an interplanetary voyage. This is the sort of task accomplished by engineers in JPL's Navigation and Mission Design Section, whose unique, high-caliber expertise is signified by its recognition as NASA Center of Excellence.
Sending a spacecraft to another planet has been compared to throwing a dart at a moving target - only the thrower is also on a moving platform, the Earth. It is further complicated by the fact that the Sun's gravity curves the trajectory of the dart. At launch, the spacecraft is aimed to arrive at the point the planet will be months from now.
Getting to the planet Mars, rather than just to its orbit, requires that the spacecraft be inserted into its interplanetary trajectory at the correct time so it will arrive at the Martian orbit when Mars will be there. This task might be compared to throwing a dart at a moving target. You have to lead the aim point by just the right amount to hit the target. The opportunity to launch a spacecraft on a transfer orbit to Mars occurs about every 26 months.
To be captured into a Martian orbit, the spacecraft must then decelerate relative to Mars using a retrograde rocket burn or some other means. To land on Mars, the spacecraft must decelerate even further using a retrograde burn to the extent that the lowest point of its Martian orbit will intercept the surface of Mars. Since Mars has an atmosphere, final deceleration may also be performed by aerodynamic braking direct from the interplanetary trajectory, and/or a parachute, and/or further retrograde burns. (From "The Basics of Spaceflight". http://www.jpl.nasa.gov/basics/bsf4-1.html)
The Revolving Door to Mars
"We have launch opportunities to Mars every 25 1/2 months because of the repetitive relative alignment of the planets," says Dan Johnston, an engineer in JPL's Navigation and Mission Design Section and mission design manager for the 2005 Mars Reconnaissance Orbiter. Opportunities to Mars this decade are in 2003, 2005, 2007 and 2009. "These are the opportunities where our launch vehicles have enough energy to send a spacecraft to Mars."
A tremendous number of calculations considering multiple variables must be performed to discover all the possible trajectories available and their unique characteristics in a given launch opportunity. Porkchop plots are visualizations that allow mission planners to view key parameters that must be considered, says Johnston.
Based on porkchop plots developed several years ago for the 2005 Mars launch opportunity, the Mars Reconnaissance Orbiter team developed a "reference mission" on which to base its project planning. That planning is now well-underway, and on October 3, NASA selected contractor Lockheed Martin Astronautics in Denver, Colo., to build the spacecraft and provide mission operations support.
What a porkchop plot really represents, says Johnston, is a solution to some orbital mechanics equations known as Lambert's theorem, which he sums up thusly: "If I know where the Earth is and where Mars is on some given day, and I know how long I would like to take to get to Mars, then I can compute the departure conditions I need at Earth to be able to get to Mars in the desired time."
"When you use Lambert's theorem to compute this, you come up with a launch and arrival date pair that gives you a single, unique trajectory solution for getting to Mars," says Johnston. "Some we call 'Type 1', which are short transfers of about seven months. Some are 'Type 2', which are longer duration transfers of about 10 months." The shorter path to Mars is not necessarily the best, Johnston explained. Some missions, such as orbiters, for example, may benefit from a longer trajectory that delivers the spacecraft to the planet at a lower arrival velocity. That way, less fuel is needed to brake the spacecraft when it arrives.
Roller Coaster to Mars
"The porkchop plots for the 2005 Mars opportunity show that for our launch vehicles, this is one of the most challenging opportunities to get to Mars in this decade," says Johnston. The interplanetary doorway to Mars open at that time requires the spacecraft to be delivered to a spot above Earth's higher latitudes. This high-latitudinal orbit requires more launch energy - a bigger rocket - than most launches.
In terms of physics, the price paid for poising a spacecraft at that jumping-off point can be compared to that paid for a bigger, steeper roller coaster ride, says Johnston: "The cars have to be pulled up a longer hill to get to the highest point before the roller coaster's first plunge," he says. "In effect, we need a steeper plunge to get onto our flight path in 2005. We've got to push ourselves up higher to get onto the Mars trajectory for this opportunity."
Nearly half the Reconnaissance Orbiter's entire 1,800 kilogram mass (3,968 pounds) is propellant that will be used to brake the spacecraft's speed when it reaches Mars, allowing it to be captured into orbit. The spacecraft also carries a heftier science package with about twice the mass of the instruments carried by its recent predecessors. All these factors mean that a rocket the size of an Atlas 3 or a Delta 3 will be required to send the Reconnaissance Orbiter on its way. (By comparison, its sibling spacecraft Mars Odyssey, at about half the mass, was lofted on its voyage by a Delta 2.)
Save Your Baggage Allowance for Science
Porkchop plotting of the mission trajectory options "starts the very first day, before you start anywhere else" with a project, says Johnston. "The porkchop plots let you determine the basic requirements of the launch vehicle you'll need as well as a preliminary estimate of the propellant load necessary for the spacecraft. If the spacecraft cannot carry all the propellant it needs to reach its desired science orbit, a mission option using aerobraking may have to be employed." After all, he says, "We don't want to use up our baggage allowance just flying fuel to Mars; we want to fly instruments that send information back to us."
The porkchop plot also provides information on the conditions of the capture orbit when the spacecraft arrives, including the time of day it will be on Mars when the northbound spacecraft crosses the equator in its first orbits. This is particularly important for the 2005 Mars Reconnaissance Orbiter, because the scientific instruments onboard require that the spacecraft end up in a nearly-polar, Sun-synchronous 3 p.m. orbit. That means that the territory beneath the orbiter as it crosses the equator of Mars will always be illuminated by a 3 p.m. Sun, providing optimum shadow and brightness characteristics on the surface for the cameras and spectrometers onboard. The 200 by 400 kilometer-high orbit (about 125 by 250 miles) will provide opportunities for extraordinary close-ups of the surface.
When the Mars Reconnaissance Orbiter arrives at Mars, it will initially enter an 8:30 p.m. orbit. But up to six months of aerobraking, followed by small propulsive trimming of the orbit, will bring the spacecraft to the desired, nearly circular, Sun synchronous 3 p.m. orbit.
Another factor that's important to mission planners is selecting an arrival date that will require the smallest propulsive maneuver to brake the spacecraft's speed to allow it to be captured into orbit around Mars. "We try to minimize the overall delta-V (change in velocity) necessary to get into orbit," says Johnston.
When traveling among the planets, it's a good idea to minimize the propellant mass needed by your spacecraft and its launch vehicle. That way, such a flight is possible with current launch capabilities, and costs will not be prohibitive. The amount of propellant needed depends largely on what route you choose. Trajectories that by their nature need a minimum of propellant are therefore of great interest.
To launch a spacecraft from Earth to an outer planet such as Mars using the least propellant possible, first consider that the spacecraft is already in solar orbit as it sits on the launch pad. This existing solar orbit must be adjusted to cause it to take the spacecraft to Mars: The desired orbit's perihelion (closest approach to the Sun) will be at the distance of Earth's orbit, and the aphelion (farthest distance from the Sun) will be at the distance of Mars' orbit. This is called a Hohmann Transfer orbit. The portion of the solar orbit that takes the spacecraft from Earth to Mars is called its trajectory. (From "The Basics of Spaceflight", http://www.jpl.nasa.gov/basics/bsf4-1.html)
Extreme Parallel Parking
Getting a spacecraft to Mars is one challenge, says Johnston, but making one stay there is quite another. Imagine driving at 100 miles per hour, slamming on your brakes and maneuvering perfectly into a tiny parallel parking spot. That roughly compares to the difficulty of delivering a spacecraft to Mars with a trajectory, orbit insertion maneuver and aerobraking scheme that places it in the particular orbit needed for scientific observations.
"We can get to Mars, but these circular, low-altitude science orbits are tremendously difficult to reach," Johnston says. "It's very difficult to achieve a low-altitude orbit propulsively because of the tremendous burden of carrying the propellant. To supplement the onboard propulsive capability, the Reconnaissance Orbiter mission plans to use aerobraking. If we did not use aerobraking we would need even more propellant and a much larger launch vehicle to initiate the journey to Mars."
If the science team was content to stay in the original highly elliptical orbit in which the spacecraft was captured, that would be difficult enough, says Johnston. But the task will be even tougher for the Mars Reconnaissance Orbiter flight team because of the mission requirement for a low, circular orbit.
The actual execution of that task will be another story. But the initial parameters to accomplish it were foretold in a porkchop.