Screenshot from NASA Webcast of the PACE launch. Dog gone it. It’s dark. Again. I miss sunlight
Mission Rundown: SpaceX Falcon 9 - PACE
Written: February 8, 2024
Keep up the PACE
NASA is set to launch the satellite PACE: Which is shorthand for (Plankton, Aerosol, Cloud, ocean Ecosystem), an Earth observation space vehicle, using a Falcon 9 rocket. The third attempted Lift Off took place on Thursday, February 8, 2023 at 01:33:36 ET - 06:33:36 UTC from SLC-40 at Cape Canaveral Space Force Station.
The Falcon 9 rocket will be composed of booster B1081-4, its 306th second stage and old reused fairings containing the PACE satellite weighing in at 1694 Kg.
The Falcon 9 didn’t perform a static fire test of the engines. This has been omitted many times due to Falcon 9’s increasing reliability. Only after engine swabs and issues with the importance of the payload does a static fire test become necessary.
PACE bends it like Beckham around the state of Florida. But the Cuban’s better take cover
B1081-4 will have made its fourth flight after launching its next mission:
After separating from the second stage, the booster B1081-4 will do a boost back burn and return to Landing Zone LZ-1 about 9 kilometers from the launch site.
After refurbishment of the booster, it will be designated as B1081-5. The second stage will after payload deployment be deorbited in the Pacific Ocean one orbit after the launch.
The fairings are both reused, flying for the ‘ninth’ and ‘tenth’ time with no known previous missions flown together. Bob will recover them 475 km downrange north of Cuba.
The PACE payload
PACE (Plankton, Aerosol, Cloud, ocean Ecosystem) is a NASA Earth observation satellite mission that hopes to improve ocean health. The satellite will operate in Sun-synchronous orbit and the expected mission duration is 3-10 years.
PACE has two primary scientific goals: “To extend key systematic ocean color, aerosol, and cloud data records for Earth system and climate studies; To address new and emerging science questions using its advanced instruments, surpassing the capabilities of previous and current missions.”
NASA’s Goddard Space Flight Centre is the design and manufacturing location for the main instrument and bus. For the spacecraft, the mass objective is not to exceed 1700 kilograms with fuel. Its size is 1.5 m x 1.5m x 3.2 m and it will be powered with 1 kW. A dual communication band will be deployed, with the S-band utilized for command and telemetry and Ka-band transmitting the science data.
PACE will be a polar-orbiting mission with an ocean color sensor and possibly an aerosol cloud polarimeter. The mission will be capable of performing radiometric and possibly polarimetric ocean and atmosphere surveys, returning a range of geophysical data from which properties of the ocean and atmosphere can be produced to add to other critical climate and Earth system variables.
The PACE mission has multiple scientific goals: making climate-quality global ocean color measurements that are essential for understanding the carbon cycle and global ocean ecology and determining how the ocean’s role in global biogeochemical (carbon) cycling and ocean ecology both affects and is affected by climate change.
Illustration of the PACE Earth observatory (image credit: NASA) It’s a small box packed to the gills
The rocket launch
A typical Starlink mission begins with the countdown that has a traditional 35-minute long propellant load sequence which begins with RP-1 (a refined form of kerosene) loading on both stages and liquid oxygen (LOX) loading on the first stage only.
Loading of RP-1 on the second stage wraps up first at the T-20 minute mark followed by the usual “T-20 minute vent” as the oxygen purging begins on the pipelines of the Falcon 9 Transporter/Erector (T/E) that supplies fluids and power to the vehicle. LOX load on the second stage begins about four minutes after that at T-16 minutes.
Engine chill commences at the T-7 minute mark with a small flow of LOX going into the turbopumps on all nine Merlin engines on the first stage. RP-1 loading on the booster then wraps up about a minute later at the T-6 minute mark.
LOX load on the first and second stages ends at around the T-3 minute and T-2 minute mark respectively, and the rocket takes control of the countdown at the T-1 minute mark.
Engine ignition is commanded at T-3 seconds allowing them to achieve maximum thrust and pass final checks before committing to launch and if engine checks look correct, the ground clamps release the rocket for liftoff at the expected T0 time.
After liftoff, Falcon 9 climbs away from the launch site, pitching downrange as it maneuvers along its pre-programmed trajectory. Approximately 72 seconds into the flight, the vehicle passes through Max-Q — the point of maximum dynamic pressure, where mechanical stresses on the rocket are the greatest.
The nine first-stage engines continue to power Falcon 9 for the first two minutes and 30 seconds of the mission, until the time of main engine cutoff (MECO), at which point all nine engines shut down nearly simultaneously.
Stage separation normally occurs 3-4 seconds later, with the ignition of the second stage’s Merlin Vacuum engine coming about seven seconds after staging.
While the second stage continues onward to orbit with its payload, the first stage coasts upward to apogee — the highest point of its trajectory — before beginning its trip back to Earth. The booster refines its course toward the landing zone before attempting to softly touch down on the deck of one of SpaceX’s three drone ships.
Two or three burns are required to secure the safe return and landing of a Falcon 9 booster depending on the chosen landing site. A boost back burn nullifies the horizontal speed from about 7000 km/h plus to a 1000 km/h negative if a return to launch site is chosen.
Normally a free fall trajectory is chosen which requires a re-entry burn designed to break the speed into the denser atmosphere. The Merlin 1D# engines start in a 1-3-1 sequence with the center engine 9 starting 4 seconds before lighting up engine 1 and 5 in a burn lasting 14-16 seconds ending with a 2 second center engine solo burn.
The re-entry burn last 20-22 seconds and the booster is now falling and steering through the denser atmosphere with the 6x8 feet grid fins. A last landing burn performed by the Merlin 1D# center engine is timed to the last millisecond securing the aiming and breaking of the boosters speed. Booster landings have now been performed 270 times.
Using a drone ship for booster recovery allows SpaceX to launch more mass in a payload on Falcon 9 than it would be able to launch on a return-to-launch-site mission.
In the meantime, the second stage carries on with the primary mission. After stage separation and Merlin Vacuum engine ignition, the payload fairing halves are jettisoned, thereby exposing the satellites to space.
Much akin to the Falcon 9 first stage, the fairing halves can be recovered and reused, using a system of thrusters and parachutes to make a controlled descent into the ocean where they will be picked up by a recovery vessel.
Second-stage engine cutoff (SECO-1) takes place just over eight and a half minutes into the flight. Other engine burns to modify or increase the deployment orbit will follow if the mission requires it. This science mission used a second burn before deploying the PACE satellite into a sun synchronous polar orbit
After spacecraft separation, the second stage will, if it’s still in low earth orbit, perform a deorbit burn for proper disposal, ensuring that reentry takes place in the south Pacific or Indian Ocean. Second stages in high geostationary transfer or geostationary insertion orbits - GTO and GEO - will become one of now 35 pieces of derelict space debris.
When the second stage comes in contact with ground control station in either Hawthorne, California or Boca Chica, Texas after one orbit. The deorbit burn and a blow out command is given simultaneously to brake and empty the propellant tanks. 40-45 minutes later the second stage re-enters and crashes into the Indian/Pacific Ocean.
The rocket vehicle
The Falcon 9 Block 5 is SpaceX’s partially reusable two-stage medium-lift launch vehicle. The vehicle consists of a reusable first stage, an expendable second stage, and, when in payload configuration, a pair of reusable fairing halves.
The Falcon 9 first stage contains 9 Merlin 1D# sea level engines. Each engine uses an open gas generator cycle and runs on RP-1 and liquid oxygen (LOx). Each engine produces 845 kN of thrust at sea level, with a specific impulse (ISP) of 285 seconds, and 934 kN in a vacuum with an ISP of 313 seconds.
Due to the powerful nature of the engine, and the large amount of them, the Falcon 9 first stage is able to lose an engine right off the pad, or up to two later in flight, and be able to successfully place the payload into orbit.
The Merlin engines are ignited by triethylaluminum and triethylborane (TEA-TEB), which instantaneously burst into flames when mixed in the presence of oxygen. During static fire and launch the TEA-TEB is provided by the ground service equipment. However, as the Falcon 9 first stage is able to propulsively land, three of the Merlin engines (E1, E5, and E9) contain TEA-TEB canisters to relight for the boost back, reentry, and landing burns.
The Falcon 9 second stage is the only expendable part of the Falcon 9. It contains a singular MVacD engine that produces 992 kN of thrust and an ISP of 348 seconds. The Falcon 9 can put some or many payloads in different orbits on missions with many burns and/or long coasts between burns, the second stage is able to be equipped with a mission extension package.
When the second stage has this mission extension package it has a gray strip, which helps keep the RP-1 warm in sunlight, an increased number of composite-overwrapped pressure vessels (COPVs) for pressurization control, and additional TEA-TEB.
SpaceX is the first entity ever that recovers and reflies its fairings. After being jettisoned, the two fairing halves will use cold gas thrusters to orientate themselves as they descend through the atmosphere. Once at a lower altitude, they will deploy drogue chutes and parafoils to help them glide down to a soft landing for recovery.
The Falcon 9’s fairing consists of two dissimilar reusable halves. The first half (the half that faces away from the transport erector) is called the active half, and houses the pneumatics for the separation system. The other fairing half is called the passive half.
Comparison of Type 1 and 2 with measurements based on pixels - Type 2 are 5-6 inches thicker
As the name implies, this half plays a purely passive role in the fairing separation process, as it relies on the pneumatics from the active half.
SpaceX used boats with giant suspended nets to attempt to catch the fairing halves, however, at the end of 2020 this program was canceled due to safety risks and a low success rate. On the PACE mission, SpaceX will recover the fairing halves from the ocean north of Cuba with the recovery vessel Bob.
There are three known types of 34 x 17 foot fairings used by SpaceX to protect payload during ascent through the atmosphere. The first type had 10 evenly spaced ventilation ports in a circle on the bottom part of the fairings. This type was not aerodynamic enough to carry a parachute and ACS - Attitude Control System.
The aerodynamic balance during descent must have made them prone to stalling, or they burned up too easily. ACS gas tanks, flight orientation computers and ACS thrusters must have helped with these problems during development of type 2 fairings.
The second type is a slightly thicker fairing with only 8 evenly spaced ventilation ports in a circle on the bottom part of the fairings. The ventilation ports release the pressurized Nitrox gas during ascent, but let seawater in which makes it harder to refurbish the fairings after recovery from the ocean.
In 2021, SpaceX started flying a new “upgraded” version of the Falcon 9 fairing. The third type has 8 ventilation ports in pair’s near the edge of the fairings.
Some old type 2 fairings have been rebuilt and reused in Starlink launches. That have been a test program to develop the type 3 fairings to prevent saltwater from the ocean from flooding and sinking the fairing, and makes refurbishment toward the next flight easier.
Lately it’s apparent that the fairings are actively aiming for the droneship in order to speed up the recovery process and cut corners of the time table. The fairings are breaking their speed during reentry and before deploying the parachute at altitude or the last moment.
Another solution is a ‘vertical’ boost lifting the fairings apogee so the ballistic trajectory is changed aiming for a landing nearer the droneship. It’s equivalent to raising the angle on a water hose giving the water stream an higher arc but giving it a shorter reach.
It’s not clear whether or not the cold gas nitrogen thrusters alone are capable of doing a ‘boost back’ or a ‘push up’ so the fairings can alter their forward momentum mid-flight. Every landing within 50 km of the ASDS seems to be an aimed fairing landing.
The PACE mission won’t be utilizing this ‘push up’ fairing recovery program, which seems not to be in use anymore due to the danger of a wayward fairing half landing to close.
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